The Golden Age of Thermally Activated Delayed Fluorescence Materials: Design and Exploitation

Abstract

Since the seminal report by Adachi and co-workers in 2012, there has been a veritable explosion of interest in the design of thermally activated delayed fluorescence (TADF) compounds, particularly as emitters for organic light-emitting diodes (OLEDs). With rapid advancements and innovation in materials design, the efficiencies of TADF OLEDs for each of the primary color points as well as for white devices now rival those of state-of-the-art phosphorescent emitters. Beyond electroluminescent devices, TADF compounds have also found increasing utility and applications in numerous related fields, from photocatalysis, to sensing, to imaging and beyond. Following from our previous review in 2017 ( Adv. Mater. 2017, 1605444), we here comprehensively document subsequent advances made in TADF materials design and their uses from 2017-2022. Correlations highlighted between structure and properties as well as detailed comparisons and analyses should assist future TADF materials development. The necessarily broadened breadth and scope of this review attests to the bustling activity in this field. We note that the rapidly expanding and accelerating research activity in TADF material development is indicative of a field that has reached adolescence, with an exciting maturity still yet to come.

1

(a) Structure and operational mechanism of an OLED. (b) Fermionic spin statistics of exciton generation in the OLED, showing that they are formed in a 3:1 ratio of triplets to singlets.

2

CIE diagram displaying two different color gamuts, where the sRGB color points are highlighted as circles (connected by solid white lines) and the Rec. 2020 as squares (connected by dotted white lines). Pure (0.33, 0.33) and warm (0.45, 0.41) white coordinates are identified by triangles.

3

Exciton formation mechanisms in different classes of OLEDs and associated maximum internal quantum efficiency in the device, from fluorescent OLEDs (F-OLEDs) to inverted singlet-triplet gap OLEDs (INVEST-OLEDs).

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Timeline of key milestones and structures of TADF materials (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

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Overview of categories of TADF materials and the applications that benefit from their use.

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Diagram illustrating the TADF mechanism following a) photoexcitation and b) electrical excitation. S₀ and S₁ are the ground and the excited singlet states, respectively; T₁ is the triplet state; S_n refers to the higher-lying singlet state; ISC is the intersystem crossing and RISC the reverse intersystem crossing processes; F and DF are the prompt and delayed fluorescence, respectively; NR is the nonradiative transition process; and VR refers to vibrational relaxation.

7

Schematic representation of different classifications of excitons based on MO overlap between initial (blue) and final (red) molecular orbitals in a hypothetical molecule with two different moieties, A and B, connected covalently to each other.

8

An example of modulation of the ³CT and ³LE energy levels, achieved by replacing two carbazole donor groups with 3,6-diphenylcarbazole groups, for faster k _RISC (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

9

Two examples where reducing the conformational flexibility of the emitter results in a change in the photophysics. Here, a) highlights the shutdown of TADF with the addition of ⁱPr groups and b) k _RISC is slowed upon substitution of methyl groups for adamantyl groups within the donor (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

10

Illustration showing different trajectories of the generated photons following exciton formation within the emitting layer of an OLED.

11

Schematic of a D-A TADF emitter design, with examples of widely used donors and acceptors and their respective HOMO and LUMO values calculated in the gas phase using DFT (PBE0/6-31G(d,p)). The blue color signifies donor moieties, while the red color signifies acceptor moieties.

12

Timeline, structures, properties, and device data of key milestones in D-A TADF emitter development from 2011 to 2022 (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

13

Simplified representation of the calculations of a) vertical and b) adiabatic ΔE _ST, where λ_S1 and λ_T1 are the S₁ and T₁ relaxation energies, respectively.

14

DFT calculations of 4CzIPN: a) HOMO-LUMO, b) NTO corresponding to the S₁-S₀ transition, c) attachment/detachment densities associated with S₁, and d) difference density S₁-S₀ plots at the TDA-PBE0/6-31G(d,p) level. Isovalue = 0.02 for a and b, 0.001 for c and d. The nature of the emissive S₁ state is long-range charge transfer (LRCT).

15

Excited-state difference density plot of a MR-TADF (DABNA-1) emitter (left) and D-A TADF (PTZ-DBTO2) emitter (right). Taken and adapted with permission from ref . Copyright [2022/Journal of Chemical Theory and Computation] American Chemical Society.

16

Calculated vs experimental ΔE _ST data of DOBNA (left) and DABNA-1 (right). The ΔE _ST values from the coupled cluster calculations (SCS-CCS) are circled in green. The structure of TABNA, a model MR-TADF compound used in this study, is also displayed. Figure taken and adapted with permission from ref . Copyright [2020/Advanced Functional Materials] John Wiley & Sons under Creative Commons Attribution 4.0 International License https://creativecommons.org/licenses/by/4.0/.

17

Plots of experimental vs calculated a) ΔE _ST and b) S₁ energies of modelled MR-TADF emitters at the SCS-CC2/cc-pVDZ level. Taken and adapted with permission from ref . Copyright [2022/Journal of Chemical Theory and Computation] American Chemical Society.

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Structures of three D-A TADF emitters with fast k _RISC involving higher-lying T_n states (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

19

Structures of MR-TADF compounds, where the structure of the emitters DABNA-1 and ICzMes₃ have been expanded in the emitters v-DABNA and DiICzMes₄ containing a decreased T₁-T₂ gap (the blue color signifies donor atoms, while the red color signifies acceptor atoms).

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Structures of three D-A TADF systems where k _RISC and SOC were investigated computationally (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

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MR-TADF emitters for which SOC and k _RISC have been investigated computationally (the blue color signifies donor atoms/functional groups, while the red color signifies acceptor atoms/functional groups).

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Examples of four emitters where the impacts of conformational and vibronic effects on k _RISC have been probed (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

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Structures of D-A TADF emitters whose properties have been computed in the condensed state and for which the solid-state polarization effects have been explicitly taken into account (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

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Examples of MR-TADF compounds where the emission spectra and vibrational contributions have been simulated (the blue color signifies donor atoms/functional groups, while the red color signifies acceptor atoms/functional groups).

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Examples of emitters where their excited-state dynamics have been investigated computationally (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

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Examples of emitters generated from ML models and high-throughput screening techniques (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

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Timeline and structures of key blue TADF emitters preceding this review (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

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Structures of blue TADF emitters featuring TRZ as the acceptor moiety and single donor groups (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

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Structures of blue TADF emitters featuring TRZ as the acceptor moiety with sterically restricted donor groups resulting from substitution either on the bridging aryl moiety or on the donors themselves (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

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Structures of blue TADF emitters featuring modified acridine donor or TRZ acceptor groups (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

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Structures of blue TADF emitters featuring TRZ acceptor moieties attached to multiple donors (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

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Structures of blue TADF emitters featuring donor macrocycle-substituted TRZ acceptor moieties or tristriazolotriazines as acceptors (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

33

CIE color coordinates of blue D-A TADF emitters containing triazine acceptors. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted illustrating the structure of the emitter of the “bluest” device, the structure of the emitter used in the device showing the highest EQE_max and the structure of the emitter associated with the device showing the lowest efficiency roll-off. Only TADF OLEDs where the λ_EL < 490 nm are included. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for blue, (0.131, 0.046), is defined as the “bluest”. The most efficient device is quantified by the highest EQE_max. The efficiency roll-off is quantified as the change in efficiency between EQE_max and EQE₁₀₀₀. In the chemical structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

34

Molecular structures of pyrimidine- and pyrazine-based blue TADF emitters (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

35

CIE color coordinates of blue D-A TADF emitters containing nitrogen heterocycle acceptors. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structure of the emitter of the “bluest” device, the structure of the emitter used in the device showing the highest EQE_max and the structure of the emitter associated with the device showing the lowest efficiency roll-off. Only TADF OLEDs where the λ_EL < 490 nm are included. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for blue, (0.131, 0.046), is defined as the “bluest”. The most efficient device is quantified by the highest EQE_max. The efficiency roll-off is quantified as the change in efficiency between EQE_max and EQE₁₀₀₀. In the chemical structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

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Structures of blue TADF emitters featuring unfused boron acceptors (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

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Structures of blue TADF emitters featuring fully fused boron acceptors (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

38

CIE color coordinates of blue TADF emitters containing boron acceptors. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structure of the emitter of the “bluest” device, the structure of the emitter used in the device showing the highest EQE_max and the structure of the emitter associated with the device showing the lowest efficiency roll-off. Only TADF OLEDs where the λ_EL < 490 nm are included. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for blue, (0.131, 0.046), is defined as the “bluest”. The most efficient device is quantified by the highest EQE_max. The efficiency roll-off is quantified as the change in efficiency between EQE_max and EQE₁₀₀₀. In the chemical structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

39

Molecular structures of nitrile-based blue TADF emitters discussed here (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

40

CIE color coordinates of blue D-A TADF emitters containing nitrile acceptors. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structure of the emitter of the bluest device, the structure of the emitter used in the device showing the highest EQE_max and the structure of the emitter associated with the device showing the lowest efficiency roll-off. Only TADF OLEDs where the λ_EL < 490 nm are included. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for blue, (0.131, 0.046), is defined as the “bluest”. The most efficient device is quantified by the highest EQE_max. The efficiency roll-off is quantified as the change in efficiency between EQE_max and EQE₁₀₀₀. In the chemical structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

41

Molecular structures of oxadiazole-based blue TADF emitters discussed here (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

42

CIE color coordinates of blue D-A TADF emitters containing oxadiazole acceptors. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structure of the emitter of the bluest device, the structure of the emitter used in the device showing the highest EQE_max and the structure of the emitter associated with the device showing the lowest efficiency roll-off. Only TADF OLEDs where the λ_EL < 490 nm are included. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for blue, (0.131, 0.046), is defined as the “bluest”. The most efficient device is quantified by the highest EQE_max. The efficiency roll-off is quantified as the change in efficiency between EQE_max and EQE₁₀₀₀. The blue color signifies donor moieties, while the red color signifies acceptor moieties.

43

Molecular structures of sulfone-based blue TADF emitters discussed here (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

44

CIE color coordinates of blue D-A TADF emitters containing sulfone acceptors. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structure of the emitter of the bluest device, the structure of the emitter used in the device showing the highest EQE_max and the structure of the emitter associated with the device showing the lowest efficiency roll-off. Only TADF OLEDs where the λ_EL < 490 nm are included. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for blue, (0.131, 0.046), is defined as the “bluest”. The most efficient device is quantified by the highest EQE_max. The efficiency roll-off is quantified as the change in efficiency between EQE_max and EQE₁₀₀₀. In the chemical structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

45

Molecular structures of ketone-based blue TADF emitters discussed here (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

46

CIE color coordinates of blue D-A TADF emitters containing ketone acceptors. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structure of the emitter of the bluest device, the structure of the emitter used in the device showing the highest EQE_max and the structure of the emitter associated with the device showing the lowest efficiency roll-off. Only TADF OLEDs where the λ_EL < 490 nm are included. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for blue, (0.131, 0.046), is defined as the “bluest”. The most efficient device is quantified by the highest EQE_max. The efficiency roll-off is quantified as the change in efficiency between EQE_max and EQE₁₀₀₀. In the chemical structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

47

Molecular structures of blue emitters with one of phosphine oxide, trifluoromethyl, indolocarbazole, or quinoxaline as the acceptor moiety (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

48

CIE color coordinates of blue D-A TADF emitters containing acceptors illustrated in Figure . The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structure of the emitter of the bluest device, the structure of the emitter used in the device showing the highest EQE_max and the structure of the emitter associated with the device showing the lowest efficiency roll-off. Only TADF OLEDs where the λ_EL < 490 nm are included. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for blue, (0.131, 0.046), is defined as the “bluest”. The most efficient device is quantified by the highest EQE_max. The efficiency roll-off is quantified as the change in efficiency between EQE_max and EQE₁₀₀₀. In the chemical structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

49

a) Selected structures of the emitters of the best performing blue OLEDs summarized in this section, with respect to color purity, device lifetime, and maximum efficiency (the blue color signifies donor moieties, while the red color signifies acceptor moieties). b) EQE_max vs CIE_y coordinate of all the blue OLEDs reviewed in this section. Different colors act as a visual guide (non-literal) of the device emission color.

50

Molecular structure of 4CzIPN, one of the first notable green TADF emitters. Note the twisted donor carbazole groups with respect to the isophthalonitrile acceptor. The blue color signifies donor moieties, while the red color signifies acceptor moieties.

51

a) Molecular structures of green D-A TADF emitters containing nitrile acceptors and b) CIE color coordinates of green D-A TADF emitters containing nitrile acceptors. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structure of the emitter of the “greenest” device and the structure of the emitter used in the device showing the highest efficiency and the lowest efficiency roll-off. Only D-A TADF OLEDs where the λ_EL = 490–580 nm that show EQE_max > 20% or have minimal efficiency roll-off are included. The most efficient device is quantified by the highest EQE_max. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for green, (0.170, 0.797), is defined as the “greenest”. In the chemical structures the blue color signifies donor moieties, while the red color signifies acceptor moieties.

52

a) Molecular structures of green D-A TADF emitters containing boron acceptors and b) CIE color coordinates of green D-A TADF emitters containing boron acceptors. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structure of the emitter of the “greenest” device and the structure of the emitter used in the device showing the highest efficiency and the lowest efficiency roll-off. Only D-A TADF OLEDs where the λ_EL = 490–580 nm that show EQE_max > 20% or have minimal efficiency roll-off are included. The most efficient device is quantified by the highest EQE_max. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for green, (0.170, 0.797), is defined as the “greenest”. In the chemical structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

53

a) Molecular structures of green D-A TADF emitters containing sulfone acceptors and b) CIE color coordinates of green D-A TADF emitters containing sulfone acceptors. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structure of the emitter of the “greenest” device and the structure of the emitter used in the device showing the highest efficiency and the lowest efficiency roll-off. Only D-A TADF OLEDs where the λ_E = 490–580 nm that show EQE_max > 20% or have minimal efficiency roll-off are included. The most efficient device is quantified by the highest EQE_max. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for green, (0.170, 0.797), is defined as the “greenest”. In the chemical structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

54

a) Molecular structures of green D-A TADF emitters containing triazine acceptors and b) CIE color coordinates of green D-A TADF emitters containing triazine acceptors. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structure of the emitter of the “greenest” device and the structure of the emitter used in the device showing the highest efficiency and the lowest efficiency roll-off. Only D-A TADF OLEDs where the λ_EL = 490–580 nm that show EQE_max > 20% or have minimal efficiency roll-off are included. The most efficient device is quantified by the highest EQE_max. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for green, (0.170, 0.797), is defined as the “greenest”. In the chemical structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

55

a) Molecular structures of green D-A TADF emitters containing diazine acceptors and b) CIE color coordinates of green D-A TADF emitters containing pyrimidine or pyridine acceptors. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structure of the emitter of the “greenest” device, the structure of the emitter used in the device showing the highest efficiency and the structure of the emitter with the lowest efficiency roll-off. Only D-A TADF OLEDs where the λ_EL = 490–580 nm which show EQE_max > 20% or have minimal roll-off are included. The most efficient device is quantified by the highest EQE_max. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for green, (0.170, 0.797), is defined as the “greenest”. In the chemical structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

56

a) Molecular structures of green D-A TADF emitters containing other N-heterocycle acceptors and b) CIE color coordinates of green D-A TADF emitters containing other N-heterocycle acceptors. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structure of the emitter of the “greenest” device, the structure of the emitter used in the device showing the highest efficiency and the structure of the emitter with the lowest efficiency roll-off. Only D-A TADF OLEDs where the λ_EL = 490–580 nm which show EQE_max > 20% or have minimal roll-off are included. The most efficient device is quantified by the highest EQE_max. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for green, (0.170, 0.797), is defined as the “greenest”. In the chemical structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

56

a) Molecular structures of green D-A TADF emitters containing other N-heterocycle acceptors and b) CIE color coordinates of green D-A TADF emitters containing other N-heterocycle acceptors. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structure of the emitter of the “greenest” device, the structure of the emitter used in the device showing the highest efficiency and the structure of the emitter with the lowest efficiency roll-off. Only D-A TADF OLEDs where the λ_EL = 490–580 nm which show EQE_max > 20% or have minimal roll-off are included. The most efficient device is quantified by the highest EQE_max. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for green, (0.170, 0.797), is defined as the “greenest”. In the chemical structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

57

a) Molecular structures of green D-A TADF emitters containing carbonyl acceptors and b) CIE color coordinates of green D-A TADF emitters containing carbonyl acceptors. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structure of the emitter of the “greenest” device, the structure of the emitter used in the device showing the highest efficiency quantified by the EQE_max and the structure of the emitter with the lowest efficiency roll-off. Only D-A TADF OLEDs where the λ_EL = 490–580 nm which show EQE_max > 20% or have minimal roll-off are included. The most efficient device is quantified by the highest EQE_max. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for green, (0.170, 0.797), is defined as the “greenest”. In the chemical structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

57

a) Molecular structures of green D-A TADF emitters containing carbonyl acceptors and b) CIE color coordinates of green D-A TADF emitters containing carbonyl acceptors. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structure of the emitter of the “greenest” device, the structure of the emitter used in the device showing the highest efficiency quantified by the EQE_max and the structure of the emitter with the lowest efficiency roll-off. Only D-A TADF OLEDs where the λ_EL = 490–580 nm which show EQE_max > 20% or have minimal roll-off are included. The most efficient device is quantified by the highest EQE_max. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for green, (0.170, 0.797), is defined as the “greenest”. In the chemical structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

58

a) Molecular structures of green D-A TADF emitters containing other acceptors and b) CIE color coordinates of green D-A TADF emitters containing other acceptors. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structure of the emitter of the “greenest” device, the structure of the emitter used in the device showing the highest efficiency and the structure of the emitter with the lowest efficiency roll-off. Only D-A TADF OLEDs where the λ_EL = 490–580 nm which show EQE_max > 20% or have minimal roll-off are included. The most efficient device is quantified by the highest EQE_max. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for green, (0.170, 0.797), is defined as the “greenest”. In the chemical structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

59

Acceptor motifs commonly used in the design of red and NIR TADF emitters, ordered according to the acceptor strength and π-conjugation length. The LUMO values were calculated at the PBE0/6-31G(d,p) level in the gas phase.

60

a) Molecular structures of red D-A TADF emitters containing terephthalonitrile acceptors, b) molecular structures of red D-A TADF emitters containing pyridine-3,5-dicarbonitrile acceptors, and c) CIE color coordinate of the most efficient emitter based on a pyridine-3,5-dicarbonitrile acceptor. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structure of the emitter of the “reddest” device and the structure of the emitter used in the device showing the highest efficiency and the lowest efficiency roll-off. Only D-A TADF OLEDs where the λ_EL > 580 nm that are high performing are included. The most efficient device is quantified by the highest EQE_max. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for red, (0.708, 0.292), is defined as the “reddest”. In the chemical structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

61

a) Molecular structures of red D-A TADF emitters containing quinoxaline acceptors and b) CIE color coordinates of red D-A TADF emitters containing quinoxaline acceptors. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structure of the emitter of the “reddest” device and the structure of the emitter used in the device showing the highest efficiency and the lowest efficiency roll-off. Only D-A TADF OLEDs where the λ_EL > 580 nm that are high performing are included. The most efficient device is quantified by the highest EQE_max. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for red, (0.708, 0.292), is defined as the “reddest”. In the chemical structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

62

a) Molecular structures of red D-A TADF emitters containing acenaphtho[1,2-b]pyrazine acceptors and b) CIE color coordinates of red D-A TADF emitters containing acenaphtho[1,2-b]pyrazine acceptors. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structure of the emitter of the “reddest” device and the structure of the emitter used in the device showing the highest efficiency and the lowest efficiency roll-off. Only D-A TADF OLEDs where the λ_EL > 580 nm that are high performing are included. The most efficient device is quantified by the highest EQE_max. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for red, (0.708, 0.292), is defined as the “reddest”. In the chemical structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

63

Structures of red TADF emitters using a) di­ben­zo[f,h]quin­oxa­line-2,3-di­car­bo­nitrile acceptor with two donor groups, b) di­ben­zo[f,h]quin­oxa­line-2,3-di­car­bo­nitrile acceptor with one donor group, and c) di­ben­zo[a,c]phen­azine-3,6-di­car­bo­nitrile acceptor (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

64

Molecular structures of red TADF emitters featuring pyrazino-phenanthrene acceptors. The blue color signifies donor moieties, while the red color signifies acceptor moieties.

65a

Molecular structures of red D-A TADF emitters containing either pyra­zin­yl­phen­an­threne or pyra­zin­yl­phen­an­thro­line acceptors. In the molecular structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

65b

CIE color coordinates of red D-A TADF emitters containing phenanthrene acceptors. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structure of the emitter used in the device showing a high efficiency which is quantified by the EQE_max, the structure of a near-IR emitter (λ_EL ∼ 780 nm) used in a device that showed a high efficiency which is quantified by the EQE_max and the structure of the emitter with the lowest efficiency roll-off, which was accomplished in a non-doped device with high efficiency. Only D-A TADF OLEDs where the λ_EL > 580 nm which are high performing are included. In the molecular structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

66

a) Molecular structures of red D-A TADF emitters containing phen­an­thro­[4,5-abc]phen­azine-11,12-di­car­bo­nitrile acceptors, b) molecular structures of red D-A TADF emitters containing phen­an­thro­[4,5-fgh]quin­oxa­line-10,11-di­car­bo­nitrile acceptors, and c) CIE color coordinates of red D-A TADF emitters containing phen­an­thro­[4,5-abc]phen­azine-11,12-di­car­bo­nitrile and phen­an­thro­[4,5-fgh]quin­oxa­line-10,11-di­car­bo­nitrile acceptors. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structure of the emitter of the “reddest” device and the structure of the emitter used in the device showing the highest efficiency. Only D-A TADF OLEDs where the λ_EL > 580 nm which are high performing are included. The most efficient device is quantified by the highest EQE_max. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for red, (0.708, 0.292), is defined as the “reddest”. In the molecular structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

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a) Molecular structures of red D-A TADF emitters containing 1,8-naphthalimide acceptors and b) CIE color coordinates of red D-A TADF emitters containing 1,8-naphthalimide acceptors. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structure of the emitter of the “reddest” device, the structure of the emitter used in the device showing the highest efficiency and the lowest efficiency roll-off. Only D-A TADF OLEDs where the λ_EL > 580 nm which are high performing are included. The most efficient device is quantified by the highest EQE_max. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for red, (0.708, 0.292), is defined as the “reddest”. In the molecular structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

68

Chemical structure of trinaphtho[3,3,3]propellane (TNP) and molecular engineering pathway. Taken and adapted with permission from ref . Copyright [2022/Nature Communications] Springer Nature under Creative Commons Attribution 4.0 International License https://creativecommons.org/licenses/by/4.0/.

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Chemical structures of red TADF emitters using other miscellaneous boron-containing acceptor moieties. The blue color signifies donor moieties, while the red color signifies acceptor moieties.

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a) Molecular structures of red D-A TADF emitters containing other acceptors and b) CIE color coordinates of red D-A TADF emitters containing other acceptors. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structure of the emitter of the “reddest” device and the structure of the emitter used in the device showing the highest efficiency and the lowest efficiency roll-off. Only D-A TADF OLEDs where the λ_EL > 580 nm which are high performing are included. The most efficient device is quantified by the highest EQE_max. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for red, (0.708, 0.292), is defined as the “reddest”. In the chemical structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

71

EQE_max vs λ_EL for selected emitter structures from high-performance red TADF OLEDs reviewed in this section.

72

A summary of strategies to achieve TADF-based WOLEDs and their associated performance metrics for the best-forming examples.

73

Molecular structures of components used in hybrid TADF WOLEDs: a) blue TADF emitters, b) R/O/G/B phosphorescence emitters, c) exciplex-type materials. In a, the blue color signifies donor moieties/atoms, while the red color signifies acceptor moieties/atoms.

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Molecular structures of components used in TADF WOLEDs: a) TADF emitters, b) fluorescent emitters, c) host and exciplex-type TADF hosts. The blue color signifies donor moieties, while the red color signifies acceptor moieties.

75

Molecular structures of emitters used in single emitter material TADF WOLEDs (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

76

Structures of CP-TADF emitters containing stereogenic centers and their respective |g_PL| (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

77

Structures of CP-TADF emitters possessing axial chirality and their respective |g_PL| (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

78

Structures of CP-TADF emitters with planar chirality and their respective |g_PL| (the blue color signifies donor moieties/atoms/functional groups, while the red color signifies acceptor moieties/atoms/functional groups).

79

Structures of CP-TADF emitters featuring chiral perturbation and their respective |g_PL| (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

80

Structures of chiral TADF exciplexes and LC emitters and their respective |g_PL| (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

81

g_EL vs EQE_max comparison for reported CP TADF OLEDs reviewed in this section (the color points represent the emission color of the devices).

82

Simplified energy level diagram of donor and acceptor molecules and their interaction in a bimolecular exciplex. On the left is a representation of exciton formation, and on the right is a Jablonski diagram showing the relative energies of the singlet, S_E, and the triplet, T_E, states of the exciplex.

83

Chemical structures of electron donor molecules used in TADF exciplex systems.

84

Chemical structures of electron acceptor molecules used in TADF exciplex systems.

85

Chemical structures of donor and acceptor molecules employed in exciplexes with a TADF molecule as a component of the exciplex (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

86

Structures of donor materials used to understand and improve the performance of exciplex systems discussed in this section.

87

Structures of acceptor materials used to understand and improve the performance of exciplex systems. The structure of Ir(tptpy)₂(acac) was misdrawn in the original publication.

88

Structures of donor and acceptor materials used in either exciplex hosts or solution-processed TADF exciplex systems.

89

Luminescent, phosphorescent, TADF, and host materials used as dopants in either exciplex hosts or solution-processed TADF exciplex systems. The blue color signifies donor moieties, while the red color signifies acceptor moieties for TADF compounds.

90

Molecular structures of D/A compounds bound by electrostatic interactions, photoluminescent compounds, donor and acceptor compounds, D-A type compounds and spacers used in WOLEDs or applied toward fundamental studies of TADF exciplex systems (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

91

CIE color coordinates of high-performance TADF exciplex devices. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structures of the emitters of the “bluest”, “greenest”, “reddest”, and “whitest” devices and the structures of the emitters used in the devices showing the highest efficiency blue, green, red, NIR, and white emission. The most efficient devices are quantified by the EQE_max at λ_EL < 490 nm for blue, λ_EL = 490–580 nm for green, λ_EL > 580 nm for red, and CIE coordinates close to (0.33, 0.33) for white. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for blue, (0.131, 0.046), is defined as the “bluest”. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for green, (0.170, 0.797), is defined as the “greenest”. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for red, (0.708, 0.292), is defined as the “reddest”. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for white, (0.33, 0.33), is defined as the “whitest”. In the chemical structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

92

Periodic table with metals that feature in TADF-active materials colored in blue.

93

Timeline of milestones achieved using metal-containing (Cu and Au) TADF emitters discussed in the Introduction. The OLED EQE_max has increased from 4.8% to 27.4% over a period of 10 years.

94

Diagrams representing the different forms of metal-containing TADF complexes. a) Organic Donor-Acceptor TADF molecule for comparison. b) Metal-containing TADF emitter with a metal directly involved in the charge transfer excited state. c) Metal-bridged TADF complexes, in which the metal bridges donor and acceptor moieties where, respectively, the HOMO and LUMO are located. d) Metal complex with a D-A TADF ligand. Taken and adapted with permission from ref . Copyright [2020/Advanced Optical Materials] John Wiley & Sons.

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a) Representation of the excited states involved in emission for metal complexes that exhibit both TADF and phosphorescence at room temperature, taken from ref . Copyright [2015/Journal of the American Chemical Society] American Chemical Society. b) Example of a comparison between emission lifetime and temperature, along with the derived τ(S₁) and ΔE_ST via the Boltzmann equation shown in equation , taken from Inorg. Chem. 2015, 54, 4322 with permission. c) Simulation of the emission fractions from TADF and phosphorescence and their temperature dependence for Cu₂Cl₂(N^∧P)₂, taken from ref . Copyright [2015/Journal of the American Chemical Society] American Chemical Society.

96

Cationic bis-diimine copper(I) complexes [Cu(N^∧N)₂]⁺ having TADF properties.

97

Cationic diimine/diphosphine copper(I) complexes [Cu(N^∧N)(P^∧P)]⁺ having TADF properties. POP = DPEPhos = bis((2-di­phen­yl­phos­phino)­phen­yl)­ether, xant = xantphos.

98

Neutral tetrahedral copper(I) complexes having TADF properties.

99

Tetrahedral copper(I) complexes that can switch between neutral and cationic forms having TADF properties.

100

Trigonal planar copper(I) complexes having TADF properties.

101

Linear copper(I) complexes having TADF properties.

102

Co-deposited copper(I) halide/ligand films having TADF properties.

103

Dinuclear copper(I) complexes having TADF properties.

104

Larger copper(I) clusters having TADF properties.

105

Chemical structures of monometallic silver(I) complexes having TADF properties. Cz = carbazole.

106

Chemical structures of dinuclear silver(I) complexes having TADF properties.

107

Chemical structures of multinuclear silver clusters having TADF properties.

108

Chemical structures of gold(I) complexes having TADF properties.

109

Chemical structures of TADF gold(I) complexes containing and NHC ligand and acyclometalating ligand based on MR-TADF motifs having TADF properties (ImIDz is a phosphorescent emitter, dipp = 1,3-di(4-imi­da­zo­lino­phen­oxyl)­pro­pane).

110

Chemical structures of TADF gold(III) complexes containing tridentate pincer ligands.

111

Chemical structures of tetracoordinate TADF gold(III) complexes.

112

Chemical structures of linear carbene metal amide (CMA) silver(I), copper(I) and gold(I) and complexes having TADF properties published up to 2020.

113

Chemical structures of linear carbene metal amide (CMA) silver(I), copper(I) and gold(I) complexes having TADF properties.

114

Chemical structures of TADF palladium(II) complexes.

115

Chemical structures of TADF platinum(II) complexes.

116

Chemical structures of TADF zinc(II) complexes.

117

Chemical structures of tin porphyrin and other porphyrin-based group 14 metal complexes having TADF properties.

118

Chemical structures of TADF tungsten and zirconium complexes.

119

Chemical structures of TADF alkali metal and aluminum complexes.

120

Chemical structures of TADF iridium(III) complexes.

121

Schematic diagram and example chemical structures of conjugated polymers with pendant TADF emitter groups. All examples consist of a host type motif incorporated in the polymer backbone along with known TADF moieties attached to the backbone as side chain (purple).

122

Schematic diagram and chemical structures of a) non-conjugated polymers containing pendant TADF groups and b) non-conjugated polymers containing pendant TADF and host groups. In both cases, the polymer backbone consists of a non-conjugated chain on which a known TADF emitter is the side chain (purple). In b, these TADF subunits are installed alternating with host units to improve the charge transport properties of the polymer.

123

Schematic diagram and chemical structures of a) donor-backbone (blue) TADF polymers containing pendant acceptor groups (red) and b) donor-acceptor-backbone (blue) TADF polymers containing pendant acceptor groups (red).

124

Schematic diagram and chemical structures of TADF polymers with chiral pendant groups (green) that are located on the TADF emitter (S-P and R-P) or on a separate side chain (P10) with the TADF emitter being anchored within the backbone (purple).

125

Schematic diagram and chemical structure of ASFCN (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

126

Schematic diagram and chemical structures of TADF polymers with donor (blue) and acceptor (red) emissive units within the main chain.

127

Schematic diagram and chemical structures of through-space charge transfer (TSCT) TADF polymers (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

128

Chemical structures of conjugated TADF dendrimers consisting of a central acceptor unit (red) and peripheral donor dendrons (blue).

129

a) π-Stacked emitters with donor (D)-acceptor (A) architectures; b) chemical structures of π-stacked through-space donor–acceptor TADF dendrimers. Taken and adapted with permission from ref . Copyright [2021/Angewandte Chemie International Edition] John Wiley & Sons.

130

Schematic diagram and chemical structures of non-conjugated TADF dendrimers, consisting of a central donor (blue)-acceptor (red) emitter with peripheral donor dendrons connected with a non-conjugated linker.

131

a) Simulated 460 nm emission spectra with FWHM of 100 nm (red) and 20 nm (black), and simulated 375 nm emission spectrum with FWHM of 100 nm (blue); b) Corresponding CIE color coordinates.

132

Schematic representation of the difference density plots (S₀-S₁) for the short-range charge transfer excited states in triangulene-based MR-TADF compounds, containing either a central acceptor (left) or donor atom (right).

133

a) Computed difference density plot (top) and the schematic representation of the difference density distribution of DABNA-1 (bottom), b) CIE coordinates of OLEDs with DABNA derivatives, and c) structures of early DABNA derivatives and HF-OLED assistant dopants (the blue color signifies donor moieties/atoms/functional groups, while the red color signifies acceptor moieties/atoms/functional groups). Difference density plots calculated at the SCS-CC2/cc-pVDZ level in the gas phase; is-value = 0.01.

134

a) CIE color coordinates of OLEDs with substituted DABNA emitters, and b) structures of the substituted DABNA emitters and HF-OLED assistant dopants. The white circles of the CIE diagram illustrate the spread of the emission color of the devices. In the chemical structures, the blue color signifies donor moieties/atoms/functional groups, while the red color signifies acceptor moieties/atoms/functional groups.

135

a) CIE color coordinates of OLEDs with DABNA derivatives bearing multiple acceptor atoms as emitters and b) structures of chalcogen derivatives of DOBNA. The white circles of the CIE diagram illustrate the spread of the emission color of the device. In the chemical structures, the blue color signifies donor atoms/functional groups, while the red color signifies acceptor atoms/functional groups.

136

a) CIE color coordinates of OLEDs with DABNA derivatives bearing multiple acceptor atoms as emitters and b) structures of DABNA emitters with multiple acceptor atoms. The white circles of the CIE diagram illustrate the spread of the emission color of the device. In the chemical structures, the blue color signifies donor atoms/functional groups, while the red color signifies acceptor atoms/functional groups.

137

Structures of HF-OLED assistant dopants used alongside emitters in Figure (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

138

a) Computed difference density plot and the schematic representation of the difference density distribution of Dt­Bu­CzB, b) CIE color coordinates OLEDs with CzBN derivatives, and c) structures of unsubstituted CzBN emitters and HF-OLED assistant dopants. Difference density plots calculated at the SCS-CC2/cc-pVDZ level in the gas phase; is-value = 0.01. The white circles of the CIE diagram illustrate the spread of the emission color of the device. In the chemical structures, the blue color signifies donor moieties/atoms/functional groups, while the red color signifies acceptor moieties/atoms/functional groups.

139

a) CIE color coordinates of OLEDs with CzBN derivatives containing acceptor moieties and b) structures of acceptor substituted CzBN emitters and HF-OLED assistant dopants. The white circles of the CIE diagram illustrate the spread of the emission color of the device. In the chemical structures, the blue color signifies donor moieties/atoms/functional groups, while the red color signifies acceptor moieties/atoms/functional groups).

140

a) CIE color coordinates of OLEDs with CzBN derivatives containing donor moieties and b) structures of donor substituted CzBN emitters and HF-OLED assistant dopants. The white circles of the CIE diagram illustrate the spread of the emission color of the device for para disposed D-D, the white squares of the CIE diagram illustrate the spread of the emission color of the device for para disposed D-A, and the white triangles of the CIE diagram illustrate the spread of the emission color of the device for para disposed D-D and para disposed D-A. In the chemical structures, the blue color signifies donor moieties/atoms/functional groups, while the red color signifies acceptor moieties/atoms/functional groups.

141

a) CIE color coordinates of OLEDs with CzBN derivatives containing both donor and acceptor moieties and b) structures of donor and acceptor substituted CzBN emitters and HF-OLED assistant dopants. The white circles of the CIE diagram illustrate the spread of the emission color of the device. In the chemical structures, the blue color signifies donor moieties/atoms/functional groups, while the red color signifies acceptor moieties/atoms/functional groups.

142

a) CIE color coordinates of substituted CzBN emitters designed to mitigate ACQ and b) structures of substituted CzBN emitters designed to mitigate ACQ and HF-OLED assistant dopants. The white circles of the CIE diagram illustrate the spread of the emission color of the device. In the chemical structures, the blue color signifies donor moieties/atoms/functional groups, while the red color signifies acceptor moieties/atoms/functional groups.

143

a) CIE color coordinates of OLEDs with substituted CzBN emitters where the substituents modulate either the SOC or the nature of the emissive S₁ state and b) structures of substituted CzBN emitters where the substituents modulate either the SOC or the nature of the emissive S₁ state, and the structure of the HF-OLED assistant dopant and an emitter that was not TADF active. The white circles of the CIE diagram illustrate the spread of the emission color of the device. In the chemical structures, the blue color signifies donor moieties/atoms/functional groups, while the red color signifies acceptor moieties/atoms/functional groups.

144

a) CIE color coordinates of OLEDs with fused indolocarbazole boron acceptor emitters and b) structures of reported fused indolocarbazole boron acceptor emitters and HF-OLED assistant dopants. The white circles of the CIE diagram illustrate the spread of the emission color of the device. In the chemical structures, the blue color signifies donor moieties/atoms/functional groups, while the red color signifies acceptor moieties/atoms/functional groups.

145

a) CIE color coordinates of OLEDs with CzBN emitters with multiple acceptor atoms and b) structures of reported CzBN emitters with multiple acceptor atoms and a derivative that is not TADF active. The white circles of the CIE diagram illustrate the spread of the emission color of the device. In the chemical structures, the blue color signifies donor atoms/functional groups, while the red color signifies acceptor atoms/functional groups.

146

Structures of HF-OLED assistant dopants used alongside emitters in Figure (the blue color signifies donor moieties/atoms/functional groups, while the red color signifies acceptor moieties/atoms/functional groups).

147

a) Computed difference density plot (top) and the schematic representation of the difference density distribution (bottom) of BN-DMAC, b) CIE color coordinates of OLEDs with bridged BN emitters, and c) structures of reported bridged BN emitters. Computational picture calculated S₁ excited state from SCS-CC2/cc-pVDZ; isovalue = 0.001. The white circles of the CIE diagram illustrate the spread of the emission color of the device. In the chemical structures, the blue color signifies donor moieties/atoms/functional groups, while the red color signifies acceptor moieties/atoms/functional groups. Difference density plots calculated at the SCS-CC2/cc-pVDZ level in the gas phase; is-value = 0.01.

148

a) CIE color coordinates of reported asymmetric BN emitters with nitrogen donor atoms and b) structures of reported asymmetric BN emitters with nitrogen donor atoms and HF-OLED assistant dopants. The white circles of the CIE diagram illustrate the spread of the emission color of the device. In the chemical structures, the blue color signifies donor moieties/atoms/functional groups, while the red color signifies acceptor moieties/atoms/functional groups.

149

Originally reported structure of TBN-TPA, and the confirmed asymmetric structure CzDABNA-NP-TB (the blue color signifies donor moieties/atoms/functional groups, while the red color signifies acceptor moieties/atoms/functional groups).

150

a) CIE color coordinates of OLEDs with asymmetric CzBN emitters with mixtures of donor atoms and b) structures of reported asymmetric CzBN emitters with mixtures of donor atoms and HF-OLED assistant dopants. The white circles of the CIE diagram illustrate the spread of the emission color of the device. In the chemical structures, the blue color signifies donor moieties/atoms/functional groups, while the red color signifies acceptor moieties/atoms/functional groups.

151

a) CIE color coordinates of OLEDs with MR-TADF emitters containing a four-coordinate boron atom and b) structures of reported four-coordinate boron MR-TADF emitters and HF-OLED assistant dopants. The white circles of the CIE diagram illustrate the spread of the emission color of the device. In the chemical structures, the blue color signifies donor moieties/atoms/functional groups, while the red color signifies acceptor moieties/atoms/functional groups.

152

a) Computed difference density plot (top) and the schematic representation of the difference density distribution (bottom) of ADBNA-Me-Mes, b) CIE color coordinates of OLEDs with central nitrogen BN MR-TADF emitters, and c) structures of reported central nitrogen BN MR-TADF emitters, HF-OLED assistant dopants, a derivative which was not TADF active and emitters which showed D-A emission and not MR-TADF. Difference density plots calculated at the SCS-CC2/cc-pVDZ level in the gas phase; is-value = 0.01. The white circles of the CIE diagram illustrate the spread of the emission color of the device. In the chemical structures, the blue color signifies donor moieties/atoms/functional groups, while the red color signifies acceptor moieties/atoms/functional groups.

153

a) CIE color coordinates of OLEDs with single donor and acceptor MR-TADF emitters and b) structures of reported single donor and acceptor MR-TADF emitters, HF-OLED assistant dopants and a derivative that is not TADF active. The white circles of the CIE diagram illustrate the spread of the emission color of the device. In the chemical structures, the blue color signifies donor moieties/atoms/functional groups, while the red color signifies acceptor moieties/atoms/functional groups.

154

a) Computed difference density plot (top) and the schematic representation of the difference density distribution (bottom) of QAO, b) CIE color coordinates of OLEDs with QAO derivatives, and c) structures of reported QAO-based emitters, HF-OLED assistant dopants and emitters which showed D-A emission and not MR-TADF. Difference density plots calculated at the SCS-CC2/cc-pVDZ level in the gas phase; is-value = 0.01. The white circles of the CIE diagram illustrate the spread of the emission color of the device. In the chemical structures, the blue color signifies donor moieties/atoms/functional groups, while the red color signifies acceptor moieties/atoms/functional groups.

155

a) CIE color coordinates of OLEDs with “exotic” derivatives of QAO and b) structures of reported exotic derivatives of QAO and emitters that exhibit LRCT emission and not SRCT emission associated with MR-TADF emitters. The white circles of the CIE diagram illustrate the spread of the emission color of the device. In the chemical structures, the blue color signifies donor moieties/atoms/functional groups, while the red color signifies acceptor moieties/atoms/functional groups.

156

a) Computed difference density plot (top) and the schematic representation of the difference density distribution (bottom) of TOAT, b) CIE color coordinates of OLEDs with TOAT derivatives, and c) structures of reported TOAT-based emitters and emitters which showed D-A emission and not MR-TADF. Difference density plots calculated at the SCS-CC2/cc-pVDZ level in the gas phase; is-value = 0.01. The white circles of the CIE diagram illustrate the spread of the emission color of the device. In the chemical structures, the blue color signifies donor atoms/functional groups, while the red color signifies acceptor atoms/functional groups.

157

a) Computed difference density plot (top) and the schematic representation of the difference density distribution (bottom) of tBisICz, b) CIE color coordinates of OLEDs with acceptor free MR-TADF emitters, and c) structures of reported acceptor free MR-TADF emitters, HF-OLED assistant dopant and derivatives that are not TADF active. Difference density plots calculated at the SCS-CC2/cc-pVDZ level in the gas phase; is-value = 0.01. The white circles of the CIE diagram illustrate the spread of the emission color of the device. In the chemical structures, the blue color signifies donor moieties/atoms/functional groups, while the red color signifies acceptor moieties/atoms/functional groups.

158

CIE color coordinates of all reported MR-TADF OLEDs. The white circles illustrate the spread of the emission color of the device. Rec. 2020 points are connected by a black line. Selected devices and their associated CIE coordinates represented by gray squares are highlighted, illustrating the structure of the emitter used in the “bluest”, “greenest”, and “reddest” device. Selected devices and their associated CIE coordinates represented by gray triangles are highlighted, illustrating the structure of the emitter of the highest efficiency blue, green, and red OLEDs quantified by the EQE_max. Selected devices and their associated CIE coordinates represented by gray stars are highlighted, illustrating the structure of the emitter with the fastest k _RISC. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for blue, (0.131, 0.046), is defined as the “bluest”. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for green, (0.170, 0.797), is defined as the “greenest”. The device with the CIE coordinates closest to the Rec. 2020 defined coordinates for red, (0.708, 0.292), is defined as the “reddest”. In the chemical structures, the blue color signifies donor atoms/functional groups, while the red color signifies acceptor atoms/functional groups.

159

Structures of TSCT TADF emitters containing non-conjugated bridges (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

160

Structures of TSCT TADF emitters containing spiro-fluorene bridges (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

161

Structures of TSCT emitters containing a carbazole bridge (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

162

Structures of TSCT TADF emitters using other types of bridges (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

163

Structures of TADF emitters featuring homoconjugation between the donor and the acceptor (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

164

Structures of sulfone-based AIE-TADF emitters (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

165

Structures of carbonyl-containing AIE-TADF emitters (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

166

Structures of other carbonyl-containing AIE-TADF emitters (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

167

Structures of AIE-TADF emitters based on acceptors other than those containing carbonyl or sulfonyl groups (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

168

Simplified mechanism of ESIPT emission combined with TADF characteristics; RPT is rapid proton transfer.

169

Structures of the ESIPT TADF emitter (HPI-Ac) and the fluorescent emitter MeOPI-Ac.

170

Molecular structure and ESIPT mechanism of TQB.

171

Molecular structure of the TADF based ESIPT and non-ESIPT emitters reported in ref (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

172

Molecular structure and ESIPT mechanism of the TADF ESIPT emitter TPXZBM and the non-ESIPT TADF emitter BPXZBM reported in ref (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

173

Molecular structures of the emitters HBI, BrA-HBI, Br-HBI, A-HBI and methylated BrA-HBI.

174

Excited-state intramolecular proton transfer (ESIPT) and ground-state intramolecular proton transfer (GSIPT) mechanisms operational in HL and Zn(HL)Cl₂ .

175

Structures of MCL emitters containing PTZ donors (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

176

MCL properties of PTZ-DBPHZ (1). Taken and adapted with permission from ref . Copyright [2019/Journal of Materials Chemitry C] Royal Society of Chemistry.

177

Structures of TADF materials undergoing MCL (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

178

a) Pressure-dependent PL spectra and microphotographs and b) PL spectra taken upon releasing the pressure and microphotographs for R-DOBP; c) Pressure-dependent PL spectra and microphotographs and d) PL spectra taken upon releasing the pressure and their micrographs for R-HDOBP. Taken and adapted with permission from ref . Copyright [2021/Angewandte Chemie International Edition] John Wiley & Sons.

179

Structure of the TADF emitter undergoing mechanically excited emission reported in ref (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

180

A comparison of the device structure of LECs and OLEDs.

181

Illustration of the potential profiles and electronic/ionic charge distribution in an LEC during steady-state operation. Potential profiles and charge distributions as predicted by the a) ECD and b) ED models. The thick blue line represents the potential profile across the device; electronic and ionic charge carriers are represented by cyan (negatively charged) and red (positively charged) symbols. High- and low-field regions in the bulk are highlighted in orange and yellow, respectively. In the low-field regions, negative and positive centres are mutually compensated. Taken and adapted with permission from . Copyright [2010/Journal of the American Chemical Society] American Chemical Society.

182

Structures of ionic TADF emitters used in LECs (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

183

Structures of hosts and neutral TADF emitters used in LECs (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

184

Structures of materials used in exciplex TADF LECs, including co-dopants, hosts, and transporting materials.

185

Structures of cationic MR-TADF emitters used in LECs (the blue color signifies donor atoms/functional groups, while the red color signifies acceptor atoms/functional groups).

186

Structures of widely investigated families of copper(I) complexes: a) a typical [Cu(N^∧N)(P^∧P)]⁺ complex, b) the structure of common P^∧P ligands used in TADF LECs, and c) the structure of common N^∧N ligands used in TADF LECs.

187

Structures of copper(I) TADF complexes containing a combination of N^∧N and P^∧P ligands or a combination of N^∧N and NHC ligands used in LECs.

188

Schematic illustration of the mechanism of TADF-sensitized emission using a TADF assistant dopant and a fluorescent terminal emitter, both embedded in a host matrix (aka Hyperfluorescence^TM).

189

Chemical structures of TADF assistant dopants and fluorescent terminal emitters used in reported HF-OLEDs (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

190

Chemical structure of the TADF assistant dopant CzAcSF (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

191

Chemical structures of the TADF assistant dopants and fluorescent terminal emitters. The terminal emitter BPPyA and the TADF assistant dopant DMAC-DMT were used in the high-performance HF-OLED, while KCTBC as the terminal emitter and 4CzFCN as the TADF assistant dopant were used in a solution-processed HF-OLED. The blue color signifies donor moieties, while the red color signifies acceptor moieties.

192

Chemical structures of TADF assistant dopants and fluorescent terminal emitters containing bulky moieties featuring both tert-butyl and methyl groups (the blue color signifies donor moieties, while the red color signifies acceptor moieties). In these reports, the bulky moieties were used to modulate intermolecular spacing.

193

Chemical structures of TADF assistant dopants and fluorescent terminal emitters (the blue color signifies donor moieties, while the red color signifies acceptor moieties). These HF-OLEDs show suppressed DET between the assistant dopant and the terminal emitter.

194

Chemical structures of TADF assistant dopants and fluorescent terminal emitters containing bulky groups, except for the compound terminal emitter PAD. The blue color signifies donor moieties, while the red color signifies acceptor moieties.

195

Chemical structures of orange-emitting TADF assistant dopants reported in ref (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

196

Chemical structure of the TADF assistant dopant DMAC-DPS used in refs and (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

197

Chemical structures of TADF assistant dopants and fluorescent (DCJTB) and phosphorescence (PtOEP) terminal emitters in HF-OLEDs (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

198

Chemical structures of TADF assistant dopants used with the AnTP terminal emitter in a study examining the impact of the TADF sensitizer’s dihedral angle on FRET efficiency, used in ref . The blue color signifies donor moieties, while the red color signifies acceptor moieties.

199

Chemical structures of green-emitting TADF assistant dopants DC-TC and DC-ACR used in ref .

200

Chemical structures of the TADF assistant dopant OSTFB and the TADF terminal emitter OPTA-BT-CN used in the HF-OLEDs reported in ref (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

201

Chemical structures of the TADF dendrimer BPS as assistant dopants and the iridium(III) and Cu(II) phosphorescent terminal emitters (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

202

Chemical structures of TADF assistant dopants and the terminal emitter BPPC-Ph (aka BPPC) used in NIR HF-OLEDs in refs and . The blue color signifies donor moieties, while the red color signifies acceptor moieties.

203

Chemical structures of the TADF assistant dopant tBuCz-σ-NI and the TADF terminal emitter tBuCz-π-NI used in the narrowband emitting HF-OLED from ref (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

204

Chemical structures of the organometallic TADF assistant dopants and the MR-TADF terminal emitters used in refs and (the blue color signifies donor moieties/atoms, while the red color signifies acceptor moieties/atoms).

205

Chemical structure of the doublet terminal emitter TTM-3PCz from ref (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

206

CIE color coordinates of high-performance HF-OLEDs. The white circles illustrate the spread of the emission color of the device. Selected devices and their associated CIE coordinates are highlighted, illustrating the structures of the emitter and HF-OLED assistant dopants of the highest efficiency blue, green, and red emission quantified by the EQE_max. In the chemical structures, the blue color signifies donor moieties, while the red color signifies acceptor moieties.

207

Chemical structures of TADF hosts used with phosphorescent emitters (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

208

Chemical structures of the TADF hosts used with fluorescent emitters (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

209

Chemical structures of the exciplex TADF hosts used with fluorescent emitters (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

210

Chemical structures of the TADF hosts used with other TADF emitters (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

211

Derivatives of CzBP incorporated into different supramolecular assemblies: a) A Pd(II) M₆L₁₂ metallocage. b) Combined with diacids to form gels. c) Assemblies with either one or two macrocyclic rings to give rotaxanes (the blue color signifies donor moieties, while the red color signifies acceptor moieties). Taken and adapted with permission form ref . Copyright [2018/ACS Applied Energy Materials], American Chemical Society.

212

The structures of the organic components commonly used in TADF MOFs. Where relevant, electron donors have been colored blue, with electron acceptors in red.

213

Incorporation of BTZPy into a TADF platinum(II) metallocycle for use as a dual chemo- and photodynamic therapy drug (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

214

H co-ordinating to cobalt(II) centres to give supramolecular photocatalyst H-Co. The carbazole electron donors have been colored blue, while the dicyanobenzene acceptor has been colored red. Structure taken and adapted with permission from ref . Copyright [2019/Dalton Transactions] Royal Society of Chemistry.

215

Examples of compounds used in the construction of TADF organic/carbon dots (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

216

Structure of emitter CDPA, which forms TADF nanorod needles in the neat films.

217

Structures of α-CD, DPA-1, DPA-2, and DPA-3, showing long-range charge transfer upon formation of TADF host⊂guest complexes.

218

Structures of donor host C[3]A (blue) and acceptor guest DCB (red), which can co-crystallise to give a TADF host⊂guest complex (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

219

Structures of acridine yellow and C₇₀ used in a TADF-based temperature sensor.

220

a) Structures of TADF compounds designed as biological oxygen sensors reported in ref . b) Images and total emission spectra of BF₂dnm(I)PLA in air and N₂. Photographs were taken with UV lamp excitation (λ_exc = 354 nm); delayed emission images were captured after the UV lamp was turned off. Taken and adapted with permission from ref . Copyright [2015/Macromolecules] American Chemical Society.

221

Structures of a, c) anthraquinone (a1 to a7 and e) and b) carbazole-substituted dicyanobenzenes (b, c, d1, and d2) emitters and their photophysical properties in toluene and immobilized in PS (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

222

Structures of TADF Pt, Pd, and Zn benzoporphyrins and their photophysical properties in toluene.

223

Photoluminescence properties of Pd-containing TADF sensors whose structures are shown in Figure . a) Temperature dependence of the emission intensity and spectra of Pd-T-1 in polystyrene, and photographic images of the same material at 23 and 118 °C excited with a UV-Lamp at 365 nm (all under N₂ atmosphere). b) Normalized emission spectra of Pd-T-S, Pd-O-S, Pd-T-1, and Pd-TPTBP at 23–25 °C and c) at 116–130 °C in PS under N₂. Taken and adapted with permission from ref . Copyright [2017/ACS Applied Materials & Interface] American Chemical Society.

224

Response of luminescence decay time τ a, b) and the intensity ratio (I_DF/PF) c, d) for Zn-OS in response to changes in temperature a, c) and oxygen b, d). The response is exemplified for two different temperatures and oxygen partial pressures. Taken and adapted with permission from ref . Copyright [2020/ACS Sensors] American Chemical Society.

225

Structures of TADF Zn-1 and Zn-2 Schiff base complexes and their photophysical properties in toluene and polystyrene (PS) at 25 °C.

226

Structures of ACR-ODA, PXZ-ODA, PTZ-ODA, PAZ-ODA and TTAC-ODA and their photophysical properties in co-polymers with 15% TADF monomer content (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

227

a) Illustration of pseudoaxial and pseudoequatorial conformers of PTZ-ODA and their respective emission wavelengths. b) Stern–Volmer plot calibrating I₅₁₆/I₃₉₆ emission ratios against partial pressures of O₂ for a thin film of PTZ-ODA_0.15 . c) Fluorescence emission response of PTZ-ODA_0.15 to O₂ concentrations. PF = prompt fluorescence. Taken and adapted with permission from refs and . Copyright [2020/ACS Applied Materials & Interfaces] American Chemical Society.

228

Structures of temperature-responsive linear and star-shaped TADF polymers P1 to P17 and their photophysical properties (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

229

Temperature-dependent emission spectra of a) P12 and b) P13. Ratiometric plot of I₃₉₀/I₆₆₀ vs temperature for c) P12 and d) P13. e) Schematic representation of the thermal response of these materials. Taken and adapted with permission from ref . Copyright [2020/ACS Applied Materials & Interfaces] American Chemical Society.

230

Temperature-dependent emission spectra of a) P14 and b) P15. c) Ratiometric plot of I₄₆₀/I₆₆₀ vs temperature for P14 and P15. d) CIE plot of P14 at measured temperature points. e) Visual representation of P14 color changes at various temperatures. f) Schematic representation of the thermal response for these materials. Taken and adapted with permission from ref . Copyright [2020/ACS Applied Materials & Interfaces] American Chemical Society.

231

Top: Structures of TADF diphenyl sulfone-based solvent polarity sensors 1, 2, and 3 and their photophysical properties. Bottom: Correlation of wavelength and lifetime of TADF and FL emission with polarity. a) Emission spectra of 3 in solvents of differing polarity under ambient conditions (λ_exc = 300 nm) and corresponding photographs of 3 under UV light (λ_exc = 365 nm). b) Linear fitting of the log of wavelength and lifetime ratios (TADF to FL) as a function of solvent polarity. c) Time-resolved PL decays of FL and TADF bands in different solvents. Taken and adapted with permission from ref . Copyright [2019/Nature Communications] Springer Nature.

232

The sensing mechanism of DCF-MPYM-lev to [SO₃]^2– ions, forming the TADF emitter DCF-MPYM.

233

Chemical structures and the photophysical properties of the TADF emitters PhTRZ-OCHO and PhTRZ-OCH₃ used as sensors for metal ion sensing (i.e., Ba⁺, Ca⁺, Cd²⁺, Co²⁺, Cr²⁺, Cu²⁺, Fe³⁺, Hg²⁺, K⁺, Mg²⁺, Mn²⁺, Na⁺, Ni²⁺, Pb⁺).

234

a) Schematic mechanism of X-ray-induced emission in organic TADF scintillators. b) Production ratio of S and T excited states in an organic TADF scintillator under X-ray irradiation. c) Molecular structures of anthracene, DMAC-TRZ, 4CzIPN, and 4CzTPN-Bu (the blue color signifies donor moieties, while the red color signifies acceptor moieties). Taken and adapted with permission from ref. Copyright [2020/Nature Materials] Springer Nature.

235

Photographs and X-ray images of TADF scintillator screens used for imaging. a) photographs under X-ray irradiation of 10 wt% DMAC-TRZ:SO, 4CzIPN:SO, and 4CzTPN-Bu:SO scintillator screens. b) X-ray images of an encapsulated metallic spring collected using the same scintillator screens. c) Bright field (left) and X-ray (right) images of a microchip and a fish using a 0.5wt% DMAC-TRZ:SO scintillators screen. d) Modulation transfer functions (MTFs) of X-ray images. Taken and adapted with permission from ref. Copyright [2022/Nature Materials] Springer Nature.

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a) Molecular structures of 4CzIPN (TADF-H) derivatives TADF-Cl, TADF-Br, and TADF-I and their photophysical properties in 60 wt% doped films in PMMA. b) X-ray absorption spectra of TADF-H, TADF-Cl, TADF-Br, and TADF-I measured as a function of X-ray energy. c) RL spectra of these four TADF chromophores at the optimal thickness compared with the reference scintillator LYSO:Ce (dose rate, 174 μGy s^–1). d) Detection limits of the TADF-H, TADF-Cl, TADF-Br, and TADF-I emitters. e) and f) Bright- and darkfield photographs of a pen e) and an electronic chip f) before and after X-ray exposure (dose rate, 174 μGy s^–1). Taken and adapted with permission from ref . Copyright [2022/Nature Photonics] Spring Nature.

237

Schematic representation of the radioluminescence mechanism of Zr-fcu-BADC-MOF-4CzTPN-Bu nanocomposite materials. Illustration of highly efficient energy transfer from the Zr-fcu-BADC-MOF to 4CzTPN-Bu under ultraviolet light irradiation (bottom left) and the significantly enhanced radioluminescence efficiency of 4CzTPN-Bu by combining the efficient energy transfer from the Zr-fcu-BADC-MOF and its direct harnessing of the singlet and triplet excitons upon X-ray radiation (upper right). Taken and adapted with the permission from ref . Copyright [2022/ Mater] Elsevier under Creative Commons Attribution 4.0 International License https://creativecommons.org/licenses/by/4.0/.

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a) Spectral overlap between the emission spectrum of the Zr-fcu-BADC-MOF nanoparticles (D) and the absorption spectrum of 4CzTPN-Bu (A). b). Emission spectra of the nanocomposite films containing different D to A ratios (D-A_n, where n is the wt% of the TADF chromophore in PMMA and the concentration of D is 1 wt% in PMMA). Inset shows corresponding photo images of D-A nanocomposite films. c) CIE 1931 coordinates of emission from (D-A_0.0 ) and (D-A_0.4 ). d) Ratios of I_580 nm/I_480 nm under the excitation of UV and X-rays. e) RL intensity at 580 nm of the D-A_0.4 nanocomposite film under continuous X-ray irradiation at a dose rate of 174 mGy s^–1. f) Detection limit of the D-A_0.4 nanocomposite film (black line) and A_0.4 film (red line) g) Bright- and dark-field photographs of a steel framework (left) and electronic component (right) before and after X-ray exposure (dose value: 174 mGy/s). Taken and adapted with permission from ref. Copyright [2022/Matter] Elsevier under Creative Commons Attribution 4.0 International License https://creativecommons.org/licenses/by/4.0/.

239

Chemical structures of the components of polyvinyltoluene (PVT) based cross-linked plastic scintillators – vinyltoluene (monomer), divinylbenzene (cross-linker), CMB, 4CzIPN, and tBuCzDBA – and photophysical properties of 4CzIPN and tBuCzDBA (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

240

a) Chemical structure of DCF-MPYM. b) Time-resolved photoluminescence and c) steady-state photoluminescence (λ_exc = 510–560 nm) images of MCF-7 stained with DCF-MPYM (20 μM) and BSA (40 μL, 10 mM) at 37 °C. Taken and adapted with permission from ref . Copyright [2014/Journal of the American Chemical Society] American Chemical Society.

241

Chemical structures of organic TADF molecules used as imaging agents with the assistance of BSA/HSA (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

242

Top: chemical structure of CPy and DSPE-PEG2000. Bottom: confocal fluorescence images of zebrafish: a–c) zebrafish injected with CPy-Odots; d–f) zebrafish reference non-TADF Odots. Taken and adapted with permission from ref . Copyright [2017/Advanced Science] John Wiley & Sons under Creative Commons Attribution 4.0 International License https://creativecommons.org/licenses/by/4.0/.

243

a) Schematic illustration of the nanoprecipitation for nanoparticle preparation, taken and adapted with permission from ref . Copyright [2020/Chemical Science] The Royal Society of Chemistry. b) Chemical structures of amphiphilic copolymer and c) organic TADF molecules used for fluorescence imaging applications (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

244

a) Chemical structures of materials for Odot preparation. b) SEM images of 6 wt% 4CzIPN/mCP glassy Odots. c) Photo-degradation properties of 6 wt% 4CzIPN/mCP glassy Odots under various preparation conditions: neat 4CzIPN Odots and blue Qdots in water (air saturated); the monitored emission wavelengths were λ_PL = 515, 548, and 450 nm, respectively; λ_exc = 300–400 nm; excitation light intensity 5 mWcm^–2. Taken and adapted with permission from ref . Copyright [2019/Chemical Communications] The Royal Society of Chemistry.

245

a) Synthesis of g-Odots with examples of isolated nanoparticle suspensions shown for dopant HAP-3MeOTPA, where A is photographed under ambient room lighting and B under 365 nm irradiation. μW = microwave irradiation; ))) = ultrasonication. b) Single-photon excitation (λ_exc = 473 nm, λ_em = 485–545 nm) and multi-photon excitation (λ_exc = 905 nm, λ_em = 575–630 nm) of HEK293 cells incubated with HAP-3MeOTPA g-Odots (+g-Odots) for 24 h. Corresponding control samples (−g-Odots) are shown as well. SNR and SBR calculated with N = 8 cells for 473 nm excitation samples and N = 22 cells for 905 nm excitation samples. Taken and adapted with permission from ref . Copyright [2022/Advanced Functional Materials] John Wiley & Sons. Taken and adapted with permission from ref . Copyright [2022/Advanced Functional Materials] John Wiley & Sons.

246

a) Chemical structures of CzBN-co-DtaB and CzBN-co-HmatB. b) Schematic diagram representing the preparation of lipid-encapsulated Odots for in vivo vascular and bone imaging. Taken and adapted with permission from ref . Copyright [2022/Chemical Science] Royal Society of Chemistry.

247

Schematic illustration showing the cell-penetrating NPs assembled from the amphiphilic peptide [F6G6(rR)3R2] and TADF molecules 4CzIPN, NAI-DPAC, or BTZ-DMAC. Taken and adapted with permission from ref . Copyright [2018/Journal of the American Chemical Society] American Chemical Society.

248

a) Chemical structures of materials for Odot preparation. b) Schematic illustration of the preparation procedure for F, NCDs@SiO₂ and photographs of F, NCDs@SiO₂ in aqueous solution after LED excitation. c) Schematic illustration of the possible structural formation. d) Delayed fluorescence mechanism of F, NCDs@SiO₂ . Taken and adapted with permission from ref . Copyright [2021/Chemical Engineering Journal] Journal of the American Chemical Society] Elsevier.

249

a) Schematic illustration of the preparation of blue/green/red FONs by a reprecipitation method for one- and two-photon cellular imaging. b) Cellular imaging and localization of the three FONs, monitored with a fluorescence microscope (one-photon λ_exc: 380–420 nm) and a laser scanning confocal fluorescence microscope (two-photon λ_exc: 800 nm) in an A549 cell (final concentration: 8 μg/mL): left column, bright-field channel; middle column, FON channel; right column, overlay of the bright and FON images. The scale bar is 50 μm. Taken and adapted with permission from ref . Copyright [2016/Journal of the American Chemical Society] American Chemical Society.

250

Chemical structure of the amphiphilic TADF monomer (AI-Cz-AM) based on AI-Cz and the design of the single component self-assembled TADF nanoprobe AI-Cz-NP. Taken and adapted with permission from ref . Copyright [2020/Chemical Communications] The Royal Society of Chemistry.

251

a) Schematic illustration for maximizing aggregation of organic fluorophores to prolong fluorescence lifetime for two-photon FLIM. b) In vitro two-photon confocal fluorescence imaging and two-photon FLIM of HepG2 cells stained with TXO NPs (10 μg mL^–1) after 2 h incubation. Using λ_exc = 760 nm the fluorescence was recorded between 600–650 nm. c) In vivo two-photon confocal fluorescence imaging and two-photon FLIM of zebrafish stained with TXO NPs (10 μg mL^–1) after 4 h incubation. The λ_exc = 760 nm, and fluorescence emissions were recorded 600–650 nm. Taken and adapted with permission from ref . Copyright [2018/Advanced Healthcare Materials] Wiley & Sons.

252

Chemical structures of organic TADF molecules having aggregation-induced delayed fluorescence used as imaging reagents (the blue color signifies donor moieties, while the red color signifies acceptor moieties).

253

a) Design and proposed uptake mechanism of NID-TPP for TRLI of mitochondria in HeLa cells. b) Two-photon luminescent images of HeLa cells incubated with 10 × 10^–6 M NID-TPP for 5 min, 11 min, and 17 min. λ_exc = 810 nm, λ_PL = 540–660 nm. Taken and adapted with permission from ref . Copyright [2020/Advanced Optical Materials] John Wiley & Sons.

254

Polymer dots formed by self-assembly of an amphiphilic block copolymer containing a water-soluble cell-penetrating guanidine unit, and a hydrophobic host material and TADF emitter used to deliver TADF emitters to biological targets. Taken and adapted with permission from ref . Copyright [2021/Journal of the American Chemical Society] American Chemical Society.

255

Schematic of photochromism and long-lived luminescent “double-check” bioimaging using the TADF polymeric nanoparticle PDFPNs. Taken and adapted with permission from ref . Copyright [2022/Advanced Optical Materials] John Wiley & Sons.

256

TADF molecules used in lasing applications. a) TADF molecules as gain media; b) TADF molecules as triplet harvesters (the arrows represent the direction of the FRET process); c) lasing from TADF molecules. The blue color signifies donor moieties/atoms/functional groups, while the red color signifies acceptor moieties/atoms/functional groups.

257

Reported resonator structures for lasing using TADF molecules: a) 4 wt% CAZ-A doped in CBP thin films with microring array; b) 3 wt% 4CzTPN doped in PS thin films with microsphere array; c) faceted MOON, ON, and MOCN single-crystalline microcrystals; d) DCzBF₂ 1D single-crystalline microwires. Taken and adapted with permission from ref (Copyright [2019/ACS Photonics] American Chemical Society); ref (Copyright [2020/Angewandte Chemie International Edition] John Wiley & Sons); ref (Copyright [2021/Nano Letters] American Chemical Society); ref (Copyright [2022/Angewandte Chemie International Edition] John Wiley & Sons), respectively.

258

General photocatalytic cycle showing the possible photon induced energy and electron transfer events, where D = donor, A = acceptor, sub = substrate, SET = single electron transfer, and PEnT = photoinduced energy transfer.

259

Structures of commonly used visible light organometallic PCs, and a range of popular and recently used TADF PCs. References correspond to first reported publication of the compound, except for Eosin Y, whereby the reference corresponds to the first reported use of this organic dye as a PC. The blue color signifies donor moieties, while the red color signifies acceptor moieties.

260

Examples of photocatalysis reactions for which CDCB PCs typically outperform commonly used transition metal PCs. The yields given reflect the highest yielding TADF PC and the highest yielding non-TADF PC. Yields given are obtained from the PC screen; further optimization may have occurred in some cases. CFL = compact fluorescent light.