Comprehensive Review on the Structural Diversity and Versatility of Multi-Resonance Fluorescence Emitters: Advance, Challenges, and Prospects toward OLEDs

Abstract

Fluorescence emitters with a multiple-resonant (MR) effect have become a research hotspot. These MR emitters mainly consist of polycyclic aromatic hydrocarbons with boron/nitrogen, nitrogen/carbonyl, and indolocarbazole frameworks. The staggered arrangement of the highest occupied molecular orbital and the lowest unoccupied molecular orbital facilitates MR, resulting in smaller internal reorganization energy and a narrower emission bandwidth. Optimal charge separation suppresses the energy gap between singlet and triplet excited states, favoring thermally activated delayed fluorescence (TADF). These MR-TADF materials, due to color purity and high emission efficiency, are excellent candidates for organic light-emitting diodes. Nevertheless, significant challenges remain; in particular, the limitation imposed by the alternated core configuration hinders their diversity and versatility. Most existing MR-TADF materials are concentrated in the blue-green range, with only a few in red and near-infrared spectra. This review provides a timely and comprehensive screening of MR emitters from their pioneering work to the present. Our goal is to gain understandings of the MR-TADF structure-performance relationship from both basic and advanced perspectives. Special emphasis is placed on exploring the correlations between chemical structure, photophysical properties and electroluminescent performance in both depth and breadth with an aim to promote the future development of MR emitters.

1. Key Themes of This Review

1

(a) Electron density distribution of representative MR emitters. (b) The multiple resonance effect facilitates a reduction in the reorganization energy (λ) for the S₀ and S₁ states. (c) MR emitters require lower energy peak emission to meet the same CIE _y , owing to their superior efficiency and narrower bandwidth compared to phosphorescence and TADF counterparts.

2. Borylation Methods of MR Emitters and Illustrative Examples

3. Intramolecular Cyclization of Nitrogen/Carbonyl-Type MR Emitters and Illustrative Examples

4. Intramolecular Cyclization of N-PAHs-Type MR Emitters and Illustrative Examples

2

Chemical structures and photophysical properties of unconventional MR emitters based on one-boron in a toluene solution.

3

Chemical structures and photophysical properties of multiple boron-based MR-emitters.

4

Chemical structures and photophysical properties of nitrogen/carbonyl-type MR emitters.

5

(a) The molecule structures of ICz-PAHs. (b) The design concept of 3IDCz with an N-π-N extended molecular structures. (c) The diagram of the formation of delocalized excited states. , Reproduced with permission from ref (copyright, 2021, John Wiley and Sons) and ref (copyright, 2022, John Wiley and Sons).

6

(a) The design strategy proposed for deep blue MR-TADF emitters. (b) The design strategy of carbonyl-fused organoboron PAHs. Reproduced with permission from ref . Copyright, 2023, John Wiley and Sons.

7

(a) UV–vis, absorption spectra (green line) and PL spectra (blue line) of MR molecules in dilute n-hexane solution at room temperature (inset: the corresponding molecular structures). (b) Single crystal structures, contours of HOMO and LUMO, and nature transition orbitals (“hole” and “electron”) of tBCzHSPO (647) and tBCzP2PO (101). (c) NTO orbitals of azaphosphinines 2PO (101) and C3PO (648) in S₁ state. Reproduced with permission from refs , (copyright, 2023, John Wiley and Sons) and ref (copyright, 2025, American Chemical Society).

8

(a) Molecules based on ICT strength modulating emission colors and FWHM. (b) The optimized configurations, HOMO and LUMO energies, and distributions of BNIP-tBuCz (116), BNIP-tBuDPAC (117), BNIP-CzDPA (118), and BNDIP (119). Adapted from ref . Reproduced with permission from ref . Copyright, 2023, John Wiley and Sons.

9

(a) Paradigm in polycyclization of B-N-containing MR parent core, frontier molecular orbitals population, and model molecule BN-TP (123), and photophysical properties of compound BN-TP-Nx (x = 1, 2, 3, 4); adapted from ref . (b) MR emitters based on conjugation modulating emission colors and FWHM. Photophysical properties measured in toluene. Reproduced with permission from ref . Copyright, 2023, John Wiley and Sons.

10

Modulating colors based on analogues of DABNA-1 (1).

11

Modulating colors based on analogues of Cz-BN (102).

12

Modulating colors based on analogues of BCz-BN (5).

13

Modulating colors based on the principle of para-positioned B-π-B and D-π-D.

14

MR emitters based on the meta-positioned boron framework.

15

MR emitters based on (a) the hybrid double boron framework, (b) dimerization strategy, and (c) boron fusion locker. Reproduced with permission from ref . Copyright, 2023, John Wiley and Sons.

16

(a) UV/vis absorption and fluorescence spectra of H-tetraazacyclophane (655) and HBN (656) in toluene solutions. (b) Δ-DABNA-TB (298) achieving a wide range of wavelength red-shift via B-doping “core–shell” strategy. (c) Diboron-based TADF compounds CzDBA (657) and tBuCzDBA (658). Adapted from refs , . Reproduced with permission from ref (copyright, 2025, John Wiley and Sons) and ref (copyright, 2024, American Chemical Society).

17

Modulating colors based on QAO (2) via single bond-linked phenyl moieties.

18

Modulating colors via dimerization or fusing PAHs segments.

19

Modulating colors via peripheral decoration of MR-core with varying donor segments. Permission from ref . Reproduced with permission from ref . Copyright, 2022, American Chemical Society.

20

Modulating colors based on TOAT (74) backbone.

21

Modulating colors based on N-PAHs-type MR emitters.

22

Some representative molecules of addressing ACQ for B/N-type MR emitters.

23

Representative molecules of addressing ACQ for MR emitters based on BCz-BN (5) framework.

24

Representative strategies of addressing ACQ via twisted conformations, space-confined donor–acceptor (SCDA), and dendrimer.

25

Representative molecules of addressing ACQ for N/CO and N-PAHs-type MR emitters.

26

Exciton utilization of MR emitters. (a) The proposed mechanism for the exciplex formation enhancing ISC/RISC in mCBP/DABNA-1 (1) system. (b) Upper: schematic illustration and molecule structures of cholic acid (CLA) and BCzBN(5) for constructing narrowband organic long-persistent luminescence (LPL) in amorphous. Lower: the preparation of the LPL composite of BCzBN (5)/ gCLA and proposed strategy in realizing narrowband LPL through dissociation and recombination (EDR) and EDR-based Förster resonance energy transfer (FRET). The gCLA (664) is the glassy CLA. (c) Natural transition orbital calculation results and spin-vibronic-coupling-assisted TADF emission mechanism. NAC stands for nonadiabatic coupling. (d) Left: conventional TADF-RISC mechanism; Middle: novel ideal superimposed fluorescence (SF) mechanism; Right: CzBO (418) and CzBS (419) exhibit conventional TADF-RISC characteristics, while CzBSe (417) demonstrates an SF mechanism. (e) Left: transient PL decay spectra in 2.0 wt % ICz-BO (420) doped CBP film. Insert: magnetic field effect on the EL of ICz-BO (420)-doped device at different voltages. Right: linear correlation of the orientation polarization (f) of solvent media with Stokes shift (ν_a–ν_f). Reproduced with permission from refs , (copyright, 2021 Springer Nature) and refs , , (copyright, 2024, 2022, 2022, 2024 John Wiley and Sons).

27

Schematic strategy to accelerate the RISC process of MR-TADF molecules by reducing the ΔE _ST and enhancing the SOC matrix elements. Reproduced with permission from ref . Copyright, 2023, John Wiley and Sons.

28

Some representative B/N type of MR emitters, where the MR-cores are surrounded by additional lone-pair (n) electron groups to accelerate spin-flipping RISC.

29

Some representative deep-blue MR emitters with high spin-flipping RISC.

30

Some representative MR emitters of acceleration spin-flipping RISC by heavy atom effects.

31

Some representative MR emitters accelerate spin-flipping RISC through fused distorted π-conjugated molecular design. Reproduced with permission from ref . Copyright, 2024, Springer Nature.

32

Some representative nitrogen/carbonyl or N-PAHs-type MR emitters with accelerated spin-flipping RISC.

33

Schematic diagram of the significance of circularly polarized-organic light-emitting diodes (CP-OLED). Reproduced with permission from ref . Copyright, 2023, Elsevier.

34

Representative molecules of CPMR-TADF.

35

Some representative custom-designed hosts for MR-emitter-based OLEDs.

36

OLED performance. (a) Device structure and ionization potentials and electron affinities (in eV) for each material. (b) Molecular structures used in the emitting layer. (c) Normalized EL spectra and device in operation. Inset: electroluminescence of the device. (d) Current density and luminance versus driving voltage characteristics. (e) EQE versus luminance characteristics. (f) Current and power efficiency versus luminance characteristics. Reproduced with permission from ref . Copyright, 2019, Springer Nature.

37

Molecular structures (No. 551–564) without reference in the above text.

38

(a) The energy transfer process in (a) TADF-sensitized MR emitters (FRET: Förster energy transfer, DET: Dexter energy transfer). (b) The diagram of absorption (dashed green lines) and emission (solid green lines) of MR dopant and the emission spectra of sensitizer (blue). The shaded area indicates the spectral overlap of sensitizer emission and MR-dopant absorption. Reproduced with permission from ref . Copyright, 2020, Chinese Chemical Society.

39

Molecular structures (No. 565–605) referred to in the above text.

40

Comparison of photophysical characteristics and emission mechanisms. (a) Comparison for Chan et al.’s work. (b) Comparison for Jeon et al.’s work. Insets show the chemical structures of the sensitizers (HDT-1 (577) and PPCzTrz (580)). Reproduced with permission from ref . Copyright, 2021, Springer Nature.

41

Energy transfer process in phosphor-sensitized MR emitters (FRET: Förster energy transfer, DET: Dexter energy transfer).

42

Molecular structures of organometallic sensitizer.

43

Some molecular structures in the above text.

44

Doublet-sensitized fluorescence (DSF) mechanism (upon photoexcitation) with specific dynamic rates. Reproduced with permission from ref . Copyright, 2024, John Wiley and Sons.

45

Molecular structures (No. 632–642) in the text.

46

(a) Schematic and Jablonski diagram for the suppression of Dexter triplet transfer from a TADF host via emitter encapsulation. (b) Structures of the mDICz (4) and encapsulated NB-1 (638) luminophores; relevant absorption and PL spectra for NB-1 (563) and DMAC-DPS (637). (c) Synthesis schemes and structures of NB-1 (638) and NB-2 (639) with X-ray single-crystal structures (H atoms are omitted for clarity). (d) Matrix-free HF system based on antiquenching TADF host SpiroAC-TRZ (621) and traditional concentration-sensitive MR-TADF emitter. , Reproduced with permission from ref (copyright, 2024, Springer Nature) and ref (copyright, 2024, John Wiley and Sons).