Enhancing the performance of pure organic room-temperature phosphorescent luminophores

Abstract

Once considered the exclusive property of metal complexes, the phenomenon of room-temperature phosphorescence (RTP) has been increasingly realized in pure organic luminophores recently. Using precise molecular design and synthetic approaches to modulate their weak spin-orbit coupling, highly active triplet excitons, and ultrafast deactivation, organic luminophores can be endowed with long-lived and bright RTP characteristics. This has sparked intense explorations into organic luminophores with enhanced RTP features for different applications. This Review discusses the fundamental mechanism of RTP in pure organic luminophores, followed by design principles, enhancement strategies, and formulation methods to achieve highly phosphorescent and long-lived organic RTP luminophores even in aqueous media. The current challenges and future directions of this field are also discussed in the summary and outlook.

Fig. 1

Fundamental mechanism of organic room-temperature phosphorescence (RTP) phenomenon. a Jablonski diagram illustrating the different photophysical relaxation processes, particularly the intersystem crossing (ISC) between singlet and triplet states, which forms the basis for phosphorescence of organic luminophores. b Schematic illustration of the El-Sayed’s rule for ISC and its utilization for controlling phosphorescence decay rate based on the molecular-orbital hybridization of the lowest triplet states. b is adapted with permission from ref. . Copyright (2016) Cell Press

Fig. 2

Major design strategies in realizing pure organic RTP based on halogen bonding, H-aggregation, and n–π transition. a Chemical structure of MBB. b Photographs of the different states of MBB crystals under normal light (left) and UV light (right) illuminations at ambient condition. c Molecular packing structure of MBB crystal. d Chemical structure of luminophore Br6A. e Photograph of Br6A crystal under UV light irradiation (left) and the molecular packing structure of Br6A crystal, illustrating the halogen bond between carbonyl oxygen and bromine (right). f Schematic illustration of the realization of phosphorescence of Br6A in crystal form. g General chemical structures of different organic molecules designed to control the lifetime of excited states through the incorporation of alkyl or aromatic substituents to facilitate H-aggregate formation and stabilize triplet exciton. R₁ and R₂ can be H, Cl, C₂H₅, C₁₂H₈N, etc. h Schematic illustration of the generation of lower-lying energy states for stabilization of the lowest triplet excited state (T₁) to achieve long organic phosphorescence. i Chemical structure of DPhCzT. j Crystal structure of DPhCzT depicting the H-aggregate formation through the measured angle of 80.9^o between the interconnected axis and transition dipoles. k Chemical structure of DCzPhP. l Steady-state photoluminescence (left) and phosphorescence (right) profiles of DCzPhP. Insets illustrate the different states of DCzPhP before (left) and after (right) switching off the 365-nm excitation under room temperature. m Chemical structure of CPhCz. n Luminescence decay of CPhCz after a 5-min visible light excitation. o Chemical structures of rationally designed organic molecule with carbonyl group and non-bonding electrons (left) and BDBF (right). p Photographs of the different states of BDBF in solution, amorphous, and crystal forms at 77 and 300 K. q Calculated energy levels with the proportion of α_n of the different excited states of BDBF. r Chemical structure of CIBDBT. s Crystal structure of CIBDBT. t Photographs of the different states of CIBDBT before (top) and after (bottom) switching off the 365-nm excitation source. u Calculated energy levels, electronic transitions, and frontier molecular orbitals of the highest occupied molecular orbitals H, H-2, and the lowest unoccupied molecular orbital L. a, b, and c are adapted/reproduced with permission from ref. . Copyright (2010) American Chemical Society. d, e, and f are adapted/reproduced with permission from ref. . Copyright (2011) Springer Nature. g is adapted with permission from ref. ^,. Copyright (2015) Springer Nature and (2017) Wiley-VCH Verlag GmbH & Co. h, i, j, k, and l are adapted/reproduced with permission from ref. . Copyright (2015) Springer Nature. m and n are adapted/reproduced with permission from ref. . Copyright (2017) Wiley-VCH Verlag GmbH & Co. o, p, and q are adapted with permission from ref. . Copyright (2016) Cell Press. r, s, t, and u are adapted/reproduced with permission from ref. . Copyright (2017) Springer Nature

Fig. 3

Enhancement of organic RTP based on co-crystallization and rigid matrix host–guest system. a Chemical structures of a pair of aldehyde luminophore (BrnA) and analogous host (Brn) used in co-crystallization, where X are CHO and Br for luminophore and host, respectively, and N refers to the alkoxy chain length with varying amount of carbon atoms. b Schematic illustration of the mechanism of RTP enhancement based on co-crystallization, where luminophores (i.e., brominated aromatic aldehydes) are substituted into the host crystals (i.e., dibrominated analogs to the luminophores) and isolated to minimize the excimer-induced self-quenching and enhance the overall RTP quantum yield. c Photographs of the co-crystals formed from BrnA (i.e., Br5A, Br6A, Br7A, and Br8A) and Brn (i.e., Br5, Br6, Br7, and Br8) with different alkoxy chain lengths consisting of five to eight carbon atoms, under 365 nm UV light excitation. The numerical values within the co-crystals indicate their photoluminescence quantum efficiencies. d Chemical structures of different luminophore/host pairs, i.e., BrC6A/BrC6, BrS6A/BrS6, and Np6A/Np6. e Photographs of the co-crystals mixed from Br6A/Br6, BrC6A/BrC6, BrS6A/BrS6, and Np6A/Np6 combinations under 365-nm UV light irradiation. f Chemical structures of 1,4-DITFB and Naphthalene. g Packing structure of the co-crystal of 1,4-DITFB and Naphthalene (Nap-DITFB). h Excitation (in blue) and emission (in green) profiles of Nap-DITFB co-crystal. Inset shows the co-crystal of 1,4-DITFB and Naphthalene under 365-nm UV light irradiation. i Chemical structure of the host PMMA. j Photoluminescence profiles of pure Br6A luminophore and Br6A embedded in different variants of PMMA. k Chemical structures of the guest G1 and the host PVA. l Schematic illustration of the mechanism of RTP enhancement based on rigid matrix host–guest system, where G1 is embedded within PVA and the PVA–PVA hydrogen bonds, the G1-PVA hydrogen bonds, as well as the G1–G1 halogen bonds collectively contribute to organic RTP enhancement. a and c are adapted/reproduced with permission from ref. . Copyright (2014) American Chemical Society. b, d, and e are adapted with permission from ref. . Copyright (2011) Springer Nature. f and h are adapted with permission from ref. . Copyright (2012) The Royal Society of Chemistry. g is reproduced with permission from ref. . Copyright (2015) The Royal Society of Chemistry. i and j are reproduced with permission from ref. . Copyright (2013) American Chemical Society. k and l are adapted/reproduced with permission from ref. . Copyright (2014) Wiley-VCH Verlag GmbH & Co

Fig. 4

Enhancement of organic RTP based on structural modified host–guest system and dopant system. a Chemical structures of the cyclodextrin-modified host BrNp-β-CD and the guest AC. b Photographs of different states of the individual guest and host, as well as the host–guest system under daylight (left) and UV light excitation (right). c Chemical structures of the secondary amino-substituted deuterated guest and the host β-estradiol. d Sum of the non-radiative triplet deactivation rate (k_nr) and triplet quenching rate (k_q) of a range of host–guest films in which the host was β-estradiol and the secondary amino-substituted aromatic hydrocarbon guests were either deuterated (open circles) or not (solid circles). e Photographs of different states of the deuterated host–guest system under UV light excitation and after switching it off over time. f Chemical structures of the donor (guest) TMB and the acceptor (host) PPT. g Schematic illustration of the mechanism of RTP enhancement based on dopant system where charge-transfer states are formed during photoexcitation (i and ii), followed by the formation of charge-separation states (iii) and the eventual exciplex emission (iv and v). a and b are adapted with permission from ref. . Copyright (2018) American Chemical Society. c, d, and e are adapted with permission from ref. . Copyright (2013) Wiley-VCH Verlag GmbH & Co. f and g are reproduced with permission from ref. . Copyright (2017) Springer Nature

Fig. 5

Nanocrystallization of amorphous nanoaggregates to yield organic RTP contrast agents for bioimaging. a Schematic illustration of the seed-assisted nanocrystallization process. b Chemical structure of C-C4-Br with red persistent RTP. c Scanning electron microscopy image of the C-C4-Br nanocrystals. Inset shows the selected area electron diffraction pattern of the nanocrystals. d Photoluminescence of the C-C4-Br amorphous nanoaggregates and nanocrystals (both in orange) and fluorescein (green) under various illumination states. e Confocal laser scanning microscopy images of breast cancer cells treated with amorphous nanoaggregates (left) and nanocrystals (right) of C-C4-Br. a is adapted with permission from ref. . Copyright (2017) Wiley-VCH Verlag GmbH & Co. b, c, d, and e are adapted with permission from ref. . Copyright (2017) Wiley-VCH Verlag GmbH & Co

Fig. 6

Nanoencapsulation and surface functionalization of nanocrystals and amorphous nanoaggregates to yield organic RTP contrast agents for bioapplications. a Schematic illustration of the nanoencapsulation of BDBF RTP nanocrystals using saponin. b Confocal laser scanning microscopy images of HeLa cells treated with pure BDBF nanocrystals (left) and saponin-encapsulated BDBF nanocrystals (right). c Schematic illustration of the syntheses of amorphous organic semiconducting nanoparticles with ultralong RTP, i.e., OSN-T and OSN-B, using top-down (top) and bottom-up (down) approaches, respectively. d Fluorescence (left) and ultralong phosphorescence (right) imaging of lymph node in living mice using intradermally administered OSN-T. a and b are adapted with permission from ref. . Copyright (2017) American Chemical Society. c and d are adapted with permission from ref. . Copyright (2017) Wiley-VCH Verlag GmbH & Co