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Commun Chem
Commun Chem
Communications Chemistry
2399-3669
Nature Publishing Group UK London

39294436
1301
10.1038/s42004-024-01301-4
Article
Donor-only substituted benzene achieves thermally activated delayed fluorescence
http://orcid.org/0000-0003-0555-2894
Mamada Masashi mamada@kuchem.kyoto-u.ac.jp

1
Yada Sawako 2
http://orcid.org/0000-0002-2405-9897
Hayakawa Masahiro 1
Uchida Ryota 1
http://orcid.org/0000-0003-4100-9995
Katagiri Hiroshi 3
http://orcid.org/0000-0002-7483-9525
Hatakeyama Takuji 1
http://orcid.org/0000-0001-6117-9604
Adachi Chihaya adachi@cstf.kyushu-u.ac.jp

24
1 https://ror.org/02kpeqv85 grid.258799.8 0000 0004 0372 2033 Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto, 606-8502 Japan
2 https://ror.org/00p4k0j84 grid.177174.3 0000 0001 2242 4849 Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, Motooka, Nishi, Fukuoka, 819-0395 Japan
3 https://ror.org/00xy44n04 grid.268394.2 0000 0001 0674 7277 Graduate School of Organic Materials Science, Yamagata University, Yonezawa, Yamagata, 992-8510 Japan
4 https://ror.org/00p4k0j84 grid.177174.3 0000 0001 2242 4849 International Institute for Carbon Neutral Energy Research (I2CNER), Kyushu University, 744 Motooka, Nishi, Fukuoka, 819-0395 Japan
18 9 2024
18 9 2024
2024
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6 9 2024
© The Author(s) 2024
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Thermally activated delayed fluorescence (TADF) is a promising mechanism for harvesting triplet excitons in organic light-emitting diodes (OLEDs). The donor–acceptor (D–A) design is the most conventional strategy for developing efficient TADF emitters. A subsequently emerged approach, known as the multiple resonance (MR) effect, also employs electron-donating and electron-withdrawing functional groups. Thus, developing TADF materials has traditionally relied on ingenuity in selecting and combining two functional units. Here, we have realized a TADF molecule by utilizing only a carbazole donor moiety. This molecule is an unusual example in the family of TADF materials and offers better insight into the electronic structures in the excited states for luminescent materials.

Thermally activated delayed fluorescence (TADF) is a promising mechanism for harvesting triplet excitons in organic light-emitting diodes, but TADF molecules typically rely on multiple functional units, such as both an electron donor and an electron acceptor. Here, the authors develop a TADF molecule using only benzene and carbazole donor moieties.

Subject terms

Photochemistry
Optical materials
https://doi.org/10.13039/501100001691 MEXT | Japan Society for the Promotion of Science (JSPS) 23K20039 Adachi Chihaya issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

Purely organic thermally activated delayed fluorescent (TADF) materials have been widely developed in the past decade primarily for use in organic light-emitting diodes (OLEDs)1. Exciton dynamics, including the triplet-to-singlet upconversion through reverse intersystem crossing (RISC), enables the harvesting of dark triplet excitons for high electroluminescence efficiencies in OLEDs2. In addition to the OLED application, the luminescent feature with long-lived delayed fluorescence, which is quenched by oxygen, makes TADF materials suitable for time-resolved imaging3. Further, TADF molecules have been employed in photocatalytic synthesis, photodynamic therapy, and X-ray imaging scintillators4–8. The most essential aspect of TADF materials is the small energy gap between the first singlet and triplet excited states (S1 and T1) referred to as ΔEST, which allows the electronic transitions with spin conversion. The fundamental design guideline for creating molecules with a small ΔEST is understood by electron exchange integral K of the frontier molecular orbitals (FMOs) responsible for the transition9. The electron exchange interaction destabilizes the singlet states, while it stabilizes the triplet states, indicating that smaller K leads to smaller ΔEST. Thus, the smaller spatial overlap between FMOs of the transition, which are normally between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), the smaller ΔEST. A spatially separated HOMO and LUMO in a molecule is easily achieved in charge-transfer (CT)-type excited states. The molecules with electron donor (D) and electron acceptor (A) units strongly localise HOMO and LUMO on each unit, respectively, and induce the electron transfers from D to A in the excited states. Thus, D–A-based molecular design has played a pivotal role in developing TADF materials10–14. A typical D–A-type TADF molecule, 4CzIPN, is shown in Fig. 12. In general, the D and A units need a weak conjugation by increasing the dihedral angle or introducing a spacer unit between D–A units for better CT characteristics15–18. Another important class of molecular design is known as the multiple resonance (MR) effect, which realises short-range CT in the rigid π-conjugated framework as represented by the DABNA scaffold19–24. The alternating resonance effects by electron-donating and electron-withdrawing substituents in the ortho relation localise HOMO and LUMO at different carbon atoms in benzene rings. With few exceptions25–34, the TADF materials have been designed by D–A or MR strategies, indicating that electronically disparate building blocks are considered to be necessary. The classification of D and A is based on a relative scale and is sometimes complicated because of independent inductive and resonance effects as seen in halogen substitution on benzene ring. In 2020, the first example of TADF materials (DBF-DMAc) using the strong-D and weak-D units as the D–A system was reported by Tsuchiya et al.35. In 2022, Hall et al. reported an MR compound (DiICzMes4) without an electron withdrawing group and they described that MR-TADF with no acceptor groups is possible36, followed by several works using the same indolo[3,2,1-jk]carbazole (ICz) and similar indolo[3,2,1-de]acridine structures37–40. Interestingly, the ICz unit could be capable as an electron acceptor in the D–A type TADF materials (ICzAc)41. As mentioned above, a wide variety of TADF materials have provided a better understanding of electronic structures and excited state dynamics of TADF materials. Therefore, it is highly desired to expand molecular design by developing unexpected systems.Fig. 1 Examples of TADF molecules.

A typical D–A molecule 4CzIPN has carbazole donors and isophthalonitrile acceptor. A typical MR molecule DABNA1 has electron-donating nitrogen and electron-withdrawing boron atoms. Strong D–weak D act as D–A, respectively. Fused ICz derivatives show MR characteristics without the electron-withdrawing atom. ICz also acts as the acceptor unit against the strong donor for forming D–A. 6MeOCzPh developed in this work is an additional TADF molecule without explicit acceptor moiety. Unconventional TADF materials are like neither D–A nor MR design, while the electronic structures of these molecules may follow these types; for example, Eosin may have MR effect and TQB showing excited state intramolecular proton transfer (ESIPT) forms CT excited states similar to D–A type. The n-π* transition observed in quinones, thiones, and TCA_C4 also leads to the small spatial overlap of FMOs.

In this work, we demonstrate TADF from a simplified molecular structure with 3,6-dimethoxy-carbazole donor-only substituted benzene (6MeOCzPh, Fig. 1). We recently revealed that hexacarbazolylbenzene (6CzPh) is useful as the host molecule because of its high T1 energy level and improvement of horizontal orientations of guest emitters42. Compared to 1,3-di(9-carbazolyl)benzene (mCP or m2CzPh), the ΔEST value of 6CzPh was markedly reduced. Although 6CzPh does not show TADF, we further investigated a series of 6CzPh derivatives for potential TADF application. By increasing the donor strength of carbazole, the ΔEST values were decreased to a sufficiently low level, exhibiting TADF in both solution and film states.

Results and discussion

The synthesis of 6CzPh derivatives having 3,6-di-tert-butylcarbazole and 3,6-dimethoxycarbazole (6tBuCzPh and 6MeOCzPh, Fig. 2) followed the method for 6CzPh by nucleophilic aromatic substitution (SNAr) reaction from hexafluorobenzene (Supplementary Methods S1). The impurities with unreacted fluorine atoms were removed by column chromatography and vacuum sublimation, and the absence of fluorine atoms was confirmed by 19F NMR. A single crystal suitable for X-ray structure analysis was obtained for 6tBuCzPh by sublimation and 6MeOCzPh by recrystallization from chloroform and acetonitrile. Similar to 6CzPh, the molecules have the C1 point group symmetry (Supplementary Fig. S1). Because of the rigid structures, 6CzPh derivatives did not melt in the standard melting point apparatus.Fig. 2 Photophysical properties of 6CzPh derivatives in solution. Inset: Chemical structures.

a, b Fluorescence (solid lines) and phosphorescence (dashed lines) spectra for 6CzPh, 6tBuCzPh, and 6MeOCzPh in toluene and chloroform, respectively. c–e Transient PL decay curves of 6MeOCzPh in toluene, chloroform, and DMF, respectively.

The photophysical properties of 6CzPh derivatives were first evaluated in toluene and chloroform. Photoluminescence (PL) spectra of 6CzPh at room temperature were broad and structureless, similar to CT-type emission (Fig. 2). The phosphorescence spectra measured at 77 K showed vibrational structures, which are closely related to the local triplet excited state of carbazole moiety. Since the T1 energies of 6CzPh (~3.0 eV) were slightly decreased compared to those of carbazole (~3.1 eV), and vibrational bands of 6CzPh and carbazole are apparently different (Supplementary Fig. S2), the distributions of molecular orbitals extend through the intramolecular interactions. Because of the relatively high S1 energy (>3.35 eV) with the emission maxima at about 400 nm, 6CzPh has ΔEST values of >0.3 eV (Table S1) which are larger than the standard threshold (<0.2 eV) for TADF to be observed. The S1 energies of 6tBuCzPh with 3,6-di-tert-butylcarbazole (tBuCz) donors were slightly decreased, while the T1 energies of tBuCz were also decreased, keeping large ΔEST values. On the other hand, the stronger donor of 3,6-dimethoxycarbazole showed much shifted S1 to the lower energies. As a result, the ΔEST of 6MeOCzPh reached 0.2 eV in toluene (Table 1), and a very weak TADF was observed in time-resolved PL measurements. With increasing the solvent polarity, the delayed components became clear because of the smaller ΔEST values (Fig. 2 and Supplementary Fig. S3–S4). The prompt and delayed emission in DMF showed typical temperature dependence behavior of TADF (Supplementary Fig. S5–S6)43, indicating that TADF from only donor-substituted benzene is possible.Table 1 Photophysical characteristics of 6MeOCzPh in solution and blend films with PS and PMMA

Condition	λPL [nm]	S1 [eV]	T1 [eV]a	ΔEST [eV]	ΦPL [-]b	Φp/Φd [-]b	τp [ns]b	τd [µs]b	kr [107 s−1]	kISC [107 s−1]	kRISC [104 s−1]	
In toluene	448	3.11	2.91	0.20	0.25	0.21/0.04	8.7	19	2.5	9.0	1.2	
In chloroform	464	3.08	2.91	0.17	0.13	0.12/0.01	7.4	4.8	1.6	12	2.7	
In acetone	478	3.04	2.91	0.13	0.16	0.14/0.02	9.2	7.9	1.5	9.4	2.2	
In DMF	485	3.00	2.86	0.14	0.12	0.10/0.02	11	5.2	0.90	8.2	4.6	
In PSc	444	3.14	2.85	0.29	0.11	0.09/0.02	4.2	31	2.2	22	0.63	
In PMMAd	452	3.12	2.85	0.27	0.19	0.16/0.03	6.0	20	2.6	14	1.2	
aMeasured at 77 K.

bMeasured under N2 in solution or Ar in films.

cPolystyrene.

dPoly(methyl methacrylate).

The TADF properties were further characterized in doped thin-film states. A polystyrene (PS) film with 6 wt% doping of 6MeOCzPh showed the emission maxima similar to that in toluene (Table 1). Because of the low polarity of PS, the ΔEST and delayed lifetime (τd) were relatively large as with the results in toluene. The use of slightly polar poly(methyl methacrylate) (PMMA) hosts exhibited better results, although the spectra were essentially similar (Fig. 3 and Supplementary Fig. S7). The transient PL spectra revealed clear prompt and delayed components with the same spectra shape, supporting delayed fluorescence for emission in the microsecond range. The temperature dependence of the rate constants of RISC (kRISC), which follows the Arrhenius equation kRISC = exp(−ΔE/kbT) (Supplementary Fig. S8), indicated thermal activation of the process. These results also support the TADF in 6MeOCzPh.Fig. 3 Photophysical properties of 6MeOCzPh in polymethylmethacrylate (PMMA).

a Fluorescence (blue line) and phosphorescence (green line) spectra. b Temperature dependence of transient PL decay curves. c Prompt and delayed emission spectra at room temperature.

The distinctive aspect of the molecular and electronic structures behind the observation of TADF in 6MeOCzPh will be discussed below. Although the structure only includes 3,6-dimethoxycarbazole and benzene moieties, the broad emission spectra suggested the possibility of the CT-type excited state. Thus, the single benzene ring might behave as an acceptor. Although the LUMO level of benzene itself is much higher than that of carbazole (Fig. 4a), it decreases with an increasing the number of carbazole substituents because of the electron-withdrawing inductive effect of the nitrogen atoms (Supplementary Fig. S9). This effect is clearly observed in the 13C NMR spectra exhibiting large downfield shifts of the carbon atoms connected to the carbazole moiety (Fig. 4b)44. As the result of the multiple substitutions, the LUMO level of 6CzPh is lower than that of carbazole and similar to that of benzonitrile. In addition, the non-bonding orbital interactions of nitrogen 2p orbitals with the p orbitals of the benzene π-system stabilized benzene π* orbital energy. Thus, the LUMO is spreading over the central benzene ring and completely separated from the HOMO distributions (Fig. 4c). Although the inductive effects of tert-butyl and methoxy substituted carbazoles seem to be similar (Supplementary Fig. S10), the strong donors shifted both HOMO and LUMO, and decreased energy gap, resulting in the realisation of the sufficiently close S1 level against the local T1 level. These results suggest that the D–A interactions cause the TADF property in 6MeOCzPh despite the structure lacking an acceptor moiety. We sometimes overlook the negative inductive effect of the donor groups. Still, it has played an essential role as observed in the comparison between benzonitrile and 5CzBN and must be carefully considered in the molecular designs.Fig. 4 Analysis of the electronic structures and intramolecular interactions.

a Calculated HOMO-LUMO energy levels at the B3LYP/6-31+G(d,p). b 1H NMR and 13C NMR spectra for 6CzPh and fragment molecules. c HOMO and LUMO of 6MeOCzPh. d The reduced density gradient (RDG) isosurface map (iso value of 0.5) for 6CzPh. e RDG scatter diagram for 6CzPh.

We should also consider particular through-space intramolecular interactions and molecular orbital overlap with multiple carbazole rings tethered around the central benzene, which may form the three-dimensional π-delocalization for loop electron migration45–48. Indeed, the S1 energy of 1,2,4,5-tetra(9-carbazolyl)benzene appeared at a much higher level than that of 6CzPh regardless of the linear decrease of the LUMO energies with increasing the number of carbazoles (Supplementary Fig. S11)49. Besides that, the fluorescence spectrum of 6CzPh is already broad despite a relatively weak D–A interaction confirmed by the small solvatochromism of emission. The NMR peaks for carbazole rings in 6CzPh are markedly shifted from those in o2CzPh with the dicarbazole at the ortho position. In addition, the theoretical investigations visualised the non-covalent interactions between carbazole rings (Fig. 4d, e and Supplementary Fig. S12). The reduced density gradient (RDG) analysis exhibited intramolecular van der Waals interactions in green color50–52. The RDG isosurface map of 6CzPh indicated the intramolecular attractive interactions among hexad carbazoles to form toroidal π-delocalization. As the co-facial π-interaction significantly affects the excited state energies, for example, commonly observed shifts of energy levels in the solid states, the intramolecular interactions in the donut-shaped toroidal structure might contribute to a decrease in ΔEST values. The molecular design considering three-dimensional interactions holds the potential for additional strategy to control excited-state characteristics in future material developments.

Conclusions

In this work, we have demonstrated TADF in the hexacarbazolylbenzene derivative. Apart from the standard strategy for developing TADF materials that combines two functional units, the molecular structure only includes carbazole-donor and benzene moieties without explicit electron-acceptor units. A single benzene substituted with the relatively strong donor units of 3,6-dimethoxy-carbazole, 6MeOCzPh, exhibited typical TADF characteristics in both solution and film states. The physical insight of the molecular design is that the electron acceptor is induced by donor units. In addition, the toroidal π-delocalization by through space intramolecular interactions might have an impact on the excited states. These ideas would open new paths in luminophore developments and further study is expected to advance our understanding of excited state dynamics.

Methods

General

Commercially available materials for the synthesis were used as received from the suppliers. Details of instruments and physical measurements are given in Supplementary Note S1.

Synthesis and characterizations

The synthetic procedures and characterization data for each material are described in Supplementary Methods S1, and NMR and MS spectra are given in Supplementary Fig. S13–S18 and Fig. S19–S21, respectively.

X-ray single crystal analysis

X-ray crystallographic information files (CIFs) are available. CCDC 2369634–2369635 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/cif.

Theoretical calculations

The computations were mainly performed using the computer facilities at the Research Institute for Information Technology, Kyushu University. Molecular orbital calculations were performed using the program Gaussian 16. The geometries were optimized at the B3LYP/6-31+G(d,p) (Supplementary Table S2–S5). The natural population analysis (NPA) and natural bond orbital (NBO) analysis were calculated at the B3LYP/6-31+G(d,p) with NBO 7.0 program using optimized geometries51. The RDG analysis was performed on Multiwfn52,53, and plotted by VMD54.

Supplementary information

Peer Review File

Supporting Information

Description of Additional Supplementary Files

Supplementary Data 1

Supplementary information

The online version contains supplementary material available at 10.1038/s42004-024-01301-4.

Acknowledgements

We acknowledge support from KAKENHI Grant Numbers 23K04879 and 23K20039.

Author contributions

M.M. and C.A. conceptualized the research. M.M. and S.Y. synthesized and characterized the materials. M.M., S.Y., M.H., R.U., and H.K. performed the measurements and the data analysis. M.M. wrote the manuscript. M.M., T.H., and C.A. obtained the funding. All authors edited the manuscript.

Peer review

Peer review information

Communications Chemistry thanks Shaolong Gong, Dong-Ying Zhou and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

Data availability

The data that support the findings of this study are available in the supplementary material of this article. Source data are provided in Supplementary Data 1 with this paper. Additional information is available from the authors on request.

Competing interests

The authors declare no competing interests.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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References

1. Hong C A brief history of OLEDs—emitter development and industry milestones Adv. Mater. 2021 33 2005630 10.1002/adma.202005630
Hong, C. et al. A brief history of OLEDs—emitter development and industry milestones. Adv. Mater. 33, 2005630 (2021).
2. Uoyama H Goushi K Shizu K Nomura H Adachi C Highly efficient organic light-emitting diodes from delayed fluorescence Nature 2012 492 234 238 10.1038/nature11687 23235877
Uoyama, H., Goushi, K., Shizu, K., Nomura, H. & Adachi, C. Highly efficient organic light-emitting diodes from delayed fluorescence. Nature 492, 234–238 (2012).23235877
3. Ni F Li N Zhan L Yang C Organic thermally activated delayed fluorescence materials for time-resolved luminescence imaging and sensing Adv. Opt. Mater. 2020 8 1902187 10.1002/adom.201902187
Ni, F., Li, N., Zhan, L. & Yang, C. Organic thermally activated delayed fluorescence materials for time-resolved luminescence imaging and sensing. Adv. Opt. Mater. 8, 1902187 (2020).
4. Bryden MA Zysman-Colman E Organic thermally activated delayed fluorescence (TADF) compounds used in photocatalysis Chem. Soc. Rev. 2021 50 7587 7680 10.1039/D1CS00198A 34002736
Bryden, M. A. & Zysman-Colman, E. Organic thermally activated delayed fluorescence (TADF) compounds used in photocatalysis. Chem. Soc. Rev. 50, 7587–7680 (2021).34002736
5. Chen W Song F Thermally activated delayed fluorescence molecules and their new applications aside from OLEDs Chin. Chem. Lett. 2019 30 1717 1730 10.1016/j.cclet.2019.08.032
Chen, W. & Song, F. Thermally activated delayed fluorescence molecules and their new applications aside from OLEDs. Chin. Chem. Lett. 30, 1717–1730 (2019).
6. Wang J-X Heavy-atom engineering of thermally activated delayed fluorophores for high-performance X-ray imaging scintillators Nat. Photon. 2022 16 869 875 10.1038/s41566-022-01092-x
Wang, J.-X. et al. Heavy-atom engineering of thermally activated delayed fluorophores for high-performance X-ray imaging scintillators. Nat. Photon. 16, 869–875 (2022).
7. Yang W Dynamic reversible full-color piezochromic fluorogens featuring through-space charge-transfer thermally activated delayed fluorescence and their application as X-ray imaging scintillators Angew. Chem. Int. Ed. 2024 63 e202402704 10.1002/anie.202402704
Yang, W. et al. Dynamic reversible full-color piezochromic fluorogens featuring through-space charge-transfer thermally activated delayed fluorescence and their application as X-ray imaging scintillators. Angew. Chem. Int. Ed. 63, e202402704 (2024).
8. Zhang, G. et al. Guest-induced thermally activated delayed fluorescence organic supramolcular macrocycle scintillators for high-resolution X-ray imaging. Adv. Funct. Mater. 2404123, https://onlinelibrary.wiley.com/doi/abs/10.1002/adfm.202404123 (2024).
9. Chen X-K Kim D Brédas J-L Thermally activated delayed fluorescence (TADF) path toward efficient electroluminescence in purely organic materials: molecular level insight Acc. Chem. Res. 2018 51 2215 10.1021/acs.accounts.8b00174 30141908
Chen, X.-K., Kim, D. & Brédas, J.-L. Thermally activated delayed fluorescence (TADF) path toward efficient electroluminescence in purely organic materials: molecular level insight. Acc. Chem. Res. 51, 2215 (2018).30141908
10. Yang Z Recent advances in organic thermally activated delayed fluorescence materials Chem. Soc. Rev. 2017 46 915 1016 10.1039/C6CS00368K 28117864
Yang, Z. et al. Recent advances in organic thermally activated delayed fluorescence materials. Chem. Soc. Rev. 46, 915–1016 (2017).28117864
11. Liu Y Li C Ren Z Yan S Bryce MR All-organic thermally activated delayed fluorescence materials for organic light-emitting diodes Nat. Rev. Mater. 2018 3 18020 10.1038/natrevmats.2018.20
Liu, Y., Li, C., Ren, Z., Yan, S. & Bryce, M. R. All-organic thermally activated delayed fluorescence materials for organic light-emitting diodes. Nat. Rev. Mater. 3, 18020 (2018).
12. Wu T-L Diboron compound-based organic light-emitting diodes with high efficiency and reduced efficiency roll-off Nat. Photon. 2018 12 235 240 10.1038/s41566-018-0112-9
Wu, T.-L. et al. Diboron compound-based organic light-emitting diodes with high efficiency and reduced efficiency roll-off. Nat. Photon. 12, 235–240 (2018).
13. Cui L-S Fast spin-flip enables efficient and stable organic electroluminescence from charge-transfer states Nat. Photon. 2020 14 636 642 10.1038/s41566-020-0668-z
Cui, L.-S. et al. Fast spin-flip enables efficient and stable organic electroluminescence from charge-transfer states. Nat. Photon. 14, 636–642 (2020).
14. Chan C-Y Stable pure-blue hyperfluorescence organic light-emitting diodes with high-efficiency and narrow emission Nat. Photon. 2021 15 203 207 10.1038/s41566-020-00745-z
Chan, C.-Y. et al. Stable pure-blue hyperfluorescence organic light-emitting diodes with high-efficiency and narrow emission. Nat. Photon. 15, 203–207 (2021).
15. Cui L-S Controlling singlet–triplet energy splitting for deep-blue thermally activated delayed fluorescence emitters Angew. Chem. Int. Ed. 2017 56 1571 1575 10.1002/anie.201609459
Cui, L.-S. et al. Controlling singlet–triplet energy splitting for deep-blue thermally activated delayed fluorescence emitters. Angew. Chem. Int. Ed. 56, 1571–1575 (2017).
16. Méhes G Nomura H Zhang Q Nakagawa T Adachi C Enhanced electroluminescence efficiency in a spiro-acridine derivative through thermally activated delayed fluorescence Angew. Chem. Int. Ed. 2012 51 11311 11315 10.1002/anie.201206289
Méhes, G., Nomura, H., Zhang, Q., Nakagawa, T. & Adachi, C. Enhanced electroluminescence efficiency in a spiro-acridine derivative through thermally activated delayed fluorescence. Angew. Chem. Int. Ed. 51, 11311–11315 (2012).
17. Tsujimoto H Thermally activated delayed fluorescence and aggregation induced emission with through-space charge transfer J. Am. Chem. Soc. 2017 139 4894 4900 10.1021/jacs.7b00873 28345346
Tsujimoto, H. et al. Thermally activated delayed fluorescence and aggregation induced emission with through-space charge transfer. J. Am. Chem. Soc. 139, 4894–4900 (2017).28345346
18. Wada Y Nakatawa H Matsumoto S Wakisaka Y Kaji H Organic light emitters exhibiting very fast reverse intersystem crossing Nat. Photon. 2020 14 643 649 10.1038/s41566-020-0667-0
Wada, Y., Nakatawa, H., Matsumoto, S., Wakisaka, Y. & Kaji, H. Organic light emitters exhibiting very fast reverse intersystem crossing. Nat. Photon. 14, 643–649 (2020).
19. Hirai H One-step borylation of 1,3-diaryloxybenzenes towards efficient materials for organic light-emitting diodes Angew. Chem. Int. Ed. 2015 54 13581 13585 10.1002/anie.201506335
Hirai, H. et al. One-step borylation of 1,3-diaryloxybenzenes towards efficient materials for organic light-emitting diodes. Angew. Chem. Int. Ed. 54, 13581–13585 (2015).
20. Hatakeyama T Ultrapure blue thermally activated delayed fluorescence molecules: efficient HOMO–LUMO separation by the multiple resonance effect Adv. Mater. 2016 28 2777 2781 10.1002/adma.201505491 26865384
Hatakeyama, T. et al. Ultrapure blue thermally activated delayed fluorescence molecules: efficient HOMO–LUMO separation by the multiple resonance effect. Adv. Mater. 28, 2777–2781 (2016).26865384
21. Yuan Y at al. The design of fused amine/carbonyl system for efficient thermally activated delayed fluorescence: Novel multiple resonance core and electron acceptor Adv. Opt. Mater. 2019 7 1801536 10.1002/adom.201801536
Yuan, Y. at al. The design of fused amine/carbonyl system for efficient thermally activated delayed fluorescence: Novel multiple resonance core and electron acceptor. Adv. Opt. Mater. 7, 1801536 (2019).
22. Suresh SM Hall D Beljonne D Olivier Y Zysman-Colman E Multiresonant thermally activated delayed fluorescence emitters based on heteroatom-doped nanographenes: recent advances and prospects for organic light-emitting diodes Adv. Funct. Mater. 2020 30 1908677 10.1002/adfm.201908677
Suresh, S. M., Hall, D., Beljonne, D., Olivier, Y. & Zysman-Colman, E. Multiresonant thermally activated delayed fluorescence emitters based on heteroatom-doped nanographenes: recent advances and prospects for organic light-emitting diodes. Adv. Funct. Mater. 30, 1908677 (2020).
23. Kim HJ Yasuda T Narrowband emissive thermally activated delayed fluorescence materials Adv. Opt. Mater. 2022 10 2201714 10.1002/adom.202201714
Kim, H. J. & Yasuda, T. Narrowband emissive thermally activated delayed fluorescence materials. Adv. Opt. Mater. 10, 2201714 (2022).
24. Mamada M Hayakawa M Ochi J Hatakeyama T Organoboron-based multiple-resonance emitters: synthesis, structure–property correlations, and prospects Chem. Soc. Rev. 2024 53 1624 1692 10.1039/D3CS00837A 38168795
Mamada, M., Hayakawa, M., Ochi, J. & Hatakeyama, T. Organoboron-based multiple-resonance emitters: synthesis, structure–property correlations, and prospects. Chem. Soc. Rev. 53, 1624–1692 (2024).38168795
25. Boudin S Phosphorescence des solutions glycériques d’éosine influence des iodures J. Chim. Phys. 1930 27 285 290 10.1051/jcp/1930270285
Boudin, S. Phosphorescence des solutions glycériques d’éosine influence des iodures. J. Chim. Phys. 27, 285–290 (1930).
26. Parker CA Hatchard CG Triplet-singlet emission in fluid solutions. phosphorescence of eosin Trans. Faraday Soc. 1961 57 1894 1904 10.1039/tf9615701894
Parker, C. A. & Hatchard, C. G. Triplet-singlet emission in fluid solutions. phosphorescence of eosin. Trans. Faraday Soc. 57, 1894–1904 (1961).
27. Carlson SA Hercules DM Delayed thermal fluorescence of anthraquinone in solutions J. Am. Chem. Soc. 1971 93 5611 5616 10.1021/ja00751a003
Carlson, S. A. & Hercules, D. M. Delayed thermal fluorescence of anthraquinone in solutions. J. Am. Chem. Soc. 93, 5611–5616 (1971).
28. Maciejewski A Szymanski M Steer RP Thermally activated delayed S1 fluorescence of aromatic thiones J. Phys. Chem. 1986 90 6314 6318 10.1021/j100281a051
Maciejewski, A., Szymanski, M. & Steer, R. P. Thermally activated delayed S1 fluorescence of aromatic thiones. J. Phys. Chem. 90, 6314–6318 (1986).
29. Berberan-Santos MN Garcia JMM Unusually strong delayed fluorescence of C70 J. Am. Chem. Soc. 1996 118 9391 9394 10.1021/ja961782s
Berberan-Santos, M. N. & Garcia, J. M. M. Unusually strong delayed fluorescence of C70. J. Am. Chem. Soc. 118, 9391–9394 (1996).
30. Park S Kwon O-H Lee Y-S Jang D-J Park SY Imidazole-based excited-state intramolecular proton-transfer (ESIPT) materials:  observation of thermally activated delayed fluorescence (TDF) J. Phys. Chem. A 2007 111 9649 9653 10.1021/jp072212p 17760426
Park, S., Kwon, O.-H., Lee, Y.-S., Jang, D.-J. & Park, S. Y. Imidazole-based excited-state intramolecular proton-transfer (ESIPT) materials:  observation of thermally activated delayed fluorescence (TDF). J. Phys. Chem. A 111, 9649–9653 (2007).17760426
31. Endo A Thermally activated delayed fluorescence from Sn4+–porphyrin complexes and their application to organic light emitting diodes — a novel mechanism for electroluminescence Adv. Mater. 2009 21 4802 4806 10.1002/adma.200900983 21049498
Endo, A. et al. Thermally activated delayed fluorescence from Sn4+–porphyrin complexes and their application to organic light emitting diodes — a novel mechanism for electroluminescence. Adv. Mater. 21, 4802–4806 (2009).21049498
32. Li J Zhang Q Nomura H Miyazaki H Adachi C Thermally activated delayed fluorescence from 3nπ* to 1nπ* up-conversion and its application to organic light-emitting diodes Appl. Phys. Lett. 2014 105 013301 10.1063/1.4887346
Li, J., Zhang, Q., Nomura, H., Miyazaki, H. & Adachi, C. Thermally activated delayed fluorescence from 3nπ* to 1nπ* up-conversion and its application to organic light-emitting diodes. Appl. Phys. Lett. 105, 013301 (2014).
33. Mamada M Highly efficient thermally activated delayed fluorescence from an excited-state intramolecular proton transfer system ACS Cent. Sci. 2017 3 769 777 10.1021/acscentsci.7b00183 28776019
Mamada, M. et al. Highly efficient thermally activated delayed fluorescence from an excited-state intramolecular proton transfer system. ACS Cent. Sci. 3, 769–777 (2017).28776019
34. Pander P Thermally activated delayed fluorescence mediated through the upper triplet state manifold in non-charge-transfer star-shaped triphenylamine−carbazole molecules J. Phys. Chem. C. 2018 122 23934 23942 10.1021/acs.jpcc.8b07610
Pander, P. et al. Thermally activated delayed fluorescence mediated through the upper triplet state manifold in non-charge-transfer star-shaped triphenylamine−carbazole molecules. J. Phys. Chem. C. 122, 23934–23942 (2018).
35. Tsuchiya Y Molecular design based on donor-weak donor scaffold for blue thermally-activated delayed fluorescence designed by combinatorial DFT calculations Front. Chem. 2020 8 403 10.3389/fchem.2020.00403 32435635
Tsuchiya, Y. et al. Molecular design based on donor-weak donor scaffold for blue thermally-activated delayed fluorescence designed by combinatorial DFT calculations. Front. Chem. 8, 403 (2020).32435635
36. Hall D Diindolocarbazole – achieving multiresonant thermally activated delayed fluorescence without the need for acceptor units Mater. Horiz. 2022 9 1068 1080 10.1039/D1MH01383A 35067689
Hall, D. et al. Diindolocarbazole – achieving multiresonant thermally activated delayed fluorescence without the need for acceptor units. Mater. Horiz. 9, 1068–1080 (2022).35067689
37. Lee HL Multiple-resonance extension and spin-vibronic-coupling-based narrowband blue organic fluorescence emitters with over 30% quantum efficiency Adv. Mater. 2022 34 2202464 10.1002/adma.202202464
Lee, H. L. et al. Multiple-resonance extension and spin-vibronic-coupling-based narrowband blue organic fluorescence emitters with over 30% quantum efficiency. Adv. Mater. 34, 2202464 (2022).
38. Meng G Highly efficient and stable deep-blue OLEDs based on narrowband emitters featuring an orthogonal spiro-configured indolo[3,2,1-de]acridine structure Chem. Sci. 2022 13 5622 5630 10.1039/D2SC01543A 35694343
Meng, G. et al. Highly efficient and stable deep-blue OLEDs based on narrowband emitters featuring an orthogonal spiro-configured indolo[3,2,1-de]acridine structure. Chem. Sci. 13, 5622–5630 (2022).35694343
39. Peng Q Indolocarbazole-based deep-blue multiple-resonance narrowband emitters and efficient organic light-emitting diodes Chem. Eng. J. 2023 466 143423 10.1016/j.cej.2023.143423
Peng, Q. et al. Indolocarbazole-based deep-blue multiple-resonance narrowband emitters and efficient organic light-emitting diodes. Chem. Eng. J. 466, 143423 (2023).
40. Oner S Bryce MR A review of fused-ring carbazole derivatives as emitter and/or host materials in organic light emitting diode (OLED) applications Mater. Chem. Front. 2023 7 4304 4338 10.1039/D3QM00399J
Oner, S. & Bryce, M. R. A review of fused-ring carbazole derivatives as emitter and/or host materials in organic light emitting diode (OLED) applications. Mater. Chem. Front. 7, 4304–4338 (2023).
41. Seo J-A Im Y Han SH Lee CW Lee JY Unconventional molecular design approach of high-efficiency deep blue thermally activated delayed fluorescent emitters using indolocarbazole as an acceptor ACS Appl. Mater. Interfaces 2017 9 37864 37872 10.1021/acsami.7b09351 28980471
Seo, J.-A., Im, Y., Han, S. H., Lee, C. W. & Lee, J. Y. Unconventional molecular design approach of high-efficiency deep blue thermally activated delayed fluorescent emitters using indolocarbazole as an acceptor. ACS Appl. Mater. Interfaces 9, 37864–37872 (2017).28980471
42. Madushani B Hexacarbazolylbenzene: an excellent host molecule causing strong guest molecular orientation and the high-performance OLEDs Adv. Mater. 2024 36 2402275 10.1002/adma.202402275
Madushani, B. et al. Hexacarbazolylbenzene: an excellent host molecule causing strong guest molecular orientation and the high-performance OLEDs. Adv. Mater. 36, 2402275 (2024).
43. Goushi K Yoshida K Sato K Adachi C Organic light-emitting diodes employing efficient reverse intersystem crossing for triplet-to-singlet state conversion Nat. Photon. 2012 6 253 258 10.1038/nphoton.2012.31
Goushi, K., Yoshida, K., Sato, K. & Adachi, C. Organic light-emitting diodes employing efficient reverse intersystem crossing for triplet-to-singlet state conversion. Nat. Photon. 6, 253–258 (2012).
44. Mamada M Highly efficient deep-blue organic light-emitting diodes based on rational molecular design and device engineering Adv. Funct. Mater. 2024 32 2204352 10.1002/adfm.202204352
Mamada, M. et al. Highly efficient deep-blue organic light-emitting diodes based on rational molecular design and device engineering. Adv. Funct. Mater. 32, 2204352 (2024).
45. Rosokha SV Neretin IS Sun D Kochi JK Very fast electron migrations within p-doped aromatic cofacial arrays leading to three-dimensional (toroidal) π-delocalization J. Am. Chem. Soc. 2006 128 9394 9407 10.1021/ja060393n 16848475
Rosokha, S. V., Neretin, I. S., Sun, D. & Kochi, J. K. Very fast electron migrations within p-doped aromatic cofacial arrays leading to three-dimensional (toroidal) π-delocalization. J. Am. Chem. Soc. 128, 9394–9407 (2006).16848475
46. Furukawa S Double aromaticity arising from σ- and π-rings Commun. Chem. 2018 1 60 10.1038/s42004-018-0057-4
Furukawa, S. et al. Double aromaticity arising from σ- and π-rings. Commun. Chem. 1, 60 (2018).
47. Bakouri OE Three-dimensional fully π‑conjugated macrocycles: when 3D-aromatic and ehen 2D-aromatic-in-3D? J. Am. Chem. Soc. 2022 144 8560 8575 10.1021/jacs.1c13478 35523019
Bakouri, O. E. et al. Three-dimensional fully π‑conjugated macrocycles: when 3D-aromatic and ehen 2D-aromatic-in-3D? J. Am. Chem. Soc. 144, 8560–8575 (2022).35523019
48. Nishiuchi T Synthesis, properties, and intermolecular interactions in the solid states of π-congested X-shaped 1,2,4,5-tetra(9-anthryl)benzenes Bull. Chem. Soc. Jpn. 2022 95 1591 1599 10.1246/bcsj.20220257
Nishiuchi, T. et al. Synthesis, properties, and intermolecular interactions in the solid states of π-congested X-shaped 1,2,4,5-tetra(9-anthryl)benzenes. Bull. Chem. Soc. Jpn. 95, 1591–1599 (2022).
49. Feng H-T Tuning molecular emission of organic emitters from fluorescence to phosphorescence through push-pull electronic effects Nat. Commun. 2020 11 2617 10.1038/s41467-020-16412-4 32457319
Feng, H.-T. et al. Tuning molecular emission of organic emitters from fluorescence to phosphorescence through push-pull electronic effects. Nat. Commun. 11, 2617 (2020).32457319
50. Johnson ER Revealing noncovalent interactions J. Am. Chem. Soc. 2010 132 6498 6506 10.1021/ja100936w 20394428
Johnson, E. R. et al. Revealing noncovalent interactions. J. Am. Chem. Soc. 132, 6498–6506 (2010).20394428
51. Glendening, E. D. et al. Theoretical Chemistry Institute, University of Wisconsin, Madison (2018).
52. Lu T Chen F Multiwfn: a multifunctional wavefunction analyzer J. Comput. Chem. 2012 33 580 592 10.1002/jcc.22885 22162017
Lu, T. & Chen, F. Multiwfn: a multifunctional wavefunction analyzer. J. Comput. Chem. 33, 580–592 (2012).22162017
53. Lu T Chen Q Interaction region indicator: a simple real space function clearly revealing both chemical bonds and weak interactions Chem.–Methods 2021 1 231 239 10.1002/cmtd.202100007
Lu, T. & Chen, Q. Interaction region indicator: a simple real space function clearly revealing both chemical bonds and weak interactions. Chem.–Methods 1, 231–239 (2021).
54. Humphrey W Dalke A Schulten K VMD: Visual molecular dynamics J. Mol. Graph. 1996 14 33 38 10.1016/0263-7855(96)00018-5 8744570
Humphrey, W., Dalke, A. & Schulten, K. VMD: Visual molecular dynamics. J. Mol. Graph. 14, 33–38 (1996).8744570
