Intermolecular Interaction-Induced Thermally Activated Delayed

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Intermolecular Interaction Induced Thermally Activated Delayed Fluorescence Based on Thiochromone Derivative Jianjun Liu, Taiping Hu, Zhiyi Li, Xiaofang Wei, Xiaoxiao Hu, Honglei Gao, Guanhao Liu, Yuanping Yi, Yukiko Yamada-Takamura, Chun-Sing Lee, Pengfei Wang, and Ying Wang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00512 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 3, 2019

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The Journal of Physical Chemistry Letters

Intermolecular Interaction Induced Thermally Activated Delayed Fluorescence Based on Thiochromone Derivative Jianjun Liu†, ¶,, Taiping Hu‡,, Zhiyi Li†, ¶, Xiaofang Wei†, ¶, Xiaoxiao Hu†, ¶, Honglei Gao†, ¶, Guanhao Liu†, ¶, Yuanping Yi*, ‡, Yukiko Yamada-Takamura§, Chun-Sing Lee∥, Pengfei Wang†, ¶ and Ying Wang*, †, ¶ †Key

Laboratory of Photochemical Conversion and Optoelectronic Materials and CityU-CAS

Joint Laboratory of Functional Materials and Devices, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, China ‡Beijing

National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids,

Institute of Chemistry, Chinese Academy of Sciences, Beijing,100190, China ¶University

§School

of Chinese Academy of Sciences, Beijing, 100049, China

of Materials Science, Japan Advanced Institute of Science and Technology, Ishikawa,

923-1292, Japan ∥ Center

of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong,

Hong Kong SAR, P. R. China

ACS Paragon Plus Environment

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Corresponding Author * Y. Wang: [email protected] *Y. Yi: [email protected] These

authors contributed equally to this work.


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ABSTRACT Exploration of new extrinsic ways to modulate thermally activated delayed fluorescence (TADF) for achieving high exciton utilization efficiency in organic light-emitting diodes (OLEDs) is highly desirable. A newly thiochromone derivative 2,3-bis(4-(9H-carbazol-9yl)phenyl)-4H-thiochromen-4-1,1-dioxide (THI-PhCz) with tunable photophysical properties from crystals to amorphous states is reported. THI-PhCz shows molecular-packing-dependent TADF in different aggregation states based on the differences of intermolecular interactions. Furthermore, it is observed that THI-PhCz doped in amorphous films of different hosts also show host-dependent TADF with a short delay lifetime (108 ns), which is interpreted as the effect of host-guest intermolecular interaction on 3CT state except for the effect on 1CT state in reported references. This work provides a new perspective for generation of TADF by tuning intermolecular interactions in crystals and amorphous films except for molecular design, which is expected to contribute in achieving low efficiency roll-off OLEDs with effective exciton utilization efficiency.

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With their superior characteristics as a new generation of the full-color flat-panel display and solid-state lighting technologies, organic light-emitting diodes (OLEDs) have been widely studied by researchers and even developed far beyond being a lab curiosity1-2. In an OLED device, since the injected charges are uncorrelated to generate the limit of spin statistics, it is critical to find suitable methods to make full use of both singlet and triplet excitons to fabricate more efficient OLEDs. Although the OLEDs based on phosphorescent emitters have been widely studied because of their 100 % exciton utilization efficiency of electroluminescence (EL) in OLEDs, the high device fabrication costs, serious efficiency roll-off and lack of blue emitters with excellent properties of phosphorescent OLEDs impede them from the application3-4. Thus, choosing appropriate emitters with efficient luminescence, low cost, and excellent chemical stability are very necessary. Furthermore, the exciton utilization efficiency of purely organic fluorescent OLEDs can be enhanced by the limited processes affecting spin statistics, which are the reported triplet-triplet annihilation (TTA)5, exciton-polaron interaction (EPI)6 or thermally activated delayed fluorescence (TADF)7. Nevertheless, the TTA and EPI with their bimolecular processes cannot fully harvest triplet excitons, which are more challenging to modulate in devices and achieve high efficiency OLEDs. In contrast, TADF can manipulate the intrinsic electronic properties based on effective molecular design with the separation of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), providing small singlet-triplet splitting (EST) for efficient reverse intersystem crossing (RISC) and full exciton utilization based on a unimolecular process8. Thus, enormous efforts have been endeavored to design suitable molecules with the maximization of TADF9. However, the TADF mechanism of achieving high exciton utilization efficiency is mainly based on intrinsic molecular electronic properties by molecular design. It is noteworthy that its efficiency can be influenced actually by


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the surroundings10-12, because the charge transfer (CT) excited states characteristic in common donor-acceptor(D-A) TADF molecules are very sensitive to their local environment, which can affect EST, the rate of RISC and further change TADF property. Adachi and Monkman reported the solvent polarity can influence the TADF property for phenothiazine derivatives by changing donor-acceptor conformation. Wang et al. investigated that TADF for xanthone derivatives can be tuned by supramolecular structures. However, the studies on it are scarce13-14, especially for the research on the emitters in host-guest system for application in OLEDs15-17. Based on this background and the limited ways of improving exciton utilization efficiency, whether there exists a new extrinsic method except for molecular design to enhance RISC rate, eventually achieving effective TADF property and high exciton utilization efficiency in OLEDs is still needed us to further investigation. To explore the new extrinsic ways to regulate photophysical properties and improve exciton utilization efficiency, we designed and synthesized a new D-A-D molecule (Figure 1(a)), THIPhCz, based on the thiochromone 1,1-dioxide (THI) acceptor and the N-phenylcarbazole (PhCz) donor to research its tunable photophysics by surroundings. THI-PhCz shows molecularpacking-dependent TADF properties in crystals and host-dependent TADF with a short TADF lifetime in amorphous doped films. These are attributed to the different intermolecular interactions induced by different packing modes and the effects of host-guest interaction on T1 state (charge transfer (CT) state), respectively. Our results provide new insight for generating TADF by modulating intermolecular interactions on top of conventional molecular design, which is promising in OLEDs with high exciton utilization efficiency.


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Figure 1 Molecular structure (a) of THI-PhCz. Confocal laser microscopy images of crystal A, crystal B and powder (b). Absorption spectra of THI-PhCz in chloroform solution and fluorescence spectra of crystal A, crystal B, powder and chloroform solution of THI-PhCz (c). In THI-PhCz, the sulfonyl and carbonyl groups provide strong electron withdrawing ability in THI7, which is first used as TADF acceptor. The adjacent PhCz with steric hindrance are beneficial to separate HOMO and LUMO. As expected, density functional theory (DFT) calculation results show that the HOMO mainly disperses over the PhCz moieties, while the LUMO is localized on THI (Figure S7). THI-PhCz was synthesized by a four-step route (Scheme S1). Structure and purity were verified with 1H NMR,

13C

NMR, mass spectra and elemental

analyses (Figure S1-S6). Photophysical properties of THI-PhCz in solutions were investigated. In different polar solvents, THI-PhCz exhibits similar absorption bands in 300-500 nm. The bands at less than 350 nm belong to -* and n-* transitions. The broad bands at around 410 nm can be assigned to


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the intramolecular CT (ICT) transition of the D-A conjugated framework (Figure 1(c) and S8(a)). It is noteworthy that the emission spectrum consists of two broad emissions with structureless peaks around 519 nm and 655 nm in chloroform solution (Figure 1(c)). These emission peaks gradually red-shift with increasing solvent polarity (Figure S8(b)), suggesting a typical ICT feature of the two emission peaks18-19. The dual ICT emission in solutions has been described in detail by Monkman11 and Adachi10. In here, THI-PhCz exhibits the analogous properties with the reported compounds, which shows that the emission contribution can be controlled by excitation energy (Figure S9). However, Time-dependent DFT (TD-DFT) (Figure S10) calculation shows two different CT excited states, which produces the different Natural Transition Orbital (NTO) pairs distributions with different CT transitions rather than the reported LE and ICT transitions (Figure S10(a))20-22. Meanwhile, time-resolved emission spectra and transient photoluminescence (PL) decay of THI-PhCz in solutions (Figure S11-S12) were also measured to investigate its TADF property, but no conclusive evidence could be found23.

Figure 2 The temperature-dependent transient PL spectra for crystal A (a) and crystal B (b). PL emission for crystal A excited at different laser pulse power (c). Delayed PL intensity dependence for crystal A on laser pulse power (d).


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Interestingly, two kinds of high-quality crystals for THI-PhCz were successfully prepared by solvent diffusion (Figure 1(b)). Crystal A was prepared by slow diffusion from CH2Cl2 to methanol, but the diffusion from CHCl3 into methanol formed crystal B. Photophysical properties of two crystals were investigated and showed significant differences. The emission bands of crystal A (max,em = 533 nm) and crystals B (max,em = 563 nm) lied in the yellow and orange region, respectively, which are different from those of the solid powder (max,em =581 nm, no TADF(Figure S13)) (Figure 1(c)). Meanwhile, emission of Crystal A displays a short lifetime of 3.84 ns and a long-lived emission of 543 s at 300 K (Figure 2(a)). The temperaturedependent transient PL decay showed that the proportion of delayed component was improved with the increase of temperature, and the long lifetime was lengthened from 436 s at 200 K to 543 s at 300 K. The emission maximum was also gradually strengthened with increasing temperature (Figure S14(a)). All temperature-dependent tests can confirm the TADF behavior of crystal A. The intensities of the delayed fluorescence for crystal A at different excitation dose were investigated24. A clear linear dependence with the slope of 1 for delayed fluorescence can be observed throughout the entire excitation range (Figure 2(c-d)), demonstrating the TADF mechanism in crystal A, not the triplet-triplet annihilation (the slope of 2)25. However, the crystal B showed only a short lifetime (3.81 ns at 300 K) and no conclusive TADF evidence according to the temperature-dependent experiments (Figure 2(b), Figure S14 (b)). The EST, further determined from the fluorescence and phosphorescence spectra recorded at 77 K (Figure S15), are 0.05 eV and 0.22 eV for crystal A and B, respectively, which is basically consistent with their crystal-dependent TADF property. To investigate the factors influencing the luminescence in crystals, crystal structures of crystal A and B were measured with single-crystal X-ray diffraction. Unlike the reported phenothiazine


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derivatives with equatorial and axial conformers14, there are only differences of torsion angles between THI and PhCz in monomers. Crystal A shows a conformation with torsion angles of 72.7 and 125.9(Figure S16), which is different from the conformation of Crystal B monomers. Also, the theoretical calculation based on monomers provides the different EST (Figure 3(c))26. The EST of 0.13 eV in crystal A is lower than that of 0.19 eV in crystal B, which is attributed to the different distributions of HOMO in monomers and their ICT difference. This calculated EST in crystal B (0.19 eV) is consistent with the experimental value (0.22 eV), which can prove the absence of TADF emission in Crystal B is mainly attributed to its intramolecular interaction in the monomer. However, the experimental EST for crystal A (0.05 eV) and crystal-dependent TADF for THI-PhCz cannot be fully explained from the monomers because of the difference in EST between theory and experiment, which is inferred that there are other reasons to affect crystal A luminescence aside from the intramolecular interaction27. We further investigate the influence of molecular stacking modes on TADF properties. In crystal A (Figure 3(a)), two adjacent molecules form a dimer with the anti-parallel conformation by intermolecular C-H···π (2.630, 2.725 and 2.853 Å) contacts. The C-H···C-H (3.106, 3.370 Å) contacts between dimers form a supermolecular chain along the crystallographic c-axis with parallel packing. Meanwhile, there is intermolecular π···π interaction on adjacent dimers (3.607 Å, overlapping area: 31 %). It is believe that the strong intermolecular interactions and π···π stacking can efficiently suppress vibrational relaxation and thus preserve the triplet excited states and resulting in strong TADF characteristics in Crystal A28. On consideration of intermolecular interactions, the result of TD-DFT calculation based on the dimer in crystal A (Figure 3(c)) displays well-separated HOMOs and LUMOs compared to its monomer. Consequently, the calculated EST of 0.06 eV in the dimer is in agreement with the experimental result (0.05 eV).


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Therefore, intermolecular interactions in crystal A play a decisive role in its TADF property by decreasing EST to improve RISC rate and preserving the triplet excited states27.

Figure 3 The molecular packing modes in crystal A (a) and Crystal B (b) along the c-axis direction. Calculated HOMO and LUMO levels for THI-PhCz monomers in crystal A and crystal B and dimer in crystal A (c).


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However, in crystal B, two monomers are arranged along the a-axis with different manners (B1B1-B2B2) (Figure 3(b)). There are only weak C-H···π (3.350 Å) and C=O-H (2.503 Å) contacts for B1B1 to form a supermolecular chain along c-axis and no dimer for B1B1 with long molecular distance (5.412 Å). Although the C-H···π (2.591-3.379 Å) contacts on adjacent molecules in B2B2 is also observed, the crystal B display weak intermolecular interactions compared to Crystal A, which can be used to explain why EST of crystal B comes from its monomer and why crystal B does not show TADF. A rational explanation is that its triplet excited states may undergo nonradiative decay via vibrational relaxation because of lack of effective intermolecular interactions for restricting molecular vibrations, leading to the disappearance of TADF13. Also, the Natural Transition Orbital (NTO) of T1S0 in Crystal B (Figure S17(b)) show a strong nonradiative vibrational relaxation, which can explain that why the crystal B fails to show TADF even though its EST less than 0.3 eV29. As a result, the difference of intermolecular interactions (packing modes and - stacking) in crystal A and B result in the molecular-packing-dependent TADF in THI-PhCz. Although the TADF in crystal A shows supramolecular aggregation state emission with strong intermolecular interactions between guest molecules, the emitting layers in OLEDs usually adopt host-guest doped amorphous systems with a low concentration of guest, which is guest singlemolecule emission rather than guest supramolecular aggregation state emission. Therefore, it is necessary to further investigate the doped films of THI-PhCz to generate the TADF emission of guest molecule for application in OLEDs with high exciton utilization efficiency. X-ray diffraction (XRD) patterns of THI-PhCz in different solid states are shown in Figure 4(a). The spectrum from THI-PhCz powder shows much boarder peaks compared to diffraction peaks from crystals, suggesting their disordered amorphous nature. But, it is noteworthy that the amorphous


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powder shows diffraction peak positions similar to Crystal B. Also, there are similar PL spectra and no TADF emission in amorphous powder and Crystal B. Therefore, we conjecture that THIPhCz films based on amorphous state may also not show TADF. Therefore, this motivates us to further study how to achieve TADF in amorphous doped films based on other intermolecular interaction for the applications in OLEDs.

Figure 4 XRD spectra of THI-PhCz in different solid states (a). The PL emission spectra of THIPhCz in different hosts with 20 % doped amount (b). The transient PL spectra of THI-PhCz doped films (c). Schematic mechanism of THI-PhCz in TPB3 and mCP (d). Different polarity hosts, polymethyl methacrylate (PMMA), Polystyrene (PS), 1,3,5-Tri(pyren1-yl)benzene (TPB3) and 1,3-Bis(carbazol-9-yl)benzene (mCP), were selected (Figure S18). To realize the effective energy transfer, the spin-coating films with a THI-PhCz doped concentration of 20 % were prepared (Figure S19 and S20)30. Figure 4(b) shows that all doped films exhibit


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featureless blue-shifted emission compared to that of the neat film (606 nm), which are closer to the emission of Crystal B. Cyclic voltammogram of hosts and guest in Figure S21 and the reported energy levels also indicate that the emissions from the films are not from exciplexes because of nor matched HOMO and LUMO between hosts and guest31. The transient PL decay characteristics of these films show that only the THI-PhCz in TPB3 display two-component decays, that is, a fast prompt lifetime of 20.85 ns and a delayed lifetime of 108.14 ns, which can be assigned to ﬂuorescence and TADF decay, respectively (Figure 4(c)). The temperaturedependent transient PL showed that the proportion of delayed component and emission maximum slightly improved with the increase of temperature because of the fast TADF lifetime and RISC rate (Figure S22). However, other hosts just display a fast decay lifetime of 6.54-9.10 ns and no TADF property. Therefore, THI-PhCz doped films achieve host-dependent TADF with the change of host materials. Furthermore, we investigate the effect of host-guest interaction on host-dependent TADF property. THI-PhCz in nonpolar PS or PMMA matrix without TADF can be caused by the nonpolar solid environment, which is similar to its property in solutions32. However, for mCP and TPB3 host, the fluorescence and phosphorescence spectra in doped film at 77 K were measured to calculate S1 and T1 energy levels (Figure S23). All emissions show structureless broad peaks, which can infer the CT characteristic of all S1 and T1 states. The S1 and T1 level of THI-PhCz are 2.22 eV and 2.02 eV in mCP doped film, respectively, but they are 2.31 eV and 2.29 eV in TPB3 doped film, respectively. So, the EST values are 0.2 eV and 0.02 eV in mCP and TPB3 doped film, respectively, which is conformed to the host-dependent TADF for THIPhCz. Also, the nearly zero value of EST in TPB3 host can explain its negligible temperaturedependent transient PL property (Figure S22) and its very short delayed lifetime (108 ns). The


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CT singlet states (1CT) are slightly decreased (0.09 eV) with increasing the polarity of hosts because of the dipole-dipole interaction, which is consistent with the reported explanation17, 33. However, if we only consider the change of 1CT, the small difference is not enough to cause the TADF to turn on/off property because of its small change of energy levels. It is noteworthy that there is an obvious change (0.27 eV) of CT triplet state (3CT) because of the CT characteristic of T1 state in doped films, which can magnificently affect the EST and RISC (Figure 4(d)). Therefore, the host-dependent TADF for THI-PhCz is most ascribed to the change of 3CT energy level except for 1CT based on the different host-guest interaction (dipole-dipole interaction). We emphasize the importance of 3CT for regulating TADF property in doped films, which has guiding significance for choosing suitable hosts to achieve effective TADF property in hostguest systems. The host-dependent TADF property with very fast TADF lifetime by regulating the CT state motivates us to design analogous molecules with high PLQY to achieve highefficiency OLEDs with low-efficiency roll-off and high exciton utilization efficiency in future. In summary, we investigated how to achieve TADF in different environments by modulating intermolecular interactions based on a D-A-D molecule THI-PhCz, which is a new extrinsic way to potentially achieve effective TADF property and high exciton utilization efficiency. THI-PhCz shows a dual CT state emission and no TADF in solutions. However, THI-PhCz can generate two different crystals, and achieve the molecular-packing-dependent TADF. The result emphasizes the influence of intermolecular interactions on TADF though molecular packing in crystals. To achieve TADF emission of guest molecule in doped films rather than supramolecular aggregation for further application, the amorphous doped films based on THI-PhCz were studied and exhibit host-dependent TADF by modulating the polarity of hosts. It is noteworthy that the effect of dipole-dipole interaction on 3CT state plays a decisive role in host-dependent TADF


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except for the reported effect on 1CT state. This work provides a new perspective method for the realization of TADF property based on intermolecular interactions from the crystals to amorphous films rather than intrinsic molecular design, and has a potential to achieve low efficiency roll-off OLEDs with high exciton utilization efficiency. ASSOCIATED CONTENT Supporting Information. Additional details of the synthesis, supplementary figures and tables, CIF formats of single-crystal data, and other information. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Y. W.) *E-mail: [email protected] (Y. Yi.) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Science Foundation of China (Grant No. 61420106002, No.51373189, No. 91833304 and No. 21772209), the National Key Research and Development Project (No. 2016YFB0401004), and the National Program for Support of Topnotch Young Professionals. REFERENCES (1) Yang, Z.; Mao, Z.; Xie, Z.; Zhang, Y.; Liu, S.; Zhao, J.; Xu, J.; Chi, Z.; Aldred, M. P. Recent Advances in Organic Thermally Activated Delayed Fluorescence Materials. Chem. Soc. Rev. 2017, 46, 915-1016.


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Intermolecular Interaction-Induced Thermally Activated Delayed

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