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Single Molecule Based White-Light Emissive Organic Solids with Molecular Packing Dependent Thermally-Activated Delayed Fluorescence Yuewei Zhang, Yang Miao, Xiaoxian Song, Yu Gao, Zuolun Zhang, Kaiqi Ye, and Yue Wang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02213 • Publication Date (Web): 20 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017

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The Journal of Physical Chemistry Letters

Single Molecule Based White-Light Emissive Organic Solids with Molecular Packing Dependent Thermally-Activated Delayed Fluorescence Yuewei Zhang, Yang Miao, Xiaoxian Song, Yu Gao, Zuolun Zhang, Kaiqi Ye,* and Yue Wang* State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China Corresponding Authors [email protected] (Y. W.) [email protected] (K. Y.)

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ABSTRACT. White-light emitting single molecules have attracted broad attention because of their great potential for use in flat-panel displays and future light sources. Herein, we report a unique molecule of 3-(diphenylamino)-9H-xanthen-9-one (3-DPH-XO), which was found to exhibit bright white-light emission in the solid state caused by the spontaneous formation of a mixture with different polymorphs. Single-crystal analyses demonstrate that non-covalent interactions (such as π···π stacking, hydrogen bonding, and C−H···π interactions) induce different stacking arrangements (polymorphs A, B, and C) with different photophysical properties in a molecular solid. In addition, crystals B and C with the acceptor···acceptor stacking feature show the thermally-activated delayed fluorescence (TADF) characteristics, indicating that appropriate non-covalent interactions could enhance the reverse intersystem crossing (RISC) process and consequently lead to delayed fluorescence. This discovery provides an effective strategy for the design of new white-light emitting single molecules as well as TADF materials.

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Organic white-light emitting materials and devices are of great interest due to their wide applications in the next-generation displays and light sources.1-3 Most of the reported white-light organic solids were achieved by rational combination of red/green/blue or blue/orange emitters, which results in the emission spectrum covering the entire visible-light region.4-7 In addition, to achieve high photoluminescence (PL) and electroluminescence (EL) quantum efficiencies, an emitter was generally doped into an appropriate host, and different emitters needed different hosts. As a result, the organic white-light solids were often constructed based on the complex material systems. To minimize the manufacture cost of materials and simplify the device fabrication process, the construction of high-performance organic white-light solids based on simple material systems is essential. However, it is still challenging. In this context, the design and construction of organic white-light solids based on a single molecular component have attracted great attention. To achieve white-light emission based on a single organic molecule, the molecular chromophore should be able to generate dual emissions (blue and orange) or triple emissions (red, green and blue). Therefore, to achieve a singlemolecule white-light organic solid, the critical issue is to simultaneously promote multi-type exited states with radiative transition. Recently, some strategies for realizing single molecule based white-light organic solids were proposed and performed: (i) monomer and excimer fluorescence;8,9 (ii) excited-state intramolecular proton transfer (ESIPT);10-13 (iii) prompt and delayed fluorescence based on molecular heredity;14,15 (iv) fluorescence and phosphorescence;1618

and (v) locally and charge-transfer (CT) excited state emissions induced by the change of

molecular conformation.19,20 So far, the highly efficient single-molecule organic white-light solids with appropriate Commission Internationale de l’Eclairage (CIE) still remain very rare.14,19 Moreover, all of the single-component white-light organic solids reported so far were prepared


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based on the design and optimization of molecular structures. Therefore, the new and efficient approaches to construct single-molecule organic white-light solids are highly desirable. In addition, to harvest both of singlet and triplet excited states in organic light-emitting devices (OLEDs), the single-molecule white-light organic solids should be endowed with the thermallyactivated delayed fluorescence (TADF) or phosphorescence feature. Recently, we demonstrated that the supramolecular structures and intermolecular non-covalent interactions sometimes dominate the excited state and emission properties.21-24 It should be a promising and unique pathway to construct single-molecule white-light organic solids based on the concept of non-covalent interaction dependent emission. Therefore, our study was focused on the non-covalent regulation toward multiple emissions of organic solids composed of a single molecule, which can be used as a strategy to construct single-molecule white-light organic solids. In this contribution, we report an efficient white-light solid constructed by a classic donoracceptor (D-A) molecule of 3-(diphenylamino)-9H-xanthen-9-one (3-DPH-XO) with the polymorph-dependent TADF property (Figure 1a).

Figure 1. Molecular structure (a) and single-crystal structure (b) of 3-DPH-XO. Calculated LUMO (c) and HOMO (d) spatial distributions of 3-DPH-XO.


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Xanthone (XO) and diphenylamine (DPH) were selected as the electron acceptor and donor, respectively, to construct a D-A type molecule. The planar feature of the XO moiety (for π···π stacking), the electron negative property of the oxygen atoms on XO moiety (for hydrogen bonding), and the large hindrance and free rotations of the DPH moiety (for changes in molecular conformations) are benefit to the construction of D-A molecule with unique property (Figure 1b).25-27 Moreover, the D-A structure also plays an important role in the formation of polymorphs. Due to the conjugation with a strong electron accepter, the protons on the phenyl groups on DPH moiety are highly polarized (electron-poor) thus can form strong hydrogenbonds with the oxygen atoms on the XO moiety on the adjacent 3-DPH-XO molecule. Theoretical calculations showed that the HOMO of 3-DPH-XO is predominantly located on the diphenylamine moiety and the benzene ring attached to it, while the LUMO is mainly distributed on the XO moiety (Figure 1c and 1d), suggesting the intramolecular charge-transfer (ICT) transition feature for this molecule. The limited spatial overlap between the HOMO and LUMO is beneficial to the generation of TADF. 3-DPH-XO was synthesized in a high yield (73%) by the Ullmann coupling reaction of 3bromo-9H-xanthen-9-one (3-Br-XO) and diphenylamine. The chemical structure and purity were verified by 1H NMR, mass spectra and elemental analyses (see Supporting Information). In dilute toluene solution (10-5 M), 3-DPH-XO showed two UV absorption bands at around 280 and 370 nm that could be assigned to the π–π* and ICT transitions of the D-A conjugated framework, respectively (Figure S1). The emission spectrum was structureless and exhibited a large Stokes shift and a broader full width at half maximum (FWHM), indicating the ICT luminescence feature.28-31 Moreover, when the solvent was changed from non-polar cyclohexane to highly polar acetonitrile, the emission maximum was gradually red shifted from 407 to 570 nm (Figure


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S2). This remarkable solvatochromic shift also demonstrates a typical ICT feature for the excited state of 3-DPH-XO molecules.32-37 In dilute toluene solution, 3-DPH-XO displayed intense blue emission with the maximum at 452 nm and a high PL quantum yield (Φf-sol) of 77%, and delayed fluorescence was not detected. In addition, under air and nitrogen atmospheres, the dilute toluene solution of 3-DPH-XO showed almost identical Φf-sol and fluorescence lifetime (around 5 ns) (Figure S3).

Figure 2. (a) The emission spectrum (combination result of the 20 emission spectra) of 3-DPHXO solid powder under excitation of 365 nm light; Inset: photo image (1) and fluorescence


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microscopy image (2) of the solid under 365 nm UV irradiation. (b) Emission spectra (20 random measurements) and the corresponding color coordinates (CIE 1931) of the 3-DPH-XO solid powder under excitation of 365 nm UV light. Interestingly, the crystalline powder of 3-DPH-XO, which was prepared by the quick solvent evaporation of the dichloromethane solution of DPH-XO under reduced pressure, exhibited bright white emission with the PL quantum yield (Φf-pow) of 40% and the CIE coordinates of (0.27,0.35) (Inset 1 in Figure 2a). Although bright white emission was observed by naked eyes when the crystalline powder was irradiated by UV light, the fluorescence microscope (FM) image recorded for the same crystalline powder clearly showed that diversified emission colors were displayed by different micro-crystals (Inset 2 in Figure 2a). Obviously, the white-light powder was composed of a large amount of small blue-, green-, and yellow-emissive crystals. A CCD (charge-coupled device) detector was employed to randomly collect the emission spectra of different 20 positions of the powder surface. The emission spectrum presented in Figure 2a is the normalized combination result of the 20 emission spectra. Obviously, the 20 emission spectra display different profiles (Figure 2b), which agree with FM image. The PL spectra of 3-DPH-XO solid powder exhibited three emission maxima (λmax) at 441, 528 and 549 nm. Remarkably, this solid is composed by different crystalline phases of 3-DPH-XO molecules. In the other words, it is the mixture of different polymorphs. To figure out structural characteristics of the polymorphs that compose the white-light solids, three kinds of 3-DPH-XO-based crystals were grown and obtained under different conditions. As shown in Figure 3, the blue-emitting crystal A (λmax-A: 441 nm, Φf-A: 58%) was prepared by slow diffusion of petroleum ether vapor into CHCl3 solutions of 3-DPH-XO. Similarly, greenemissive crystal B (λmax-B: 525 nm, Φf-B: 49%) was obtained by slow diffusion of methanol vapor


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into CHCl3 solutions of 3-DPH-XO. Heating the saturated toluene solution of 3-DPH-XO at 60 °C for several minutes followed by slowly cooling down to room temperature resulted in the formation of yellow-emissive crystal C (λmax-C: 544 nm, Φf-C: 44%).

Figure 3. Photo images (a) and normalized fluorescence spectra (b) of the polymorphs A, B, and C recorded under 365 nm UV irradiation at room temperature. To understand the dependence of emission properties on supramolecular structures, single crystal X-ray structures of these polymorphs were carefully studied. In crystal A, 3-DPH-XO molecules have two kinds of conformations, A1 and A2 (A1:A2 = 1:1), which exhibit different torsion angles (Figure S4) and are arranged along the c-axis with an alternate manner (A1A1A2A2-A1A1-A2A2) based on multiple C−H···π, O=C···H−C, and C−H···O hydrogen bonds (Figure 4a and Figure 5a). For crystal A, the absence of π···π stacking interactions and a


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relatively isolated state of 3-DPH-XO molecules can promote short-wavelength emission. In crystal B, 3-DPH-XO molecules form abundant isomers with eight different conformations (namely B1 to B8) being observed (Figure S5). Crystal B is formed based on extended weak intermolecular π···π interactions (contact distance: 3.55 Å, overlapping area: 20%) between XO planes (Figure 4b). In addition, intermolecular C−H···π and O=C···H−C interactions also exist in crystal B (Figure 5b). Different from crystals A and B, there is only one molecular conformation in crystal C (Figure S6). The most notable structural feature in crystal C is that 3DPH-XO molecules aggregate into dimers based on strong intermolecular π···π stacking interactions between XO planes, and the interplanar separation distance and overlapping area are around 3.45 Å and 44%, respectively (Figure 4c and Figure 5c). It was demonstrated that intermolecular π···π stacking interactions can induce the emission red shift and the decrease of Φf.21-24,38-42 This conclusion explains the longer-wavelength emission and lower Φf of crystals B and C compared with crystal A. We note that the ground-state energies for all of the molecules with different conformations are very similar (Figures S4-S6), so they may form simultaneously within the same flask when the quick crystallization process is employed.15,43 In addition, the superimposed emission spectrum of crystals A, B, and C shown in Figure S7 is similar to that of the crystalline powder. Thus, the white-emissive powder should be formed by the simultaneous crystallization of phases A, B, and C with a certain ratio. The superimposed emission spectrum is not identical to that of the crystalline powder suggesting that in the crystalline powder the fraction of crystal A, B, and C is not equal. The PL quantum yield (40%) of crystalline powder is lower than that of each crystal A (58%), B (49%), and C (44%). These results may be attributed to that the powder, which was prepared by the quick solvent evaporation of 3-DPH-XO solution, was not a high quality crystalline sample that often displayed lower PL quantum yield. The


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comparison between the powder X-ray diffraction (PXRD) pattern of 3-DPH-XO crystalline powder and the simulated patterns from the single-crystal diffraction data of phases A, B, and C (Figure S8) also suggests that the white-emissive powder of 3-DPH-XO should be composed of the three polymorphs. Apparently, the three polymorphs with different emission characteristics coexist, leading to the formation of white-light organic solid.

Figure 4. Molecular packing modes of 3-DPH-XO in the single crystals of A (a), B (b), and C (c).


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Figure 5. Intermolecular non-covalent interactions in crystals A (a), B (b), and C (c). The theoretical calculations demonstrated that the excited state of 3-DPH-XO molecule exhibits much more twisted configuration compared with its ground state (Figure S9). Therefore, 3-DPH-XO molecule has twisted intramolecular charge transfer (TICT) feature. Since the ICT state is accompanied by intramolecular rotation between the donor and acceptor units, the restriction of intramolecular rotation is an effective way to suppress the transformation from the locally excited state to the ICT state.44,45 At 77 K, the solution of 3-DPH-XO in low-polarity toluene was frozen into a glassy state with high rigidity, and the intramolecular rotation of 3DPH-XO was efficiently inhibited. Therefore, the resulting emission spectrum (Figure S10) was assigned to the emission from the locally excited state.31,45 Considering the close peak wavelengths of the emission spectra of the glassy toluene solution (434 nm) and crystal A (441 nm), the emission of crystal A should also originate from the locally excited state. In contrast to crystal A, crystals B and C exhibit ICT feature. For crystal B, the emission band with the maximum at around 440 nm should result from the locally excited state emission, which is


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similar to the case of crystal A. As the emission band with the maximum at around 525 nm is structureless and broad (FWHM: 102 nm), it is ascribed to the ICT emission. Obviously, crystal C displays only the ICT emission with the maximum at around 540 nm and large FWHM of 105 nm. For the crystals with π···π stacking interactions, excimeric states might be induced and thus crystals B and C exhibit longer lifetimes for the prompt fluorescence compared with crystal A (Figure S11a-c). The particular dual emission of crystal B from both the locally and ICT excited states suggests that crystal B can be regarded as the intermediate between crystals A and C. For the crystalline powder of 3-DPH-XO, the transient decay spectra were recorded at the three different emission maxima of 441, 528 and 549 nm (Figure S11d). As expected, the prompt fluorescence lifetimes corresponding to these wavelengths were different, being 2.29, 7.09 and 8.70 ns, respectively. Notably, these lifetimes were respectively close to those of crystalline phases A, B, and C measured with the emission maximum as the monitored wavelength. These experimental results demonstrate that the white-light emission of 3-DPH-XO powder is attributed to the spontaneous formation of the mixture of polymorphs. Crystal A displayed only one kind of prompt fluorescence with a short lifetime of 2.48 ns (Figure S11a). However, detailed studies of transient emission spectra demonstrated that crystals B and C showed long-lived delayed fluorescence (τ: 174 µs for B, and 221 µs for C) (Figure S11b and S11c), suggesting that the two crystals may have the TADF feature. The temperaturedependent transient PL measurements revealed that the delayed component was improved (Figure S12) upon increasing the temperature, which confirmed the typical TADF behavior of crystals B and C.46-48 Crystal A didn’t show delayed fluorescence behavior. The energy gap (∆Est) between the singlet (S1) and triplet (T1) states, calculated from the onsets of the fluorescence and phosphorescence spectra recorded at 77 K (Figure S13), are 0.38, 0.08, and 0.02 eV for A, B,


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and C, respectively. A possible explanation for the different photoluminescent properties is that in crystal A, as the lack of effective intermolecular interactions for restricting molecular vibrations, the triplet excited state of molecules may easily undergo non-radiative decay via vibrational relaxation, leading to the disappearance of TADF.49 For crystals B and C, the existence of intermolecular π···π stacking (acceptor···acceptor) (Figure 5b and c) interactions and various hydrogen bonds (such as C−H···O and O=C···H−C) (donor···acceptor) could efficiently suppress the vibrational relaxation and thus preserve the triplet exited states at room temperature. Recently, our studies demonstrated that, for some crystals based on the D-A type of organic molecules, appropriate intermolecular donor···donor or acceptor···acceptor interactions facilitate the TADF process.23 It is worth to note that the solid thin film of a 3-DPH-XO derivative displays white light emission and can be employed to fabricate white light OLEDs, which will be reported elsewhere. In conclusion, an organic molecule 3-DPH-XO with polymorphism and TADF properties was designed and synthesized. The 3-DPH-XO crystalline powder displayed bright white-light emission.

Detailed

studies

on

polymorph

preparation,

single-crystal

structures

and

photoluminescence properties demonstrated that the white-light crystalline powder was composed of 3-DPH-XO-based polymorphs with different emission colors. Three kinds of 3DPH-XO-based single crystals with different supramolecular structures and emission properties were achieved. These polymorphs exhibit distinguished intermolecular non-covalent interaction (such as π···π stacking, hydrogen bonding and C−H···π interactions) features, which determine the emission characteristics. Crystal A without intermolecular π···π interactions exhibited only locally exited state emission. Crystal B with weak intermolecular π···π interactions displayed both locally and ICT exited state emissions. Crystal C with strong intermolecular π···π


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interactions showed only ICT exited state emission. Polymorphs B and C with the acceptor···acceptor stacking feature showed the TADF characteristics, indicating that appropriate non-covalent interactions could enhance the reverse intersystem crossing (RISC) process and thus lead to delayed fluorescence. Therefore, in this study, we developed an efficient and simple approach to construct white-light organic solids with TADF property. ASSOCIATED CONTENT Supporting Information Available: Additional details of the synthesis, supplementary figures and tables, PDF and CIF formats of single-crystal data and other information.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Y. W.) *E-mail: [email protected] (K. Y.) Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (91333201) and the National Basic Research Program of China (2015CB655000). REFERENCES (1) Kido, J.; Kimura, M.; Nagai, K. Multilayer White Light-Emitting Organic Electroluminescent Device. Science 1995, 267, 1332–1334.


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