Conformational Effect of Polymorphic Terfluorene on Photophysics

In general, when the efficient light-emitting π-conjugated molecules ... CDCl3) δ 7.85 (d, J = 8.0 Hz, 2H), 7.82 (d, J = 8.0 Hz, 2H), 7.78 (d, J = 7...
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Conformational Effect of Polymorphic Terfluorene on Photophysics, Crystal Morphologies, and Lasing Behaviors Chang-Jin Ou,†,‡,∥ Xue-Hua Ding,†,∥ Yin-Xiang Li,‡ Can Zhu,§ Meng-Na Yu,‡ Ling-Hai Xie,*,‡ Jin-Yi Lin,†,‡ Chun-Xiang Xu,*,§ and Wei Huang*,†,‡ †

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University, Nanjing 211816, China ‡ Center for Molecular Systems and Organic Devices (CMSOD), Key Laboratory for Organic Electronics & Information Displays (KLOEID) and Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, Nanjing 210046, China § State Key Laboratory of Bioelectronics, School of Electronic Science and Medical Engineering, Southeast University, Nanjing 210096, China S Supporting Information *

ABSTRACT: Molecular conformation is an important factor in flexible organic molecules and deeply influences their physical and chemical properties. Here, a model fluorene trimer, 2,2′:7′,2″-ter(9,9dimethylfluorene) (TDMeF), was designed and synthesized to investigate the effect of conformational diversity of oligofluorene on the morphologies of microcrystals and photophysical properties. Single crystal X-ray diffraction analysis indicates that TDMeF has four polymorphs with different molecular conformations, and crystalline polymorphism is first observed in oligofluorenes. Moreover, the slight change in molecular conformation leads to form different crystal morphologies, namely, ribbon and rodlike microcrystals, in virtue of different intermolecular interactions. Finally, although both microcrystals display deep blue lasing behaviors, the rodlike microcrystal shows a threshold of 114 W/cm2, which is twice times lower than that of the ribbon-like one due to the effect of molecular orientation and optical microcavity.

1. INTRODUCTION The excitonic and electronic properties of organic/polymer semiconductors are collectively determined by the four interrelated elements, namely, electronic structure of conjugated backbone, steric strain and hindrance, conformational topology, as well as supramolecular interaction.1−6 Due to weak intermolecular interaction, π-conjugated organic molecules/ polymers can usually self-organize into at least two polymorphs in the solid state.7,8 More importantly, due to different packing motifs, and/or molecular conformations, the polymorphs exhibit different physical and chemical properties, which provide an opportunity to deeply investigate the effects of solid-state packing on the optical and electronic properties, and hence have received much attention in the exploration of optical, electronic, and smart materials.9−11 3(5)-(9-Anthryl)pyrazole (NAP) and 2,7-diphenylfluorenone derivatives showed polymorphism-dependent luminescent properties and stimuli response since the arrangement of chromophores varied along with the change of hydrogen bonding and π−π stacking interactions.12,13 Commonly, the polymorphism can be divided into packing polymorph and conformation polymorph, in which the conformationally flexible organic molecules/poly© XXXX American Chemical Society

mers have more chances to display conformational polymorphs.14 Furthermore, conformational change will redistribute the π-electrons of the conjugated backbone and modify the molecular arrangement, which modulates frontier molecular orbitals and electronic overlap, as well as optical and electronic properties. In comparison with the twisted conjugated backbone, the small molecules or polymers with planar conformation exhibited higher carrier mobility and power conversion efficiency.15−20 The conformational change from disorder to π-stacked conformation of poly(N-vinylcarbazole) and its derivative resulted in dramatic enhancement in electrical conductivity.21,22 The pure conformational alteration of dibenzofulvene derivatives made the emission color shift from blue to green and displayed outstanding optical waveguide behaviors.23 Thus, molecular conformation plays an important role in tuning packing behaviors, optical and electronic properties of functional materials. Received: April 9, 2017 Revised: June 8, 2017 Published: June 13, 2017 A

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Figure 1. Synthetic route and assigned 1H spectrum of TDMeF.

In general, when the efficient light-emitting π-conjugated molecules self-assemble into stable nano/microcrystals with regular morphology via supramolecular interactions, the crystals may simultaneously serve as gain media and optical resonator in organic lasers.24−28 Therefore, crystal polymorphism has an inevitable influence not only on optical microcavities but also on lasing behaviors, such as wavelength, threshold, and quality factor. First of all, the microcrystal shapes determining the optical microcavity of lasers is Fabry-Pérot (FP) or whisperinggallery-mode (WGM) resonators.28,29 Moreover, the wavelength-controlled organic microlasers or multicolor amplified spontaneous emission (ASE) can be successfully demonstrated via crystal polymorphism-dependent intramolecular chargetransfer or excited-state intramolecular proton transfer process.30−32 Besides, Wang et al. found that 4,4′-(thiazolo[5,4-d]-thiazole-2,5-diyl)bis(N,N-diphenylaniline) had four crystal forms and displayed conformation- and packingdependent luminescent properties and multicolor emission, while only needle crystals showed a low ASE threshold.33 The results indicate that molecular conformation and packing modes significantly influence stimulated emission properties of crystals, while it still lacks molecular models to investigate the effect of conformational polymorphs on microlasers to date. Oligo/polyfluorenes as typical conformationally flexible systems have displayed conformational diversity and rich polymorphs in films, such as amorphous phase, liquid crystal phase, β-phase, and crystalline phase, and exhibited conformation-dependent optoelectronic properties.34−39 First, a small amount of conformational planarization in conjugated backbones of polyfluorenes gives an intriguing β-phase, which can improve spectral and thermal stability, hole mobility, and luminescent efficiency.40,41 In addition, β-phase domains are excellent energy acceptors, and laser modes can be modulated by controlling the β-phase content.39,42,43 Second, conformational bending in tetrafluorene results in green band emission, which can partially explain the origin of low energy emission in

polyfluorenes.44 Third, the spectral shift in some fluorene-based oligomers and dendrimers may also be related to the change of dihedral angles.45−48 Herein, a model compound, 2,2′:7′,2″ter(9,9′-dimethylfluorene) (TDMeF), was synthesized to deeply investigate the effect of conformational diversity on the microlaser behaviors. We observed crystal polymorphs with different molecular conformations in oligofluorenes and correlated the relationship of their conformation and photoluminescent properties. Moreover, conformation change led to form different microcrystal morphologies, and the as-prepared microcrystals showed dramatic differences in thresholds of microlasers due to the effect of molecular orientation and optical microcavity.

2. EXPERIMENTAL SECTION All of the chemicals were purchased from J&K Scientific Co. Ltd. and were used without further purification unless otherwise state. The synthesis of 2-(9,9-dimethyl-9H-fluoren2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane was according to the reported literature.49 2.1. Synthesis. 2,2′:7′,2″-Ter(9,9′-dimethylfluorene) (TDMeF). Under an inert atmosphere, a mixture of 2,7dibromo-9,9′-dimethylfluorene (0.70 g, 2.0 mmol), 2-(9,9dimethyl-9H-fluoren-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (0.96 g, 3.0 mmol), Pd(PPh3)4 (0.115 g, 0.1 mmol), 2 M K2CO3 solution (3.0 mL), and toluene/THF (20 mL, 1:1 v/v) was stirred and heated 90 °C for 12 h. The mixture was extracted with dichloromethane, and then the combined extracts were dried with anhydrous MgSO4. The solvent was evaporated under vacuum condition. The crude product was purified by silica gel column chromatography to afford a white solid (0.93 g) with 80.2% yield. mp: 293 °C; 1H NMR (400 MHz, CDCl3) δ 7.85 (d, J = 8.0 Hz, 2H), 7.82 (d, J = 8.0 Hz, 2H), 7.78 (d, J = 7.2 Hz, 2H), 7.73 (s, 2H), 7.72 (s, 2H), 7.67 (dd, J = 8.0, 1.2 Hz, 4H), 7.48 (d, J = 7.2 Hz, 2H), 7.39−7.32 (m, 4H), 1.66 (s, 6H), 1.59 (s, 12H). 13C NMR (100 MHz, B

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The Journal of Physical Chemistry C CDCl3) δ 154.63, 154.33, 153.93, 140.86, 140.84, 138.92, 138.42, 138.09, 127.24, 127.04, 126.40, 126.29, 122.62, 121.43, 121.41, 120.35, 120.29, 120.06, 47.15, 47.00, 27.41, 27.29. HRMS: m/z 579.3032 [M + H]+ (calculated for C45H38 578.30). 2.2. Characterization. 1H NMR and 13C NMR spectra were measured on a Varian Mercury Plus 400 spectrometer with tetramethylsilane as the internal standard. Absorption and photoluminescence (PL) emission spectra were measured in toluene solution using a PerkinElmer Lambda 35 spectrophotometer and a PerkinElmer LS55 spectrophotometer, respectively. The thin films were prepared by spin-coating on a quartz plate at a spin-coating speed of 1200 rpm from 8 mg/mL in toluene solution. Thermogravimetric analysis (TGA) was undertaken with a Shimadzu thermogravimetry and differential thermal analysis DTG-60H at a heating rate of 10 °C/min under N2. Differential scanning calorimetry (DSC) measurements were performed under a N2 atmosphere at both heating and cooling rates of 10 °C/min, using a Shimadzu DSC-60A. The absolute fluorescence quantum efficiency of microcrystals was measured by an Edinburgh Instruments FLS-920 in integrating sphere. The single crystal data collection was performed at 100 or 295 K on a Bruker 2000 CCD area detector using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). All structures were solved by direct methods using SHELXS-2014 and refined against F2 using SHELXL-2014. Hydrogen atoms were fixed geometrically and refined isotropically. 2.3. Microlaser Characterization. The lasing spectra of the microcrystals were excited by a focused 325 nm laser through a confocal μ-PL system (OLYMPUS BX53). The size of the light spot was about 20 μm in diameter. The luminescent light was collected through an optical multichannel analyzer (Princeton, Acton SP2500i), and the spectral resolution of the spectrometer is 0.167 nm. All measurements were performed at ambient atmosphere.

Figure 2. Photo images of the crystal forms of TDMeF. Crystal forms A and D were achieved from dichloromethane/ethanol and chloroform/hexane systems, respectively. Crystal forms B and C were simultaneously obtained from tetrahydrofuran/isopropanol system. (a) Crystal structure, torsion angle, and packing motif of crystal form A, view from c-axis. (b) Crystal structure, torsion angle, and packing motif of crystal form B, view from b-axis.

fluorenes are 40.1°. Furthermore, TDMeF exhibits slip-stacked packing and forms a 1D molecular chain along the crystallographic a-axis through multiple same C-H···π interactions (2.562 Å). Then, such chains assemble into a molecular layer along the c-axis via weak C-H···C short contacts (2.879 Å), and the molecular layers are packing in a layer-by-layer fashion along the b-axis due to weak van de Waals (VDW) forces (Figure S6). Although TDMeF exhibits H-like aggregate behaviors, the large torsion conformation and steric hindrance of methyl groups impede face-to-face packing, which weakens intermolecular electronic coupling and can keep deep blue emission with high fluorescent efficiency. Needle crystal B belongs to the monoclinic system with a space group of P21/n, and with Z = 4 and Z′ = 1, respectively. Interestingly, as shown in Figure 2b, the molecular conformation of B is slightly different from A, and the both torsion angles of B (37.3° and 39.7°) are smaller than that of A. B has similar packing motifs with A, forming a 1D molecular chain along the crystallographic a-axis via multiple C-H···π interactions (2.629 and 2.630 Å) in B, but the distance of CH···π interactions becomes weaker. On account of asymmetric intermolecular interaction, the molecular chain of B may first organize into a double chain structure through multiple C-H··· C short contacts (2.855 Å), and then the double chains may further form the 3D structure via VDW forces (Figure S7). In hexagonal form C, TDMeF crystallizes in the monoclinic system with a space group of C2/c. There are eight TDMeF molecules and four tetrahydrofuran molecules in the asymmetric unit, in which two different conformational TDMeF molecules are involved. Different from A and B, the both molecules in C adopt cis−trans conformations with torsion angles of −154.9° and −29.0°, and 149.3° and 19.5°, respectively (Figure S8). Accordingly, the molecules in C have more planar conformations than that in A and B. As shown in Figure S9, TDMeF in C adopts a face-to-edge (Tshape) packing motif, which results in a small overlap of πsystems between adjacent molecules, so the photophysical properties were determined by the intramolecular effective conjugation. Finally, slow diffusion of hexane into chloroform solution gave platelet form D. The single crystal XRD study

3. RESULTS AND DISCUSSION 3.1. Synthesis and Thermal Behaviors. The synthetic route of TDMeF is shown in Figure 1. TDMeF was synthesized in 80.2% yield via coupling of 2-(9,9′-dimethylfluoren-2-yl)4,4,5,5-tetramethyl-1,3,2-dioxaborolane with 2,7-dibromo-9,9′dimethylfluorene. As illustrated in Figure 1, all the hydrogens of TDMeF could be assigned clearly, which confirmed the chemical structure. TDMeF displayed a high thermal decomposition temperature (Td) of 386 °C (Figure S4). In addition, TDMeF showed a melting point (Tm) and cold crystallization at 293 and 214 °C (Figure S5), indicating that TDMeF has high thermal stability. 3.2. Crystal Structures. To investigate molecular conformation and packing behaviors of TDMeF, we tried to cultivate single crystals via solution diffusion. Single crystals of TDMeF were obtained by diffusing poor solvents into good solvents (for details, see in Figure 2), and performed by X-ray diffraction. TDMeF has at least four crystal forms, A, B, C, and D (CCDC Nos. 1523096, 1534753, 1523109, and 1523095), which were achieved from different solvent systems. The crystal form A belongs to the orthorhombic system with a space group of Pbcn, and four molecules in the unit cell (Z = 4) and one-half a molecule in the asymmetric unit (Z′ = 0.5), respectively. The crystal structure of A is shown in Figure 2a, in which, unexpectedly, TDMeF adopts a cis−cis conformation and bent structure, and all the methyl groups locate on the same side of backbone. The both torsion angles between the adjacent C

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Figure 3. (a) Fluorescence microscope image of ribbon microcrystals (form A) excited with UV band using mercury lamp as source. The scale bar is 100 μm. (b) SEM image of ribbon microcrystals; the scale bar is 100 μm. (c) XRD of ribbon microcrystal; the simulated XRD was calculated from single crystal data. (d) Fluorescence microscope image of rod microcrystals (form B) excited with UV band using mercury lamp as source. The scale bar is 100 μm. (e) SEM image of rod microcrystals; the scale bar is 50 μm. (f) XRD of microcrystal of form B; the simulated XRD was calculated from single crystal data.

lowest growth rate, resulting in the increased exposure of (020) facets.29 From another point of view, the molecules form layerby-layer packing along the b-axis direction via weak VDW force, which is weaker than the C-H···π and C-H···C interactions of aand c-axis directions, respectively. Theoretical simulation also indicates that A forms a ribbon-like crystal (Figure S12b), which is matching well with the data of XRD. With the combination of XRD data and supramolecular analysis, we can further confirm that ⟨100⟩ is the preferential growth direction, which makes A forms 1D ribbon-like microcrystals. TDMeF easily formed plate microcrystals (microcrystal of C) with uninform size and morphology at room temperature in tetrahydrofuran/isopropanol solution, and when the temperature was increased to 65 °C, the plate microcrystals could completely change into rodlike microcrystals with two sharp ends (microcrystal of B) (Figure 3d,e). Different from the ribbon-like microcrystal of A, the rod microcrystal of B has four strong diffraction peaks at 7.6, 10.9, 15.1, and 16.1°, corresponding to (002), (020), (004), and (014) crystal planes, respectively. A set of weak diffraction peaks indicate that more crystal planes are exposed. As listed in Table S2, the crystal facet of {002} has the maximum Eatt of −55.47 kcal/mol, and the energies lie in the same order of magnitude among crystal facets of {011} {012} {013} {020}, so B forms rod microcrystals, as indicated in Figures 3e and S12d. As shown in supramolecular analysis of B, except for the a-axis direction, the self-assembly driving forces of the other directions are weak VDW forces, and ⟨100⟩ is the preferential growth direction. Above all, the small conformational change between A and B leads to the slight difference in intermolecular interactions and molecular packing, and further results in the apparent difference in microcrystal morphologies, which will significantly influence the lasing behaviors. 3.4. Photophysical Properties. As shown in Figure 4, in solution and film, TDMeF shows only one unstructured main absorption band with the maximum absorption peak of 349 and 348 nm, respectively, originating from π−π* transition of the

indicates that platelet D has similar crystal parameters and molecular conformation with C (Figures S10 and S11, Table S1). The major difference is that they contain different solvents in the unit cell (chloroform in D), which leads to slightly modify the torsional angles (the change less than 3°), yet does not change the molecular packing motif. In all, in consequence of two changeable torsion angles and low intramolecular strained energy, TDMeF has four crystal forms with different conformations, and crystalline polymorphism is first observed in oligofluorene systems. Conformational change between A and B results in a slight change in molecular packing and intermolecular interaction, and they provide us a platform to investigate the effect of molecular conformation and packing motif on the crystal morphology, optical and electronic properties of fluorene-based semiconductors. 3.3. Influences of Conformational Change on Microcrystals. To study the effect of conformational change on the microcrystal morphology, the microcrystals of A and B were prepared by a reprecipitation method. TDMeF first selfassembled into rhomboid microplates from dichloromethane/ ethanol, while the microplates were unstable in air, maybe containing dichloromethane solvents, and could spontaneously transform into thermal stable 1D ribbon-like microcrystals (microcrystals of A) in solution at room temperature (Figure 3a,b). XRD study of the ribbon microcrystals showed two strong X-ray diffraction peaks at 7.6° and 15.2°, corresponding to the crystal plane of (020) and (040) of A, and three weak diffraction peaks at 9.4, 16.2, and 18.7°, corresponding to crystal planes of (021), (041), and (042), respectively (Figure 3c). Those results indicate that the ribbon microcrystal adopts a lamellar structure with the crystal plane of (020) parallel to the substrate. The attachment energies (Eatt) of crystal facets of {020}, {021}, {022}, {023}, and {002} are −18.08, −78.86, −85.95, −124.93, and −128.93 kcal/mol, respectively (Table S2). Since the Eatt of {020} is maximum, and the growth rate is inversely proportional to Eatt, the crystal facet {020} has the D

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units.47,48 In our case, the similar absorption spectra in solution and film indicate that TDMeF has a largely twisted conformation in the ground state of solution and film, and the theoretical calculation shows that the minimal energy of the ground state is at torsion angles ±40° and ±140°.50 Since TDMeF also has large torsion angles in A and B, which are close to the theoretical result, we can exclude the contribution of the change of torsion angles to the blue shift in PL spectra. Therefore, we deduce that the difference in emission spectra between amorphous film and A or B is mainly derived from the difference in excited state properties. The tightly packing and stronger supramolecular interaction in a crystal restrict intramolecular rotation, and prevent the excited-state conformational planarization, which may suppress Kasha’s rule and undergo radiative transition from a higher excited state than S1. While amorphous film has lower packing density and loose molecular packing,51 TDMeF experiences excited-state conformational planarization, and finally adopts a planar conformation in the excited state, which was demonstrated in solution and the solid state of oligofluorenes.52,53 As a result, although they have similar torsion angles in film and crystal, the emission peak of amorphous film is apparently bathochromic. Moreover, the film and crystals also show dramatic differences in fluorescent quantum efficiency (Φ) and lifetime. The Φ of spin-coated film is only 18%, while the microcrystals have high Φ of 83% and 80% for A and B, respectively. As described in Figure S13, the PL decays of film, A, and B indicate biexponential decay with an averaged lifetime (τ) of 0.47, 0.90, and 0.81 ns, yielding a radiative rate of Kr = Φ/τ = 0.38, 0.92, and 0.99 ns−1, respectively. Due to the regular crystal morphology, and high Φ and Kr, the microcrystals of A and B can serve as gain media for microlasers. 3.5. Microlasing Properties. To further investigate conformation change on optical properties, lasing behaviors of microcrystals were measured. Lasing oscillation was observed in the individual ribbon and rod microcrystals. Figure 5a,d shows the PL spectra of the ribbon and rod microcrystals as a

Figure 4. (a) Absorption and emission spectra of TDMeF in dichloromethane solution with a concentration of 10−5 mol/L; the film was prepared via spin-coating from toluene solution with a concentration of 8 mg/mL onto quartz. (b) Emission spectra of TDMeF crystal polymorphs. All the emission spectra were excited by the excitation wavelength of 350 nm.

conjugated terfluorene backbone. However, the emission spectra are well-structured with three apparent emission bands at 395, 416, and 440 nm for TDMeF in solution, corresponding to 0-0, 0-1, and 0-2 vibronic progressions. Compared with solution, the PL spectrum of the spin-coating film shows a spectral red shift and the maximum emission peak is bathochromic to 426 nm (0-1 transition) due to the change of dielectric constant. Furthermore, TDMeF exhibits conformation-dependent PL properties in single crystals. The forms A and B nearly have the same PL spectra with the maximum emission peak at 413 nm, corresponding to the 0-1 transition of crystals, indicating that a small difference in conformation has little influence on PL spectra. Since forms C and D have a more planar conformation (Figures S8 and S10), the 0-1 transition emission peaks of C and D are bathochromic to around 426 and 429 nm, respectively. In comparison with solution and amorphous film, there are 3 and 13 nm blue shifts in the 0-1 transition emission peak of A and B. This blue shift was also observed in pentafluorene and fluorene dendrimers, and the authors speculated that the blue spectral shift in emission of annealed film may might originate from the increase of the dihedral angles between adjacent fluorene

Figure 5. PL spectra of ribbon microcrystal (a) and rod microcrystal (d) of TDMeF under different pump fluence excitation at 325 nm UV laser. (b) Integrated intensity of 0-1 peaks of ribbon crystal (b) and rod microcrystal (e) as a function of pump fluence. (c) Schematic diagram to elucidate the influence of molecular orientation on the formation of the FP cavity between two lateral faces of ribbon microcrystal (c) and rod microcrystal (f). E

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The Journal of Physical Chemistry C function of pump fluence. At low pump fluence excitation, both microcrystals display a broad spontaneous emission. When the pump fluence exceeds the threshold, a set of sharp lasing peaks appear on the top of the 0-1 transition with line width Δλ = 0.52 and 0.36 nm, and centered on 409 and 412 nm for ribbon and rod microcrystals, respectively. Therefore, the cavity quality factor (Q = λ/Δλ) can be estimated to be as high as ∼786 and 1144 for ribbon and rod microcrystals, respectively. Figure 5b,e displays the curves of the integrated intensity of the 0-1 transition peak versus pump fluence. It clearly shows that the laser thresholds are 296 and 114 W/cm2 for ribbon and rod microcrystals, respectively. Since both microcrystals belong to 1D microstructures with two flat facets (Figure S12), the ribbon and rod microcrystals are Fabry-Pérot (FP) optical microcavities.9 According to the crystal morphology of the microcrystal, it is the two lateral facets that function as reflectors in both microcrystals (Figure 5c,f). Since the transition dipole moment (TDM) is along the long axis of the molecule, the input laser beam is perpendicular to the TDM in the ribbon microcrystal, and yet inclined to the TDM with a certain degree in rod crystals. Hence, rod microcrystals may have higher absorption efficiency, which leads to lower lasing threshold. Consequently, the conformation change affects lasing behaviors by the way of supramolecular interaction tuning the microcrystal morphology and molecular orientation.

Author Contributions

4. CONCLUSION In conclusion, a flexible TDMeF displays conformational polymorphs with four crystal forms, and conformationdependent emission properties. More importantly, due to slight differences in molecular conformation and intermolecular interaction, the A and B exhibit significant differences in microcrystal morphologies, ribbon and rodlike microcrystals for A and B, respectively. As a result, they show an apparent difference in microlasing properties in virtue of the effect of molecular orientation and optical microcavity, in which the rod microcrystal has a low lasing threshold of 114 W/cm2 and high Q of 1144. Molecular conformation has an important influence on microcrystal lasers, and should receive much attention in material design.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The project was supported by the National Natural Science Foundation of China (U1301243, 21504041), the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, PAPD (YX03002), the Excellent Science and Technology Innovation Team of Jiangsu Higher Education Institutions (2013), the Six Peak Talents Foundation of Jiangsu Province (XCL-CXTD-009), the Natural Science Foundation of Jiangsu Province (BM2012010), Synergetic Innovation Center for Organic Electronics and Information Displays, the Doctoral Fund of Ministry of Education of China (No. 20133223110007), National Synergistic Innovation Center for Advanced Materials (SICAM), the Open Project from State Key Laboratory of Supramolecular Structure and Materials at Jilin University (No. sklssm201710), the Natural Science of the Education Committee of Jiangsu Province (15KJB430019), the China Postdoctoral Science Foundation (2015M580419), Jiangsu Planned Projects for Postdoctoral Research Funds (1501019B, 1601042B). REFERENCES

(1) Xie, L.-H.; Yin, C.-R.; Lai, W.-Y.; Fan, Q.-L.; Huang, W. Polyfluorene-Based Semiconductors Combined with Various Periodic Table Elements for Organic Electronics. Prog. Polym. Sci. 2012, 37, 1192−1264. (2) Gu, P.-Y.; Gao, J.; Lu, C.-J.; Chen, W.; Wang, C.; Li, G.; Zhou, F.; Xu, Q.-F.; Lu, J.-M.; Zhang, Q. Synthesis of Tetranitro-Oxacalix[4]arene with Oligoheteroacene Groups and Its Nonvolatile Ternary Memory Performance. Mater. Horiz. 2014, 1, 446−451. (3) Li, J.; Li, P.; Wu, J.; Gao, J.; Xiong, W.-W.; Zhang, G.; Zhao, Y.; Zhang, Q. [4 + 2] Cycloaddition Reaction to Approach Diazatwistpentacenes: Synthesis, Structures, Physical Properties, and Self-Assembly. J. Org. Chem. 2014, 79, 4438−4445. (4) Gu, P.-Y.; Zhao, Y.; He, J.-H.; Zhang, J.; Wang, C.; Xu, Q.-F.; Lu, J.-M.; Sun, X. W.; Zhang, Q. Synthesis, Physical Properties, and LightEmitting Diode Performance of Phenazine-Based Derivatives with Three, Five, and Nine Fused Six-Membered Rings. J. Org. Chem. 2015, 80, 3030−3035. (5) Ou, C.-J.; Ren, B.-Y.; Li, J.-W.; Lin, D.-Q.; Zhong, C.; Xie, L.-H.; Zhao, J.-F.; Mi, B.-X.; Cao, H.-T.; Huang, W. Excimer-Based White Electroluminescence from Supramolecular Bulk Effects of DumbbellShaped Molecules Via Attractor-Repulsor Molecular Design. Org. Electron. 2017, 43, 87−95. (6) Yu, M.-N.; Ou, C.-J.; Liu, B.; Lin, D.-Q.; Liu, Y.-Y.; Xue, W.; Lin, Z.-Q.; Lin, J.-Y.; Qian, Y.; Wang, S.-S.; Cao, H.-T.; Bian, L.-Y.; Xie, L.H.; Huang, W. Progress in Fluorene-based Wide-bandgap Steric Semiconductors. Chin. J. Polym. Sci. 2017, 35, 155−170. (7) Chung, H.; Diao, Y. Polymorphism as an Emerging Design Strategy for High Performance Organic Electronics. J. Mater. Chem. C 2016, 4, 3915−3933. (8) Jones, A. O. F.; Chattopadhyay, B.; Geerts, Y. H.; Resel, R. Substrate-Induced and Thin-Film Phases: Polymorphism of Organic Materials on Surfaces. Adv. Funct. Mater. 2016, 26, 2233−2255. (9) Varghese, S.; Das, S. Role of Molecular Packing in Determining Solid-State Optical Properties of Π-Conjugated Materials. J. Phys. Chem. Lett. 2011, 2, 863−873. (10) Yang, B.; Xiao, J.; Wong, J. I.; Guo, J.; Wu, Y.; Ong, L.; Lao, L. L.; Boey, F.; Zhang, H.; Yang, H. Y.; Zhang, Q. Shape-Controlled Micro/Nanostructures of 9,10-Diphenylanthracene (DPA) and Their

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b03366. NMR spectra, TG−DSC data, LR-MS, supramolecular analysis, PL decay curves of film and crystals (PDF) X-ray crystallographic data for form A (CIF) X-ray crystallographic data for form B (CIF) X-ray crystallographic data for form C (CIF) X-ray crystallographic data for form D (CIF)

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C.-J. Ou and X.-H. Ding contributed equally to this work.

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Corresponding Authors

*Fax: +86 25 8586 6999/8349 2333. Tel: +86 25 8588 2333/ 8586 6008. E-mail: [email protected] (L.-H.X.). *E-mail: [email protected] (W.H.). *E-mail: [email protected] (C.-X.X.). ORCID

Chun-Xiang Xu: 0000-0001-8116-2869 Wei Huang: 0000-0001-7004-6408 F

DOI: 10.1021/acs.jpcc.7b03366 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.7b03366 J. Phys. Chem. C XXXX, XXX, XXX−XXX