Adjusting emission behaviors of molecules based 2,1,3-benzothiadiazole by intermolecular interaction and mechano-responsive luminescence Enhanced Publication

Adjusting emission behaviors of molecules based 2,1,3-benzothiadiazole by intermolecular interaction and mechano-responsive luminescence 分子间相互作用调节苯并噻二唑基分子的发光，以及刺激响应特性 1 2 first-author 董书凡(1999-)，男，江西景德镇人，硕士研究生，2016年于太原理工大学获得学士学位，主要从事有机发光小分子与电致发光二极管制备的研究。 http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029460&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029462&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029461&type= 24.57342529 35.68777466 董书凡 (1999-)，男，江西景德镇人，硕士研究生，2016年于太原理工大学获得学士学位，主要从事有机发光小分子与电致发光二极管制备的研究。 1 2 1 2 1 2 1 2 corresp corresp E-mail： xuhuixia@tyut.edu.cn E-mail： xuhuixia@tyut.edu.cn xuhuixia@tyut.edu.cn xuhuixia@tyut.edu.cn 许慧侠(1981-)，女，河南长垣人，博士，副教授，研究生导师，2010年于太原理工大学获得博士学位，主要从事有机光电材料与器件的研究. http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029478&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029480&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029479&type= 26.05773926 34.57034302 许慧侠 (1981-)，女，河南长垣人，博士，副教授，研究生导师，2010年于太原理工大学获得博士学位，主要从事有机光电材料与器件的研究. xuhuixia@tyut.edu.cn 1 2 1 2 1 2 太原理工大学 新材料界面科学与工程教育部重点实验室， 太原 山西　030024 太原理工大学 新材料界面科学与工程教育部重点实验室， 太原 山西　030024 Key Laboratory of Interface Science and Engineering in Advanced Materials， Ministry of Education， Taiyuan University of Technology， Taiyuan 030024， China Key Laboratory of Interface Science and Engineering in Advanced Materials， Ministry of Education， Taiyuan University of Technology， Taiyuan 030024， China 山西大同大学 微结构功能材料固体物理研究所山西省重点实验室， 大同 山西　037009 山西大同大学 微结构功能材料固体物理研究所山西省重点实验室， 大同 山西　037009 Shanxi Province Key Laboratory of Microstructure Functional Materials Institute of Solid-State Physical， School of Physics and Electronic Science， Shanxi Datong University， Datong， 037009， China Shanxi Province Key Laboratory of Microstructure Functional Materials Institute of Solid-State Physical， School of Physics and Electronic Science， Shanxi Datong University， Datong， 037009， China It is a feasible way to adjust emission behaviors by molecular conformation and intermolecular interactions. Herein， two multifunctional materials， 4，4'-（benzo［c］［1，2，5］thiadiazole-4，7-diyl）bis（N，N-diphenylaniline） and 4-（7-（9，9-dimethylacridin-10（9H）-yl）benzo［c］［1，2，5］thiadiazol-4-yl）-N，N-diphenylaniline， namely TBT and DBT ， with donor-acceptor-donor （D-A-D） and D-A-D’ structures composed of 2，1，3-benzothiadiazole （BT） acceptor and the different donors of triphenylamine （TPA） and 9，9-dimethyl-9，10-dihydroacridine （DMAC） were synthesized. These materials exhibit not only polymorphism-dependent emission but also multicolor luminescence switch in response to the external stimulus. The high-contrast crystal-dependent emission behaviors of TBT （ TBT-O ： λ PL =593 nm and TBT-R ： λ PL =616 nm） and DBT （ DBT-Y ： λ PL =570 nm， DBT-O ： λ PL =605 nm and DBT-R ： λ PL =642 nm） were observed. TBT with double TPA groups exhibits four reversible color switches， while DBT with TPA and DMAC groups shows irreversible bicolor change. 通过分子构型和分子间相互作用是调节材料的发光颜色和光物理性能的重要途径之一。在本文中，我们以苯并噻二唑为受体（A），三苯胺（TPA）和二甲基吖啶（DMAC）为给体（D），设计并合成了两种D-A-D和D-A-D’型的发光分子，分别为 TBT 和 DBT 。两个分子均表现出同质多晶现象（ TBT -O： λ PL =593 nm 和 TBT -R： λ PL =616 nm） 和 （ DBT-Y ： λ PL =570 nm， DBT-O ： λ PL =605 nm and DBT-R ： λ PL =642 nm）。外界刺激下，对称结构的 TBT 为四色的可逆光色转换，非对称的 DBT 分子则表现为不可逆光色转换。我们的研究为基于苯并噻二唑类刺激响应发光材料提供重要的分子设计思路。 苯并噻唑 机械响应 多色发光 Benzothiadiazole Mechanochromic responses Multicolor luminescence 1　Introduction 1 Development of organic luminescent material is of great importance for their applications in organic light-emitting diodes (OLEDs), sensors, bioimaging and displays [ 1 1 - 3 3 1-3 ] . Multicolor emission in a single molecule, resulting from external stimuli such as solvent, aggregate state and external forces, endows organic compound unique applications, such as ink security, sensors and disease detectors [ 4 4 - 7 7 4-7 ] . Mechanochromic responses luminescent (MRL) mainly dependent on molecular conformations and packing structures in solid state [ 8 8 , 9 9 ] . This kind of materials usually have donor (D) with flexible structure and acceptor (A) units, which facilitate adjustment of emission colors. In addition, the luminescent behaviors of D-A-type emitter are inclined to be affected by surrounding environment owing to their special charge transfer (CT) transition [ 10 10 , 11 11 ] . Consequently, their excited-state properties are also sensitive to external stimuli [ 12 12 - 15 15 12-15 ] . For example, asymmetrical molecule with two different chromophores reported by Chi achieved various excited state natures including room-temperature phosphorescent (RTP), thermally activated delayed fluorescence (TADF) and mechano-responsive luminescence (MRL) in a single molecule due to the different dominant chromophore under excitation [ 16 16 ] . The D-A-type of TRZ-c-BPXZ exhibited on-off switchable crystal-dependent TADF feature [ 17 17 ] . Strong afterglow and dual emissive were achieved successfully by bridging dibenzofure moieties [ 18 18 ] . Simultaneously, these materials with abundant excited state nature and emission colors benefit to investigate the relationship between molecular packing and configuration [ 19 19 , 20 20 ] . 2,1,3-benzothiadiazole (BT) and its derivatives are promising types of acceptor units, owing to their strong electron-withdrawing property, intense light absorption and good photochemical stability, which usually couples with a variety of electron-rich groups to form low band gap emitting materials [ 21 21 , 22 22 ] . BT derivatives have exhibited huge potential applications in organic photovoltaics (OPVs) [ 23 23 ] , dye-sensitized solar cells, organic field effect transistor (OFETs) [ 24 24 ] and OLEDs [ 25 25 ] . Our previous work indicates that incorporating triphenylamine (TPA) can enhance OLED device performance [ 26 26 ] . DMAC unit in emitter with two methyl generally undergo fold along C-N axis [ 16 16 ] , endowing emitters with a great diversity of molecular conformations. Therefore, two molecules with D-A-D and D-A-D’ structure were designed and synthesized, expecting to achieve emission colors changes by switching from one chromophore to another. Recently, TBT with BT as an acceptor and two TPA as donor were reported, which possess a unique hybridized local and charge-transfer (HLCT) [ 27 27 , 28 28 ] . However, its interesting high-contrast crystal-dependent emission ( TBT-G : λ PL =570 nm, TBT-Y : λ PL =593 nm and TBT-R : λ PL =616 nm) and MRL behaviors have not been revealed. To study further the emission-changing mechanism and the role of TPA, DBT with D-A-D’ structure using DMAC to replace TPA were also synthesized. The results show emission colors of DBT are also highly dependent on its conformations and three crystals were obtained, namely DBT-Y : λ PL =570 nm, DBT-O : λ PL =605 nm and DBT-R : λ PL =642 nm. The asymmetrical structure for DBT exhibit selective of dominant chromophore result from the fold motions of DMAC. Yang and co-workers reported that DBD molecule using BT as acceptor and DMAC as donor with D-A-D structure was a typical TADF emitter [ 29 29 ] . While, DBT exhibits no delayed florescent. Their molecular structures are listed in Scheme 1 Scheme 1 . This research provided an important strategy to investigate the relationship between molecular configuration and photophysical properties. 2　Results and discussions 1 2.1　Synthesis and characterization 2 DBT was prepared via simple Suzuki coupling reaction, as shown in Scheme S1. The chemical structures of target compounds were confirmed by 1 H NMR, 13 C NMR (Figure S1 and S2) and X-ray single crystal diffraction. While the synthesis and structure characteristics for TBT were reported [ 27 27 ] . Its polymorphism-dependent and mechanochromic luminescent properties have not revealed thus far. The single crystals of TBT-R and DBT-O were grown from tetrahydrofuran (THF) / methanol. The crystals of TBT-Y were prepared from dichloromethane (DCM)/ methanol and DBT-R were obtained from the DCM/ methanol/ ethanol. 2.2　Photophysical properties 2 The ultraviolet-visible (UV-vis) absorption and photoluminescence (PL) spectra of TBT and DBT were measured in DCM solution (1×10 -5 M) (in Figure 1 Figure 1 a) and the data were listed in Table 1 Table 1 . The intense absorption peaks at 305 nm for TBT and 312 nm for DBT , which should be attributed to the π-π* transition. While the broad low-energy absorption band at 458 nm for TBT and 438 nm for DBT could be assigned to intramolecular charge transfer (ICT) transition from electron-donating unit to BT acceptor [ 30 30 ] . The PL spectra of TBT and DBT in DCM solution displayed broad emission with peaks at 662 and 652 nm. The large stokes shifts indicated that their emission could be origin from the charge transfer (CT) states in the single molecular state. This result also had been proven by their PL spectra in different solvents (Figure. S3). With increasing polarity from hexane to acetonitrile, the emission spectra of TBT and DBT turned to be broaden and weaken in PL intensity. 1,2; λ abs a （nm） λ em b （nm） T d /T g （℃） E g c （eV） HOMO d （eV） LUMO （eV） DBT 304，437 /315， 472 662/615 360/112 2.26 -5.34 -3.08 TBT 315，460 /312，456 651/609 375/100 2.28 -5.2 -2.92 7 a the absorption peak in DCM/film; b the emission peak in DCM solution/film, c energy gap; d energy of HOMO For the aggregate state, the PL emission peaks of TBT and DBT in the films were located at 609 and 615 nm with the similar profiles ( Figure 1 Figure 1 b). To verify emission behaviors of two molecules, the transient PL spectra in films were measured, as can be seen in Figure S4. The fluorescent lifetime (τ) of TBT at 609 nm was 5.71 ns. While the emission at 615 nm for DBT was fitted to biexponential with τ 1 =3.84 ns (27.22%) and τ 2 =8.03 ns (72.78%). Therefore, the emission of TBT and DBT in films originated from the different excited states. As illustrated in Figure S5, the levels of highest occupied molecular orbitals (HOMOs) were estimated to be -5.20 and -5.34 eV for TBT and DBT . Their optical energy gaps (E g ) were calculated to be 2.26 and 2.28 eV by absorption cutoffs. Accordingly, the levels of lowest unoccupied molecular orbitals (LUMOs) were -2.92 and -3.08 eV. 2.3　High-contrast and crystal-dependent emission behaviours 2 X-ray single crystal analyses were carried out to get insight into the relationship between emission and configurations, and the single-crystal data were summarized in Table S1. For compound DBT , three different crystals of DBT-Y ( <math id="M1"><msub><mrow><mi>λ</mi></mrow><mrow><mi>P</mi><mi>L</mi></mrow></msub></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029538&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029537&type= 4.57200003 3.72533321 = 570 nm), DBT-O ( <math id="M2"><msub><mrow><mi>λ</mi></mrow><mrow><mi>P</mi><mi>L</mi></mrow></msub></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029497&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029496&type= 4.57200003 3.72533321 = 605 nm) and DBT-R ( <math id="M3"><msub><mrow><mi>λ</mi></mrow><mrow><mi>P</mi><mi>L</mi></mrow></msub></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029538&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029537&type= 4.57200003 3.72533321 = 642 nm) were obtained by solvent evaporation method and can be divided in two groups: 1) DBT-Y and DBT-R with the folded DMAC around　N-C axis; 2) DBT-O with a planar configuration of DMAC, as shown in Figure 2 Figure 2 and Figure 3 Figure 3 . DBT-Y and DBT-R were monoclinic system with the space group of P2 1 /c, presenting the rod like shapes while DBT-O displayed block shape. The absolute photoluminescence quantum yield ( Φ PL ) of DBT-Y , DBT-O and DBT-R were measured to be 12，10.6 and 6%, respectively. Luminescent properties are not only origin from the single-molecule structure, but also derived from the packing modes in aggregate state [ 31 31 ] . The molecular packing of DBT-Y and DBT-R were presented in Figure S3 and Fig.3 Fig.3 . Their intramolecular and intermolecular interactions were listed in Table S3. The adjacent intermolecular π-π stacking occurred between BT units through tail-to-head antiparallel arrangement with the distance larger than 4.07 Å. DBT-R exhibited a little shorter distances than that of the DBT-Y , indicating a compact packing. The folded angles of DMAC in DBT-Y and DBT-R were 150 and 148°. These differences in molecular interactions and folded angles resulted in the different photophysical properties [ 32 32 ] . For DBT-O , it exhibited the orthorhombic system with Pbca space group and a planar DMAC unit. The strong π-π interaction of 3.02, 3.13 and 3.24 Å by parallel arrangement between benzene rings of TPA were observed in crystalline stacking. For DBT-O molecule, there was not only intramolecular interaction C-H…N of 2.279 Å, but also the intermolecular interaction C-H…N of 2.939 Å. The single crystal of DBT-Y and DBT-O exhibited single exponential with 10.84 and 6.81 ns, while the decay curve DBT-R fitted to biexponential with τ 1 =5.77 ns (24.71%) and τ 2 =4.53 ns (75.29 %), as exhibited in Figure S7. For compound TBT with the symmetrical structure, two different crystals with high-contrast emission were also prepared by solvent evaporation methods, TBT-Y ( <math id="M4"><msub><mrow><mi>λ</mi></mrow><mrow><mi>P</mi><mi>L</mi></mrow></msub></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029538&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029537&type= 4.57200003 3.72533321 = 593 nm) and TBT-R ( <math id="M5"><msub><mrow><mi>λ</mi></mrow><mrow><mi>P</mi><mi>L</mi></mrow></msub></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029538&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029537&type= 4.57200003 3.72533321 = 617 nm), as shown in Fig.4 Fig.4 . The monomer TBT have been reported with <math id="M6"><msub><mrow><mi>λ</mi></mrow><mrow><mi>P</mi><mi>L</mi></mrow></msub></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029511&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029510&type= 4.57200003 3.72533321 = 615 nm [ 27 27 ] . They were all triclinic system and P-1 space group. TBT-G with <math id="M7"><msub><mrow><mi>λ</mi></mrow><mrow><mi>P</mi><mi>L</mi></mrow></msub></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029513&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029512&type= 4.57200003 3.72533321 = 580 nm also have been reported but no single-crystal data [ 22 22 ] . For TBT-Y and TBT-O , there existed four different molecules in one single crystal assembling together by close H-hydron intermolecular interaction ( Fig.5 Fig.5 ). Their torsions angle of each molecule have been listed in Figure S8 and S9. The short contacts were listed in Table S4. In TBT-Y , two molecules were distributed in one dimension through the head-to-head pattern of BT, and the third molecule was parallel to their plane. The fourth molecule exhibit the “ax” model, which the BT unit is perpendicular to the adjacent molecules, as shown in Figure 5 Figure 5 a1. In a single molecule, the intramolecular interactions of C-H…N were not same, indication TBT was not strictly symmetrical as its chemical structure. There was only intramolecular C-H…N interaction. While, for TBT-O , three molecules were localized at horizontal direction, and the fourth molecule seated at the vertical direction by BT group. The intramolecular and intermolecular actions in TBT-R were shorted than that of in TBT-Y , suggesting a close packing and a red shifted emission. The crystal data for DBT and TBT have been deposited in the Cambridge Crystallographic Data Centre (CCDC) with the numbers (CCDC2160951-2160953, 2052993, 2174557 and 2174564). 2.4　Emission behaviors responding to the external stimuli 2 It was feasible to adjust the emission colors of these crystals through external force stimuli. As shown in Figure 6 Figure 6 a, the emission peak of single crystal DBT-O occurred the red-shift from 609 to 618 nm with the enhancement of emission intensity by grinding. In contrast, upon grinding, the crystal of DBT-R displayed a blue-shift emission from 641 to 621 nm with sharply increasing PL intensity ( Fig. 6 Fig. 6 b). The single crystals of DBT-O and DBT-R have the obvious and intensity diffraction peaks, but the peaks disappeared ( Fig.7 Fig.7 a) after grinding. This indicated that the transform from crystal to amorphous. The opposite MRL nature of DBT-O and DBT-R may be resulted from their different molecular conformation and intermolecular interactions. DBT-O shows compact staking, which were difficult to destroy to form a new packing mode. While, DBT-R possess loosely packing, which was easily collapsed when we gave it an external stimulus and resulting a blue-shifted emission. To further study the MRL properties of DBT , differential scanning calorimetry (DSC) were performed. As presented in Fig.7 Fig.7 b, pristine powder and DBT-R show endothermic peaks at 230 and 232 ℃, which are assigned to their melting points. And there are endothermic peaks at 112 ℃, which are identical as glass transition point. DBT-R displayed an exothermic peak at 160 ℃, accompanied with the obvious emission peak change from 640 to 620 nm. Thus, this point may be ascribed to the thermal phase transition process. This result suggests that DBT-R was metastable phase and could be turn to a more stable state. No obvious exothermic or endothermic peaks for DBT-O were observed during whole heating process, which suggest that it is difficult to change the packing mode of DBT-O . Therefore, DBT-O only show 10 nm red shift in response to external stimuli no matter fuming, heating or grinding. For TBT , upon grinding pristine powder with a mortar and pestle, the emission shifted from 572 nm to 598 nm with the increasing the emission intensity (in Fig.8b). XRD pattern showed that the diffraction peaks sharply reduced after grinding samples, suggesting a transform from well-order crystalline phase to amorphous. When fuming the grinded sample with DCM vapor, it turned to yellow light with the PL peak at 580 nm with some intense diffraction peaks appeared again, indicating that it is a microcrystalline. Then, grinding the fumed sample again, the emission of TBT red-shifted to 598 nm. When heating this grinded samples at 120 ℃ in the air, it converted back into pristine microcrystals with emission maximum of 573 nm. Thereafter, the pristine sample were heated to the melt point. The bright red light with the emission peak at 612 nm were observed. Thus, TBT achieves a reversible four colors transition. TBT showed four colors reversible switches in response to the external stimuli with a large span from yellow to red. Except for DBT-Y (A few of single crystal DBT-Y were obtained, it is difficult to investigate the MCL properties.), DBT-O and DBT-R exhibit bicolor change with the smaller emission shift in emission peaks and their MRL emissive behaviors is irreversible. 2.5　Density Function Theory Calculations 2 The density functional theory calculations were performed on the two compounds to get insight about the luminescent mechanism by using the single crystal data as input file. The ground state (S 0 ) optimization of TBT and DBT were carried out by using the basis set B3LYP-631G (d, p) by employing crystal data as input file. The results demonstrated that, for DBT-Y and DBT-R , HOMOs were all mainly localized on the TPA moiety, which was the dominant chromophore (Figure. S10). While, HOMO of DBT-O was all distributed on the DMAC unit. The LUMOs of three different crystals were on the electron-withdrawing BT groups. The planar DMAC exhibits stronger donating-electron ability than TPA. The calculated HOMO/LUMO of DBT-Y , DBT-O and DBT-R are -5.11/-2.54, -4.87/-2.46 and -5.12/-2.56 eV, respectively. For TBT , their HOMOs distributed on the whole molecular skeleton, while the LUMOs were mainly on the BT acceptor, as shown in Figure. S10. The difference was that the HOMO and LUMO of tetramer TBT-O and TBT-R were appeared on the different molecules, suggesting that the transition from HOMO to LUMO were intermolecular. 3　Conclusions 1 In conclusion, we have developed two crystal-dependent emitting materials of symmetrical TBT and asymmetric DBT . Compound TBT exhibited two kinds crystals of TBT-O ( <math id="M8"><msub><mrow><mi>λ</mi></mrow><mrow><mi>P</mi><mi>L</mi></mrow></msub></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029538&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029537&type= 4.57200003 3.72533321 = 593 nm) and TBT-R ( <math id="M9"><msub><mrow><mi>λ</mi></mrow><mrow><mi>P</mi><mi>L</mi></mrow></msub></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029538&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029537&type= 4.57200003 3.72533321 = 617 nm) with the different packing structures. DBT possessed three kinds crystals of DBT-Y ( <math id="M10"><msub><mrow><mi>λ</mi></mrow><mrow><mi>P</mi><mi>L</mi></mrow></msub></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029534&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029533&type= 4.57200003 3.72533321 = 570 nm), DBT-O ( <math id="M11"><msub><mrow><mi>λ</mi></mrow><mrow><mi>P</mi><mi>L</mi></mrow></msub></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029538&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029537&type= 4.57200003 3.72533321 = 605 nm) and DBT-R ( <math id="M12"><msub><mrow><mi>λ</mi></mrow><mrow><mi>P</mi><mi>L</mi></mrow></msub></math> http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029538&type= http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=43029537&type= 4.57200003 3.72533321 = 642 nm), respectively. The results showed that the different emission colors resulted from the folded angles of DMAC units. Upon external stimuli, their emission colors in single crystals can be adjusted. Crystals DBT shows reverse bicolor change behavior, while TBT exhibit four reversible color-switching properties. The relationship between molecular configuration and emission were deeply explored. The emission peaks of TBT and DBT were located at 662 and 652 nm in DCM, 609 and 615 nm in films. Our work will promote development the multicolor emitting materials based on BT groups. References XU Y ， WANG Q ， CAI X ， et al .， Highly efficient electroluminescence from narrowband green circularly polarized multiple resonance thermally activated delayed fluorescence enantiomers ［J］， Adv. 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