Adjusting Emission Behaviors of Molecules Based Benzothiadiazole by Intermolecul

DONG Shufan，YANG Tingting，WEN Zhengjie，WANG Yating，XU Huixia*，DONG Wenjian，ZHAO Song，WANG Hua

（1.Key Laboratory of Interface Science and Engineering in Advanced Materials，Ministry of Education，Taiyuan University of Technology，Taiyuan030024，China；2.Shanxi Province Key Laboratory of Microstructure Functional Materials Institute of Solid-state Physical，School of Physics and Electronic Science，Shanxi Datong University，Datong037009，China）

Abstract：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（TBTO：λPL=593 nm and TBTR：λPL=616 nm）and DBT（DBTY：λPL=570 nm，DBTO：λPL=605 nm and DBTR：λPL=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.

Key words：benzothiadiazole；mechanochromic responses；multicolor luminescence

1 Introduction

Development of organic luminescent material is of great importance for their applications in organic light-emitting diodes（OLEDs）, sensors, bioimaging and displays[1-3]. Multicolor emission in a single mol⁃ecule, resulting from external stimuli such as sol⁃vent, aggregate state and external forces, endows or⁃ganic compound unique applications, such as ink se⁃curity, sensors and disease detectors[4-7]. Mechano⁃chromic responses luminescence（MRL）mainly de⁃pends on molecular conformations and packing structures in solid state[8-9]. 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 DA-type emitter are inclined to be affected by sur⁃rounding environment owing to their special charge transfer（CT）transition[10-11]. Consequently, their ex⁃cited-state properties are also sensitive to external stimuli[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 mole⁃cule due to different dominant chromophore under excitation[16]. The D-A-type of TRZ-c-BPXZ exhibit⁃ed on-off switchable crystal-dependent TADF fea⁃ture[17]. Strong afterglow and dual emissive were achieved successfully by bridging dibenzofure moi⁃eties[18]. Simultaneously, these materials with abun⁃dant excited state nature and emission colors benefit to investigate the relationship between molecular packing and configuration[19-20].

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[21-22]. BT de⁃rivatives have exhibited huge potential applications in organic photovoltaics（OPVs）[23], dye-sensitized so⁃lar cells, organic field effect transistor（OFETs）[24]and OLEDs[25]. Our previous work indicates that in⁃corporating triphenylamine （TPA）can enhance OLED device performance[26]. DMAC unit in emitter with two methyl generally undergoes fold along C-N axis[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 col⁃ors 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）[27-28].However, its interesting high-contrast crystal-depen⁃dent emission（TBT⁃G:λPL=570 nm, TBT⁃Y:λPL=593 nm and TBT⁃R:λPL=616 nm）and MRL behav⁃iors 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 emis⁃sion colors of DBT are also highly dependent on its conformations and three crystals were obtained,namely DBT⁃Y:λPL=570 nm, DBT⁃O:λPL=605 nm and DBT⁃R:λPL=642 nm. The asymmetrical structure for DBT exhibited selective of dominant chro⁃mophore 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-AD structure was a typical TADF emitter[29]. While,DBT exhibits no delayed florescent. Their molecu⁃lar structures are listed in Scheme 1. This research provided an important strategy to investigate the rela⁃tionship between molecular configuration and photo⁃physical properties.

Scheme 1 Molecular structures of TBT and DBT

2 Experiment

2.1 Materials and Synthesis

All reagents and solvents for synthesis and characterization were purchased from commercial companies and without further purification. DBT was preparedviasimple Suzuki coupling reaction, as shown in Scheme S1. The chemical structures of tar⁃get compounds were confirmed by1H NMR,13C NMR（Fig. S1 and S2）. While the synthesis and structure characteristics for TBT were reported[27]. Its poly⁃morphism-dependent and mechanochromic luminescent properties have not revealed thus far. The sin⁃gle 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 Characterizations

1H-NMR and13C-NMR spectra were measured with a Switzerland Bruker DR×600 at 600 MHz and 151 MHz using tetramethylsilane（TMS）as the inter⁃nal standard. UV-Vis spectra were recorded using a Hitachi U-3900 spectrometer. Photoluminescence（PL）spectra were recorded with a Horiba Fluoro⁃Max-4 spectrometer. The transient photoluminescence decay was carried out using an Edinburgh In⁃strument FLS980 spectrometer. Thermal gravimet⁃ric analysis was performed on a Netzsch TG 209F3 under dry nitrogen atmosphere. Differential scan⁃ning calorimetry（DSC）was measured on DSC Q2000. Powder X-ray diffraction were recorded us⁃ing Bruker APEX- Ⅱ CCD diffractometer with a graphite-monochromated Mo Kα radiation（Bruker Corporation, Billerica, MA, USA）. Molecular structures were determined by direct methods with SHELXS-97/SHELXL-97.

The electrochemical properties were obtainedviacyclic voltammetry（CV）measurement by using a CHI 660E voltammetry analyzer. Tetrabutylammo⁃niumhexafluorophosphate（TBAPF6）in anhydrous dichloromethane（0.1 mol/L）was used as the elec⁃trolyte. A platinum wire was used as the working electrode. A platinum electrode was the counter and a Ag/AgCl system was used as reference electrode with ferrocenium-ferrocene（Fc+/Fc）as the internal standard.EHOMO=-4.8-e(Eocx-Eofx)V,ELUMO=EHOMO+Eg,Eg=Theoretical simulations were performed using the Gaussian 09 package. Geometry optimization was performed by density functional theory（DFT）in B3LYP/6-31 G（d）basis sets.

3 Results and Discussions

3.1 Photophysical Properties

The UV-Vis absorption and PL spectra of TBT and DBT were measured in DCM solution （1×10-5mol/L）（in Fig. 1（a））and data were listed in Tab. 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）transi⁃tion from electron-donating unit to BT acceptor[30]. The PL spectra of TBT and DBT in DCM solution dis⁃played broad emission with peaks at 662 nm and 652 nm. The large Stokes shifts indicated that their emis⁃sion 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 sol⁃vents（Fig. S3）. With increasing polarity from hexane to acetonitrile, the emission spectra of TBT and DBT turned to be broaden and weaken in PL intensity.

Tab.1 Photophysical data of DBT and TBT

Fig.1 UV-Vis absorption and PL spectra measured in DCM solution（a）and in film（b）of TBT and DBT

For the aggregate state, the PL emission peaks of TBT and DBT in the films were located at 609 nm and 615 nm with the similar profiles（Fig. 1（b））. To verify emission behaviors of two mole⁃cules, the transient PL spectra in films were mea⁃sured, as can be seen in Fig. 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 biexpo⁃nential withτ1=3.84 ns（27.22%）andτ2=8.03 ns（72.78%）. Therefore, the emission of TBT and DBT in films originated from different excited states. As illustrated in Fig. S5, the levels of the highest occupied molecular orbitals（HOMOs）were estimated to be -5.20 eV and -5.34 eV for TBT and DBT. Their optical energy gaps（Eg）were calcu⁃lated to be 2.26 eV and 2.28 eV by absorption cut⁃offs. Accordingly, the levels of the lowest unoccu⁃pied molecular orbitals（LUMOs）were -2.92 eV and -3.08 eV.

3.2 High⁃contrast and Crystal⁃dependent Emis⁃sion Behaviours

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 Tab. S1. For compound DBT, three different crystals of DBT⁃Y（λPL=570 nm）, DBT⁃O（λPL=605 nm）and DBT⁃R（λPL=642 nm）were obtained by solvent evaporation method and can be divided into 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 Fig. 2 and Fig. 3. DBT⁃Y and DBT⁃R were mono⁃clinic system with the space group ofP21/c, present⁃ing the rod like shapes while DBT⁃O displayed block shape.

Fig.2 （a）PL spectra of DBT crystal. Luminescence images of DBT⁃Y（b），DBT⁃O（c）and DBT⁃R（d）.

The absolute photoluminescence quantum yield（ΦPL）of DBT⁃Y, DBT⁃O and DBT⁃R were mea⁃sured to be 12%，10.6% and 6%, respectively. Lu⁃minescent properties are not only origin from the sin⁃gle-molecule structure, but also derived from the packing modes in aggregate state[31]. The molecular packing of DBT⁃Y and DBT⁃R were presented in Fig. S3 and Fig. 3. Their intramolecular and inter⁃molecular interactions were listed in Tab. S3. The adjacent intermolecular π ⁃ π stacking occurred between BT units through tail-to-head antiparallel ar⁃rangement with the distance larger than 0.407 nm.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 different photophysical properties[32]. For DBT-O, it exhibited the ortho⁃rhombic system withPbcaspace group and a planar DMAC unit. The strong π-π interaction of 0.302，0.313，0.324 nm 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 0.227 9 nm, but also the intermolecular interaction C—H…N of 0.293 9 nm. The single crystal of DBT-Y and DBT-O exhibited single exponential with 10.84 ns and 6.81 ns, while the decay curve DBT-R fitted to biexponential withτ1=5.77 ns（24.71%）andτ2=4.53 ns（75.29 %）, as exhibited in Fig.S7.

Fig.3 Single-crystal structures and molecular packing of DBT⁃Y（a，a1），DBT⁃O（b，b1）and DBT⁃R（c，c1）.

For compound TBT with the symmetrical structure, two different crystals with high-contrast emis⁃sion were also prepared by solvent evaporation meth⁃ods, TBT-Y（λPL=593 nm）and TBT-R（λPL=617 nm）, as shown in Fig. 4. The monomer TBT have been reported withλPL=615 nm[27]. They were all tri⁃clinic system andP-1 space group. TBT⁃G withλPL=580 nm also have been reported but no singlecrystal data[22].

Fig.4 （a）PL spectra of TBT crystals. Luminescence images of TBT⁃Y（b）and TBT⁃R（c）.

Fig.5 The single-crystal structures and packing of TBT⁃Y（（a），（a1），（a2））and TBT⁃O（（b），（b1），（b2））

For TBT⁃Y and TBT⁃O, there existed four dif⁃ferent molecules in one single crystal assembling to⁃gether by close H-hydron intermolecular interaction（Fig. 5）. Their torsions angle of each molecule have been listed in Fig. S8 and S9. The short contacts were listed in Tab. S4. In TBT⁃Y, two molecules were distributed in one dimension through the headto-head pattern of BT, and the third molecule was parallel to their plane. The fourth molecule exhibits the “ax” model, while the BT unit is perpendicular to the adjacent molecules, as shown in Fig. 5（a1）.In a single molecule, the intramolecular interactions of C—H…N were not the same, indicating TBT was not strictly symmetrical as its chemical structure.There was only intramolecular C—H…N interaction. While, for TBT⁃O, three molecules were local⁃ized at horizontal direction, and the fourth molecule seated at the vertical direction by BT group. The in⁃tramolecular and intermolecular actions in TBT⁃R were shorted than that of in TBT⁃Y, suggesting a close packing and a red shifted emission. The crys⁃tal data for DBT and TBT have been deposited in the Cambridge Crystallographic Data Centre（CCDC）with the number（sCCDC 2160951-2160953, 2052993,2174557 and 2174564）.

3.3 Emission Behaviors Responding to Exter⁃nal Stimuli

It was feasible to adjust the emission colors of these crystals through external force stimuli. As shown in Fig. 6（a）, the emission peak of single crys⁃tal DBT⁃O occurred the red-shift from 609 nm 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 nm to 621 nm with sharply increasing PL intensity（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（a））after grind⁃ing. This indicated the transform from crystal to amorphous. The opposite MRL nature of DBT⁃O and DBT⁃R may be resulted from their different mo⁃lecular 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 stimu⁃lus and resulting a blue-shifted emission.

Fig.6 Fluorescent photographs under UV irradiation with 365 nm and PL spectra of DBT⁃O（a）and DBT⁃R（b）in response to external stimuli

To further study the MRL properties of DBT,differential scanning calorimetry （DSC）were performed. As presented in 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 dis⁃played an exothermic peak at 160 ℃, accompanied with the obvious emission peak change from 640 nm to 620 nm. Thus, this point may be ascribed to the thermal phase transition process. This result sug⁃gests that DBT⁃R was metastable phase and could be turned 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 re⁃sponse 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. 8（b））. XRD pattern showed that the dif⁃fraction peaks sharply reduced after grinding sam⁃ples, suggesting a transform from well-order crystal⁃line 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 dif⁃fraction 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 pris⁃tine sample was heated to the melt point. The bright red light with the emission peak at 612 nm was ob⁃served. Thus, TBT achieves a reversible four colors transition.

Fig.8 Fluorescent photographs（a）under UV irradiation with 365 nm，PL spectra response to external stimuli（b），XRD pat⁃terns in different states for TBT（c）.

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 sin⁃gle crystal DBT⁃Y were obtained, it is difficult to in⁃vestigate the MCL properties）, DBT⁃O and DBT⁃R exhibit bicolor change with the smaller emission shift in emission peaks and their MRL emissive be⁃haviors are irreversible.

3.4 Density Function Theory Calculations

The density functional theory calculations were performed on the two compounds to get insight about the luminescent mechanism by using the single crys⁃tal data as input file. The ground state（S0）optimiza⁃tion 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 local⁃ized on the TPA moiety, which was the dominant chromophore（Fig. S10）. While, HOMO of DBT⁃O was all distributed on the DMAC unit. The LUMOs of three different crystals were on the electron-with⁃drawing 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 eV, -4.87/-2.46 eV 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 Fig. S10.The difference was that the HOMO and LUMO of tet⁃ramer TBT⁃O and TBT⁃R were appeared on different molecules, suggesting that the transition from HOMO to LUMO were intermolecular.

4 Conclusions

In conclusion, we have developed two crystaldependent emitting materials of symmetrical TBT and asymmetric DBT. Compound TBT exhibited two kinds crystals of TBT⁃O（λPL=593 nm）and TBT⁃R（λPL=617 nm）with different packing structures.DBT possessed three kinds crystals of DBT⁃Y（λPL=570 nm）, DBT⁃O（λPL=605 nm）and DBT⁃R（λPL=642 nm）, respectively. The results showed that differ⁃ent 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 configura⁃tion and emission were deeply explored. The emis⁃sion peaks of TBT and DBT were located at 662 nm and 652 nm in DCM, 609 nm and 615 nm in films.Our work will promote the development of the multi⁃color emitting materials based on BT groups.

Supplementary Information and Response Letter are available for this paper at: http://cjl.lightpublishing.cn/thesisDetails#10.37188/CJL.20230022.