Synthesis, Crystal Structure And Photophysical Properties Of A Series Of Bi-1,3,4-oxadiazole Derivatives

Organic semiconductor materials are under the spotlight attract much attentionbecause of their broad application prospects. The performance of this material on theoptoelectronic devices, such as solid light-emitting efficiency and carrier mobility, notonly depends on the electronic structure of the molecule itself, but also on theelectronic coupling between molecules. Crystal has clear structure and arrangement,which provides an ideal model system for studying on the relationship between theintermolecular forces and molecular arrangement and the solid-state light-emittingefficiency and carrier mobility. Based on the previous studies by our group, we havedesigned and synthesized a series of bi-1,3,4-oxadiazole derivatives, which areBOXD, BOXD-o-F, BOXD-m-F, BOXD-p-F, BOXD-o-OCH3, BOXD-o-NO2,BOXD-p-NO2and TBOXD. We change the substitution position and the substituentspecies on the benzene ring, to regulate structure and arrangement of the molecular.Detailed study on the effect of substituents on photophysical properties of moleculesin solution and solid state has been done. Combined with theoretical calculation, weattempt to reveal the molecular geometry on the ground state, the nature of molecularorbital and electronic transitions. Through adjusting the crystal growth conditionsextensively, we have got four kinds of single crystals, which are BOXD, BOXD-o-F,BOXD-m-F and BOXD-o-OCH3. We have explored deeply the effects of differentsubstituents on the molecular arrangement in the crystal and luminescence properties.The main results of this paper are as follows:1. The UV-vis absorption spectra show that the molecules with resonance donor oracceptor groups (-OCH3,-NO2) as well as the thienyl group (TBOXD) exhibit a largered-shift in the absorption maximums λabscompared with BOXD, while the moleculeswith inductive donor or acceptor groups (F) do not show significant variation in λabs.Similar trends are shown in the fluorescence spectra. All the bi-1,3,4-oxadiazolederivatives exhibit high fluorescence quantum yield at low concentration. The absorption bands become wider, with the substitution position from Para-to Ortho-.Ortho-methoxy and nitro substituted oxadiazoles show two absorption bands, one isred-shift and the other is blue shift. All the bi-1,3,4-oxadiazole derivatives exhibithigh fluorescence quantum yield at low concentration expect for nitro substitutedmolecules. The absorption maximums λabsof BOXD-o-OCH3varies from323nm in avery polar solvent (ethanol (ETO)) to315nm in a less polar solvent (cyclohexane(CHEX)). Only about8nm red-shifts in wavelength reveals that the ground state andFrank Condon (FC) excited states of the molecular structure are very similar.Furthermore, the emission spectra shows a large red-shift in their maximums, with theincrease of solvent polarity. This phenomenon is attributed to the excited stateintramolecular charge transfer.2. Optimizing the geometry of these molecules in the ground state by DFT method,BOXD-o-F, BOXD-m-F, BOXD-o-OCH3and BOXD-o-NO2have two conformationsand other derivatives have only one. For most molecules, HOMO to LUMO transitionis the major transition mode in S0-> S1except for BOXD-p-NO2, BOXD-o-NO2-1and BOXD-o-NO2-2, which appear HOMO to LUMO transition in S0-> S3.Theanalysis results show that the theoretical calculation and actual test results of UVabsorption and luminescence properties of these derivatives are basically consistent.The theoretical calculation results indicate that the double peaks in the UV-visabsorption spectra of BOXD-o-NO2and BOXD-o-OCH3is due to existing twoconformations.3. Using the method of solvent slow evaporation, four kinds of single crystal havebeen got, which are BOXD, BOXD-o-F, BOXD-m-F and BOXD-o-OCH3. The resultsshow that the dimers BOXD, BOXD-o-F and BOXD-m-F are J-type aggregates, butBOXD-o-OCH3is H-type aggregate. Fluoro and methoxy substituents have an effecton the coplanarity and dislocation parallel degree of the molecule. The molecules ofBOXD, BOXD-o-F and BOXD-m-F in single crystal states are planar conformations,but BOXD-o-OCH3is twist conmformation, which has a twist angle about9.85degrees between the benzene and oxazole ring. So we believe that there are two kindsof conformations for BOXD-o-OCH3, one is twist conformation and the other is planar conformation, causing the double peaks in the UV-vis absorption spectra.