Design,Synthesis And Optoelectronic Properties Of 1,3,4-Oxadiazole Derivatives With D-A-A-D Structure

Organic conjugated small molecules with D-A structure have wide applications in many fields,such as organic light-emitting diodes,dye-sensitized solar cells,organic fluorescence sensors and nonlinear optical materials due to its easy adjustive structures,rapid photo response,good nonlinear optical properties and high optical threshold.Developing organic conjugated small molecules with excellent intramolecular charge transfer properties and high solid state fluorescence quantum efficiency have always been the goal of scientists.It is well known that the intramolecular charge transfer characteristic and solid state fluorescence quantum efficiency are highly dependent on molecular structures.Therefore,to obtain donor-acceptor type organic conjugated small molecules with excellent performance,it is necessary to study the molecular structure-property relationships and realize the controllable of material structures.Herein,we designed and synthesized a series of1,3,4-oxadiazile derivatives with D-A-A-D structurs,carefully studied the relationship between molecular structures and intramolecular charge transfer characteristic or optoelectronic properties,and successfully realized the tuning of materials aggregate structure,which could provide guidance in the development of organic luminescent materials with excellent performance.1.The substituent effect on intramolecular charge transfer characteristics of a series methoxy substituted bi-1,3,4-oxadiazole derivatives with D-A-A-D structure have been studied.It is found that there are almost no red shifts in the absorption spectra of these methoxy substituted compounds,but different degrees of fluorescence red-shifts with increasing solvent polarity.The changes of molecular dipole moment between the ground state and charge transfer?CT?excited state are calculated to be 7.56 D in BOXD-o-OCH3,12.07 D in BOXD-m-OCH3,7.38 D in BOXD-p-OCH3,14.79 D in BOXD-D1 and 16.80 D in BOXD-T1.Theoretical calculations at Density Functional Theory level reveal that the charges have transferred from the terminal methoxy phenyl group to the central bi-oxadiazole group.The analysis of charge transfer based on electron density exhibit that the charge transfer distance is longer in BOXD-m-OCH3 as compared with the other two mono-substituted molecules,and the transferred charges are much more in poly-substituted molecules BOXD-D1 and BOXD-T1,thus BOXD-m-OCH3,BOXD-D1 and BOXD-T1 exhibit stronger intramolecular charge transfer strength than other compounds.2.Two diphenylamine substituted symmetric 1,3,4-oxadiazole derivatives?DPAOXD and DPAOXDBEN?were designed and synthesized,and their intramolecular charge transfer characteristic was studied through a combination of experimental techniques and theoretical calculations.The experimental results found that only a small red shift in the absorption spectra?¹5 nm?but a very large red shift in the emission spectra?¹14 nm for DPAOXD and¹40 nm for DPAOXDBEN?with increasing solvent polarity.The increase of molecular dipole moment from the ground state to the charge transfer excited state is calculated to be22.10 D in DPAOXD and 26.67 D in DPAOXDBEN,respectively.As compared to methoxy substituted compounds,the substitution by a diphenylamine group increase the transferred charge,which substantially strengthens the intramolecular charge transfer of DPAOXD;furthermore,the introduction of a central benzene ring in DPAOXDBEN increase the transferred distance,and then enhance the charge transfer strength of DPAOXDBEN.3.The crystal packing,intermolecular interactions,charge-transport and photophysical properties of a series symmetrical alkoxy substituted bi-1,3,4-oxadiazole derivatives are carefully investigated based on single crystal X-ray diffraction and/or theoretical calculation.The single crystal structure analyses reveal that these crystals all exhibit similar layer structures,while their molecular conformations,displacements of the nearest adjacent molecules in?-stacking and molecular packing modes can be effectively tuned by changing the positions,amounts or length of the alkoxy groups.Theoretical analysis shows that the?-?intermolecular interactions are the main forces in the formation of single crystal.Other intermolecular interactions such as C-H…N and C-H…O interaction also play a nonnegligible contribution in the different packing modes of these compounds.The absorption and emission maxima of these single crystals exhibit different extent red shifts as compared in solutions.The fluorescence quantum efficiency of BOXD-T1 crystal in which molecules adopt a cross packing modes is 0.49.4.The molecular self-assembly behavior of BOXD-6 in different states have been carefully studied.The aggregation modes of BOXD-6 in solutions are dependent on the concentration of solutions.It is found that BOXD-6 molecules would self-assemble into both H-and J-aggregates at moderate concentration(from5×10^-6 M to 2×10^-4 M)and then transit to exclusive J-aggregates at higher concentration(>4×10^-4 M)in THF solutions.Two types of crystal structures can be obtained by using slow evaporation method at room temperature.BOXD-6-?single crystals are found in chloroform,tetrahydrofuran and dichloromethane solutions,in which the solubility of BOXD-6 are 20 mg/mL,7 mg/mL and 4 mg/mL,respectively.While BOXD-6-?single crystals are found in the saturated solutions of ethyl acetate,acetone,acetonitrile and ethanol in which the solubility of BOXD-6 are all about 0.5mg/mL.In addition,two kinds of films with different structures and distinguished morphologies could be successfully prepared through solution casting method of BOXD-6 saturated tetrahydrofuran and ethyl acetate solutions.