Investigation On The Ultrafast Dynamics Of Excited State Intramolecular Charge Transfer Process Of Diphenylamine-substituted Oxadiazole Derivatives

D-A(donor-acceptor)organic conjugated small molecules have wide applications in many fields,such as organic optoelectronic devices,fluorescent sensors,due to their variable structure,easy control of the spectrum,strong stability,and rapid photo response.At present,the development of D-A type organic conjugated small molecule materials with high luminous efficiency and charge transfer properties is the goal that people are constantly pursuing.The study of molecular excited state decay process is particularly important for revealing the influence of molecular structure on photoelectric properties,perfecting the theory of organic electronic structure,and developing materials with high luminous efficiency.Therefore,we studied the excited state kinetics of two diphenylamine substituted1,3,4-oxadiazole derivatives by transient absorption spectroscopy.And discussed the influence of the charge transfer process in the excited state on the luminous efficiency and provide guidance for the future design of D-A type organic materials with high luminous efficiency.1.Theexcitedstatedecayprocessof2,2-bis[4-4-(N,N-diphenylamino)]-1,3,4-oxadiazole(DPAOXD)molecule have been studied by transient absorption spectroscopy.The results show that in the non-polar cyclohexane(CHEX),the molecule is excited to the locally excited(LE)state and then quickly transform to the ICT state with a relaxation time of 1.25 ps.In the medium polarity tetrahydrofuran(THF)solvent,the ICT state is stabilized by solvation due to the interaction between the solvent and the polar solute molecules.Therefore,besides the LE-ICT transition,it can also observed that the ICT to the solvent stable ICT state(SSICT)transition,relaxation time are 0.5 ps and 1.92 ps,and finally decays back to the ground state.In the highly polar acetonitrile(ACN)solvent,there are also LE-ICT and ICT-SSICT state transitions,with a relaxation time of 1 ps and 181.2 ps,respectively,and finally decays back to the ground state.It is found that the transition of ICT-SSICT in ACN solvent is much slower than that in THF solvent,which makes ICT-SSICT in a disadvantageous position in the competition with the non-radiation relaxation process.More molecules return to the ground state through non-radiation transition.This may be the reason for the lower fluorescence quantum efficiency in acetonitrile solvent.Theoretical calculation studies have shown that for DPAOXD molecules,from the ground state to the first excited state,the torsion angle between the donor and the acceptor changes little,and the change curve of the torsion angle of the first excited state on the potential energy is the same as that of the ground state.This indicates that the progress from the ground state excitation to the first excited state,the molecular structure has not been twisted,and it is a planar type of intramolecular charge transfer.2.In the same way,the excited state relaxation process of the1,4-bis{[4-4-(N,N-diphenylamino)]-1,3,4-oxadiazole}benzene(DPAOXDBEN)molecule have been studied.The results show that DPAOXDBEN is consistent with DPAOXD,showing a LE-ICT transition in non-polar CHEX solvent with a relaxation time of 1.7 ps;in a medium polar THF,it exhibits LE-ICT transition and ICT-SSICT transition with a relaxation time of 0.6 ps and 1.59 ps,respectively;in highly polar ACN,the relaxation time of two transition are about 0.42 ps and 16.73 ps,respectively.Similarly,it is found that the transition of ICT-SSICT in ACN solvent is much slower than that in THF solvent,which makes ICT-SSICT in a disadvantageous position in the competition with the non-radiation relaxation process.More molecules return to the ground state through non-radiation transition.This may be the reason for the lower fluorescence quantum efficiency in acetonitrile solvent.Theoretical calculation studies have also proved that the intramolecular charge transfer process of DPAOXDBEN is planar type.3.In view of the strong electron donating ability of diphenylamine and symmetrical structure are beneficial to enhance the charge transfer properties of the molecule,we further designed and synthesized a star-shaped diphenylamine tri-substituted1,3,4-oxadiazolederivative1,3,5-tris{[4-4-(N,N-diphenylamino)]-1,3,4-oxadiazole}benzene(TDPAOXDBEN),the molecular structure was confirmed by FT-IR,~1HNMR and elemental analysis.Photophysical experimental studies have shown that with the increase of solvent polarity,the absorption spectra of the TDPAOXDBEN molecule changes very little,moving only 13 nm.But the fluorescence emission spectra of the molecule has a large red shift from non-polar cyclohexane solvent to highly polar acetonitrile solvent(115 nm),proving that the molecule has strong intramolecular charge transfer properties.At the same time,the fluorescence quantum efficiency of TDPAOXDBEN in chloroform solvent is 0.8,and it can maintain 0.53 in tetrahydrofuran,showing high luminous efficiency.