Study On The Third-order Nonlinear Optical Property Of A-D-A Type Small Molecule Acceptor Material

Due to their advantages of highly conjugated ?-electron structure,strong intramolecular charge transfer(ICT)and high electron mobility,the fused-ring A-D-A small molecule acceptors used in organic photovoltaic(OPV)devices exhibit superior nonlinear optical(NLO)performance,but the investigations are still in their infancy.Herein,several fused-ring A-D-A small molecule acceptors are chosen to investigate the third-order NLO properties by ultrafast Z-scan and 4f phase-coherent imaging system in solution and thin film,respectively.(1)We select planar molecule O-IDTBR and twisted molecule(IDFBR)with a dihedral angle of 33°to explore the relationship of structure-property.Femtosecond Z-scan results show that both O-IDTBR and IDFBR exhibit reverse saturated absorption(RSA),and planar molecule O-IDTBR exhibit better nonlinear optical(NLO)properties from 800 nm to 1030 nm.Especially,the two-photon absorption(TPA)cross-section of O-IDTBR is 8811 GM,over4 times that of IDFBR(1902 GM)at 800 nm.The results of AFM and the 4f phase-coherent imaging system indicate that the planar molecule films show more ordered morphology and a larger nonlinear refractive index.We conclude that the large NLO refractive index is ascribed to the combination of strong ICT and well-ordered morphology.In addition,femtosecond transient absorption spectroscopy is conducted to investigate the dynamic processes after photoexcitation.(2)We select two A-D-A molecules with different thiophene ratios(IHIC and ITIC)to perform a comparative study.Interestingly,the results of femtosecond Z-scan show that although the conjugation length of IHIC is smaller than ITIC,IHIC with a higher ratio of thiophene exhibits better NLO properties than that of ITIC at 900 nm in solution.The results of the 4f system and AFM show that ITIC film show more ordered morphology and better NLO properties.Therefore,thiophene can more effectively enhance the conjugation length and improve the NLO performance in solution,and ordered morphology is the main factor to regulate the NLO properties in film.Through the analysis of the transient absorption spectrum,dynamic decay process is investigated.