Study of third-order microscopic optical nonlinearities in sequentially built and systematically derivatized structures

With a goal of understanding the structure-property relationship for third-order microscopic optical nonlinearity, we have investigated the nonlinearities of a number of sequentially built and systematically derivatized π-conjugated structures by using degenerate four-wave mixing. To examine the nature of effective conjugation, we have measured the third-order microscopic nonlinearities, γ, for several para polyphenyls and compared the dependence on the number of repeat units with that observed in the α-thiophene oligomers. Our results show that while the limiting conjugation length in each conjugated series may be different, it becomes much shorter for polyphenyls than that for polythiophenes. Systematically derivatized α-terthiophene structures have also been investigated. The substitution of a pyrrole or a benzene unit in the place of the central thiophene ring in the α-terthiophene structure reduces the γ value. The substitution of polarizable iodine atoms at the end α positions of α-terthiophene increases the γ value. Even larger increases in the γ values are observed with the mononitro and dinitro substitutions at the a positions in the α-terthiophene structure. Possible theoretical reasons for such changes in the γ values for derivatized structures are discussed.