Spectroscopic studies of nonlinear optical organic polymer : a twisted intramolecular charge transfer conformation in P, P'-disubstituted diphenyl hexatriene systems
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The photophysical properties of 1,6-diphenyl-1,3,5- hexatriene (referred to as DPH) and a series of p,p’- disubstituted 1,6-diphenyl-l,3,5-hexatrienes (referred to as D,A-DPH) are investigated, where D and A are the electron-donating and electron-accepting groups, respectively: -OCHg, -N(CH3)2» and -NO2• In all solvents used, a dual fluorescence is observed for D,A-DPH containing the groups of -N(CH3>2 or -NO2 which are capable of internal rotation, suggesting that the a fluorescence originates from a twisted intramolecular charge transfer (TICT) state. It is found that the twisted structure of D,A-DPH exists in the ground state and is stabilized by a liquid crystal-like intermolecular charge interaction. A self-absorption phenomenon is detected in DPH in a study of concentration effects. This together with the fluorescence lifetime measurements indicate that the lowest excited singlet state for DPH is a Ag and not a Bu, even in a polar medium. An optical quality single crystal of DPH is obtained and used to characterize the optical transition moment. It is found that the optical transition moment is directed along the r-conjugated system in the DPH single crystal. The D,A-DPH is a promising compound with large nonlinear optical (NLO) properties. The TICT state is thought to play an important role in optical nonlinearity.