Laser photochemistry of polymers

Abstract

Absorption spectra of poly(N-vinyl-carbazole) and related polymers in the excited singlet, triplet, cationic, and anionic states were measured by nanosecond and picosecond laser photolysis methods and compared with those of model bichromophoric compounds. Absorption spectral shape of the excited singlet and cationic states is a function of polymer structure and sensitive to relative geometrical structure of two carbazolyl groups. The excited singlet species in poly(N-vinylcarbazole) are the sandwich excimer, the excited dimer with the partial overlap structure between two chromophores, and the so-called second excimer. The formation process of the sandwich excimer consists of an instantaneous rise and a slow one with a time-constant of a few hundred ps. The absorption spectrum of poly(N-vinylcarbazole) cation can be reproduced by a superposition of absorption bands of the sandwich dimer cation, the dimer cation with the partial overlap, and the dimer cation with the open form. On the other hand, absorption spectra of the triplet and anionic polymers are similar to those of the corresponding monomer model compound, showing a very weak interchromophoric interaction. All these results indicate that electronic excitation and charge in these polymers are not delocalized over several chromophores but trapped in some monomer and dimer sites. Dynamic behavior such as conformational change, intersystem crossing, photochemical formation yield of ion radicals, and their decay kinetics was also studied in detail.