Picosecond time-resolved fluorescence studies of poly(N-vinylcarbazole) using a pulse-radiolysis technique

The formation processes of two distinct excimers of poly(N-vinylcarbazole)(PVCZ) have been directly observed by using a picosecond single-pulse radiolysis system with 10 ps time resolution. The primary processes of radiation-induced excitation in the polymer solutions have been studied. A so-called second excimer (λ_max of fluorescence = 375 nm) of PVCZ is almost completely formed immediately after a 10 ps electron pulse. Most of the sandwich-type excimer (λ_max of fluorescence = 420 nm) is formed in several nanoseconds. Most of the second excimer was confirmed to be formed directly in preexisting sites. Most of the sandwich-type excimer was confirmed to be formed through structural change of the excited carbazolyl and the ground-state carbazolyl chromophore to the sandwich conformation. Both the concentration of the preexisting trap sites and the efficiency of energy migration along the polymer chain before relaxation or solvation of the excited state are considered to be most important in the formation of the second excimer. Most of the primary excited states of PVCZ in irradiated toluene solutions of PVCZ are formed immediately after the 10 ps electron pulse. These very fast formation processes of excited states of solute molecules agree with the recent experimental results observed in irradiated hydrocarbon solutions of aromatic molecules in the picosecond time domain.