Time-resolved fluorescence studies of carbazole and poly(N-vinylcarbazole) for elucidating intramolecular excimer formation

We have studied the excited-state properties of carbazole and poly(N-vinylcarbazole) (PNVC) by means of femtosecond time-resolved fluorescence and time-dependent density-functional theory (TD DFT) calculations. When we excite the polymer at 280 nm, the polymer is shown to exhibit normal fluorescence peaks at 351 and 362 nm and an excimer peak above 400 nm. The lifetimes of the normal and excimer S1 states for PNVC were measured to be τF = 5.34 ± 0.03 ns for normal fluorescence and τF = 6.98 ± 0.03 ns for excimer fluorescence. In the DFT calculation, the chromophores in the ground-state PNVC are shown to be parallel. Owing to the intramolecular hindrance, the chromophores in the singlet excited-state PNVC may not be parallel, giving rise to a slightly red-shifted excimer peak. TD DFT calculations indicate that the vertical excitation energies are almost independent of the conformational changes and the degree of polymerization.