Photoinduced intramolecular charge separation in poly(alkyl methacrylate) solids studied by using a fluorescent polarity probe

The rate of photoinduced intramolecular charge separation (CS) k(CS) in polymer solids was evaluated by the fluorescence lifetime measurement over a temperature range 100-400 K. Nine kinds of carbazole-spacer-acceptor dyads (Cz-S-A) were embedded in a series of poly(alkyl methacrylate) films with various glass transition temperatures (T-g's). The rate k(CS) of Cz-S-A increased above T-g or melting temperature T-m of each polymer solid. The temperature dependence of k(CS) in a poly(butyl methacrylate) glassy solid was in agreement with that of the dielectric constant at higher frequencies, indicating that orientational motions faster than the CS rate are effectively coupled to the CS event. The effective dielectric constant (epsilon') for CS was evaluated with a novel fluorescent probe of carbazole-terephthalate cyclophane (Cz-TP). The free energy gap and temperature dependence of k(CS) were well explained by a single quantum mode electron transfer (ET) formula with epsilon' rather than a static dielectric constant epsilon(S) in a poly(butyl methacrylate) glassy solid. In conclusion, CS in poly(alkyl methacrylate)s is promoted above T-g or T-m by enhancement in orientational motions of ester groups with a relaxation time comparable to the CS event.