Photoinduced electron transfer of carbazole-acceptor dyads in solution and in a polymer solid

Photoinduced charge separation (CS) of carbazole-acceptor dyads (Cz-S-A) in solutions and in a polymer solid was examined by the measurement of fluorescence decay. For the discussion of CS from the viewpoint of thermal fluctuations in a solution and in a polymer solid, the separation distance between the Cz donor moiety and acceptor moiety was fixed with a rigid spacer. The photoinduced CS was observed for various solutions with different dielectric constants ranging from 3.06 to 37.5 at room temperature. The rate constant k(CS) increased with an increase in the free energy gap of -DeltaG(CS), indicating that CS is in the normal region of the Marcus theory. The temperature dependence of CS in a solution from 183 to 296 K was quantitatively explained by an electron transfer (ET) formula where solvent motions are treated as a classical mode and vibrational motions of the reactant are treated as a quantum mode. On the other hand, the photoinduced CS was also observed for a polymer solid with polar cyano groups over a wide temperature range from 100 to 400 K, although most motions are highly restricted compared with those in a solution. Above the glass transition temperature (T-g), CS was explained by the same ET formula as that in a solution with dielectric constants measured at a high frequency. Below T-g, CS was independent of temperature, indicating that CS is caused by nuclear tunneling at low temperatures.