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

Abstract

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 −ΔG_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.