Photoinduced charge separation and recombination processes in fine particles of oligothiophene-C₆₀ dyad molecules

Photoinduced charge separation and recombination processes in fine particles of octathiophene-C₆₀ and dodecathiophene-C₆₀ dyad molecules were investigated by the subpicosecond laser photolysis method. Sizes of the fine particle samples were estimated to be 60-140 nm using SEM observation and dynamic light scattering measurements. Charge separated states were generated within 1 ps upon the 150 fs laser excitation in both fine particle samples of the dyad molecules. Generation yields of the charge separated states were about a half of those of the corresponding dyad molecules in polar solvents, indicating the existence of the competitive deactivation pathways of the singlet excited states, such as exciton migration in each fine particle. On the other hand, the charge separated states deactivated via the two-step-decay processes. Direct charge recombination in a dyad molecule and indirect charge recombination after migration of radical cation (hole) and anion (electron) in fine particles were observed as fast and slow decay components, respectively. The faster charge recombination rates than those of previously reported for aniline-fullerene dyad cluster will result from high hole-mobility in the oligothiophene-moieties of the present dyad molecules composing fine particles.