Photoinduced electron transfer in a donor-acceptor dyad oriented by an aligned nematic liquid crystal solvent

Ultrafast photoinduced charge separation in the donor-acceptor dyad, N-(10-phenyl-10-H-phenothiazine)-9-cyano-1,6-bis(3,5-di-tert-butylphenoxy)-perylene-3,4-dicarboximide (CNPMI-PTZ) occurs readily in both toluene and benzonitrile. When CNPMI-PTZ is dissolved in the nematic liquid crystal 4-(n-pentyl)-4′-cyanobiphenyl (5CB) and the mixture is incorporated into a cell whose interior surfaces are coated with a rubbed polymer, both the liquid crystal director and CNPMI-PTZ align parallel to the rub direction. This geometry allows selective excitation of the CNPMI chromophores aligned along the director using linearly polarized light. Transient absorption measurements using aligned CNPMI-PTZ are compared to those obtained in isotropic solvents. It is found that both charge separation and charge recombination are adiabatic in the aligned 5CB and the charge transfer kinetics are slow relative to the motions of the aligned donor-acceptor molecule. The adiabaticity leads to enhanced ion pair lifetimes, which most likely will prove useful for designing systems for the photoconversion of solar energy.