Controlling the adiabaticity of electron-transfer reactions using nematic liquid-crystal solvents

Transient absorption studies of intramolecular photoinduced charge separation and thermal charge recombination were carried out on a molecule consisting of a 4-(N-pyrrolidinyl)naphthalene-1,8-imide donor (PyrNI) covalently attached to a pyromellitimide acceptor (PI) dissolved in the liquid crystal N-(4-methoxybenzylidene)-4-butylaniline (MBBA). In the isotropic phase, just above the nematic to isotropic phase transition, ordered microdomains exist that give rise to solvation dynamics that consist of collective reorientational motions of the microdomains and relatively faster reorientations of molecules within the domains. The different temperature dependence of these responses permits the assignment of collective or molecular solvent motions as mediators of the charge separation and recombination reactions. We find for PyrNI-PI in MBBA that charge separation is adiabatic and controlled by the collective microdomain reorientations, whereas the charge recombination is nonadiabatic and controlled by the faster molecular motions within the domains. The data are compared to results for charge separation and recombination within PyrNI-PI dissolved in 4′-pentyl-4-cyanobiphenyl (5CB) under similar conditions. In 5CB, the opposite behavior was observed, i.e., the charge separation is nonadiabatic while the charge recombination is adiabatic. The results are interpreted as arising from the different signs of the dielectric anisotropies of the two liquid crystals, where that of MBBA is negative while that of 5CB is positive.