Photoinduced charge separation and thermal charge recombination reactions in a series of structurally related donor-acceptor molecules based on the 4-(N-pyrrolidinyl)naphthalene-1,8-dicarboximide (5ANI) and 4-(N-piperidinyl)naphthalene-1,8-dicarboximide (6ANI) chromophores were studied in the nematic liquid crystals 4-cyano-4′-(n-pentyl)biphenyl (5CB) and N-(4′-methoxybenzylidene)-4-(n-butyl)aniline (MBBA) as well as in the isotropic solvents pyridine, 2-methyltetrahydrofuran, and pentyl propionate over a temperature range of 293-353 K. The photoexcited 5ANI and 6ANI chromophores donate an electron to pyromellitimide (PI), which is attached to the donors via a N-N single bond between their imide groups to give 5ANI-PI and 6ANI-PI, respectively. The photoexcited 6ANI chromophore also accepts electrons from p-methoxyaniline, which is attached to the 4-position of the naphthalene-1,8-dicarboximide as a N′-(4′-methoxyphenyl)piperazinyl substituent to give MeOAn-6ANI. Attachment of a secondary PI acceptor to MeOAn-6ANI via an imide-imide N-N single bond yields MeOAn-6ANI-PI. Photoinduced charge separation to form MeOAn+-6ANI--PI is followed by thermal electron transfer to produce MeOAn+-6ANI-PI-. The charge recombination reactions in all three molecules have negative activation energies in the three isotropic Debye solvents pyridine, methyltetrahydrofuran, and pentyl propionate. This is a result of non-Condon behavior due to the internal rotational dynamics of the five- and six-membered cyclic amines about the C-N bond joining them to the naphthalene ring in 5ANI-PI, 6ANI-PI, and MeOAn-6ANI-PI on the time scale of the charge recombination reaction. This effect is not observed in 5CB and MBBA, in which slower solvent motions are strongly coupled to the electron transfer process as indicated by the fact that the electron transfer reaction rates were solvent-controlled in the liquid crystals.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry