Rigid intramolecular Donor-Acceptor(1)Acceptor(2)-X trichromophoric arrays based on a 4,5-diamino-xanthene bridge B were prepared in which the xanthene bridge orients A2 relative to D, so that nonbonded interactions between 1*D and A2 can be studied. The electron donor, D, is 4-(N-piperidinyl)naphthalene-1,8-dicarboximide (ANI), and the acceptors A1 and A2 are pyromellitimide (PI) and naphthalene-1,8:4,5-bis-(dicarboximide) (NI), respectively, and X is either n-C8H17 or H. The torsional angles of the xanthene-imide single bonds orient the NI and PI acceptors approximately cofacial to one another. Femtosecond transient absorption measurements show that electron transfer from 1*ANI to NI occurs by nonbonded superexchange interactions that include contributions from both the n-C8H17 substituent on the NI acceptor and nearby solvent molecules. The structural rigidity of these compounds allows evaluation of the distance dependence of both charge separation and recombination. For charge separation, β = 1.1-1.3 Å-1, and evidence exists for solvent contributions to superexchange in BzCN. The distance dependence of the charge recombination rates also correlates strongly with contributions from solvent molecules mediating this process via a hole-transfer mechanism in toluene, where β = 0.3 Å-1. Our findings indicate that mediation of electron transfer by nonbonded interactions can compete effectively with electron transfer via bonded pathways.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry