Electron Transfer Involving Nonbonded Superexchange Interactions in Rigid Donor−Acceptor Arrays

Rigid intramolecular trichromophoric arrays based on a 4,5-diaminoxanthene bridge B were prepared in which the xanthene bridge orients A 2 relative to D, so that nonbonded interactions between 1 *D and A 2 can be studied. The electron donor, D, is 4-( N -piperidinyl)naphthalene-1,8-dicarboximide (ANI), and the acceptors A 1 and A 2 are pyromellitimide (PI) and naphthalene-1,8:4,5-bis(dicarboximide) (NI), respectively, and X is either n -C 8 H 17 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 -C 8 H 17 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.