Electron transfer involving nonbonded superexchange interactions in rigid Donor-Acceptor arrays

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.