Stepwise versus Direct Long-Range Charge Separation in Molecular Triads

Abstract

Trifunctional electron donor−donor−acceptor molecules are described in which photoinduced charge separation, D2−D1−A* → D2−D1+−A-, is followed by a charge migration step D2−D1+−A- (CS1) → D2+−D1−A- (CS2), leading to a relatively long-lived charge-separated state. The rate of the charge migration process could be determined in a range of solvents of low polarity. In benzene and dioxane, reversibility of the process allowed the determination of the free energy difference between CS1 and CS2. The relative energy of the CS2 state is much lower than expected from simple electrostatic models. An increase of the charge migration rate was found with increasing solvent polarity within a series of alkyl ethers or alkyl acetates. However, an apparent preferential stabilization of the CS1 state in acetates relative to ethers leads to discontinuities in the solvatochromic shift behavior of the CT fluorescence from the CS1 state, and in the increase of the charge migration rate as a function of dielectric constant. In a reference compound lacking the intermediate redox unit, direct long-range charge separation yielding a D2+−bridge−A- charge-separated state can occur, but the yield is significantly lower than in the triads.