Stepwise versus direct long-range charge separation in molecular triads

Trifunctional electron donor-donor-acceptor molecules are described in whic h photoinduced charge separation, D-2-D-1-A* --> D-2-D-1(+)-A(-), is follow ed by a charge migration step D-2-D-1(+)-A(-) (CS1) --> D-2(+)-D-1-A- (CS2) , leading to a relatively long-lived charge-separated state. The rate of th e charge migration process could be determined in a range of solvents of lo w polarity. In benzene and dioxane, reversibility of the process allowed th e determination of the free energy difference between CS1 and CS2. The rela tive energy of the CS2 state is much lower than expected from simple electr ostatic models. An increase of the charge migration rate was found with inc reasing solvent polarity within a series of alkyl ethers or alkyl acetates. However, an apparent preferential stabilization of the CS1 state in acetat es 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 refere nce compound lacking the intermediate redox unit, direct long-range charge separation yielding a D-2(+)-bridge-A(-) charge-separated state can occur, but the yield is significantly lower than in the triads.