Solvent-Mediated Intramolecular Electron Transfer in U-Shaped Systems with Different “Bite Sizes”

The rate of photoinduced charge separation is measured as a function of solvent for four donor (D)−bridge−acceptor (A) systems:  DMN[10]DCV, DMN[10nb]DCV, DMN[10cy]DCV, and DMAN[10cy]DCV. In the first three members of this series, the D/A pair is kept constant and contains the strong dimethoxynaphthalene (DMN) donor which enables detection of electron transfer over a wide range of solvent polarity. In the fourth member, DMN is substituted by a dimethoxyanthracene (DMAN) unit, which decreases the driving force for photoinduced charge separation by about 0.58 eV and thereby limits the occurrence of electron transfer to polar solvents. In all systems the bridge is held at a length of 10 σ bonds. The configuration of the bridge is, however, varied to increase its bending in the series, which leads to center-to-center D/A distances decreasing from 13.4 Å in the first system to 9.54 Å in the second, and 7.50 Å in the latter two. In DMN[10nb]DCV, the rate of intramolecular charge separation over 9.54 Å is always smaller than that over 13.4 Å in DMN[10]DCV, which is in line with a dominant through-bond mechanism that is more efficient via an extended array of σ bonds. However, in DMN[10cy]DCV, the rate is as high as or even higher than that in DMN[10]DCV. Although changes in the driving force are also important, as shown, for example, by the dramatic rate decrease in DMAN[10cy]DCV as compared to that of DMN[10cy]DCV, the high rates observed for DMN[10cy]DCV in polar aromatic solvents as well as in acetonitrile strongly indicate an important contribution of through-solvent interaction across the 7.5 Å D/A distance, which in principle allows the intercalation of a single solvent molecule in close contact with both D and A. At the longer distance of 9.54 Å in DMN[10nb]DCV, a smaller contribution of through-solvent interaction can still be detected for polar aromatic solvents but not for acetonitrile. The inherently discontinuous distance dependence of through-solvent interaction and its possible interesting dependence on molecular structure and temperature are discussed.