Nr. Lokan et al., Solvent-mediated intramolecular electron transfer in U-shaped systems withdifferent "bite sizes", J AM CHEM S, 122(21), 2000, pp. 5075-5081
The rate of photoinduced charge separation is measured as a function of sol
vent for four donor (D)-bridge-acceptor (A) systems: DMN[10]DCV, DMN[10nb]D
CV, DMN[10cy]DCV, and DMAN[10cy]-DCV. In the first three members of this se
ries, the D/A pair is kept constant and contains the strong dimethoxynaphth
alene (DMN) donor which enables detection of electron transfer over a wide
range of solvent polarity. In the fourth member, DMN is substituted by a di
methoxyanthracene (DMAN) unit, which decreases the driving force for photoi
nduced 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 sigma bonds. The configuration of the bridge is, however,
varied to increase its bending in the series, which leads to center-to-cen
ter D/A distances decreasing from 13.4 Angstrom in the first system to 9.54
Angstrom in the second, and 7.50 Angstrom in the latter two. In DMN[10nb]D
CV, the rate of intramolecular charge separation over 9.54 Angstrom is alwa
ys smaller than that over 13.4 Angstrom in DMN[10]DCV, which is in line wit
h a dominant through-bond mechanism that is more efficient via an extended
array of sigma bonds. However, in DMN[10cy]DCV, the rate is as high as or e
ven higher than that in DTVIN[10]DCG. Although changes in the driving force
are also important, as shown, for example, by the dramatic rate decrease i
n DMAN[10]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 stro
ngly indicate an important contribution of through-solvent interaction acro
ss the 7.5 Angstrom D/A distance, which in principle allows the intercalati
on of a single solvent molecule in close contact with both D and A. At the
longer distance of 9.54 Angstrom in DMN[10nb]DCV, a smaller contribution of
through-solvent interaction can still be detected for polar aroInatic solv
ents but not for acetonitrile. The inherently discontinuous distance depend
ence of through-solvent interaction and its possible interesting dependence
on molecular structure and temperature are discussed.