The catalysis of the [2 + 2] cycloreversion of the anti-o,o'-benzene dimer
1 and the syn-o,o'-naphthalene-benzene dimer 2 through thermal and photoind
uced electron transfer is studied using experimental and computational meth
ods. The reaction of the radical cations formed by electron transfer is at
least 10(5) times faster than the thermal background reaction. It is demons
trated that the photoinduced electron transfer catalyzed reaction proceeds
via an electron transfer sensitized pathway and that the observed inverse s
econdary deuterium isotope effect of 0.91 +/- 0.02 on the reaction is due t
o the equilibrium isotope effect on the electron transfer step. The relevan
ce of these findings on the mechanism of the electron transfer catalyzed [2
+ 2] cycloreversion of the biologically important cis,syn-cyclobutane-thym
ine dimer is also discussed.