The rates for ionic photodissociation of malachite green leucocyanide
to form cyanide ion and a malachite green carbonium ion were measured
as a function of solvent and temperature. The observed rates in mixtur
es of polar and nonpolar solvents all had an activation energy of abou
t 2 kcal/mol for a wide range of dielectric constants. This dissociati
ve intramolecular electron transfer (DIET) is unusual because it is th
e first example where solvent configurational entropy changes are requ
ired to enable a large amplitude molecular distortion leading to a non
adiabatic electron transfer and ionic dissociation. This solvent gated
intramolecular electron-transfer mechanism is supported by analysis o
f the preexponential and activation energy trends in dipolar aprotic s
olvent mixtures and alcohol solvents. The large amplitude motion is no
t separately measurable due to the slow gating rates, but viscosity ef
fects on both the preexponential and the activation energy are analyze
d to demonstrate consistency with a barrierless diffusion model having
a structural dependence on electron-transfer rate. The rate has an in
verse dependence on viscosity raised to the 0.53 power.