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 mixtures of polar and nonpolar solvents all had an activation energy of about 2 kcal/mol for a wide range of dielectric constants. This dissociative intramolecular electron transfer (DIET) is unusual because it is the first example where solvent configurational entropy changes are required to enable a large amplitude molecular distortion leading to a nonadiabatic electron transfer and ionic dissociation. This solvent gated intramolecular electron-transfer mechanism is supported by analysis of the preexponential and activation energy trends in dipolar aprotic solvent mixtures and alcohol solvents. The large amplitude motion is not separately measurable due to the slow gating rates, but viscosity effects on both the preexponential and the activation energy are analyzed to demonstrate consistency with a barrierless diffusion model having a structural dependence on electron-transfer rate. The rate has an inverse dependence on viscosity raised to the 0.53 power.