SOLVENT GATING OF INTRAMOLECULAR ELECTRON-TRANSFER

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.