Does the radiationless relaxation of Soret-excited metalloporphyrins follow the energy gap law?

The model metalloporphyrin, ZnTPP, has been excited in its Soret absorption band at 400 nm in several solvents and its decay dynamics measured by fluorescence upconversion. The linear relationship between the Lorenz–Lorentz polarizability function and S2–S1 electronic energy gap strongly suggests that the only significant radiative state is the 2(1Eu) state directly populated on Soret excitation. The energy gap law of radiationless transition theory has been shown to apply to the limited case of the relaxation rates of the S2 state of ZnTPP in these fluid solvents at room temperature, but does not apply to other zinc porphyrins.