D2–D0 Fluorescence of pseudo-D3h symmetric radical cations in solution and their `invisibleʹ D1 state as a route to non-radiative decay

Photophysical properties of three radical cations with a pseudo-D3h electronic structure in acetonitrile were studied by laser flash photolysis and their tandem excitation to their D2 state. The theoretical calculations of their electronic levels by the INDO/1 method have indicated that their D1 and D0 states deviate from the degenerate state and D1←D0 transition has excitation energies of 0.5–1.1 eV with oscillator strengths of nearly zero. The rate constants for their non-radiative decays from the D2 state increased exponentially with the decrease of the calculated D2–D1 energy gap obeying the energy gap law.