The photoinduced ring opening reaction of benzo(2H)chromenes: a kinetic and thermodynamic approach

The pyran-ring opening process in the benzo(2H)chromenes depends on the degree and nature of vibronic excitation in the first two or three excited electronic states when fundamental, harmonic and combination bands of two optically active modes are selectively excited by monochromatic radiation. Based on a previously proposed model, these findings are interpreted as being the result of competition between vibrational relaxation and photochemistry at each vibronic level. In this work, the model is further expanded and a new equation is developed to describe the relaxation from combination bands, which allows the quantum yields of the individual processes which contribute to deactivation of vibronically excited levels to be determined. To gain more insights into the relaxation dynamics of the excited states, the Heisenberg uncertainty principle was applied to evaluate the upper limit values of the rate constants for the relaxation processes of the excited states from the widths of the vibronic peaks. The energy barriers between reactants and photoproducts were estimated by using an Arrhenius-type relationship. The low barriers that were found (∼2 kJ mol-1) satisfactorily explain the competition between the reaction path and the energy degradation processes.