[en] A photochemical and photophysical investigation has been carried out on 2,2-dimethyl-7,8-benzo-2H-chromene and 2,2-dimethyl-5,6-benzo-2H-chromene. Absolute quantum yields of fluorescence [Φ _F(n)] and photochemistry [Φ _PC(n)] have been determined and are functions of the individual states, modes, and vibrational levels (n) excited and thus, there are many quantum yields of each of the foregoing processes. We have found that photochemistry and vibrational relaxation (non-radiative decay) are competitive processes at each vibrational level of each mode of all states. Using photochemistry as a probe, we have determined: (1) the pathways of vibrational relaxation and photochemical processes and have quantitatively evaluated the competitive nature of these; (2) that vibrational relaxation only occurs within the mode excited, there is no crossing between different modes during relaxation; (3) significant radiationless relaxation occurs directly between any pure excited state, S _m(0), and the ground state; (4) the photochemical yield is largely determined by the mode excited, not the energy of excitation

[en] 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