Nonlinear photoprocesses in bichromophores. I. Two-photon and cooperative excitation and nonlinear quenching

We consider nonlinear photoprocesses in a bichromophore, that is, in a molecule consisting of two chromophoric groups (monomers). The phenomena considered include stepwise excitation and collective processes (summation of the excitation energy of two monomers in one of them and nonlinear quenching), as well as energy transfer between the monomers. A system of kinetic (balance) equations describing the evolution of level populations in a bichromophore is obtained. Unlike the equations used previously, which are quadratic in the populations, the equations obtained by us are linear. Nonetheless, these equations account exactly for the correlation between level populations of two monomers in a bichromophore, which is induced by energy transfer. Stationary level populations of a chromophore have been found. It is shown that the collective phenomena always deplete population of the first singlet but can either enhance or suppress excitation of the higher (second) singlet.