Decay pathways of charge-separated states in strongly fluorescent electron donor-acceptor compounds

The nonradiative decay pathways are studied of the 1-cyano-4-cyclohexylidenenaphthalene chromophore and electron donor acceptor compounds containing the same chromophore. It is argued that the rapid decay at room temperature of the locally excited singlet state of the electron acceptor chromophore involves twisting of the double bond via a low barrier, and a polar transition state. In non-polar solvents the charge transfer excited states also decay mostly via a thermally activated decay process, possibly via a similar pathway as the locally excited state of the acceptor unit. In more polar solvents, decay via direct charge recombination to the ground state obeys the energy gap law, but in contrast to the prediction of semiclassical Marcus theory an activation energy of 1-2 kcal mol(-1) is found.