VIBRONIC EFFECTS IN PHOTOCHEMISTRY - COMPETITION BETWEEN INTERNAL CONVERSION AND PHOTOCHEMISTRY

An emission spectroscopic investigation was carried out on molecules capable of undergoing photochemistry as well as on those that cannot. Where photochemistry does occur, the quantum yield of fluorescence (φF) depends upon the particular vibrational level excited within a vibrational manifold of an excited electronic state. That is, φF generally decreases with excitation into successively higher vibrational levels. The mechanism responsible for these results involves competition between internal conversion and photochemistry at each vibrational level within a vibrational manifold. Based on this mechanism, corresponding equations have been developed from which the ratio of the rate constant of internal conversion to that of photochemistry, kφIC/kφPC, can be evaluated. The value of this ratio is dependent upon the particular vibration involved as well as the particular excited electronic singlet state involved. It appears as if internal conversion may proceed only within a particular vibrational manifold.