Some Effects of Intramolecular Vibrational Energy Transfer in Complex Fluorescent Molecules

Abstract

The effects of intramolecular redistribution of vibrational energy on both the fate of an excited complex molecule and the appearance of its electronic spectra are discussed in terms of a three-dimensional energy surface. I t is suggested that continuous absorption be used a s a criterion for the application of classical methods to intra-and inter-molecular vibrational energy transfer in those cases where fluorescence emission is observed. INTRODUCTIONThe selection rules require that comparatively few vibrations of a polyatomic molecule are excited during an electronic transition (13) and that if the transition is allowed these are totally symmetric (8). Since totally symmetric vibratioils lead to the simultaneous rupture of more than one bond if excited sufficiently, it is concluded that ilormal dissociation into two fragments is brought about by non-totally symmetric vibrations, the ground state energy of which is preserved during the electronic transition (25). If a forbidden transition to the fluorescent state is observed owing to the excitation of a non-totally symmetric vibration a s in benzene (26) and naphthalene (24), this vibration is rarely excited sufficiently to cause direct dissociation. Thus the normal photodissociation of polyatomic molecules involves an internal transfer of energy from totally symmetric t o non-totally symmetric vibrations. T H E THREE-DINIENSIONAL E N E R G Y SURFACELet us make the generalization that the majority of vibrations of a complex molec~rle will not be excited during an electronic transition but will be associated with dissociation along one coordinate, whilst the few optically excited vibrations require a much higher energy for a dissociation which produces more than two fragments; these will be referred to as optically inert or dissociative vibrations and optically active vibratioils respectively. The energy of the former will be related to the vapor temperature T , whilst that of the latter will be determined by the energy h c~ of the absorbed quantum.If the vibrations of the excited n-atomic molecule are assumed to be harmonic, its total vibratiollal energy V is given by where vi and F~ are the level and frequeilcy of the ith vibration and V A and VD are the energy sums of optically active and optically inert vibrations respectively. If we now consider the molecule as a single harmonic oscillator we may write 1iWanttscript received Septe?nber 84, 1057.

The Photochemical Decomposition of Aryldiazonium Salts I—Stability and Quantum Yields

“…Sukigari and Kikuchi (8), on the basis of calculations of electronic states, have shown that the positive charge is mainly present on the nitrogen of the >C-N in the diazonium group. The C-N stretching mode is excited to a higher vibrational level (9) and, provided the energy of the transition state is greater than the C-N bond energy, nitrogen elimination can occur. Photochemical decompositions can then take place by radiation-induced transitions corresponding to transition moments along the long axis of the molecule.…”