ChemInform Abstract: Rotational Effects on Intramolecular Radiationless Transitions. The Story of Pyrazine

Abstract

The experimental rotational effects on the S1 (1B3u) excited state dynamics of pyrazine are briefly summarized. These Experiments impose severe constraints on the validity of the general radiationless transitions theory in the limit of intermediate level structure. A model presented accounted for all the problems confronted by the theory in terms of rotationally induced intra-triplet vibrational energy redistribution, This new type of intramolecular radiationless transition called vibrational crossing is an interstate process with intrastate vibronic coupling features. From this model two major predictions emerge, which are shown to be fulfilled. We demonstrate that coriolis coupling is an important and dominant vibronic coupling leading into intramolecular vibrational energy redistribution. The absorption spectrum, fluorescence excitation spectrum and the resulting emission quantum yield of pyrazine were studied using dye laser with 2 GHz resolution as a light source. The emission quantum yield decreased with the rotational state. The absorption contour of a single rotational state was broader than the excitation contour and a low quantum yield absorption is shown between adjacent rotational transitions. These experimental results combined with the quantitative fit of the inverse emission quantum yield and A+/A- were predicted by the vibrational crossing model and thus strongly support its validity.