Internal conversion in aromatic and N-heteroaromatic molecules

Abstract

A previous semi-empirical approach to the calculation of the rate of internal conversion, regarded as a tunnelling process, is reformulated on a sounder theoretical basis. Following Robinson and Frosch, tunnelling rates are correlated with Franck-Condon factors for the associated transition. The total Franck-Condon factor, S2max, is a product of three terms, associated respectively with skeletal stretching, CH stretching, and skeletal angle bending vibrations. The value of S2max may be controlled by one term only of the product, or by two or more (mixed tunnelling). Tunnelling rates should be slowed down by deuteration only when CH vibrations participate significantly; specific predictions are made here. Skeletal-angle bending vibrations are of negligible importance in internal conversion in aromatic molecules, but they are very significant in transitions between states of π,π* and n,π* type in heteroaromatics. Correlation between S2max and estimated tunnelling rates is encouraging for four aromatic molecules and certain monocyclic azines; but the failure of pyridine and pyridazine to show luminescence is still unexplained. The case of pyrazine is discussed in detail.

Ultraviolet photodissociation action spectroscopy of the N-pyridinium cation

“…Deuteration is know to greatly decrease the rate of radiationless transitions 72 and it has been shown that skeletal bending and stretching vibrations are important participants in the internal conversion of aromatic nitrogen heterocycles. 73 This observation may be consistent with an IC-mediated process.…”

From Jabłoński To Femtoseconds. Evolution of Molecular Photophysics

“…One negative aspect in several cases of stated theories [32,35,37,40,41] is the assumption that vibrational relaxation is rapid compared with radiationless transition rates. Thus, the slower processes, e.g., S1 S0 and S1 T1 and T1 S0 are covered by their theories, but not Sn -S1 or Tn T1 .…”

“…In this case also ultrafast lasers have a role in defining the mechanism of excitation in specific cases. Not only is this of interest in the case of aromatic hydrocarbons, in which there is an observable competition between S1 and T1 state excitations [16,32], but the case of N-heteromatic molecules poses additional complexities in competitive excitation dynamics [35].…”

“…-Ross and co-workers theory [32] of solely intramolecular radiationless transitions, focussing on tunneling phenomena and Franck-Condon factors [35]. -Siebrand's study [40] of Franck-Condon factors in the Robinson-Frosch theory.…”

Magnetic Circular Dichroism of Molecules in Dense Media: Benzene

“…It should be noted that to obtain Eq. (8), we assumed that the change in normal frequencies between the two electronic states is negligible and that the normal vibration is harmonic, assumptions that are reasonable for the A,0g 992 cm-' mode (11,12).…”