Structural changes upon electronic excitation in 1,2-dimethoxybenzene from Franck-Condon fits of the fluorescence emission spectra

The altered electron distribution after electronic excitation gives rise to structural changes of the heavy atom frame of aromatic molecules. These changes can be influenced by the nature of substituents as well as by their relative position in the aromatic ring. We determined the changes of the structure of 1,2-dimethoxybenzene upon electronic excitation to the first excited singlet state, using a fit of line intensities from the fluorescence emission spectra obtained through various vibronic bands and the rotational constant changes upon excitation from rotationally resolved electronic spectroscopy. The structure fit showed that the benzene ring expands unsymmetrically upon excitation. Clearly, the ring expansion can to some extent be described as ortho-quinoidal. Additionally, a vibrational analysis of the fluorescence emission spectrum based on comparison to an ab initio normal mode analysis at the coupled cluster second order perturbation SCS-CC2/cc-pVTZ level of theory is presented. (C) 2020 Published by Elsevier B.V.