Fluorescence Spectra of 7,8-Benzoquinoline Isolated in the Supersonic Jet Expansion - An Ab Initio Analysis

I. Deperasińskaa, J. Prochorowaand Y. Stepanenkob aInstitute of Physics, Polish Academy of Sciences, al. Lotników 32/46, 02-668 Warsaw, Poland bInstitute of Physical Chemistry and The Laser Center, Polish Academy of Sciences, ul. M. Kasprzaka 44, 01-224 Warsaw, Poland

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Received: 30 06 2004; in final form 16 08 2004;

The optimized equilibrium geometry of 7,8-benzoquinoline molecule in its first excited, S1, singlet state was computed with the use of ab initio RCIS/6-31G(d) method. It was found that the electronic transition to the first excited state in 7,8-benzoquinoline is not confined to the neighborhood of nitrogen atom, but is delocalized over entire aromatic rings system, much alike in the molecule of phenanthrene. With the optimized geometry of the ground and excited state of the molecule, the frequencies of the vibrational fundamental modes were computed, together with their displacement parameters (geometry changes of vibrations between the excited and the ground state). These frequencies are in good agreement with vibrational frequencies present in the fluorescence spectrum of 7,8-benzoquinoline observed recently under jet-cooling conditions in supersonic beam expansion. In comparison to the fluorescence spectrum of phenanthrene, the calculated, as well as experimental fluorescence spectra of 7,8-benzoquinoline contain much more vibrational features, and this increased vibronic activity is related to the symmetry break caused by the introduction of N-heteroatom into the aromatic ring system of phenanthrene.

DOI: 10.12693/APhysPolA.106.535 PACS numbers: 82.20.Wt, 31.15.Ar, 33.20.Tp