A Theoretical Study: Solvent Effects on the Electronic Excitations of (N,N)-bridged 4-Aminobenzonitriles

Rudolf Schamschule^1,+, Andreas B. J. Parusel¹, Gottfried Köhler^1,2

¹ Institut für Theoretische Chemie und Strahlenchemie, University of Vienna, Althanstr. 14, 1090 Vienna, Austria.
² Austrian Society for Aerospace Medicine - Institute for Space Biophysics, Lustkandlgasse 52, 1090 Vienna, Austria. ⁺ Corresponding author.

Keywords: Aminobenzonitriles, Charge transfer, Dual fluorescence, Solvent Effects

Abstract

The influence of polar environments on the excited state properties is investigated for a series of four to six membered N,N-bridged 4-aminobenzonitriles. The self consistent reaction field model (SCRF), with an extension for the description of solvent effects on the electronically excited states, is used in a semiempirical AM1 based method. The rotational isomerization of the alkyl-amino group, a pseudo Jahn-Teller mechanism, and the rehybridization at the cyano carbon are investigated as possible explanations for the observed occurrence of dual fluorescence. Based on these solvent calculations, fluorescence wavelengths are compared to experimental data from literature. The RICT (Rehybridization by Intramolecular Charge Transfer) and WICT (Wagging Intramolecular Charge Transfer) models are rejected as explanations of the dual fluorescence phenomenon for this class of donor-acceptor compounds. The TICT (Twisted Intramolecular Charge Transfer) is found to explain the dual fluorescence of each compound for itself. A sufficiently small energy gap is found between the S₁ and S₂ excited states, making vibronic coupling, as proposed by the pseudo Jahn-Teller hypothesis, an accurate hypothesis for the description of the dual fluorescence behaviour of these compounds.