A CIS study of the solvent effects on the electronic absorption spectra of push-pull ethylenes

The lowest energy electronic transitions of two push-pull compounds, 1,1-bisdimethylamino-2-cyano-2-para-fluorophenylethene (PP-F) and 1,1-bisdimethylamino-2-cyano-2-para-nitrophenylethene (PP-NO2) were investigated as a function of solvent polarity using experimental and quantum mechanical techniques. Ab initio calculations at the unrestricted Hartree-Fock (UHF) level were employed for structure optimization with the solvent effects simulated via a self-consistent reaction field (SCRF) using the Onsager model. The configuration interaction singles (CIS) method was used to calculate the transition energies using various solute-solvent interaction models. Experimentally, while the principal absorption for both compounds exhibited a red shift with increasing solvent polarity, PP-NO2 demonstrated a more pronounced shift. The predicted shifts from the CIS calculations were in good agreement with the experimental data. The different behavior for the two systems with solvent was attributed to qualitative differences in the nature of their electronic transitions. While the transition of PP-F was attributed to a pi(C=C) to pi*(C=C) transition, that of PP-NO2 was identified as an intramolecular charge transfer transition from pi(C=C) to pi*(-para-Ph-NO2). (C) 1998 Elsevier Science B.V.