Investigation of the Nile Red spectra by semi-empirical calculations and spectrophotometric measurements

The spectroscopic behavior of Nile Red (NR) in solutions was investigated by spectrophotometric measurements and was calculated with the semiempirical intermediate neglect of differential overlap for spectroscopy (INDO/S) method. Two parametrizations implemented, respectively, by Pople and by Da Motta and Zerner, were employed; the solvent effects were simulated according to the dielectric continuum model [self-consistent reaction field (SCRF)], as well as with the explicit inclusion of solvent molecules. The calculations simulated reasonably well the experimental spectra and the intense solvatochromism of NR because the dipole moments were calculated to be higher in the first excited state than in the ground state. In addition, INDO/S calculations were carried out for partially optimized twisted conformers, whose electron donor moiety was orthogonal to the aromatic acceptor one and charge transfer was observed for excited states known as twisted intramolecular charge-transfer (TICT) states. In polar media, INDO/S+SCRF calculations drastically stabilized the TICT state, reducing its energy down to the first excited state. Despite this intense solute-solvent stabilization, it is still less stable than the first excited state of the nontwisted, fully optimized conformer, and the limitations of the semi-empirical methods could not predict its role, if any, on the spectroscopic behavior of NR, except for the fact that the solvatochromism was explained independent of its presence. (C) 2006 Wiley Periodicals, Inc.