Solvent effects on the electronic spectrum of Reichardt's dye

Abstract

The extreme sensitivity of the absorption spectrum to small changes in the medium polarity has made Reichardt's dyes useful molecular probes in the study of micelle/solution interfaces and phospholipid bilayers. This work reports preliminary results of semiempirical quantum chemical calculations on some conformations of 2,6-diphenyl-(2,4,6-triphenyl-1-pyridinium)-N-phenoxide betaine (Reichardt's betaine, RB), which exhibits negative solvatochromic effects. We have used the AM1 Hamiltonian of Dewar in the geometry optimizations, and the Intermediate Neglect of Differential Overlap method parameterized for spectroscopy (INDO/S). For RB, two low-lying conformations have been found. The small difference in energy between them suggests that both forms may be present in solution, an observation confirmed by calculations on the spectra using the SCRF model: the superposition of the calculated spectra for these two forms matches the experimental spectra very well. For nonpolar solvents, the general pattern consists of variation of Et(30) concurrent with variation of the dielectric constant. We have also carried out calculations for solvents which form specific (e.g., H-bond) binding to the solute, namely methanol and water, using a supermolecule approach. Our results are in excellent agreement with the experiment and present an accurate description of the spectra. © 1994 John Wiley & Sons, Inc.