SOLVENT EFFECTS ON THE ELECTRONIC-SPECTRUM OF REICHARDTS DYE

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 chemi cal calculations on some conformations of iphenyl-(2,4,6-triphenyl-1-p yridinium)-N-phenoxide betaine (Reichardt's betaine, RB), which exhibi ts negative solvatochromic effects. We have used the AMI Hamiltonian o f 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 differe nce in energy between them suggests that both forms may be present in solution, an observation confirmed by calculations on the spectra usin g the SCRF model: the superposition of the calculated spectra for thes e two forms matches the experimental spectra very well. For nonpolar s olvents, the general pattern consists of variation of Et(30) concurren t with variation of the dielectric constant. We have also carried out calculations for solvents which form specific (e.g., H-bond) binding t o the solute, namely methanol and water, using a supermolecule approac h. Our results are in excellent agreement with the experiment and pres ent an accurate description of the spectra. (C) 1994 John Wiley & Sons , Inc.