Semiempirical INDO/S study on the solvatochromism of merocyanine dyes

We report results of semiempirical intermediate neglect of differential ove rlap (INDO/S) calculations on the spectroscopy of 4-hydroxy-1-methylstilbaz olium betaine (HMSBB). Solvent effects were included using both the continu um and supermolecule approaches. The calculated spectra are compared with e xperimental ultraviolet-visible spectra for several solvents. The odd batho chromic shift of the low energy band (for nonpolar solvents) is tentatively discussed in terms of the contributions of different solvation mechanisms, and an assignment based on INDO/S calculated spectra is presented. We star t from the assumption that in solution, conformations other than the comple tely planar structure (A) are possible. For protic solvents, in which hydro gen bonding constitutes the main contribution to solvation, calculations us ing the planar fully optimized AMI geometry reproduce fairly well the obser ved spectra. The intense, pi --> pi* band is calculated close to 20,000 cm( -1), in good agreement with the experimental values in water and methanol. For the very polar formamide, the INDO/S+SCRF result is also very accurate, suggesting that formamide's strong polarity induces HMSBB to adopt the pla nar structure A. For polar solvents with nonspecific interactions such as a cetonitrile or dimethylformamide, the situation is not as clear. We specula te that the experimental numbers can be reproduced only by a very large sup ermolecule. Finally, for nonpolar solvents such as cyclohexane, benzene and CCI4, the experimental number of 16,141 cm(-1) (previously obtained from e xtrapolation of solvent mixtures by Jacques and coworkers) is reproduced by the INDO/S method only considering structures B and C. These results can b e considered a confirmation of Jacques' suggestion that the solvatochromism of HMSBB can be explained by the solvents inducing a geometry change in HM SBB. (C) 2001 John Wiley & Sons, Inc.