LCAO-MO-berechnungen an schwefelhaltigen π-elektronensystemen—XXXIV : Nichtbindende S,S-wechselwirkung in polymethinfarbstoffen

Sulfur-containing polymethine dyes exhibit anomalous chromophoric properties. The vinylene shift of the first two homologues is often considerably larger or smaller than the shift between higher members of the series. Moreover, the absorption wavelengths of non-symmetrical dyes with sulfur-containing groups, as calculated by the mean value rule differ from the experimental data, but not in the predicted manner.This deviation is caused by an interannular no-bond SS-interaction in the monomethine ion. This conclusion is supported by the spectra of appropriately-substituted dyes and is substantiated by theoretical considerations. Starting from the well-known symmetry properties of polymethine MO's the simple PMO-method reproduces correctly the effects of the p_π-type overlap between atomic orbitals of adjacent S atoms at long wavelength absorptions: The SS-interactions lead to a hypsochromic shift in monomethines with (4N + 2)π-electrons (N = 1,2…) and to a bathochromic shift in monomethines with (4N)π-electrons.