Complex formation between symmetrical thiacyanine dyes and aromatic heterocycles: evidence for molecular recognition

Abstract

Complexes between three thiacarbocyanine dyes and imidazole, Im, benzimidazole, Bz, and 2-methylbenzimidazole, 2-MeBz, have been detected by perturbation of the dyes' fluorescence. In some cases 1 : 2, as well as 1 : 1 complexes, can be observed. This assignment has been confirmed by time-resolved laser flash spectroscopy, detected by transient absorption spectroscopy. Based on a preconception of complex geometry, we selected a series of α,ω-bis(2-benzimidazolyl)alkanes of chain length n, as complexing agents which we anticipated might exhibit molecular recognition towards the cyanine chromophores. A modicum of molecular recognition was indeed observed when ( n−4) matched the number of carbon atoms in the polymethine chain of the dye. Computational chemical studies using the Merck molecular force field and AM1 semi-empirical molecular orbital calculation were carried out to elucidate complex structure. Geometry of the Im and Bz complexes turned out to be quite different. The calculated geometry of the latter complexes turned out to be incompatible with the anticipated basis for molecular recognition. The computational studies suggested the basis for dye-complexing agent interaction in the ground state to be principally electrostatic, i.e., with little or no dispersion or charge transfer contributions.