Stability and Self-Association of styryl hemicyanine dyes in water studied by 1H NMR spectroscopy

The stability in water of five styryl hemicyanine dyes containing benzoxazole, benzothiazole, benzosele-nazole and oxazolopyridne rings with different substituents at different pH values has been investigated. All compounds are stable at acidic and neutral pH, however, at basic pH the oxazole dyes undergo a ring opening reaction. The stability at basic pH is increased by substituting the benzooxazolium with oxa-zolopyridium function in the oxazole dyes 3a and 3e. H-1 and H-1 DOSY solution NMR spectroscopy prove that slight structural changes result in significant differences in the process of self-association. The asso-ciation constants for dimerization have been determined by curve fitting the concentration dependent H-1 NMR spectra based on the isodesmic model. Van't Hoff analysis has been used to determine the thermo-dynamic parameters of the self-association process. The aggregation in water decreases with decreasing electronegativity and increasing the charge delocalization in the row (E)-2-(4-(dimethylamino)styryl)-3-methylbenzo[d]oxazol-3-ium iodide (3a), (E)-2-(4-(dimethylamino)styryl)-3-methylbenzo[d]thiazol-3-ium iodide (3b), (E)-2-(4-(dimethylamino)styryl)-3-methylbenzo[1,3-d]selenazol-3-ium iodide (3d) in the oxazole/thiazole/selenazole analogues. Introducing methyl groups in the benzothiazole ring enhances stacking effects in water due to stronger hydrophobic character. This derivative is characterized by the highest self-association constant (k(a) = 299 L mol-1) and the lowest Gibbs energy (delta G =-14 kJ mol(-1)). Substituting the phenyl with pyridyl ring in the oxazole analogue enhances significantly the solubility in water and thus decreases the self-association constant. Concentration dependent H-1 ROESY spectra provide information about the orientation of the molecules in the stack. (c) 2022 Elsevier B.V. All rights reserved.