Weak acid triggers the ring opening of an otherwise long-lived triangle terthiazole closed isomer

Abstract

The stability of the colored cyclized form of a compound from the new class of photochromic triangle terthiazoles was found to be strongly decreased by the presence of trifluoroacetic acid (TFA) in acetonitrile at room temperature. The apparent first-order process rate constant of the observed bleaching is of the same order of magnitude as if the acid-free sample was heated to about 100 °C. Association constants with TFA were determined from the kinetics analysis and by NMR titration and were found to be moderate for the colored form and very weak for the colorless form. These results were interpreted considering the basicity of the molecules and the solvent effect on the acid strength. The structural consequences of the protonation on the ring-closed compound and on the entire ring-opening process were examined by DFT calculations. Protonated closed and open forms were found to be geometrically “closer” than their non-protonated parents and the altered bond-length alternation may explain the diminution of the activation barrier. The puzzling instability of the protonated terthiazole was compared to a related thermally stable imidazolium homologue. From structural DFT considerations, it was concluded that this different behavior may be explained in term of charge stabilization, the sulfur being less efficient than the nitrogen.