Catalytic Isomerization of Photoisomers of Merocyanine Derivatives in Protic Solventst

Abstract

The rate of thermal isomerization of merocyanine deriv atives and the mechanism of catalytic reaction with proton were investigated in various solvents. The rate contstants (kd), in methanol significantly depended on the number of sulfur atoms introduced into the merocyanine molecule. The activation energy decreased by ca.3.5 kcalmol^-1 per sulfur atom. The increase in the isomerization rate by addition of HC1 was proportional to t he proton activity in the solution as expressed by k_d=k_o+k_H⋅a_Hvu, where ko is the rate constant characteristic of the solvent used.
It was found that the activation energy of ko and kH was the same, since the activation energy of kd did not show any pH-dependence at all. This indicates that the catalytic intermediates in neutral and alkaline regions are protonated merocyanines accompanied by counter anions, MDH⁺…OR^-, rather than more weakly interacted species through hydrogen bonds, MD…HOR.
We proposed that a_o=k_o/k_H would be a convenient measure of the intrinsic proton activity in protic solvents. The possible application of the r49rocyanine photoisomers was also discussed as memory devices and proton probes for biological systems. t Spectroscopic Studies on Merocyanine Photoisomers. N.