Proton transfer across four-member hydrogen bonded network prefers over six-member hydrogen bonded network: Lactim-Lactam vs Imine-Amine photoisomerization

In this article, lactim-lactam vs imine-amine photoisomerization in two molecules namely 3-(benzo[d]thiazol-2-yl)pyridin-2-ol (BTPO) and 3-(1H-benzo[d]imidazol-2-yl)pyridin-2-ol (BIPO) has been studied using steady-state and time-resolved spectroscopic methods. The solved structure obtained from single crystal XRD data, revealed the lactam form of these molecule as the most stable structure which was further supported by theoretical calculation using Density Functional Theory (DFT) method. Experimental results are in favour of preferential occurrence of lactim-lactam tautomerism over imine-amine tautomerism. From structural view point, as evident from the crystallographic structure, the proton acceptor nitrogen atom involving imine-amine tautomerism is oriented opposite to the proton donor oxygen atom hence structural inhibition plays a major role in the determining the preferentiality of the two competitive proton transfer pathways. The potential energy curves for the two different proton transfer paths in the ground and excited state were computed using Density functional theory (DFT) at B3LYP/6-311++g(d,p) level to corroborate with experimental findings.