Proton transfer across a four-membered hydrogen-bonded network is preferred over a six-membered hydrogen-bonded...

In this study, the competitive 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), is explored using steady-state and time-resolved spectroscopies. Single-crystal XRD data predict the presence of the most stable lactam structure in the solid state, which is further supported by theoretical calculations using the density functional theory (DFT) method. Experimental results support the occurrence of lactim-lactam tautomerism over imine-amine tautomerism. As is evident from the crystallographic structure, the proton acceptor nitrogen atom involving imine-amine tautomerism is oriented opposite to the proton donor oxygen atom, and hence, structural inhibition plays a major role in determining the direction of the two competitive proton transfer pathways. The potential energy curves for the two different proton transfer paths in the ground and excited state are computed using density functional theory (DFT) at the B3LYP/6-311++g(d,p) level in the gas phase and in acetonitrile solvent. Theoretically, a highly excited state proton transfer barrier (>25 kcal mol^-1) does not favor lactim-lactam isomerization via a concerted pathway. Even if the calculated barrier energy along the imine-amine isomerization is low (<3 kcal mol^-1) in the excited state, this channel is not allowed due to the high lactim to imine conversion barrier at room temperature.