Mechanism of hydrogen bond assisted cis-trans isomerization in the excited state of 2-naphthol

Stationary points on the ground and first two excited state potential energy surfaces of 2-naphthol and the hydrogen bonded complex 2-naphtholNH(3) have been located using an approximate method (time dependent density functional based tight-binding) based on time dependent density functional theory (TDDFT). For the obtained extrema, single point calculations at the full TDDFT level (PW916-311+G(**)) were performed for validation purposes. The energetical ordering of the cis and trans forms of 2-naphthol in the different electronic states is found to be in good agreement with available experimental data. Interestingly, also the optical spectrum is correctly predicted by TDDFT in contrast to calculations on unsubstituted acenes in the literature. Besides the stationary points, also the minimum energy paths connecting them are calculated using an adaptive nudged elastic band approach in order to estimate isomerization barriers. While the increase of barrier height going from the ground to the first excited state is already documented, the authors find as a new result a significant barrier reduction upon hydrogen bond formation in the S(2). The observed effect can contribute to the explanation of recent experiments on infrared laser induced cis-trans isomerization in 2-naphthol.