→ photoisomerisation of azobenzene: a fresh theoretical look.

The → photo-isomerisation mechanism of azobenzene, after excitation to the nπ* and ππ* states, is revisited using high-level surface hopping mixed quantum-classical dynamics in combination with multi-reference CASSCF electronic structure calculations. A reduction of photoisomerisation quantum yield of 0.10 on exciting to the higher energy ππ* state compared to the lower energy nπ* state is obtained, in close agreement with the most recent experimental values [Ladányi , , 2017, , 1757-1761] which re-examined previous literature values which showed larger changes in quantum yield. By direct comparison of both excitations, we have found that the explanation for the decrease in quantum yield is not the same as for the reduction observed in the → photoisomerisation. In contrast to the → scenario, S → S decay does not occur at 'earlier' C-NN-C angles along the central torsional coordinate after ππ* excitation, as in the → case the rotation about this coordinate occurs too rapidly. The wavelength dependency of the quantum yield is instead found to be due to a potential well on the S surface, from which either or -azobenzene can be formed. While this well is accessible after both excitations, it is more easily accessed after ππ* excitation - an additional 15-17% of photochromes, which under nπ* excitation would have exclusively formed the isomer, are trapped in this well after ππ* excitation. The probability of forming the isomer when leaving this well is also higher after ππ* excitation, increasing from 9% to 35%. The combination of these two factors results in the reduction of 0.10 of the quantum yield of photoisomerisation on ππ* excitation of -azobenzene, compared to nπ* excitation.