cis → trans photoisomerisation of azobenzene: a fresh theoretical look

The cis -> trans photo-isomerisation mechanism of azobenzene, after excitation to the n pi* and pi pi* states, is revisited using high-level ab initio 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 pi pi* state compared to the lower energy n pi* state is obtained, in close agreement with the most recent experimental values [Ladanyi et al., Photochem. Photobiol. Sci., 2017, 16, 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 trans -> cis photoisomerisation. In contrast to the trans -> cis scenario, S-1 -> S-0 decay does not occur at 'earlier' C-N-N-C angles along the central torsional coordinate after pi pi* excitation, as in the cis -> trans 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-2 surface, from which either cis or trans-azobenzene can be formed. While this well is accessible after both excitations, it is more easily accessed after pi pi* excitation - an additional 15-17% of photochromes, which under n pi* excitation would have exclusively formed the trans isomer, are trapped in this well after pi pi* excitation. The probability of forming the cis isomer when leaving this well is also higher after pi pi* 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 pi pi* excitation of cis-azobenzene, compared to n pi* excitation.