Issue 35, 2021

cistrans photoisomerisation of azobenzene: a fresh theoretical look

Abstract

The cistrans photo-isomerisation mechanism of azobenzene, after excitation to the nπ* and ππ* 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 ππ* state compared to the lower energy nπ* state is obtained, in close agreement with the most recent experimental values [Ladányi 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 transcis photoisomerisation. In contrast to the transcis scenario, S1 → S0 decay does not occur at ‘earlier’ C–N[double bond, length as m-dash]N–C angles along the central torsional coordinate after ππ* excitation, as in the cistrans 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 S2 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 ππ* excitation – an additional 15–17% of photochromes, which under nπ* excitation would have exclusively formed the trans isomer, are trapped in this well after ππ* excitation. The probability of forming the cis 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 cis-azobenzene, compared to nπ* excitation.

Graphical abstract: cis → trans photoisomerisation of azobenzene: a fresh theoretical look

Supplementary files

Article information

Article type
Paper
Submitted
28 apr 2021
Accepted
23 jun 2021
First published
01 jul 2021

Phys. Chem. Chem. Phys., 2021,23, 19155-19165

cistrans photoisomerisation of azobenzene: a fresh theoretical look

I. C. D. Merritt, D. Jacquemin and M. Vacher, Phys. Chem. Chem. Phys., 2021, 23, 19155 DOI: 10.1039/D1CP01873F

To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page.

If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given.

If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page.

Read more about how to correctly acknowledge RSC content.

Social activity

Spotlight

Advertisements