Thermal isomerization of phenylazoindoles: Inversion or rotation? That is the question

Azoheteroarenes represent an attractive group of photochromes exhibiting a large structural variability and tunability of photoswitching characteristics. The thermal back-isomerization can proceed via inversion or rotation mechanisms, depending on the functionalization and environment. However, the distinction between the two remains a challenge for both experiment and theory. Here, four experimentally fully characterized phenylazoindoles are studied to establish the mechanism of back-reaction in solvent using density functional theory (DFT), spin-flip time-dependent (TD-)DFT, mixed-reference TD-DFT, and restricted ensemble Kohn-Sham approaches as well as CASPT2 and CCSD(T). While the inversion is consistently described by all methods, the rotation mechanism requires multireference approaches including dynamic correlation. The balanced description of both pathways becomes even more important in solvent which apparently affects the mechanism. For the present set, the range-separated functionals combined with continuum models appear to be the most consistent with experiment in terms of the substitutional and solvent effects on thermal halftimes.