A comparative study on the isomerization pathways of heteroatom (S, O)-bridged 9-membered and 8-membered cyclic azobenzene photoswitches

The current computational study, mostly based on the Mixed-Reference Spin-Flip Time-Dependent Density Functional Theory (MRSF-TDDFT) method, investigates two experimentally well-studied azobenzene-based photoswitches. The study highlights the differences in the photo-isomerization pathways of the 9-membered and 8-membered 2,2'-linked heteroatom-bridged cyclic azobenzene (AZB) systems, designated as R-AZB-(CH2-X-CH2) and AZB-(CH2-X) (where R = tert-butyl, H and X = S, O), respectively. As we move from the 9-membered to the 8-membered system, the stable ground state isomer switches from trans to cis. In R-AZB-(CH2-X-CH2), the strongly allowed S0-S2 transition from the trans-ground state predominantly populates the S2 state. This subsequently relaxes through the rotational and planar S2 minima. The latter minimum, a possible fluorescent geometry, appears along the N=N stretching mode, and it is connected to the S2/S1-conical intersection (CI) channel. The relaxation pathway along the S1 surface passes through a minimum and torsional S0/S1-CI to form the cis-photoproduct. A barrier of 4 kcal/mol has been detected along this path. In contrast, the forward isomerization of AZB-(CH2-X) initiates through equally allowed S0-S1 and S0-S2 transitions from the cis-ground state. No energy minimum (on the S2 or S1 surface) has been noticed along this cis → trans isomerization path, and a low barrier (∼1 kcal/mol) path leading to the S0/S1-CI has been identified. These indicate a comparatively more efficient isomerization pathway in the 8-membered ring system, which is in line with their reported superior photo-switching ability. In both cases, the reverse photo-isomerization pathways are barrierless. These reverse isomerization pathways corresponding to the thermal processes have also been investigated in this study for both these systems.