Solving the Azobenzene Entropy Puzzle: Direct Evidence for Multi-State Reactivity

Abstract

A solution to the azobenzene “entropy puzzle” [J. Phys.: Condens. Matter201729314002] is provided. Previous computational studies of the thermal Z → E(back-)isomerization of azobenzene could not describe the experimentally observed large negative activation entropies. Here it is shown that the experimental results are only compatible with a more complicated multistate rotation mechanism that involves a triplet excited state. Using nonadiabatic transition state theory, close to perfect agreement is achieved between all calculated and experimental Eyring parameters. We also provide new experiments that indicate the presence of a noticeable external heavy-atom effect, which is a direct result of spin–orbit coupling effects being important in the proposed mechanism. These results suggest a reexamination of the mechanisms of related thermal double bond isomerizations in other systems in cases when an excited state of triplet (or other) multiplicity becomes thermally accessible during a rotation process.

Multifunctional fluorescent diarylethene: time-resolved study of photochemistry

“…like in the case of azobenzene and its derivatives. 20,[60][61][62][63] At the moment we have no approved mechanism of this photoprocess. However, the difference in the spectra of precursors supports the idea that pand ap-conformers of E-D1-o are involved in principally different photochemical reactions.…”

Singlet and Triplet Pathways Determine the Thermal Z/E Isomerization of an Arylazopyrazole-Based Photoswitch

“…This puzzle was finally resolved showing that the mechanism of isomerization requires the involvement of triplet states , otherwise neglected. The participation of triplet states in the thermal isomerization of azobenzene was actually proposed almost 20 years ago , but it has been largely overlooked while studying many azobenzene derivatives, with few recent exceptions. − ,, …”

“…Based on current literature about azobenzene, ,, the thermal isomerization of PATP should be also discussed in terms of inversion and rotation mechanisms involving singlet and triplet electronic states. Figure a shows plausible transition states (TSs) associated with the in-plane inversion of either the pyrazole or aryl moieties around their neighboring azo nitrogen (TS iPy with NNC angle α′ ≈ 180° and TS iAr with CNN angle α ≈ 180°) and the out-of-plane rotation around the azo-bond (TS r with CNNC dihedral angle d ≈ 90°), respectively.…”

“…Previous studies on heteroarylazoswitches have either neglected the pathway associated with TS r regardless of spin, ,, or considered rotation but neglected triplet states, ,,− or considered only the T 1 minimum (not the MECPs) to calculate the barrier, or just suggested the triplet mechanism qualitatively. , By contrast, the most recent work , on azobenzene argued that the thermal isomerization proceeds through the rotational triplet mechanism, Path rT1 . In this work, we want to investigate all four thermal isomerization pathways on the same footing in order to clarify whether the triplet pathway is also the dominant mechanism in PATP, as proposed for azobenzene.…”

Finding Excited-State Minimum Energy Crossing Points on a Budget: Non-Self-Consistent Tight-Binding Methods

“…To illustrate the capabilities of the introduced scheme, azobenzene provides a helpful benchmarking example. From theoretical studies, mainly, the MECIs and MECPs of azobenzene are known to be dependent on the CNNC dihedral twist of the molecule, and the S 0 / T 1 MECP and S 0 / S 1 MECI, in particular, are located at around 90°. − Performing a scan along the dihedral coordinate reveals the existence of such points at the semiempirical GFN0-xTB level, as shown in Figure .…”