Computational Design and Synthesis of a Deeply Red-Shifted and Bistable Azobenzene

TLDR

The study designed a di‑ortho‑fluoro di‑ortho‑chloro azobenzene and computationally demonstrated its superiority over tetra‑ortho‑chloro azobenzene. Computational analysis of ortho‑chlorinated azobenzenes guided the design of this variant, which was then synthesized via 2‑fold C–H activation and characterized by UV–vis, NMR, and X‑ray spectroscopy. X‑ray data showed that trans‑tetra‑ortho‑chloro azobenzene is conformationally flexible, enabling red‑shifted geometries that extend the n→π* band, while the synthesized dfdc azobenzene exhibits near‑bistable isomers with a larger n→π* band separation, allowing high cis‑isomer yields under green/yellow and red light in the biooptical window.

Abstract

We computationally dissected the electronic and geometrical influences of ortho-chlorinated azobenzenes on their photophysical properties. X-ray analysis provided the insight that trans-tetra-ortho-chloro azobenzene is conformationally flexible and thus subject to molecular motions. This allows the photoswitch to adopt a range of red-shifted geometries, which account for the extended n → π* band tails. On the basis of our results, we designed the di-ortho-fluoro di-ortho-chloro (dfdc) azobenzene and provided computational evidence for the superiority of this substitution pattern to tetra-ortho-chloro azobenzene. Thereafter, we synthesized dfdc azobenzene by ortho-chlorination via 2-fold C–H activation and experimentally confirmed its structural and photophysical properties through UV–vis, NMR, and X-ray analyses. The advantages include near-bistable isomers and an increased separation of the n → π* bands between the trans- and cis-conformations, which allows for the generation of unusually high levels of the cis-isomer by irradiation with green/yellow light as well as red light within the biooptical window.