Electrochemical synthesis and photochromic properties of eight-member cyclic azobenzenes

Abstract

Cyclic azobenzenes are capable of undergoing reversible cis-trans isomerization upon visible light irradiation, making them promising candidates as molecular photoswitches for applications in biological systems. However, their synthesis remains challenging due to their unique structural features. In this study, two eight-membered cyclic azobenzenes, namely 12H-dibenzo[b,f][1,4,5]thiadiazocine (13) and 12H-dibenzo[b,f][1,4,5] oxadiazocine (14), were synthesized through electrochemical reduction of 2’-nitrobenzyl-2-nitrophenyl-sulfide (1) and 1-nitro-2-((2’-nitrobenzyl)oxy)benzene (2), respectively, in the presence of CO₂. The synthesis yielded high conversion rates (13: 85.98%, 14: 93.76%). Using in situ infrared spectroscopy, NMR, and MS, detailed reduction mechanisms were elucidated, revealing an irreversible 8-electron transfer process and CO₂ involvement in a four-step reaction pathway. UV-Vis spectroscopic analysis showed that both 13 and 14 undergo trans-to-cis isomerization upon exposure to yellow (520 nm) or red (630 nm) light, in agreement with theoretical calculations. Additionally, S0→S1 excitations of both cis and trans isomers are predominantly governed by n→π* transitions. It is found that the cis conformation of compounds 13 and 14 is more stable than the trans conformation. Under yellow or red light irradiation, they undergo rapid trans-to-cis isomerization. No photochemical fatigue was observed in the cyclic experiments at room temperature, indicating their excellent photostability and potential application in photopharmacology. This work offers a sustainable strategy for the synthesis of cyclic azobenzenes and CO₂ capture.