Ten-Membered Cyclic Azobenzene: Electrochemical Synthesis and Photochromic Properties

Abstract

Cyclic azobenzenes generally exhibit enhanced photophysical properties compared to conventional linear azobenzenes, such as visible-light activation and exceptional thermal stability. However, their broader application has been hindered by synthetic challenges. Herein, the electrochemical synthesis of a ten-membered cyclic azobenzene, 6,7-dihydrodibenzo[e,i][1,4,7,8]dioxadiazecine (DDEI), is reported through the reduction of 1,2-bis(2-nitrophenoxy)ethane (BNPOE) in the presence of CO₂. The reduction mechanism, involving an irreversible eight-electron process and CO₂ capture, is elucidated using in situ fourier transform infrared spectroscopy (FT-IR) spectroelectrochemistry, complemented by ¹³C NMR, ¹H NMR, and mass spectrometry analyzes. Both experimental results and density functional theory calculations show that DDEI undergoes highly efficient trans-to-cis isomerization under green light (500 nm) irradiation, achieving a remarkable yield of 97%, with both isomers exhibiting excellent thermal stability. In contrast to 8- and 9-membered cyclic azobenzenes and in line with conventional linear azobenzenes, the trans configuration of DDEI is more stable than the cis configuration. This combination of synthetic accessibility and superior photophysical properties makes DDEI a promising candidate for various applications, including those in living systems.