Hole Catalysis as a General Mechanism for Efficient and Wavelength-Independent Z → E Azobenzene Isomerization

Abstract

Whereas the reversible reduction of azobenzenes has been known for decades, their oxidation is destructive and as a result has been notoriously overlooked. Here, we show that a chain reaction leading to quantitative Z / E isomerization can be initiated before reaching the destructive anodic peak potential. This hole-catalyzed pathway is accessible to all azobenzenes, without exception, and offers tremendous advantages over the recently reported reductive, radical-anionic pathway because it allows for convenient chemical initiation without the need for electrochemical setups and in the presence of air. In addition, catalytic amounts of metal-free sensitizers, such as methylene blue, can be used as excited-state electron acceptors, enabling a shift of the excitation wavelength to the far red of the azobenzene absorption (up to 660 nm) and providing quantum yields exceeding unity (up to 200%). Our approach will boost the efficiency and sensitivity of optically dense liquid-crystalline and solid photoswitchable materials.

Oxidative and reductive cyclization in stiff dithienylethenes

“…As such, the bis(4-(9,9-dimethyl-9 H -fluoren-2-yl)phenyl)-substituted DTE-PhFluorene undergoes reductive as well as oxidative cyclization ( Fig. 5 ), a phenomenon that we have recently also observed for reductively and oxidatively induced azobenzene Z → E isomerization [ 44 – 45 ].…”

Interplay of arene radical cations with anions and fluorinated alcohols in hole catalysis

“…Recently, researchers proposed that radical cations from styrene derivatives react with alcohols and carbonyl groups to realize radical addition via hole catalytic manner, where carbonyl group works as a intermolecular radical acceptor for the first time 27 . Hole catalytic transformations of molecular switches, such as diarylethene or azobenzene, are also been reported 28,29 .…”

Electrocatalytic oxidative Z/E isomerization of a stilbene favoured by the presence of an electroactive persistent radical

“…In fact, electron-transfer-induced (E/Z) isomerizations have been demonstrated in hydrazone 11,12 or stilbene derivatives, either through the formation of radical cations 13,14 or radical anions. [15][16][17] Furthermore, fast isomerizations can be achieved when catalytic mechanisms involving radical species are activated, as proved for azobenzenes 18,19 and stilbene-based structures. 20 Consequently, recent advances in molecular machines point at geometric isomerizations triggered by redox reactions as an alternative to photoinduced stimulation.…”

“…), an electrocatalytic nature of the oxidative isomerization was considered, as equally described for stilbenes 20 or azobenzenes. 18,19 Finally, a high excess of oxidant (42 eq.) provoked a second color evolution when the orange/reddish solution of [(E)-PTM-DMA] turned into dark blue.…”