Electrocatalytic Z → E Isomerization of Azobenzenes

Abstract

A variety of azobenzenes were synthesized to study the behavior of their E and Z isomers upon electrochemical reduction. Our results show that the radical anion of the Z isomer is able to rapidly isomerize to the corresponding E configured counterpart with a dramatically enhanced rate as compared to the neutral species. Due to a subsequent electron transfer from the formed E radical anion to the neutral Z starting material the overall transformation is catalytic in electrons; i.e., a substoichiometric amount of reduced species can isomerize the entire mixture. This pathway greatly increases the efficiency of (photo)switching while also allowing one to reach photostationary state compositions that are not restricted to the spectral separation of the individual azobenzene isomers and their quantum yields. In addition, activating this radical isomerization pathway with photoelectron transfer agents allows us to override the intrinsic properties of an azobenzene species by triggering the reverse isomerization direction (Z → E) by the same wavelength of light, which normally triggers E → Z isomerization. The behavior we report appears to be general, implying that the metastable isomer of a photoswitch can be isomerized to the more stable one catalytically upon reduction, permitting the optimization of azobenzene switching in new as well as indirect ways.

Efficient Sensitized Z→E Photoisomerization of an Iridium(III)‐Azobenzene Complex over a Wide Concentration Range

“…Subsequent increase of the applied negative potential promotes the reduction of the electrochemically generated E ‐ 1 to the radical anion (bottom inset, Figure ) with the corresponding spectral signature appearing at 448 nm and 575 nm. The same behavior was observed for the reference compound Z ‐ 3 (Figure S19 in the Supporting Information), suggesting that the electrochemical reduction of the Z ‐AB moiety in 1 is indeed a catalytic process, as shown by us recently . This process can be promoted under an applied negative potential at the conditions employed in the SEC and CV measurements.…”

“…As implied from the top inset of Figure , at lower negative potentials a small reduction process occurs (top inset, Figure ), which, once initiated at very low current values, converts the Z ‐ 1 solution quantitatively to E ‐ 1 ,. Subsequent increase of the applied negative potential promotes the reduction of the electrochemically generated E ‐ 1 to the radical anion (bottom inset, Figure ) with the corresponding spectral signature appearing at 448 nm and 575 nm.…”

“…In the case of AB photoswitches, Laviron and Mugnier already in 1978 showed that in aprotic solvents the Z ‐isomer could be reduced to the radical anion, which rapidly isomerized to the E ‐isomer . These observations implied that catalytic amounts of electrons suffice to trigger quantitative Z → E conversion, which was recently proven by us and shown to be a general feature of AB photoswitches . Furthermore, controlling the electrochemical switching process by coupling it to an indirect source of electrons, for example, a photosensitizer, enabled the control of the electrocatalytic Z → E isomerization with light.…”

“…When the CV of compound Z ‐ 1 was measured, only the cathodic peak assigned to the pure E ‐isomer was detected . Following Hapiot's methodology, very fast scan rates were employed and combined with spectroelectrochemistry, to record the spectral evolution upon increasing applied negative potential for Z ‐ 1 (Figure ).…”

“…Furthermore, controlling the electrochemical switching process by coupling it to an indirect source of electrons, for example, a photosensitizer, enabled the control of the electrocatalytic Z → E isomerization with light. However, the efficiency of the system was found to be dependent on the concentration of the photosensitizer due to the intermolecular nature of the electron transfer process, requiring a threshold amount of the photosensitizer (10 mol % corresponding to 10 −4 m ) to exert the maximum electrocatalytic effect …”