Enhancing thermal isomerization rates of redox auxiliary-appended azobenzenes via redox auxiliary catalysis

Abstract

Azobenzene exists in E or Z isomeric forms having different three-dimensional structures, chemical properties, optical properties and electronic properties. Azobenzene can switch between E and Z conformations using light and has been incorporated in chemical systems to impart photoswitchable characteristics. Thermal conversion from the Z to E often occurs slowly and is unideal when fast dynamics is desired. ZE photoswitching is typically incomplete, leaving a significant percentage of molecules in the Z conformation. The Blackstock group has covalently attached an easily oxidized aryl amine “redox auxiliary” (RA) to the azobenzene scaffold to facilitate rapid and complete ZE conversion by adding a catalytic amount of oxidant. We hypothesize that the oxidized Z isomer of a RA-appended azobenzene (RA-azo) will rapidly isomerize to the E conformation because oxidizing the RA greatly reduces the energetic barrier to ZE isomerization. The newly formed (E) RA•+ azo molecule will oxidize a neutral (Z) RA azo, generating a (Z) RA•+ azo radical cation. This (Z) RA•+ azo will undergo rapid ZE isomerization to yield another (E) RA•+ azo molecule, which will oxidize another neutral (Z) RA azo. To increase the electron catalysis turnover number, we incorporated a new RA. This new RA-azo underwent complete and rapid ZE switching using a lower electrocatalytic loading than other RA-azos previously studied. Rapid ZE conversion was also achieved in a photo-electrical cell device (PED), and several EZE switching cycles were achieved using light and voltage in tandem. Attaching two azobenzene moieties to the new RA allowed for electrocatalytic ZE switching of both moieties. The azobenzene ZE thermal isomerization half-life is ~2 days, but adding the RA (or most other substituents) reduces this half-life to hours or minutes. By developing an RA-azo with ortho fluorines, we produced an RA-azo exhbiting an exceptionally long half-life (t1/2 = 40 days) and the capability to undergo rapid and complete ZE switching upon addition of 0.11 mol% oxidant. This results in a 6,900,000 fold ZE rate acceleration. Extrapolating to the case of 100 mol% oxidant gives a 6,300,000,000 fold ZE rate acceleration, which is by far the greatest RA-facilitated ZE rate acceleration observed for any RA-azo studied by the Blackstock group to date.