Electron‐Transfer‐Catalyzed cis → trans Isomerization of 1,1′‐Azonorborane. Prototype of a reversible two‐stage storage system

Abstract

~~~~ ~ ~ESR and cyclic voltammetry investigations show that isomerization of the radical cation of cis-1,l'-azonorbornane (cis-1) to the trans-radical ion proceeds too fast in solution for direct investigation of the cis-radical ion even at -78". The facile isomerization of the radical cation is in agreement with PM3 calculations proposing an activation barrier of only 17 kJ/mol. As a consequence, quantitative cis + trans isomerization of 1,l'-azonorbornane can effectively be accomplished by addition of catalytic amounts of one-electron oxidants. This is the first evidence for a radical-cation-catalyzed cis -+ frans isomerization of azo compounds.

An Investigation into the Initial Degradation Steps of Four Major Dye Chromophores: Study of Their One-Electron Oxidation and Reduction by EPR, ENDOR, Cyclic Voltammetry, and Theoretical Calculations

“…This is compatible with the high and irreversible oxidation potential of +1360 mV vs Ag/AgCl, which indicates the low thermodynamic and kinetic stability of the AO7 radical cation. Only in rare cases, with protected aliphatic substituents, it has been possible to observe radical cations of intact azo compounds by EPR. − Photoinduced electron transfer has been shown to cause an immediate destruction of AO7 …”

Photo-Electroswitchable Arylaminoazobenzenes

“…The more general azobenzene Z → E switching by hole catalysis demonstrated by the Hecht group relies on transitory oxidation at the Z-azo linkage followed by rapid isomerization/reduction to avoid long resident lifetimes of the azo radical cations, which have limited stability. The first demonstration of ET catalyzed Z → E azo isomerization was reported by Gescheidt and co-workers for azonorbornane . Our RA approach to the Z → E azobenzene ET catalysis allows residence of the radical cation at the RA center on a longer time scale without concern for chemical degradation.…”

To Isomerize or not to Isomerize? E/Z Isomers of Cyclic Azobenzene Derivatives and Their Reactivity Upon One‐Electron Reduction

“…Accordingly, photo‐induced Z ↔ E photoisomerization causes activation of functionalities in systems containing bridged azo moieties as photoresponsive units. Moreover, it has been shown that azo compounds with a strained geometry and those with aliphatic substituents form rather persistent radical ions upon one‐electron reduction (or oxidation) [14,21–32] . Whereas it has been shown that electron transfer causes efficient and rapid Z ↔ E conversion in non‐bridged azo compounds with advantageous reduction potentials, [26,33–40] the changes in molecular configuration upon one‐electron reduction in bridged azobenzene derivatives have not been addressed.…”

“…Moreover, it has been shown that azo compounds with a strained geometry and those with aliphatic substituents form rather persistent radical ions upon one-electron reduction (or oxidation). [14,[21][22][23][24][25][26][27][28][29][30][31][32] Whereas it has been shown that electron transfer causes efficient and rapid Z $ E conversion in non-bridged azo compounds with advantageous reduction potentials, [26,[33][34][35][36][37][38][39][40] the changes in molecular configuration upon one-electron reduction in bridged azobenzene derivatives have not been addressed.…”