Azo- and quinone-conjugated redox complexes - photo- and proton-coupled intramolecular reactions based on d-pi interaction

Novel photo- and proton-coupled behavior of azo- and quinone-conjugated metal complexes based on the d-pi electronic interaction is described. The quantum yield for photoisomerization of azobenzene-bridged bis(terpyridine) complexes significantly depends on the nature of the metal, counterions, and solvents. The trans-to-cis isomerization of azoferrocene proceeds not only by the photoexcitation of the pi-pi* transition band but also by the photoexcitation of the MLCT (d-pi* transition) band. The oxidation of the ferrocene moieties occurs at 0.3 V less positive potential, and the electronic interaction between the ferrocene moieties is significantly weaker in cis-azoferrocene than in trans-azoferrocene. In a vinylene-bridged ferrocene-hydroquinone complex, the oxidation of the ferrocenyl site promotes the elimination of the hydroxyl and the vinylene protons followed by intramolecular electron transfer to form an allene and a quinonoid structure. The protonation to carbonyl groups of the anthraquinone moiety in 1-ferrocenylethynylanthraquinone causes intramolecular electron transfer leading to a novel structural change into a eta(6)-fulvene-cumulene Fe(II) complex. (C) 2002 Elsevier Science B.V. All rights reserved.