Combination of redox- and photochemistry of azo-conjugated metal complexes

Recent studies on azobenzene-combined photochromic metal complexes in which the redox chemistry of the metal complex moiety and photochemistry of the azobenzene moiety are coupled, are reviewed. Tris(azobenzene-bound bipyridine)cobalt compounds, [Co(p-abbpy)3](BF4)n and [Co(m-abbpy)3](BF4)n (n = 2, 3) (p-dbbpy and m-dbbpy are 4-(4-(4-tolylazo)phenyl-2,2′-bipyridine and 4-(3-(4-tolylazo)phenyl-2,2′-bipyridine, respectively), undergo reversible trans-cis isomerization using a combination of the CoIII/CoII redox change and single UV light (366 nm) irradiation exciting the π-π* transition. The trans-cis conversion yield is higher for the meta isomers, [Co(m-abbpy)3](BF4)n, than for the para isomers, [Co(p-abbpy)3](BF4)n. The trans-cis photoisomerization behavior of 4-[4-(tolylazo)phenyl]-6,6′-dimethyl-2,2′-bipyridine, dmabbpy, is synchronized with coordination of the bipyridine moiety to copper, and the trans/cis isomerization can be controlled reversibly through the CuII/CuI redox process and single UV light (365 nm) irradiation. 3-Ferrocenylazobenzene (3-FcAB), undergoes reversible trans-cis isomerization using a single green light (546 nm) source and the ferrocenium/ferrocene redox change. These results indicate that several kinds of photo-redox combined multi-functionalities can be realized for the azo-conjugated transition metal complexes by tuning the interaction between the azo moiety and the metal complex unit.