Multi-mode molecular switching properties and functions of azo-conjugated metal complexes

Recent Studies on the azo-group-combined photochromic metal complexes are reviewed. These studies show unique trans-cis isomerization behaviors that have not been observed in regular organic azobenzenes. In the case of azobenzene-bound bis(terpyridine) complexes of transition metals, photoisomerization behavior depends strongly on the central metals, counterions, and solvents. Rh and Co complexes undergo photoisomerization smoothly, but the isomerization of Ru and Fe complexes is substantially retarded Clue to the energy transfer from the pi-pi* excited state to the MLCT transition. The photoluminescence property of an azobenzene-attached Pt complex is switched by photoisomerization of the azo moiety. Tris(azobenzene-bound bipyridine)cobalt undergoes reversible trans-cis isomerization using a combination of the Co-III/Co-II redox change and a single UV light source exciting the pi-pi* transition. The trans-cis conversion yield is higher for the meta isomer (with respect to the position of the azo group against the pyridyl group) than for the para isomer. The trans-cis photoisomerization behavior of a 6,6'-dimethyl-substituted azobenzene-bipyridine ligand is synchronized with coordination of the bipyridine moiety to copper, and the trans/cis isomerization can be controlled reversibly through Cu-II/Cu-I redox and a single UV light irradiation. Photoreaction of azoferrocene occurs not only by UV-light irradiation but also by green light irradiation that excites the MLCT transition. 3-Ferrocenylazobenzene undergoes reversible trans-cis isomerization using a single green light source and the Fe-III/Fe-II redox change. Azo-conjugated metalladithiolenes of Ni-II, Pd-II, and Pt-II with diphenylphosphinoethane Lis a co-ligand show facile photoisomerization. The energy of the reversible trans-to-cis photoisomerization is considerably lower than that of azobenzene. The thermal stability of the cis form is, however, much higher than that of the organic azobenzene derivatives showing similar low-energy trans-to-cis photoisomerization. A novel proton response of the azo group occurs, and the combination of photoisomerization and protonation leads to a novel proton-catalyzed cis-to-trans isomerization, These results indicate that several kinds of multi-photo-functionalities can be realized for the azo-conjugated transition metal complexes by tuning the interaction between the azo moiety and the metal complex unit.