Synthesis, characterization, and photochemical properties of azobenzene-conjugated Ru(II) and Rh(III) bis(terpyridine) complexes

We synthesized azobenzene-conjugated bis(terpyridine) Ru(II) and Rh(III) mo nonuclear and dinuclear complexes and investigated their photochemical prop erties on excitation of the azo pi-pi* pb upon 366 run light irradiation. T he Ru mononuclear complex underwent trans-to-cis photoisomerization to reac h the photostationary state with only 20% of the cis form, while the Ru din uclear complex did not isomerize at all photochemically. On the other hand, the mononuclear and dinuclear Rh complexes showed almost complete trans-to -cis photoisomerization behavior. Cis forms of the Rh complexes thermally r eturned to the trans form at a much slower rate than those of organic azobe nzenes, but they did not isomerize photochemical ly. The reduction potentia l of the cis forms was 80 mV more negative than that of the trans forms. Th e photoisomerization quantum yields of the Rh complexes were strongly depen dent on the polarity, viscosity, and donor site of the solvents as well as the size of the counterions. We investigated the photoisomerization process of these complexes using femtosecond absorption spectroscopy. For the Rh c omplexes, we observed S-n <-- S-2 and S-n <-- S-1 absorption bands similar to those of organic azobenzenes. For the Ru complexes, we observed very fas t bleaching of the MLCT band of the Ru complex, which indicated that the en ergy transfer pathway to the MLCT was the primary cause of the depressed ph otoisomerization. The electronic structures, which were estimated from ZIND O molecular orbital calculation, supported the different photochemical reac tion behavior between the Ru and Rh complexes.