Photochemical Behavior of Azobenzene-Conjugated CoII, CoIII, and FeII Bis(terpyridine) Complexes

Azobenzene-conjugated mononuclear and dinuclear terpyridyl complexes of CoII, CoIII, and FeII were synthesized, and their photoisomerization behavior was investigated. CoII and CoIII complexes, [tpyCo(tpy-AB)]Xn and [(Cotpy)2(tpy-AB-tpy)]Xn (tpy-AB = C15N3H10-C6H4-N=NC6H5, tpy-AB-tpy = C15N3H10-C6H4-N=NC6H4-C15N3H10, X = PF6 or BPh4), exhibit trans-to-cis photoisomerization by irradiation at 366 nm, and this behavior is dependent on solvents and counterions. For the CoII complexes, BPh4 salts undergo cis-to-trans isomerization in propylene carbonate by both photoirradiation with visible light (435 nm) and heat, indicating that reversible trans-cis isomerization has occurred. [Co(tpy-AB)2](BPh4)2 shows a two-step trans-tocis isomerization process. The trans-cis isomerization behavior of CoIII complexes was observed only in the solvents with a low donor number such as 1,2-dichloroethane. FeII complexes, [tpyFe(tpy-AB)]Xn (X = PF6 or BPh4), exhibit slight trans-to-cis photoisomerization due to the energy transfer from the azobenzene moiety to Fe(tpy)2 moieties.