Photochemical behavior of azobenzene-conjugated CoII, CoIII, and FeII bis(terpyridine) complexes

Azobenzene-conjugated mononuclear and dinuclear terpyridyl complexes of Co-II, Co-III, and Fe-II were synthesized, and their photoisomerization behavior was investigated. Co-II and Co-III complexes, [tpyCo(tpy-AB)]X-n and [(Cotpy)(2)(tpy-AB-tpy)]X-n (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 Co-II 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-to-cis isomerization process. The trans-cis isomerization behavior of Co-III complexes was observed only in the solvents with a low donor number such as 1,2-dichloroethane. Fe-II complexes, [tpyFe(tpy-AB)]X-n (X = PF6 or BPh4), exhibit slight trans-to-cis photoisomerization due to the energy transfer from the azobenzene moiety to Fe(tpy)(2) moieties.