Redox-conjugated reversible isomerization of ferrocenylazobenzene with a single green light

A new reversible isomerization cycle for meta-ferrocenylazobenzene accomplished by combination of a single green light (546 nm) and redox change between Fe(II) and Fe(III) was discovered. In the Fe(II) state, trans-to-cis isomerization proceeded upon the green light irradiation exciting the metal-to-ligand charge transfer (MLCT) in a high quantum yield (Phit-->c = 0.51) which exceeds that of azobenzene (Phit-->c = 0.12 (313 nm excitation)). The cis molar ratio reached 35% in the photostationary state. The oxidation to the Fe(III) state followed by irradiation with the same green light led to the cis-to-trans back-reaction to recover almost all of the trans-form. The "on-off switching" of the MLCT character played an important role in the redox-dependent response to the green light for the isomerization. The photoisomerization behavior of ferrocenylazobenzenes was strongly dependent on the substitution position of the ferrocenyl moiety on the benzene ring. The MLCT excitation was not effective for the trans-to-cis conversion in para-ferrocenylazobenzene. Time-dependent density functional theory (TD-DFT) calculations for meta- and para-ferrcenylazobenzene showed that the origin of the visible band (MLCT band) is different. The initial orbital for the MLCT in meta-ferrocenylazobenzene is delocalized over Fe and the Cp ring, while that in para-ferrocenylazobenzene is localized on the iron.