Ultrafast Photodynamics of an Azopyridine-Functionalized Iron(II) Complex: Implications for the Concept of Ligand-Driven Light-Induced Spin Change.

We report on the ultrafast photodynamics of an iron(II) complex with a photoisomerizable pentadentate azo-tetrapyridylamino ligand after irradiation with ultraviolet light. The results of femtosecond transient electronic absorption spectroscopy performed on the low-spin (LS) form of the title complex show that initial excitation of the ππ* state of the azopyridine unit in the ligand at λpump = 312 nm is followed by an ultrafast intersystem crossing (ISC) that leads to the formation of a metal-centered (MC) 5T state, in competition with the intended photoswitching of the azopyridine unit. Additional measurements carried out upon excitation of the singlet metal-to-ligand charge-transfer (1MLCT) transition at λpump = 455 nm suggest that this energy transfer occurs via an MLCT state. The resulting high-spin (HS) 5T state of the complex is metastable and recovers to the LS ground state with a time constant of ∼3 ns. The implications of these observations on the ligand-driven light-induced spin change concept are discussed.