Influence of Transition-Metal Ions on the Photoisomerization of a Pentadentate Azopyridine Ligand: Interplay of Charge-Transfer and Metal-Centered States.

The influence of ZnII, FeII, and CoIII metal centers on the photoisomerization of a pentadentate azopyridine ligand, a potential scaffold for ligand-driven light-induced spin change (LD-LISC) systems, is investigated. Steady-state UV-vis and NMR spectroscopy reveal a systematic increase in the cis-isomer fraction in the photostationary state from the FeII to the CoIII to the ZnII complex. Time-resolved electronic absorption spectroscopy links these trends to distinct competing ultrafast relaxation pathways. While the ZnII complex exhibits predominantly ligand-centered (LC) excited-state dynamics, the FeII analogue shows rapid population of low-lying metal-centered (MC) quintet states, as we reported previously. The isoelectronic CoIII complex exhibits an alternative relaxation pathway that partially restores the desired photoswitching behavior; however, weak signatures of a long-lived excited-state species remain detectable. Spectroelectrochemical measurements combined with quantum chemical calculations identify a 3MC state as the most likely origin of this residual signal. These findings further highlight the critical role of charge-transfer states in controlling photoswitching efficiency and underscore fundamental limitations in the design of LD-LISC systems.