The design and photophysics of a trinuclear iron triad

Abstract

Earth‑abundant alternatives to noble‑metal photosensitizers are urgently sought for solar‑driven catalysis and photoredox chemistry. This thesis describes the design, synthesis, and comprehensive characterization of a fully iron‑based triad that melds a pyrazine‑functionalized Fe(II) N‑heterocyclic carbene (Fe‑NHC) chromophore with two {Fe(CN)5} acceptor units. Guided by ligand‑field considerations, the strongly σ‑donating NHC scaffold raises the iron eg orbitals, while the pyrazine contributes low‑lying π* levels, collectively stabilizing the metal‑to‑ligand charge‑transfer (MLCT) manifold. Stepwise coordination of the pentacyanoiron fragments broadens the absorption cross‑section into the visible‑to‑near‑IR region and establishes an electronically coupled Fe-Fe-Fe architecture that facilitates energy transfer between metal centers. Femtosecond transient‑absorption spectroscopy reveals sub‑picosecond intersystem crossing, inter-ligand charge transfer that bridges multiple MLCT states, energy transfer between metal centers with 380nm and 500nm excitation and exhibits ~70 ps excited state lifetime with 650nm excitation, accompanied by a non‑decaying excited‑state absorption persisting beyond the 5 ns experimental window. These findings establish a modular strategy for constructing high‑performance, all‑iron photosensitizers and provide fundamental insight into inter‑metal electronic communication within polynuclear architectures.