Highly Strained Iron(II) Polypyridines: Exploiting the Quintet Manifold To Extend the Lifetime of MLCT Excited States

Halogen substitution at the 6 and 6 '' positions of terpyridine (6,6 ''-Cl-2-2,2:6',2 ''-terpyridine = dctpy) is used to produce a room-temperature high-spin iron(II) complex [Fe(dctpy)(2)](BF4)(2). Using UV-vis absorption, spectroelectrochemistry, transient absorption, and TD-DFT calculations, we present evidence that the quintet metal-to-ligand charge-transfer excited state ((MLCT)-M-5) can be accessed via visible light absorption and that the thermalized (MLCT)-M-5,7 is long-lived at 16 ps, representing a > 100 fold increase compared to the (MLCT)-M-1,3 within species such as [Fe(bpy)(3)](2+). This result opens a new strategy for extending iron(II) MLCT lifetimes for potential use in photoredox processes.