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″-Cl2-2,2:6',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 ((5)MLCT) can be accessed via visible light absorption and that the thermalized (5,7)MLCT is long-lived at 16 ps, representing a > 100 fold increase compared to the (1,3)MLCT 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.