Iron(II) bis-terpyridyl complexes have been synthesized which contain either fluorine ([Fe(dftpy)]2+), chlorine ([Fe(dctpy)]2+), or bromine ([Fe(dbtpy)]2+) at the 6 and 6” positions. All three complexes have been found to exhibit significant high-spin character due to substituent-induced intramolecular strain. The ground and excited state properties of these unique complexes have been explored using X-ray crystallography, SQUID magnetometry, UV-Vis absorption spectroscopy, spectroelectrochemistry, and transient absorption spectroscopy. The choice of halogen serves to alter the relative state energies as evidenced by the emergent spin-crossover phenomenon in [Fe(dftpy)2]2+. By altering the excitation wavelength, it is possible to selectively excite the high-spin or low-spin fraction and observe the dynamics of each manifold. Within the high-spin manifold, a long-lived quintet metal-to-ligand charge transfer state is accessible via visible light absorption at 400 nm. The lifetime of this state has been found to exceed 10 ps, a &gt; 100-fold improvement over the ~100 fs lifetime of the low-spin parent, iron(II) bis-terpyridine. Furthermore, the altering of the state energies manifests as a slowing of non-radiative charge transfer state decay with [Fe(dbtpy)2]2+ showing the largest effect and achieving a 17.4 ps MLCT lifetime. This promising result provides a novel pathway for extending iron(II) MLCT lifetimes for potential use in photocatalysis.

Tuning spin states and quintet MLCT excited states in Fe(II) polypyridines using sterically demanding substituents

254th National Meeting (2017)

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