A Synthetically Tunable System To Control MLCT Excited-State Lifetimes and Spin States in Iron(II) Polypyridines

2,2':6',2″-Terpyridyl (tpy) ligands modified by fluorine (dftpy), chlorine (dctpy), or bromine (dbtpy) substitution at the 6- and 6″-positions are used to synthesize a series of bis-homoleptic Fe(II) complexes. Two of these species, [Fe(dctpy)₂]²⁺ and [Fe(dbtpy)₂]²⁺, which incorporate the larger dctpy and dbtpy ligands, assume a high-spin quintet ground state due to substituent-induced intramolecular strain. The smaller fluorine atoms in [Fe(dftpy)₂]²⁺ enable spin crossover with a T_1/2 of 220 K and a mixture of low-spin (singlet) and high-spin (quintet) populations at room temperature. Taking advantage of this equilibrium, dynamics originating from either the singlet or quintet manifold can be explored using variable wavelength laser excitation. Pumping at 530 nm leads to ultrafast nonradiative relaxation from the singlet metal-to-ligand charge transfer (¹MLCT) excited state into a quintet metal centered state (⁵MC) as has been observed for prototypical low-spin Fe(II) polypyridine complexes such as [Fe(tpy)₂]²⁺. On the other hand, pumping at 400 nm excites the molecule into the quintet manifold (⁵MLCT ← ⁵MC) and leads to the observation of a greatly increased MLCT lifetime of 14.0 ps. Importantly, this measurement enables an exploration of how the lifetime of the ⁵MLCT (or ⁷MLCT, in the event of intersystem crossing) responds to the structural modifications of the series as a whole. We find that increasing the amount of steric strain serves to extend the lifetime of the ^5,7MLCT from 14.0 ps for [Fe(dftpy)₂]²⁺ to the largest known value at 17.4 ps for [Fe(dbtpy)₂]²⁺. These data support the design hypothesis wherein interligand steric interactions are employed to limit conformational dynamics and/or alter relative state energies, thereby slowing nonradiative loss of charge-transfer energy.