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 T1/2 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.