Communication pubs.acs.org/JACS
Highly Strained Iron(II) Polypyridines: Exploiting the Quintet Manifold To Extend the Lifetime of MLCT Excited States Samuel G. Shepard,†,‡ Steven M. Fatur,†,‡ Anthony K. Rappé,§ and Niels H. Damrauer*,† †
Department of Chemistry and Biochemistry, University of Colorado Boulder, Boulder, Colorado 80309, United States Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States
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S Supporting Information *
derivatives.16 While it is argued that a LF is reached such that the T2 and 3T1 excited states switch their energetic ordering (a phenomenon that is rare), the deactivation of the MLCT remains ultrafast. In a different approach, Wärnmark et al.18,19 and then Gros et al.20 increased the LF using cyclometalated Fe(II) complexes based on tridentate ligands that incorporate two nheterocyclic carbenes. Both groups have now demonstrated efficient electron injection from related Fe(II) molecular sensitizers into TiO2,19,20 thus supporting the idea that MLCT excited states are not being deactivated on an ultrafast time scale. In acetonitrile (MeCN) solution phase studies of their sensitizers, where the ligands are functionalized with electronwithdrawing carboxylic acid substituents, record MLCT lifetimes of 16−18 ps are reported by both groups,19,20 representing a 360fold increase relative to [Fe(bpy)3]2+ (vide supra).10 We became interested in a different strategy for MLCT lifetime extension based on recent work in our group involving bis-tridentate Ru(II) systems where the terpyridyl ligands (6,6″Br2-2,2′:6′,2″-terpyridine) contain significant steric bulk at positions enhancing interligand repulsion.21 We argued based on constrained density functional theory (DFT) results that these interligand interactions hinder the kinds of motions that engender coupling between the MLCT and LF manifolds.21 However, as will be described in this current work, bis-tridentate Fe(II) complexes of the related ligand 6,6″-Cl2-2,2′:6′,2″terpyridine (hereafter called dctpy) do not adopt a low-spin ground-state structure. Rather, analogous to what has been seen by Constable et al. in related methyl and phenyl-substituted systems,22 interligand repulsions push [Fe(dctpy)2]2+ into a quintet ground state with no evidence of singlet character at room temperature. This situation presents questions of interest to us that have not, to our knowledge, been considered in the literature. Namely, can the 5MLCT manifold (which has been discussed in theoretical23−26 and low-temperature crystal work27) be directly accessed from a quintet ground state, and do the structural properties within the Franck−Condon state significantly alter its subsequent nonradiative decay dynamics? Herein we argue that MLCT excitation is indeed feasible and that its lifetime, determined from our exploration of this initial system, effectively matches the longest that is known for an Fe(II) polypyridyl system.19,20 The dctpy ligand was synthesized using a Suzuki coupling protocol shown to be effective for similar systems,28 wherein two chlorinated pyridyl boronic acid pinacol esters couple to a central
ABSTRACT: 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 (5MLCT) can be accessed via visible light absorption and that the thermalized 5,7MLCT 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.
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ontrolling nonradiative relaxation pathways is an essential component in the design of molecular systems that engage in the conversion of light to chemical or electrical potential. Ruthenium(II) polypyridyl complexes, and in particular 2,2′bipyridine (bpy) derivatives, have found broad use in applications tied to solar energy conversion1,2 as well as the burgeoning field of photoredox catalysis.3−5 This utility is due to the energetic ordering of metal-to-ligand charge-transfer (MLCT) excited states below ligand field (LF) excited states whose structural perturbations, via population of metal−ligand antibonding orbitals, provide facile pathways for energy loss. Iron(II) polypyridyl complexes, on the other hand, have lower energy LF excited states due to a decrease in metal−ligand covalency. With these in play, MLCT deactivation is extremely rapid.6−10 Whereas the 3MLCT of [Ru(bpy)3]2+ survives for ∼1 μs in room-temperature fluid solution, the 1,3MLCT manifold in [Fe(bpy)3]2+ is lost to a 3LF state in 150 fs, according to recent xray fluorescence measurements.8 That state is further deactivated to a lower energy 5LF state (∼25% of the 1MLCT energy) in 50 fs.8 Alternatively, recent transient absorption (TA) experiments argue that the 5LF state is impulsively populated in
