Bis(tridentate) Iron(II) Complexes with a Cyclometalating Unit: Photophysical Property Enhancement with Combinatorial Strong Ligand Field Effect

A series
of bis­(tridentate) cyclometalated oligopyridine complexes [FeII(C∧N∧N)­(L)]+ have been synthesized via the reactions of [FeII(C∧Npy∧Npy)­Br­(PMe3)2] (1, HC∧Npy∧Npy = 6-(phenyl)-2,2′-bipyridine)
and [FeII(C∧Nppy∧Nisoquin)­Br­(PMe3)2] (2, HC∧Nppy∧Nisoquin = 3-(4,6-diphenylpyridin-2-yl)­isoquinoline) with tridentate ligands
L, affording [FeII(C∧Npy∧Npy)­(4′-R-tpy)]+ (3a–e, R = H, Cl, p-ClC6H4, p-MeOC6H4, p-tol, respectively; tpy = 2,2′:6′,2″-terpyridine),
[FeII(C∧Nppy∧Nisoquin)­(tpy)]+ (4), [FeII(C∧Npy∧Npy)­(6-(Ph2PCH2)-bpy)]+ (5, bpy = 2,2′-bipyridine), and [FeII(C∧Npy∧Npy)­(6-NHC-bpy)]+ (6, NHC = N-heterocyclic carbene). Complexes 3–6 exhibit bathochromically shifted absorption
bands, markedly cathodically shifted E1/2(FeIII/FeII) values, and narrowed electrochemical
band gaps in comparison to those of [FeII(tpy)2]2+, as revealed by UV–vis absorption spectroscopy
and cyclic voltammetry. DFT and TDDFT calculation studies revealed
strong mixing of MLCT with LLCT transitions as origins to the major
absorption bands. Complex 6 incorporating a cyclometalating
unit and an NHC donor exhibits panchromic absorption, and 5 shows markedly enhanced molar absorptivity in the visible region.
A femtosecond transient absorption spectroscopic study of 6, coupled with DFT and TDDFT calculations and spectroelectrochemical
measurements, gave evidence for the possible involvement of a 3MLCT state with a lifetime of 21.4 ps.