Boosting Vis/NIR Charge‐Transfer Absorptions of Iron(II) Complexes by N‐Alkylation and N‐Deprotonation in the Ligand Backbone

Abstract

Reversing the metal-to-ligand charge transfer ( 3 MLCT)/metal-centered ( 3 MC) excited state order in iron(II) complexes is ac hallengingo bjective, yet would finally result in long-sought luminescent transition-metal complexes with an earth-abundant central ion. One approacht oa chieve this goal is based on low-energy charge-transfer absorptions in combination with as trong ligand field. Coordinating electron-rich and electron-poor tridentate oligopyridine ligands with large bite angles at iron(II) enables both low-energy MLCT absorption bands around 590 nm and as trong ligand field. Variations of the electron-rich ligand by introducing longer alkyl substituents destabilizes the iron(II) complex towards ligand substitution reactions while hardly affecting the optical properties. On the other hand, N-deprotonation of the ligand backbone is feasible andr eversible, yielding deep-green complexes with charge-transfer bands extending into the near-IRr egion.T ime-dependent density functional theory calculations assign these absorption bands to transitions with dipole-allowed ligand-to-ligand charget ransfer character.T his unique geometric and electronic situation establishes af urtherr egulating screw to increase the energy gap betweenp otentially emitting charge-transfer states and the non-radiativeligand field states of iron(II) dyes.

Is Iron the New Ruthenium?

“…[11] The approach of obtaining luminescent LLCT (insteado fM LCT) states in push-pull complexes with Fe II seems very promising with a variant of ddpd in which the aminesa re not methylated,b ut merely bear ah ydrogen atom. [21] Concept III:A ccessing 5…”

Deuterated Molecular Ruby with Record Luminescence Quantum Yield

“…[4][5][6][7][8] Milsmann and co-workers reported aluminescent zirconium-(IV) complex bearing 2,6-bis(pyrrolyl)pyridine ligands. [9] The" molecular ruby" 1 3+ Scheme 1. [5] Wengersd 6 metal(0) isocyanide chelate complexes display MLCT luminescence in solution (Scheme 1).…”

“…[6,7] Thec hromium(III) complex [Cr(ddpd) 2 ] 3+ (1 3+ ) [8] with ad 3 electron configuration is based on the tridentate strongfield ligand N,N'-dimethyl-N,N'-dipyridine-2-yl-pyridine-2,6diamine (ddpd) introduced into coordination chemistry by the Heinze group (Scheme 1). [9] The" molecular ruby" 1 3+ Scheme 1. Luminescent complexeswith earth-abundant-metal centers (d n electron configuration; n < 10;atroom temperature, in solution).…”

A Strongly Luminescent Chromium(III) Complex Acid

“…The NH group provides a chemically reactive unit, which should enable the electronic properties of the resulting complexes to be influenced by chemical means, resulting, for example, in luminescence switching. Homoleptic and heteroleptic H 2 tpda iron(II) complexes [Fe(H 2 tpda) 2 ] 2+ and [Fe(dcpp)(H 2 tpda)] 2+ have been reported (dcpp=2,6‐bis(2‐carboxypyridyl)pyridine) . Deprotonation to their conjugate bases [Fe(dcpp)(Htpda)] + and [Fe(dcpp)(tpda)] 0 leads to Vis/NIR absorbing dyes .…”

“…Homoleptic and heteroleptic H 2 tpda iron(II) complexes [Fe(H 2 tpda) 2 ] 2+ and [Fe(dcpp)(H 2 tpda)] 2+ have been reported (dcpp=2,6‐bis(2‐carboxypyridyl)pyridine) . Deprotonation to their conjugate bases [Fe(dcpp)(Htpda)] + and [Fe(dcpp)(tpda)] 0 leads to Vis/NIR absorbing dyes . Yet, deprotonation of H 2 tpda to tpda 2− prior to complexation to metal ions yields polynuclear linear chain complexes M 5 (tpda) 4 X 2 with chromium(II) and many other divalent and some trivalent metal ions, for example, Co 2+ , Ni 2+ , Cu 2+ , Ru 2+ , Rh 2+…”