Luminescent First-Row Transition Metal Complexes

Abstract

Precious and rare elements have traditionally dominated inorganic photophysics and photochemistry, but now we are witnessing a paradigm shift toward cheaper and more abundant metals. Even though emissive complexes based on selected first-row transition metals have long been known, recent conceptual breakthroughs revealed that a much broader range of elements in different oxidation states are useable for this purpose. Coordination compounds of V, Cr, Mn, Fe, Co, Ni, and Cu now show electronically excited states with unexpected reactivity and photoluminescence behavior. Aside from providing a compact survey of the recent conceptual key advances in this dynamic field, our Perspective identifies the main design strategies that enabled the discovery of fundamentally new types of 3d-metal-based luminophores and photosensitizers operating in solution at room temperature.

Figure 1

(a) Illustration of the overlap between metal d-orbitals and ligand orbitals. (b) Exemplary single configurational coordinate diagram for a 3d-metal complex (c) Exemplary single configurational coordinate diagram for a 4d- or 5d-metal complex. Q is a nuclear coordinate.

Figure 2

Red: Elements for which luminescent complexes have long been known but important conceptual progress has been made recently. Yellow: A few selected luminescent complexes were known for these elements, and a few more examples have been added recently. Green: New elements on the landscape of emissive molecular complexes in solution at room temperature. Gray: Many luminescent Zn^II complexes are known, but they typically feature ligand-centered emission. White: Elements for which room temperature emission in solution still remains a challenge and no recent reports have appeared. Spin states and appertaining oxidation states of so far known examples of luminescent molecular complexes in solution at room temperature are listed below the individual elements.

Figure 3

(a) Linear molecular structure of two-coordinate CAAC-Cu^I-amido complexes (at the top) and structures of exemplary CAAC and amido ligands. (b) Key frontier orbitals in CAAC-Cu^I-amido complexes and direction of charge transfer upon LLCT excitation. (c) Renner–Teller distortion upon MLCT excitation of linear d¹⁰ complexes. (d) Energy-level diagram illustrating emission processes of CAAC-Cu^I-amido complexes. (e) Molecular structures of linear Cu^I complexes based on cyclic mono- or diamidocarbene ligands. (f) Molecular structures of some exemplary CAArC-Cu^I complexes. R′ = H, CN. R″ = H, CN. R^a = CH₃, Ph. R^b = H, CH₃. ^CNCz, when R′ = CN and R″ = H.

Figure 4

(a) Tanabe–Sugano diagram for the d⁶-electron configuration in an octahedral ligand field. (b) Single configurational coordinate diagram involving some key electronic states in [Fe(bpy)₃]²⁺. (c) Molecular structures of the isocyanide ligands L^tBu, L^bi, L^pyr, and L^tri. (d) Generic structures of homoleptic tris(diisocyanide) and bis(triisocyanide) complexes. M = Cr⁰, n = 0; Mn^I, n = 1.^,, (e) Molecular structures of Ni^II complexes showing luminescence that strongly resembles ligand fluorescence (R = COOEt, COOMe; R′ = Me, CF₃). (f) Molecular structure of [Ni^II(dpb)(Cz)].

Figure 5

Molecular structures of (a) [M(PhB(MeIm)₃)₂]ⁿ⁺ (with M = Mn^IV, n = 2; M = Fe^III, n = 1; M = Co^III, n = 1)^,, and (b) [Fe(btz)₃]³⁺. (c) Tanabe–Sugano diagram for the d⁵ electron configuration in an octahedral ligand field. (d) Single configurational coordinate diagram with key electronic states in [Fe(btz)₃]³⁺. Molecular structures of (e) [Fe(Imp)₂]⁺ and (f) [Co(dgpy)₂]³⁺ (X = CH) as well as [Co(dgpz)₂]³⁺ (X = N).^,

Figure 6

Tanabe–Sugano diagrams for the d² (a) and the d³ (b) electron configurations in an octahedral ligand field. (c) Molecular structure of [M(ddpd)₂]³⁺; M = Cr^III or V^III.^,,−, (d) Molecular structure of [Cr(bpmp)₂]³⁺. (e) Molecular structure of [Cr(^Rdqp)₂]³⁺, where R = H, OMe, Br or C≡CH.^,, (f) Enantiomeric pair PP-[Cr(^Brdqp)₂]³⁺ (upper) and MM-[Cr(^Brdqp)₂]³⁺ (lower). The Cr^III ion is orange, N atoms are blue, and the two ^Brdqp ligands are in red and green, respectively. (g) Molecular structure of [Cr(tpe)₂]³⁺. (h) Molecular structure of [Cr(dpc)₂]⁺.

Figure 7

Recent key developments summarized graphically.