[Cr(ddpd)₂]³⁺: A Molecular, Water‐Soluble, Highly NIR‐Emissive Ruby Analogue

Abstract

Bright, long-lived emission from first-row transition-metal complexes is very challenging to achieve. Herein, we present a new strategy relying on the rational tuning of energy levels. With the aid of the large N-Cr-N bite angle of the tridentate ligand ddpd (N,N'-dimethyl-N,N'-dipyridine-2-ylpyridine-2,6-diamine) and its strong σ-donating capabilities, a very large ligand-field splitting could be introduced in the chromium(III) complex [Cr(ddpd)2](3+), that shifts the deactivating and photoreactive (4)T2 state well above the emitting (2)E state. Prevention of back-intersystem crossing from the (2)E to the (4)T2 state enables exceptionally high near-infrared phosphorescence quantum yields and lifetimes for this 3d metal complex. The complex [Cr(ddpd)2](BF4)3 is highly water-soluble and very stable towards thermal and photo-induced substitution reactions and can be used for fluorescence intensity- and lifetime-based oxygen sensing in the NIR.

A Molybdenum(0) Isocyanide Analogue of Ru(2,2′‐Bipyridine)32+: A Strong Reductant for Photoredox Catalysis

“…[5] Aside from such d 6 metal diimines,m any Pt II and Au I/III complexes have favorable luminescence properties, [6] but emissive complexes made from earth-abundant metals are more difficult to obtain. [7] Notable exceptions are complexes based on Zn II or Cu I , [8] but their MLCT excited states often undergo strong geometrical distortion, [9] and nonradiative relaxation to the ground state can be rapid.…”

Deuterated Molecular Ruby with Record Luminescence Quantum Yield

“…[1,2] Except for copper(I) complexes, [2] strongly luminescent molecular complexes of earth-abundant-metal ions were essentially unknown until very recently.N ear-IR (NIR) emission with mononuclear copper(I) complexes is still very rare. [4][5][6][7][8] Milsmann and co-workers reported aluminescent zirconium-(IV) complex bearing 2,6-bis(pyrrolyl)pyridine ligands. [3] Ah andful of examples of luminescent complexes based on earth-abundant-metal ions with d n electron configuration (n < 10) only became available in the last three years (Scheme 1).…”

“…They include ligand-to-metal charge transfer (LMCT), metal-to-ligand (MLCT), and spin-flip emitters. [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). [4] A unique low-spin hexacarbene iron(III) complex with LMCT emission has been reported by Wärnmark.…”

“…[4] A unique low-spin hexacarbene iron(III) complex with LMCT emission has been reported by Wärnmark. [4][5][6][7][8] [*] C. Wang displays asharp,unprecedentedly intense spin-flip emission in the NIR with aphotoluminescence quantum yield of F = 0.11 and al uminescence lifetime of t = 898 msi nH 2 Oa tr oom temperature. [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).…”

Blue‐Emissive Cobalt(III) Complexes and Their Use in the Photocatalytic Trifluoromethylation of Polycyclic Aromatic Hydrocarbons

“…[3] Many strategies have been used to improve the photophysical properties of these complexes with the most popular focusing on the manipulation of the energies of non-emissive 3 MC states relative to emissive triplet metal-to-ligand charge transfer ( 3 MLCT) states. Following these approaches,Ru II , [5,6] Cr III , [7] and Fe((III), low spin) [8] complexes with strongly s-donating tridentate ligands have been shown to have dramatically longer emission lifetimes.A nother popular strategy to induce emission in 1 st row transition metals is to use d [10] metal-ions to avoid nonemissive d-d transition, for example,Cu I , [9] Ni 0 , [10] and Zn II ; [11] however, their MLCT excited states often undergo strong geometrical distortion [12] and non-radiative relaxation to the ground state can be rapid. Following these approaches,Ru II , [5,6] Cr III , [7] and Fe((III), low spin) [8] complexes with strongly s-donating tridentate ligands have been shown to have dramatically longer emission lifetimes.A nother popular strategy to induce emission in 1 st row transition metals is to use d [10] metal-ions to avoid nonemissive d-d transition, for example,Cu I , [9] Ni 0 , [10] and Zn II ; [11] however, their MLCT excited states often undergo strong geometrical distortion [12] and non-radiative relaxation to the ground state can be rapid.…”