The overlooked NIR luminescence of Cr(ppy)₃

Abstract

The chromium(III) complex Cr(ppy)3, a structural analog of the green phosphorescent iridium(III) complex Ir(ppy)3 (from a metal-to-ligand charge-transfer state), emits even in solution at room temperature from a weakly distorted...

Molecular ruby: exploring the excited state landscape

“…Following the report of [Cr(ddpd) 2 ] 3+ the underlying design principles gave rise to a large number of highly luminescent Cr III complexes. 19,33–35,36 Current efforts are directed to shifting the emission energy of molecular ruby derivatives to lower or higher energy by ligand modifications 19,37 and by modification of the central metal and electron configuration from d 3 -Cr III to d 2 -V III . 38 Variation of the counter ions of [Cr(ddpd) 2 ] 3+ and surrounding matrix as well as shielding of the metal center by sterically demanding groups proved to be fruitful strategies to enhance quantum yields and lower oxygen sensitivity.…”

Complex-as-Ligand Strategy as a Tool for the Design of a Binuclear Nonsymmetrical Chromium(III) Assembly: Near-Infrared Double Emission and Intramolecular Energy Transfer

“…Chromium polypyridyl complexes have received increasing attention during the past decade because of their robustness and their long-lived near-infrared (NIR) emission. − Important efforts were provided to tune the spin-flip emission energy − and to extend the excited-state lifetime to the millisecond range at room temperature. − Recently, the chirality of Cr III complexes has been harnessed to implement circularly polarized luminescence (CPL), which results in impressive dissymmetry factors approaching those of rare-earth centers. ,,,− Even if chromium­(III) polypyridyl chromophores display appealing features as mononuclear complexes, their incorporation into polymetallic architectures is required for the exploitation of novel emission properties such as directional NIR to NIR downshifting , or CPL emission boosted by indirect sensitization . However, the synthetic pathways able to combine several chromium­(III) polypyridyl units within a single (supra)­molecule are scarce, and we are only aware of (i) the self-assembly of CrLnCr triple helices displaying molecular-based upconversion − and (ii) the design of centrosymmetrical binuclear chromium­(III) polypyridyl complexes separated by back-to-back bis­(terpyridine) bridging ligands showing weak-to-negligible intermetallic communication .…”

Doublet Ground State in a Vanadium(II) Complex: Redox and Coordinative Noninnocence of Tripodal Ligand Architecture

“…25−29 The low-lying 2 E excited state of d 3 complexes is classified as a spin-flip transition whose energy is approximately independent of ligand field strength but proportional to the interelectronic repulsion between its metal-centered electrons. 30,31 Weakening the interelectronic repulsion, via metal−ligand covalency, has been recently demonstrated by Wenger and Piguet 32 and Heinze 33 as a robust design principle to impact the energy of the 2 E state of a Cr 3+ complex. The ability to modulate the energies of states of different multiplicities in first-row transition metals impacts their ability to perform catalysis.…”

“…Attributable to the weakened covalency of their metal–ligand bonds, 3d complexes possess many low-lying excited states that can have distinct electronic spin multiplicities. While these near-degenerate states are advantageous with respect to molecular magnetism, − they convolute direct mechanistic pathways for catalytic transformations. − Octahedral Cr 3+ polypyridyl complexes have been demonstrated to facilitate photocatalytic and photoredox processes. − The low-lying 2 E excited state of d 3 complexes is classified as a spin-flip transition whose energy is approximately independent of ligand field strength but proportional to the interelectronic repulsion between its metal-centered electrons. , Weakening the interelectronic repulsion, via metal–ligand covalency, has been recently demonstrated by Wenger and Piguet and Heinze as a robust design principle to impact the energy of the 2 E state of a Cr 3+ complex. The ability to modulate the energies of states of different multiplicities in first-row transition metals impacts their ability to perform catalysis …”