Luminescence and Light‐Driven Energy and Electron Transfer from an Exceptionally Long‐Lived Excited State of a Non‐Innocent Chromium(III) Complex

Abstract

Photoactive metal complexes employing Earth‐abundant metal ions are a key to sustainable photophysical and photochemical applications. We exploit the effects of an inversion center and ligand non‐innocence to tune the luminescence and photochemistry of the excited state of the [CrN6] chromophore [Cr(tpe)2]3+ with close to octahedral symmetry (tpe=1,1,1‐tris(pyrid‐2‐yl)ethane). [Cr(tpe)2]3+ exhibits the longest luminescence lifetime (τ=4500 μs) reported up to date for a molecular polypyridyl chromium(III) complex together with a very high luminescence quantum yield of Φ=8.2 % at room temperature in fluid solution. Furthermore, the tpe ligands in [Cr(tpe)2]3+ are redox non‐innocent, leading to reversible reductive chemistry. The excited state redox potential and lifetime of [Cr(tpe)2]3+ surpass those of the classical photosensitizer [Ru(bpy)3]2+ (bpy=2,2′‐bipyridine) enabling energy transfer (to oxygen) and photoredox processes (with azulene and tri(n‐butyl)amine).

Spin Crossover and Long‐Lived Excited States in a Reduced Molecular Ruby

“…One key to these properties lies in the strong‐field ligand ddpd forming six‐membered chelate rings. Photophysical and redox properties contrast those of the chromium(III) complexes [Cr(bpy) 3 ] 3+ , [Cr(phen) 3 ] 3+ , [Cr(tpy) 2 ] 3+ and [Cr(tpe) 2 ] 3+ featuring electron‐poor pyridine ligands (bpy=2,2′‐bipyridine, phen=1,10‐phenanthroline, tpy=2,2′:6′,2′′‐terpyridine, tpe=1,1,1‐tris(pyrid‐2‐yl)ethane) . Consequently, the latter complexes show ligand‐centered redox chemistry yielding for example, [Cr III (tpy ⋅− )(tpy)] 2+ , but no low‐spin chromium(II) .…”

“…Consequently, the latter complexes show ligand‐centered redox chemistry yielding for example, [Cr III (tpy ⋅− )(tpy)] 2+ , but no low‐spin chromium(II) . This ligand‐based redox chemistry also enables a rich photo‐redox chemistry . On the other hand, [Cr(ddpd) 2 ] 3+ gives the genuine chromium(II) complex [Cr(ddpd) 2 ] 2+ upon reduction .…”

“…The ligand‐centered photoredox chemistry of [Cr(bpy) 3 ] 3+ , [Cr(phen) 3 ] 3+ , [Cr(tpy) 2 ] 3+ and [Cr(tpe) 2 ] 3+ contrasts the metal‐centered redox and photoredox chemistry of [Cr‐ (ddpd) 2 ] 3+ . This implies different electronic couplings of the various chromium(III) complexes and a reducing agent and consequently different kinetic barriers to the electron transfer.…”

“…Finally, to probe conceivable long‐lived excited states of [Cr(ddpd) 2 ] 2+ at low and high temperature, step‐scan FT‐IR spectra of KBr pellets of [Cr(ddpd) 2 ][BF 4 ] 2 were recorded at 290 K (ca. 50 % low‐spin) and at 20 K (>98 % low‐spin) between 0–750 ns after excitation with a 355 nm pulse (Figure , Supporting Information, Figure S16).…”

Luminescence and Light‐Driven Energy and Electron Transfer from an Exceptionally Long‐Lived Excited State of a Non‐Innocent Chromium(III) Complex

“…In all Figure S6. [89] 6 ]c oordination sphere as well. [45] Counterions and acetonitrile molecules occupy the six pockets spanned by the pyridyl rings of the tripodal ligands.…”