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).

Gaining chemical control over the thermodynamics and kinetics of photoexcited states is paramount to an efficient and sustainable utilization of photoactive transition metal complexes in a plethora of technologies. In contrast to energies of charge transfer states described by spatially separated orbitals, the energies of spin-flip states cannot straightforwardly be predicted as Pauli repulsion and the nephelauxetic effect play key roles. Guided by multireference quantum chemical calculations, we report a novel highly luminescent spin-flip emitter with a quantum chemically predicted blue-shifted luminescence. The spin-flip emission band of the chromium complex [Cr­(bpmp)2]3+ (bpmp = 2,6-bis­(2-pyridyl­methyl)­pyridine) shifted to higher energy from ca. 780 nm observed for known highly emissive chromium­(III) complexes to 709 nm. The photoluminescence quantum yields climb to 20%, and very long excited state lifetimes in the millisecond range are achieved at room temperature in acidic D2O solution. Partial ligand deuteration increases the quantum yield to 25%. The high excited state energy of [Cr­(bpmp)2]3+ and its facile reduction to [Cr­(bpmp)2]2+ result in a high excited state redox potential. The ligand’s methylene bridge acts as a Brønsted acid quenching the luminescence at high pH. Combined with a pH-insensitive chromium­(III) emitter, ratiometric optical pH sensing is achieved with single wavelength excitation. The photophysical and ground state properties (quantum yield, lifetime, redox potential, and acid/base) of this spin-flip complex incorporating an earth-abundant metal surpass those of the classical precious metal [Ru­(α-diimine)3]2+ charge transfer complexes, which are commonly employed in optical sensing and photo­(redox) catalysis, underlining the bright future of these molecular ruby analogues.

Sensitized triplet‐triplet annihilation upconversion (sTTA‐UC) mainly relies on precious metal complexes thanks to their high intersystem crossing (ISC) efficiencies, excited state energies, and lifetimes, while complexes of abundant first‐row transition metals are only rarely utilized and with often moderate UC quantum yields. [Cr(bpmp)2]3+ (bpmp=2,6‐bis(2‐pyridylmethyl)pyridine) containing earth‐abundant chromium possesses an absorption band suitable for green light excitation, a doublet excited state energy matching the triplet energy of 9,10‐diphenyl anthracene (DPA), a close to millisecond excited state lifetime, and high photostability. Combined ISC and doublet‐triplet energy transfer from excited [Cr(bpmp)2]3+ to DPA gives 3DPA with close‐to‐unity quantum yield. TTA of 3DPA furnishes green‐to‐blue UC with a quantum yield of 12.0 % (close to the theoretical maximum). Sterically less‐hindered anthracenes undergo a [4+4] cycloaddition with [Cr(bpmp)2]3+ and green light.

Photoactive metal complexes employing Earthabundant metal ions are ak ey to sustainable photophysical and photochemical applications.W ee xploit the effects of an inversion center and ligand non-innocence to tune the luminescence and photochemistry of the excited state of the [CrN 6 ] 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 (t = 4500 ms) reported up to date for am olecular polypyridyl chromium(III) complex together with av ery high luminescence quantum yield of F = 8.2 %a t room temperature in fluid solution. Furthermore,t he tpe ligands in [Cr(tpe) 2 ] 3+ are redox non-innocent, leading to reversible reductive chemistry.The excited state redoxpotential 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).Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.org/10.

No abstract