Strongly Red-Emissive Molecular Ruby [Cr(bpmp)₂]³⁺ Surpasses [Ru(bpy)₃]²⁺

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