Strongly Red-Emissive Molecular Ruby [Cr(bpmp)_2]~(3+) Surpasses [Ru(bpy)_3]~(2+)

摘要

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 multi-reference 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-pyridyl-methyl)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 D_2O 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 BrΦnsted 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.