Heteroleptic Ter-Bidentate Cr(III) Complexes as Tunable Optical Sensitizers

To exploit Cr(III) coordination complexes as sensitizers in supramolecular energy-converting devices, the latter optical relays should display long-lived excited states, broad emission bands, and tunable spatial and electronic connections to activator units. An ad-hoc versatile strategy has been therefore developed for the preparation of a family of luminescent pseudo-octahedral [CrN₆] chromophores made up of ter-bidentate heteroleptic [Cr(phen)₂(N-N'')]³⁺ complexes, where phen is 1,10-phenanthroline, and N-N' stands for α,α'-diimine ligands possessing peripheral substituents compatible with both electronic tuning and structure extensions. As long as the ligand field in these [CrN₆] chromophores remains sufficiently strong to avoid back-intersystem crossing, photophysical studies indicate that the lifetime of the near-infrared emissive Cr(²E) excited state is poorly sensitive to ligand-based electronic effects. On the contrary, a drop in symmetry, the coupling with high frequency oscillators, and the implementation of sterical constraints in heteroleptic [Cr(phen)₂(N-N')]³⁺ complexes affect both Cr(²E → ⁴A₂) energies and Cr(²E) lifetimes. Altogether, [Cr(phen)₂(phenAlkyn)]³⁺ (phenAlkyn = 5-ethynyl-1,10-phenanthroline) and [Cr(phen)₂(dpma)]³⁺ (dpma = di(pyrid-2-yl)(methyl)amine) complexes mirror the favorable photophysical properties of homoleptic [Cr(phen)₃]³⁺ and thus emerge as the best heteroleptic candidates for acting as sensitizers at room temperature, and below 100 K, respectively, in more complicated architectures.