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 pseudooctahedral
[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 backintersystem
crossing, photophysical studies indicate that the lifetime of the nearinfrared
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.