Near-IR Absorbing Ruthenium Complexes of Non-Innocent 6,12-Di(pyridin-2-yl)indolo[3,2-b]carbazole: Variation as a Function of Co-Ligands.

This article deals with the hitherto unexplored metal complexes of deprotonated 6,12-di(pyridin-2-yl)-5,11-dihydroindolo[3,2-b]carbazole (H2 L). The synthesis and structural, optical, electrochemical characterization of dimeric [{RuIII (acac)2 }2 (μ-L.- )]ClO4 ([1]ClO4 , S=1/2), [{RuII (bpy)2 }2 (μ-L.- )](ClO4 )3 ([2](ClO4 )3 , S=1/2), [{RuII (pap)2 }2 (μ-L2- )](ClO4 )2 ([4](ClO4 )2 , S=0), and monomeric [(bpy)2 RuII (HL- )]ClO4 ([3]ClO4 , S=0), [(pap)2 RuII (HL- )]ClO4 ([5]ClO4 , S=0) (acac=σ-donating acetylacetonate, bpy=moderately π-accepting 2,2'-bipyridine, pap=strongly π-accepting 2-phenylazopyridine) are reported. The radical and dianionic states of deprotonated L in isolated dimeric 1+ /23+ and 42+ , respectively, could be attributed to the varying electronic features of the ancillary (acac, bpy, and pap) ligands, as was reflected in their redox potentials. Perturbation of the energy level of the deprotonated L or HL upon coordination with {Ru(acac)2 }, {Ru(bpy)2 }, or {Ru(pap)2 } led to the smaller energy gap in the frontier molecular orbitals (FMO), resulting in bathochromically shifted NIR absorption bands (800-2000 nm) in the accessible redox states of the complexes, which varied to some extent as a function of the ancillary ligands. Spectroelectrochemical (UV/Vis/NIR, EPR) studies along with DFT/TD-DFT calculations revealed (i) involvement of deprotonated L or HL in the oxidation processes owing to its redox non-innocent potential and (ii) metal (RuIII /RuII ) or bpy/pap dominated reduction processes in 1+ or 22+ /3+ /42+ /5+ , respectively.