Sensitized near-infrared luminescence of NdIII, YbIII and ErIII complexes by energy transfer from a ruthenium antenna

• Synthesis of three new NIR emitting bimetallic ruthenium–lanthanide complexes.
• Ru(II) complex acts as an antenna which permits energy transfer to Ln(III).
• Nd(III) is the most effective quencher of the 3MLCT luminescence of the Ru(II) component.
• The rate of energy transfer and energy transfer efficiency is the maximum.

A series of new ruthenium–lanthanide bimetallic complexes bridged by bibenzimidazole (BibzImH2) group were synthesized. The luminescent ruthenium bibenzimidazole complex, [(bpy)2Ru(BibzImH2)]2+, with a vacant diimine site, was used as a ‘complex ligand’ to prepare three new heterodinuclear d–f complexes by attaching a {Ln(dik)3} fragment (Ln = Nd, Yb, or Er); (dik = tta(trifluoro theonyl acetone)) at the vacant site. All of the prepared complexes were characterized by a variety of spectral techniques. Upon excitation at the MLCT absorption band of RuII, due to efficient energy transfer, sensitized emission occurs from lanthanide centre in the near-infrared region. The lanthanide centre has low-energy f–f excited states, which are capable of accepting energy from the 3MLCT excited state of the Ru(II) centre, leading to the quenching of 3MLCT luminescence of the Ru(II) centre and subsequent sensitized lanthanide(III)-based luminescence in the near-IR region. Because BibzImH2 is a short bridging ligand, it is able to assist in faster energy transfer from Ru(II) to Ln (III). The Nd(III) species was found to be the most effective for quenching the 3MLCT luminescence of the ruthenium component because of the high density of the f–f excited states of the appropriate energy, which make it as effective an energy-acceptor as Er(III) and Yb(III) complexes.

Three new d–f dyads were prepared, in which a Ru(II) polypyridine chromophore is connected to a Ln(III) centre by bibenzimidazole ligand. Upon excitation, the Ru(II) centre acts as an antenna, and the bibenzimidazole being a short bridge, permits energy transfer to Ln(III), thereby resulting in sensitized NIR emission.Figure optionsDownload as PowerPoint slide