Visible light sensitized near-infrared luminescence of ytterbium via ILCT states in quadruple-stranded helicates

Visible light excitation is especially important for the NIR emitting materials that are employed in biochemistry and cell biology, since living tissues are generally damaged under UV light irradiation. Here, two new bis--diketones, L-1 and L-2 (L-1 = 4,4-di(4,4,4-trifluoro-1,3-dioxobutyl)-triphenylamine; L-2 = 4-(N,N-dimethylamino)-4,4-di(4,4,4-trifluoro-1,3-dioxobutyl)-triphenylamine), featuring intra-ligand charge transfer (ILCT) excited states, have been designed and prepared for the synthesis of a series of anionic quadruple-stranded dinuclear helicates (HNEt3)(2)[M2L41] and (HNEt3)(2)[M2L42], where M = Yb, Gd and Y. H-1 NMR and electrospray mass spectrometry confirm the formation of single complex species in solution. According to the comprehensive spectral characterization and in combination with quantum chemical modelling, the ILCT feature of L-1 and L-2 is confirmed. The presence of ILCT allows the use of lower energy excitation wavelengths in the visible spectral region to sensitize ytterbium NIR luminescence. In (HNEt3)(2)[Yb2L42], the introduction of strong electron donating N,N-dimethyl leads to a remarkable red shift of the absorbance edge to 560 nm compared with that in (HNEt3)(2)[Yb2L41] at 450 nm. Upon excitation with blue light, the two ytterbium helicates show excellent NIR luminescence in the range of 900-1100 nm, with the luminescence quantum yields reaching 1.1% for (HNEt3)(2)[Yb2L41] and 1.5% for (HNEt3)(2)[Yb2L42] in CH3CN. Luminescence mechanism experiments show that the ILCT singlet path and classic triplet state path together participate in sensitizing Yb(iii) ion NIR luminescence.