NIR-emission from Yb(III)- and Nd(III)-based complexes in the solid state sensitized by a ligand system absorbing in a broad UV and visible spectral window

•New heteroleptic complexes of Yb(III) and Nd(III) have been synthesized and characterized.•Yb(III) and Nd(III) NIR emission is sensitized upon excitation in a broad UV/Vis spectral window.•A ligand-to-metal charge transfer state is involved in the sensitization of Yb(III) luminescence. In this contribution, we present the synthesis, characterization and spectroscopic investigation of the heteroleptic (R,R)-YbL1(tta) and (R,R)-NdL1(tta) complexes (with tta = 2-thenoyltrifluoroacetonate and L1 = N,N′-bis(2-(8-hydroxyquinolinate)methylidene)-1,2-(R,R or S,S)-cyclohexanediamine) in the solid state. The f-f metal-centered NIR luminescence emission of Nd(III) and Yb(III) is efficiently sensitized by both chromophoric ligands in a very broad range of wavelengths [from 250 to 600 nm, in the case of Nd(III) and from 250 to 650 nm, for Yb(III)]. A possible energy transfer mechanism is proposed: for (R,R)-NdL1(tta) complex a classical Ligand-to-Metal Energy Transfer (LMET) mechanism (antenna effect) is suggested, whilst in the case of the (R,R)-YbL1(tta) complex, the presence of a ligand-to-metal charge transfer (LMCT) state determines the sensitization of Yb(III) luminescence. We propose that this level is populated by the singlet and triplet excited states belonging to π → π * and n → π * transitions of both ligands and it can transfer the excitation energy to 2F5/2.