NIR Lanthanide Luminescence by Energy Transfer from Appended Terpyridine-Boradiazaindacene Dyes

Mononuclear trivalent lanthanide complexes with formula [Ln(L)(NO3)3] [in which L=4,4-difluoro-8-(2:2;6:2-terpyridin-4-yl)-1,3,5,7-tetramethyl-2,6-diethyl-4-bora-3a,4a-diaza-s-indacene (Boditerpy)] are reported for Ln=Yb, Nd, Er, La and Gd. According to the crystal structure of the Yb complex, the lanthanide ion is bound to the terdentate terpyridine and the inner coordination sphere of the nine-coordinate lanthanide ion is completed by three bidentate nitrate anions. The coordination polyhedron can be described as a distorted tricapped antiprism. The terpyridine chelate is almost planar and tilted by nearly 60° from the indacene subunit. FT-IR spectra confirm the bidentate binding mode of the nitrate anions for the other complexes. NMR and ES-MS spectra (through characteristic isotopic patterns) confirm the chemical formulation. The complexes have high molar absorption coefficients in the visible spectral region (65 000 M-1 cm-1 at 529 nm) and display sizeable NIR luminescence (900 to 1600 nm, for Ln=Yb, Nd and Er), upon irradiation through the electronic state of the indacene moiety at 514 nm. Crystal-field splitting was analysed at low temperature. The quantum yield of the Yb solution (10-4 M) in dichloromethane amounts to 0.31 %, corresponding to a sensitisation efficacy of the ligand of ca. 63 %.