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-bora3a,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 degrees 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 formu- lation. The complexes have high molar absorption coefficients in the visible spectral region (65000M(-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 %.