Energy transfer (ET) from ferrocene or ruthenium tris(bipyridine) functionalized light‐harvesting units allows the first observation of sensitized near‐infrared (NIR) lanthanide (Ln) luminescence (Lum.; see scheme, Exc.=excitation). Since the donating triplet state of the ruthenium tris(bipyridine) antenna is luminescent, the sensitization process could be studied in detail.

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The design of sensitizer-functionalized lanthanide complexes that exhibit efficient luminescence in the near-infrared (NIR) region is currently of interest because of potential applications in fluoroimmuno-assays, [1] in laser systems, [2] and in polymer-based optical signal amplifiers. [3,4] NIR luminescence of complexes of the lanthanide ions erbium(iii), neodymium(iii), and ytterbium(iii) is readily observed in solution at room temperature, only when the ions are excited indirectly by energy transfer from the triplet state [5] of an incorporated organic chromophore (the antenna or sensitizer). Direct excitation is more demanding because of the forbidden optical transitions within the 4f subshells of lanthanide ions. As a result, the absorption coefficients are very low (typically 1 ± 10 m À1 cm À1 ) and the lifetimes of the excited states are relatively long (microseconds to milliseconds).Herein, we report the first example of Nd III and Yb III luminescence sensitized by the transition metal complexes ruthenium(ii) tris(bipyridine) ([Ru(bpy) 3 ] 2 ) and ferrocene. Investigations of sensitizers for NIR lanthanide luminescence have focused on conjugated organic molecules, nevertheless, there are only a few examples of sensitizers that enable visible light excitation instead of near-UV excitation. [1,4,6,7] The [Ru(bpy) 3 ] 2 complex not only has an intersystem-crossing quantum yield near unity, but it should enable excitation of Nd III and Yb III with visible light of up to 500 nm. Compared to [Ru(bpy) 3 ] 2 , ferrocene has weaker absorption bands in the visible region, [8] but its low-lying triplet state (E T 13 300 cm À1 ) [9] closely matches the receiving luminescent states of Nd III (11 300 cm À1 ) and Yb III (10 250 cm À1 ).Our systems [(Ln 2 -Ru)1] 2 and (Ln)2 are based on mterphenyl-based lanthanide (Ln) complexes [10±12] that are covalently linked to the transition metal complexes. Experimental and physical data can be found in the Supporting Information.Excitation of the [(Nd 2 -Ru)1] 2 and [(Yb 2 -Ru)1] 2 complexes in deoxygenated [D 6 ]DMSO (10 À5 m) with visible light resulted in Ln III -centered NIR luminescence. Figure 1 shows the emission bands of [(Nd 2 -Ru)1] 2 at 1060 and 1330 nm corresponding to the 4 F 3/2 3 4 I 11/2 and 4 I 13/2 transitions of Nd III , respectively, and the emission band of [(Yb 2 -Ru)1] 2 at [4] a) [**] We thank Roel Fokkens and Nico Nibbering (University of Amsterdam) for recording and discussing the MALDI-TOF mass spectra. Martijn Werts (University of Amsterdam) is gratefully acknowledged for his support with the time-resolved luminescence measurements.