Triplet energy transfer from quantum dots increases Ln(III) photoluminescence, enabling excitation at visible wavelengths

Europium(III) complexes are promising for bioimaging because of their long-lived, narrow emission. The photoluminescence (PL) from europium(III) complexes is usually low. Thus, the effective utilization of low-energy light >400 nm and enhancement of PL are long-standing goals. Here, we show for the first time that 1-naphthoic acid triplet transmitter ligands bound to CdS quantum dots (QDs) and europium(III) complexes create an energy transfer cascade that takes advantage of the strong QD absorption. This is confirmed by transient absorption spectroscopy, which shows hole mediated triplet energy transfer from QDs to 1-NCA, followed by triplet transfer from 1-NCA to europium(III) complexes with an efficiency of 65.9 +/- 7.7%. Smaller CdS QDs with a larger driving force lead to higher triplet transfer efficiency, with Eu(III) PL intensity enhanced up to 21.4 times, the highest value ever reported. This hybrid QD system introduces an innovative approach to enhance the brightness of europium complexes.