Energy Transfer from Colloidal Quantum Dots to Near-Infrared-Absorbing Tetraazaporphyrins for Enhanced Light Harvesting

We investigate the mechanisms of energy transfer from CdSe quantum dots (QDs) to porphyrin derivatives as a potential antenna system with enhanced light-harvesting efficiency. Two ferrocenyl-containing tetraazaporphyrin derivatives, namely, magnesium 2(3),7(8),12(13),17(18)-tetraferrocenyl-5,10,15,20-tetraazaporphyrin (TAP^FcMg) and magnesium 2(3),7(8),­12(13),­17(18)-tetra­cyano-3(2),8(7),13(12),18(17)-tetraferrocenyl-5,10,15,20-tetraazaporphyrin (TAP^FcCNMg), are used as energy acceptors in this proposed antenna system along with size-dependent QDs as donors. Our approach includes Förster resonance energy transfer (FRET) calculations as well as photoluminescence (PL) intensity and lifetime quenching measurements. Our FRET calculations indicate that higher energy transfer efficiency can be achieved with smaller quantum dot size. However, PL intensity and lifetime measurements suggest that energy transfer efficiency in QD/tetraazaporphyrin complexes is regulated by a competing trap-assisted ultrafast quenching mechanism, which is more dominant with smaller QD size. Furthermore, it is found that the trap-assisted quenching process is more active in QD/TAP^FcMg than QD/TAP^FcCNMg complexes. As a result, high efficiency energy transfer can be achieved in the complexes combining large QDs and TAP^FcCNMg, where trap-assisted quenching mechanism is suppressed. Our study suggests that CdSe quantum dots can be promising energy transfer donors for NIR-absorbing tetraazaporphyrins to form antenna systems with enhanced light-harvesting efficiency.