Two-photon absorption enhancement for organic acceptor molecules with QD antennas.

The photophysics of an inorganic/organic hybrid system was studied by time-resolved optical spectroscopy, focusing on the goal of increasing the two-photon efficiency of photoresponsive systems. The hybrid system consists of CdS/ZnS core/shell quantum dots (QDs) as energy donor and coumarin derivatives as energy acceptor molecules. The spectral overlap of QD emission and coumarin 343 absorption promotes a Förster resonance energy tranfer (FRET) mechanism leading to a FRET efficiency up to nearly 90%. Additionally, time-correlated single photon counting showed a faster fluorescence decay while acceptor molecules were attached to the QD surface. Femtosecond transient absorption measurements demonstrated an ultrafast FRET reaction. Importantly, FRET was observed also after two-photon excitation of the QDs indicating that the chosen QDs can act as two-photon antennas.