Quantitative Analysis of Singlet Oxygen (1O2) Generation via Energy Transfer in Nanocomposites Based on Semiconductor Quantum Dots and Porphyrin Ligands

We report on the results of a detailed quantitative experimental exciton relaxation pathways as well as direct measurement of singlet oxygen (O-1(2)) generation efficiencies for CdSe/ZnS quantum dot (QD)- porphyrin nanocomposites in toluene at 295 K QD photoluminescence. quenching in nanocomposites is caused. by two main factors: electron tunneling in the quantum confined QD.(efficiency 0.85-0.90) and Forster resonance energy transfer (FRET) QD -> porphyrin (quenching efficiency 0.10-0.15). Efficiencies of O-1(2) generation gamma(Delta) by nanocomposites are essentially higher with respect to those obtained for QDs alone. For nanocomposites, the nonlinear decrease of O-1(2) generation efficiency gamma(Delta) on the laser pulse energy is caused by nonradiative intraband Auger processes, realized in the QD counterpart. Finally, FRET efficiencies found from the direct sensitization data for porphyrin fluorescence in, nanocomposites (Phi(FRET) = 0.14 +/- 0.02) are in good agreement with the corresponding values obtained via the direct O-1(2) generation measurements at low laser excitation (Phi(Delta)(FRET) = 0.12 +/- 0.03). The obtained quantitative results provide for the first time strong evidence that a FRET process QD -> porphyrin is the reason for singlet oxygen generation by nanocomposites.