Cooperative and FRET-Assisted Brightness Enhancement in Oligo(phenylene ethynylene): Quantum Dot Organic-Inorganic Nanohybrids.

Herein, we combine the ideas of concerted emission from fluorophore ensembles and its further amplification through FRET in an organic-inorganic hybrid approach. Spherical and highly fluorescent organic nanoparticles (FONPs, Φf =0.38), prepared by the self-assembly of oligo(phenylene ethynylene) (OPE) molecules, were selected as a potential donor material. This organic core was then decorated with a shell of fluorescent CdSe/ZnS core-shell quantum dots (QDs; <d>≅5.5 nm, Φf =0.27) with the aid of a bifunctional ligand, mercaptopropionic acid. Its high extinction coefficient (ϵ≈4.1×105  m-1  cm-1 ) and good spectral match with the emission of the FONPs (J(λ)≈4.08×1016  m-1  cm-1  nm4 ) made them a better acceptor candidate to constitute an efficient FRET pair (ΦFRET =0.8). As a result, the QD fluorescence intensity was enhanced by more than twofold. The fundamental calculations carried out indicated an improvement in all the FRET parameters as the number of QDs around the FONPs was increased. This, together with the localization of multiple QDs in a nanometric dimension (volume≈1.8×106  nm3 ), gave highly bright reddish luminescent hybrid particles as visualized under a fluorescence microscope.