The
energy transfer efficiencies of organic–inorganic nanohybrids
comprised of two structurally similar squaraine dyes and CdSe nanoparticles
were studied in detail and compared. Carbazole based unsymmetrical
squaraine dyes (CTSQ-1 and CTSQ-2) having modified absorption characteristics
were considered for modulating the effect of the overlap integral
on energy transfer rate with the designed QDs. CTSQ-2 with ∼1.75
times higher molar extinction coefficient and 35 nm red-shift in absorption
resulted in an ∼2.4 times faster energy transfer rate with
QD. The calculated energy transfer rates (k
T = 1.35 × 108 s–1 and 3.26 ×
108 s–1 respectively for QD:CTSQ-1 and
QD:CTSQ-2 nanohybrids) are at least one order of magnitude higher
than both radiative (k
r = 5.97 ×
106 s–1) and nonradiative decay rate
constants (k
nr = 1.89 × 107 s–1) of QDs yielding very high FRET efficiency.
The Stern–Volmer analysis of the quenching data indicated mainly
static interaction of dyes with the QDs thus suggesting formation
of organic–inorganic nanohybrids. When incorporated in dye-sensitized
solar cells, the nanohybrids with 93% FRET efficiency, exhibited an
overall 43% improvement in the photovoltaic performance. Among the
two architectures employed for device fabrication the one with the
smallest donor–acceptor distance delivered the best performance.
Due to increased contribution from QDs, the IPCE spectra clearly indicate
panchromatic response from the visible to NIR region. Thus, photovoltaic
performance of NIR absorbing dyes were successfully improved by constructing
panchromatic organic–inorganic nanohybrid materials.
The polarity difference between nanoparticle's surface and medium is utilized for studying electrostatically driven self-assembly; moreover, diminishing the repulsive forces via charge neutralization fosters the self-organization of QDs into 2D sheets.
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, Φ =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; ≅5.5 nm, Φ =0.27) with the aid of a bifunctional ligand, mercaptopropionic acid. Its high extinction coefficient (ϵ≈4.1×10 m cm ) and good spectral match with the emission of the FONPs (J(λ)≈4.08×10 m cm nm ) made them a better acceptor candidate to constitute an efficient FRET pair (Φ =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≈1.8×10 nm ), gave highly bright reddish luminescent hybrid particles as visualized under a fluorescence microscope.
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