Acceptor Concentration Dependence of Förster Resonance Energy Transfer Dynamics in Dye–Quantum Dot Complexes

Abstract

The dynamics of the photoinduced Forster resonance energy transfer (FRET) in a perylene diimide−quantum dot organic−inorganic hybrid system has been investigated by femtosecond time-resolved absorption spectroscopy. The bidentate binding of the dye acceptor molecules to the surface of CdSe/CdS/ZnS multishell quantum dots provides a well-defined dye-QD geometry for which the efficiency of the energy transfer reaction can be easily tuned by the acceptor concentration. In the experiments, the spectral characteristics of the chosen FRET pair facilitate a selective photoexcitation of the quantum dot donor. Moreover, the acceptor related transient absorption change that occurs solely after energy transfer is utilized for the determination of the energy transfer dynamics. Our time-resolved measurements demonstrate that an increase of the acceptor concentration accelerates the donor−acceptor energy transfer. Considering a Poisson distribution of acceptor molecules per quantum dot, the dependence of the energy transfer rate on its mean value is linear. The results of the presented spectroscopic experiments allow for determining the relative and absolute acceptor/donor ratio in the investigated FRET system without any parameters intrinsic to Forster theory. ■ INTRODUCTIONSemiconductor quantum dots (QD) are nanometer sized luminescent particles with remarkable optical properties. Since the QD dimension is in the range of the Bohr exciton radius, the electronic transition energies become size dependent which leads to a tunability of the absorption and emission properties. 1 QD have received broad and multidisciplinary research interest which ranges from the elucidation of their fundamental photophysical properties such as blinking, 2−5 homogeneous line width, 6−8 and exciton relaxation dynamics 9−12 to the implementation as light absorbers or emitters in photovoltaic devices, 13−16 LEDs, 17−19 and lasers. 20−22 The reduced size of QD inevitably leads to a high surface to volume ratio and consequently to a strong impact of the surface composition on the QD properties. The QD fluorescence quantum yield can be significantly increased by growing an inorganic shell on the core nanoparticles. The stronger fluorescence of core/shell particles is explained by the saturation of surface associated trap states, and fluorescence quantum yields as high as 85% have been observed. 23,24 High photostability together with strong and narrow emission has motivated the application of QD in biological sensing 25−29 and imaging. 30,31 Sensing applications are typically based on the modulation of the QD emission as response to the presence of a target molecule. 28 The modulation can be achieved by quenching mechanisms such as Forster resonance energy transfer (FRET) 25−28,32,33 or charge transfer (CT). 34−38 In QD-based FRET systems the inorganic nanoparticles are predominantly applied as energy donors. Yet, in some recent reports QD have been successfully utilized as acceptors in conjugation with naphthalimide dyes 39 as well as with light harves...

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