Electron transfer (ET) processes are one of the most researched topics for applications ranging from energy conversion to catalysis. An exciting variation would be utilizing colloidal semiconductor nanostructures to explore such processes. Semiconductor quantum dots (QDs) are emerging as a novel class of light harvesting, emitting and charge-separation materials for applications such as solar energy conversion. Detailed knowledge of the quantitative dissociation of the photo-generated excitons and the interfacial charge- (electron/hole) transfer is essential for optimization of the overall efficiency of many such applications. Numerous organic and inorganic molecules have been utilized as electron donor/acceptors to probe the interfacial ET reaction following photoexcitation of QDs; in comparison very few studies have focused on QD-free radical systems. Organic free radicals could be attractive counterparts for studying ET to/from QDs because these undergo single-electron transfer steps in reversible fashion. Nitroxides are an exciting class of stable organic free radicals, which have recently been demonstrated to be efficient as redox mediators in dye-sensitized solar cells, making them even more interesting for the afore mentioned studies.<br/>We have been investigating the interaction between nitroxide free radical (4-Amino TEMPO) and II-VI semiconductor (CdSe and CdTe) QDs. The nature of interaction in these hybrids has been examined through ground-state UV-Vis absorbance, Steady state and Time-resolved photoluminescence (PL) spectroscopy. It has been found that 4-Amino TEMPO quenches the PL of QDs by an efficient electron transfer process. The detailed analysis of the PL quenching indicates that there are contributions from both dynamic (diffusion-related) and static (QD-radical bound species) processes. Equilibrium constants for the formation of QD-radical complexes have been derived from the PL quenching and electron paramagnetic (EPR) measurements. Transient Absorbance (TA) measurements have been used to derive the ultrafast ET rate constants for these hybrids. Temperature dependence of this ET rate is also being explored through TA measurements to gain further insight about the activation energy barrier and other thermodynamic parameters of this system. Further TA measurements will be conducted on different QDs to study the size-dependence of this ET rate and evaluate the kinetics of the process under investigation; in terms of the Marcus theory of ET.

Photoinduced electron donor/acceptor processes between colloidal CdSe quantum dots and nitroxide free radicals

251st National Meeting (2016)

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