Overcoming the Complex Excited-State Dynamics of Colloidal Cadmium Selenide Nanocrystals Involved in Energy Transfer Processes

Abstract

Semiconductor nanocrystals are often characterized by complex excited-state dynamics which reflect the inhomogeneous character of ensemble of nanocrystals. A new hybrid inorganic–organic donor–acceptor system involving CdSe nanocrystals and paramagnetic nitronyl nitroxide free radicals is shown to lead to efficient Förster (dipolar) resonance energy transfer. This transfer process, which is monitored by steady-state and time-dependent photoluminescence quenching experiments, occurs on a time scale similar to that of the intrinsic recombination in CdSe nanocrystals, allowing to unravel some of the complexity associated with the excited-state photophysics of semiconductor nanocrystals. A Stern–Volmer formalism that can handle the multiexponential nature of the time-dependent excited-state kinetics of CdSe nanocrystals is developed, leading to excellent agreement between steady-state and time-dependent photoluminescence data when a log-normal distribution model is used for the intrinsinc recombination rate constant and a Poisson distribution for the number of bound quenchers per emitter.

Quantum Efficiency of Charge Transfer Competing against Nonexponential Processes: The Case of Electron Transfer from CdS Nanorods to Hydrogenase

“…The ensemble ET efficiency, ⟨ϕ⟩, is found by summing ϕ­( N ) over the distribution of subpopulations in the ensemble ,,− After substituting in eqs and , eq can be simplified to give . We can integrate by parts to obtain to a compact expression for the ensemble quantum efficiency of electron transfer where J 0 ( t ) = −d S 0 ( t )/d t .…”

“…Typically, this quantity must be extracted from models used to fit experimental data. Attaining an accurate measure of ϕ in photoexcited nanocrystals is complicated by the fact that they often exhibit excited-state decays that contain multiple decay components over a broad range of timescales due to population heterogeneity as well as intrinsically nonexponential relaxation. ,− In systems where Cd-chalcogenide nanocrystals are coupled to enzymes and other charge acceptors, there is a distribution in the number of acceptors bound to a particular donor in the ensemble, which imparts an additional layer of complexity to the extraction of ϕ from experimental data. ,− …”

Mechanistic Insights into Selective Sensing of Pb2+ in Water by Photoluminescent CdS Quantum Dots

“…Stern–Volmer plots, using I 0 / I as well as τ 0 /τ are nonlinear (Figure C). The downward deviation of Stern–Volmer plots is indicative of restricted access of quencher Pb 2+ ions to the PL emitter and or charge/shielding effect. , Modified Stern–Volmer plots are nonlinear as well (Figure S5B). Hence, it may be inferred that not only does the quencher have restricted access to the QDs, but also the mechanism at play cannot be adequately described by the conventional approach.…”

Redox Reactions at Colloidal Semiconductor Nanocrystal Surfaces

“…There have also been a number of elegant experiments using EPR spectroscopy of NC ligands that display unpaired electrons. Separately, Scaiano and Beaulac have studied CdSe NCs modified with radical ligands (4-carboxyphenylnitronyl nitroxide and 4-amino-TEMPO radicals). − These show an interaction between the unpaired electron of the ligand and the NC which results in fast PL quenching. For these systems, EPR spectroscopy was primarily used to confirm that the radical-containing ligands were unmodified prior to photoexcitation.…”