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This paper describes the mechanism behind the pH response of the photoluminescence (PL) of aqueous dihydrolipoic acid (DHLA)-capped PbS quantum dots (QDs). The PL spectra of ensembles of PbS-DHLA Q...

This paper describes spectroscopic evidence for the photoinduced transfer of a hole from the biexcitonic state of a CdS quantum dot (QD) to a phenothiazine (PTZ) molecular acceptor, covalently link...

Defects in the organic ligand layers on the surfaces of colloidal quantum dots (QDs) provide pathways for corrosive molecules to penetrate to the QD core. This paper describes the decrease in the p...

Colloidal semiconductor nanocrystals, or "quantum dots" (QDs), have several optical and chemical properties that give them the potential to enable nonincremental increases in the efficiencies of many types of photocatalytic reactions relevant for energy conversion and organic synthesis. Colloidal photocatalysts have many desirable characteristics of both heterogeneous and homogeneous catalysts but come with their own particular set of challenges. This viewpoint outlines some of the obstacles one first encounters when driving reactions with these colloids and offers some strategies for overcoming these obstacles, including ways to extend their excited state lifetimes, prevent corrosion by photogenerated holes, and choose a surface chemistry and buffering system for maximum colloidal stability over a range of environmental conditions.

This paper describes the use of electrostatic assemblies of negatively charged colloidal CuInS2/ZnS quantum dot (QD) sensitizers and positively charged, trimethylamino-functionalized iron tetraphenylporphyrin catalysts (FeTMA) to photoreduce CO2 to CO in water upon illumination with 450 nm light. This system achieves a turnover number (TON) of CO (per FeTMA) of 450 after 30 h of illumination, with a selectivity of 99%. Its sensitization efficiency (TON per Joule of photons absorbed) is a factor of 11 larger than the previous record for photosensitization of an iron porphyrin catalyst for this reaction, held by a system in which both QDs and metal porphyrin were uncharged. Steady-state and time-resolved optical spectroscopy provides evidence for electrostatic assembly of QDs and FeTMA. Control of the size of the assemblies with addition of a screening counterion, K+, and a correlation between their measured size and their catalytic activity, indicates that the enhancement in performance of this system over the analogous uncharged system is due to the proximity of the FeTMA catalyst to multiple light-absorbing QDs and the selective formation of QD-FeTMA contacts (rather than QD-QD or FeTMA-FeTMA contacts). This system therefore shows the ability to funnel photoinduced electrons to a reaction center, which is crucial for carrying out reactions that require multistep redox processes under low photon flux, and thus is an important advance in developing artificial photocatalytic systems that function in natural light.

Photosensitization of molecular catalysts to reduce CO2 to CO is a sustainable route to storable solar fuels. Crucial to the sensitization process is highly efficient transfer of redox equivalents from sensitizer to catalyst; in systems with molecular sensitizers, this transfer is often slow because it is gated by diffusion-limited collisions between sensitizer and catalyst. This article describes the photosensitization of a meso-tetraphenylporphyrin iron(III) chloride (FeTPP) catalyst by colloidal, heavy metal-free CuInS2/ZnS quantum dots (QDs) to reduce CO2 to CO using 450 nm light. The sensitization efficiency (turnover number per absorbed unit of photon energy) of the QD system is a factor of 18 greater than that of an analogous system with a fac-tris(2-phenylpyridine)iridium sensitizer. This high efficiency originates in ultrafast electron transfer between the QD and FeTPP, enabled by formation of QD/FeTPP complexes. Optical spectroscopy reveals that the electron-transfer processes primarily responsi...

This paper describes control of the rate constant for near-infrared excitonic energy transfer (EnT) within soluble aqueous assemblies of PbS quantum dots, cross-linked by Zn2+, by changing the length of the mercapto-alkanoic acid (MAA) that serves as the cross-linking ligand. Sequestration of Zn2+ by a chelating agent or zinc hydroxide species results in deaggregation of the assemblies with EnT turned “off”. Upon decreasing the number of methylene groups in MAAs from 16 to 3, the interparticle separation decreases from 5.8 nm to 3.7 nm and the average observed EnT rate increases from ∼(150 ns)−1 to ∼(2 ns)−1. A master equation translates intrinsic (single-donor–single-acceptor) EnT rate constants predicted for each ligand length using Forster theory to observed average rate constants. For interparticle distances greater than ∼4 nm, the point dipole approximation (PDA) implementation of Forster theory agrees with experimentally measured rates. At shorter interparticle distances, the PDA drastically underes...

Lead chalcogenide quantum dots (QDs) are promising acceptors for photovoltaic devices that harness the singlet fission (SF) mechanism. The rate of singlet fission of polyacenes in the presence of QDs is a critical parameter in determining the performance of such devices. The present study demonstrates that the rates of SF in a pentacene derivative, 6,13-diphenylanthracene (DPP), are modulated by forming coaggregates with PbS QDs in aqueous dispersions. PbS QDs generally accelerate SF within DPP aggregates, and the extent of acceleration depends on the size of the QD. The average rate of SF increases from 0.074 ps-1 for DPP-only aggregates to 0.37 ps-1 within DPP-D co-aggregates for QDs with radius 2.2 nm, whereas co-aggregation with the smallest (r = 1.6 nm) and largest (r = 2.7 nm) QDs we tried only slightly change the SF rate. The rate variation is associated with (i) the density of surface ligands, which is influenced by the faceting of the PbS surface, and (ii) the local dielectric constant for DPP. To accelerate SF, the ligands should be dense enough to provide sufficient affinity for DPP aggregates and effectively perturb the perpendicular alignment of DPP monomers within aggregates to increase the intermolecular coupling that promotes SF, but should not be too dense so as to form a low dielectric environment that disfavors SF. The study suggests that it is critical to consider the influence of the microenvironment of the QD surface on photophysical processes when fabricating QD/organic hybrid devices.

Light-driven [2+2] cycloaddition is the most direct strategy to build tetrasubstituted cyclobutanes, core components of many lead compounds for drug development. Significant advances in the chemoselectivity and enantioselectivity of [2+2] photocycloadditions have been made, but exceptional and tunable diastereoselectivity and regioselectivity (head-to-head vs. head-to-tail adducts), required for synthesis of bioactive molecules, have not yet been achieved. Here we show that colloidal quantum dots (QDs) serve as visible-light chromophores, photocatalysts, and reusable scaffolds for homo- and hetero-intermolecular [2+2] photocycloadditions of 4-vinylbenzoic acid derivatives, including aryl-conjugated alkenes, with up to 98% switchable regioselectivity and 98% diastereoselectivity for the previously minor syn-cyclobutane products, including the syn-head-to-tail cyclobutane, which has never before been accessed as the major product of a hetero-photocycloaddition. Transient absorption spectroscopy confirms that our system is the first example of catalysis triggered by triplet-triplet energy transfer from a QD. The precisely controlled triplet energy levels of QD photocatalysts facilitate efficient and selective heterocoupling, a major challenge in direct cyclobutane synthesis.

This paper describes the pH dependence of the excitonic energies and dynamics of CdS quantum dots (QDs) capped with phosphonopropionate (PPA) in water. QDs capped with PPA carry a negative charge on their surfaces upon deprotonation of PPA above pH ∼ 8.5; the resultant electric field induces large changes in the QD’s optical properties. Between pH 5.6 and 12.0, an increase in pH is accompanied by a 47-meV bathochromic shift in the bandgap of the QDs and a decrease in the Stokes shift by ∼4.3 meV/pH unit. An increase in the radiative recombination rate by a factor of 20.9 occurs on increasing the pH from 5.6 to 9.4. These observations are attributed to a shifting of the energy levels within the first exciton manifold, and are simulated using time-dependent density functional theory calculations on model Cd29S29 clusters surrounded by point charges.

This paper describes the enhancement of the rate of hole transfer from a photoexcited CdS quantum dot (QD), with radius R = 2.0 nm, to a molecular acceptor, phenothiazine (PTZ), by linking the donor and acceptor through a phenyldithiocarbamate (PTC) linker, which is known to lower the confinement energy of the excitonic hole. Upon adsorption of PTC, the bandgap of the QD decreases due to delocalization of the exciton, primarily the excitonic hole, into interfacial states of mixed QD/PTC character. This delocalization enables hole transfer from the QD to PTZ in300 fs (within the instrument response of the laser system) when linked by PTC, but not when linked by a benzoate group, which has a similar length and conjugation as PTC but does not delocalize the excitonic hole. Comparison of the two systems was aided by quantification of the surface coverage of benzoate and PTC-linked PTZ by (1)H NMR. This work provides direct spectroscopic evidence of the enhancement of the rate of hole extraction from a colloidal QD through covalent linkage of a hole acceptor through an exciton-delocalizing ligand.

Excitonic energy transfer (EnT) is the mechanism by which natural photosynthetic systems funnel energy from hundreds of antenna pigments to a single reaction center, which allows multielectron redox reactions to proceed with high efficiencies in low-flux natural light. This paper describes the use of electrostatically assembled CdSe quantum dot (QD) aggregates as artificial light harvesting–reaction center units for the photocatalytic reduction of H + to H 2 , where excitons are funneled through EnT from sensitizer QDs (sQDs) to catalyst QDs (cQDs). Upon increasing the sensitizer-to-catalyst ratio in the aggregates from 1:2 to 20:1, the number of excitons delivered to each cQD (via EnT) per excitation of the system increases by a factor of nine. At the optimized sensitizer-to-catalyst ratio of 4:1, the internal quantum efficiency (IQE) of the reaction system is 4.0 ± 0.3%, a factor of 13 greater than the IQE of a sample that is identical except that EnT is suppressed due to the relative core sizes of the sQDs and cQDs. A kinetic model supports the proposed exciton funneling mechanism for enhancement of the catalytic activity.

Reversible coupling of glutathione-capped PbS quantum dots through coordination with Zn2+ cations forms water-soluble aggregates. These assemblies mediate multi-step hopping of near-infrared excitons through unity-quantum yield Forster resonance energy transfer. The photoluminescence spectra from these samples are sensitive indicators of average interparticle distance.

The subject of this review is the colloidal quantum dot (QD) and specifically the interaction of the QD with proximate molecules. It covers various functions of these molecules, including (i) ligands for the QDs, coupled electronically or vibrationally to localized surface states or to the delocalized states of the QD core, (ii) energy or electron donors or acceptors for the QDs, and (iii) structural components of QD assemblies that dictate QD-QD or QD-molecule interactions. Research on interactions of ligands with colloidal QDs has revealed that ligands determine not only the excited state dynamics of the QD but also, in some cases, its ground state electronic structure. Specifically, the article discusses (i) measurement of the electronic structure of colloidal QDs and the influence of their surface chemistry, in particular, dipolar ligands and exciton-delocalizing ligands, on their electronic energies; (ii) the role of molecules in interfacial electron and energy transfer processes involving QDs, including electron-to-vibrational energy transfer and the use of the ligand shell of a QD as a semipermeable membrane that gates its redox activity; and (iii) a particular application of colloidal QDs, photoredox catalysis, which exploits the combination of the electronic structure of the QD core and the chemistry at its surface to use the energy of the QD excited state to drive chemical reactions.