Electronic Structure and Photophysics of Low Spin d5 Metallocenes.

The electronic structure and photophysics of two low spin metallocenes, decamethylmanganocene (MnCp*2) and decamethylrhenocene (ReCp*2), were investigated to probe their promise as photoredox reagents. Computational studies support the assignment of 2E2 ground state configurations and low energy ligand-to-metal charge transfer transitions for both complexes. Weak emission is observed at room temperature for ReCp*2 with τ = 1.8 ns in pentane, whereas MnCp*2 is not emissive. Calculation of the excited state reduction potentials for both metallocenes reveal their potential potency as excited state reductants (E°'([MnCp*2]+/0*) = -3.38 V and E°'([ReCp*2]+/0*) = -2.61 V vs Fc+/0). Comparatively, both complexes exhibit mild potentials for photo-oxidative processes (E°'([MnCp*2]0*/-) = -0.18 V and E°'([ReCp*2]0*/-) = -0.20 V vs Fc+/0). These results showcase the rich electronic structure of low spin d5 metallocenes and their promise as excited state reductants.

Noncovalent Control of the Electrostatic Potential of Quantum Dots through the Formation of Interfacial Ion Pairs.

This paper describes the role of tetraalkylammonium counterions [NR4+, R = -CH3, -CH2CH3, -(CH2)2CH3, or -(CH2)3CH3] in gating the electrostatic potential at the interface between the 6-mercaptohexanoate (MHA) ligand shell of a PbS quantum dot (QD) and water. The permeability of this ligand shell to a negatively charged anthraquinone derivative (AQ), measured from the yield of electron transfer (eT) from the QD core to AQ, increases as the steric bulk of NR4+ increases (for a given concentration of NR4+). This result indicates that bulkier counterions screen repulsive interactions at the ligand/solvent interface more effectively than smaller counterions. Free energy scaling analysis and molecular dynamics simulations suggest that ion pairing between the ligand shell of the QD and NR4+ results from a combination of electrostatic and van der Waals components, and that the van der Waals interaction promotes ion pairing with longer-chain counterions and more effective screening. This work provides molecular-level details that dictate a nanoparticle's electrostatic potential and demonstrates the sensitivity of the yield of photoinduced charge transfer between a QD and a molecular probe to even low-affinity binding events at the QD/solvent interface.

Enhancing the Rate of Quantum-Dot-Photocatalyzed Carbon-Carbon Coupling by Tuning the Composition of the Dot's Ligand Shell.

This Communication describes the photoredox catalysis of a C-C coupling reaction between 1-phenylpyrrolidine (PhPyr) and phenyl trans-styryl sulfone by visible-light-absorbing colloidal CdS quantum dots (QDs), without a sacrificial oxidant or reductant, and without a co-catalyst. Simple kinetic analysis reveals that photo-oxidation of PhPyr by the QDs is the rate-limiting step. Disordering of the ligand shell of the QDs by creating mixed monolayers of oleate and octylphosphonate increases the initial rate of the reaction by a factor of 2.3, and the energy efficiency (mol product/joule of incident photons) of the reaction by a factor of 1.6, by facilitating the hole-transfer step.

Ultrafast Two-Electron Transfer in a CdS Quantum Dot-Extended-Viologen Cyclophane Complex.

Time-resolved optical spectroscopies reveal multielectron transfer from the biexcitonic state of a CdS quantum dot to an adsorbed tetracationic compound cyclobis(4,4'-(1,4-phenylene) bipyridin-1-ium-1,4-phenylene-bis(methylene)) (ExBox(4+)) to form both the ExBox(3+•) and the doubly reduced ExBox(2(+•)) states from a single laser pulse. Electron transfer in the single-exciton regime occurs in 1 ps. At higher excitation powers the second electron transfer takes ∼5 ps, which leads to a mixture of redox states of the acceptor ligand. The doubly reduced ExBox(2(+•)) state has a lifetime of ∼10 ns, while CdS(+•):ExBox(3+•) recombines with multiple time constants, the longest of which is ∼300 µs. The long-lived charge separation and ability to accumulate multiple charges on ExBox(4+) demonstrate the potential of the CdS:ExBox(4+) complex to serve as a platform for two-electron photocatalysis.

Organic-to-Aqueous Phase Transfer of Cadmium Chalcogenide Quantum Dots using a Sulfur-Free Ligand for Enhanced Photoluminescence and Oxidative Stability.

This paper describes a procedure for transferring colloidal CdS and CdSe quantum dots (QDs) from organic solvents to water by exchanging their native hydrophobic ligands for phosphonopropionic acid (PPA) ligands, which bind to the QD surface through the phosphonate group. This method, which uses dimethylformamide as an intermediate transfer solvent, was developed in order to produce high-quality water soluble QDs with neither a sulfur-containing ligand nor a polymer encapsulation layer, both of which have disadvantages in applications of QDs to photocatalysis and biological imaging. CdS (CdSe) QDs were transferred to water with a 43% (48%) yield using PPA. The photoluminescence (PL) quantum yield for PPA-capped CdSe QDs is larger than that for QDs capped with the analogous sulfur-containing ligand, mercaptopropionic acid (MPA), by a factor of four at pH 7, and by up to a factor of 100 under basic conditions. The MPA ligands within MPA-capped QDs oxidize at Eox ~ +1.7 V vs. SCE, whereas cyclic voltammograms of PPA-capped QDs show no discerible oxidation peaks at applied potentials up to +2.5 V vs. SCE. The PPA-capped QDs are chemically and colloidally stable for at least five days in the dark, even in the presence of O2, and are stable when continuously illuminated for five days, when oxygen is excluded and a sacrificial reductant is present to capture photogenerated holes.

The role of ligands in determining the exciton relaxation dynamics in semiconductor quantum dots.

This article reviews the mechanisms through which molecules adsorbed to the surfaces of semiconductor nanocrystals, quantum dots (QDs), influence the pathways for and dynamics of intra- and interband exciton relaxation in these nanostructures. In many cases, the surface chemistry of the QDs determines the competition between Auger relaxation and electronic-to-vibrational energy transfer in the intraband cooling of hot carriers, and between electron or hole-trapping processes and radiative recombination in relaxation of band-edge excitons. The latter competition determines the photoluminescence quantum yield of the nanocrystals, which is predictable through a set of mostly phenomenological models that link the surface coverage of ligands with specific chemical properties to the rate constants for nonradiative exciton decay.

Identification of a minimal region of the HIV-1 5'-leader required for RNA dimerization, NC binding, and packaging.

Assembly of human immunodeficiency virus type 1 (HIV-1) particles is initiated in the cytoplasm by the formation of a ribonucleoprotein complex comprising the dimeric RNA genome and a small number of viral Gag polyproteins. Genomes are recognized by the nucleocapsid (NC) domains of Gag, which interact with packaging elements believed to be located primarily within the 5'-leader (5'-L) of the viral RNA. Recent studies revealed that the native 5'-L exists as an equilibrium of two conformers, one in which dimer-promoting residues and NC binding sites are sequestered and packaging is attenuated, and one in which these sites are exposed and packaging is promoted. To identify the elements within the dimeric 5'-L that are important for packaging, we generated HIV-1 5'-L RNAs containing mutations and deletions designed to eliminate substructures without perturbing the overall structure of the leader and examined effects of the mutations on RNA dimerization, NC binding, and packaging. Our findings identify a 159-residue RNA packaging signal that possesses dimerization and NC binding properties similar to those of the intact 5'-L and contains elements required for efficient RNA packaging.

NMR detection of structures in the HIV-1 5'-leader RNA that regulate genome packaging.

The 5'-leader of the HIV-1 genome regulates multiple functions during viral replication via mechanisms that have yet to be established. We developed a nuclear magnetic resonance approach that enabled direct detection of structural elements within the intact leader (712-nucleotide dimer) that are critical for genome packaging. Residues spanning the gag start codon (AUG) form a hairpin in the monomeric leader and base pair with residues of the unique-5' region (U5) in the dimer. U5:AUG formation promotes dimerization by displacing and exposing a dimer-promoting hairpin and enhances binding by the nucleocapsid (NC) protein, which is the cognate domain of the viral Gag polyprotein that directs packaging. Our findings support a packaging mechanism in which translation, dimerization, NC binding, and packaging are regulated by a common RNA structural switch.