Spin-selective charge recombination in complexes of CdS quantum dots and organic hole acceptors.

This paper describes the mechanisms of charge recombination on both the nanosecond and microsecond time scales in a donor-acceptor system comprising thiol-modified bis(diarylamino)4,4'-biphenyl (TPD) molecules attached to a CdS quantum dot (QD) via the thiolate linker. Transient absorption measurements, in conjunction with EPR and magnetic field effect studies, demonstrate that recombination on the nanosecond time scale is mediated by radical pair intersystem crossing (RP-ISC), as evidenced by the observation of a spin correlated radical ion pair, the formation of the localized (3)*TPD state upon charge recombination, and the sensitivity of the yield of (3)*TPD to an applied magnetic field. These experiments show that the radical spins of the donor-acceptor system have weak magnetic exchange coupling (|2J| < 10 mT) and that the electron donated to the QD is trapped in a surface state rather than delocalized within the QD lattice. The microsecond-time scale recombination is probably gated by diffusion of the trapped electron among QD surface states. This study demonstrates that magneto-optical studies are useful for characterizing the charge-separated states of molecule-QD hybrid systems, despite the heterogeneity in the donor-acceptor geometry and the chemical environment of the radical spins that is inherent to these systems.

Photoisomerisation and ligand-controlled reversible aggregation of azobenzene-functionalised gold nanoparticles.

The photochemical behaviour of functionalised gold nanoparticles (AuNPs) carrying azobenzenethiolate-alkylthiolate monolayers was investigated. Repeated trans-cis and cis-trans isomerisation cycles could be performed in all cases with high efficiency. Reversible photoinduced aggregation was observed when azothiolates with long alkyl spacers (≥C7) were combined with short (C5) alkylthiolate coligands. The choice of a coligand thus offers control over the aggregation properties of the nanoparticles.

The chemical environments of oleate species within samples of oleate-coated PbS quantum dots.

A combination of FT-IR, (1)H NMR, nuclear Overhauser effect (NOESY), and diffusion-ordered (DOSY) NMR spectroscopies shows that samples of oleate-coated PbS quantum dots (QDs) with core radii ranging from 1.6 to 2.4 nm, and purified by washing with acetone, contain two species of oleate characterized by the stretching frequencies of their carboxylate groups, the chemical shifts of their protons, and their diffusion coefficients. One of these oleate species exists primarily on the surfaces of the QDs and either chelates a Pb(2+) ion or bridges two Pb(2+) ions. The ratio of bridging oleates to chelating oleates on the surfaces of the QDs is approximately 1:1 for all sizes of the QDs we studied. The second oleate species in these samples bridges two Pb(2+) ions within clusters or oligomers of lead oleate (with a hydrodynamic radius of ~1.4 nm), which are byproducts of the QD synthesis. The concentration of these clusters increases with increasing size of the QDs because larger QDs are produced by increasing the concentration of the oleic acid ligand in the reaction mixture. The oleate molecules on the surfaces of the QDs and within the lead oleate clusters are in rapid exchange with each other. Additional washes with methanol progressively eliminate the contaminating clusters from the PbS QD samples. This work quantitatively characterizes the distribution of binding geometries at the inorganic/organic interface of the nanocrystals and demonstrates the utility of using organic ligands as probes for the composition of a colloidal QD sample as a function of the preparation procedure.

Spontaneous multielectron transfer from the surfaces of PbS quantum dots to tetracyanoquinodimethane.

This paper describes an investigation of the interfacial chemistry that enables formation of a multielectron ground-state charge-transfer (CT) complex of oleate-coated PbS quantum dots (QDs) and tetracyanoquinodimethane (TCNQ) in CHCl3 dispersions. Thermodynamically spontaneous electron transfer occurs from sulfur ions on the surfaces of the QDs (radius = 1.6 nm) to adsorbed TCNQ molecules and creates indefinitely stable ion pairs that are characterized by steady-state visible and mid-infrared absorption spectroscopy of reduced TCNQ and by NMR spectroscopy of the protons of oleate ligands that coat the QDs. The combination of these techniques shows that (i) each QD reduces an average of 4.5 TCNQ molecules, (ii) every electron transfer event between the QD and TCNQ occurs at the QD surface, (iii) sulfur ions on the surfaces of the QDs (and not delocalized states within the QDs) are the electron donors, and (iv) some TCNQ molecules adsorb directly to the surface of the QDs while others adsorb upon displacement of oleate ligands.

Gating of hole transfer from photoexcited PbS quantum dots to aminoferrocene by the ligand shell of the dots.

Photoinduced hole transfer from PbS quantum dots (QDs) to aminoferrocene only occurs if the ligand shell of the QD allows aminoferrocene to gain direct access to the inorganic core of the QD; the permeability of the ligand shell is therefore more important than its conductivity in determining the probability of interfacial charge transfer.

Dual-time scale photoinduced electron transfer from PbS quantum dots to a molecular acceptor.

A combination of picosecond and microsecond transient absorption dynamics reveals the involvement of two mechanisms by which 1,4-benzoquinone (BQ) induces the decay of the excited state of PbS quantum dots (QDs): (i) electron transfer to BQ molecules adsorbed to the surfaces of PbS QDs and (ii) collisionally gated electron transfer to freely diffusing BQ. Together, these two mechanisms quantitatively describe the quenching of photoluminescence upon addition of BQ to PbS QDs in dichloromethane solution. This work represents the first quantitative study of a QD-ligand system that undergoes both adsorbed and collisionally gated photoinduced charge transfer within the same sample. The availability of a collisionally gated pathway improves the yield of electron transfer from PbS QDs to BQ by an average factor of 2.5 over that for static electron transfer alone.

Excited-state dynamics and dye-dye interactions in dye-coated gold nanoparticles with varying alkyl spacer lengths.

Gold nanoparticles (ca. 3 nm in diameter) coated with bis(diarylamino)biphenyl-based thiols with two different alkyl spacers (propyl and dodecyl) between the chromophore and the surface-anchoring thiol group have been prepared and characterized with a variety of techniques. The excited-state dynamics of the dyes in close proximity to the nanoparticle surface were studied using the time-correlated single-photon counting technique and near-IR fs transient absorption spectroscopy. The excited states of the dyes in the hybrid metal/organic systems exhibit an ultrafast (<5 ps) deactivation as evidenced by the fs transient absorption measurements. The length of the alkyl spacer between the dye and the thiol group has a profound effect on the ultrafast dynamics of the photoexcited systems. An ultrafast formation (ca. 0.5 ps) of a cation-like species has been recorded for the system incorporating the propyl spacer but not for the dodecyl-linker system. The formation of the cation-like species has been shown to be less efficient in a mixed-ligand system in which the bis(diarylamino)biphenyl-based thiol was diluted on the surface with dodecanethiol. Additionally, the ultrafast formation (ca. 1 ps) of a cation-like species with a similar spectroscopic signature has been observed in the solid state of the dye. A combination of the ultrafast dynamics and (1)H NMR spectroscopic data has been used to discuss the observed behavior in terms of dye-dye interactions in the nanoparticle systems. Due to the surface curvature of the nanoparticle, the propyl spacer imposes a closer dye-dye distance than the dodecyl spacer, thus facilitating dye-dye interactions that lead to the formation of a charge-transfer species involving two or more dye molecules.

Preparation and characterization of 4'-donor substituted stilbene-4-thiolate monolayers and their influence on the work function of gold.

Self-assembled monolayers of E-stilbene-4-thiolate (SAM1), E-4'-(ethoxy)stilbene-4-thiolate (SAM2), and E-4'-(dimethylamino)stilbene-4-thiolate (SAM3) on Au(111) have been obtained from reaction of ethanol solutions of the corresponding S-acetyl derivatives with gold substrates. A combination of X-ray photoelectron spectroscopy, ellipsometry, and infrared reflection absorption spectroscopy indicates that the monolayers are dense (ca. 3.3 x 10(14) molecules/cm(2)) and that the long molecular axes of the thiolates are approximately perpendicular to the surface. Ultraviolet photoelectron spectroscopy shows that formation of these monolayers decreases the work function of pristine Au by 0.9-1.3 eV, in part due to a bond dipole of ca. 4.4 D/molecule formed upon adsorption and partly due to the molecular dipole moment arising from the 4'-pi-donor substituents. However, the extent of the work function variation between SAM1, 2, and 3 is smaller than anticipated from purely electrostatic considerations.