Light-Controlled Nanoparticle Collision Experiments.

Electrochemical monitoring of catalytically amplified collisions of individual metal nanoparticles (NP) with ultramicroelectrodes (UME) has been extensively used to study electrocatalysis, mass-transport, and charge-transfer processes at the single NP level. More recently, photoelectrochemical collision experiments were carried out with semiconductive NPs. Here, we introduce two new types of light-controlled nanoimpact experiments. The first experiment involves localized photodeposition of catalyst (Pt) on TiO2 NPs with a glass-sheathed carbon fiber simultaneously serving as the light guide and collector UME. The collisions of in situ prepared Pt@TiO2 NPs with the carbon surface produced blips of water oxidation current, while the activity of pristine TiO2 NPs was too low to yield measurable signal. In another experiment, collisions of catalytic (Ir oxide) NPs with the semiconductor (Nb doped n-type TiO2 rutile single crystal) electrode are monitored by measuring the photocurrent of water oxidation.

Photo-Scanning Electrochemical Microscopy on the Nanoscale with Through-Tip Illumination.

Scanning electrochemical microscopy (SECM) has previously been employed in probing photoelectrochemical processes at semiconductor surfaces. However, the spatial resolution of these studies has not yet matched the nanoscale SECM resolution attained without substrate illumination. Herein, we introduce nanoscale photo-SECM with a glass-sealed, polished tip simultaneously serving as a nanoelectrode and a light guide to produce a microscopic light spot on the substrate surface. The advantages of this approach are demonstrated by comparing current transients obtained using through-tip and global illumination of the sample. The spot of light on the substrate surface facing the nanotip was sufficiently bright to measure the diffusion-controlled positive feedback current in good agreement with the theory. We employed this approach for high-resolution photoelectrochemical mapping of ferrocenemethanol oxidation and oxygen evolution reactions at the Nb:TiO2 rutile (110) single crystal surface. The images obtained using 40-50 nm radius tips showed only minor and random variations in photoelectrochemical reactivity for both processes, pointing to essentially uniform distribution of the Nb dopant over the TiO2 surface and no measurable segregation on the ∼50 nm scale.

Electronic Processes within Quantum Dot-Molecule Complexes.

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.

Control of the Redox Activity of PbS Quantum Dots by Tuning Electrostatic Interactions at the Quantum Dot/Solvent Interface.

This paper describes the control of electron exchange between a colloidal PbS quantum dot (QD) and a negatively charged small molecule (9,10-anthraquinone-2-sulfonic acid sodium salt, AQ), through tuning of the charge density in the ligand shell of the QD, within an aqueous dispersion. The probability of electron exchange, measured through steady-state and time-resolved optical spectroscopy, is directly related to the permeability of the protective ligand shell, which is a mixed monolayer of negatively charged 6-mercaptohexanoate (MHA) and neutral 6-mercaptohexanol (MHO), to AQ. The composition of the ligand shell is quantitatively characterized by (1)H NMR. The dependence of the change in Gibbs free energy, ΔGobs, for the diffusion of AQ through the charged ligand shell and its subsequent adsorption to the QD surface is well-described with an electrostatic double-layer model for the QD/solvent interface. Fits of the optical data to this model yield an increase in the free energy for transfer of AQ from bulk solution to the surface of the QD (where it exchanges electrons with the QD) of 154 J/mol upon introduction of each additional charged MHA ligand to the ligand shell. This work expands the set of chemical parameters useful for controlling the redox activity of QDs via surface modification and suggests strategies for the use of nanoparticles for molecular and biomolecular recognition within chemically complex environments and for design of chemically stable nanoparticles for aqueous photocatalytic systems.

Description of the Adsorption and Exciton Delocalizing Properties of p-Substituted Thiophenols on CdSe Quantum Dots.

This work describes the quantitative characterization of the interfacial chemical and electronic structure of CdSe quantum dots (QDs) coated in one of five p-substituted thiophenolates (X-TP, X = NH2, CH3O, CH3, Cl, or NO2), and the dependence of this structure on the p-substituent X. (1)H NMR spectra of mixtures of CdSe QDs and X-TPs yield the number of X-TPs bound to the surface of each QD. The binding data, in combination with the shift in the energy of the first excitonic peak of the QDs as a function of the surface coverage of X-TP and Raman and NMR analysis of the mixtures, indicate that X-TP binds to CdSe QDs in at least three modes, two modes that are responsible for exciton delocalization and a third mode that does not affect the excitonic energy. The first two modes involve displacement of OPA from the QD core, whereas the third mode forms cadmium-thiophenolate complexes that are not electronically coupled to the QD core. Fits to the data using the dual-mode binding model also yield the values of Δr1, the average radius of exciton delocalization due to binding of the X-TP in modes 1 and 2. A 3D parametrized particle-in-a-sphere model enables the conversion of the measured value of Δr1 for each X-TP to the height of the potential barrier that the ligand presents for tunneling of excitonic hole into the interfacial region. The height of this barrier increases from 0.3 to 0.9 eV as the substituent, X, becomes more electron-withdrawing.

Composition and Permeability of Oleate Adlayers of CdS Quantum Dots upon Dilution to Photoluminescence-Relevant Concentrations.

This paper describes the changes in surface chemistry that occur in oleate-capped CdS quantum dots (QDs) upon dilution from NMR-relevant concentrations (10 µM) to photoluminescence (PL)-relevant concentrations (0.1 µM) and the consequences these changes have on the relative probabilities of radiative and nonradiative decay of the QD exciton. Characterization of the QD surface by nuclear magnetic resonance (NMR) spectroscopy reveals that upon dilution in three solvents, C6D6, C6D12, and CDCl3, oleate ligands, in the form of cadmium oleate and Cd(x)OA(y) clusters, desorb. Changes in the ligand coverage by 30-40% do not impact the solubility of the QDs, do not have measurable influence on the absorption or PL line widths, produce small (±0.05), nonmonotonic changes in the relative PL quantum yield, and produce small, nonmonotonic changes the relative partitioning between band-edge and "trapped" exciton emission. Desorption of surface ligands as a result of dilution of the QDs does, however, make the QDs more redox-active with respect to a small-molecule photooxidant, benzoquinone (BQ), because less dense organic adlayers allow a greater number of BQs to permeate the ligand shell and adsorb to the QD surface. Unlike previous studies, in which the QD concentrations used for NMR characterization were more than a factor of 10 higher than those used for optical measurements, this study directly correlates the surface composition of the QDs to their photophysical properties.

Contrasting electrogenerated chemiluminescence for a dissolved and surface-attached carbazole thiophene cyanoacrylate dye.

The electrogenerated chemiluminescence (ECL) of a carbazole thiophene cyanoacrylate dye ((2-cyano-3-[5"'-(9-ethyl-9H-carbazol-3-yl)-3',3",3"',4-tetra-n-hexyl-[2,2',5',2",5",2"']-quarter-thiophenyl-5yl]acrylate) = MK-2) has been investigated in solution, where the maximum ECL wavelength occurs at 640 nm, and in a thin film on an ITO surface, where the ECL is substantially red-shifted to 730 nm. The ECL intensity for the solution annihilation reaction is relatively weak, whereas a much higher ECL intensity is measured with oxalate as a co-reactant. This result is attributed to the two Nernstian reversible oxidation waves of the thiophene moiety of MK-2, whereas the reduction is stabilized by the unblocked carbazole and cyanoacrylate groups.

Templated homoepitaxial growth with atomic layer deposition of single-crystal anatase (101) and rutile (110) TiO2.

Homoepitaxial growth of highly ordered and pure layers of rutile on rutile crystal substrates and anatase on anatase crystal substrates using atomic layer deposition (ALD) is reported. The epilayers grow in a layer-by-layer fashion at low deposition temperatures but are still not well ordered on rutile. Subsequent annealing at higher temperatures produces highly ordered, terraced rutile surfaces that in many cases have fewer electrically active defects than the substrate crystal. The anatase epitaxial layers, grown at 250 °C, have much fewer electrically active defects than the rather impure bulk crystals. Annealing the epilayers at higher temperatures increased band gap photocurrents in both anatase and rutile.

Influence of the aggregation of a carbazole thiophene cyanoacrylate sensitizer on sensitized photocurrents on ZnO single crystals.

Dye sensitization of zinc oxide single crystals by a carbazole thiophene cyanoacrylate (MK-2) sensitizer deposited from THF and mixtures of THF and water was investigated. AFM images show the formation of larger aggregates, with the maximum size of 20-30 nm from mixtures of THF and water, compared with 8-12 nm from pure THF. Sensitized photocurrent spectra were correlated with the morphological results from AFM imaging and indicate that aggregation in water results in less efficient sensitization of the ZnO substrate. The presence of the aggregation in solution due to water content was confirmed by absorbance and fluorescence spectroscopies.

Electrogenerated chemiluminescence of BODIPY, Ru(bpy)3(2+), and 9,10-diphenylanthracene using interdigitated array electrodes.

Interdigitated array electrodes (IDAs) were used to produce steady-state electrogenerated chemiluminescence (ECL) by annihilation of oxidized and reduced forms of a substituted boron dipyrromethene (BODIPY) dye, 9,10-diphenylanthracene (DPA), and ruthenium(II) tris(bypiridine) (Ru(bpy)3(2+)). Digital simulations were in good agreement with the experimentally obtained currents and light outputs. Coreactant experiments, using tri-n-propylamine and benzoyl peroxide as a sacrificial homogeneous reductant or oxidant, show currents corresponding to electrode reactions of the dyes and not the oxidation or reduction of the coreactants. The results show that interdigitated arrays can produce stable ECL where the light intensity is magnified due to the larger currents as a consequence of feedback between generator and collector electrodes in the IDA. The light output for ECL is around 100 times higher than that obtained with regular planar electrodes with similar area.

Synthesis, Photophysics, Electrochemistry and Electrogenerated Chemiluminescence of PEG-Modified BODIPY dyes in Organic and Aqueous Solutions.

A set polyethylene glycol (PEG) appended BODIPY architectures (BOPEG1 - BOPEG3) have been prepared and studied in CH2Cl2, H2O:CH3CN (1:1) and aqueous solutions. BOPEG1 and BOPEG2 both contain a short PEG chain and differ in substitution about the BODIPY framework. BOPEG3 is comprised of a fully substituted BODIPY moiety linked to a PEG polymer that is roughly 13 units in length. The photophysics and electrochemical properties of these compounds have been thoroughly characterized in CH2Cl2 and aqueous CH3CN solutions. The behavior of BOPEG1 - BOPEG3 correlates with established rules of BODIPY stability based on substitution about the BODIPY moiety. ECL for each of these compounds was also monitored. BOPEG1, which is unsubstituted at the 2- and 6-positions dimerized upon electrochemical oxidation while BOPEG2, which contains ethyl groups at the 2- and 6-positions, was much more robust and served as an excellent ECL luminophore. BOPEG3 is highly soluble in water due to the long PEG tether and demonstrated modest ECL activity in aqueous solutions using tri-n-propylamine (TPrA) as a coreactant. As such, BOPEG3 represents the first BODIPY derivative that has been shown to display ECL in water without the need for an organic cosolvent, and marks an important step in the development of BODIPY based ECL probes for various biosensing applications.

Scanning electrochemical microscopy study of ion annihilation electrogenerated chemiluminescence of rubrene and [Ru(bpy)3]2+.

Scanning electrochemical microscopy (SECM) was used for the study of electrogenerated chemiluminescence (ECL) in the radical annihilation mode. The concurrent steady-state generation of radical ions in the microgap formed between a SECM probe and a transparent microsubstrate provides a distance-dependent ECL signal that can provide information about the kinetics, stability, and mechanism of the light emission process. In the present study, the ECL emission from rubrene and [Ru(bpy)(3)](2+) was used to model the system by carrying out experiments with the SECM and light-detecting apparatus inside an inert atmosphere box. We studied the influence of the distance between the two electrodes, d, and the annihilation kinetics on the ECL light emission profiles under steady-state conditions, as well as the ECL profiles when carrying out cyclic voltammetry (CV) at a fixed d. Experimental results are compared to simulated results obtained through commercial finite element method software. The light produced by annihilation of the ions was a function of d; stronger light was observed at smaller d. The distance dependence of the ECL emission allows the construction of light approach curves in a similar fashion as with the tip currents in the feedback mode of SECM. These ECL approach curves provide an additional channel to describe the reaction kinetics that lead to ECL; good agreement was found between the ECL approach curve emission profile and the simulated results for a fast, diffusion-limited second-order annihilation process (k(ann) > 10(7) M(-1) s(-1)). In the CV mode at fixed distance, the ECL emission of rubrene showed two distinct signals at different potentials when fixing the substrate to generate the radical cation and scanning the tip to generate the radical anion. The first signal (pre-emission) corresponded to an emission well before reaching the generation of the radical anion and was more intense on Au than on Pt. The second ECL signal showed the expected steady-state behavior from the second-order annihilation reaction and agreed well with the simulation. A comparison of the emission obtained with rubrene and [Ru(bpy)(3)](2+) to test the direct formation of lower energy triplets directly at the electrode showed that triplets are not the cause of the pre-emission observed. Wavelength selection experiments for the rubrene system showed that the pre-emission ECL signal also appeared slightly red-shifted with respect to the main luminophore emission; a possible explanation for this phenomenon is inverse photoemission, where the injection of highly energetic holes by the oxidized species into the negatively biased tip electrode causes emission of states in the metal that appear at a different wavelength than the singlet emission from the ECL luminophore.

Electrochemistry and electrogenerated chemiluminescence of BODIPY dyes.

BODIPY (boron dipyrromethene) dyes are unique materials with spectroscopic and electrochemical properties comparable to those of aromatic hydrocarbons. Electrochemical studies are useful in understanding the redox properties of these materials and finding structure-stability relations for the radical ions; along with spectroscopy, these studies help researchers design novel compounds with desired properties. This Account represents our attempt at a full description of the electrochemical and electrogenerated chemiluminescence (ECL) properties of the BODIPY dyes. When the dyes are completely substituted with alkyl or other groups, the radical ions of BODIPY dyes are highly stable. But if they include unsubstituted positions, the radical ions can undergo dimerization or other reactions. BODIPY dyes also show unusually large separations, ~1.0 V, between the first and second cyclic voltammetric (CV) waves for both oxidation and reduction half-reactions. Alkyl-substituted BODIPY dyes show good photoluminescence (PL) quantum efficiencies, and radical ion electron transfer annihilation in these molecules produces electrogenerated chemiluminescence (ECL), the intensity of which depends on the structure of the dye. The large separation between waves and the presence of strong ECL signals are both important in the design of stable ECL-based materials. The ECL spectra provide a fast method of monitoring the electrochemical formation of dimers and aggregates from the monomers. BODIPY dyes are particularly good systems for studying stepwise electron transfer in their chemically synthesized oligomers and polymers because of the small separation between the first oxidation and first reduction waves, generally about 2.0-2.4 V, and their relative ease of reduction compared with many other aromatic compounds. The larger separation between consecutive waves for oxidation compared with reduction is noticeable for all BODIPY dimers and trimers. We also observe a more difficult addition or extraction of a third electron compared with the second for the trimers, signaling the importance of electrostatic interactions. In general, BODIPY dyes combine interesting electrochemical and spectroscopic properties that suggest useful analytical applications.

Chemical and electrochemical dimerization of BODIPY compounds: electrogenerated chemiluminescent detection of dimer formation.

The electrochemistry of several difluoroboradiaza-s-indacene (BODIPY) compounds lacking substituent groups in the meso (8)- and/or 3 (α)-positions was investigated. Chemical and electrochemical dimerization was demonstrated, and the dimerization depended on the character of substitution. The chemical dimerization was achieved by oxidative coupling using FeCl(3) in CH(2)Cl(2) at 0 °C. The electrochemical dimerization proceeded via anodic oxidation to the radical cation and monitored by both cyclic voltammetry (CV) and electrogenerated chemiluminescence (ECL). An available open 3-position was important for the formation of the dimer. The resulting 3,3'-dimer produced a second peak in the CV oxidation and also the appearance of a longer wavelength ECL peak at 656 nm, which is considerably shifted from the parent peak at 532 nm. No dimerization was seen for BODIPY molecules in which only the meso 8-position was unsubstituted, either by chemical or electrochemical means, demonstrating that dimerization occurs at position 3.

Synthesis, photophysical, electrochemical, and electrogenerated chemiluminescence studies. Multiple sequential electron transfers in BODIPY monomers, dimers, trimers, and polymer.

Synthesis of the C(8) BODIPY monomers, dimers, and trimers, a C(8) polymer, and N(8) aza-BODIPY monomer and dimer was carried out. Methyl and mesityl C(8)-substituted monomers, dimers, and trimers were used. Dimers, trimers, and polymer were formed chemically through the ß-ß (2/6) positions by oxidative coupling using FeCl(3). A red shift of the absorbance and fluorescence is observed with addition of monomer units from monomer to polymer for C(8) dyes. The aza-BODIPY dye shows red-shifted absorbance and fluorescence compared with the C(8) analogue. Cyclic voltammetry shows one, two, and three one-electron waves on both reduction and oxidation for the monomer, dimer, and trimer, respectively, for the C(8) BODIPYs. The separation for the reduction peaks for the C(8) dimers is 0.12 V compared with 0.22 V for the oxidation, while the trimers show separations of 0.09 V between reduction peaks and 0.13 V for oxidation peaks. The larger separations between the second and third peaks, 0.25 V for the oxidation and 0.2 V for the reduction, are consistent with a larger energy to remove or add a third electron compared with the second one. The BODIPY polymer shows the presence of many sequential one-electron waves with a small separation. These results provide evidence for significant electronic interactions between different monomer units. The aza-BODIPY dye shows a reduction peak 0.8 V more positive compared to the C(8) compound. Aza-BODIPY dimer shows the appearance of four waves in dichloromethane. The separation between two consecutive waves is around 0.12 V for reduction compared with 0.2 V for oxidation, which is comparable with the results for the C(8) dyes. Electrogenerated chemiluminescence (ECL) of the different species was obtained, including weak ECL of the polymer.

Synthesis, Photophysics, Electrochemistry and Electrogenerated Chemiluminescence of a Homologous Set of BODIPY-Appended Bipyridine Derivatives.

Two new 2,2'-bipyridine (bpy) based ligands with ancillary BODIPY chromophores attached at the 4 and 4'-positions were prepared and characterized, which vary in the substitution pattern about the BODIPY periphery by either excluding (BB1) or including (BB2) a ß-alkyl substituent. Both absorb strongly throughout the visible region and are strongly emissive. The basic photophysics and electrochemical properties of BB1 and BB2 are comparable to those of the BODIPY monomers on which they are based. The solid-state structures and electronic structure calculations both indicate that there is negligible electronic communication between the BODIPY moieties and the intervening bpy spacers. Electrogenerated chemiluminescence spectra of the two Bpy-BODIPY derivatives are similar to their recorded fluorescence profiles and are strongly influenced by substituents on the BODIPY chromophores. These 2,2'-bipyridine derivatives represent a new set of ligands that should find utility in applications including light-harvesting, photocatalysis, and molecular electronics.

Dependence of electrochemical and electrogenerated chemiluminescence properties on the structure of BODIPY dyes. Unusually large separation between sequential electron transfers.

Electrochemistry and electrogenerated chemiluminescence (ECL) of selected substituted BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) dyes have been studied. The location and nature of substituents on positions 1-8 are important in predicting the behavior, and especially the stability, of the radical ions formed on electron transfer. Dyes with unsubstituted positions 2, 6, and 8 show a kinetic contribution to both oxidation and reduction. Dyes with only unsubstituted positions 2 and 6 and a substituted 8 position show chemically reversible reduction but irreversible oxidation. Unsubstituted positions 2 and 6 tend to show dimer formation on oxidation. Completely substituted dyes show nernstian oxidation and reduction. Oxidation and reduction studies of simple BODIPY dyes show an unusually large separation between the first and second reduction peaks and also the first and second oxidation peaks, of about 1.1 V, which is very different from that observed for polycyclic hydrocarbons and other heteroaromatic compounds, where the spacing is usually about 0.5 V. Electronic structure calculations confirmed this behavior, and this effect is attributed to a greater electronic energy required to withdraw or add a second electron and a lower relative solvation energy for the dianion or dication compared with those of the polycyclic hydrocarbons. ECL was generated for all compounds either by annihilation or by using a co-reactant.