Single-Molecule Kinetics of Interfacial Electron Transfer

Single-Molecule Study of Redox Reaction Kinetics by Observing Fluorescence Blinking

Recent advances in fluorescence microscopy allow us to track chemical reactions at the single-molecule level. Single-molecule measurements make it possible to minimize the amount of sample needed for analysis and diagnosis. Signal amplification is often applied to ultralow-level biomarker detection. Polymerase chain reaction (PCR) is used to detect DNA/RNA, and enzyme-linked immunosorbent assay (ELISA) can sensitively probe antigen-antibody interactions. While these techniques are brilliant and will continue to be used in the future, single-molecule-level measurements would allow us to reduce the time and cost needed to amplify signals.The kinetics of chemical reactions have been studied mainly using ensemble-averaged methods. However, they can hardly distinguish time-dependent fluctuations and static heterogeneity of the kinetics. The information hidden in ensemble-averaged measurements would be extractable from a single-molecule experiment. Thus, single-molecule measurement would provide unique opportunities to investigate unrevealed phenomena and to elucidate the questions in chemistry, physics, and life sciences. Redox reaction, which is triggered by electron transfer, is among the most fundamental and ubiquitous chemical reactions. The redox reaction of a fluorescent molecule results in the formation of radical ions, which are normally nonemissive. In single-molecule-level measurements, the redox reaction causes the fluctuation of fluorescence signals between the bright ON-state and the dark OFF-state, in a phenomenon called blinking. The duration of the OFF-state (τ_OFF) corresponds to the lifetime of the radical ion state, and its reaction kinetics can be measured as 1/τ_OFF. Thus, the kinetics of redox reactions of fluorescent molecules can be accessed at the single-molecule level by monitoring fluorescence blinking. One of the key aspects of single-molecule analysis based on blinking is its robustness. A blinking signal with a certain regular pattern enables single fluorescent molecules to be distinguished and resolved from the random background signal.In this Account, we summarize the recent studies on the single-molecule measurement of redox reaction kinetics, with a focus on our group's recent progress. We first introduce the control of redox blinking to increase the photostability of fluorescent molecules. We then demonstrate the control of redox blinking, which allows us to detect target DNA by monitoring the function of a molecular beacon-type probe, and we investigate antigen-antibody interactions at the single-molecule level. By tracing the time-dependent changes in blinking patterns, redox blinking is shown to be adaptable to tracking the structural switching dynamics of RNA, the preQ₁ riboswitch. This Account ends with a discussion of our ongoing work on the control of fluorescent blinking. We also discuss the development of devices that allow single-molecule-level analysis in a high-throughput fashion.

Electron Transfer in a Naphthalene Diimide System Studied by Single-Molecule Delayed Fluorescence

Electron transfer (ET) is a key chemical reaction in nature and has been extensively studied in bulk systems, but remains challenging to investigate at the single-molecule level. A previously reported naphthalene diimide (NDI)-based system (Higginbotham et al., Chem. Commun. 2013, 49, 5061–5063) displays delayed fluorescence with good quantum yield (~0.5) and long-lived (nanoseconds) prompt and delayed fluorescence lifetimes, providing an opportunity to interrogate the underlying ET processes in single molecules. Time-resolved single-molecule fluorescence measurements enabled forward and reverse ET rate constants to be calculated for 45 individual molecules embedded in poly(methylmethacrylate) (PMMA) film. Interpretation of the results within the framework of Marcus–Hush theory for ET demonstrates that variation in both the electronic coupling and the driving force for ET is occurring from molecule to molecule within the PMMA film and over time for individual molecules.

Single-molecule photoredox catalysis

Photocatalytic dehalogenation by a common dyestuff under aqueous conditions is driven by energy-additive absorption of two photons on the single-molecule level.

The chemistry of life is founded on light, so is it appropriate to think of light as a chemical substance? Planck's quantization offers a metric analogous to Avogadro's number to relate the number of particles to an effective reaction of single molecules and photons to form a new compound. A rhodamine dye molecule serves as a dehalogenating photocatalyst in a consecutive photoelectron transfer (conPET) process which adds the energy of two photons, with the first photon inducing radical formation and the second photon triggering PET to the substrate molecule. Rather than probing catalytic heterogeneity and dynamics on the single-molecule level, single-photon synthesis is demonstrated: the light quantum constitutes a reactant for the single substrate molecule in a dye–driven reaction. The approach illustrates that molecular diffusion and excited-state internal conversion are not limiting factors in conPET reaction kinetics because of catalyst–substrate preassociation. The effect could be common to photoredox catalysis, removing the conventional requirement of long excited-state lifetimes.

Single-Molecule Electrochemistry on a Porous Silica-Coated Electrode

Here we report the direct observation and quantitative analysis of single redox events on a modified indium-tin oxide (ITO) electrode. The key in the observation of single redox events are the use of a fluorogenic redox species and the nanoconfinement and hindered redox diffusion inside 3-nm-diameter silica nanochannels. A simple electrochemical process was used to grow an ultrathin silica film (∼100 nm) consisting of highly ordered parallel nanochannels exposing the electrode surface from the bottom. The electrode-supported 3-nm-diameter nanochannels temporally trap fluorescent resorufin molecules resulting in hindered molecular diffusion in the vicinity of the electrode surface. Adsorption, desorption, and heterogeneous redox events of individual resorufin molecules can be studied using total-internal reflection fluorescence (TIRF). The rate constants of adsorption and desorption processes of resorufin were characterized from single-molecule analysis to be (1.73 ± 0.08) × 10^-4 cm·s^-1 and 15.71 ± 0.76 s^-1, respectively. The redox events of resorufin to the non-fluorescent dihydroresorufin were investigated by analyzing the change in surface population of single resorufin molecules with applied potential. The scan-rate-dependent molecular counting results (single-molecule fluorescence voltammetry) indicated a surface-controlled electrochemical kinetics of the resorufin reduction on the modified ITO electrode. This study demonstrates the great potential of mesoporous silica as a useful modification scheme for studying single redox events on a variety of transparent substrates such as ITO electrodes and gold or carbon film coated glass electrodes. The ability to electrochemically grow and transfer mesoporous silica films onto other substrates makes them an attractive material for future studies in spatial heterogeneity of electrocatalytic surfaces.

Single-molecule fluorescence resonance energy transfer in molecular biology

Single-molecule fluorescence resonance energy transfer (smFRET) is a powerful technique for studying the conformation dynamics and interactions of individual biomolecules. In this review, we describe the concept and principle of smFRET, illustrate general instrumentation and microscopy settings for experiments, and discuss the methods and algorithms for data analysis. Subsequently, we review applications of smFRET in protein conformational changes, ion channel open-close properties, receptor-ligand interactions, nucleic acid structure regulation, vesicle fusion, and force induced conformational dynamics. Finally, we discuss the main limitations of smFRET in molecular biology.

Electron transfer-based single molecule fluorescence as a probe for nano-environment dynamics

Electron transfer (ET) is one of the most important elementary processes that takes place in fundamental aspects of biology, chemistry, and physics. In this review, we discuss recent research on single molecule probes based on ET. We review some applications, including the dynamics of glass-forming systems, surface binding events, interfacial ET on semiconductors, and the external field-induced dynamics of polymers. All these examples show that the ET-induced changes of fluorescence trajectory and lifetime of single molecules can be used to sensitively probe the surrounding nano-environments.

Imaging fluorescent nanoparticles to probe photoinduced charging of a semiconductor-solution interface

Optically transparent semiconductors allow simultaneous control of interfacial electrical potential and spectroscopic observation of chemistry near the electrode surface. Care must be taken, however, to avoid unwanted photoexcitation-induced charging of the semiconductor electrode that could influence the results. In this work, we investigate the in situ surface charging by photoexcitation well below the band gap of an optically transparent semiconductor, indium-tin oxide (ITO) electrode. Using total-internal-reflection fluorescence microscopy, the population of ~100-nm negatively charged carboxylate-polystyrene fluorescent nanoparticles at an ITO-aqueous solution interface could be monitored in situ. At positive applied potentials (~0.7 V versus Ag/AgCl), nanoparticles accumulate reversibly in the electrical double-layer of the ITO surface, and the interfacial nanoparticle populations increase with 488-nm excitation intensity. The potential sensitivity of nanoparticle population exhibited no dependence on excitation intensity, varied from 0.1 to 10 W cm(-2), while the onset potential for particle accumulation shifted by as much as 0.3 V. This shift in surface potential appears to be due to photoexcitation-induced charging of the ITO, even though the excitation radiation photon energy, ~2.4 eV, is well below the primary band gap of ITO, >3.5 eV. A kinetic model was developed to determine the photon order of electron-hole generation relative to the electron-hole recombination. The photoexcitation process was found to be first-order in photon flux, suggesting one-photon excitation of an indirect band gap or defect sites, rather than two-photon excitation into the direct band gap. A control experiment was conducted with red-fluorescent carboxylate-polystyrene particles that were counted using 647-nm excitation, where the photon energy is below the indirect band gap or defect site energy and where the optical absorption of the film vanishes. Red illumination between 1 and 15 W cm(-2) produced no detectable shifts in the onset accumulation potential, which is consistent with the negligible optical absorption of the ITO film at this longer wavelength.

Single-molecule DNA digestion in various alkanethiol-functionalized gold nanopores

This paper presents the alkanethiol-functionalized environmental effects of individual DNA molecules in nanopores on enzyme digestion at the single-molecule level. A template consisting of gold deposited within a solid-state nanoporous polycarbonate membrane was used to trap individual λ-DNA and enzyme molecules. The gold surfaces were modified with various functional groups (-OH, -COOH, -NH3). The enzyme digestion rates of single DNA molecules increased with decreasing nanopore diameters. Surprisingly, the digestion rates in the l-cysteine chemisorbed nanopores were 2.1-2.6 times faster than in the mercaptoethanol chemisorbed gold nanopores, even though these nanopores had equivalent interspacial areas. In addition, the membrane of chemisorbed cysteamine with ionized functional groups of H3N(+) at pH 8.2 had a greater positive influence on the enzyme digestion rate than the membrane of chemisorbed mercaptoproponic acid with ionized carboxyl groups (COO(-)). These results suggest that the three-dimensional environment effect is strongly correlated with the functional group in confined nanopores and can significantly change the enzyme digestion rates for nanopores with different internal areas.

Light-driven water oxidation for solar fuels

Light-driven water oxidation is an essential step for conversion of sunlight into storable chemical fuels. Fujishima and Honda reported the first example of photoelectrochemical water oxidation in 1972. In their system, TiO2 was irradiated with ultraviolet light, producing oxygen at the anode and hydrogen at a platinum cathode. Inspired by this system, more recent work has focused on functionalizing nanoporous TiO2 or other semiconductor surfaces with molecular adsorbates, including chromophores and catalysts that absorb visible light and generate electricity (i.e., dye-sensitized solar cells) or trigger water oxidation at low overpotentials (i.e., photocatalytic cells). The physics involved in harnessing multiple photochemical events for multielectron reactions, as required in the four-electron water oxidation process, has been the subject of much experimental and computational study. In spite of significant advances with regard to individual components, the development of highly efficient photocatalytic cells for solar water splitting remains an outstanding challenge. This article reviews recent progress in the field with emphasis on water-oxidation photoanodes inspired by the design of functionalized thin film semiconductors of typical dye-sensitized solar cells.

Photophysical behaviors of single fluorophores localized on zinc oxide nanostructures

Single-molecule fluorescence spectroscopy has now been widely used to investigate complex dynamic processes which would normally be obscured in an ensemble-averaged measurement. In this report we studied photophysical behaviors of single fluorophores in proximity to zinc oxide nanostructures by single-molecule fluorescence spectroscopy and time-correlated single-photon counting (TCSPC). Single fluorophores on ZnO surfaces showed enhanced fluorescence brightness to various extents compared with those on glass; the single-molecule time trajectories also illustrated pronounced fluctuations of emission intensities, with time periods distributed from milliseconds to seconds. We attribute fluorescence fluctuations to the interfacial electron transfer (ET) events. The fluorescence fluctuation dynamics were found to be inhomogeneous from molecule to molecule and from time to time, showing significant static and dynamic disorders in the interfacial electron transfer reaction processes.

Revealing time bunching effect in single-molecule enzyme conformational dynamics

In this perspective, we focus our discussion on how the single-molecule spectroscopy and statistical analysis are able to reveal enzyme hidden properties, taking the study of T4 lysozyme as an example. Protein conformational fluctuations and dynamics play a crucial role in biomolecular functions, such as in enzymatic reactions. Single-molecule spectroscopy is a powerful approach to analyze protein conformational dynamics under physiological conditions, providing dynamic perspectives on a molecular-level understanding of protein structure-function mechanisms. Using single-molecule fluorescence spectroscopy, we have probed T4 lysozyme conformational motions under the hydrolysis reaction of a polysaccharide of E. coli B cell walls by monitoring the fluorescence resonant energy transfer (FRET) between a donor-acceptor probe pair tethered to T4 lysozyme domains involving open-close hinge-bending motions. Based on the single-molecule spectroscopic results, molecular dynamics simulation, a random walk model analysis, and a novel 2D statistical correlation analysis, we have revealed a time bunching effect in protein conformational motion dynamics that is critical to enzymatic functions. Bunching effect implies that conformational motion times tend to bunch in a finite and narrow time window. We show that convoluted multiple Poisson rate processes give rise to the bunching effect in the enzymatic reaction dynamics. Evidently, the bunching effect is likely common in protein conformational dynamics involving in conformation-gated protein functions. In this perspective, we will also discuss a new approach of 2D regional correlation analysis capable of analyzing fluctuation dynamics of complex multiple correlated and anti-correlated fluctuations under a non-correlated noise background. Using this new method, we are able to map out any defined segments along the fluctuation trajectories and determine whether they are correlated, anti-correlated, or non-correlated; after which, a cross correlation analysis can be applied for each specific segment to obtain a detailed fluctuation dynamics analysis.

Seeing the forest for the trees: fluorescence studies of single enzymes in the context of ensemble experiments

Enzymes are remarkable molecular machines that make many difficult biochemical reactions possible under mild biological conditions with incredible precision and efficiency. Our understanding of the working principles of enzymes, however, has not reached the level where one can readily deduce the mechanism and the catalytic rates from an enzyme's structure. Resolving the dynamics that relate the three-dimensional structure of an enzyme to its function has been identified as a key issue. While still challenging to implement, single-molecule techniques have emerged as one of the most useful methods for studying enzymes. We review enzymes studied using single-molecule fluorescent methods but placing them in the context of results from other complementary experimental work done on bulk samples. This review primarily covers three enzyme systems--flavoenzymes, dehydrofolate reductase, and adenylate kinase--with additional enzymes mentioned where appropriate. When the single-molecule experiments are discussed together with other methods aiming at the same scientific question, the weakness, strength, and unique contributions become clear.

Photoinduced electron transfer of oxazine 1/TiO2 nanoparticles at single molecule level by using confocal fluorescence microscopy

Kinetics of photoinduced electron transfer (ET) from oxazine 1 dye to TiO(2) nanoparticles (NPs) surface is studied at a single molecule level by using confocal fluorescence microscopy. Upon irradiation with a pulsed laser at 630 nm, the fluorescence lifetimes sampled among 100 different dye molecules are determined to yield an average lifetime of 2.9 +/- 0.3 ns, which is close to the value of 3.0 +/- 0.6 ns measured on the bare coverslip. The lifetime proximity suggests that most interfacial electron transfer (IFET) processes for the current system are inefficient, probably caused by physisorption between dye and the TiO(2) film. However, there might exist some molecules which are quenched before fluorescing and fail to be detected. With the aid of autocorrelation analysis under a three-level energy system, the IFET kinetics of single dye molecules in the conduction band of TiO(2) NPs is evaluated to be (1.0 +/- 0.1) x 10(4) s(-1) averaged over 100 single molecules and the back ET rate constant is 4.7 +/- 0.9 s(-1). When a thicker TiO(2) film is substituted, the resultant kinetic data do not make a significant difference. The trend of IFET efficacy agrees with the method of fluorescence lifetime measurements. The obtained forward ET rate constants are about ten times smaller than the photovoltage response measured in an assembled dye-sensitized solar cell. The discrepancy is discussed. The inhomogeneous and fluctuation characters for the IFET process are attributed to microenvironment variation for each single molecule. The obtained ET rates are much slower than the fluorescence relaxation. Such a small ET quantum yield is yet feasibly detectable at a single molecule level.

Single-molecule fluorescence imaging of TiO(2) photocatalytic reactions

Heterogeneous photocatalysts have both potential and demonstrated applications for use in the water-splitting reaction that produces hydrogen, the degradation of organic pollutants, the surface wettability conversion, etc. In this feature article, we have focused on the in-site observation of various reactive oxygen species (ROS), such as singlet oxygen ((1)O(2)) and the hydroxyl radical ((*)OH), generated by the photoexcitation of TiO(2) nanomaterials using single-molecule fluorescence spectroscopy. The spatially resolved photoluminescence (PL) imaging techniques enable us to determine the location of the (photo)catalytically active sites that are related to the heterogeneously distributed defects on the surface. We also present the results that revealed the formation and reaction dynamics of the photogenerated charge carriers in individual TiO(2) nanoparticles. Furthermore, we introduce the single-molecule single-mismatch detection of the nucleotide sequence upon the photoexcitation of a novel nanoconjugate consisting of TiO(2) and DNA on the basis of the mechanistic aspects. Notably, the present conjugates can recognize the difference in a single nucleotide. Consequently, this article provides a significant opportunity to understand the temporal and spatial distributions of ROS generated during the photoirradiation of TiO(2) nanomaterials and directly explore the microscopic world in many fields ranging from fundamental physics and chemistry to practical applications.

Controlling the fluorescence of ordinary oxazine dyes for single-molecule switching and superresolution microscopy

Fluorescent molecular switches have widespread potential for use as sensors, material applications in electro-optical data storages and displays, and superresolution fluorescence microscopy. We demonstrate that adjustment of fluorophore properties and environmental conditions allows the use of ordinary fluorescent dyes as efficient single-molecule switches that report sensitively on their local redox condition. Adding or removing reductant or oxidant, switches the fluorescence of oxazine dyes between stable fluorescent and nonfluorescent states. At low oxygen concentrations, the off-state that we ascribe to a radical anion is thermally stable with a lifetime in the minutes range. The molecular switches show a remarkable reliability with intriguing fatigue resistance at the single-molecule level: Depending on the switching rate, between 400 and 3,000 switching cycles are observed before irreversible photodestruction occurs. A detailed picture of the underlying photoinduced and redox reactions is elaborated. In the presence of both reductant and oxidant, continuous switching is manifested by "blinking" with independently controllable on- and off-state lifetimes in both deoxygenated and oxygenated environments. This "continuous switching mode" is advantageously used for imaging actin filament and actin filament bundles in fixed cells with subdiffraction-limited resolution.

Single-molecule photophysics of oxazines on DNA and its application in a FRET switch

The role and interplay of triplet states and radical ion states in single-molecule fluorescence spectroscopy has recently been elaborated providing us with new insights into the photophysics and photobleaching pathways of fluorescent dyes. Adjustment of fluorophore redox properties in combination with specific redox properties of the environment, i.e. addition of reducing and oxidizing agents, allows control of the emission properties: it has become possible to suppress blinking and to also induce blinking in single-molecule fluorescence transient by selectively opening and closing specific excited state pathways. Induced blinking is, for example, of interest for super-resolution fluorescence microscopy based on the subsequent localization of single fluorophores. For oxazines this control even allowed the separation of the influence of reducing and oxidizing agents, enabling switching the fluorescence of single fluorophores. Here, we study the factors that contribute to the kinetics of the photophysical pathways more closely with a focus on the photophysics of the oxazine ATTO655 labeled to DNA. Our data show that the oxazine ATTO655 interacts with DNA, shielding it efficiently from reagents in solution. Besides redox reactions, the pH also influences the blinking kinetics and especially the off-times. Moreover, we present the extension of ATTO655 as a single-molecule redox sensor to a ratiometric fluorescence-resonance-energy-transfer based sensor. Therefore, we designed FRET probes that showed the highest possible contrast of FRET changes and demonstrate reversible FRET-switching of Cy3B-ATTO655 DNA constructs.

2D regional correlation analysis of single-molecule time trajectories

We report a new approach of 2D regional correlation analysis capable of analyzing fluctuation dynamics of complex multiple correlated and anticorrelated fluctuations under a noncorrelated noise background. Using this new method, by changing and scanning the start time and end time along a pair of fluctuation trajectories, we are able to map out any defined segments along the fluctuation trajectories and determine whether they are correlated, anticorrelated, or noncorrelated; after which, a cross-correlation analysis can be applied for each specific segment to obtain a detailed fluctuation dynamics analysis. We specifically discuss an application of this approach to analyze single-molecule fluorescence resonance energy transfer (FRET) fluctuation dynamics where the fluctuations are often complex, although this approach can be useful for analyzing other types of fluctuation dynamics of various physical variables as well.

Clay nanoparticle-supported single-molecule fluorescence spectroelectrochemistry

Here we report that clay nanoparticles allow formation of a modified transparent electrode, spontaneous adsorption of fluorescent redox molecules on the clay layer, and thus the subsequent observation of single-molecule fluorescence spectroelectrochemistry. We can trace single-molecule fluorescence spectroelectrochemistry by probing the fluorescence intensity change of individually immobilized single redox molecules modulated via cyclic voltammetric potential scanning. This work opens a new approach to explore interfacial electron transfer mechanisms of redox reactions.

Single Dibenzoterrylene Molecules in an Anthracene Crystal: Main Insertion Sites

We present a spectroscopic study of the properties of the two principal insertion sites (at 785.1 and 794.3 nm) of single dibenzoterrylene molecules in anthracene single crystals at cryogenic temperatures. We measured the temperature dependence of the line width, the orientation of the transition dipole moments, and the Stark effect. We performed molecular dynamics simulations, which show that one dibenzoterrylene molecule preferably replaces three anthracene molecules. From simulated annealing, we derive the molecular conformations in the most stable insertion sites and the orientations of the transition dipole moments. The good agreement between the spectroscopic results and the simulations allows us to propose unambiguous structures for the two principal spectroscopic sites.

Single Dibenzoterrylene Molecules in an Anthracene Crystal: Spectroscopy and Photophysics

We study single dibenzoterrylene molecules in an anthracene single crystal at 1.4 K in two insertion sites at 785.1 and 794.3 nm. The single-molecule zero-phonon lines are narrow (about 30 MHz), intense (the detected fluorescence rates at saturation reach 100,000 counts s(-1)), and very photostable. The intersystem-crossing yield is extremely low (10(-7) or lower). All of these features are hallmarks of an excellent system for high-resolution spectroscopy and nanoscale probing at cryogenic temperatures.

Single-Molecule Spectroelectrochemistry (SMS-EC)

We introduce single-molecule spectroelectrochemistry (SMS-EC), a powerful new technique for studying electrochemical kinetics in highly heterogeneous systems. This technique uses fluorescence single-molecule spectroscopy to indirectly measure electrochemical kinetics one molecule at a time, offering for the first time the distribution of key electrochemical variables, such as the half-wave potential, E1/2, not just the ensemble averages. In SMS-EC, the potential of the working electrode of an electrochemical cell is linearly scanned while simultaneously measuring the florescence intensity, Ifl(t), of individual single molecules as a function of time in a wide-field microscope. SMS-EC is used herein to study the oxidation at an indium tin oxide (ITO) electrode of single molecules of the organic conjugated polymer F8BT. The results reveal both excited singlet state and ground state oxidation of F8BT. The latter process occurs over a narrow distribution of single-molecule half-wave potential values, indicating a relatively uniform electrochemical potential at the electrode.

Solvent reorganization of electron transitions in viscous solvents

We develop a model of electron transfer reactions at conditions of nonergodicity when the time of solvent relaxation crosses the observation time window set up by the reaction rate. Solvent reorganization energy of intramolecular electron transfer in a charge-transfer molecule dissolved in water and acetonitrile is studied by molecular dynamics simulations at varying temperatures. We observe a sharp decrease of the reorganization energy at a temperature identified as the temperature of structural arrest due to cage effect, as discussed by the mode-coupling theory. This temperature also marks the onset of the enhancement of translational diffusion relative to rotational relaxation signaling the breakdown of the Stokes-Einstein relation. The change in the reorganization energy at the transition temperature reflects the dynamical arrest of the slow, collective relaxation of the solvent related to the relaxation of the solvent dipolar polarization. An analytical theory proposed to describe this effect agrees well with both the simulations and experimental Stokes shift data. The theory is applied to the analysis of charge-transfer kinetics in a low-temperature glass former. We show that the reorganization energy is substantially lower than its equilibrium value for the low-temperature portion of the data. The theory predicts the possibility of discontinuous changes in the dependence of the electron transfer rate on the free energy gap when the reaction switches between ergodic and nonergodic regimes.

Reorganization Energy of Electron Transfer in Viscous Solvents above the Glass Transition

We present a molecular-dynamics study of the solvent reorganization energy of electron transfer in supercooled water. We observe a sharp decrease of the reorganization energy at a temperature identified as the temperature of structural arrest due to cage effect as discussed by the mode coupling theory. Both the heat capacity and dielectric susceptibility of the pure water show sharp drops at about the same temperature. This temperature also marks the onset of the enhancement of translational diffusion relative to rotational relaxation signaling the breakdown of the Stokes-Einstein relation. The change in the reorganization energy at the transition temperature reflects the dynamical arrest of the slow, collective relaxation of the solvent related to Debye relaxation of the solvent dipolar polarization.

Probing Inhomogeneous Vibrational Reorganization Energy Barriers of Interfacial Electron Transfer

An atomic force microscopy (AFM) and confocal Raman microscopy study of the interfacial electron transfer of a dye-sensitization system, i.e., alizarin adsorbed upon TiO(2) nanoparticles, has revealed the distribution of the mode-specific vibrational reorganization energies encompassing different local sites ( approximately 250-nm spatial resolution). Our experimental results suggest inhomogeneous vibrational reorganization energy barriers and different Franck-Condon coupling factors of the interfacial electron transfer. The total vibrational reorganization energy was inhomogeneous from site to site; specifically, mode-specific analyses indicated that energy distributions were inhomogeneous for bridging normal modes and less inhomogeneous or homogeneous for nonbridging normal modes, especially for modes far away from the alizarin-TiO(2) coupling hydroxyl modes. The results demonstrate a significant step forward in characterizing site-specific inhomogeneous interfacial charge-transfer dynamics.