Rate Of Photoinduced Charge Transfer Research Articles

How good can a simple artificial neural network predict the medium reorganization energy and the free energy gap from a steady-state fluorescence spectrum?

Medium reorganization energy, Erm, and free energy gap, ΔG, are key characteristics in determining the photoinduced charge transfer rate. They are strongly related to the width and position of the steady-state fluorescence and absorption spectra of organic dyes in solvents. Steady-state spectroscopy is one of the most standard techniques for studying organic dyes. Here, we present a simple machine learning model for cost-effective prediction the medium reorganization energy and the free energy gap from the steady-state fluorescence spectra. Fluorescence spectra with corresponding reference values of physical quantities are calculated using an explicit mathematical expression that addresses electronic-vibrational transitions, intramolecular and solvent reorganizations. An approach to the separate formation of data sets for training and validation is proposed. Gaussian noise is used in training to improve predictions. This study shows that Erm and ΔG can be predicted with the mean absolute error ∼0.024 eV and ∼0.009 eV, respectively. Robustness testing is performed on the experimental spectra of coumarin-153, coumarin-307, and coumarin-522B in a series of solvents.

Photoelectrocatalytic Activity of ZnO-Modified Hematite Films in the Reaction of Alcohol Degradation.

Thin-film nanocrystalline hematite electrodes were fabricated by electrochemical deposition and loaded with electrodeposited zinc oxide in various amounts. Under visible light illumination, these electrodes demonstrate high activity in the photoelectrochemical degradation of methanol, ethylene glycol and, in particular, glycerol. Results of intensity-modulated photocurrent spectroscopy show that the photoelectrocatalysis efficiency is explained by the suppression of the electron-hole pair recombination and an increase in the rate of photo-induced charge transfer. Thus, zinc oxide can be considered an effective modifying additive for hematite photoanodes.

Thin-Film Nanocrystalline Zinc Oxide Photoanode Modified with CdO in Photoelectrocatalytic Degradation of Alcohols

Thin-film nanocrystalline zinc oxide electrodes were fabricated by electrochemical deposition of ZnO on FTO-coated glass slides. ZnO electrodes were promoted by CdO coating on top of ZnO in amounts corresponding to 0.8, 0.1, and 0.05 C cm−2 in electric units. Modification of ZnO by a small amount of CdO, corresponding to 0.05 C cm−2, shifts the photoactivity of the composite photoanode into the visible part of the solar spectrum. It is shown that the ZnO/(0.05C)CdO/FTO electrode demonstrates high efficiency in photoelectrochemical degradation of methanol, ethylene glycol, and glycerol when irradiated by a simulated sunlight. According to intensity-modulated photocurrent spectroscopy (IMPS), the effect is due to suppression of the electron–hole pairs recombination and increase in the rate of photo-induced charge transfer. Therefore, thin-film photoanodes based on zinc oxide modified by CdO can be used for photoelectrochemical degradation of byproducts of biofuel production glycerol, and of other alcohols.

(Keynote) Photoinduced Charge Transfer from Quantum Dots Measured By Cyclic Voltammetry

Measuring and modulating charge transfer processes at quantum dot interfaces are crucial steps in the development of quantum dot photocatalysis. In this work, cyclic voltammetry under illumination is demonstrated to measure the rate of photoinduced charge transfer from CdS quantum dots by directly probing the changing oxidation states of a library of model charge acceptors. The voltammetry data demonstrates the presence of long-lived electron donor states generated by native photodoping of the QDs as well as the relationship between driving force and rate of charge transfer. Changes to the voltammograms under illumination follow mechanistic predictions from classic zone diagrams and electrochemical modeling allows for measurement of the rate of productive electron transfer, giving rate constants (104 M-1s-1) that are distinct from the ps dynamics measured by conventional optical spectroscopy methods and that are more closely connected to the quantum yield of photoinduced chemical transformations.

Photoinduced Charge Transfer from Quantum Dots Measured by Cyclic Voltammetry.

Measuring and modulating charge-transfer processes at quantum dot interfaces are crucial steps in developing quantum dots as photocatalysts. In this work, cyclic voltammetry under illumination is demonstrated to measure the rate of photoinduced charge transfer from CdS quantum dots by directly probing the changing oxidation states of a library of molecular charge acceptors, including both hole and electron acceptors. The voltammetry data demonstrate the presence of long-lived charge donor states generated by native photodoping of the quantum dots as well as a positive correlation between driving force and rate of charge transfer. Changes to the voltammograms under illumination follow mechanistic predictions from the ErCi' zone diagram, and electrochemical modeling allows for measurement of the rate of productive electron transfer. Observed rates for photoinduced charge transfer are on the order of 0.1 s-1, which are distinct from the picosecond dynamics measured by conventional transient optical spectroscopy methods and are more closely connected to the quantum yield of light-mediated chemical transformations.

Donor-length dependent photoninduced charge transfer in two-photon absorption.

In this paper, we study the donor-length dependent photoinduced charge transfer in two-photon absorption (TPA). In the donor-acceptor (D-A) system of oligo-thiophene-fullerene, two kind of lengths of oligo-thiophene are chosen to study the donor-length dependent photoinduced charge transfer in TPA. It is found that donor length can significantly influence the rate of charge transfer. When the donor length is quarter-thiophene, the rate of charge transfer is less than 50%; while with the increase of donor-length to octant-thiophene, the rate of charge transfer can be reach up to 100%. The transition channels of photoinduced charge transfer in TPA are visualized with transition density matrix and charge difference density. Our results promote deeper understanding and rational design for donor-acceptor system with large rate of photoinduced charge transfer in TPA.

Photoinduced charge transfer rate of Cy3/C60 blend material.

The rates of charge separation and charge recombination of the cyanine dye/C60 heterojunction solar cell in an external electric field were provided using the Marcus and Marcus-Levich-Jortner formalisms. The vibrational mode as another influencing factor was also introduced into the rate expression for the planar heterojunction solar cell. Detailed theoretical analysis of the excited-state of the Cy3/C60 blend was achieved using density functional theory and time-dependent density functional theory. The Gibbs free energy was regulated by an external electric field, while the reorganisation energy presented the opposite conclusion. Frequency analysis was utilised to demonstrate the energy stability of the obtained structures. The rate calculated using the Marcus formalism was greater than that obtained by the Marcus-Levich-Jortner formalism. Consideration of the calculated rates in all vibration modes and at different external electric field strengths indicated that vibrational mode and external electric field played important roles in determining the rates of charge separate and charge recombination, which could provide a more accurate theoretical rate for organic photovoltaic devices.

Integration of plasmonic effect into spindle-shaped MIL-88A(Fe): Steering charge flow for enhanced visible-light photocatalytic degradation of ibuprofen

High electron-hole separation efficiency and wide spectrum absorption are desired for improving the photocatalytic activity. In this research, we integrate plasmonic Ag/AgCl with a typical semiconductor-like metal-organic framework MIL-88A(Fe) to form Ag/AgCl@MIL-88A(Fe) (ACMA) nanocomposites by a facile one-pot solvothermal method. The results demonstrate that ACMA nanocomposites not only broaden the visible-light absorption of MOFs, but also greatly accelerate photo-induced charge transfer rate. EDS mapping and HRTEM results suggest that Ag/AgCl is uniformly and densely dispersed on the surface of MOF, which is beneficial for charge flow and expedites the charge migration. By virtue of the structural and compositional features, these unique ACMA nanocomposites perform excellent photocatalytic activity on ibuprofen (IBP) in terms of high degradation efficiency, high mineralization and excellent cycling stability. ACMA-2 (Fe:Ag = 2:1) displays highly efficient photocatalytic activity on IBP under visible light irradiation, and the corresponding photo-degradation rate is 10.8 and 40.4 folds higher than that of MIL-88A(Fe) and Ag/AgCl, respectively. Total organic carbon (TOC) removal of IBP photodegradation reaches 91% by the catalysis of ACMA-2. Combined with the electrochemical tests, quencher experiments and analysis of related degradation products identified by ion chromatography (IC) and LC-MS-MS, oxidation by superoxide radicals (O2−) as well as holes (h+) and electrons (e−) is the dominant degradation process in the photocatalytic degradation of IBP. This work provides a new perspective for the preparation of environment-stable and efficient MOF-based photocatalysts by steering charge flow.

Excitation Frequency Dependence of Ultrafast Photoinduced Charge Transfer Dynamics

The dependence of the photoinduced charge transfer rate constant on the pump pulse carrier frequency is shown to be strong, and it is considerably affected by the value of the reorganization energy of low-frequency modes at the stage of excitation. In the area of small values of the reorganization energy, the dependence of the charge transfer rate constant on the pump pulse carrier frequency is strongly nonmonotonic that is caused by vibrational resonances and variation of the initial position of the wave packet on the term of the locally excited state. Increasing the reorganization energy smoothes the dependence. The smoothing is caused by the broadening of the vibrational resonances and their overlapping. The high-frequency vibrational mode excitation typically accelerates the charge transfer in both areas of high and low exergonicity and decelerates it in the vicinity of the Marcus barrierless region.

Tetramethoxybenzene is a Good Building Block for Molecular Wires: Insights from Photoinduced Electron Transfer.

Two donor bridge-acceptor molecules with terminal triarylamine and Ru(bpy)3(2+) (bpy = 2,2'-bipyridine) redox partners were synthesized and investigated by cyclic voltammetry, optical absorption, luminescence, and transient absorption spectroscopy. The two dyads differ only by the central bridging unit, which was tetramethoxybenzene (tmb) in one case and unsubstituted phenylene (ph) in the other case. Photoirradiation of the Ru(bpy)3(2+) complex of the two dyads triggers intramolecular electron transfer from the triarylamine to the (3)MLCT-excited metal complex, and this process occurs with time constants of 1.5 and 6.8 ns for the tmb- and ph-bridged dyads, respectively. Thermal electron transfer in the reverse direction then leads to disappearance of the photoproduct with a time constant of 10 ns in both dyads. The faster rate of photoinduced charge transfer in the tmb-bridged dyad can be understood in the framework of a hole-tunneling model in which the electron-rich tmb bridge imposes a more shallow barrier than the less electron-rich ph spacer. Until now tmb-based molecular wires have received very little attention, and alkoxy substituents have been mostly used for improving the solubility of oligo-p-phenylene vinylene (OPV) and oligo-p-phenylene ethynylene (OPE) wires. Our study illustrates how four alkoxy-substituents on a phenylene backbone can have a significant influence on the charge-transfer properties of a molecular wire, and this is relevant in the greater context of a future molecular electronics technology.

Effect of Excitation Pulse Carrier Frequency on Ultrafast Photoinduced Charge Transfer Kinetics: Effect of Intramolecular High Frequency Vibrational Mode Excitation

Influence of excitation pulse carrier frequency on photoinduced ultrafast intramolecular charge transfer kinetics is investigated in the framework of a multichannel stochastic point transition model involving an excited state formation. It is supposed that an intramolecular high frequency vibrational mode being active at the excitation stage also accepts the energy at the stage of photoinduced charge transfer. A strong dependence of the photoinduced charge transfer rate constant on excitation pulse carrier frequency is uncovered. Since this dependence is associated with charge separation from different excited states of an intramolecular high frequency vibrational mode, it is named the vibrational spectral effect. The simulations show that the effect may be both positive (the photoinduced charge transfer rate constant increases with increasing the excitation pulse carrier frequency) and negative (opposite trend). In the area of low exergonicity of the photoinduced charge transfer, the effect is mostly positive, while in the field of strong exergonicity, it is mostly negative. The amplitude of the vibrational spectral effect predicted by the model is rather large and can be observed in experiments even if the vibrational relaxation/redistribution time constant is as short as 100 fs.

External Electric Field-Dependent Photoinduced Charge Transfer in a Donor–Acceptor System for an Organic Solar Cell

External electric field is incorporated into the generalized Mulliken–Hush model and Marcus theory. With this new development, we have investigated the field-dependent electronic structure and rate of photoinduced charge transfer in organic donor–acceptor dyad using time-dependent density functional theory and extensive multidimensional visualization techniques. The model is used to evaluate the influence of the external electric field on the electronic coupling between donor and acceptor. The reorganization energy and the free energy change of the electron transfer were calculated. It was found that the major effects in the external electric field dependent rate of the charge transfer originate from changes in the electronic coupling. The new theoretical approach not only promotes a deeper understanding of the connection between the external electric field, chemical structure, and optical and electronic properties of the donor–acceptor system, but also can be used for rational design of novel donor–acceptor system for organic solar cells.

Enhancement of the photocatalytic activity of modified TiO2 nanoparticles with Zn2+. correlation between structure and properties

Zinc-doped and undoped TiO2 photocatalysts were synthesized via sol-gel techniques. Doping of TiO2 with M2+ (M-Zn) was intended to create tail states within the band gap of TiO2. These can subsequently be employed as efficient photocatalysts which can effectively decompose organic contaminants only with visible light activation. The structure, physico-chemical and optical properties of the products were characterized by using the X-ray diffraction (XRD), Raman spectra, UV-vis diffuse reflectance spectroscopy (DRS) and electrochemical impedance spectroscopy (EIS) techniques. Doping shifts the optical absorption edge to the visible region and decreases the charge-transfer resistance. Under visible light, the composite nanoparticles very efficiently catalyze the MB dye. Results implied that Zn doping increased photoinduced charge transfer rate, and the use of described methods is a powerful tool toward predicting and understanding the photocatalytic processes and behaviors.