Bimolecular Process Research Articles

Time-resolved Fourier-transform infrared spectroscopy reveals the hidden bimolecular process of the ferrioxalate actinometer.

Step-scan Fourier-transform infrared spectroscopy was used to monitor the photochemical reactions following the 266 nm-photolysis of aqueous ferrioxalate solutions on microsecond-to-millisecond time scales. Together with most recent observations from ultrafast infrared spectroscopy the reported results finally disclose the full molecular-level mechanism of a photochemical system that is widely known as the Hatchard-Parker actinometer.

Light-induced EPR study of spin-assisted charge transport in PFOT:PC61BM composite

Magnetic resonance, relaxation and dynamic parameters of polaron spins and methanfullerene radical anions, initiated by the infrared-visible-ultraviolet photons in bulk heterojunctions of the composite formed by narrow-band poly[(9,9-dioctylfluorenyl-2.7-diyl)-co-(bithiophene)] (PFOT, F8T2) copolymer and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) globules were investigated by the direct Light-Induced Electron Paramagnetic Resonance (LEPR) spectroscopy. It was shown that some of the polarons are captured by spin traps formed in the copolymer matrix due to its disordering. The number, spatial distribution and energy depth of such traps depend on the structure and morphology of composite. It is shown that the main parameters of both charge carriers are determined by the exchange interaction of the spin ensembles, as well as the energy of the photons. The formation of spin traps in the copolymer matrix and the exchange interaction between different spin packets cause the extreme sensitivity of the composite's magnetic resonance and electronic parameters to the number and energy of the initiating photons. The predominant photoinitiation of localized polarons in the copolymer matrix was demonstrated. This process is substantially accelerated when the composite is illuminated by photons with the energy lying near 1.8 and 2.7 eV. The recombination of both charge carriers can be described in terms of a bimolecular process of the second order. It was found that the contributions of polarons and methanofullerene radical anions to the effective paramagnetic susceptibility increase substantially near the photon energy of 2.6 and 2.1 eV due to the exchange interaction of these charge carriers. It is shown that the mobility of polarons varies monotonically throughout all the photon energy range, whereas the librational spin dynamics of the methanofullerene globules is substantially accelerated near the photon energy of 2.0 and 2.7 eV.

Determination of Ethofumesate‐2‐hydroxy Stereolability by Dynamic HPLC

AbstractEvaluating stereolability of target chiral chemical is pivotal when developing enantioselective studies. Ethofumesate‐2‐hydroxy is one of the main metabolites of ethofumesate, a chiral herbicide currently marketed as racemate. In order to perform enantioselective pharmacokinetic and metabolism studies, the stereolability of susceptible molecules should be evaluated. Based on that, dynamic high‐performance liquid chromatography experiments were performed to evaluate ethofumesate‐2‐hydroxy enantiomerization process on‐column. The thermodynamic results helped understand that ethofumesate‐2‐hydroxy enantiomerization is a bimolecular process assisted by acid and base catalysis. The enantiomerization barrier determined suggested that ethofumesate‐2‐hydroxy single enantiomers may undergo extensive racemization at ambient temperature turning ethofumesate‐2‐hydroxy enantioselective studies into an unfeasible task.

Perspective: The development and applications of H Rydberg atom translational spectroscopy methods.

Determining the product velocities offers one of the most direct and penetrating experimental probes of the dynamics of gas phase molecular photodissociation and bimolecular collision processes and provides an obvious point of contact with theoretical molecular dynamics simulations, potential energy surfaces, and non-adiabatic couplings between such surfaces. This perspective traces the development of the H Rydberg atom translational spectroscopy technique from a serendipitous first encounter through to the present, highlights the advances that make it the method of choice for studying many benchmark photofragmentation and photoinduced collision processes that yield H (or D) atoms amongst the products, and anticipates some future opportunities afforded by the technique.

Self-Consistency of the Theory of Elementary Stage Rates of Reversible Processes and the Equilibrium Distribution of Reaction Mixture Components

An analysis is presented of one of the key concepts of physical chemistry of condensed phases: the theory self-consistency in describing the rates of elementary stages of reversible processes and the equilibrium distribution of components in a reaction mixture. It posits that by equating the rates of forward and backward reactions, we must obtain the same equation for the equilibrium distribution of reaction mixture components, which follows directly from deducing the equation in equilibrium theory. Ideal reaction systems always have this property, since the theory is of a one-particle character. Problems arise in considering interparticle interactions responsible for the nonideal behavior of real systems. The Eyring and Temkin approaches to describing nonideal reaction systems are compared. Conditions for the self-consistency of the theory for mono- and bimolecular processes in different types of interparticle potentials, the degree of deviation from the equilibrium state, allowing for the internal motions of molecules in condensed phases, and the electronic polarization of the reagent environment are considered within the lattice gas model. The inapplicability of the concept of an activated complex coefficient for reaching self-consistency is demonstrated. It is also shown that one-particle approximations for considering intermolecular interactions do not provide a theory of self-consistency for condensed phases. We must at a minimum consider short-range order correlations.

Organic solar cells based on anthracene-containing PPE–PPVs and non-fullerene acceptors

Lately, non-fullerene acceptors (NFAs) have received increasing attention for use in polymer-based bulk-heterojunction (BHJ) organic solar cells (OSCs), as improved photovoltaic performance compared to classical polymer–fullerene blends could be demonstrated. In this study, polymer solar cells based on a statistically substituted anthracene-containing poly(p-phenylene ethynylene)-alt-poly(p-phenylene vinylene)s (PPE–PPVs) copolymer (AnE-PVstat) as donor in combination with a number of different electron accepting materials were investigated. Strong photoluminescence quenching of the polymer donor indicates intimate intermixing of both materials. However, the photovoltaic performances were found to be poor compared to blends that use fullerene as acceptor. Time-delayed collection field (TDCF) measurements demonstrate: charge generation is field-independent, but bimolecular recombination processes limit the fill factor and thus the efficiency of devices.

Transient Photocurrent Elucidating Carrier Dynamics and Potential of Bulk Heterojunction Solar Cells Fabricated by Thermal Precursor Approach

Transient photocurrents are measured to clarify carrier dynamics in a series of bulk heterojunction solar cells that are composed of [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) and diketopyrrolopyrrole‐tetrabenzoporphyrin conjugates substituted by straight alkyl groups (Cn‐DPP‐BP), of which the device parameters are largely dependent on the length of the substituted alkyl groups (Cn). The time profile of the observed transients indicates the presence of both unimolecular and bimolecular charge recombination processes in the active layer of the cells. Analysis of the photocurrent decays detected at various bias voltages using a drift model that considers charge extraction by electrodes and charge recombination in the active layer enables elucidation of the charge drift mobility, the initial photocarrier density, and the charge recombination rate constants. The bimolecular recombination rate constant is correlated with Cn and this is the main factor that determines the power conversion efficiency (η) of the Cn‐DPP‐BP:PCBM solar cells. The Cn‐dependent fill factor can be interpreted in terms of the bias voltage dependence of the ratio between the extraction and recombination charges (Qex/Qre). Comparison of η and Qex/Qre at the short‐circuit condition predicts that η for the Cn‐DPP‐BP:PCBM solar cell can be potentially improved up to 6%.

Observation of correlated excitations in bimolecular collisions.

Although collisions between atoms and molecules are largely understood, collisions between two molecules have proven much harder to study. In both experiment and theory, our ability to determine quantum-state-resolved bimolecular cross-sections lags behind their atom-molecule counterparts by decades. For many bimolecular systems, even rules of thumb-much less intuitive understanding-of scattering cross sections are lacking. Here, we report the measurement of state-to-state differential cross sections on the collision of state-selected and velocity-controlled nitric oxide (NO) radicals and oxygen (O2) molecules. Using velocity map imaging of the scattered NO radicals, the full product-pair correlations of rotational excitation that occurs in both collision partners from individual encounters are revealed. The correlated cross sections show surprisingly good agreement with quantum scattering calculations using ab initio NO-O2 potential energy surfaces. The observations show that the well-known energy-gap law that governs atom-molecule collisions does not generally apply to bimolecular excitation processes, and reveal a propensity rule for the vector correlation of product angular momenta.

Interplay of bimolecular and Auger recombination in photoexcited carrier dynamics in silicon nanocrystal/silicon dioxide superlattices

We report results of investigating carrier recombination in silicon nanocrystal/silicon dioxide superlattices. The superlattices prepared by nitrogen-free plasma enhanced chemical vapour deposition contained layers of silicon nanocrystals. Femtosecond transient transmission optical spectroscopy was used to monitor carrier mechanisms in the samples. The three-particle Auger recombination was observed in accord with previous reports. However, under high pump intensities (high photoexcited carrier densities) the bimolecular process dominated the recombination. Detailed analysis of measured data and fitting procedure made it possible to follow and quantify the interplay between the two recombination processes. The bimolecular recombination was interpreted in terms of the trap-assisted Auger recombination.

Ultrafast Exciton Dynamics in Shape-Controlled Methylammonium Lead Bromide Perovskite Nanostructures: Effect of Quantum Confinement on Charge Carrier Recombination

We report ultrafast spectroscopy investigations of photoinduced exciton dynamics in three novel CH3NH3PbBr3 perovskite nanostructures: nanocrystals (0D), nanowires (1D), and nanoplatelets (2D). Aided by analysis of UV–visible absorption and photoluminescence spectra, features in the transient absorption (TA) spectra are assigned to different charge carrier processes, time constants of which are determined in fitting the transient kinetics. Immediately after photoexcitation, the charge carrier thermalization process occurs within the instrument response function time (fwhm ∼ 350 fs) and results in a quasi-equilibrium distribution of charge carriers. It is followed by charge carrier cooling on a sub-picosecond time scale (τc ∼ 300 fs). The ensuing charge carrier recombination process obeys the rate law of a second-order reaction, which suggests that it occurs mainly via the bimolecular nongeminate recombination process. Dependence of the charge recombination rate constant (kr) on the initial charge carrier ...

Fluorescence Investigations on Interactions between 7,8-benzo-4-azidomethyl Coumarin and Ortho- and Para-phenylenediamines in Binary Solvent Mixtures of THF and Water.

Fluorescence investigations on interactions between 7,8-benzo-4-azidomethyl coumarin (7BAMC) and quenchers ortho-phenylenediamine (OPD) and para-phenylenediamines (PPD) in binary solvent mixtures (THF + water) have been reported. UV-absorption study indicated a weak hydrophobic interaction between 7BAMC and the para isomer. NMR spectral studies indicated the presence of an interaction between 7BAMC and PPD. Magnitudes of the parameters associated with FRET, showed the presence of interactions between 7BAMC and PPD quencher is more predominant than OPD. Fluorescence quenching studies reveal the role of static and dynamic quenching pathways, depending upon Stern-Volmer constant, dielectric constant and dominant non-radiative processes. Binding equilibria analysis indicates a strong interaction between 7BAMC and PPD than OPD and formation of H-bonding. Based on the magnitudes of free energy, enthalpy change and entropy, bimolecular interaction process may be considered as spontaneous and hydrophobic.

Lysosome‐Targeting Amplifiers of Reactive Oxygen Species as Anticancer Prodrugs

AbstractCancer cells produce elevated levels of reactive oxygen species, which has been used to design cancer specific prodrugs. Their activation relies on at least a bimolecular process, in which a prodrug reacts with ROS. However, at low micromolar concentrations of the prodrugs and ROS, the activation is usually inefficient. Herein, we propose and validate a potentially general approach for solving this intrinsic problem of ROS‐dependent prodrugs. In particular, known prodrug 4‐(N‐ferrocenyl‐N‐benzylaminocarbonyloxymethyl)phenylboronic acid pinacol ester was converted into its lysosome‐specific analogue. Since lysosomes contain a higher concentration of active ROS than the cytoplasm, activation of the prodrug was facilitated with respect to the parent compound. Moreover, it was found to exhibit high anticancer activity in a variety of cancer cell lines (IC50=3.5–7.2 μm) and in vivo (40 mg kg−1, NK/Ly murine model) but remained weakly toxic towards non‐malignant cells (IC50=15–30 μm).

Lysosome-Targeting Amplifiers of Reactive Oxygen Species as Anticancer Prodrugs.

Cancer cells produce elevated levels of reactive oxygen species, which has been used to design cancer specific prodrugs. Their activation relies on at least a bimolecular process, in which a prodrug reacts with ROS. However, at low micromolar concentrations of the prodrugs and ROS, the activation is usually inefficient. Herein, we propose and validate a potentially general approach for solving this intrinsic problem of ROS-dependent prodrugs. In particular, known prodrug 4-(N-ferrocenyl-N-benzylaminocarbonyloxymethyl)phenylboronic acid pinacol ester was converted into its lysosome-specific analogue. Since lysosomes contain a higher concentration of active ROS than the cytoplasm, activation of the prodrug was facilitated with respect to the parent compound. Moreover, it was found to exhibit high anticancer activity in a variety of cancer cell lines (IC50 =3.5-7.2 μm) and in vivo (40 mg kg-1 , NK/Ly murine model) but remained weakly toxic towards non-malignant cells (IC50 =15-30 μm).

Origin of Efficient Inverted Nonfullerene Organic Solar Cells: Enhancement of Charge Extraction and Suppression of Bimolecular Recombination Enabled by Augmented Internal Electric Field.

We report our effort to unravel the origin of efficient operation of nonfullerene organic solar cells (OSCs), based on a poly[4,8-bis(5-(2-ethylhexyl) thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)](PTB7-Th):3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno [1,2-b:5,6-b']dithiophene (ITIC) blend system. The effects of buildup of space charges, charge extraction, and bimolecular recombination processes on the performance and the stability of PTB7-Th:ITIC-based regular and reverse configuration OSCs are analyzed. It is found that the high-performing inverted PTB7-Th:ITIC OSCs benefit from the combined effects of (1) suppression of bimolecular recombination enabled by an augmented effective internal electric field and (2) improvement of charge extraction by avoiding the holes passing through ITIC-rich region, which would otherwise occur in a regular configuration cell. The inverted PTB7-Th:ITIC OSCs possess a significant improvement in the cell stability and a high power conversion efficiency of ∼8.0%, which is >29% higher than that of an optimized regular configuration control cell (6.1%).

Shorter Exciton Lifetimes via an External Heavy-Atom Effect: Alleviating the Effects of Bimolecular Processes in Organic Light-Emitting Diodes.

Multiexcited-state phenomena are believed to be the root cause of two exigent challenges in organic light-emitting diodes; namely, efficiency roll-off and degradation. The development of novel strategies to reduce exciton densities under heavy load is therefore highly desirable. Here, it is shown that triplet exciton lifetimes of thermally activated delayed-fluorescence-emitter molecules can be manipulated in the solid state by exploiting intermolecular interactions. The external heavy-atom effect of brominated host molecules leads to increased spin-orbit coupling, which in turn enhances intersystem crossing rates in the guest molecule. Wave function overlap between the host and the guest is confirmed by combined molecular dynamics and density functional theory calculations. Shorter triplet exciton lifetimes are observed, while high photoluminescence quantum yields and essentially unaltered emission spectra are maintained. A change in the intersystem crossing rate ratio due to increased dielectric constants leads to almost 50% lower triplet exciton densities in the emissive layer in the steady state and results in an improved onset of the photoluminescence quantum yield roll-off at high excitation densities. Efficient organic light-emitting diodes with better roll-off behavior based on these novel hosts are fabricated, demonstrating the suitability of this concept for real-world applications.

Ultrafast Excited-State Transport and Decay Dynamics in Cesium Lead Mixed Halide Perovskites

While significant research efforts directed toward characterizing the excited-state dynamics of lead halide perovskites have enabled promising advances in photovoltaic, light-emitting diode, and laser technologies, a detailed correlation between composition and functionality in this promising class of materials remains unestablished. We use pump–probe microscopy to characterize both transport and relaxation dynamics in individual crystals of CsPbI2Br, a mixed halide, all-inorganic analogue to the well-studied organic–inorganic hybrid perovskites. In contrast to the methylammonium lead tri-iodide perovskite, we find excited-state dynamics that decay primarily via first-order and Auger mechanisms. By global fitting of power-dependent kinetics collected from individual domains, we find a range of Auger rate constants between 3.3 × 10–30 and 1.5 × 10–28 cm6/s, with negligible contributions from second-order (bimolecular) processes. Direct imaging of the excited-state spatial evolution reveals an average diffu...

Novel application of simple molybdates: Catalytic hydrolysis of an organophosphate neurotoxin under mild aqueous conditions

A novel protocol for hydrolyzing phosphonothioate neurotoxins has been developed that uses a readily available, inexpensive and non-toxic molybdate (MoO42−). The target organophosphate is O,S-diethylphenyl phosphonothioate (1), a model compound that has served as an analog of the chemical warfare agent VX. Molybdate-mediated hydrolysis of 1 proceeds at room temperature (pH∼7.5) and yields a relatively safe phosphonate product through P-S specific scission. This is the first report of utilizing the molybdate, MoO42−, to degrade an organophosphate neurotoxin with turnover. A ΔS‡ of −81.5J/mol·K (−19.5cal/mol·K) indicates a bimolecular process, and 18O-labeling studies show no oxygen exchange of 1 throughout the hydrolysis by MoO42−(aq). We hypothesize primarily the monomeric molybdate is the active species at pH 7.5 where a SN2(P) mechanism takes place. Density functional theory methods suggest the molybdate oxoanion serves as the nucleophile to attack phosphonothioate 1 to form a molybdate-phosphonate anhydride that subsequently hydrolyzes to the starting molybdate and phenylethyl phosphonate.

Isoprene (co)polymers with glycidyl methacrylate via bimolecular and unimolecular nitroxide mediated radical polymerization

ABSTRACTHomo and copolymerization of isoprene with small amounts (1–10 wt %) of glycidyl methacrylate (GMA) are conducted using controlled‐living radical polymerization mediated by nitroxides at 120 °C and 1170 kPa in solution with toluene (30 wt % solids). N‐tert‐butyl‐N‐(2‐methyl‐1‐phenylpropyl)‐O‐(1‐phenylethyl) hydroxylamine is successfully used as a control agent (unimolecular process) although other controllers are also tested (TIPNO and OH‐TEMPO in a bimolecular process using BPO as initiator). Chain extension experiments demonstrate the livingness of the synthesized materials. Several additives (acetic anhydride, camphorsulfonic acid and glucose) prove effective in accelerating the reactions. All the successful polymerizations result in first‐order kinetics with respect to the monomer, yielding average molecular weights (Mn) of about 75% compared to the theoretical Mn (Mn, theo) with dispersities (Ð) ranging from 1.2 to 1.7 depending on the agent used for control. Controlled grafts of poly(isoprene‐co‐GMA) are also attached to polyisoprene via nitroxide chemistry. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45108.

Dynamics of photogenerated polarons and polaron pairs in P3HT thin films

On the basis of the analysis of the experimental data of the transient absorption spectroscopy of thin films of regioregular poly(3-hexyl thiophene) we suggest a mechanism of the polaron pairs decay based on the coherent up and down conversion cycling between hot polaron pairs and free polarons. Compared to previously adopted models, based on single molecular processes or bimolecular collisions, the suggested model reproduced well both the polaron pairs and polaron population kinetics at short time after photoexcitation. For longer times, when the polaron pairs are already thermalized, they decay via 1-D diffusion limited bi-molecular collision processes.

Unreactive Gel Networks as Versatile Confined Spaces for Enhanced Photoinduced Processes

SummaryGel materials have been recognized as promising confined media to enhance photoinduced reactions of reactants that are physically embedded in the gel network. In particular, stimuli‐responsive supramolecular self‐assembled gels that are unreactive to light irradiation constitute versatile materials to enhance both unimolecular and bimolecular photoinduced processes (in comparison to those carried out in solution) such as dimerization, isomerization and redox reactions, including multistep mechanisms.