Dynamic Solvent Effects Research Articles

Determining Kinetic Barriers in Hydrogen Electrocatalysis in Alkaline Media through Single-Crystal Voltammetry and Kinetic Isotope Effects

The sluggish hydrogen reaction kinetics observed in alkaline media compared to acidic media remain a fundamental challenge in modern electrocatalysis. Despite numerous efforts, the key parameters needed to accurately describe the slow hydrogen reaction kinetics at high pH are still unknown. Thorough understanding of the reaction mechanism and other relevant kinetic parameters is crucial for good electrocatalyst design needed for efficient energy conversion applications. In this work, we undertake an experimental and computational study to determine the effect of solvent dynamics in the kinetics of the hydrogen evolution and hydrogen oxidation reactions (HER and HOR). A possible explanation for the decreased reaction kinetics arises when considering the role of hydroxide on HER/HOR in alkaline media. The Markovic [1,2] group showed that HER/HOR activity can be significantly improved by decorating the catalyst surface with oxophilic sites (such as Ru or Ni(OH)2clusters on Pt), suggesting a “bifunctional” reaction pathway where hydroxide adsorbed on these oxophilic sites is an active participant in HER and HOR. Our past studies [3,4], however, indicate that a bifunctional mechanism where adsorbed hydrogen and hydroxide react is thermodynamically inconsistent, and that kinetic parameters must also be considered for explaining the sluggish HER and HOR kinetics in alkaline media. Other recent efforts have focused on the impact of such oxophilic clusters on the interfacial electric field and water network. The Koper and Feliu [5,6] groups showed that the introduction of Ni(OH)2clusters onto the Pt surface also decreases the ordering of the water network at the interface to accommodate charge transfer through the double layer, the energetics of which are controlled by how strongly water interacts with the interfacial electric field. The introduction of Ni(OH)2clusters therefore decreases the interfacial electric field strength and allows for a more dynamic water network near the interface. In another study, Lia et. al. [7] showed the importance of alkali cations for anchoring reactive species in the double layer region, in accordance with other reports by the Markovic and Koper groups [8,9]. While these studies highlight the profound impact that the interfacial water network rigidity and alkali cations have on the system, the fundamental role of solvent dynamics on adsorption processes and overall reaction kinetics is yet to be deciphered. In this work, we examine through the means of single-crystal voltammetry and microkinetic modeling the effect of solvent dynamics on the overall alkaline hydrogen reaction kinetics via a kinetic isotope effect. By comparing experimentally measured differences between H2O and D2O, we are able to probe the influence of interfacial water dynamics and electrolyte cation choice on adsorption processes (e.g. the Volmer step) and overall reaction kinetics. In addition, we focus on the reaction’s surface sensitivity by comparing Pt(111) and Pt(110) single crystals. Our preliminary results indicate that the HER and HOR kinetics are slower in D2O by approximately 20%, further showing that hydrogen and hydroxide binding energies are not the sole descriptors for HER and HOR in alkaline media, and that solvent reorganization kinetics are important limiting factors. Peak splitting measurements and comparisons with our microkinetic model suggest that the rate-determining step must be hydroxide adsorption, in agreement with the Koper and Feliu groups. The results presented herein reinforce the importance of local barriers and interfacial solvent dynamics for adsorption processes and overall reaction kinetics. [1]Subbaraman et. al. Nat. Mater. 2012, 550-557. [2] Strmcnik et. al. Nat. Chem. 2013, 5, 300–306. [3] Intikhab et. al. ACS Cat. 2017, 7, 8314−8319. [4] Rebollar et. al. JES, 2018, 165 (15), 3209-3221. [5] Ledezma et. al. Nat. Energy 2017, 2(4). [6] Sarabia et. al. ACS App. Mater. Interfaces 2019 11 (1), 612-623. [7] Lia et. al. J. Am. Chem. Soc., 2019, 141 (7), 3232–3239. [8] Subbaraman et. al. Science. 334 (2011) 334, 6060, 1256-1260. [9] Sarabia et. al. ACS App. Mater. Interfaces 2019 11 (1), 612-623.

Electrochemistry of Electroactive Species Loaded inside Polymeric Hydrogels. Effect of Viscoelastic Properties on Mass Transport and Heterogeneous Charge Transfer

The loading and mass transfer of electroactive species inside polymeric hydrogels is of interest in drug delivery, analytical chemistry and water remediation. The electrochemical properties of the species could be affected by the viscoelastic properties of the gel, local ionic force and pH, as well as interactions (e.g. hydrophobic) between the ions and the polymer chains. Usually, a thin film of the hydrogel is deposited on the electrode, loaded with electroactive species and electrochemically investigated. However, the mass transfer is controlled by finite diffusion and depends of an unknown, the thickness of the layer in the experimental conditions. On the other hand, we show here that a simple set-up, consisting of a disk electrode pressed on the soft hydrogel, allows performing electrochemistry of electroactve species (redox complexes, arsenite, nitrite) loaded inside the hydrogel matrix. The redox complexes are present as dilute solutions and the hydrogel dimensions fulfill the semi-infinite diffusion boundary conditions. Therefore, the data analysis can be performed using the theoretical framework developed for electrochemical measurements in liquid solutions. The physicochemical properties of the bulk hydrogels are evaluated by measuring the swelling kinetics, in the same conditions of the electrochemical measurements. Redox cations (e.g. tris(phenanthroline)iron(II) (TPFeII)) or anions (e.g. Fe(CN)6 -3, (FeCN) are loaded into anionic, neutral and cationic hydrogels showing no Donnan exclusion effects. The cyclic voltammetry of the loaded species show a quasireversible electron transfer mechanism (Eqre). The electrochemical parameters (diffusion coefficient and charge transfer constant) inside hydrogels are measured using chronoamperometry and digital simulation of the cyclic voltammetry. The Stokes-Einstein equation is used to calculate the effective viscosity of the hydrogel matrixes using the diffusion coefficients of redox complexes determined inside the hydrogels and in aqueous solutions (of known viscosities). The calculated viscosities correlate, with a negative slope, with the swelling rate constant of the hydrogel matrix. The heterogeneous charge transfer constants correlate with the calculated viscosities, revealing the effect of solvent dynamics on the charge transfer, according to Marcus theory for strongly adiabatic electron transfer. In that way, it is shown that the electrochemical measurements are able to monitor the local solvation properties of the hydrogel matrix. The method can be applied in electrochemical sensing of environmental contaminants. In the case of nitrite, the charge transfer is slow on glassy carbon (GC) electrodes. Therefore a redox catalyst (TPFeII) and nitrite are simultaneously absorbed in an anionic hydrogel and the redox electrocatalysis occurs inside the hydrogel matrix. On the other hand, the sensing of arsenite is made using Co oxide nanoparticles adsorbed on the GC electrode with arsenite loaded inside the hydrogel.

Dynamical Solvent Effects on the Charge and Reactivity of Ceria-Supported Pt Nanoclusters

Supported Pt nanoparticles are key components in heterogeneous catalysis for energy and environment applications that involve vapor and wet conditions. In the latter case, the reaction proceeds at the catalyst–water interface where the solvent actively participates in the reaction mechanism. In this work, ab initio molecular dynamics simulations shed light on the effects of solvation on the reactivity and electronic properties of Pt6 nanocatalysts supported by ceria (CeO2), a highly reducible oxide. The calculated trajectories show that H2O molecules spontaneously dissociate at both the supported Pt6 cluster and at the ceria surface already at T = 350 K. Water dissociation leads to hydroxylation of the ceria surface and, most importantly, to the selective decoration of the metal–oxide periphery with hydroxide ions, which are stabilized by solvent-induced electronic effects and which quickly diffuse to the interfacial Pt sites via Grotthus-like proton chains. The periphery of the metal–oxide interface is thus identified as the active region of ceria-supported Pt clusters in wet environments. Solvation is shown to drive dynamic charge transfers across the metal/oxide interface that modify the cluster charge, a key parameter of the catalyst reactivity.

Ultrafast Ground-State Intramolecular Proton Transfer in Diethylaminohydroxyflavone Resolved with Pump-Dump-Probe Spectroscopy.

4'- N, N-Diethylamino-3-hydroxyflavone (DEAHF), due to excited-state intramolecular proton transfer (ESIPT) reaction, exhibits two solvent-dependent emission bands. Because of the slow formation and fast decay of the ground-state tautomer, its population does not accumulate enough for its detection during the normal photocycle. As a result, the details of the ground-state intramolecular proton-transfer (GSIPT) reaction have remained unknown. The present work uses femtosecond pump-dump-probe spectroscopy to prepare the short-lived ground-state tautomer and track this GSIPT process in solution. By simultaneously measuring femtosecond pump-probe and pump-dump-probe spectra, ultrafast kinetics of the ESIPT and GSIPT reactions are obtained. The GSIPT reaction is shown to be a solvent-dependent irreversible two-state process in two solvents, with estimated time constants of 1.7 ps in toluene and 10 ps in the more polar tetrahydrofuran. These results are of great value in both fully describing the photocycle of this four-level proton transfer molecule and for providing a deeper understanding of dynamical solvent effects on tautomerization.

Solvent effect on the electrochemical oxidation of N,N,N′,N′-tetramethyl-1,4-phenylenediamine. New insights into the correlation of electron transfer kinetics with dynamic solvent effects

Electrochemical oxidation of N,N,N′,N′-tetramethyl-1,4-phenylenediamine (TMPD) to its cation radical (TMPD•+) and dication (TMPD++) in various nonaqueous solvents was studied using the cyclic voltammetry technique on a glassy carbon electrode. The variation of the half-wave potentials (E1/2) with the physical properties of the solvents was interpreted using multi-parametric correlation. It is shown that polarity-polarizability index, viscosity and longitudinal relaxation of solvents play an undeniable role in the E1/2 value. The electron transfer rate constants were simulated and analyzed using the MLR regression method to separate the effect of longitudinal relaxation time from other effective parameters according to the Marcus theory. Finally, a new correlation method was developed which involve solvent's Pekar factor, longitudinal and Debye relaxation time, viscosity and relative permittivity. The computational method was used for calculating the standard of the Gibbs free energy of solvation and oxidation process.

Computational Analysis of Solute–Solvent Coupling Magnitude in the Z/E Isomerization Reaction of Nitroazobenzene and Benzylideneanilines

The dynamic solvent effect often arises in solution reactions, where coupling between chemical reaction and solvent fluctuation plays a decisive role in the reaction kinetics. In this study, the Z/E isomerization reaction of nitoroazobenzene and benzylideneanilines in the ground state was computationally studied by molecular dynamics simulations. The non-equilibrium solvation effect was analyzed using two approaches: (1) metadynamics Gibbs energy surface exploration and (2) solvation Gibbs energy evaluation using a frozen solvation droplet model. The solute–solvent coupling parameter (Ccoupled) was estimated by the ratio of the solvent fluctuation Gibbs energy over the corresponding isomerization activation Gibbs energy. The results were discussed in comparison with the ones estimated by means of the analytical models based on a reaction–diffusion equation with a sink term. The second approach using a frozen solvation droplet reached qualitative agreement with the analytical models, while the first metadynamics approach failed. This is because the second approach explicitly considers the non-equilibrium solvation in the droplet, which consists of a solute at the reactant geometry immersed in the pre-organized solvents fitted with the solute at the transition state geometry.

Novel adenine/thymine photodimerization channels mapped by PCM/TD-DFT calculations on dApT and TpdA dinucleotides.

Despite the biological relevance of AT-rich DNA sequences, the excited state paths associated with the photochemical reactions involving adenine and thymine stacked pairs have never been characterized, and the structure of the most abundant photoproduct in DNA is unknown. PCM/TD-M052X calculations on dApT and TpdA unveil the paths leading to the main photoproduct in TpdA, provide new insights into the reasons why it is not formed in dApT and show the existence of a new photochemical path, which could produce the precursor of the most abundant genomic AT/TA photoproduct. Our calculations confirm that anti/anti conformers are photochemically active and show that the dynamical solvation effects could significantly modulate the reaction yields.

Effect of solvent and protein dynamics in ligand recognition and inhibition of aminoglycoside adenyltransferase 2″-Ia.

The aminoglycoside modifying enzyme (AME) ANT(2″)-Ia is a significant target for next generation antibiotic development. Structural studies of a related aminoglycoside-modifying enzyme, ANT(3″)(9), revealed this enzyme contains dynamic, disordered, and well-defined segments that modulate thermodynamically before and after antibiotic binding. Characterizing these structural dynamics is critical for in situ screening, design, and development of contemporary antibiotics that can be implemented in a clinical setting to treat potentially lethal, antibiotic resistant, human infections. Here, the first NMR structural ensembles of ANT(2″)-Ia are presented, and suggest that ATP-aminoglycoside binding repositions the nucleotidyltransferase (NT) and C-terminal domains for catalysis to efficiently occur. Residues involved in ligand recognition were assessed by site-directed mutagenesis. In vitro activity assays indicate a critical role for I129 toward aminoglycoside modification in addition to known catalytic D44, D46, and D48 residues. These observations support previous claims that ANT aminoglycoside sub-class promiscuity is not solely due to binding cleft size, or inherent partial disorder, but can be controlled by ligand modulation on distinct dynamic and thermodynamic properties of ANTs under cellular conditions. Hydrophobic interactions in the substrate binding cleft, as well as solution dynamics in the C-terminal tail of ANT(2″)-Ia, advocate toward design of kanamycin-derived cationic lipid aminoglycoside analogs, some of which have already shown antimicrobial activity in vivo against kanamycin and gentamicin-resistant P. aeruginosa. This data will drive additional in silico, next generation antibiotic development for future human use to combat increasingly prevalent antimicrobial resistance.

Verification of Nonequilibrium Mechanism of Ultrafast Charge Recombination in Excited Donor–Acceptor Complexes

Control of charge transfer requires knowledge of its detailed mechanism. Due to the large number of known mechanisms, the identification of the mechanism in specific systems is a challenge so far. In this article we propose the idea of how to distinguish between thermal and nonequilibrium modes of charge recombination in excited donor-acceptor complexes. Simulations of the effect of solvent relaxation time scale on ultrafast charge recombination kinetics in photoexcited donor-acceptor complexes within the framework of the multichannel stochastic model have shown that a series of regularities inherent to the thermal and nonequilibrium charge transfer can strongly differ. Among them there are opposite regularities, for example, the dependence of the dynamic solvent effect on the free energy gap. In particular, theory predicts that in ultrafast charge recombination of excited donor-acceptor complexes the dynamic solvent effect is weak in the area of weak exergonicity and becomes stronger in the area of stronger exergonicity whereas for the thermal reactions an opposite trend is expected. Comparison of such trends with experimental data implemented in this article allowed establishing the regime in which the reaction proceeds. It is shown that observation of dynamic solvent effect in the region of strong exergonicity for ultrafast charge recombination is decisive evidence in favor of nonequilibrium mechanism.

Hole Transfer Processes in meta- and para-Conjugated Mixed Valence Compounds: Unforeseen Effects of Bridge Substituents and Solvent Dynamics.

To address the question whether donor substituents can be utilized to accelerate the hole transfer (HT) between redox sites attached in para- or in meta-positions to a central benzene bridge, we investigated three series of mixed valence compounds based on triarylamine redox centers that are connected to a benzene bridge via alkyne spacers at para- and meta-positions. The electron density at the bridge was tuned by substituents with different electron donating or accepting character. By analyzing optical spectra and by DFT computations we show that the HT properties are independent of bridge substituents for one of the meta-series, while donor substituents can strongly decrease the intrinsic barrier in the case of the para-series. In stark contrast, temperature-dependent ESR measurements demonstrate a dramatic increase of both the apparent barrier and the rate of HT for strong donor substituents in the para-cases. This is caused by an unprecedented substituent-dependent change of the HT mechanism from that described by transition state theory to a regime controlled by solvent dynamics. For solvents with slow longitudinal relaxation (PhNO2, oDCB), this adds an additional contribution to the intrinsic barrier via the dielectric relaxation process. Attaching the donor substituents to the bridge at positions where the molecular orbital coefficients are large accelerates the HT rate for meta-conjugated compounds just as for the para-series. This effect demonstrates that the para-meta paradigm no longer holds if appropriate substituents and substitution patterns are chosen, thereby considerably broadening the applicability of meta-topologies for optoelectronic applications.

Rich Dynamics Underlying Solution Reactions Revealed by Sampling and Data Mining of Reactive Trajectories.

Efficient sampling in both configuration and trajectory spaces, combined with mechanism analyses via data mining, allows a systematic investigation of the thermodynamics, kinetics, and molecular-detailed dynamics of chemical reactions in solution. Through a Bayesian learning algorithm, the reaction coordinate(s) of a (retro-)Claisen rearrangement in bulk water was variationally optimized. The bond formation/breakage was found to couple with intramolecular charge separation and dipole change, and significant dynamic solvent effects manifest, leading to the “in-water” acceleration of Claisen rearrangement. In addition, the vibrational modes of the reactant and the solvation states are significantly coupled to the reaction dynamics, leading to heterogeneous and oscillatory reaction paths. The calculated reaction rate is well interpreted by the Kramers’ theory with a diffusion term accounting for solvent–solute interactions. These findings demonstrated that the reaction mechanisms can be complicated in homogeneous solutions since the solvent–solute interactions can profoundly influence the reaction dynamics and the energy transfer process.

Free-Energy-Gap Law for Ultrafast Charge Recombination of Ion Pairs Formed by Intramolecular Photoinduced Electron Transfer.

In this article, regularities of ultrafast charge recombination (CR) kinetics in photoinduced intramolecular electron transfer in polar solvents are studied. The kinetics of charge separation and ensuing ultrafast CR are simulated within the framework of the multichannel stochastic model. This model accounts for the reorganization of both the solvent and a number of intramolecular high-frequency vibrational modes. The solvent relaxation is described in terms of two relaxation modes. For ultrafast CR, the free-energy-gap law strongly depends on the parameters: the electronic coupling, reorganization energy of intramolecular high-frequency vibrational modes, and the vibrational and solvent relaxation times. The semilog dependence of the CR rate constant on the free-energy gap varies from a parabolic shape to a nearly linear one with increasing the electronic coupling and decreasing the vibrational relaxation time. The dynamic solvent effect in CR is predicted to be large in the area of strong exergonicity and small in the area of weak exergonicity. This regularity is opposite to that observed for the thermal reactions.

Dynamic Solvent Effect on Ultrafast Charge Recombination Kinetics in Excited Donor-Acceptor Complexes.

Manifestation of the dynamic solvent effect (DSE) on the charge recombination (CR) kinetics of photoexcited donor-acceptor complexes in polar solvents has been investigated within the framework of the multichannel stochastic model. The model takes into account the reorganization of both the solvent and a number of intramolecular high-frequency vibration modes as well as their relaxation. The non-Markovian solvent dynamics is described in terms of two relaxation modes. The similarities and differences inherent to ultrafast charge transfer reactions occurring in the nonequilibrium and thermal regimes have been identified. The most important differences are as follows: (1) the DSE is strong in the area of weak exergonicity and is weak in the area of strong exergonicity for thermal reactions, whereas for the nonequilibrium reactions, the regions of strong and weak DSEs are reversed; (2) an increase in the electronic coupling value results in a decrease in the magnitude of DSE for nonequilibrium electron transfer and in its increase for the thermal reactions; and (3) the two-staged regime most clearly manifests if the reorganization energy of the relaxation modes noticeably exceeds the CR free-energy gap. With an increase in electronic coupling, the kinetics approaches the exponential regime because in the limit of strong electronic coupling, the reaction includes only single, nonequilibrium, stage.

Electroactive polymers made by loading redox ions inside crosslinked polymeric hydrogels. Effects of hydrophobic interactions and solvent dynamics

Electroactive polymers are made by loading redox complexes inside different polymeric crosslinked hydrogels. A redox cation (tris(phenanthroline)iron(II) (TPFeII) is sorbed in anionic, neutral and cationic hydrogels while redox anions (ferricyanide) is taken up inside an anionic hydrogel. Moreover, a redox anion (nitrite) and a cation (TPFeII) are simultaneously absorbed in an anionic hydrogel. Homopolymeric hydrogels of 2-acrylamidopropanesulfonic acid (PAMPS), acrylic acid (PAA) or (3-acrylamidopropyl)trimethylammonium chloride) (PAPMTAC), crosslinked with bisacrylamide, are synthesized by radical polymerization. The same method is used to produce a 1:1 copolymeric hydrogel of acrylic acid and 2-acrylamidopropanesulfonic acid (PAA-co-AMPS). The physicochemical properties of the hydrogels are evaluated by measuring the swelling kinetics, in the same conditions of the electrochemical measurements. The cyclic voltammetric response of all electroactive polymers show quasireversible electron transfer (Eqrev mechanism) while the hydrogel loaded with TPFeII and nitrite show a catalyzed oxidation (EqrevC mechanism). The electrochemical parameters (diffusion coefficient and charge transfer constant) of TPFeII loaded inside PAMPS, PAA and PAA-co-AMPS are measured using chronoamperometry and digital simulation of the cyclic voltammetry. The Stokes-Einstein equation is used to calculate the effective viscosity of the hydrogel matrixes using the diffusion coefficients of a redox complex (TPFeII) determined inside the hydrogels and the same parameters determined in aqueous solution. The calculated viscosity correlates, with a negative slope, with the swelling rate constant of the hydrogel matrix. The heterogeneous charge transfer of the redox complex inside ionomers (PAMPS and PAA-co-PAMPS) is nearly as fast as in solution, while the charge transfer inside the neutral PAA is ca. 100 times smaller. The measured charge transfer constants correlate with the calculated viscosity, revealing the effect of solvent dynamics on the charge transfer, according to Marcus theory for strongly adiabatic electron transfer. In that way, it is shown that the electrochemical measurements are able to monitor the local solvation properties of the hydrogel matrix. These results suggest novel ways to produce electroactive polymers by loading redox active substances inside hydrogels, even when the hydrogel matrix is neutral or bear the same charge than the redox probe. The redox complexes are present as dilute solutions and the hydrogel dimensions fulfill the semi-infinite diffusion boundary conditions. Therefore, the data analysis can be performed using the theoretical framework for electrochemical measurements in liquid solvents.

First Observation for Dynamic Solvent Effect in Ionic Liquids

We observed pressure effects on the rate of thermal fading of colored chromene 1 photochemically generated from 2 in ionic liquids. The reaction rates were retarded with increasing pressure in [C4-...

Nile blue and Nile red optical properties predicted by TD-DFT and CASPT2 methods: static and dynamic solvent effects

The absorption and emission spectra of the fluorescent dyes Nile blue (NB) and Nile red (NR), widely used in biology and histology, were simulated with different methods, considering the effect of water as solvent. The aforementioned dyes are extremely significant because they also act as DNA sensitizers and hence can be used in photodynamic therapy. Especially, time dependent-density functional theory (TD-DFT) including different functionals, and ab initio single-state- and multi-state-complete active space perturbation theory (SS- and MS-CASPT2) including the effect of the basis set, were considered. The solvent environment was taken into account statically and dynamically: static optical properties were calculated with the polarizable continuum model as vertical transitions from the ground state equilibrium geometry, while dynamic properties were obtained by performing ground state molecular dynamics of NB and NR in explicit water, followed by hybrid quantum mechanics/molecular mechanics calculations of a statistical number of geometries along the trajectory. The results show that a dynamic treatment is required in order to reproduce the experimental absorption spectra, since the static approach gives rise to a hypsochromic shift of ca. 0.3 eV for NB and 0.2 eV for NR, at the TD-DFT and CASPT2 level of theory. This can be explained in terms of out-of-plane vibrational normal modes, which are properly taken into account only in the dynamic approach. Moreover, an exhaustive description of the charge transfer character in the excited state is given, at both TD-DFT and CASPT2 levels of theory.

Molecular features in complex environment: Cooperative team players during excited state bond cleavage.

Photoinduced bond cleavage is often employed for the generation of highly reactive carbocations in solution and to study their reactivity. Diphenylmethyl derivatives are prominent precursors in polar and moderately polar solvents like acetonitrile or dichloromethane. Depending on the leaving group, the photoinduced bond cleavage occurs on a femtosecond to picosecond time scale and typically leads to two distinguishable products, the desired diphenylmethyl cations (Ph2CH+) and as competing by-product the diphenylmethyl radicals (). Conical intersections are the chief suspects for such ultrafast branching processes. We show for two typical examples, the neutral diphenylmethylchloride (Ph2CH–Cl) and the charged diphenylmethyltriphenylphosphonium ions () that the role of the conical intersections depends not only on the molecular features but also on the interplay with the environment. It turns out to differ significantly for both precursors. Our analysis is based on quantum chemical and quantum dynamical calculations. For comparison, we use ultrafast transient absorption measurements. In case of Ph2CH–Cl, we can directly connect the observed signals to two early three-state and two-state conical intersections, both close to the Franck-Condon region. In case of the , dynamic solvent effects are needed to activate a two-state conical intersection at larger distances along the reaction coordinate.

A State-Specific PCM–DFT method to include dynamic solvent effects in the calculation of ionization energies: Application to DNA bases

A State-Specific PCM–DFT method taking into account dynamical solvent effects in the calculation of ionization energies (IEs) is reported. Mono-methylated nucleobases, representative of DNA blocks, are used as a test case. The accuracy of several functionals is tested, the M05-2X functional providing the most consistent performance. The inclusion of some molecules of the first hydration shell does not significantly affect the computed IE. The obtained vertical and adiabatic IEs are in good agreement with the available experimental and computational results, supporting the reliability of our method, enabling the calculation of first and upper ionization energies by using a single approach.

Investigations of the Degenerate Intramolecular Charge Exchange in Symmetric Organic Mixed Valence Compounds: Solvent Dynamics of Bis(triarylamine)paracyclophane Redox Systems

Triarylamines are important hole-transport components in optoelectronic devices. Understanding the factors controlling their intra- and intermolecular electron transfer properties is crucial to the application and optimization of organic hole conductors. Here, we report on the degenerate intramolecular electron exchange reactions of several purely organic mixed valence compounds based on the bis(triarylamine)paracyclophane structural unit, which are archetypical molecular wires. Different bridging moieties are compared, and the foremost impact of the solvent environment on the rate of electron transfer is demonstrated. Comparing the rate constants found for many different solvents, we find that surprisingly the electron transfer reaction is limited by the solvent dynamic effect and not strongly impacted by the peculiarities of the bridging moiety, a finding which was not anticipated for this type of long-range, thermally activated intramolecular charge transfer from previous studies. Rate constants are measured by dynamic electron paramagnetic resonance spectroscopy. Our insight was possible using various solvents spanning a wide range of longitudinal relaxation times (0.24 ps ≤ τL ≤ 516 ps) and Pekar factors (0.298 ≤ γ ≤ 0.526).

Investigation of solvent dynamic effects on the electron self-exchange in two thianthrene couples with large inner reorganization energies.

The large structural difference between thianthrene radical cations and their neutral parent molecules can possibly affect their electron self-exchange reactions. Before this can be investigated experimentally, it is necessary to first understand the influence of the solvent on such electron transfer reactions. To achieve this, the rate constants of the electron self-exchange reactions of the Th˙(+)/Th and MTh˙(+)/MTh (Th = thianthrene, MTh = 2,3,7,8-tetramethoxythianthrene) couples were investigated by means of ESR line broadening experiments in different solvents at 293 K. The diffusion corrected rate constants cover a range of 7.2 × 10(8)≤ket≤ 44 × 10(8) M(-1) s(-1) for Th˙(+)/Th and 2.0 × 10(8)≤ket≤ 11.6 × 10(8) M(-1) s(-1) for MTh˙(+)/MTh, respectively. The results were analysed within the framework of the Marcus Theory and the characteristic reorganization energy, λ, was determined. Both couples clearly show a solvent dynamic effect controlled by the longitudinal relaxation time τL of the solvents. However, the influence of the structural changes, in terms of λ, was smaller than expected at room temperature.