Adiabatic Approximation in Explicit Solvent Models of RedOx Chemistry

We propose a calculation scheme that accelerates QM/MM simulations of solvated systems. This new approach is based on the adiabatic approximation whereby the solute degrees of freedom are separated from those of the solvent. More specifically, we assume that the solute electron density remains constant with respect to the relaxation of the solvent molecules. This allows us to achieve a dramatic speed-up of QM/MM calculations by discarding the slow self-consistent field cycle. We test this method by applying it to the calculation of the redox potential of aqueous transition metal ions. The root-mean-square deviation (RMSD) between the full solvation and adiabatic approximation is only 0.17 V. We find a RMSD from experimental values of 0.32 V for the adiabatic approximation as compared to 0.31 V for the full solvation model, so that the two methods are of essentially the same accuracy. Meanwhile, the adiabatic calculations are up to 10 times faster than the full solvation calculations, meaning that the method proposed here reduces the cost of QM/MM calculations while retaining the accuracy

Charge Recombination in Phosphorescent Organic Light-Emitting Diode Host–Guest Systems through QM/MM Simulations

Host–guest systems are crucial for achieving high efficiency in most organic light-emitting diode (OLED) devices. However, charge recombination in such systems is poorly understood due to complicated molecular environment, making the rational design of host–guest systems difficult. In this article, we present a computational study of a phosphorescent OLED with 2,8-bis­(triphenylsilyl)­dibenzofuran (BTDF) as the host and <i>fac</i>-tris­(2-phenylpyridine) iridium (<i>fac</i>-Ir­(ppy)₃) as the guest, using a combined quantum mechanics/molecular mechanics (QM/MM) scheme. A new reaction coordinate is introduced to measure the electrostatic interactions between the host and guest molecules. Ionization potentials and electron affinities of the host show broader distributions as the host–guest interaction increases. On the basis of these distributions, we describe a molecular picture of charge recombination on the guest and find a direct charge trapping route for this system. Our results suggest several strategies for the design of more efficient host and guest combinations

Adiabatic Approximation in Explicit Solvent Models of RedOx Chemistry

We propose a calculation scheme that accelerates QM/MM simulations of solvated systems. This new approach is based on the adiabatic approximation whereby the solute degrees of freedom are separated from those of the solvent. More specifically, we assume that the solute electron density remains constant with respect to the relaxation of the solvent molecules. This allows us to achieve a dramatic speed-up of QM/MM calculations by discarding the slow self-consistent field cycle. We test this method by applying it to the calculation of the redox potential of aqueous transition metal ions. The root-mean-square deviation (RMSD) between the full solvation and adiabatic approximation is only 0.17 V. We find a RMSD from experimental values of 0.32 V for the adiabatic approximation as compared to 0.31 V for the full solvation model, so that the two methods are of essentially the same accuracy. Meanwhile, the adiabatic calculations are up to 10 times faster than the full solvation calculations, meaning that the method proposed here reduces the cost of QM/MM calculations while retaining the accuracy

Adiabatic Approximation in Explicit Solvent Models of RedOx Chemistry

We propose a calculation scheme that accelerates QM/MM simulations of solvated systems. This new approach is based on the adiabatic approximation whereby the solute degrees of freedom are separated from those of the solvent. More specifically, we assume that the solute electron density remains constant with respect to the relaxation of the solvent molecules. This allows us to achieve a dramatic speed-up of QM/MM calculations by discarding the slow self-consistent field cycle. We test this method by applying it to the calculation of the redox potential of aqueous transition metal ions. The root-mean-square deviation (RMSD) between the full solvation and adiabatic approximation is only 0.17 V. We find a RMSD from experimental values of 0.32 V for the adiabatic approximation as compared to 0.31 V for the full solvation model, so that the two methods are of essentially the same accuracy. Meanwhile, the adiabatic calculations are up to 10 times faster than the full solvation calculations, meaning that the method proposed here reduces the cost of QM/MM calculations while retaining the accuracy

Dynamic Current Suppression and Gate Voltage Response in Metal−Molecule−Metal Junctions

We critically re-examine conductance in benzenedithiol (BDT)/gold junctions using real-time DFT simulations. Our results indicate a powerful influence of the BDT molecular charge on current, with negative charge suppressing electron transport. This effect occurs dynamically as the BDT charge and current oscillate on the femtosecond time scale, indicating that a steady-state picture may not be appropriate for this single molecule conducting device. Further, we exploit this effect to show that a gate voltage can be used to indirectly control the device current by adjusting the molecular charge. Thus, it appears that transport in even this simple molecular junction involves a level of sophistication not heretofore recognized

Non-radiative deactivation of cytosine derivatives at elevated temperature

<p>In this work, we simulate the non-radiative deactivation process of three cytosine derivatives with known S0/S1 conical intersections (cytosine, 5-fluorocytosine and 5-methylcytosine). We use quantum chemistry methods to compute the potential energy profile of each derivatives and estimate the energy barrier height between the minimum of the S1 state and the conical intersection. Although the topology of the potential surface seems to play a role in the deactivation process, we show that the magnitude of the barrier is too high to explain the picosecond timescale reported for this reaction. Instead, rates in agreement with experiments are predicted only when incorporating dynamical factors via <i>ab-initio</i> molecular dynamics and a generalised master equation approach. In particular, we find that the energy fluctuations experienced by the system after photoexcitation are key to realistically model the relaxation dynamics. In gas phase, the cytosine derivatives remain vibrationally ‘hot’ for long after the excitation, raising the effective temperature of the system. We argue that it is this elevated temperature that allows for the crossing of the energy barrier. Further, we show that the reaction kinetics are not actually dominated by the conical intersection as it is enough for the system to find an avoided crossing.</p

QM/MM Study of Static and Dynamic Energetic Disorder in the Emission Layer of an Organic Light-Emitting Diode

Static and dynamic energetic disorder in emission layers of organic light-emitting diodes (OLEDs) is investigated through combined molecular dynamics and hybrid quantum mechanics/molecular mechanics (QM/MM) calculations. The analysis is based on a comparison of ensemble and time distributions of site energies of guest and host components in an emission layer. The law of total variance is applied to decompose the total disorder into its static and dynamic contributions. It is found that both contributions are of the same order of magnitude. While the dynamic disorder is not affected by intermolecular interactions, the static disorder for both guests and hosts is determined by the polarity of host molecules. The amount of static disorder affects charge-transport properties and exciton formation pathways, which consequently influence the overall efficiency of an OLED device. The simulations indicate that the amount of static disorder induced by the host should be considered for the optimization of the emission layer

Electrostatic Effects at Organic Semiconductor Interfaces: A Mechanism for “Cold” Exciton Breakup

Exciton dissociation at organic semiconductor interfaces is an important process for the design of future organic photovoltaic (OPV) devices, but at present, it is poorly understood. On the one hand, exciton breakup is very efficient in many OPVs. On the other, electron–hole pairs generated by an exciton should be bound by Coulombic attraction, and therefore difficult to separate in materials of such low dielectric. In this paper, we highlight several electrostatic effects that appear commonly at organic/organic interfaces. Using QM/MM simulations, we demonstrate that the electric fields generated in this fashion are large enough to overcome typical electron–hole binding energies and thus explain the high efficiencies of existing OPV devices without appealing to the existence of nonthermal (“hot”) carrier distributions. Our results suggest that the classical picture of flat bands at organic/organic interfaces is only qualitatively correct. A more accurate picture takes into account the subtle effects of electrostatics on interfacial band alignment

Direct-Coupling O₂ Bond Forming a Pathway in Cobalt Oxide Water Oxidation Catalysts

We report a catalytic mechanism for water oxidation in a cobalt oxide cubane model compound, in which the crucial O–O bond formation step takes place by direct coupling between two Co^IV(O) metal oxo groups. Our results are based upon density functional theory (DFT) calculations and are consistent with experimental studies of the CoPi water oxidation catalyst. The computation of energetics and barriers for the steps leading up to and including the O–O bond formation uses an explicit solvent model within a hybrid quantum mechanics/molecular mechanics (QM/MM) framework, and captures the essential hydrogen-bonding effects and dynamical flexibility of this system

Fluorescence Quenching by Photoinduced Electron Transfer in the Zn²⁺ Sensor Zinpyr-1: A Computational Investigation

We report a detailed study of luminescence switching in the fluorescent zinc sensor Zinpyr-1 by density functional methods. A two-pronged approach employing both time-dependent density functional theory (TDDFT) and constrained density functional theory (CDFT) is used to characterize low-lying electronically excited states of the sensor. The calculations indicate that fluorescence activation in the sensor is governed by a photoinduced electron transfer mechanism in which the energy level ordering of the excited states is altered by binding Zn2+. While the sensor is capable of binding two Zn2+ cations, a single Zn2+ ion appears to be sufficient to activate moderate fluorescence in aqueous solution at physiological pH. We show that it is reasonable to consider the tertiary amine as the effective electron donor in this system, although the pyridyl nitrogens each contribute some density to the xanthone ring. The calculations illustrate an important design principle: because protonation equilibria at receptor sites can play a determining role in the sensor’s fluorescence response, receptor sites with a pKa near the pH of the sample are to be disfavored if a sensor governed by a simple PET fluorescence quenching model is desired