21 research outputs found

    Non-radiative deactivation of cytosine derivatives at elevated temperature

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    <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

    Computational Design Principles of Two-Center First-Row Transition Metal Oxide Oxygen Evolution Catalysts

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    Computational screens for oxygen evolution reaction (OER) catalysts based on Sabatier analysis have seen great success in recent years; however, the concept of using chemical descriptors to form a reaction coordinate has not been put under scrutiny for complex systems. In this paper, we examine critically the use of chemical descriptors as a method for conducting catalytic screens. Applying density functional theory calculations to a two-center metal oxide model system, we show that the Sabatier analysis is quite successful for predicting activities and capturing the chemical periodic trends expected for the first-row transition metal series, independent of the proposed mechanism. We then extend this analysis to heterodimer metallic systemsmetal oxide catalysts with two different catalytically active metal centersand find signs that the Sabatier analysis may not hold for these more complex systems. By performing a principal component analysis on the computed redox potentials, we show (1) that a single chemical descriptor inadequately describes heterodimer overpotentials and (2) mixed-metal overpotentials cannot be predicted using only pure-metal redox potentials. We believe that the analysis presented in this article shows a need to move beyond the simple chemical descriptor picture when studying more complex mixed metal oxide OER catalysts

    Self-Attractive Hartree Decomposition: Partitioning Electron Density into Smooth Localized Fragments

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    Chemical bonding plays a central role in the description and understanding of chemistry. Many methods have been proposed to extract information about bonding from quantum chemical calculations, the majority of them resorting to molecular orbitals as basic descriptors. Here, we present a method called self-attractive Hartree (SAH) decomposition to unravel pairs of electrons directly from the electron density, which unlike molecular orbitals is a well-defined observable that can be accessed experimentally. The key idea is to partition the density into a sum of one-electron fragments that simultaneously maximize the self-repulsion and maintain regular shapes. This leads to a set of rather unusual equations in which every electron experiences self-attractive Hartree potential in addition to an external potential common for all the electrons. The resulting symmetry breaking and localization are surprisingly consistent with chemical intuition. SAH decomposition is also shown to be effective in visualization of single/multiple bonds, lone pairs, and unusual bonds due to the smooth nature of fragment densities. Furthermore, we demonstrate that it can be used to identify specific chemical bonds in molecular complexes and provides a simple and accurate electrostatic model of hydrogen bonding

    The Impact of Carrier Delocalization and Interfacial Electric Field Fluctuations on Organic Photovoltaics

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    Organic photovoltaic (OPV) devices hold a great deal of promise for the emerging solar market. However, to unlock this promise, it is necessary to understand how OPV devices generate free charges. Here, we analyze the energetics and charge delocalization of the interfacial charges in poly­(<i>p</i>-phenylenevinylene) (PPV)/[6,6]-phenyl-C<sub>61</sub>-butyric acid methyl ester (PCBM) and poly­(3-hexylthiophene-2,5-diyl) (P3HT)/PCBM devices. We find that, in the PPV system, the interface does not produce molecular disorder, but an interfacial electric field is formed upon the inclusion of environmental polarization that promotes charge separation. In contrast, the P3HT system shows a significant driving force for charge separation due to interfacial disorder confining the hole. However, this feature is overpowered by the polarization of the electronic environment, which generates a field that inhibits charge separation. In the two systems studied herein, electrostatic effects dominate charge separation, overpowering interfacially induced disorder. This suggests that, when balancing polymeric order with electrostratic effects, the latter should take priority

    Prediction of Excited-State Energies and Singlet–Triplet Gaps of Charge-Transfer States Using a Restricted Open-Shell Kohn–Sham Approach

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    Organic molecules with charge-transfer (CT) excited states are widely used in industry and are especially attractive as candidates for fabrication of energy efficient OLEDs, as they can harvest energy from nonradiative triplets by means of thermally activated delayed fluorescence (TADF). It is therefore useful to have computational protocols for accurate estimation of their electronic spectra in order to screen candidate molecules for OLED applications. However, it is difficult to predict the photophysical properties of TADF molecules with LR-TDDFT, as semilocal LR-TDDFT is incapable of accurately modeling CT states. Herein, we study absorption energies, emission energies, zero–zero transition energies, and singlet–triplet gaps of TADF molecules using a restricted open-shell Kohn–Sham (ROKS) approach instead and discover that ROKS calculations with semilocal hybrid functionals are in good agreement with experimentsunlike TDDFT, which significantly underestimates energy gaps. We also propose a cheap computational protocol for studying excited states with large CT character that is found to give good agreement with experimental results without having to perform any excited-state geometry optimizations

    Salt Effect Accelerates Site-Selective Cysteine Bioconjugation

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    Highly efficient and selective chemical reactions are desired. For small molecule chemistry, the reaction rate can be varied by changing the concentration, temperature, and solvent used. In contrast for large biomolecules, the reaction rate is difficult to modify by adjusting these variables because stringent biocompatible reaction conditions are required. Here we show that adding salts can change the <i>rate constant over 4 orders of magnitude</i> for an arylation bioconjugation reaction between a cysteine residue within a four-residue sequence (π-clamp) and a perfluoroaryl electrophile. Biocompatible ammonium sulfate significantly enhances the reaction rate without influencing the site-specificity of π-clamp mediated arylation, enabling the fast synthesis of two site-specific antibody–drug conjugates that selectively kill HER2-positive breast cancer cells. Computational and structure–reactivity studies indicate that salts may tune the reaction rate through modulating the interactions between the π-clamp hydrophobic side chains and the electrophile. On the basis of this understanding, the salt effect is extended to other bioconjugation chemistry, and a new regioselective alkylation reaction at π-clamp cysteine is developed

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

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    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)<sub>3</sub>) 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

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    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

    Insights into Magneto-Optics of Helical Conjugated Polymers

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    Materials with magneto-optic (MO) properties have enabled critical fiber-optic applications and highly sensitive magnetic field sensors. While traditional MO materials are inorganic in nature, new generations of MO materials based on organic semiconducting polymers could allow increased versatility for device architectures, manufacturing options, and flexible mechanics. However, the origin of MO activity in semiconducting polymers is far from understood. In this paper, we report high MO activity observed in a chiral helical poly-3-(alkylsulfone)­thiophene (<b>P3AST</b>), which confirms a new design for the creation of a giant Faraday effect with Verdet constants up to (7.63 ± 0.78) × 10<sup>4</sup> deg T<sup>–1</sup> m<sup>–1</sup> at 532 nm. We have determined that the sign of the Verdet constant and its magnitude are related to the helicity of the polymer at the measured wavelength. The Faraday rotation and the helical conformation of <b>P3AST</b> are modulated by thermal annealing, which is further supported by DFT calculations and MD simulations. Our results demonstrate that helical polymers exhibit enhanced Verdet constants and expand the previous design space for polythiophene MO materials that was thought to be limited to highly regular lamellar structures. The structure–property studies herein provide insights for the design of next-generation MO materials based upon semiconducting organic polymers

    Charge Transfer or J‑Coupling? Assignment of an Unexpected Red-Shifted Absorption Band in a Naphthalenediimide-Based Metal–Organic Framework

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    We investigate and assign a previously reported unexpected transition in the metal–organic framework Zn<sub>2</sub>(NDC)<sub>2</sub>(DPNI) (<b>1</b>; NDC = 2,6-naphthalenedicarboxylate, DPNI = dipyridyl-naphthalenediimide) that displays linear arrangements of naphthalenediimide ligands. Given the longitudinal transition dipole moment of the DPNI ligands, J-coupling seemed possible. Photophysical measurements revealed a broad, new transition in <b>1</b> between 400 and 500 nm. Comparison of the MOF absorption spectra with that of a charge transfer (CT) complex formed by manual grinding of DPNI and H<sub>2</sub>NDC led to the assignment of the new band in <b>1</b> as arising from an interligand CT. Constrained density functional theory utilizing a custom long-range-corrected hybrid functional was employed to determine which ligands were involved in the CT transition. On the basis of relative oscillator strengths, the interligand CT was assigned as principally arising from π-stacked DPNI/NDC dimers rather than the alternative orthogonal pairs within the MOF
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