21 research outputs found
Investigation of the Second Harmonic Generation at the Water–Vacuum Interface by Using Multi‐Scale Modeling Methods
No full textAbstract Invited for this month′s cover picture are Dr. Tárcius N. Ramos and Prof. Benoît Champagne at the University of Namur (Belgium). The cover picture shows the interfacial selectivity of second harmonic generation at the water‐vacuum interface, which is targeted in this work. In more details, the molecular first hyperpolarizability responses have been calculated by combining classical molecular dynamics and quantum chemistry simulations, and our model was able to distinguish between the bulk and the interfacial contributions. Read the full text of their Research Article at 10.1002/open.202200045
A theoretical study of the low-lying excited states and the photophysics of dimethoxy curcumin in cyclohexane and acetonitrile
No full textInvestigation of the Second Harmonic Generation at the Water-Vacuum Interface by Using Multi-Scale Modeling Methods
Full text linkThe Sequential Quantum Mechanics/Molecular Mechanics scheme has been enacted to perform a systematic investigation of the polarizability (α) and first hyperpolarizability (β) re- sponses at the water–vacuum interface. After performing classical molecular dynamics simulations to provide snapshots of the structures, quantum chemistry calculations of the linear and nonlinear optical responses have been performed for clusters of five water molecules at the time-dependent DFT level in combination with different embedding schemes, ranging from point charges to polarizable point charges, withand without local field effects. When going from the bulk to the interface, the main observations of these calculations encom- pass i) a modest increase of the average polarizability but an increase by about a factor of two of its anisotropy, ii) an increase by about 20 % of the βHRS response, accompanied by a small increase of its depolarization ratio, and iii) a net increase of the component of the β tensor normal to the interface (βzzz) as well as of β//. Globally, the interfacial effects on β are localized at the first molecular layer while they are observed up to the fourth molecular layer on α.The Sequential Quantum Mechanics/Molecular Mechanics scheme has been enacted to perform a systematic investigation of the polarizability (α) and first hyperpolarizability (β) responses at the water–vacuum interface. After performing classical molecular dynamics simulations to provide snapshots of the structures, quantum chemistry calculations of the linear and nonlinear optical responses have been performed for clusters of five water molecules at the time-dependent DFT level in combination with different embedding schemes, ranging from point charges to polarizable point charges, with and without local field effects. When going from the bulk to the interface, the main observations of these calculations encompass i) a modest increase of the average polarizability but an increase by about a factor of two of its anisotropy, ii) an increase by about 20 % of the βHRS response, accompanied by a small increase of its depolarization ratio, and iii) a net increase of the component of the β tensor normal to the interface (βzzz) as well as of β//. Globally, the interfacial effects on β are localized at the first molecular layer while they are observed up to the fourth molecular layer on α
Unraveling the Electric Field-Induced Second Harmonic Generation Responses of Stilbazolium Ion Pairs Complexes in Solution Using a Multiscale Simulation Method
Full text linkThe electric field-induced second harmonic generation (EFISHG) response has been largely used to describe the first β and the second γ hyperpolarizabilities in solution. Although the EFISHG technique cannot be applied to charged compounds (due to the external static electric field), it can be used to describe ion pairs as neutral complexes. A multiscale computational approach is required to generate representative geometrical configurations of such kinds of complexes (using classical force fields), to compute the electronic structure of each configuration (using quantum mechanics methods), and to perform statistical analyses describing the behavior of the nonlinear optical properties. In this work, we target solvated neutral ion pairs complexes, of which the cation is an organic chromophore, and we estimate their EFISHG and hyper-Rayleigh scattering responses. It is shown that the anion–cation relative spatial distribution determines the permanent dipole moment of the complexes, and therefore the relative distance controls the EFISHG response. On the other hand, the β tensor is independent of the dipole moment and it shows a weak linear correlation with the π-electron conjugation length of the cations. The γ contributions in the global EFISHG response range from 5% to 15%, which is mostly due to the variations of amplitude of the μβ∥ contribution, which results from differences in the μ and β vectors’ orientations. The applied multiscale approach provides reasonable results compared with experimental ones, although additional efforts are still required to improve such comparison mainly to consider the possible dissociation effects
Modeling second harmonic generation at alcohol/air interfaces. A molecular multi-layer approach
No full textThe liquid/air interface asymmetry leads to specific molecular interactions, compared to those observed in the bulk, and its characterization has significant implications for describing and controlling chemical and physical processes inherent to catalysis and environmental sciences. The second harmonic generation (SHG) is an interfacial selective spectroscopy often used to characterize interfaces. In this study, a computational multiscale methodology that combines molecular dynamics and quantum chemistry calculations has been worked out to scrutinize the SHG responses of 1-alcohols/air interfaces, and to unravel the role of the aliphatic chain length by exploring alcohols ranging from ethanol to pentanol. First, their molecular responses (β, the first hyperpolarizability) and then their macroscopic responses (χ 2, the second-order NLO susceptibility) have been computed and analyzed by decomposing the global responses into the contributions of successive molecular layers, providing relationships between the structure of the interfacial molecules and their dipolar SHG responses. The results show (i) an enhancement of the β ZZZ component (the component perpendicular to the interface) attributed to collective responses arising from the structural alignment of the interfacial molecules. Furthermore, (ii) longer aliphatic chains tend to create a bilayer structure, which was clearly observed at the pentanol/air interface where the polar heads of the molecules in the first and second molecular layers point toward each other. Such structural organization leads to a partial centrosymmetry at the interface, which is detrimental to the dipolar SHG response. On the other hand, (iii) owing to their smaller number density, the χ ZZZ 2 response of longer alcohols is smaller than for the smaller ones. Furthermore, (iv) a correlation between χ ZXX 2 and the aliphatic chain length is observed for the third molecular layer.</p
Disentangling the molecular polarizability and first hyperpolarizability of methanol–air interfaces
Full text linkLiquid-air interfaces have extensive implications in different areas of interest because the dynamical processes at the interface can be different from those in bulk. Thus, its characterization, understanding, and control may be pivotal in advancing discoveries. However, characterizing the interface requires special and selective tools to avoid signals from the bulk region. This surface specificity and versatility is achieved by using the second harmonic generation (SHG) responses. This study adopts multiscale simulation methods to evaluate the surface SHG responses of methanol-air interfaces with submonolayer resolution tackled by sequentially using classical molecular dynamics simulations under different temperatures and then employing quantum chemistry methods to compute the molecular first hyperpolarizabilities (β). This approach ensures the configurational diversity required to evaluate the average β values. The main achievements are (i) a quasi-absence of surface sensitivity of the mean polarizability 〈α〉 with values about 2% larger than those obtained in bulk, (ii) conversely, smooth variations on the polarizability anisotropy Δα are observed up to the fourth molecular layer at around 20 Å from the interface, and (iii) narrow interfacial effects on the SHG responses, β(−2ω;ω,ω), which are limited to the first molecular layer (∼3.0 Å) and characterized by a high contrast in the β ZZZ(−2ω;ω,ω) tensor component between the first and the subsequent layers. Similar trends are obtained at different temperatures or when increasing the number of methanol molecules treated at the quantum chemistry level, indicating the robustness of the approach for describing the dipolar molecular responses of air-liquid interfaces.</p
Second harmonic generation signatures of supramolecular assemblies based on amide moieties
No full textTargeting the use of the second harmonic generation (SHG) as a bioimaging technique to unravel the formation of aggregates, the SHG first hyperpolarizabilities ((Figure presented.)) of assemblies of benzene-1,3,5-tricarboxamide derivatives have been evaluated at the density functional theory level. Calculations have revealed that i) the assemblies exhibit SHG responses and the total first hyperpolarizability responses of the aggregates are evolving with their size. The largest aggregation effect is a 18-times increase for (Figure presented.) of B4 when going from the monomer to the pentamer, that ii) the intrinsic SHG responses described by the hyper-Rayleigh Scattering (Figure presented.) are enhanced in presence of iodine atoms on the phenyl core, that iii) the side chains affect the relative orientation of the dipole moment and first hyperpolarizability vectors, which impacts more the EFISHG quantities than their moduli, and that iv) the radial component to (Figure presented.) is dominant for the compounds having the largest responses. These results have been obtained using the sequential molecular dynamics then quantum mechanics approach to account for dynamic structural effects on the SHG responses.</p
Second harmonic generation responses of ion pairs forming dimeric aggregates
Full text linkA sequential approach combining molecular dynamics and density functional theory calculations has been worked out to unravel the second harmonic generation responses of anion–cation (AC) pairs when they form dimeric aggregates, where the cation is a stilbazolium derivative and the anions range from small inorganic iodide to medium-size organic p-toluenesulfonate. These complexes showed a strong self-aggregation behavior in molecular dynamics simulations within high-concentration conditions and formed stable dimeric aggregates, (AC)2, which can adopt different structural shapes from stacked, Λ, to head-to-head configurations. These various structures are associated with different symmetries, which are shown to modulate the second- and third-order nonlinear optical (NLO) responses. By consolidating the NLO results of this work with those previously obtained for single AC pairs [ J. Chem. Inf. Model. 2020, 60, 4817−4826], we have been able to explain the experimentally observed variations of the electrical-field-induced second harmonic generation (EFISHG) responses of these complexes as a function of concentration [ ChemPhysChem 2010, 11, 495−507]. Moreover, results have highlighted that (i) the second-order contribution, μβ//, dominates the global EFISHG response; (ii) the μβ// responses of dimers are about half of those computed for the parent AC pairs, while the third-order contributions, γ//, are reduced by only 10%; (iii) these distinct trends are ascribed to the formation of dimers adopting mainly Λ and head-to-head shapes, increasing the centrosymmetric character, in comparison to the monomers, a situation in which the second-order response cancels out as well as influences the dipole moment on μβ//; (iv) the presence of a strong amino donor group in the cation enhances the μβ// response by 1 order of magnitude and γ// by about a factor of 2; and finally, (v) dimeric aggregation has similar effects on the hyper-Rayleigh scattering response, βHRS, as on μβ//, while it reduces the one-dimensional character of βHRS. This work constitutes a step forward for the modeling of the NLO responses of AC aggregates in solution
Self-aggregation of stilbazolium ion pairs in liquid chloroform. A molecular dynamics study
No full textMolecular aggregation plays an important role on optical phenomena, and optimizing the latter requires understanding the aggregation behavior as a function of the molecular structures. The formation of molecular aggregates composed of stilbazolium chromophores and small to medium-size anions is studied by carrying out classical molecular dynamics (MD) simulations, shedding light on the dynamics of spontaneous self-aggregation of anion-cation (AC) pairs and estimating their equilibrium aggregation constants. First, analyses based on atom-atom distances and on contact mapping (Voronoi tessellation) highlight that the interactions occur at the level of the methylpyridinium groups of the cations, which enabled monitoring the clustering dynamics. Then, simulations show that clusters containing up to 5 AC pairs dominate the populations. Among the AC pairs, two of them (where the stilbazolium bears a strong donor substituent and when the anion is SCN- or pTS-) aggregate into small clusters even at high concentration, corroborating related experimental results [ChemPhysChem. 11 (2010) 495–507]. Still, these MD simulations do not reveal strict relationships between the aggregate size and the nature of the anion. Nevertheless, AC pairs bearing iodide anions require longer MD simulation times to reach an asymptotic equilibrium tendency of the average number of clusters. Finally, the aggregates are shown to adopt a preferential Λ-shape with contributions from head-to-head and stacked forms. Complementarily, the analyses of the isoperimetric quotient reinforce the AC pairs are not forming spherical aggregates even for clusters including more than 10 AC pairs. So, when the stilbazolium bears a strong donor substituent and when the anion is pTS-, stacked aggregates are formed, at both low and high concentrations, resulting in excellent agreement with the measured aggregation equilibrium constant
Investigation of the Second Harmonic Generation at the Water–Vacuum Interface by Using Multi‐Scale Modeling Methods
No full textAbstract The Sequential Quantum Mechanics/Molecular Mechanics scheme has been enacted to perform a systematic investigation of the polarizability (α) and first hyperpolarizability (β) responses at the water–vacuum interface. After performing classical molecular dynamics simulations to provide snapshots of the structures, quantum chemistry calculations of the linear and nonlinear optical responses have been performed for clusters of five water molecules at the time‐dependent DFT level in combination with different embedding schemes, ranging from point charges to polarizable point charges, with and without local field effects. When going from the bulk to the interface, the main observations of these calculations encompass i) a modest increase of the average polarizability but an increase by about a factor of two of its anisotropy, ii) an increase by about 20 % of the βHRS response, accompanied by a small increase of its depolarization ratio, and iii) a net increase of the component of the β tensor normal to the interface (βzzz) as well as of β//. Globally, the interfacial effects on β are localized at the first molecular layer while they are observed up to the fourth molecular layer on α
