Calculated Absorption Spectra Research Articles

The interplay of vibronic and spin-orbit coupling in the fluorescence quenching in trans-dithionated PDI.

Organic chromophores such as the thionated derivatives of perylene diimides (PDIs) show prolonged triplet-excited state lifetimes in contrast to their pristine parent PDI molecule, which shows near unity fluorescence quantum yield. The excited state dynamics in the trans-dithionated PDI (S2-PDI) are studied here. Unlike PDI, the photo absorbing ππ* state of S2-PDI is in close proximity to quasi-degenerate nπ* states. The latter exhibits an interesting vibronic problem leading to the breaking of orbital symmetry mediated through non-totally symmetric vibrations. The time-dependent quantum dynamics are studied with a diabatic model Hamiltonian involving three singlet and three triplet states coupled via 22 vibrational modes. A combined effect of multiple internal-conversion and inter-system crossing (ISC) pathways leads to population transfer from the 1ππ* state to the 3ππ* state via the nπ* states, with an overall ISC rate of 0.70ps that compares well with the experimental value. The calculated absorption spectra for PDI and S2-PDI reproduce the essential vibronic features in the observed experimental spectra. The dominant vibronic progressions are found to have significant contributions from the vinyl stretching modes of the PDI core.

Theoretical predictions and electronic properties of thiolate-protected gold nanocluster: Au44(SR)32

Thiolate-protected gold nanoclusters have received extensive attention both experimentally and theoretically due to their excellent physical and chemical properties. However, comprehending the geometric structure of thiolate-protected gold nanoclusters remains a challenging task. Predicting the structural configuration of these ligand-protected gold nanoclusters can enhance our understanding of this class of materials. Due to the current lack of knowledge regarding the structure of Au44(SR)32, we employed the grand unified model (GUM) for gold core stacking and the ring model to describe interfacial interactions between gold cores and the protection motifs. Based on these models, we successfully predicted the geometric structure of Au44(SR)32 clusters and demonstrated partial reproducibility between the calculated absorption spectra and experimental results. Furthermore, by analyzing the energy levels and molecular orbitals, we have identified that the absorption spectra of Au44(SR)32 arise from d→d and d→sp transitions, with the low-energy state primarily resulting from Au44-to-core transitions and the high-energy state from Au44-to-ligand transitions. These findings not only enhance our understanding of Au44(SR)32 clusters but also validate the effectiveness and applicability of GUM and ring model in predicting stable structures of thiolate-protected gold nanoclusters.

Algebraic Diagrammatic Construction Schemes Employing the Intermediate State Formalism: Theory, Capabilities, and Interpretation.

Algebraic diagrammatic construction (ADC) schemes represent a family of ab initio methods for the calculation of excited electronic states and electron-detached and -attached states. All ADC methods have been demonstrated to possess great potential for molecular applications, e.g., for the calculation of absorption or photoelectron spectra or electron attachment processes. ADC originates from Green's function or propagator theory; however, most recent ADC developments heavily rely on the intermediate state representation or effective Liouvillian formalisms, which comprise new ADC methods and computational schemes for high-order properties. The different approaches for the calculation of excitation energies, ionization potentials, and electron affinities are intimately related, and they provide a coherent description of these quantities at equivalent levels of theory and with comparable errors. Most quantum chemical program packages contain ADC methods; however, the most complete ADC suite of methods can be found in the recent release of Q-Chem.

Composition-Defined Optical Properties and the Direct-to-Indirect Transition in Core-Shell In1-xGaxP/ZnS Colloidal Quantum Dots.

Semiconductors are commonly divided into materials with direct or indirect band gaps based on the relative positions of the top of the valence band and the bottom of the conduction band in crystal momentum (k) space. It has, however, been debated if k is a useful quantum number to describe the band structure in quantum-confined nanocrystalline systems, which blur the distinction between direct and indirect gap semiconductors. In bulk III-V semiconductor alloys like In1-xGaxP, the band structure can be tuned continuously from the direct- to indirect-gap by changing the value of x. The effect of strong quantum confinement on the direct-to-indirect transition in this system has yet to be established because high-quality colloidal nanocrystal samples have remained inaccessible. Herein, we report one of the first systematic studies of ternary III-V nanocrystals by utilizing an optimized molten-salt In-to-Ga cation exchange protocol to yield bright In1-xGaxP/ZnS core-shell particles with photoluminescence quantum yields exceeding 80%. We performed two-dimensional solid-state NMR studies to assess the alloy homogeneity and the extent of surface oxidation in In1-xGaxP cores. The radiative decay lifetime for In1-xGaxP/ZnS monotonically increases with higher gallium content. Transient absorption studies on In1-xGaxP/ZnS nanocrystals demonstrate signatures of direct- and indirect-like behavior based on the presence or absence, respectively, of excitonic bleach features. Atomistic electronic structure calculations based on the semi-empirical pseudopotential model are used to calculate absorption spectra and radiative lifetimes and evaluate band-edge degeneracy; the resulting calculated electronic properties are consistent with experimental observations. By studying photoluminescence characteristics at elevated temperatures, we demonstrate that a reduced lattice mismatch at the III-V/II-VI core-shell interface can enhance the thermal stability of emission. These insights establish cation exchange in molten inorganic salts as a viable synthetic route to nontoxic, high-quality In1-xGaxP/ZnS QD emitters with desirable optoelectronic properties.

Fluorine derivatives of piceatannol for photobiological and photophysical applications by DFT approach

In the present work, we have calculated the electrical and optical properties of pure piceatannol (PI) and its six fluorinated derivatives (F1-F6). The first-principles calculations are performed in the framework of density functional theory employing the full-potential linearized augmented plane wave (FP-LAPW) method using PBE-GGA. It is found that the PI and its F-substituted derivatives are semiconductors with a HOMO-LUMO bandgap of 0.38 to 2.29 eV. Calculated partial density of states (DOS) of carbon and oxygen atoms show that the nature of C-O bonds is polar at PI. The effect of F substitution on the bandgap of PI are analyzed by comparing the DOS spectra. The size of the band gap can be controlled by changing the F sites in the PI. It is found that the band gap value decreases by substation of F at the F1, F2, F3, and F4 sites along with the F-O bonds at the LUMO state. However, in F5 and F6 sites, the change of the band gap is not significant. The dielectric behavior of the PI is observed at the y-direction. The calculated dielectric function spectra show the optical anisotropy at the x and the y-directions. The optical spectra revealed that F substitution in PI led to reduction of the optical band gap values and a red-shift occurs in the absorption spectra. The calculated absorption spectra of PI are in good agreement with the experimental data.

Spectrophotometric thermodynamic analysis of methylene blue aggregation in water by chemometrics; what comes in the presence of different cyclodextrins?

Spectrophotometry has been utilized to characterize the thermodynamic/dynamic properties of self-aggregation of methylene blue (MB) in water, particularly while interacting with a modulator like different cyclodextrins (α-, β-, hydroxypropyl-β- (HP-β-), and γ-CDs). These systems comprise many interactions that make such chemical systems sophisticated. We developed a mathematical modeling-fitting analysis for the simultaneous quantitative analysis of thermodynamic parameters of chemical reactions, relying on the fitting algorithm. Through analyzing simulated photometric titration data, we demonstrate the simultaneous determination of thermodynamic parameters of the different guest/host interactions. This first has brought the need for the calculation of the visible-light absorption spectrum and the thermodynamic parameters for the pure dimerization system. Therefore, the multiwavelength spectral-mole ratio data of aqueous solutions of MB over a concentration range of 2.5 × 10−5 to 4.5 × 10−5 M while temperature is changing; or being titrated with CDs solutions at various temperatures were collected, augmented, and then have been fed to solid mathematical routines to determine the potential existence of dimeric aggregates. The results of thermodynamics indicated that the positions of the monomer/dimer equilibria do not alter by the presence of α-CD. The apparent dimerization was suppressed upon addition of β- or HP-β-CDs, while the addition of γ-CD enhanced the dimerization.

Coupled Potential Energy Surfaces Strongly Impact the Lowest-Energy Spin-Flip Transition in Six-Coordinate Nickel(II) Complexes.

Luminescent complexes of earth-abundant first-row transition metals are of renewed, broad interest due to their spectroscopic and photochemical properties as well as emerging applications. New strong-field polypyridine ligands have led to six-coordinate 3d3 chromium(III) complexes with intense spin-flip luminescence in solution at room temperature. The ground and emissive states both arise from the (t2)3 electron configuration involving the dπ levels (O point group symmetry labels). Pseudoctahedral 3d8 nickel(II) complexes with such strong ligands are a priori also promising candidates for spin-flip luminescence. In contrast, the relevant electron configurations involve the dσ orbitals and (e)2 configurations. We have prepared the known nickel(II) complexes [Ni(terpy)2]2+, [Ni(phen)3]2+, and [Ni(ddpd)2]2+ as well as the novel complexes [Ni(dgpy)2]2+ and [Ni(tpe)2]2+ forming a series with increasing ligand field strengths (terpy = 2,2':6',2″-terpyridine; phen = 1,10-phenanthroline; ddpd = N,N'-dimethyl-N,N'-dipyridine-2-ylpyridine-2,6-diamine; dgpy = 2,6-diguanidylpyridine; tpe = 1,1,1-tris(pyrid-2-yl)ethane). The lowest-energy singlet and triplet excited states of these nickel(II) complexes are analyzed based on absorption spectra using ligand field theory and CASSCF-NEVPT2 calculations for vertical transition energies and a model based on coupled potential energy surfaces, leading to calculated absorption spectra in good agreement with the experimental data. No photoluminescence signal was observed in the wavelength ranges identified through the analyses of the absorption spectra. The models provide insight into key differences between the nickel(II) complexes and their strongly luminescent chromium(III) analogues.

Electric-field-induced semiconductor-semimetal phase transition of GeTe/SnSe van der Waals heterojunction

The electronic structure and optical property of GeTe/SnSe van der Waals heterojunction are investigated by first-principles method. We find GeTe/SnSe van der Waals heterojunction is a type-II heterojunction with an indirect band gap of 0.71 eV. The band gap can be tuned and semiconductor-semimetal phase transition is observed under both positive and negative electric field. The band offset of GeTe/SnSe van der Waals heterojunction can be controlled, and thus the potential barrier can be controlled by applying a gate voltage. Combined with the calculation of effective mass and absorption spectrum we predict that GeTe/SnSe van der Waals heterojunction has important applications in the fields of solar cell and photodetector devices.

Enhanced Device Absorption Efficiency and Photocatalytic Response of Monolayer PtX2 (X = S, Se, Te) for Photocatalysis and Solar Cell Applications

Using density functional theory, the electronic and optical properties of monolayer platinum dichalcogenides (PtX2) are calculated. It is observed that PtS2, PtSe2, and PtTe2 have indirect bandgap of 2.70, 1.94, and 0.82 eV, respectively, indicating their potential in light harvesting. According to the calculated absorption spectra, PtX2 (X = S, Se, Te) absorbs the light extremely well in the visible range (300–700 nm), implying potential as third‐generation solar absorber in tandem configuration. The calculated device absorption efficiency increases above 90% at smaller wavelengths (300–350 nm). As a result, strong absorption across the entire light spectrum, from visible to UV, is exhibited. Interestingly, the top cell in tandem architecture requires a wider bandgap and strong device absorption efficiency, and these conditions are perfectly fulfilled by the materials PtS2 and PtSe2. The narrower gap (0.82 eV) and strong absorption of PtTe2 make it an excellent candidate for bottom cell in tandem architecture. Furthermore, it is observed that PtX2 family is useful for oxidizing H2O into O2 but fails to reduce H+ to H2. Thus, suitable bandgap, photocatalytic property for splitting water, and strong absorption efficiency make PtX2 an efficient candidate for application in optoelectronic devices, photocatalysis, and solar cells.

An Indium Synthetic Etioporphyrin for Organic Electronics: Aggregation and Photoconductivity in Thin Films.

A new complex of indium(III)chloride with etioporphyrin-I was synthesized and characterized. As with naturally occurring extraligated etioporphyrins, the InCl-EtioP-I spectrum in solution has a very strong B-band and a more than an order of magnitude weaker Q-band, but this difference diminishes in solid films of InCl-EtioP-I obtained by thermal evaporation in vacuum. In a solid, molecules have a tight convex-convex arrangement in a 2D double layered structure with interplane distance of 3.066 Å. The conductivity of films can easily be activated by the action of temperature or light. In the cells with symmetrical lateral contacts the photocurrent exceeds the dark current by about three orders of magnitude, with the contribution of photons in the Q-band range being greater than expected from the experimental or calculated absorption spectrum. The Q-bands contribute significantly to the photovoltaic effect in the ITO/InCl-EtioP-I/Al sandwich cells. Such cells show an untypically strong signal in the photodiode regime, which yields the spectral detectivity of 10^12 Jones.

Time‐dependent coupled‐cluster theory

AbstractRecent years have witnessed an increasing interest in time‐dependent coupled‐cluster (TDCC) theory for simulating laser‐driven electronic dynamics in atoms and molecules, and for simulating molecular vibrational dynamics. Starting from the time‐dependent bivariational principle, we review different flavors of single‐reference TDCC theory with either orthonormal static, orthonormal time‐dependent, or biorthonormal time‐dependent spin orbitals. The time‐dependent extension of equation‐of‐motion coupled‐cluster theory is also discussed, along with the applications of TDCC methods to the calculation of linear absorption spectra, linear and low‐order nonlinear response functions, highly nonlinear high harmonic generation spectra and ionization dynamics. In addition, the role of TDCC theory in finite‐temperature many‐body quantum mechanics is briefly described along with a few other application areas.This article is categorized under: Electronic Structure Theory > Ab Initio Electronic Structure Methods Theoretical and Physical Chemistry > Spectroscopy Software > Simulation Methods

Atomically Dispersed Silver Atoms Embedded in NiCo Layer Double Hydroxide Boost Oxygen Evolution Reaction.

Bimetallic layered double hydroxides (LDHs) are promising catalysts for anodic oxygen evolution reaction (OER) in alkaline media. Despite good stability, NiCo LDH displays an unsatisfactory OER activity relative to the most robust NiFe LDH and CoFe LDH. Herein, a novel NiCo LDH electrocatalyst modified with single-atom silver grown on carbon cloth (AgSA -NiCo LDH/CC) that exhibits exceptional OER activity and stability in 1.0m KOH is reported. The AgSA -NiCo LDH/CC catalyst only requires a low overpotential of 192mV to reach a current density of 10mA cm-2 , obviously boosting the OER activity of NiCo LDH/CC (410 mV@10mA cm-2 ). Inspiringly, AgSA -NiCo LDH/CC can maintain its high activity for up to 500h at a large current density of 100mA cm-2 , exceeding most single-atom OER catalysts. In situ Raman spectroscopy studies uncover that the in situ formed NiCoOOH during OER is the real active species. Hard X-ray absorption spectrum (XAS) and density functional theory (DFT) calculations validate that single-atom Ag occupying Ni site increases the chemical valence of Ni elements, and then weakens the adsorption of oxygen-contained intermediates on Ni sites, fundamentally accounting for the enhanced OER performance.

Quantum mechanics/fluctuating charge calculations for absorption spectra of aqueous green fluorescent protein chromophore

AbstractWe applied the quantum mechanics/fluctuating charges (QM/FQ) method to simulate the absorption spectra of three protonated forms of p‐hydroxybenzylidene imidazolone (p‐HBDI) in water. Configurations of solute‐solvent were generated from molecular dynamics (MD) simulation within the context of a hybrid scheme in which solute and solvents were treated at QM and molecular mechanics, respectively. Nonperiodic spherical boundary condition was used. To examine the influence on simulated spectra coming from the geometrical relaxation of solute, the MD simulations were carried out for two cases: frozen and flexible molecular geometry of solute. As a result of our calculations, the simulations using the flexible solute's structure delivered vertical excitation energies closer to experimental data than those of a frozen solute, especially for the neutral case. For spectral broadening, changing from fixed geometry of solute to a flexible one led to a significant increase for both neutral and anionic forms of p‐HBDI, but no effect happened for the cationic one.

DFT and experimental study on the spectral and nonlinear optical properties of a highly conjugated imine

In this study, (2-((1E,2E)-3-phenylallylidene) amino)-4,5,6,7-tetrahydrobenzo[b]thiophene-3-carbonitrile) [referred from here onwards as HCIM that is a highly conjugated imine] was synthesized via a condensation reaction between cinnamaldehyde and a 2-aminothiophene derivative, and subjected to computational and experimental approaches. At first, the IR, UV–Vis spectral, and HOMO–LUMO analysis of the synthesized compound were carried out by density functional theory (DFT) at B3LYP/6-311++G(d,p) level. Secondly, experimental study on the nonlinear optical (NLO) properties (third order optical coefficients) of compound HCIM in methanol was conducted by Z-can technique. The IR results indicated that the computational and experimental of vibrational frequencies were in good agreement with each other. The calculated HOMO-LUMO gap (HLG) in gas phase and methanol is 3.14 and 3.085 eV, respectively, and this parameter was used to interpret Z-scan data’s. The values of calculated absorption spectra of the compound HCIM in gas phase were close to them in solution phase (methanol). Also, the experimental results showed that the main origin of NLO refraction is thermal lensing effect and positive nonlinear absorption is due to electrostriction effect. Electrostriction is a characteristic of organic molecules with low HOMO-LUMO energy gap about 3.1 eV, that exhibits high electrical polarizability. Finally, the results suggest that compound HCIM is advantageous for NLO applications.

Modelling the Structure and Optical Properties of Reduced Graphene Oxide Produced by Laser Ablation: Insights from XPS and Time-Dependent DFT

Graphene derivatives such as reduced graphene oxide and graphene-based composites are regarded as highly promising materials for optoelectronics and photodetection applications. Recently, considerable interest has arisen in using facile and environmentally friendly methods of graphene production. Despite significant progress in experimental studies of graphene and graphene-based composites, some fundamental questions about their structures, and the interaction between components in these systems, remain open. In the present work, several atomistic models of oxidized graphene fragments have been proposed based on XPS compositional analysis and DFT calculations, representing reduced graphene oxide produced by laser irradiation. The composition of the oxygen-containing functional groups, their equilibrium configuration and influence on the electronic and optical properties of graphene sheets were determined. The nature of the low-lying excited states, as well as the photoactive regions, has also been studied for the proposed models. The calculated absorption spectra of the graphene sheets were compared with experimental UV-Vis spectrum of rGO produced by laser ablation.

Hydrogen sulfide measurement of combustion gaseous product using ultraviolet absorption spectroscopy

Measurement of hydrogen sulfide (H2S) in gaseous product can provide direct reference for high-temperature corrosion of industrial combustion equipment. Aiming at the interference effect of overlapping measurement of absorption spectrum for H2S and other combustion gas components in UV absorption spectroscopy, the simulation calculation of ultraviolet band absorption spectrum of the typical gaseous products of coal combustion were carried out. The cross interference effects of combustion gaseous products were analyzed in detail. Due to the SO2 and H2S gas under typical combustion condition have equivalent absorption strength in the 228 nm-233 nm band, the influence of SO2 on H2S concentration inversion must be emphatically considered, and the inversion algorithm of H2S gas concentration under the background interference of combustion gaseous products SO2 was put forward. In order to validate the measurement accuracy by the inversion algorithm, the measurement experiments of gas mixture of H2S and SO2 were carried out. The results show that the algorithm can effectively solve the problem of cross interference between the detection of H2S and SO2 in the combustion gaseous products by UV absorption spectroscopy. On this basis, the sampling, pre-treatment and H2S gas concentration measurement system for boiler flue gas near the wall was developed and applied to measure H2S concentration of flue gas near the wall in a 1000 MW ultra-supercritical coal-fired power generation boiler. The results can provide important references for combustion adjustment and high-temperature corrosion prevention of boiler.

Eliminating Artificial Boundary Conditions in Time-Dependent Density Functional Theory Using Fourier Contour Deformation.

We present an efficient method for propagating the time-dependent Kohn-Sham equations in free space, based on the recently introduced Fourier contour deformation (FCD) approach. For potentials which are constant outside a bounded domain, FCD yields a high-order accurate numerical solution of the time-dependent Schrödinger equation directly in free space, without the need for artificial boundary conditions. Of the many existing artificial boundary condition schemes, FCD is most similar to an exact nonlocal transparent boundary condition, but it works directly on Cartesian grids in any dimension, and runs on top of the fast Fourier transform rather than fast algorithms for the application of nonlocal history integral operators. We adapt FCD to time-dependent density functional theory (TDDFT), and describe a simple algorithm to smoothly and automatically truncate long-range Coulomb-like potentials to a time-dependent constant outside of a bounded domain of interest, so that FCD can be used. This approach eliminates errors originating from the use of artificial boundary conditions, leaving only the error of the potential truncation, which is controlled and can be systematically reduced. The method enables accurate simulations of ultrastrong nonlinear electronic processes in molecular complexes in which the interference between bound and continuum states is of paramount importance. We demonstrate results for many-electron TDDFT calculations of absorption and strong field photoelectron spectra for one and two-dimensional models, and observe a significant reduction in the size of the computational domain required to achieve high quality results, as compared with the popular method of complex absorbing potentials.

Molecular γ-amino butyric acid and its crystals: Structural, electronic and optical properties

This work presents the formation of monoclinic γ-aminobutyric acid (GABA) crystals grown from an aqueous ethanol solution with the dimensions 4 ​× ​1 ​× ​0.6 ​mm3. The structural properties of the grown crystal were evaluated via X-ray diffraction analysis of the powder and single crystal, which confirmed a monoclinic crystal system with space group P21/c. Thermal stability and the melting point of the synthesized GABA crystal were investigated using TG-DSC measurements. We also characterized experimentally monoclinic and molecular GABA solvated in water, obtaining its optical absorption spectrum in the UV-VIS region. Time-dependent DFT calculations were performed for the GABA molecule in the neutral and zwitterion forms to understand their optical absorption features. Structural, electronic, and optical properties of four γ-aminobutyric acid (GABA) crystal polymorphs (monoclinic, tetragonal, hexagonal, and monohydrate) were achieved through DFT calculations employing a dispersion corrected exchange-correlation functional. Differences in the electronic and optical properties between polymorphs are discussed. We predict the GABA monoclinic crystal to be an indirect gap semiconductor with a gap value of 5.02 ​eV, in good agreement with our experimental measurements. Considering the other three polymorphs, their fundamental gap values range from 4.6 ​eV to 5.16 ​eV (being direct for all of them, except the monohydrate). The absorption spectra calculations reveal a significant optical anisotropy for all GABA crystals, with the highest optical absorption for the monoclinic structure in the 5–6 ​eV energy range.

Optical study on mesa structures with adjustable absorption based on Mn1.56Co0.96Ni0.48O4 thin films

In order to improve the infrared detection performance, mesa structures of Mn1.56Co0.96Ni0.48O4/Aluminum/Silicon dioxide/Silicon (MCN/Al/SiO2/Si) were prepared by chemical solution deposition method. Raman spectroscopy results indicate that increasing the thickness causes a change in the Mn3+–O band stretching vibration and the Mn4+–O band bending vibration. Spectroscopic elllipsometry was used to obtain the dielectric function of the films by fitting the experiment data using the classical middle-infrared dispersion formula. The evaluated real part of the dielectric constants ε1 and refractive index n decreased as the wavelength increase, the imaginary part ε2 and extinction coefficient k monotonically increased as the wavelength increase in the entire measuring range of 2–10 μm. The calculated absorption spectra of the MCN/Al/SiO2/Si structure showed that the absorption peak position could be adjusted by varying the thickness of the MCN films. Our results are important in the design and application of MCN film infrared detectors.

New accurate diabatic potential energy surfaces for the two lowest 1A'' states of H2S and photodissociation dynamics in its first absorption band.

In this work, state-to-state photodissociation dynamics of H2S in its first absorption band has been studied quantum mechanically with a new set of coupled potential energy surfaces (PESs) for the first two 1A'' excited states, which were developed at the explicitly correlated internally contracted multi-reference configuration interaction level with the cc-pVQZ-F12 basis set and a large active space. The calculated absorption spectrum, product state distributions, and angular distributions are in excellent agreement with available experimental data, validating the accuracy of the PESs and the non-adiabatic couplings. Detailed analysis of the dynamics reveals that there are strong non-adiabatic couplings between the bound 11B1 and dissociative 11A2 states around the Franck-Condon region, leading to very fast predissociation to ro-vibrationally cold SH(X̃) fragments, during which marginal angular anisotropy of the PESs is involved. This study provides quantitatively accurate characterization of the electronic structure and detailed fragmentation dynamics of this prototypical photodissociation system, which is desirable for improving astrochemical modelling.