Time-dependent Density Functional Theory Approach Research Articles

Modification of benzoindenothiophene-based organic dye with fused thiophenes for efficient dye-sensitized solar cells

Recently low-energy-gap benzoindenothiophene (BIT)-based organic dyes have been experimentally sensitized to dye-sensitized solar cells (DSSCs) with impressive 10.9% power conversion efficiency. This paper presents a computational study of the modification of BIT-based dyes with fused thiophene moieties to build novel low band gap sensitizers. Density functional theory (DFT), tight-binding DFT, and time dependent DFT (TDDFT) approaches are used to demonstrate the electronic and optical properties of the BIT dyes and dye/(TiO2)46 complexes. Our calculations show that the structural modification by using fused thiophenes can effectively lower the band gap of the BIT dyes by 0.07–0.12 eV and affect the optical properties of BIT dyes. Enlarging the thiophene unit in BIT with thienothiophene and dithienothiophene improves the oscillator strength by 14%–25%, while the lowest-energy absorption peak basically remains at 559 nm. The incorporation of cyclopentadithiophene unit leads to a significant 47 nm red-shift of absorption peak and a 25% enhanced oscillator strength, compared to the original BIT dye. Those fused thiophenes modified BIT dyes also demonstrate ideal molecular orbital distribution patterns and ultra-fast injection time at the dye/(TiO2)46 interface. Our calculations provide useful guidance for the molecular design of novel naphthalene-based dyes for DSSC optimizations.

Relativistic Orbital-Optimized Density Functional Theory for Accurate Core-Level Spectroscopy.

Core-level spectra of 1s electrons of elements heavier than Ne show significant relativistic effects. We combine advances in orbital-optimized density functional theory (OO-DFT) with the spin-free exact two-component (X2C) model for scalar relativistic effects to study K-edge spectra of third period elements. OO-DFT/X2C is found to be quite accurate at predicting energies, yielding a ∼0.5 eV root-mean-square error versus experiment with the modern SCAN (and related) functionals. This marks a significant improvement over the >50 eV deviations that are typical for the popular time-dependent DFT (TDDFT) approach. Consequently, experimental spectra are quite well reproduced by OO-DFT/X2C, sans empirical shifts for alignment. OO-DFT/X2C combines high accuracy with ground state DFT cost and is thus a promising route for computing core-level spectra of third period elements. We also explored K and L edges of 3d transition metals to identify limitations of the OO-DFT/X2C approach in modeling the spectra of heavier atoms.

Donor functionalized perylene and different π-spacer based sensitizers for dye-sensitized solar cell applications - a theoretical approach.

A series of perylene-based novel metal-free organic dye sensitizers are designed and optimized for dye-sensitized solar cell (DSSC) applications. The electronic and optical properties are analyzed through density functional theory (DFT) and time-dependent density functional theory (TD-DFT) approach. For perylene-based donors, the effects of additional donor units and different π-spacer positions were investigated. Cyanovinyl and thiophene are used as π-spacers, dimethylamine (DM) and N-N-dimethylaniline (DMA) are used as additional donors, and cyanoacrylic acid is used as mono acceptor unit for the designed sensitizers. Natural bonding orbitals (NBOs), frontier molecular orbitals (FMO), UV-Vis, and nonlinear orbital analysis were predicted to find the net electron transfer, energy gap, absorption spectra, and electronic charge distribution for perylene-based dye sensitizers, respectively. The electron injection and electron regeneration properties were also analyzed for perylene-based sensitizers.

Solvated Nuclear-Electronic Orbital Structure and Dynamics.

Nonadiabatic dynamical processes such as proton-coupled electron transfer and excited state intramolecular proton transfer have been the subject of much research. One of the promising theoretical methods to describe these processes is the nuclear-electronic orbital (NEO) approach. This approach inherently accounts for nuclear quantum effects within quantum chemistry calculations, and it has recently been extended to directly simulate nonadiabatic processes with the development of real-time NEO methods. These processes can also be significantly dependent on the surrounding chemical environment, however, and capturing the effects of the environment is often necessary for analyzing experimentally relevant systems. This work couples the NEO density functional theory and real-time time-dependent density functional theory approaches with solvation through the polarizable continuum model. The effects of this coupling are investigated for ground state properties, solvent-dependent vibrational frequencies, and direct excited state intramolecular proton transfer dynamics.

A Theoretical Investigation about Photoswitching of Azobenzene Adsorbed on Ag Nanoparticles

The optical properties of hybrid systems composed of silver nanoparticles (NPs) and azobenzene molecules were systematically investigated by combining the real-time time-dependent density functional theory (RT-TDDFT) approach with the classical electrodynamics finite difference time domain (FDTD) technique for the solution of Maxwell’s equations. In order to reflect the chemical interaction between azobenzene and metal more exactly, except for adsorbed molecules, a Ag cluster separated from NP was also dealt, using RT-TDDFT. We studied the different factors affecting the surface-enhanced absorption spectra. It was found that the electric field amplified by plasmon resonance of Ag NPs can have an overall enhancement to the molecular light absorption throughout the whole energy range. The resonance between the electron and the plasmon excitation results in a larger percentage of enhancement in the absorption spectrum the closer the resonance peak is. The enhancement ratio of the resonance peak is the largest. The plasmon–exciton coupling and the optical properties of different isolate isomers influence the line shape of the absorption spectra. The dipole interaction and electronic transfer between azobenzene molecules and Ag NPs also change the shape of spectroscopy from the absorption enhancement ratio and the location of the peak. Physical and chemical factors lead to photoswitching in these hybrid systems together.

Activation by Glutathione in Hypoxic Environment of an Azo-based Rhodamine Activatable Photosensitizer. A Computational Elucidation.

In the present paper, density functional theory (DFT) has been applied to the study of the activation mechanism of a new selenium azo-rhodamine (azoSeRho) in presence of the tripeptide thiol, glutathione (GSH), as potent activatable photosensitizer to be employed in photodynamic therapy. The introduction of the azo group into the conjugated system of the seleno-rhodamine dye and its reaction with GSH allow the selective formation of the active photosensitizer, SeRho. Furthermore, DFT calculations have allowed to shed light on the activation mechanism of the azoSeRho photosensitizer when molecular oxygen is present and hydrogen peroxide is formed. This study is the first theoretical investigation revealing how the reductive cleavage of the azo moiety by GSH occurs. Time-dependent DFT approach has been used to evaluate the chalcogen-substitution effect on the structures and photophysical properties of the azo derivatives and, then, on the activated photosensitizers.

Energy-based fragmentation contribution approach for calculating the fluorescence spectrum of biomacromolecules

It remains challenging to calculate the fluorescence spectra of biomacromolecules due to the large number of atoms involved. Here, we propose an efficient computation method, named the energy-based fragmentation contribution (EBFC) approach, to calculate the fluorescence spectra of super-large proteins. Specifically, an optimized molecular tailoring rule is provided to divide a super-large protein into smaller fragments, then the extended tight binding method and the simplified time-dependent density functional theory approach are employed to calculate the fluorescence spectrum of each fragment in parallel, and finally, the fluorescence spectrum of the target protein is obtained by superimposing the contributions of all fragments. Utilizing the EBFC approach, we calculated fluorescence spectra of three different super-large proteins and the calculated spectra show excellent agreement with experimental data with the difference of less than 1.5%. This work provides an efficient and reliable approach to enhance the modeling capability of the excited states in biomacromolecules.

Effect of solvents and temperature on the structural, thermodynamic and electronic properties of capped phenylalanine: A computational study

Effects of solvents and temperature on the structural, thermodynamic, and electronic properties of L-configuration of N-acetyl-phenylalaninyl amide (NAPA) were studied using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) approach. Enthalpy (H), entropy (S) and specific heat capacity (Cv) were found to increase with the increase of temperature (100 K-1600 K) because of the increasing intensities of molecular vibration. On the contrary, Gibb’s free energy (G) was found to decrease with the increase of temperature. The UV-light absorption maximum, λmax, is red-shifted in the presence of polar protic, aprotic and non-polar solvents. On the other hand, calculation shows that dipole moment in polar solvents (protic and aprotic), non-polar solvents, and the gas phase are ~3.35, ~3.0, and 2.5 D, respectively. However, no significant change was found in the HOMO-LUMO energy gap in the presence of different types of solvents. J. Bangladesh Acad. Sci. 45(2); 205-215: December 2021

Steric hindrance effect on the excited-state proton transfer process: TDDFT study on the fluorescent sensing mechanism of a fluoride sensor

An understanding of the excited-state process and the sensing mechanism for specific anions can be helpful for the design and synthesis of fluorescent sensors in analytical chemistry and biotechnology. Here, we theoretically investigated the fluorescent response mechanism of a reported acylhydrazone-based fluorescent sensor (Soft Matter, 2019, 15, 6690) for fluoride recognition using the time-dependent density functional theory approach. At the M06/TZVP/SCM level, the vertical excitation energies, which were calculated based on the ground state and first singlet-state geometries of the sensor molecule, agreed well with the experimental ultraviolet–visible and fluorescence spectra. Therefore, the time-dependent density functional theory method was considered reasonable and effective. According to the frontier orbital analysis and an excited-state potential energy scan, we proposed an excited-state proton transfer mechanism for the sensor-fluorine complex, where the steric hindrance leads to a high potential barrier. The excited-state proton transfer process facilitates sensor molecule deprotonation, alleviates its steric hindrance effect and expands its conjugated system. As a result, the fluorescence emission band of the sensor molecule was red-shifted significantly with the addition of fluoride anion. Based on this fluorescence difference, the sensor could be used for fluoride anion identification. This work provides a strategy to study sensor–analyte interactions in the excited state and offers an approach to tune the fluorescence emission wavelength of sensor molecules in anionic environments.

Simulations of x-ray absorption spectra for CO desorbing from Ru(0001) with transition-potential and time-dependent density functional theory approaches.

The desorption of a carbon monoxide molecule from a Ru(0001) surface was studied by means of X-ray Absorption Spectra (XAS) computed with Transition Potential (TP-DFT) and Time Dependent (TD-DFT) DFT methods. By unraveling the evolution of the CO electronic structure upon desorption, we observed that at 2.3 Å from the surface, the CO molecule has already predominantly gas-phase character. While C 1s XAS is quite insensitive to changes in the C–O bond length, the O 1s excitation is very sensitive with the π* coming down in energy upon CO bond stretching, which competes with the increase in orbital energy due to the repulsive interaction with the metallic surface. We show in a systematic way that the TP-DFT method can describe the XAS rather well at the endpoints (chemisorbed and gas phase) but is affected by artificial charge transfer and/or incorrect spin treatment in the transition region in cases like CO, where there are low-lying π* orbitals and large exchange interactions between the core 1s and valence-acceptor π* orbitals. As an alternative, we demonstrate by comparing with experimental data that a linear response approach using TD-DFT employing common exchange-correlation functionals and finite-size clusters can yield a good description of the spectral evolution of the 1s → π* transition with correct spin and gas-to-chemisorbed chemical shifts in good agreement with experiment.

Noncovalently bound excited-state dimers: a perspective on current time-dependent density functional theory approaches applied to aromatic excimer models.

Excimers are supramolecular systems whose binding strength is influenced by many factors that are ongoing challenges for computational methods, such as charge transfer, exciton coupling, and London dispersion interactions. Treating the various intricacies of excimer binding at an adequate level is expected to be particularly challenging for Time-Dependent Density Functional Theory (TD-DFT) methods. In addition to well-known limitations for some TD-DFT methods in the description of charge transfer or exciton coupling, the inherent London dispersion problem from ground-state DFT translates to TD-DFT. While techniques to appropriately treat dispersion in DFT are well-developed for electronic ground states, these dispersion corrections remain largely untested for excited states. Herein, we aim to shed light on current TD-DFT methods, including some of the newest developments. The binding of four model excimers is studied across nine density functionals with and without the application of additive dispersion corrections against a wave function reference of SCS-CC2/CBS(3,4) quality, which approximates select CCSDR(3)/CBS data adequately. To our knowledge, this is the first study that presents single-reference wave function dissociation curves at the complete basis set level for the assessed model systems. It is also the first time range-separated double-hybrid density functionals are applied to excimers. In fact, those functionals turn out to be the most promising for the description of excimer binding followed by global double hybrids. Range-separated and global hybrids—particularly with large fractions of Fock exchange—are outperformed by double hybrids and yield worse dissociation energies and inter-molecular equilibrium distances. The deviation between each assessed functional and reference increases with system size, most likely due to missing dispersion interactions. Additive dispersion corrections of the DFT-D3(BJ) and DFT-D4 types reduce the average errors for TD-DFT methods but do so inconsistently and therefore do not offer a black-box solution in their ground-state parametrised form. The lack of appropriate description of dispersion effects for TD-DFT methods is likely hindering the practical application of the herein identified more efficient methods. Dispersion corrections parametrised for excited states appear to be an important next step to improve the applicability of TD-DFT methods and we hope that our work assists with the future development of such corrections.

Multi-electron excitation contributions towards primary and satellite states in the photoelectron spectrum.

The computation of Dyson orbitals and corresponding ionization energies has been implemented within the equation of motion coupled cluster singles, doubles and perturbative triples (EOM-CC3) method. Coupled to an accurate description of the electronic continuum via a time-dependent density functional approach using a multicentric B-spline basis, this yields highly accurate photoionization dynamical parameters (cross-sections, branching ratios, asymmetry parameters and dichroic coefficients) for primary (1h) states as well as satellite states of (2h1p) character. Illustrative results are presented for the molecular systems H2O, H2S, CS, CS2 and (S)-propylene oxide (a.k.a. methyloxirane).

Decomposition analysis on the excitation behaviors of thiazolothiazole (TTz)-based dyes via the time-dependent dielectric density functional theory approach.

Thiazolothiazole (TTz)-based materials have been attracting much attention because of their widespread applications. In this paper, we discuss the excited electronic behaviors of asymmetric TTz dyes in solvents based on the time-dependent dielectric density functional theory method. Based on dipole moment and charge distribution (population) analyses, we discuss large intramolecular electron transfers, which are triggered by photon excitations, toward the acceptor part of dyes. In addition, we explore the contributions of geometrical changes and solvent reorientations (reorganizations) to the solvatofluorochromic phenomena based on a decomposition technique. The decomposition analysis shows that the solvent reorientation effect mainly contributes to changes in the fluorescent spectra in response to solvents.

Quantum chemical and photovoltaic modeling of D-π-A organic dyes based on substituted arylamine electron donors in dye sensitized solar cells

In the present research, we designed four tunable triphenylamine (TPA) based organic dyes by applying various aminated electron donor groups and screened their electron donating effects on the electronic, optical and photovoltaic properties of the dyes for application in dye sensitized solar cells (DSSCs). In this respect, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) approaches were applied to investigate the enhanced electron donating effect of substituted TPA dyes on the strength of conjugation with thiophene and a C–C triple bond in the π-spacer fragment that led to a considerable effect on absorption properties, light harvesting efficiency and panchromatic sensitization of the designed dyes in DSSCs. In fact, since the most significant impediment of TPA-based organic dyes for high photovoltaic performance in DSSCs is their low absorption in the near-infrared spectrum, we aimed our modeling to introduce molecular dyes with reduced frontier molecular orbital energy gaps and hence, the favoured red-shift absorption spectra. Moreover, we have computationally concentrated on the photovoltaic performance of our novel dyes in DSSCs and demonstrated that employing aminated electron donating groups in TPA based organic dyes notably tunes the electron injection and regeneration driving forces and hence leads to higher power conversion efficiencies.

Non-fullerene acceptor IDIC based on indacinodithiophene used as an electron donor for organic solar cells: A computational study

In the present paper, a computational study was performed on a planar non-fullerene acceptor (A-D-A) type based on indacenodithiophene (noted IDIC) which is widely used in the fabrication of organic solar cells. The structural and optoelectronic properties were studied using the Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) approaches with different functionals, such as B3LYP, B3PW91, MPW1PW91. The optoelectronic properties such as HOMO and LUMO energy levels, energy gap, λmax were determined and compared with experimental results reported. Charge transfer properties were further characterized through Frontier Molecular Orbitals (FMOs) and Density of States (DOS). Transition density matrix (TDM) and hole&electron isosurface were used to illustrate the behavior of electronic excitation processes as well as the position of electron holes between the donor and acceptor units. In addition, the IDIC compound was tested as an electron donor with the fullerenes and their derivatives as electron acceptors (PCBM). Both electrochemical and photovoltaic properties were investigated and discussed in detailed. The theoretical results indicated that the B3LYP/6-31G(d,p) and its time-dependent counterpart TD-B3LYP/6-31G(d,p) methods are appropriate to predict the optoelectronic properties. The values of the open-circuit voltage (Voc) of IDIC with used acceptors range from 1.165 to 1.665 V. The results of this study showed the high potential of the IDIC compound for integratation into solar cells as an electron donor material and suggested the usefulness of studied materials as promising candidates for photovoltaics.

Spectroscopic and quantum chemical computation on molecular structure, AIM, ELF, RDG, NCI, and NLO activity of 4-VINYL benzoic acid: A DFT approach

Vibrational spectral analysis was carried out for 4-Vinyl Benzoic acid (4VBA) by using Fourier transform infrared (FT-IR) and FT-Raman spectroscopy in the range of 400–4000 and 50–3500 cm−1 respectively. The optimized geometrical parameters were computationally obtained at the DFT/B3LYP level of theory. The distribution of the vibrational bands was carried out with the help of normal coordinate analysis (NCA). The resulting harmonic wavenumbers were scaled by using NCA method. 13C and 1H NMR spectra were recorded and 13C and 1H nuclear magnetic resonance chemical shifts of the molecule were calculated using the gauge independent atomic orbital (GIAO) method. Topological analysis, such as Electron localization function (ELF), natural bond orbital analysis (NBO), reduced density gradient (RDG) and atoms in molecule (AIM) analysis, molecular electrostatic potential (MEP) have been used to evaluate the intermolecular interaction, especially the hydrogen bonds. UV-visible spectrum of the compound was recorded in the region 200–900 nm and the electronic properties and HOMO-LUMO energies were calculated by time dependent density functional theory approach.

Why Does a B12H12 Icosahedron Need Two Electrons to be Stable: A First-Principles Electron-Correlated Investigation of B12Hn (n = 6, 12) Clusters.

In this work, we present large-scale electron-correlated computations on various conformers of B12H12 and B12H6 clusters to understand the reasons behind the high stability of dianion icosahedron (Ih) and cage-like B12H6 geometries. Although the B12 icosahedron is the basic building block in some structures of bulk boron, it is unstable in its free form. Furthermore, its H-passivated entity, i.e., a B12H12 icosahedron, is also unstable in the free form. However, dianion B12H12 has been predicted to be stable as a perfect icosahedron in the free-standing form. To capture the correct picture for the stability of B12H122- and B12H6 clusters, we optimized these structures by employing the coupled-cluster singles and doubles (CCSD) approach and the cc-pVDZ basis set. We also performed the vibrational frequency analysis of the isomers of these clusters using the same level of theory to ensure the stability of the structures. For all of the stable geometries obtained from the vibrational frequency analysis, we additionally computed their optical absorption spectra using the time-dependent density functional theory (TDDFT) approach at the B3LYP/6-31G* level of theory. Our calculated absorption spectra could be probed in future experiments on these clusters.

Spin waves in doped graphene: A time-dependent spin density functional approach to collective excitations in paramagnetic two-dimensional Dirac fermion gases

In spin-polarized itinerant electron systems, collective spin-wave modes arise from dynamical exchange and correlation (xc) effects. We consider here spin waves in doped paramagnetic graphene with adjustable Zeeman-type band splitting. The spin waves are described using time-dependent spin density functional response theory, treating dynamical xc effects within the Slater and Singwi-Tosi-Land-Sj\"olander approximations. We obtain spin-wave dispersions and spin stiffnesses as a function of doping and spin polarization, and we discuss the prospects for their experimental observation.

Impact of internal (donor/acceptor) moieties and π-spacer in triphenylamine-based dyes for DSSCs

The impact of internal (donor/acceptor) groups and π-spacers (benzene/thiophene) are investigated in dye-sensitized solar cells (DSSCs) using a series of D-A-π-A and D-D-π-A type organic dyes. These dyes have a triphenylamine donor, different internal acceptor (A) or donor (D) groups and thiophene/benzene as a π-spacer combined with an acceptor cyanoacrylic acid. Density functional theory (DFT) and time-dependent DFT (TDDFT) approaches were used to investigate the dye geometries, as well as electronic excitations and charge transport characteristics. The frontier molecular orbital energies and energy gaps facilitated for the consideration of appropriate energy levels for dye regeneration, electron transfer and electron injection. The short circuit current density (JSC) calculations were achieved by using dye regeneration energy (ΔGreg), injection driving force (ΔGinj) and light harvesting efficiency (LHE). Charge transport properties like electron affinities (EA), ionization potentials (IP), along with reorganization energies (λh and λe) are also described. The outcomes show that adding an internal donor/acceptor in D-π-A arrangement and different π-bridge influences the absorption energies and enhanced photovoltaic characteristics. As a result, the fundamental goal of this research is to establish a theoretical foundation for appropriate structural changes and design in the development of efficient DSSCs.

Unveiling the Nature of Light-Triggered Hole Traps in Lead Halide Perovskites: A Study with Time-Dependent Density Functional Theory.

Structural variations of lead halide perovskites (LHPs) upon light illumination play an important role in their photovoltaic applications. However, density functional theory (DFT)-based simulations have often been performed to unveil the nature of defects in LHPs without light illumination. So far, the nature of traps in LHPs triggered by the light remains largely unexplored. In this work, hole traps induced by the halogen interstitial in LHPs are studied by combining DFT and time-dependent DFT approaches, the latter of which treats electron-hole and electron-nuclei interactions on the same footing. Both a semilocal exchange functional and hybrid functional are adopted to relax the ground-state and excited-state geometries followed by the calculations of energy levels of hole traps. The effect of the self-interaction corrections on the light-triggered geometric deformation and the electronic structure of hole traps is analyzed. Relaxation energies that correspond to the light-triggered geometric deformation are also calculated with different functionals. The relationship between the hole traps and light-triggered geometric variations are then explored.