Combined Time-dependent Density Functional Theory Research Articles

Intersystem Crossing in Boron-Based Donor-Spiro-Acceptor Organic Chromophore: A Detailed Theoretical Study.

Intersystem crossing and reverse intersystem crossing (rISC) processes were investigated in a boron-based donor-spiro-acceptor organic chromophore which shows thermally activated delayed fluorescence. Due to the perpendicular arrangement between donor and acceptor moieties, the HOMO and the LUMO are spatially separated, and the compound shows charge transfer (CT) transitions. We found both S1 and T1 excited states are CT in nature (i.e., electron and hole wave functions are localized on acceptor and donor units, respectively) and T2, which is higher in energy than S1 and T1, is locally excited in nature (i.e., both electron and hole wave functions are localized on an acceptor unit). Because of the same nature of excitation (i.e., CT here), the spin-orbit coupling matrix element between S1 and T1 is very low and insignificant exciton conversion occurs from the T1 state to the S1 state (and vice versa). Our combined time-dependent density functional theory and quantum dynamics simulation shows that the rISC process from the T1 state to the S1 state can be enhanced by the presence of a nearby local excited triplet state (i.e., T2 state here). A smaller gap between the T1 and T2 states efficiently establishes the rISC route.

High-order harmonic generation in doped diamond: comparison of light and heavy regimes

The electronic response of crystalline n-doped diamond in light and heavy concentrations, as well as high harmonic generation following irradiation by a few cycles high-intensity laser pulse are calculated by modeling the dynamics in the framework of combined time-dependent density functional theory and virtual crystal approximation. The driver pulse frequencies are above and below the band gap frequency. The comparison of transient optoelectronic properties between two regimes shows that light doping enhances the magnitude of HHG. Further, the above gap irradiated doped diamond in both regimes represents much bright harmonics than below gap one. Modulations of the transient effective mass under irradiation of femtosecond laser are analyzed for both doping rates over the Brillouin zone. Comparison of the spectrum from virtual crystal approximation with available harmonic data of dielectric materials from real defected supercell calculations shows the important contribution of ionic relaxation in the description of nonlinear optical properties.

Mechanism of Light Induced Radical Pair Formation in Coenzyme B12-Dependent Ethanolamine Ammonia-Lyase

Coenzyme B12 (adenosylcobalamin = AdoCbl)-dependent enzymes catalyze complex molecular transformations by employing radical chemistry. The initial step in the native catalytic cycle, upon substrate binding, involves homolytic cleavage of the Co–C bond of AdoCbl to form the Co(II)/Ado• radical pair (RP). Formation of Co(II)/Ado• is subsequently coupled with H atom abstraction from the substrate. Interestingly, these same RPs can be generated upon light absorption without presence of a substrate. Herein, the photochemistry associated with the mechanism of Co–C bond photocleavage inside the AdoCbl-dependent ethanolamine ammonia-lyase (EAL) was investigated using a combined time-dependent density functional theory and molecular mechanics (TD-DFT/MM) approach. Excited state potential energy surfaces (PESs), constructed as a function of axial bond lengths, were used to understand the photocleavage of the Co–C bond and to elucidate the mechanism of photodissociation for AdoCbl inside the enzyme. The S1 PES is ch...

A combined TD-DFT and spectroscopic investigation of the solute–solvent interactions of efavirenz

Efavirenz, commercially known as Sustiva® or Stocrin®, is a first-line antiretroviral treatment for HIV/AIDS. The clinical efficacy of efavirenz is, however, hindered by its solubility. We sought to investigate the solute–solvent effects of efavirenz by means of a combined qualitative study implementing UV–visible spectrophotometry, 1H NMR spectroscopy and time-dependent density functional theory (TD-DFT) calculations. The UV spectrum displayed two main absorbance maxima, band I and band II at 246–260 and 291–295nm, respectively. A general bathochromic shift was noticed from the non-polar solvent cyclohexane to the most polar solvent DMSO (≈13.69nm) in band I and a smaller bathochromic (≈2.17nm) and hyperchromic shift was observed in band II. We propose that these observations are due to the role of the amino (NH) and carbonyl (CO) functionalities which induce charge-transfer and intra- and inter-molecular hydrogen bonding. The aromatic and amine protons showed the most deshielded effects in the observed chemical shifts (δ) in the more polar DMSO-d6 solvent relative to CDCl3. The 1H NMR chemical shifts observed are due to the increased delocalization of the lone pair electrons of the amino nitrogen with increased polarity of the more polar DMSO solvent. The theoretical reproduction of the UV and 1H NMR spectra by means of TD-DFT is in good agreement with the experimental results.

Analytical expression for the excited-state force from density-functional perturbation theory

We provide an analytical expression for the energy gradient of electronically excited states within the combined time-dependent density-functional theory and the dynamical density-response formalism, called the Casida formalism. This is achieved by obtaining the nuclear coordinate derivatives of the Kohn-Sham eigenvalues and orbitals through density-functional perturbation theory (DFPT). The DFPT-based method was found successfully applied to obtain potential energy surfaces of small molecules (${\text{N}}_{2}$ and $\text{CO}$) and conduct a molecular dynamics simulation of a benzene molecule, thereby revealing the slow convergence problem associated with the singular matrix appearing in DFPT.

Four novel ketocoumarin-based chromophores: Synthesis, photoluminescent behaviors and theoretical studies

Abstract Four novel coumarin derivatives were synthesized and characterized via MS, 1 H NMR, FT-IR, and UV–vis absorption spectra. The fluorescence behaviors of the compounds in DFM solutions were observed. The compounds exhibited strong blue and green emissions under ultraviolet light excitation. Calculations performed using a combined time-dependent density functional theory (TD-DFT) and conductor-like polarizable continuum model (C-PCM) reproduced the π → π * type absorption bands of the four compounds. Resonance frequency calculations were also performed to study the IR spectra of the compounds. The calculated results are in good accordance with the experimental values.

Solvent-Induced Shift of the Lowest Singlet π → π* Charge-Transfer Excited State ofp-Nitroaniline in Water: An Application of the TDDFT/EFP1 Method

The combined time-dependent density functional theory effective fragment potential method (TDDFT/EFP1) is applied to a study of the solvent-induced shift of the lowest singlet π → π* charge-transfer excited state of p-nitroaniline (pNA) from the gas to the condensed phase in water. Molecular dynamics simulations of pNA with 150 EFP1 water molecules are used to model the condensed-phase and generate a simulated spectrum of the lowest singlet charge-transfer excitation. The TDDFT/EFP1 method successfully reproduces the experimental condensed-phase π → π* vertical excitation energy and solvent-induced red shift of pNA in water. The largest contribution to the red shift comes from Coulomb interactions, between pNA and water, and solute relaxation. The solvent shift contributions reflect the increase in zwitterionic character of pNA upon solvation.

Analytic energy gradient in combined time-dependent density functional theory and polarizable force field calculation

Formulas for evaluating analytic energy gradient are derived for combined time-dependent density functional theory (TDDFT) and polarizable force field methods that incorporate dipole polarizability tensors and linearly induced point dipoles. The Z-vector method for determining relaxed one-particle difference density matrix in regular TDDFT methods is extended to include induced dipoles. The analytic gradient of the mutual polarization energy of the force field and the TDDFT excited state can be formulated by using the TDDFT difference density-induced dipoles and the transition state density-induced dipoles. All the forces and torques involving induced dipoles can be efficiently evaluated using standard electrostatic formulas as if the induced dipoles were permanent dipoles. The formulas are given in the most general form and are applicable to various flavors of polarizable force fields. Implementation and tests with a polarizable five-point water model show that the formulas are rigorous. The carbonyl vibration modes and infrared spectrum intensities of a cluster formed by acetone and two water molecules are studied.

Comparative Theoretical Study of Rotamerism and Excited State Intramolecular Proton Transfer of 2-(2′-Hydroxyphenyl)benzimidazole, 2-(2′-Hydroxyphenyl)imidazo[4,5-b]pyridine, 2-(2′-Hydroxyphenyl)imidazo[4,5-c] pyridine and 8-(2′-Hydroxyphenyl)purine

The effect of nitrogen substitution in the benzene ring of 2-(2'-hydroxyphenyl)benzimidazole (HPBI) on the photophysics and rotamerization were examined theoretically by a comparative study of HPBI with 2-(2'-hydroxyphenyl)imidazo[4,5-b]pyridine (HPIP-b), 2-(2'-hydroxyphenyl)imidazo[4,5-c]pyridine (HPIP-c), and 8-(2'-hydroxyphenyl)purine (HPP). Density functional theory (DFT) was used for ground state calculations. Restricted configuration interaction singles (RCIS) combined time dependent DFT (TDDFT) was used for excited state calculations. The calculations reveal in the ground state all of the molecules have two stable rotameric forms, but their relative population is strongly affected by nitrogen substitution. The excitation and emission bands have been calculated theoretically for the rotamers and tautomers. Fluorescence emission and excitation spectra were recorded for HPBI in dioxane and compared with the theoretical results. Theoretical excitation and emission data are in good agreement with the available experimental data. The potential energy surface simulated for the proton transfer processes reflect that it is not favorable in S(0) state, but it is feasible in S(1) state in all of the molecules. Except in HPIP-b, HPIP-b', and HPP', in all other nitrogen substituted molecules, the energy difference between the keto and enol form along the excited state proton transfer coordinates decreases compared to that in HPBI. The study also reveals that torsional relaxation of tautomer to twisted state competes with radiative transitions and leads to fluorescence quenching. Nitrogen substitution enhances this torsional induced nonradiative process and it follows the order HPBI < HPIP-b < HPIP-c < HPP.

Theoretical investigation of ground- and excited-state properties of fluorenone–alkynyl Hg(II) complexes: The tuning of electronic spectroscopy

The electronic properties of R C C H ( 1), [R C C HgCH 3] ( 2), [CH 3Hg C C R C C HgCH 3] ( 3) and [R C C Hg C C R] ( 4) (R = fluoren-9-one) in the ground and singlet/triplet excited states were explored theoretically. Calculations indicate that geometry parameters of 1– 4 change slightly on going from ground state to excited state. Experimental spectra were well reproduced by the combined TD-DFT (time-dependent density functional theory) and PCM (Polarized-Continuum Model) calculations. With increasing methylmercury and/or fluorenone groups, the featured absorption and emission bands of complexes are redshifted. The investigation reveals such a variation can tune excitation energies but not change transition nature.