Shift Of Excitation Energies Research Articles

A discrete interaction model/quantum mechanical method for describing response properties of molecules adsorbed on metal nanoparticles

A new polarizable quantum mechanics/molecular mechanics method for the calculation of response properties of molecules adsorbed on metal nanoparticles is presented. This method, which we denote the discrete interaction model/quantum mechanics (DIM/QM) method, represents the nanoparticle atomistically which enables the modeling of the influence of the local environment of a nanoparticle surface on the optical properties of a molecule. Using DIM/QM, we investigate the excitation energies of rhodamine-6G (R6G) and crystal violet (CV) adsorbed on silver and gold nanoparticles of different quasispherical shapes and sizes. The metal nanoparticle is characterized by its static total polarizability, a reasonable approximation for frequencies far from the plasmon resonance. We observe that for both R6G and CV, the presence of the nanoparticle shifts the strongest excitation to the red approximately 40 nm and also increases the oscillator strength of that excitation. The shifts in excitation energies due to the nanoparticle surface are found to be comparable to those due to solvation. We find that these shifts decay quickly as the molecule is moved away from the surface. We also find that the wavelength shift is largest when the transition dipole moment is aligned with the edges of the nanoparticle surface where the electric field is expected to be the largest. These results show that the molecular excitations are sensitive to the local environment on the nanoparticle as well as the specific orientation of the molecule relative to the surface.

Absorption Spectra of Blue-Light-Emitting Oligoquinolines from Time-Dependent Density Functional Theory

Recently, it has been discovered that a series of four conjugated oligomers, oligoquinolines, exhibits many desirable properties of organic materials for developing high-performance light-emitting diodes: good blue color purity, high brightness, high efficiency, and high glass-transition temperatures. In this work, we investigate the optical absorption of oligoquinolines in the gas phase and chloroform (CHCl3) solution, respectively, using time-dependent density functional theory with the adiabatic approximation for the dynamical exchange-correlation potential. Our calculations show that the first peak of optical absorption corresponds to the lowest singlet excited state, whereas several quasi-degenerate excited states contribute to the experimentally observed higher-frequency peak. We find that, compared with the gas phase, there is a moderate red shift in excitation energy in solution due to the solute-solvent interaction simulated using the polarizable continuum model. Our results show that the lowest singlet excitation energies of oligoquinolines in chloroform solution calculated with the adiabatic hybrid functional PBE0 are in a good agreement with experiments. Our simulated optical absorption agrees well with the experimental data. Finally, analysis of the natural transition orbitals corresponding to the excited states in question underscores the underlying electronic delocalization properties.

Effect of Noncovalent Interactions on the n-Butylbenzene···Ar Cluster Studied by Mass Analyzed Threshold Ionization Spectroscopy and ab initio Computations

Clusters of Ar bound to isomers of the aromatic hydrocarbon n-butylbenzene (BB) have been studied using two-color REMPI (resonance enhanced multiphoton ionization) and MATI (mass analyzed threshold ionization) spectroscopy to explore noncovalent vdW interactions between these two moieties. Blue shifts of excitation energy were observed for gauche-BB...Ar clusters, and red shifts for anti-BB...Ar clusters were observed. Adiabatic ionization energies (IEs) of the conformer BB-I...Ar and BB-V...Ar were determined as 70052 and 69845 +/- 5 cm (-1), respectively. Spectral features and vibrational modes were interpreted with the aid of UMP2/cc-pVDZ ab initio calculations. Data of complexation shifts of the alkyl-benzenes and their argon clusters were collected and discussed. Using the CCSD(T) method at complete basis set (CBS) level, interaction energies for the neutral ground states of BB-I...Ar and BB-V...Ar were obtained as 650 and 558 cm (-1), respectively. Combining the CBS calculation results and the REMPI and MATI spectra allowed further the determination of the interaction energies and the energetics of BB...Ar in the excited neutral S 1 and the D 0 cationic ground states.

Spectral shifts of ozone molecule by the complex formation with a water molecule

The excitation energies of O 3–H 2O complex have been calculated by means of SAC–CI method to elucidate the spectral shifts of excitation energies of O 3 caused by the complex formation with a water molecule. The eclipsed form, where the center oxygen of O 3 and water oxygen are located on the C s molecular plane, was examined in the present study. The first and third excitation energies of O 3 were slightly blue-shifted by the complex formation with H 2O. The oscillator strength for the third excitation was not affected by the complex formation, indicating that the photo-dissociation of the O 3–H 2O complex occurs efficiently as well as free ozone molecule. The electronic states of the complex was discussed on the basis of theoretical results.

How Proteins Trigger Excitation Energy Transfer in the FMO Complex of Green Sulfur Bacteria

A simple electrostatic method for the calculation of optical transition energies of pigments in protein environments is presented and applied to the Fenna-Matthews-Olson (FMO) complex of Prosthecochloris aestuarii and Chlorobium tepidum. The method, for the first time, allows us to reach agreement between experimental optical spectra and calculations based on transition energies of pigments that are calculated in large part independently, rather than fitted to the spectra. In this way it becomes possible to understand the molecular mechanism allowing the protein to trigger excitation energy transfer reactions. The relative shift in excitation energies of the seven bacteriochlorophyll-a pigments of the FMO complex of P. aestuarii and C. tepidum are obtained from calculations of electrochromic shifts due to charged amino acids, assuming a standard protonation pattern of the protein, and by taking into account the three different ligand types of the pigments. The calculations provide an explanation of some of the earlier results for the transition energies obtained from fits of optical spectra. In addition, those earlier fits are verified here by using a more advanced theory of optical spectra, a genetic algorithm, and excitonic couplings obtained from electrostatic calculations that take into account the influence of the dielectric protein environment. The two independent calculations of site energies strongly favor one of the two possible orientations of the FMO trimer relative to the photosynthetic membrane, which were identified by electron microscopic studies and linear dichroism experiments. Efficient transfer of excitation energy to the reaction center requires bacteriochlorophylls 3 and 4 to be the linker pigments. The temporal and spatial transfer of excitation energy through the FMO complex is calculated to proceed along two branches, with transfer times that differ by an order of magnitude.

HYBRIDIZATION BETWEEN 4f-5d STATES IN ErAs(100)

Generally in both elemental and compound rare earth systems, the 4f levels are treated as core states with little interaction with the 5d hybrid conduction and valence band states. In the present study, the band dispersions of the rare-earth pnictide erbium arsenide ( ErAs ) are observed to be perturbed from this expectation. Both photoemission symmetry selection rules and ab initio calculations provide evidences of the 4f - 5d hybridization. This finding is also supported by the f–f resonant transition excitation energy shifts in characteristic electron-energy-loss spectroscopy (EELS).

Hydrogen-bonding-induced shifts of the excitation energies in nucleic acid bases: an interplay between electrostatic and electron density overlap effects.

The theoretically calculated dimerization-induced shifts of the lowest excitation energies in two model systems, adenine-thymine and guanine-cytosine base pairs, are analyzed. The applied formalism is based on first principles and allows one to study the influence of the microscopic environment of a given molecule on its ground- [Wesolowski, T. A.; Warshel, A. J. Phys. Chem. 1993, 97, 8050] and excited-state [Casida, M. E.; Wesolowski, T. A. Int. J. Quantum Chem. 2004, 96, 577] properties. The assessment of the relative importance of such effects as (a) Coulomb interactions, (b) orbital interactions, (c) electronic polarization of the environment, and (d) electron density overlap effects is straightforward in this formalism. In the applied formalism, electron density overlap effects can be further decomposed into the exchange-correlation component which provides a small attractive contribution and the repulsive kinetic energy-dependent component. It is shown that the shifts can be attributed to the electrostatic interactions and the repulsive overlap-dependent term in the embedding potential. The electronic polarization of the environment plays a significant role (up to 30% of the total shift) only in transitions involving the orbitals localized on hydrogen bond donor groups. For all analyzed shifts, the contribution of the intermolecular orbital interactions is negligible. The analysis of this work provides strong evidence supporting the use of the widely applied embedding-molecule strategy in computational studies of chromophores in a condensed phase even in such cases where only one end of the hydrogen bond is included in the quantum mechanical part.

Absorption and Fluorescence of 3-Methylindole: A Theoretical Study, Including H2O Interactions

The paper presents a computational study of the ground and excited states of 3-methylindole. The ground-state geometry is optimized using the DFT B3LYP potential in conjunction with a 6-31+G(d) basis set. In addition the ground and first and second excited singlet states are optimized, for the first time, using the CASSCF method with an ANO−S basis set. The results are compared with experiment and with the experimental and theoretical literature data for the parent indole. The calculated excitations are in agreement with the absorption data. The comparison also shows that the 3-methylindole relaxation of the second excited-state terminates in an avoided crossing. The relaxed geometry of the second excited 3-methylindole state is therefore of mixed 1La/1Lb nature. The interaction of 3-methylindole with water is also examined, revealing three stable chromophore−water complexes. The calculations are extended to the more stable chromophore−(water)2 complexes. The excitation energies of these complexes are calculated using the CASPT2 method (ANO−S type basis). Calculations using ESP-charges instead of water molecules demonstrate the usefulness of such approximation for the calculation of excitation energy shifts. Analogous results are obtained using Amber5.0 point charges.

On the performance of time-dependent density functional theory and polarizable continuum model in the study of aqueous formaldehyde

In this paper Time-Dependent Density Functional Theory (TD-DFT) with Becke's three-parameter hybrid method using the Lee–Yang–Parr correlation functional (B3LYP) in conjunction with different basis sets is applied to calculate the solvatochromic shift on the lowest singlet transition of formaldehyde in vacuum and in solution. Our results, in excellent agreement with solvent excitation energy shift of acetone in water at 298–300 K (no experimental data on formaldehyde in water are available so far) and with previous high level quantum chemical calculations, help in appreciating the potentialities of the combined DFT/PCM approach on aquo-complexes for the study of structural and spectroscopic properties of molecule in solution.

Condensation effects in K-shell excitation and de-excitation of solid neon

Condensation effects in excitation and decay of inner shell electronic states with respect to the isolated atom have been studied for solid neon, in particular excitation energy shifts for the surface and for the bulk, effects of symmetry breaking at the surface, and differential cross section changes of the core decay. For the investigation of the excitation, different probes and experimental techniques including threshold electron spectroscopy and photon stimulated desorption have been used to discriminate bulk and surface, and bound and continuum states, and to extract condensation induced modifications of their properties compared to the isolated atom. In the decay, we focus on changes of the branching ratios into spectator and shake-up/shake-down channels by condensation. We find a slight increase of the shake probability for the bulk, but not for the surface.

Molecular Response Method for Solvated Molecules in Nonequilibrium Solvation

We present an extension of the molecular response method of solvated compounds. The extension treats in a proper fashion the effects of nonequilibrium solvation on the molecular properties of the solvated compound. The molecular properties evaluated within the response formalism are properties that are measured using high-frequency electromagnetic fields. We consider the changes in the molecular properties due to nonequilibrium solvation. The nonequilibrium solvation arises from changes in the molecular charge distribution induced by the high-frequency perturbation. The optical polarization vector is at all times assumed to be in equilibrium with the charge distribution of the solute, contrary to the inertial polarization, which is not necessarily in equilibrium with the charge distribution of the solute. We calculate shifts of excitation energies and frequency-dependent polarizabilities as a function of the optical and static dielectric constants.

Reaction field calculation of the spectral shifts of indole

Abstract : The excitation energy of indole has been investigated at geometries relevant to both the absorption and emission spectra using first-order configuration interaction (FOCI) methods. The calculation focused on the shift in the relative energy between the two excited states, L sub B and L sub A, and the excitation energies with geometry. A reaction field (RF) calculation also examined the shift in excitation energies when the molecule is dissolved in water. The excited states are found to be separated in vacuo by about 4,500 /cm at both the ground state and an approximation to the equilibrium L sub B geometry, M, and by 2,500/cm at the optimized geometry of both the first triplet excited state, T, and the first excited singlet state represented by a singles-only configuration interaction (S1). In water, the excited states are essentially degenerate and axed at the ground-state geometry. However, at the excited-states geometries, M and T, the L sub A state is shifted lower in energy than the L sub B state due to the large L sub A excited-state RF. At the S1 geometry, the electronic behavior of the two excited states is different from that at the other geometries. An approximation to the L sub A state is now the lower state both in vacuo and in the presence of the RF. There is a large calculated solvent fluorescent red shift in water at any geometry as well substantial Franck-Condon (FC) shift due to the steeper energy surface in the ground state compared to the excited states. The calculated energy shifts, dipole moments, and relative transition dipoles are in qualitative accord with experiment. However, the calculated fluorescent reaction dipoles are substantially larger than estimated in earlier studies. (jg)

Collective excitations of the QED vacuum.

Using relativistic Green's-function techniques we examined single-electron excitations from the occupied Dirac sea in the presence of strong external fields. The energies of these excited states are determined taking into account the electron-electron interaction. We also evaluate relativistic transition strengths incorporating retardation, which represents a direct measure of correlation effects. The shifts in excitation energies are computed to be lower than 0.5%, while the correlated transition strengths never deviate by more than 10% from their bare values. A major conclusion is that we found no evidence for collectivity in the electron-positron field around heavy and superheavy nuclei.

Photoluminescence Properties of Graded Composition MgxZn1−xSe Crystals

ABSTRACTThe photoluminescence properties of MgxZn1−xSe solid solution grown on ZnSe substrate by a closed solid-state diffusion technique have been investigated. The depth profiles of the diffusion region shows distinguished surface layer and thick graded layer. The PL. spectra of MgxZn1−xSe side of the sample excited with a wide range of excitation energy shift the edge emission band toward longer wavelengths with decreasing excitation energy. The edge emission spectrum was found to shift toward the shorter wavelength with electric field applied parallel to the compositional gradient ofMgxZn1−xSe-ZnSe heterostructure.