Nature Of Electronic Excitations Research Articles

Colorimetric detection of clotrimazole environmental pollutant using a newly developed chemosensor; an experimental and theoretical study

ABSTRACT Clotrimazole (CTZ), as an antifungal medication, is used to treat dermatological infections in humans, and to remove fungal infections from plants in agriculture, but its widespread usage has led to pharmaceutical pollution and environmental concerns. In this study, a new bromopyrogallol red (BPR) and N i 2 + based colorimetric sensor was developed for the selective and sensitive detection of CTZ. The BPR-Ni complex showed a clear colour change from purple to violet in the presence of CTZ, and a significant change was observed in the UV-Vis spectrum. The stoichiometric ratio of 1:1 between BPR and N i 2 + was determined using the Benesi-Hildebrand method. DFT and time-dependent DFT (TD-DFT) were applied to identify the molecular and electronic structures of the BPR, BPR-Ni, and BPR-Ni-CTZ in ground and excited states. The nature of electronic excitations and charge transfers was evaluated using the natural transition orbitals (NTO). The NTO analysis reveals that the Ni atom facilitates the charge transfer process between BPR and CTZ and improves the sensitivity of the chemosensor. The quantum theory of atoms in molecules (QTAIM) was employed to determine the nature of interactions between BPR, N i 2 + , and CTZ. This study paves the way for the detection of environmentally harmful pharmaceutical pollution using simple but effective colorimetric sensors.

Nature of electronic excitations in small non-stoichiometric quantum dots

Low-energy electronic excitations in non-stoichiometric quantum dots (QDs) have a unique charge transfer (CT) character: surface-to-core in anion-rich and core-to-surface in cation-rich QDs.

Experimental and theoretical study of 4-formyl pyrrole derived aroylhydrazones

Two new 4-formyl pyrrole derived aroylhydrazones (3a, b) from ethyl 4-formyl-3,5-dimetyl-1H-pyrrole-2-carboxylate and aroylhydrazides (3,5-dinitrobenzohydrazide/2-hydrazinocarbonyl-N-phenyl-acetamide) have been synthesized and characterized by various spectroscopic techniques 1H NMR, Mass, UV–Visible and FT-IR. The calculated thermodynamic parameters show that the formation of 3a as spontaneous, whereas 3b as non-spontaneous. TD-DFT has been used to calculate the absorption wavelengths, oscillator strength (f) and the nature of electronic excitations. Natural bond orbital (NBO) analysis has been carried out to explore the various conjugative and hyperconjugative interactions and their second order stabilization energy (E(2)) within monomer and its dimer. The dimer formation of 3a, 3b due to result of intermolecular hydrogen bonding N1H30⋯O84, N1H28⋯O60 is obvious in 1H NMR, NBO and FT-IR as down field chemical shifts, n(O84)→σ∗(N1H30), n(O60)→σ∗(N1H28) interactions, vibrational red shifts, respectively. To determine the strength and nature of hydrogen bonding, topological parameters at bond critical points (BCP) have been analyzed by ‘Quantum theory of Atoms in molecules’ (QTAIM) in detail. The global electrophilicity index (ω) has been calculated to determine the relative electrophilic strength of molecules. The local reactivity descriptors analyses such as Fukui functions (fk+, fk−), local softnesses (sk+, sk−) and electrophilicity indices (ωk+, ωk−) have been performed to determine the reactive sites within molecules. The first hyperpolarizabilities (β0) of 3a, b have been computed to evaluate the non-linear optical (NLO) response of the investigated molecules.

The nature of electronic excitations at the metal–bioorganic interface illustrated on histidine–silver hybrids

We present a joint theoretical and experimental study of the structure selective optical properties of cationic and anionic histidine-silver complexes with Ag and Ag3 which were prepared in the gas phase using mass spectroscopy coupled to electrospray ion source. Our TDDFT calculations provide general insight into the nature of electronic excitations at the metal-bioorganic interface that involve π-π* excitation within bioorganic subunits, charge transfer between two subunits and intrametallic excitations. The binding of silver to histidine, one of the most important amino acids, induces red shift in the optical absorption of protonated histidine particularly for anionic species. The presence of the smallest metallic subunit Ag3 increases the intensity of low energy transitions of histidine illustrating a metal cluster-induced enhancement of absorption of biomolecules in hybrid systems. Comparison of calculated absorption spectra with experimental photofragmentation yield provides structural assignment of the measured spectroscopic patterns. Our findings may serve to establish silver-labeling as the tool for the detection of histidine or histidine-tagged proteins.

Studies on molecular structure, spectral analysis, chemical reactivity and first hyperpolarizability of a newly synthesized 1,9-bis[(4-isonicotinoyl)-hydrazonomethyl]-5-phenyl-dipyrromethane using experimental and theoretical approaches

The detailed spectroscopic analysis of a newly synthesized dipyrromethane derivative: 1,9-bis[(4-isonicotinoyl)-hydrazonomethyl]-5-phenyl-dipyrromethane (3) has been performed using experimental measurement and quantum chemical calculations. Calculated thermodynamic parameters (H, G, S) of all the reactants and products have been used to determine the nature of reaction of synthesis. Occurring of a 1H NMR singlet at 5.54ppm indicates meso-proton and formation of product molecule (3). The molecular orbital coefficients and molecular plots analysis indicates the nature of electronic excitations as π→π*. Natural bond orbitals (NBOs) analysis has been carried out to investigate the intramolecular interactions within molecule and their second order stabilization energy. The primary hyperconjugative interactions n2(O23)→σ*(N21C22)/σ*(C22C24) and n2(O48)→σ*(N46C47)/σ*(C47C49) stabilize the molecule to a greater extent ∼117.75KJ/mol. Global electrophilicity index (ω=3.64eV) shows that the title molecule (3) is a strong electrophile. Local reactivity descriptors: Fukui functions (fk+,fk-), local softnesses (sk+,sk-) and electrophilicity indices (ωk+,ωk-) analyses were performed to find out the reactive sites within molecule. The first hyperpolarizability (β0) has been computed and found to be 60.95×10−30esu indicating synthesized molecule to be suitable for non-linear optical response.

Large face to face tetraphenylporphyrin/fullerene nanoaggregates. A DFT study

Large face to face tetraphenylporphyrin/fullerene nanoaggregates containing up six C60 units and six tetraphenylporphyrin (H2TPP) or tetraphenylporphyrinato-zinc (TPP-Zn) moieties have been studied using dispersion corrected PBE/def2-SVP level of theory. It has been found that most important contribution to the binding energy between fragments comes from dispersion interactions. The binding energies for Zn containing nanoaggregates are slightly higher than those for metal free ones what is not related to the difference in dispersion contributions to the binding energy but comes from DFT term. Center to center distances for large nanoaggregates are shorter than those for the complexes H2TPP/C60 and TPP-Zn/C60 and this effect is more obvious for metal free nanoaggregates. According to the calculations, the band gap of nanoaggregates barely depends on its size being close to 2eV. The nature of electronic excitations in Zn containing nanoaggregates has strong charge transfer (CT) contribution and does not depend on nanoaggregate size, while for metal free nanoaggregate most of low energy excitations are not CT by nature. Ionization potentials and electron affinities of nanoaggregates depend strongly on their size. Polaron cations are uniformly delocalized over donor H2TPP or TPP-Zn units, while polaron anions are delocalized over acceptor C60 units. The reorganization energies calculated for hole and electron transport decreased linearly with 1/n where n is the number of repeating units in nanoaggregate.

Electronic excitations in shocked nitromethane

The nature of electronic excitations in crystalline solid nitromethane under conditions of shock loading and static compression are examined. Density-functional theory calculations are used to determine the crystal bandgap under hydrostatic stress, uniaxial strain, and shear strain. Bandgap lowering under uniaxial strain due to molecular defects and vacancies is considered. Ab initio molecular-dynamics simulations are done of all possible nearest-neighbor collisions at a shock front, and of crystal shearing along a sterically hindered slip plane. In all cases, the bandgap is not lowered enough to produce a significant population of excited states in the crystal. The nearly free rotation of the nitromethane methyl group and localized nature of the highest occupied molecular orbital and lowest unoccupied molecular orbital states play a role in this result. Dynamical effects have a more significant effect on the bandgap than static effects, but relative molecule velocities in excess of 6 km/s are required to produce a significant thermal population of excited states.

Theoretical exploration of stationary and of ultrafast spectroscopy of small clusters

Stationary spectra offer information on the interplay between the structures and the nature of electronic excitations reflecting bonding properties, as shown by comparing Sin with Agn (n=4-6) clusters. In order to study the dynamical properties, simulations and analysis of femtosecond (fs) time-resolved pump–probe or pump–dump signals have been carried out, which allows us to determine the timescales and the nature of configurational changes versus internal vibrational relaxation (IVR) in electronic ground or excited states. For this purpose we have developed a multi-state ab initio molecular dynamics (involving ground as well as adiabatic or non-adiabatic excited electronic states) on the timescale of the nuclear motion, using the time evolution of a thermal ensemble in the Wigner representation. The combination of ab initio quantum-chemical methods used for the molecular dynamics ‘on the fly’ and the Wigner-distribution approach for the description of the motion of the nuclei also allowed the accurate determination of pump–probe and pump–dump signals under temperature-dependent initial conditions. We use this novel combination of methods to investigate the dynamics in excited states of non-stoichiometric NanFn-1 clusters with a single excess electron. The timescales of the structural relaxation in excited states versus intramolecular vibrational relaxation processes have been determined, as illustrated for the example of Na4F3. This is the first study of the system with 15 degrees of freedom for which the dynamics in the excited states has been carried out without the precalculation of the energy surfaces.

The nature of electronic excitations in singly bonded C60 dimer

Studying excited states of molecules is rather demanding computationally, and available theoretical methods usually lack a transparent interpretation scheme. Interpretation of electronic spectra is especially important if two or more fragments can be identified in the system.We developed a method to characterize different excited states in dimer molecules. Localized dimer wave functions are determined in the following manner. Isolated monomer wave functions are projected into the exact subspace of dimer orbitals leading to nonorthogonal localized MOs. Orthogonal localized MOs emerge from these by Löwdin's procedure. The resulting orbitals have the characteristic feature that they are localized either on monomer “A”, or monomer “B”, but they preserve the local intramolecular symmetries within both fragments. In terms of these orbitals an all-single CI calculation is executed. Inspecting the CI eigenvectors, the excitations can easily be classified as to their localized vs charge transfer characters (A→A, B→B, A→B, and B→A). This procedure was used in studying excited states of the singly bonded covalent buckminsterfullerene dimer.

Excitonic interactions and excimer formation in pure and mixed cluster isotopomers of naphthalene

The nature of electronic excitation and subsequent excited-state dynamics in small naphthalene clusters has been elucidated using isotope labeling techniques. The S1←S0 spectra of tetramer isotopomers are characterized by structurally inequivalent site splitting which is induced by an exciton interaction in the S2 state through vibronic coupling. It is suggested that the tetramer involves a pair of nearly overlapped chromophores, which is responsible for the excitonic interaction, and other two chromophores acting as solvents. An excitonic state originating in this pair is proposed to correlate with the lowest excimer state of the cluster as the interplanar separation and dihedral angle are reduced. In contrast, the spectra of trimer isotopomers are analyzed by invoking a symmetric geometry in which three chromophores are less overlapped and thus give rise to weak excitonic effects. The isomerization dynamics of these clusters is discussed in terms of a vibrationally activated process which makes the cluster framework wobble, allowing for stronger exciton interaction.

Two new developments in the theory of high Tc superconductivity

Abstract Two new developments in the theory of high Tc superconductivity which explain experimental anomalies are described: First, an explanation of some striking anomalies in the angle resolved photoemission excitation spectrum provides experimental evidence for the composite nature of electronic excitations; and secondly, a gap equation for the superconducting state, which allows detailed calculations using phonon plus interlayer coupling, allows calculation of the large anisotropy of the gap, of the isotope effect, and of estimates of Tc and its dependence upon structure.

Luminescence excitation of pure and impure BeO single crystals using synchrotron radiation

Absorption, reflection, UV and VUV luminescence excitation and thermoluminescence excitation spectra have been measured for BeO and BeOZn crystals in the energy range from 8 to 35 eV, using synchrotron radiation. The nature of electronic excitations in the fundamental absorption edge region is discussed; the value of the forbidden-gap energy, E g, is estimated as 10.6 eV for BeO. Intrinsic luminescence with a 6.7 eV maximum for BeO and impurity luminescence with a 6.0 eV maximum for BeOZn are due to the relaxation of both optically created self-trapped or impurity-trapped excitons. The multiplication effect of electronic excitations with E>2 E g (or E≥2 E ex for 6.7 eV luminescence) for beryllium oxide is due to the inelastic scattering of hot photoelectrons or hot photoholes.

Electron excitation and luminescence in Bi 4Ge 3O 12 and Bi 4Si 3O 12 crystals

Reflection, luminescence excitation and thermoluminescence excitation spectra have been measured for Bi 4Ge 3O 12 and Bi 4Si 3O 12 crystals in the energy range from 3 to 35 eV using synchrotron radiation. The reflection data are evaluated using a modified Kramers-Kronig method for extracting the information on optical constants of these crystals. The nature of electronic excitations in the fundamental absorption edge region is discussed, the value of forbidden-gap energy E g is estimated as 5.0 eV for Bi 4Ge 3O 12 and 5.4 eV for Bi 4Si 3O 12. Intrinsic luminescence with 2.5 eV maximum for Bi 4Si 3O 12 and 2.45 eV maximum for Bi 4Ge 3O 12 is due to both the relaxation of optically created excitons and recombination processes. The multiplication effect of electron excitations in E ⩾ 2 E g for these crystals is due to the inelastic scattering of hot photoelectrons and hot photoholes.

Electronic excitations in solid ZrO 2 from reflection EELS and ESCA multipeak structures

The nature of electronic excitations in ZrO 2 is investigated by means of ESCA multipeak structures and EELS spectra obtained in reflection mode with primary electron beams having an energy of 0.2–4.5 keV. Comparison between the ESCA multipeak regions and the low-energy region of the reflection EELS spectrum obtained on the same sample discriminates the photoionization final-state structures from those due to inelastic scattering of photoelectrons.