Absorption Energy Shifts Research Articles

Difference in the hydration state of water at the hydrophobic interface of structural isomers of propanol investigated by U.V visible absorption and Raman spectroscopic study

In this work, the nature of hydration shell water around hydrophobic groups of alcohol has been investigated by studying the U.V visible absorption energy shift of p-nitro-aniline upon interaction with water-alcohol mixture in different proportion. Analysis shows that, at low mole fraction of alcohol the absorption energy shift with composition of alcohol-water mixture originates from the interaction of hydrophobically modified water with PNA. It is further observed that, the absorption energy changes nonlinearly with the chain length of alkyl group of alcohol. From Raman studies it is apparent that, the absorption energy shift originates from the changing population of dangling OH interacting with PNA. The central part of the study has been to investigate the difference in the hydrophobic environment due to variation in the arrangement of hydrophobic groups in propanol. The U.V visible absorption studies of PNA in 1-propanol and 2-propanol water mixture show that 1-propanol bears higher population of dangling OH compared to 2-propanol. In addition, we have also performed Raman spectroscopic studies in the region of OH stretch. The spectra of hydration shell water are extracted by RD-SCF method. The characteristic peak due to dangling OH was analyzed and it also shows 1-propanol to possess relatively higher amount of dangling OH compared to 2-propanol. This joint study confirms that arrangement of hydrophobic moiety in small molecules can affect the water hydrogen bonding of water at the hydrophobic interface.

Quantum chemical design of near-infrared retinal-based pigments and evaluating their vibronic/electronic properties

Some retinal base structures with controlled properties in near-infrared excitation energy are proposed. The electronic and structural properties of the new derivatives are analyzed using density functional theory methods. There is a possibility of changing the photo-switching property of our proposed pigments in the effect of the added functional groups and the electrostatic interactions with the environment. A development concept in the future is the design of material with controlled and desirable properties. Accordingly, some newly appeared materials in this work could be good candidates for biological objects, as their absorption energy shifts to less than 1.91 eV.

Unfolding Band and Absorption Energy Shift of \(\text{Si-Ge}\) Nano Crystals from First-principles Calculations

Physical properties of the Si\(_{1-x}\)Ge\(_{x}\) alloys ($x$ being the composition of Ge) can be understood and predicted from their electronic band structures. In this paper, electronic band structures of the Si\(_{1-x}\)Ge\(_{x}\) alloys are calculated using the first-principles density functional theory. The supper cell approach employed in our calculations leads to folding of electronic bands into the smaller Brillouin zone of the supercell, especially at the \(\Gamma\) point. This often leads to the misinterpretation that the materials have direct band gap. The problem can be resolved by an unfolding band technique which allows us to recover the primitive cell picture of band structure of Si\(_{1-x}\)Ge\(_{x}\). As a result, unfolded electronic bands correctly show an indirect band gap with the valence band maximum (VBM) at the $\Gamma $ point and the conduction band minimum (CBM) shifted away from \(\Gamma\). CBM is gradually shifted from a point along \(\Gamma X\) symmetry line (associated with Si) to the L point (associated with Ge) with the increased Ge composition \(x\) and the switching occurs at \(x\) in the range of 0.6\(\sim\)0.8 which is in accordance with the calculation using \textbf{\textit{kp}} method. Moreover, the additional electron pockets appear and develop at \(\Gamma\) and $L$. This provides more comprehensive understanding for our recent experimental observations on the shift of the absorption energy assigned to $E1$ direct transitions within \(L\) and \(\Gamma\) points in the Brillouin zone of Si\(_{1-x}\)Ge\(_{x}\) alloy nanocrystals.

Periodic Trends from Metal Substitution in Bimetallic Mo-Based Phosphides for Hydrodeoxygenation and Hydrogenation Reactions

Bimetallic phosphides are promising materials for biomass valorization, yet many metal combinations are understudied as catalysts and require further analysis to realize their superior properties. Herein, we provide the synthesis, characterization, and catalytic performance of a variety of period 4 and 5 solid solutions of molybdenum-based bimetallic phosphides (MMoP, M = Fe, Co, Ni, Ru). From the results, the charge sharing between the metals and phosphorus control the relative oxidation of Mo and reduction of P in the lattice, which were both indirectly observed in binding energy shifts in X-ray photoelectron spectroscopy (XPS) and absorption energy shifts in X-ray absorption near-edge spectroscopy (XANES). For MMoP (M = Fe, Co, Ni), the more oxidized the Mo in the bimetallic phosphide, the higher the selectivity to benzene from phenol via direct deoxygenation at 400 °C and 750 psig. This phenomenon was observed in the bimetallic materials synthesized across period 4, where aromatic selectivity and degr...

PH-Dependent absorption spectrum of a protein: a minimal electrostatic model of Anabaena sensory rhodopsin.

A minimal electrostatic model is introduced which aims at reproducing and analyzing the visible-light absorption energy shift of a protein with pH. It relies on the existence of a protein structure, the prediction of titratable amino-acid pKa values and a very limited set of parameters. Applied to the case of the photochromic Anabaena sensory rhodopsin protein, the model succeeds in reproducing qualitatively the reported experimental data, confirming the importance of aspartic acid 217 in the observed blue shift in the λmax of ASR at neutral pH. It also suggests for the first time the role of two other amino acids, glutamic acid 36 at basic pH and aspartic acid 120 at acidic pH.

Shift of absorption energy during thin dye film growth: interpretation by geometric models of the growth morphology

We measure absorption spectra of two different perylene molecules in thin films with thicknesses varied between 1 and 30nm physical vapor deposited on glass substrates. Atomic force microscopy (reveals the formation of crystallized needles for N, N`-dimethyl perylene tetracarboxylic acid diimide and amorphous round-shaped islands for 1,6,7,12-tetrachloro-N,N`-dimethyl perylene tetracarboxylic. For both molecules, a spectral shift of the lowest energy transition to lower energies was observed with increasing film thickness. Common models taking electric fields, electronic coupling or exciton confinement into account cannot completely describe the observed spectral shift. Here we show that the experimental values can be fitted with an advanced layering model based on the energetic difference between bulk and interface/surface materials. In contrast to the known simple d−1 model, which describes the energetic shift for the special case of a layer-by-layer growth, we extend this idea to other film morphologies.

Charging-Discharging Phenomena on P2-Na2/3Co2/3Mn1/3O2 Observed by X-ray Absorption Spectroscopy

Charging-discharging phenomena of P2-Na2/3Co2/3Mn1/3O2 prepared by a co-precipitation method were investigated by ex situ and in situ X-ray absorption spectroscopy [1, 2]. The electronic transitions at the O K-edge and the charge compensation mechanism, during the sodium intercalation process, were elucidated by combining Density Function Theory (DFT) calculations and X-ray absorption spectroscopy (XAS) data. The pre-edge of the oxygen K-edge moves to higher energy while the integrated intensity dramatically decreases, indicating that the population of holes in O 2p states is reduced with increasing numbers of sodium ions. From the K-edge and L-edge observations, the oxidation states of pristine Co and Mn were determined to be +III and +IV, respectively. The absorption energy shifts to lower positions during the discharging process for both the Co and Mn edges, suggesting that the redox pairs, i.e. Co3+/Co2+ and Mn4+/Mn3+, are both involved in the reaction.[1] J.-H. Cheng, C.-J. Pan, J.-F. Lee, J.-M. Chen, M. Guignard, C. Delmas, D. Carlier, and B.-J. Hwang, Chem. Mater., 26 (2014 ) 1219–1225.[2] D. Carlier, J. H. Cheng, R. Berthelot, M. Guignard, M. Yoncheva, R. Stoyanova, B. J. Hwang and C. Delmas, Dalton Trans.,40 ( 2011), 9306.

Probing the dominance of interstitial oxygen defects in ZnO nanoparticles through structural and optical characterizations

ZnO nanoparticles, synthesized adopting a facile chemical precipitation route, are studied here. The structural, optical and electronic properties of prepared ZnO nanoparticles were extensively investigated employing X-ray diffraction (XRD), transmission electron microscope (TEM), energy dispersive analysis by X-rays (EDAX), X-ray photoelectron spectroscopy (XPS), UV–vis absorption and fluorescence (FL) spectroscopy. The XRD analysis revealed hexagonal wurtzite phase 26.1–29.6nm size ZnO nanocrystallites. This observation gets further support from TEM images where particles of 25–30nm size are vividly seen. Interestingly, oxygen rich stoichiometry of nanoparticles is detected via zinc and oxygen emission lines of EDAX spectrum. XPS analysis establishes coexistence of lattice oxygen (OL), interstitial oxygen (Oi) and oxygen vacancy (VO) in ZnO nanoparticles. In line with EDAX analysis, XPS investigations substantiate interstitial oxygen rich composition of nanoparticles. Blue shift of absorption energy, as observed in the UV–vis spectrum of ZnO nanoparticles, typically manifests quantum confinement effect. Such transitions indicate the occurrence of various discrete energy states of prepared nanoparticles. FL spectroscopic investigations ascertain the existence of these discrete states by probing the radiative transitions arising among such states. Finally, FL study not only demonstrates visible emissions emanating from the oxygen defect states but more remarkably, in concurrence with EDAX and XPS analysis, establishes the excess of interstitial oxygen defects in prepared ZnO nanoparticles.

Ionization and dissociation dynamics of vinyl bromide probed by femtosecond extreme ultraviolet transient absorption spectroscopy

Strong-field induced ionization and dissociation dynamics of vinyl bromide, CH2=CHBr, are probed using femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy. Strong-field ionization is initiated with an intense femtosecond, near infrared (NIR, 775 nm) laser field. Femtosecond XUV pulses covering the photon energy range of 50-72 eV probe the subsequent dynamics by measuring the time-dependent spectroscopic features associated with transitions of the Br (3d) inner-shell electrons to vacancies in molecular and atomic valence orbitals. Spectral signatures are observed for the depletion of neutral C2H3Br, the formation of C2H3Br(+) ions in their ground (X̃) and first excited (Ã) states, the production of C2H3Br(++) ions, and the appearance of neutral Br ((2)P3/2) atoms by dissociative ionization. The formation of free Br ((2)P3/2) atoms occurs on a timescale of 330 ± 150 fs. The ionic à state exhibits a time-dependent XUV absorption energy shift of ∼0.4 eV within the time window of the atomic Br formation. The yield of Br atoms correlates with the yield of parent ions in the à state as a function of NIR peak intensity. The observations suggest that a fraction of vibrationally excited C2H3Br(+) (Ã) ions undergoes intramolecular vibrational energy redistribution followed by the C-Br bond dissociation. The C2H3Br(+) (X̃) products and the majority of the C2H3Br(++) ions are relatively stable due to a deeper potential well and a high dissociation barrier, respectively. The results offer powerful new insights about orbital-specific electronic processes in high field ionization, coupled vibrational relaxation and dissociation dynamics, and the correlation of valence hole-state location and dissociation in polyatomic molecules, all probed simultaneously by ultrafast table-top XUV spectroscopy.

Simultaneous Reduction of Co3+ and Mn4+ in P2-Na2/3Co2/3Mn1/3O2 As Evidenced by X-ray Absorption Spectroscopy during Electrochemical Sodium Intercalation

Sodium intercalation in P2-Na2/3Co2/3Mn1/3O2 (obtained by a coprecipitation method) was investigated by ex situ and in situ X-ray absorption spectroscopy. The electronic transitions at the O K-edge and the charge compensation mechanism, during the sodium intercalation process, were elucidated by combining Density Function Theory (DFT) calculations and X-ray absorption spectroscopy (XAS) data. The pre-edge of the oxygen K-edge moves to higher energy while the integrated intensity dramatically decreases, indicating that the population of holes in O 2p states is reduced with increasing numbers of sodium ions. From the K-edge and L-edge observations, the oxidation states of pristine Co and Mn were determined to be +III and +IV, respectively. The absorption energy shifts to lower positions during the discharging process for both the Co and the Mn edges, suggesting that the redox pairs, that is, Co3+/Co2+ and Mn4+/Mn3+, are both involved in the reaction.

Intersubband absorption energy shifts in 3-level system for asymmetric quantum well terahertz emitters

Intersubband absorption energy shifts in 3-level system stemming from depolarization and excitonlike effects are investigated. Analytically, the expressions we derive present good explanations to the conventional 2-level results and bare potential transition energy results; and numerical results show that they are more exact than the previous studies to describe the 3-level system depolarization and excitonlike shift (DES) character especially for higher carrier density (more than 8×1011 cm−2). One interesting detail we find is that the “large blue” DES becomes “slight redshift” in the low doping limit (less than 1.9×1011 cm−2), which may be neglected by the previous studies of intersubband transitions. Temperature character of DES in the step well structure is also numerically studied. Finally the above are applied to calculate asymmetric step quantum well structures. The two main functional aspects of terahertz (THz) emitters are discussed and several basic optimizing conditions are considered. By adjusting the well geometry parameters and material composition systematically, some optimized structures which satisfy all of the six conditions are recommended in tables. These optimizations may provide useful references to the design of 3-level-based optically pumping THz emitters.

The opsin shift and mechanism of spectral tuning in rhodopsin.

Molecular dynamics simulations and combined quantum mechanical and molecular mechanical calculations have been performed to investigate the mechanism of the opsin shift and spectral tuning in rhodopsin. A red shift of -980 cm(-1) was estimated in the transfer of the chromophore from methanol solution environment to the protonated Schiff base (PSB)-binding site of the opsin. The conformational change from a 6-s-cis-all-trans configuration in solution to the 6-s-cis-11-cis conformer contributes additional -200 cm(-1), and the remaining effects were attributed to dispersion interactions with the aromatic residues in the binding site. An opsin shift of 2100 cm(-1) was obtained, in reasonable accord with experiment (2730 cm(-1)). Dynamics simulations revealed that the 6-s-cis bond can occupy two main conformations for the β-ionone ring, resulting in a weighted average dihedral angle of about -50°, which may be compared with the experimental estimate of -28° from solid-state NMR and Raman data. We investigated a series of four single mutations, including E113D, A292S, T118A, and A269T, which are located near the PSB, along the polyene chain of retinal and close to the ionone ring. The computational results on absorption energy shift provided insights into the mechanism of spectral tuning, which involves all means of electronic structural effects, including the stabilization or destabilization of either the ground or the electronically excited state of the retinal PSB.

Experimental and DFT Studies: Novel Structural Modifications Greatly Enhance the Solvent Sensitivity of Live Cell Imaging Dyes

Structural modifications of previously reported merocyanine dyes (Toutchkine, A.; Kraynov, V.; Hahn, K. J. Am. Chem. Soc. 2003, 125, 4132-4145) were found to greatly enhance the solvent dependence of their absorbance and fluorescence emission maxima. Density functional theory (DFT) calculations have been performed to understand the differences in optical properties between the new and previously synthesized dyes. Absorption and emission energies were calculated for several new dyes using DFT vertical self-consistent reaction field (VSCRF) methods. Geometries of ground and excited states were optimized with a conductor-like screening model (COSMO) and self-consistent field (SCF) methods. The new dyes have enhanced zwitterionic character in the ground state and much lower polarity in the excited state, as shown by the DFT-VSCRF calculations. Consistently, the position of the absorption bands are strongly blue-shifted in more polar solvent (methanol compared to benzene), as predicted by the DFT spectral calculations. Inclusion of explicit H-bonding solvent molecules within the quantum model further enhances the predicted shifts and is consistent with the observed spectral broadening. Smaller but significant spectral shifts in polar versus nonpolar solvent are predicted and observed for emission bands. The new dyes show large fluorescence quantum yields in polar hydrogen-bonding solvents; qualitatively, the longest bonds along the conjugated chain at the excited S1 state minimum are shorter in the more polar solvent, inhibiting photoisomerization. The loss of photostability of the dyes is a consequence of the reaction with and electron transfer to singlet oxygen, starting oxidative dye cleavage. The calculated vertical ionization potentials of three dyes I-SO, AI-SO(4), and AI-BA(4) in benzene and methanol are consistent with their relative photobleaching rates; the charge distributions along the conjugated chains for the three dyes are similarly predictive of higher reaction rates for AI-SO(4) and AI-BA(4) than for I-SO. Time-dependent DFT calculations were also performed on AI-BA(4); these were less accurate than the VSCRF method in predicting the absorption energy shift from benzene to methanol.

Absorption, fluorescence, and semiempirical ASED-MO studies on a typical Brooker's merocyanine dye

Solvatochromism and Solvatofluorchromism of Brooker's merocyanine 1-methyl-4- (4′-hydroxystyryl) pyridinium betaine, M, were studied in twelve polar protic and aprotic solvents. Moderate hypsochromic fluorescence energy shifts are 4.57 kcal mole −1 while strong hypsochromic absorption energy shifts are 16.63 kcal mole −1. Decreasing of the dipole moment of M upon excitation is the factor, which is responsible for the difference between the two energy shifts. The change of both energies rectilinearly with solvent acidity scale shows the importance of oxygen atom of M as a strong basic center. The application of the atom superposition and electron delocalization molecular orbital (ASED-MO) theory reproduces geometrical and electronic structures for M, which agree well with the experimental observations. The calculations suggest strongly that the dye has a benzenoid valence structure in the ground state and shifts towards a quinonoid one upon excitation with an observed decreasing of the dipole moment. The changing of the dipole moment is explained clearly depending upon the calculated charge distribution over the whole skeleton of the molecule. The formation of a H-bond between the water molecule and the highly negative oxycyclic oxygen atom of M has slightly effect on its dipole moment in the ground state. This leads to suggest that this kind of interaction could be represented as attacking of water with acidic character on the basic site of M. Also, the calculations predict that the formation of monohydrated complex is an exothermic, down hill reaction, which is confirmed from the stabilization of the frontier molecular orbitals, oxygen lone-pair and the HOMO levels.

Reaction field spectral shifts with semiempirical molecular orbital theory

A macroscopic solution polarization free-energy functional is combined with semiempirical molecular orbital theory to study shifts of electronic absorption energies for several molecules in solution. The present method requires calculation of the induced electrostatic potential on the van der Waals surface and this calculation is implemented in a new way. The combined method is tested by calculating absorption energy shifts for several molecules of standard interest. We find the physically reasonable result that there is a correlation between the absorption energy shift and the magnitude of the dipole moments of the initial and final states involved in the absorption transition. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 143–155, 1997

Far-Infrared Impurity Absorption in Highly Dopedn-Type Silicon

Far-infrared absorption spectra of uncompensated phosphorus-doped silicon with donor concentrations between 6.8×10 17 and 2.0×10 18 cm -3 have been measured at 6 K. These crystals show an absorption edge in the photon energy region lower than the lowest (1s→2p) excitation line of isolated phosphorus atoms. The position of this absorption energy shifts to lower energy with increasing donor concentration. The thermal donor ionization energy ε 1 determined from measurements of the electrical property is situated near the low-energy threshold in the absorption cross-section spectrum, and therefore the absorption is ascribed to the ionization transition of donor electrons into the conduction band states perturbed by interactions with impurities. The absorption cross-section spectrum shows no anomaly at the photon energy equal to the thermal activation energy ε 2 , which is characteristic of the intermediate concentration range of impurity conduction.