Direct visual evidence for the chemical mechanism of surface‐enhanced resonance Raman scattering via charge transfer

We describe the chemical and electromagnetic enhancements of surface‐enhanced resonance Raman scattering (SERRS) for the pyridine molecule absorbed on silver clusters, in which different incident wavelength regions are dominated by different enhancement mechanisms. Through visualization we theoretically investigate the charge transfer (CT) between the molecule and the metal cluster, and the charge redistribution (CR) within the metal on the electronic intracluster collective oscillation excitation (EICOE). The CT between the metal and the molecule in the molecule–metal complex is considered as an evidence for chemical enhancement to SERRS. CR within the metal on EICOE is considered as an evidence for the electromagnetic enhancement by collective plasmons. For the incident wavelength from 300 to 1000 nm, the visualized method of charge difference density can classify the different wavelength regions for chemical and electromagnetic enhancement, which are consistent with the formal fragmented experimental studies. Copyright © 2008 John Wiley & Sons, Ltd.     

Direct visual evidence for the chemical mechanism of surface‐enhanced resonance Raman scattering via charge transfer: (II) Binding‐site and quantum‐size effects

We describe quantum‐size and binding‐site effects on the chemical and local field enhancement mechanisms of surface‐enhanced resonance Raman scattering (SERRS), in which the pyridine molecule is adsorbed on one of the vertices of the Ag₂₀ tetrahedron. We first investigated the influence of the binding site on normal Raman scattering (NRS) and excited state properties of optical absorption spectroscopy. Second, we investigated the quantum‐size effect on the electromagnetic (EM) and chemical mechanism from 300 to 1000 nm with charge difference density. It is found that the strong absorption at around 350 nm is mainly the charge transfer (CT) excitation (CT between the molecule and the silver cluster) for large clusters, which is the direct evidence for the chemical enhancement mechanism for SERRS; for a small cluster the strong absorption around 350 nm is mainly intracluster excitation, which is the direct evidence for the EM enhancement mechanism. This conclusion is further confirmed with the general Mie theory. The plasmon peak in EM enhancement will be red‐shifted with the increase of cluster size. The influence of the binding site and quantum‐size effects on NRS, as well as chemical and EM enhancement mechanisms on SERRS, is significant. Copyright © 2009 John Wiley & Sons, Ltd.     

Theoretical study on SERRS of rhodamine 6G adsorbed on Ag2 cluster: chemical mechanism via intermolecular or intramolecular charge transfer

The problem of the chemical enhancement of rhodamine 6G (R6G) adsorbed on silver cluster has been theoretically investigated by charge difference densities (CDDs) to show the direct charge transfer (CT) evidence. For surface‐enhanced resonance Raman scattering (SERRS) of R6G excited at 514.5 nm, the enhancements of v(151) and v(154) result from weak intermolecular (from Ag to R6G) CT and the strong intramolecular CT [similar to that of resonance Raman scattering (RRS) of R6G], respectively. The possibility of the SERRS of R6G contributed from pure intermolecular CT is also discussed, when the incident light is close to the new metal–R6G CT excited state at 1571.4 nm. Meanwhile compared with the absorption process the fluorescence yield of R6G is investigated by transition densities and CCDs. Copyright © 2008 John Wiley & Sons, Ltd.     

Experimental and theoretical evidence for the chemical mechanism in SERRS of rhodamine 6G adsorbed on colloidal silver excited at 1064 nm

The evidence for the existence of a chemical mechanism in surface‐enhanced resonance Raman scattering (SERRS) of rhodamine 6G (R6G) adsorbed on colloidal silver excited at 1064 nm is reported on the basis of experimental and theoretical analyses. A weak absorption peak at around 1060 nm for R6G‐functionalized silver nanoparticles was observed, which is not present in the individual spectra of R6G or silver nanoparticles. Theoretically, the charge difference density reveals that this weak absorption is a metal‐to‐molecule charge transfer excited state. Copyright © 2010 John Wiley & Sons, Ltd.     

The intramolecular charge transfer in a donor–π–acceptor dianion probed by resonance Raman spectroscopy and quantum chemical calculations

The dideprotonation of 4‐(4‐nitrophenylazo)resorcinol generates an anionic species with substantial electronic π delocalization. As compared to the parent neutral species, the anionic first excited electronic transition, characterized as an intramolecular charge transfer (ICT) from the CO⁻ groups to the NO₂ moiety, shows a drastic red shift of ca. 200 nm in the λ_max in the UV‐vis spectrum, leading to one of the lowest ICT energies observed (λ_max = 630 nm in dimethyl sulfoxide (DMSO)) in this class of push‐pull molecular systems. Concomitantly, a threefold increase in the molar absorptivity (ε_max) in comparison to the neutral species is observed. The resonance Raman enhancement profiles reveal that in the neutral species the chromophore involves several modes, as ν(C N), ν(NN), ν(CC) and ν_s(NO₂), whereas in the dianion, there is a selective enhancement of the NO₂ vibrational modes. The quantum chemical calculations of the electronic transitions and vibrational wavenumbers led to a consistent analysis of the enhancement patterns observed in the resonance Raman spectra. Copyright © 2009 John Wiley & Sons, Ltd.     

Study of interfacial charge transfer in nanosemiconductor–molecule composites

Interaction of sol–gel synthesized Ce–Ag‐codoped ZnO (CSZO) nanocrystals with (E)‐1‐(naphthalen‐1‐yl)‐2‐styryl‐1H‐phenanthro[9,10‐d]imidazole has been analysed. The synthesized nanocrystals and their composites with naphthyl styryl phenanthrimidazole have been characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X‐ray spectroscopy, X‐ray diffractometry, X‐ray photoelectron spectroscopy (XPS) lifetime and Fourier transform infrared spectroscopy and cyclic voltammetry. XPS shows doped silver and cerium in Ag⁰ and Ce⁴⁺ states, respectively_. SEM and TEM images of CSZO nanoparticles show that they appear to be 3D trapezoid and cocoon‐like shape. The selected area electron diffraction pattern supports the nanocrystalline character of the synthesized material. The percentages of doping of cerium and silver in CSZO are 0.54 (at.) and 0.34 (at.), respectively. From the energy levels of the materials used in the imidazole–CSZO composite, the dominant CT direction has been analysed. Theoretical investigation shows that the binding energy and energy gap of the imidazole composites are highly dependent on the nature of the silver oxide cluster and that charge transfer in the imidazole–Ag₄O₄ composite is faster than the same in other composites. Molecular docking technique has also been carried out to understand the imidazole–DNA interactions. Copyright © 2016 John Wiley & Sons, Ltd.     

Intramolecular charge transfer in donor‐bridge‐acceptor compounds with paired linearly conjugated or cross‐conjugated pathways

An understanding of intramolecular charge transfer in 2‐D linearly conjugated and cross‐conjugated compounds is necessary for the rational design of molecular electronics, improved solar energy devices, semi‐conducting polymers, and materials with nonlinear optical properties. In this work, the femtosecond transient absorption spectra and kinetics of several donor‐bridge‐acceptor compounds containing cross‐conjugated or linearly conjugated bridging groups were obtained. The veratrole group was used as the donor, and the phthalimide group was used as an acceptor. 2‐D conjugation was achieved by involving two bridging groups arranged cyclically between the donor and acceptor. The donor and acceptor were bridged by m‐phenylene in the cross‐conjugated compounds or 2,5‐thiophene in the linearly conjugated compounds. We found slower charge separation times and slower charge recombination times in the compounds containing cyclic cross‐conjugated bridging groups than in those containing the cyclic linearly conjugated groups in polar solvent. Charge separation rates that were found to be dependent on solvent were observed in the donor‐bridge‐acceptor compounds. Copyright © 2014 John Wiley & Sons, Ltd.     

Raman spectroscopy of the PTCDA–inorganic semiconductor interface: evidence for charge transfer

In the present work, we investigate the vibrational properties of a PTCDA molecule with an additional positive or negative charge using density functional theory. With respect to the calculated vibrational frequencies of the neutral molecule, some modes in particular in the region 1200–1800 cm⁻¹ show large shifts. These calculations are compared with resonant Raman spectra of sub-monolayer PTCDA films on passivated semiconductor surfaces, both before and after annealing the deposited films at elevated temperatures (350 °C). Independent of the sample treatment, the sub-monolayer Raman spectra correspond quite well to reference spectra obtained for thicker films, and we find no evidence for the strong shifts predicted in the calculations for the charged species. From the small changes in the mode frequencies it can be concluded that any charge transfer present involves significantly less than one elementary charge.     

Identification of two Ag‐2,2′:6′,2″‐terpyridine surface species on Ag nanoparticle surfaces by excitation wavelength dependence of SERS spectra and factor analysis: evidence for chemical mechanism contribution to SERS of Ag(0)–tpy

Measurement and interpretation of the excitation wavelength dependence of surface‐enhanced Raman scattering (SERS) spectra of molecules chemisorbed on plasmonic, e.g. Ag nanoparticle (NP) surfaces, are of principal importance for revealing the charge transfer (CT) mechanism contribution to the overall SERS enhancement. SERS spectra, their excitation wavelength dependence in the 445–780‐nm range and factor analysis (FA) were used for the identification of two Ag‐2,2′:6′,2″‐terpyridine (tpy) surface species, denoted Ag⁺–tpy and Ag(0)–tpy, on Ag NPs in systems with unmodified and/or purposefully modified Ag NPs originating from hydroxylamine hydrochloride‐reduced hydrosols. Ag⁺–tpy is a spectral analogue of [Ag(tpy)]⁺ complex cation, and its SERS shows virtually no excitation wavelength dependence. By contrast, SERS of Ag(0)–tpy surface complex generated upon chloride‐induced compact aggregate formation and/or in strongly reducing ambient shows a pronounced excitation wavelength dependence attributed to a CT resonance (the chemical mechanism) contribution to the overall SERS enhancement. Both the resonance (λ_exc = 532 nm) and off‐resonance (λ_exc = 780 nm) pure‐component spectra of Ag(0)–tpy obtained by FA are largely similar to surface‐enhanced resonance Raman scattering (λ_exc = 532 nm in resonance with singlet metal to ligand CT (¹ MLCT) transition) and SERS (λ_exc = 780 nm) spectra of [Fe(tpy)₂]²⁺ complex dication. Copyright © 2014 John Wiley & Sons, Ltd.     

Structure–reactivity relationship for alcohol oxidations via hydride transfer to a carbocationic oxidizing agent

Second‐order rate constants were determined for the oxidation of 27 alcohols (R¹R²CHOH) by a carbocationic oxidizing agent, 9‐phenylxanthylium ion, in acetontrile at 60 °C. Alcohols include open‐chain alkyl, cycloalkyl, and unsaturated alcohols. Kinetic isotope effects for the reaction of 1‐phenylethanol were determined at three H/D positions of the alcohol (KIE_α‐D = 3.9, KIE_β‐D3 = 1.03, KIE_OD = 1.10). These KIE results are consistent with those we previously reported for the 2‐propanol reaction, suggesting that these reactions follow a hydride‐proton sequential transfer mechanism that involves a rate‐limiting formation of the α‐hydroxy carbocation intermediate. Structure–reactivity relationship for alcohol oxidations was deeply discussed on the basis of the observed structural effects on the formation of the carbocationic transition state (C^δ+? OH). Efficiencies of alcohol oxidations are largely dependent upon the alcohol structures. Steric hindrance effect and ring strain relief effect win over the electronic effect in determining the rates of the oxidations of open‐chain alkyl and cycloalkyl alcohols. Unhindered secondary alkyl alcohols would be selectively oxidized in the presence of primary and hindered secondary alkyl alcohols. Strained C₇? C₁₁ cycloalkyl alcohols react faster than cyclohexyl alcohol, whereas the strained C₅ and C₁₂ alcohols react slower. Aromatic alcohols would be efficiently and selectively oxidized in the presence of aliphatic alcohols of comparable steric requirements. This structure–reactivity relationship for alcohol oxidations via hydride‐transfer mechanism is hoped to provide a useful guidance for the selective oxidation of certain alcohol functional groups in organic synthesis. Copyright © 2011 John Wiley & Sons, Ltd.     

The electronic delocalization in para‐substituted β‐nitrostyrenes probed by resonance Raman spectroscopy and quantum‐chemical calculations

The electronic (UV‐vis) and resonance Raman (RR) spectra of a series of para‐substituted trans‐β‐nitrostyrenes were investigated to determine the influence of the electron donating properties of the substituent (X = H, NO₂, COOH, Cl, OCH₃, OH, N(CH₃)₂, and O⁻) on the extent of the charge transfer to the electron‐withdrawing NO₂ group directly linked to the ethylenic (C = C) unit. The Raman spectra and quantum chemical calculations show clearly the correlation of the electron donating power of the X group with the wavenumbers of the ν_s(NO₂) and ν (C = C)_sty normal modes. In conditions of resonance with the lowest excited electronic state, one observes for X = OH and N(CH₃)₂ that the symmetric stretching of the NO₂, ν_s(NO₂), is the most substantially enhanced mode, whereas for X = O⁻, the chromophore is extended over the whole molecule, with substantial enhancement of several carbon backbone modes. Copyright © 2008 John Wiley & Sons, Ltd.     

Effective charge of a macroparticle in a non‐isothermal plasma within the Poisson–Boltzmann model

The electrostatic potential distribution around a charged, spherical, finite‐size macroparticle in a non‐isothermal plasma‐like medium is studied numerically within the Poisson–Boltzmann model. It is assumed that plasma consists of electrons and one species of singly charged ions. The effective charge of a macroparticle is calculated and its dependence on the electron to ion temperature ratio as well as on the particle radius and bare charge is considered. Numerical results for the effective charge in an isothermal plasma are compared with known analytical expressions.     

Study of spin–phonon coupling in LiFe1 − xMnxPO4olivines

LiFe_{1 − x}Mn_xPO₄ olivines are promising material for improved performance of Li‐ion batteries. Spin–phonon coupling of LiFe_{1 − x}Mn_xPO₄ (x = 0, 0.3, 0.5) olivines is studied through temperature‐dependent Raman spectroscopy. Among the observed phonon modes, the external mode at ~263 cm⁻¹ is directly correlated with the motions of magnetic Fe²⁺/Mn²⁺ ions. This mode displays anomalous temperature‐dependent behavior near the Néel temperature, indicating a coupling of this mode with spin ordering. As Mn doping increases, the anomalous behavior becomes clearly weaker, indicating the spin–phonon coupling quickly decreases. Our analyses show that the quick decrease of spin–phonon coupling is due to decrease of the strength of spin–phonon coupling, but not change of spin‐ordering feature with Mn doping. Importantly, we suggest that the low electrochemical activity of LiMnPO₄ is correlated with the weak spin–phonon coupling strength, but not with the weak ferromagnetic ground state. Our work would play an important role as a guide in improving the performances of future Li‐ion batteries. Copyright © 2015 John Wiley & Sons, Ltd.     

Au adsorption and Au-mediated charge transfer on the SrO-termination of SrTiO3 (0 0 1) surface

Atomic Au adsorption on the SrO-termination of SrTiO₃ (0 0 1) surface has been studied by means of the first-principles calculations based on the density functional theory (DFT). It indicates that charge polarization dominantly contributes to the bonding between Au and SrO-termination. Interfacial charge transfer induces dipole moment and changes work function. The mediating role Au played in charge transfer from electron-doped SrTiO₃:Nb to NO has been simulated. Charge transfer from SrTiO₃:Nb to Au is ascertained indicating that Au plays as an electron trapping center. SrO-termination has weak activity to NO while the molecule can be strongly adsorbed on negatively charged Au atom. It has been represented that Au mediates the charge transfer from SrTiO₃:Nb to NO. Antibonding orbital (π_2p*) of NO accommodates the charge and thus molecular bond is weakened (activated). Fukui functions demonstrate the role Au played in transiting the charge transfer from electronically excited SrTiO₃ to target species. Evidence that metal deposited on photocatalyst surface effectively separates the electron-hole pairs and improves the photocatalytic activity is presented in the current work.     

Intricate packing in the hydrophobic core of barstar through a CH–π interaction

Identification of specific packing interactions within in the hydrophobic core of proteins is important for understanding the integrity of protein structure. Finding such interactions is challenging because few tools allow monitoring of a specific interaction in the presence of several non‐specific forces that hold proteins together. It is important to understand how and when such interactions develop during protein folding. In this study, we have used the intrinsic tryptophan residue, Trp53, as an ultraviolet resonance Raman probe to elucidate the packing interactions in the hydrophobic core of the protein barstar. Barstar is extensively studied for its folding, unfolding and aggregation properties. The Trp53 residue is known to be completely buried in the hydrophobic core of the protein and is used extensively as an intrinsic probe to monitor the folding and unfolding reactions of barstar. A comparison of the resonance Raman cross sections of some bands of Trp53 with those observed for N‐acetyl‐tryptophanoamide in water suggests that Trp53 in barstar is indeed isolated from water. Intensity ratio of the Fermi doublet suggests that Trp53 is surrounded by several aliphatic amino acid residues in corroboration with the crystal structure of barstar. Importantly, we show that the side chain of Trp53 is involved in a unique CH–π interaction with CH groups of Phe56 as well as a steric interaction with the methyl group of Ile5. Copyright © 2014 John Wiley & Sons, Ltd.     

Near‐forward/high‐pressure‐backward Raman study of Zn1 − xBexSe (x ~ 0.5) – evidence for percolation behavior of the long (Zn―Se) bond

The apparently universal 1‐bond → 2‐mode percolation behavior in the Raman spectra of zincblende semiconductor alloys is generally observed for the short bond only, and not for the long one. In this work we perform a combined high‐pressure‐backward/near‐forward Raman study of the leading percolation‐type (Zn,Be)Se alloy (~50 at.% Be), which exhibits a distinct percolation doublet in the spectral range of its short Be―Se bond, in search of a Zn―Se analogue. The high‐pressure‐backward insight is not conclusive per se, but clarifies the perspective behind the near‐forward Raman study. The latter reveals an unique Zn―Se phonon–polariton. Its fair contour modeling depending on the scattering angle is achieved within the linear dielectric approach, based on ellipsometry measurement of the ZnBeSe refractive index. Somewhat surprisingly this reveals that the phonon–polariton in question is a ‘fractional’ one in that it carries only half of the available Zn―Se oscillator strength, as ideally expected in case of a BeSe‐like bimodal Raman behavior of the long Zn―Se bond. Copyright © 2015 John Wiley & Sons, Ltd.     

Discharge mechanisms modeling in LPCVD silicon nanocrystals usingC–Vand capacitance transient techniques

Charging and discharging phenomena from silicon nanocrystals have been studied by means of capacitance–voltage characteristics on P-type metal-oxide-semiconductor (P-MOS) capacitors with embedded self-assembled silicon quantum dots. The dots have a floating gate behavior as shown by the hysteresis onC –V curves. The Si-dots are charged or discharged by direct tunneling of carriers through a 3 nm thick oxide. The nanocrystals could be charged by electrons or holes, depending on the charging bias conditions. The discharge is studied by constant bias method and shows a logarithmic variation with time. Retention times higher than several hours are observed. A simple model is developed in order to evaluate the electric field within the tunneling oxide layer. Then, complete simulations are done for the different discharge paths. The barrier heights are extracted from the discharge data and possible confinement effects are discussed. The results confirm the high potentiality of silicon nanocrystal-floating gates for memory applications.     

Electron–phonon coupling and vibrational modes contributing to linear electro‐optic effect of the efficient NLO chromophore 4‐(N,N‐dimethylamino)‐N‐methyl‐4′‐toluene sulfonate (DAST) from their vibrational spectra

The optimized geometry and structural features of the most prospective electro‐optic crystal 4‐(N,N‐dimethylamino)‐N‐methyl‐4′‐toluene sulfonate (DAST), and the vibrational spectral investigations have been comprehensively described with the near infrared Fourier transform (NIR FT) Raman and Fourier transform infrared (FT‐IR) spectra supported by the density functional theoretical (DFT) computations to elucidate the contribution of vibrational modes to the linear electro‐optic (LEO) effect. Mulliken population analysis and natural bond orbital (NBO) analysis have also been carried out to analyze the effects of intramolecular charge transfer (ICT), intramolecular hydrogen bonding and hyperconjugative interactions on the geometries. The influence of CT interaction between the phenyl ring and the dimethylamino group of the nonlinear optical (NLO) chromophore on the endocyclic and exocyclic angles, and the electronic effects such as hyperconjugation and back‐donation on the methyl hydrogen atoms have been examined. The concurrent intense activation of Raman and IR activities of the effective conjugation vibrational coordinate, which significantly contributes to the LEO effect resulting from the strong electron–phonon (e/ph) coupling, has been analyzed in detail. The effects of frontier orbitals, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), transition of electron density (ED) transfer and the influence of planarity in the stilbazolium ring on the first hyperpolarizability are also discussed. Copyright © 2008 John Wiley & Sons, Ltd.     

The 2D {P} Spin-Echo-Difference Constant-Time [C, H]-HMQC Experiment for Simultaneous Determination of JH3′P and JC4′P in C-Labeled Nucleic Acids and Their Protein Complexes

A two-dimensional {³¹P} spin-echo-difference constant-time [¹³C, ¹H]-HMQC experiment (2D {³¹P}-sedct-[¹³C, ¹H]-HMQC) is introduced for measurements of ³J_C4′P and ³J_H3′P scalar couplings in large ¹³C-labeled nucleic acids and in DNA–protein complexes. This experiment makes use of the fact that ¹H–¹³C multiple-quantum coherences in macromolecules relax more slowly than the corresponding ¹³C single-quantum coherences. ³J_C4′P and ³J_H3′P are related via Karplus-type functions with the phosphodiester torsion angles β and ε, respectively, and their experimental assessment therefore contributes to further improved quality of NMR solution structures. Data are presented for a uniformly ¹³C, ¹⁵N-labeled 14-base-pair DNA duplex, both free in solution and in a 17-kDa protein–DNA complex.