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.     

Theoretical insight into the excited‐state proton transfer process: Role of the substituent ‐CN on HBT system

In this work, using density functional theory and time‐dependent density functional theory methods, we theoretically studied the excited‐state behaviors of 3 novel 2‐(2‐hydroxyphenyl)benzothiazole (HBT) derivatives (HBT‐H‐H, HBT‐CN‐H, and HBT‐CN‐CN). Analyses about primary chemical structures such as bond lengths and bond angles, we found that all the intramolecular hydrogen bonds in these 3 structures should be strengthened in the S₁ state upon the photoexcitation. Exploring the infrared vibrational spectra at the hydrogen bonds groups, we confirmed that nonsubstitutional HBT‐H‐H structure might play more important roles in the excited‐state intramolecular proton transfer (ESIPT) reaction than HBT‐CN‐H and HBT‐CN‐CN. Further, investigating vertical excitation process, it can be revealed that charge redistribution involved in hydrogen bonding moieties could facilitate the ESIPT reaction. Based on constructing potential energy curves of both S₀ and S₁ states, we confirmed that the substituents on HBT systems can reasonably regulate and control the ESIPT processes because of the different potential energy barriers. We deem that this present work not only elaborates the different excited‐state behaviors of HBT‐H‐H, HBT‐CN‐H, and HBT‐CN‐CN but also may play important roles in designing and developing new materials and applications involved in HBT systems in future.     

Charge‐transfer behavior of polyaniline single wall carbon nanotubes nanocomposites monitored by resonance Raman spectroscopy

Highly dispersed nanocomposites of polyaniline(PANI) and oxidized single wall carbon nanotubes(SWNTs) have been prepared using dodecylbenzenesulfonic acid as dispersant. The materials were characterized via resonance Raman and electronic absorption spectroscopies. The behavior of the composites as a function of the applied potential was also investigated using in situ Raman electrochemical measurements. The results obtained at E_laser = 1.17 eV suggest that a charge‐transfer process occur between PANI and semiconducting nanotubes for samples where the metallic tubes are previously oxidized. The spectroelectrochemical data show that the presence of SWNTs prevents the oxidation of PANI rings. Copyright © 2010 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman spectroscopy of 4‐tert‐butylpyridine on a silver electrode

We report the observation of large surface‐enhanced Raman scattering (SERS) (10⁶) for 4‐tert‐butylpyridine molecules adsorbed on a silver electrode surface in an electrochemical cell with electrode potential set at − 0.5 V. A decrease in electrode potential to − 0.3 V was accompanied by a decrease in relative intensities of the vibrational modes. However, there were no changes in vibrational wavenumbers. Comparison of both normal solution Raman and SERS spectra shows very large enhancement of the intensities of a₁, a₂, and b₂ modes at laser excitation of 488 nm. Enhancement of the non‐totally symmetric modes indicates the presence of charge transfer as a contributor to the enhancement. Copyright © 2011 John Wiley & Sons, Ltd.     

Adsorption study of 4‐MBA on TiO2 nanoparticles by surface‐enhanced Raman spectroscopy

In this paper, the adsorption of 4‐mercaptobenzoic acid (4‐MBA) on TiO₂ nanoparticles was studied mostly by surface‐enhanced Raman spectroscopy (SERS) and UV‐vis spectroscopy, at different pH values as well as under different temperatures and concentrations. The results show that the 4‐MBA molecules are bonded to the TiO₂ surface both through the sulfur atoms and COO⁻ groups at neutral or alkaline pH, but only through the sulfur atom at acidic pH. Furthermore, the 4‐MBA molecules possess high adsorptive stability on TiO₂ at a comparatively high temperature (150 °C). Concentration‐dependent SERS experiments show that the saturation concentration for 4‐MBA adsorbed on TiO₂ is about 10⁻³ M in natural case (pH = 6). Copyright © 2009 John Wiley & Sons, Ltd.     

Raman spectroscopy of collective modes in charge‐density‐wave systems: the mean‐field microscopic theory

The electron‐mediated coupling of external electromagnetic fields and Raman‐active oscillations is derived for a general electronic model with multiple bands using the adiabatic approach and the explicit diagrammatic approach. The theory is illustrated on the quasi‐one‐dimensional (Q1D) quarter‐filled charge‐density‐wave (CDW) model. It is shown how the long‐range Coulomb forces and the single‐electron relaxation processes affect the Raman spectroscopy of the amplitude‐oscillation mode in clean CDW systems. It is also argued that the adiabatic treatment of the photon‐phonon coupling functions can be safely used in this case. Copyright © 2008 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman spectra of the neonicotinoid pesticide thiacloprid

Thiacloprid is a widely used pesticide belonging to the neonicotinoid class, which is characterized by a selective activity against insects and a reduced acute toxicity for humans. The importance of the environmental impact of neonicotinoids is being intensively researched, in order to evaluate the danger they pose for useful insects. Physical methods which allow the characterization of neonicotinoids in diluted aqueous solutions are therefore desirable. We present a study of Raman and surface‐enhanced Raman scattering (SERS) spectroscopy on thiacloprid in solid state, in acetone solution, and adsorbed onto silver and gold hydrosols at μM concentration. Density functional theory calculations allow the individualization of the most stable molecular structure, both in gas phase and in solution, and of the corresponding Raman spectra. The vibrational assignments lead to an interpretation of the differences between SERS and ordinary Raman spectra based on the possible interactions between the molecule and the metal surface, the main one involving the iminocyano group. Formation of a charge‐transfer complex is suggested by the dependence of the SERS spectra on the laser excitation wavelength. We evaluate the applicability of SERS spectroscopy to the chemical analysis of thiacloprid comparing SERS with current analytical methods. Copyright © 2013 John Wiley & Sons, Ltd.     

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.     

Surface‐enhanced Raman scattering of molecules adsorbed on Co‐doped ZnO nanoparticles

Transition‐metal‐doped semiconductor nanoparticles (NPs) have been well studied for their optical and catalytic properties but seldom studied by surface‐enhanced Raman scattering (SERS). In this paper, transition‐metal‐doped semiconductor NPs are investigated for their SERS property. Four groups of Co‐doped (0.5, 1, 3, and 5%) ZnO (Co ZnO) NPs and pure ZnO NPs were synthesized and studied. When 4‐mercaptobenzoic acid was used as probing molecule, significant SERS signals were obtained on all the five samples. Moreover, it is very interesting to observe a relationship between the Co‐doping concentration and enhancement of the SERS signals. SERS intensities first increase with doping concentration (up to 1%), and then decrease with further increase in doping concentration (up to 5%). Charge transfer (CT) is considered to be the main contribution to this phenomenon. Different CT ratios from substrates to molecules seem to induce different intensities of the SERS signals. In our experiments, the crystalline defects of Co ZnO NPs caused by the Co dopant affect the CT ratios. A possible mechanism of CT from the valance band of Co ZnO NPs to the lower unoccupied molecular orbital of the molecules via energy of the surface states is suggested. X‐ray photoelectron spectra, UV vis spectra, and Raman spectra were used to characterize the structure and defects in Co ZnO NPs. Copyright © 2011 John Wiley & Sons, Ltd.     

DFT studies of surface‐enhanced Raman spectroscopy of 1,4‐benzenedithiol–Au2 complex under the effect of static electric fields

In this work, we demonstrate that the applied electric‐field strength and orientation can multiply modulate the Raman intensity and vibrational wavenumber of small molecule–metal complex, 1,4‐benzenedithiol–Au₂ (1,4BDT–Au₂), by density functional theory and time‐dependent density functional theory simulations. The polarizabilities are changed by the applied electric fields, leading to enhanced specific vibrational intensity and shifted vibrational wavenumber of the surface‐enhanced Raman scattering effect. The applied electric fields perturb the bonds and angles of the 1,4BDT–Au₂ complex. Owing to this reason, the peaks of Raman spectra related to these structures exhibit distinguishable responses in quasi‐static field (low‐frequency oscillating electric field). We use the visualized method of charge difference density to show that the electric fields tune the traditional excited state to pure charge‐transfer excited state. The charge‐transfer resonance transition produces enhanced Raman intensities for non‐totally symmetric modes and totally symmetric modes. These simulation results of the function of static electric field provide new guidance for the surface‐enhanced Raman scattering measurements. Copyright © 2015 John Wiley & Sons, Ltd.     

Tip‐heating‐assisted Raman spectroscopy at elevated temperatures

We demonstrate tip‐heating‐assisted enhanced Raman spectroscopy to investigate the temperature dependence of the carbon nanotube G‐band with nanoscale resolution. The controllable and nanoscale heat generated at the tip apex was used to thermally perturb and characterize a small volume in a carbon nanotube sample that is precisely positioned underneath the tip. The dependence of tip enhancement with temperature was also experimentally examined, which is in good agreement with the enhancement calculated from the electromagnetic model of isolated spheroids. The technique presented may open up opportunities in the study of controlled heat‐assisted biochemical reactions and physical transformations of nanostructures. It can also be used for thermal characterization of various materials requiring site‐selective and controllable nanoscale heat source and could enable the realization of new photothermal devices. Copyright © 2010 John Wiley & Sons, Ltd.     

Reactivity of 2‐aryl‐1,3‐dithiane anions towards neopentyl,neophyl and phenyl iodides. New evidence for an SRN1 mechanism

The reactions of 2‐(4‐Z‐phenyl)‐1,3‐dithiane anions (Z = H, OMe, Cl, CN) with neopentyl, neophyl and phenyl iodides were studied in DMSO, taking into consideration the effect of the Z substituent on the dithiane anions reactivity as well as on the product distribution. These substitution reactions proceed by an S_RN1 mechanism with radicals and radical anions as intermediates. Two competitive pathways are possible for the radical anion of the substitution product, namely electron transfer (ET) to the substrate giving the substitution product and C–S bond fragmentation to yield a distonic radical anion. ET is the main pathway for the reactions between dithiane anions bearing electron‐donor substituents and neopentyl or its analogue iodides affording the substitution products in moderate yields (41–53%). Copyright © 2011 John Wiley & Sons, Ltd.     

UV‐resonance Raman micro‐spectroscopy to assess residual chromophores in cellulosic pulps

Ultraviolet‐resonance Raman (UV‐RR) micro‐spectroscopy is an appropriate and sensitive tool to assess the chromophore structures in bleached cellulosic pulps used for papermaking. The particular selectivity in detection and identification of chromophores in pulps is achieved by acquiring the UV‐RR spectra in the solid state with laser excitation at 325 nm. This wavelength corresponds to absorption of poly‐unsaturated chromophore structures in partially bleached/fully bleached pulps, and linearly correlated with the signal at ca 1600 cm⁻¹ in the UV‐RR spectra. The characteristic vibrations from particular pulp chromophore structures have been assigned from experiments with model compounds, thus allowing the establishment of a UV‐RR database. Among the components of bleached pulp, the xylan–lignin complex was suggested to be an important source of chromophores. The monitoring of pulp bleaching by UV‐RR allowed us to suggest that it is the formation of new polysaccharide‐derived chromophores upon bleaching that hinders development of further brightness and is co‐responsible for the brightness reversion of fully bleached pulps. Copyright © 2010 John Wiley & Sons, Ltd.     

Testing of Raman spectroscopy as a non‐invasive tool for the investigation of glass‐protected pastels

Five French pastels and a sanguine drawing dating from the 17th to the 20th century were studied by Raman spectroscopy. Different operative conditions were used: the pastels were investigated through their protective glass, and the results obtained were compared with those obtained after removing the glass and after sampling a micrometric particle of pigment. Different parameters (wavelengths, powers of excitation and objectives) were tested in order to assess the optimal procedure of analysis for this fragile work of art. The results obtained for black (carbons), yellow (chrome/cobalt yellow), red (lead oxide, vermillion, orpiment), brown (red lead and chrome yellow), blue (Prussian blue, lapis lazuli/ultramarine), green (mixture of above blue and yellow pigments) and white (calcite, lead white, anatase) pigments are presented and the consistency of the pigments' period of use with the dating proposed for each pastel is evaluated. In one of the pastels, the blackening of the carnation colour made of an unstable mixture of lead white, red lead and vermilion was studied. Copyright © 2010 John Wiley & Sons, Ltd.     

Spectroscopic characterization of cadion: UV–vis,resonance Raman and DFT calculations of a versatile metal complexing agent

The spectroscopic characterization of cadion or 1‐(4‐nitrophenyl)‐3‐(4‐phenylazophenyl)triazene, its anionic species and its complexes with Hg²⁺, Cd²⁺ and Ni²⁺ in DMSO solution is presented. Resonance Raman (RR) and UV–vis spectroscopy were the techniques of choice with results supported by density functional theory calculations. As previously reported, the UV–vis data indicate distinct absorption spectra for the five species mentioned, allowing for spectrophotometric metal differentiation. In order to understand the electronic spectra of such species, we performed a detailed RR investigation, where the spectral profiles revealed the vibrational modes mainly involved in the chromophoric moieties. The distinct enhancement profile observed for each species suggests that the charge transfer from the π‐system to the electron withdrawing NO₂ group is modulated by the electron donating triazene moiety, depending on the metal attached. The results reported herein show that RR spectroscopy can be potentially used in the metal detection of multicomponent samples by the proper choice of the excitation wavelength. Copyright © 2012 John Wiley & Sons, Ltd.     

Push–pull hyperbranched molecules. A theoretical study

The electronic properties of the ground state, unrelaxed and relaxed first excited states of push–pull hyperbranched molecules bearing amino and nitro terminal groups have been studied at BB1K/cc‐pvdz//HF/6‐31g(d), TD‐BB1K/cc‐pvdz//HF/6‐31g(d) and TD‐BB1K/cc‐pvdz//CIS/6‐31g(d) levels of theory, respectively. It was demonstrated that dendritic architecture of push–pull molecules favours the charge transfer in the excited state compared to linear molecules. The possibility of adopting a plane conformation is an important condition for the charge transfer in an excited state. According to the calculations 1:1 ratio of donor and acceptor groups is another important precondition for the manifestation of strong charge separation in the excited state. In case of excess of nitro groups over the amino, some of the excitations participating in the S0 → S1 transition favour the charge transfer in the excited state in the opposite directions, thus decreasing the charge separation. 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.     

Synthesis,properties, and solvatochromism of 1,3‐dimethyl‐5‐{(thien‐2‐yl)‐[4‐(1‐piperidyl) phenyl]methylidene}‐(1H,3H)‐pyrimidine‐2,4,6‐trione

A new merocyanine dye, 1,3‐Dimethyl‐5‐{(thien‐2‐yl)‐[4‐(1‐piperidyl)phenyl]methylidene}‐ (1H, 3H)‐pyrimidine‐2,4,6‐trione 3 , has been synthesized by condensation of 2‐[4‐(piperidyl)benzoyl]thiophene 1 with N,N′‐dimethyl barbituric acid 2 . The solvatochromic response of 3 dissolved in 26 solvents of different polarity has been measured. The solvent‐dependent long‐wavelength UV/Vis spectroscopic absorption maxima, v_max, are analyzed using the empirical Kamlet–Taft solvent parameters π* (dipolarity/polarizability), α (hydrogen‐bond donating capacity), and β (hydrogen‐bond accepting ability) in terms of the well‐established linear solvation energy relationship (LSER): (1) The solvent independent coefficients s , a , and b and (v_max)₀ have been determined. The McRae equation and the empirical solvent polarity index, E_T(30) have been also used to study the solvatochromism of 3 . Copyright © 2007 John Wiley & Sons, Ltd.     

Changes in the vibrational modes of carbon nanotubes induced by electron‐beam irradiation: resonance Raman spectroscopy

Influence of electron‐beam (e‐beam) irradiation on multi‐walled (MW) and single‐walled (SW) carbon nanotube films grown by microwave chemical vapor deposition technique is investigated. These films were subjected to an e‐beam energy of 50 keV from a scanning electron microscope for 2.5, 5.5, 8.0, and 15 h, and to 100 and 200 keV from a transmission electron microscope for a few minutes to ∼2 h continuously. Such conditions resemble an increased temperature and pressure regime enabling a degree of structural fluidity. To assess structural modifications, they were analyzed prior to and after irradiation using resonance Raman spectroscopy (RRS) in addition to in situ monitoring by electron microscopy. The experiments showed that with extended exposures, both types of nanotubes displayed various local structural instabilities including pinching, graphitization/amorphization, and formation of an intramolecular junction (IMJ) within the area of electron beam focus possibly through amorphous carbon aggregates. RRS revealed that irradiation generated defects in the lattice as quantified through (1) variation of the intensity of radial breathing mode (RBM), (2) intensity ratio of D to G band (I_D/I_G), and (3) positions of the D and G bands and their harmonics (D* and G*) and combination bands (D + G). The increase in the defect‐induced D band intensity, quenching of RBM intensity, and only a slight increase in G band intensity are some of the implications. The MW nanotubes tend to reach a state of saturation for prolonged exposures, while the SW ones transform from a semiconducting to a quasi‐metallic character. Softening of the q = 0 selection rule is suggested as a possible reason to explain these results. Furthermore, these studies provide a contrasting comparison between MW and SW nanotubes. Copyright © 2006 John Wiley & Sons, Ltd.