Raman Excitation Profiles Research Articles

Vibrational dynamics and structural investigation of 4‐benzoylpyridine

AbstractThe polarized Raman spectra in different environments along with the IR counterpart of 4‐benzoylpyridine (4‐BOP) were critically analyzed to assign all of its normal modes of vibration. The knowledge of the positions of different excited electronic states (EESs)was obtained from the study of electronic absorption spectra. Measurement of Raman excitation profiles (REPs)of several normal modes was carried out to get insight into structural and symmetry properties of the molecule. All the experimental observations were substantiated and corroborated theoretically by quantum chemical calculations (QCCs). The possibility of exciton splitting of the 1La band has been explored both from theoretical and experimental points of view. Copyright © 2010 John Wiley & Sons, Ltd.

Revealing new electronic behaviours in the Raman spectra of chirality‐enriched carbon nanotube ensembles

AbstractWe present Raman spectroscopy of single‐walled carbon nanotubes (SWNTs) that are enriched in metallic species by density gradient ultracentrifugation (DGU) and enriched in single semiconducting chiralities through DNA‐based separations. Radial breathing mode (RBM) spectra demonstrate that DGU samples are highly enriched in armchair chiralities. The enrichment allows acquisition of pure G‐band spectra of the armchair SWNTs and reveals that the LO mode is absent in these structures. Raman excitation profiles for the G‐band in nearly pure (10,2) samples reveals a strong asymmetry in the intensities of the resonance coupling to incident and scattered photons. The experimental data may be fit using a four‐level molecular model for Raman scattering and the strong asymmetry can be understood as a consequence of the presence of non‐Condon effects. The result requires a reassessment of the assumption that the incident and scattered resonances are equivalent. The consequences of such non‐Condon effects on other SWNT electronic and optical processes will be an important topic for future study.

Spectroscopic Comparison of Photogenerated Tryptophan Radicals in Azurin: Effects of Local Environment and Structure

Tryptophan radicals play a significant role in mediating biological electron transfer. We report the photogeneration of a long-lived, neutral tryptophan radical (Az48W*) from the native residue tryptophan-48 in the hydrophobic core of azurin. The optical absorption, electron paramagnetic resonance, and resonance Raman spectra strongly support the formation of a neutral radical, and the data are consistent with direct electron transfer between tryptophan and the copper(II) center. Spectra of the long-lived Az48W* species are compared to those of a previously studied, solvent-exposed radical at position 108 to identify signatures of tryptophan radicals that are sensitive to the local environment. The absorption maxima of Az48W* display an approximately 23 nm hypsochromic shift in the nonpolar environment. The majority of the resonance Raman frequencies are downshifted by approximately 7 cm(-1) relative to the solvent-exposed radical, and large changes in intensity are observed for some modes. The resonance Raman excitation profiles for Az48W* exhibit distinct maxima within the absorption envelope. Electron paramagnetic resonance spectroscopy yields spectra with partially resolved lines caused by hyperfine couplings; the differences between the coupling constants for the buried and solvent-exposed radical are primarily caused by variations in structure. The insights gained by electronic, vibrational, and magnetic resonance spectroscopy enhance our fundamental understanding of the effects of protein environment on radical properties. Hypotheses for the proton transfer pathway within azurin and a deprotonation rate of approximately 5 x 10(6) s(-1) are proposed.

Raman scattering for weakened bonds in the intermediate state: enhancement of low-frequency vibrations

A multimode theory of the Raman scattering in resonance with an electronic transition causing a strong weakening of atomic bonds is proposed. Simple analytical relations between the Fourier transforms of the first- and second-order Raman amplitudes and the absorption are derived. It is predicted that the Raman scattering on low-frequency vibrations will be strongly enhanced. Besides the second-order Raman scattering is also enhanced as compared to the first-order scattering. The Raman excitation profiles show a structure caused by the Airy oscillations. The shapes of the profiles of all vibrations contributing to the weakening bond are the same as a consequence of the strong mode mixing under the virtual vibronic transition.

UV resonance Raman investigation of the conformations and lowest energy allowed electronic excited states of tri- and tetraalanine: charge transfer transitions.

UV resonance Raman excitation profiles and Raman depolarization ratios were measured for trialanine and tetraalanine between 198 and 210 nm. Excitation within the pi --> pi* electronic transitions of the peptide bond results in UVRR spectra dominated by amide peptide bond vibrations. In addition to the resonance enhancement of the normal amide vibrations, we find enhancement of the symmetric terminal COO(-) vibration. The Ala(3) UVRR AmIII(3) band frequencies indicate that poly-proline II and 2.5(1) helix conformations and type II turns are present in solution. We also find that the conformation of the interior peptide bond of Ala(4) is predominantly poly-proline-II-like. The Raman excitation profiles of both Ala(3) and Ala(4) reveal a charge transfer electronic transition at 202 nm, where electron transfer occurs from the terminal nonbonding carboxylate orbital to the adjacent peptide bond pi* orbital. Raman depolarization ratio measurements support this assignment. An additional electronic transition is found in Ala(4) at 206 nm.

Raman excitation profiles of metallic single-walled carbon nanotubes

In this work Raman excitation profiles of metallic carbon nanotubes have been calculatedand thoroughly analyzed. Suppression and vanishing of the high-energy resonance iscompletely confirmed by our calculations. The presented results clearly show that thesuppression, and finally the absence, of the resonance is caused by electron–phononinteraction and interference effects. Electron–phonon coupling for low-energy resonance issignificantly larger than for high-energy resonance. Furthermore, the transition energies ofthose two transitions are close enough to make interference effects important. The type ofinterference is determined by the sign of the electron–phonon interaction matrix elements.Constructive interference makes the low-energy resonance more intensive anddestructive interference destroys the high-energy resonance for most of the metallictubes.

Excited state distortions in a charge transfer state of a donor–acceptor [2]rotaxane

The charge transfer excited state of a mechanically interlocked [2]rotaxane (R(4+)) with a donor 1,5-dioxynaphthalene (DNP) unit in the rod and the acceptor cyclobis(paraquat-p-phenylene) (CBPQT(4+)) ring component, along with the analogous non-interlocked [2]pseudorotaxane (P(4+)), is studied by resonance Raman spectroscopy and electronic absorption spectroscopy. Resonance Raman excitation profiles are obtained, calculated quantitatively using time-dependent theoretical methods, and interpreted with the assistance of DFT calculations. The active vibrational modes are consistent with an electron transfer from the HOMO centered on the DNP unit to the LUMO on the CBPQT(4+) ring. Displacement vectors of highly distorted modes agree with the bonding changes predicted from the MO nodal pattern. Subtle changes in the frequency of some modes in the free components compared with those in R(4+) are observed. The largest distortions are found for modes involving ring breathing in the DNP unit of the rod and the paraquat units of the CBPQT(4+) ring. The individual mode contributions to the vibrational reorganization energy, as well as the total vibrational reorganization energy, are calculated. Very similar values of λ(v) for R(4+) and P(4+) are calculated (∼2910 cm(-1)), indicating that the mechanical stoppers in the interlocked system do not significantly affect the excited state properties of R(4+) compared with P(4+).

Theoretical resonant Raman spectra of nanotube (7,0) with point defects

AbstractThe Raman spectra of the nanotube (7,0) with point defects (monovacancy, divacancy, and Stone–Wales defect) were simulated in order to derive spectroscopic signatures of defective nanotubes. First, we calculated the electronic band structure and the phonon dispersion of the defective nanotubes using supercells within a non‐orthogonal tight‐binding model. We found that new optical transitions and Raman‐active phonons appeared in comparison with the perfect nanotube. Secondly, we calculated the resonance Raman excitation profile for all Raman‐active phonons of the defective nanotubes and simulated their Raman spectra at specific laser excitation energies. The predicted high‐intensity Raman lines can be used as spectroscopic signatures of the defective nanotubes.

Sorting of single‐walled carbon nanotubes by amphiphiles molecules adsorption studied by resonant Raman excitation profile

AbstractAdsorption of specifically designed polyaromatic amphiphiles were used to sort HiPCo single‐walled carbon nanotubes (SWNTs) with different helicities. The sorting is investigated by resonant Raman excitation profiles. Chiral indexes (n and m) of SWNTs present in our samples are determined by fitting the Raman peaks observed in the radial breathing modes region (RBM). Scanning over an excitation energy range between 2 and 2.2 eV with a 0.01 eV step allows to investigate mainly two families of metallic nanotubes (2n + m = 24 and 2n + m = 27). The transition energies of nanotubes measured from the Raman spectra differ from the calculated ones, leading to useful corrections of the Kataura plot. The results also display significant differences between the sorted and the reference samples, meaning that discrimination between SWNTs of different properties is possible.

Investigation of the Resonance Raman Spectra and Excitation Profiles of a Monometallic Ruthenium(II) [Ru(bpy)2(HAT)]2+ Complex by Time-Dependent Density Functional Theory

The resonance Raman (RR) properties of the [Ru(bpy)(2)(HAT)](2+) (where bpy = 2,2'-bipyridine and HAT = 1,4,5,8,9,12-hexaazatriphenylene) complex have been investigated by means of time-dependent density functional theory calculations employing the hybrid B3LYP-35 XC functional and by including the effects of the solvent within the polarizable continuum model approach. Analysis of the electronic excited-state energies has demonstrated that mainly four different metal-to-ligand charge-transfer excitations contribute to the first absorption band in vacuo and water. The simulation of the absorption spectra by including the vibronic structure of the states has shown a general agreement with the experimental spectrum recorded in water. Furthermore, significant variations of the excited-state energies and compositions have been found when the effects of the solvent are included. Calculation of the short-time-approximation RR spectra has provided the vibrational signature of each contributing state and has shown that considering only one excited state is not sufficient to accurately simulate the RR spectra for excitation frequencies in resonance with the first absorption band. A comparison of the RR spectra calculated using the vibronic theory for different excitation wavelengths with the measured spectra at 514 and 458 nm has demonstrated that inclusion of the solvent effects in the simulation scheme leads to substantial improvements of the RR intensity patterns, which allow assignment of the vibrational bands. In particular, the calculations are able to reproduce the variations of the HAT and bpy RR intensities as illustrated by their RR excitation profiles, highlighting the strong dependence of the RR intensities with respect to the excitation frequency.

Solvent effects on the electronic and vibrational properties of high-valent oxomolybdenum(V) 5,10,15-triphenylcorrole probed by UV-visible and resonance Raman spectroscopy

Solution UV-visible electronic absorption and resonance Raman (RR) scattering spectra in 25 aprotic organic solvents have been measured and analyzed for a high-valent molybdenum( V ) oxo complex of 5,10,15-triphenylcorrole, (MoVO)CorPh3 . Optical absorption data in the visible region show a significant positive solvatochromism upon increasing solvent polarizability (ca. 660 cm-1 for the violet Soret band on going from acetonitrile to carbon disulfide), described by an excellent linear correlation between the bathochromic shifts of corrole π → π* electronic transitions and the solvent refractive index [Formula: see text]. Isotope labeling of the terminal oxo group with 18 O revealed the oxo-Mo(V) stretch of (MoVO)CorPh3 to vibrate at frequency ν(MoVO) ≈ 970 cm-1 [Δ(16O-18O) = 46 cm-1] , consistent with a MoV≡O triple bond. This vibration is greatly enhanced in the RR spectra with excitations within the Soret absorption band and its frequency is sensitive to solute-solvent interactions that weaken the MoV≡O triple bond by inhibiting O2- → Mo5+ electron donation. The ν(MoVO) frequency decreases in direct proportion to the Swain's solvent polarity parameter (A + B), from 969.4 cm-1 in n-hexane solution down to 955.6 cm-1 in dimethyl sulfoxide solution. However, only weak linear correlation was found when the ν(MoVO) frequencies were plotted as a function of the Gutmann's solvent acceptor numbers (ANs). A molecular association occurs between chloroform and (MoVO)CorPh3 through MoV≡O⋯H-CCl3 hydrogen-bonding interactions, manifested as an upshift of the ν(MoVO) RR band in deuteriochloroform solution. The ν(MoVO) stretching frequencies of (MoVO)CorPh3 and its fluorinated derivative (MoVO)Cor(Ph(CF3)2)3 {Cor(Ph(CF3)2)3 = 5,10,15-tris-[3,5-bis(trifluoromethyl)phenyl]corrole} are compared, and the increased strength of the MoV≡O bond in the latter oxo-Mo(V) species is attributed to the cis effect of the electron-withdrawing -C6H3(CF3)2 groups, as judged by a 6 cm-1 elevation of the ν(MoVO) frequency. Raman excitation profiles of ν(MoVO) and corrole skeletal vibrations show the presence of charge-transfer and π → π* electronic transitions within the Soret absorption band of oxomolybdenum( V ) corroles.

Initial Excited-State Structural Dynamics of Uracil from Resonance Raman Spectroscopy Are Different from Those of Thymine (5-Methyluracil)

To explore the origin of the differences in UV photochemistry of uracil (RNA) and thymine (DNA) nucleobases, we have measured the UV resonance Raman spectra of uracil in aqueous solution at wavelengths throughout the lowest-energy absorption band and analyzed the resulting resonance Raman excitation profiles and absorption spectra using a time-dependent wave-packet formalism to obtain the initial excited-state structural changes. In contrast to thymine, which differs from uracil only by the presence of a methyl group at C(5), most of the resonance Raman intensity and resulting initial excited-state structural dynamics for uracil occur along in-plane hydrogen-bond angle deformation, ring stretching, and carbonyl vibrational modes. Weaker intensities and less significant structural dynamics are observed along the C=C stretching mode. These results suggest that the initial excited-state structural dynamics of uracil occur along a carbon pyramidalization coordinate. These dynamics are different from those of thymine, which distorts primarily along a C(5)=C(6) bond lengthening coordinate. These differences in initial excited-state structural dynamics can explain the different primary photoproducts observed for these two pyrimidine nucleobases.

Manifestation of conical intersections in resonance Raman intensities

AbstractThis paper deals with the manifestations of conical intersections (CIs), unequivocal spectroscopic signatures of which are still elusive, in the resonance Raman intensities. In particular, the results of our calculations on the ‘two state‐two vibrational mode’ and the ‘two state‐three vibrational mode’ models are presented. The models comprise two excited states of different spatial symmetry, one bright and one dark, which are coupled by a nontotally symmetric mode while the energy gap between them is tuned by one/two totally symmetric modes. Time dependent theory for vibronically coupled states is employed for the calculation and analysis of Raman excitation profiles (REPs). The manifestation of intersections in REPs is studied by extensive model calculations and the results of two specific models are presented. The feasibility of using REPs to probe the role of CIs in polyatomic systems is ascertained by multimode calculations on two polyatomic systems viz., pyrazine and trans‐azobenzene. The study also notes the importance of the pump excitation wavelength dependence in a femtosecond time‐resolved experiment probing the intersection‐induced nonadiabatic dynamics. Copyright © 2009 John Wiley & Sons, Ltd.

DFT/TDDFT Exploration of the Potential Energy Surfaces of the Ground State and Excited States of Fe2(S2C3H6)(CO)6: A Simple Functional Model of the [FeFe] Hydrogenase Active Site

Fe(2)(S(2)C(3)H(6))(CO)(6) (a) is a simple model of the [FeFe] hydrogenase catalytic site. The topology of the potential energy surface (PES) of this complex, of its cationic and anionic species (a(+) and a(-)), and of its lowest triplet state was studied using density functional theory (DFT) with BP86 and B3LYP functionals, while selected low- and high-lying singlet excited states were studied with the time-dependent density functional theory (TDDFT). The global minima of a and a(-) PESs are characterized by an all-terminal CO ligand arrangement, while the two rotated forms are transition states (TS). On the contrary, for the a(+) and lowest triplet state PES, the three forms considered are local minima, and the syn rotated form is the global minimum. The relative stability of the rotated forms and the all-terminal CO form on the a, a(+), and a(-) PESs is discussed in light of the Quantum Theory of Atoms in Molecules (QTAIM) analysis of the electron density. By comparing the Fe-Fe bond features of the three forms for each PES, we found that the global minimum structure is characterized by the shortest Fe-Fe bond distance and highest electron density at the Fe-Fe critical point. This approach gave evidence that in the a rotated forms, the weak Fe-C(mu) interaction between the Fe atom of the unrotated Fe(CO)(3) and the C atom of the semibridged CO is formed to the detriment of the Fe-Fe bond interaction. These results suggest that the stabilization of the rotated forms on the cationic PES might be due to the formation of the weak Fe-C(mu) interaction minimizing the weakening of the Fe-Fe bond. The low-lying and lowest triplet excited-state PES investigated are characterized by the stabilization of the rotated forms over the all-terminal CO ligand arrangement. On the first singlet 1(1)A'' excited-state PES, an Fe(CO)(3) semirotated structure is the lowest-energy stationary point, while the exploration of the 1(1)A' and 2(1)A'' singlet excited PESs evidences the stabilization of the rotated over the all-terminal CO forms. Singlet excited-state optimized geometry results are compared with excited-state nuclear distortions recently obtained from resonance Raman excitation profiles. Finally, the results of the exploration of the 6(1)A' and 9(1)A' high-lying excited PESs are discussed in light of the recent ultraviolet photolysis experiments on a.

Calculation of absorption and resonance Raman cross sections of ClO2 in the gas‐phase and in solution: including Duschinsky effects

AbstractThe time‐correlation function formalism has been used to calculate resonance Raman cross sections, excitation profiles, and electronic absorption spectra of the OClO molecule in the gas‐phase and in different solvents like cyclohexane, chloroform, and water. The multidimensional time domain integrals that arise in these calculations have been evaluated for the case in which an X2B1Ã2A1 electronic transition takes place between displaced‐distorted‐rotated harmonic potential energy surfaces. Ab initio calculations have been performed to provide the spectroscopic constants required for the evaluation of these integrals. The calculated absorption spectra and resonance Raman cross sections have been compared with the experimental results. Copyright © 2009 John Wiley & Sons, Ltd.

On the chemical bonding effects in the Raman response: Benzenethiol adsorbed on silver clusters

We study the effects of chemical bonding on Raman scattering from benzenethiol chemisorbed on silver clusters using time-dependent density functional theory (TDDFT). Raman scattering cross sections are computed using a formalism that employs analytical derivatives of frequency-dependent electronic polarizabilities, which treats both off-resonant and resonant enhancement within the same scheme. In the off-resonant regime, Raman scattering into molecular vibrational modes is enhanced by one order of magnitude and shows pronounced dependence on the orientation and the local symmetry of the molecule. Additional strong enhancement of the order of 10(2) arises from resonant transitions to mixed metal-molecular electronic states. The Raman enhancement is analyzed using Raman excitation profiles (REPs) for the range of excitation energies 1.6-3.0 eV, in which isolated benzenethiol does not have electronic transitions. The computed vibrational frequency shifts and relative Raman scattering cross sections of the metal-molecular complexes are in good agreement with experimental data on surface enhanced Raman scattering (SERS) from benzenethiol adsorbed on silver surfaces. Characterization and understanding of these effects, associated with chemical enhancement mechanism, may be used to improve the detection sensitivity in molecular Raman scattering.

Synthesis and Spectroscopic Identification of a μ-1,2-Disulfidodinickel Complex

Reduction of elemental sulfur by a monovalent nickel precursor leads to a trans-1,2-mu-disulfidodinickel(II) complex assigned based on a combination of advanced spectroscopic methods in conjunction with density functional theory calculations. The disulfido linkage is characterized by an intense optical S --> Ni charge transfer transition at 650 nm, which causes a significant distortion of the Ni(2)S(2) core along an isotope-sensitive v(S-S) mode at 474 cm(-1), as demonstrated by the resonance Raman excitation profile of this vibrational feature.

Surface-Enhanced Raman Excitation Spectroscopy of a Single Rhodamine 6G Molecule

The surface-enhanced Raman excitation profiles (REPs) of rhodamine 6G (R6G) on Ag surfaces are studied using a tunable optical parametric oscillator excitation source and versatile detection scheme. These experiments afford the ability to finely tune the excitation wavelength near the molecular resonance of R6G (i.e., approximately 500-575 nm) and perform wavelength-scanned surface-enhanced Raman excitation measurements of a single molecule. The ensemble-averaged surface-enhanced REPs are measured for collections of molecules on Ag island films. The relative contributions of the 0-0 and 0-1 vibronic transitions to the surface-enhanced REPs vary with vibrational frequency. These results highlight the role of excitation energy in determining the resonance Raman intensities for R6G on surface-enhancing nanostructures. Single-molecule measurements are obtained from individual molecules of R6G on Ag colloidal aggregates, where single-molecule junctions are located using the isotope-edited approach. Overall, single-molecule surface-enhanced REPs are narrow in comparison to the ensemble-averaged excitation profiles due to a reduction in inhomogeneous broadening. This work describes the first Raman excitation spectroscopy studies of a single molecule, revealing new information previously obscured by the ensemble.

Wavelength-Scanned Surface-Enhanced Resonance Raman Excitation Spectroscopy

We explore the correlation between localized surface plasmon resonance (LSPR) of triangular Ag nanoparticles, molecular resonance, and the surface-enhanced resonance Raman excitation profile of molecules adsorbed on these nanostructures. Ag nanoparticles with various LSPRs were fabricated with nanosphere lithography (NSL). A monolayer of tris(2,2′-bipyridine)-ruthenium(II) (Ru(bpy)32+) was introduced to the NSL Ag nanoparticles, and its effect on the LSPR was monitored by UV−vis spectroscopy. Wavelength-scanned surface-enhanced resonance Raman excitation spectroscopy (WS SERRES) profiles of Ru(bpy)32+ adsorbed on Ag nanoparticle arrays were measured for excitation wavelengths in the range of 400−500 nm. The WS SERRES profiles are correlated, both spatially and spectrally, with the corresponding LSPR spectra of the nanoparticle arrays and with the solution absorption spectrum of Ru(bpy)32+. The WS SERRES profile peak position depends on the relative spectral position of LSPR and the molecular resonance. Quasi-static electrodynamics modeling was applied to simulate the WS SERRES profiles, and the calculations are in agreement with the experimental results, demonstrating that the WS SERRES profiles involve multiplicative electromagnetic and resonance Raman enhancements.

Ultrafast dynamics and photochemistry of π–π* excited trans‐azobenzene—a comprehensive analysis of resonance Raman intensities

AbstractAzobenzene and its derivatives have widely been recognized as systems with potential applications such as optical switching elements, and the mechanism of trans‐cis photoisomerization, which is the basis for all these applications, has long been sought. The long‐standing controversy on the relaxation mechanism of π–π* (S2) excited trans‐azobenzene (TAB) is addressed in this paper. The role of S1S2 intersection and the possible involvement of a third state in the S2 photochemistry are investigated. The excited state structure and dynamics of TAB are extracted by modeling the Raman excitation profiles (REPs) of the S1 and S2 states and the absorption spectrum of TAB by Heller's time‐dependent formalism. The REPs of S2 are modeled in the single resonant state approximation by explicitly including the eight most Franck–Condon (FC) active modes. The obtained NN and CN bond lengths of S2 and the intial dynamics in these coordinates following photoexcitation to S2 are in agreement with recent theoretical calculations. The structural parameters of S1 are obtained by exploiting the nature of interference in the REPs of S1. From these parameters a three‐mode–two‐state vibronic coupling model, based on harmonic diabatic potentials and linear coupling, is formulated to study the S2S1 internal conversion dynamics. The results indicate that the S1S2 intersection is not directly accessible within electronic dephasing time of S2 to cause significant population decay from S2 to S1. The current conception of π–π* relaxation mechanisms in TAB is reviewed and revised on the basis of our observations and also from recently reported results of time‐resolved and theoretical studies. The role of a third state (S3), which appears to be an integral part of in the relaxation mechanism of S2, is pointed out and the feasibility of two competing relaxation pathways for S2 state is identified. One is the direct rotationless S2S1(C2h) internal conversion and the other is S2S3 internal conversion along rotational pathway followed by a branching of the S3 population to S0 and S1. Copyright © 2008 John Wiley & Sons, Ltd.