Raman excitation profiles of conjugated segments in solution (original) (raw)
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Synthetic Metals, 1991
In this paper, we present experimental and theoretical Resonance Raman studies carried out on oriented polymer systems. It is shown that different oriented trans polyacetylene samples exhibit Raman features which depend upon the stretching ratio. Also, the depolarization ratio of the Raman bands have been found to depend upon the isomerization temperature. Oriented copolymers polyacetylene/polynorbomene, (CH)x-(NBE)n-(CH)x, have also been studied. Raman spectra of the trans-(CH)x parts of these compounds have been analyzed in the frame of the bimodal distribution model. It is shown that whatever x and n values, results are rather similar in terms of conjugation length. In addition, from IR studies, one can deduce that in the stretched oriented copolymers, polynorbomene and polyacetylene segments have a different orientation degree.
A comparison of the Raman dispersion in different polyacetylenes with aromatic ring substituents
Synthetic Metals, 2006
Raman dispersion data for poly-2-ethynylpyridine, poly-4-ethynylpyridine, poly-β-naphthylacetylene, poly(2-pyridinium hydrochloride-2-pyridylacetylene) and poly(2-N-t-butylpyridiniumylacetilene)iodide were obtained and analysed using the Amplitude Mode Model (AMM).The slopes of the ∏n(ωnR/ωn0)2 versus ωL curves (D) ranged from 0.090 to 0.146eV−1, substantially larger than the values reported for other photoluminescent conjugated polymers (up to 0.020eV−1). This behaviour was interpreted as arising from the presence of a second
Journal of Raman Spectroscopy, 2001
We review resonance Raman investigations of the linear conjugated p-electron system of polydiacetylenes embedded in single-crystalline monomer matrices. Absorption, luminescence, resonance Raman and resonance CARS spectroscopy were used to investigate vibronic properties of the polymers. As an example, we discuss the theoretical model applied for the simulation of the resonance Raman line and excitation profiles obtained from the polymer chains in FBS, TS/FBS and TS6 diacetylene single crystals. The theory is based on a Franck-Condon model, which considers a chain length dependence. The electronic transition energies and matrix elements were calculated by means of an LCAO MO (linear combination of atomic orbitals molecular orbital) calculation in the Hückel approach. An interaction between ensemble and intramolecular properties due to defects can be shown. Besides the well known polymer absorption, the partially polymerized single crystals of the diacetylenes under investigation show blue-shifted absorption bands in sharply separated crystal areas. The spectroscopic investigations of this chromism revealed that most probably growth defects in f111g crystal growth sectors influence the side-group geometry of the polymer chains. Using femtosecond time-resolved CARS spectroscopy, the dynamics of the vibrational excitation in the electronic ground and excited states can be investigated. Here, we report the controlled selective excitation of the polymers. We used a CARS scheme where the difference of pump and Stokes laser wavelengths is chosen to achieve resonance with several chain modes of the polydiacetylenes. By varying the timing and the phases of the femtosecond laser pulses, the timing and the intensity of these modes can be strongly influenced.
Resonant Raman scattering from amplitude modes in trans polyacetylene: Temperature effects
Synthetic Metals, 1987
The resonantly enhanced Raman lines of polyacetylene (PAC) are investigated at different temperatures. It is found that the laser independent lines (primary lines) decrease in frequency while those which shift upward with the laser energy (satellite lines) increase in energy as the temperature decreases, thus giving rise to broadening of the Raman bands. This effect is connected with the temperature dependence of the electronic gap and is explained by the amplitude modes (AM) model.
Journal of the American Chemical Society, 1993
Sequential ring-opening metathesis polymerization of norbornene and 7,8-bis(trifluoromethyl)tricyclo[4.2.2.02~s]deca-3,7,9-triene with Mo(CHCMe3)(NAr)(OCMeJ2 (Ar = 2,6-diisopropylphenyI) followed by linking termination with a conjugated dialdehyde results in highly soluble A-B-A triblock copolymers containing an oligomer of a precursor of Durham polyacetylene as the central block. Subsequent heat treatment converts the polyacetylene precursors into model polyenes. A series of these copolymers that had conjugation lengths ranging from 4 to 16 double bonds was synthesized. The polyenes were isomerized to the predominantly all-trans isomer. The presence of the polynorbornene chains solubilizes the polyenes, allowing their hyperpolarizability to be probed with electric field induced second harmonic generation; yN(-2w;w,w,0) and y N (0) varied with conjugation length to the 3.6 and 3.2 powers, respectively.
UV-Excited Raman and Resonance Raman Spectra of Synthetic Polymers
Applied Spectroscopy, 1992
High-quality, UV-excited (218-242 nm), fluorescence-free conventional Raman spectra have been generated from UV-transparent polymers such as polytetra-fluoroethylene (Teflon ®), polyethylene, polypropylene, and polyoxymethylene (Delrin®). Spectra can be generated with the use of low exciting power (< 1 mW), but even at low power, precautions have to be taken to prevent photo-oxidation and thermal degradation of samples. Polyvinylchloride polymers especially have been found to be prone to degradation. Weakly absorbing polymers such as nylon and polybutadiene show special promise for Raman analysis, because they exhibit significant preresonance enhancement of structurally significant modes, and because they still retain information derived from conventional Raman spectra. Strongly preresonance-enhanced ethylenic modes of polybutadiene isomers can be resolved, allowing the easy identification of cis and trans isomers. In contrast, strongly absorbing polymers such as polystyrene, polycarbonate, and polyphenolics show only intense, resonance-enhanced Raman spectra. Unfortunately, Raman spectra of these materials are limited to phenyl ring modes, and all valuable conventional Raman spectra are totally lost due to internal absorption.
Polyacetylene oligomers:π-electron fluctuations, vibrational intensities, and soliton confinement
Physical Review B, 1999
We present a simple, internally consistent model for calculating ground-state properties of polyacetylene ͑PA͒ oligomers. The model describes electrons according to Hückel theory and accounts for electron-phonon coupling with an exponential dependence of hopping integrals on the bond length. Careful tuning of model parameters against the geometry and Raman frequencies of pristine PA allows the calculation of vibrational frequencies and nonresonant Raman intensities of oligomers, as well as infrared active vibrational spectra of solitons on short chains. The available experimental data are well reproduced by the proposed model. Both infrared and nonresonant Raman intensities are found to scale superlinearly with the chain length. By relating this behavior to the scaling of the optical gap, we gain insight on the coherence length of-electron motion in PA. ͓S0163-1829͑99͒04635-4͔