Polydiacetylenes Raman frequencies

Table 2. Raman frequencies and absorption maxima of amphiphilic polydiacetylenes...

I. Is it possible to observe a shift in coherent Raman scattering in the three-level system with A-type coupling We have done an experiment to obtain a femtosecond Raman gain spectrum in polydiacetylenes. The Raman spectrum is shifted to the red under increased pump (to i) intensity. By changing o>2> the amplification peak signal is to be shifted to lower frequency. If the optical Stark effect is observed, then, in principle, it should be possible to observe the effect of a high field on the coherent Raman spectrum (see Fig. 1). 

Resonant raman spectroscopy has proved to be another valuable tool for the study of the structure of the polydiacetylene chain. Due to the resonance enhancement the spectra are compared to greatly simplified, infrared spectra and show as principle feature only the in-plane modes of the polymer chain. The correlation of the CsC and C = C stretching modes and their temperature dependence have been interpreted as resonances between the mesomeric structures (I) and (II) i32) Hoy(rever, a model using simple anharmonic force constants for the acetylene structure (II) is in good agreement with the experiment, e.g, the temperature and pressure dependence of the vibration frequency and the mechanical properties... 

A low-temperature splitting of some of the lines in the resonant Raman spectrum of 234b has been reported recently. Dramatic changes in the frequency and intensity of the C—C and C=C bands of another polydiacetylene 234, R = EtNHC02(CH2)4, have been noted with changes in temperature as the polymer passes through a phase transition . 

Micromechanisms of reinforcement. It was pointed out earlier that the vibrational frequencies of certain main-chain Raman active modes are found to change with the level of applied strain (26). The biggest change is found for the C=C triple bond stretchTng frequency which is of the order of 20 cm-i/%. This property can be used to determine the strain in any polydiacetylene fibre subjected to any... 

One well-known example confirming our calculations is the vibration mode of polydiacetylene, which demonstrates three strong maximums in the Raman spectrum at = 2116cm(along the chain axis) and cd2 = 1517 cm" (along the kinks) and at cdj = 1072 cm" [25]. The deviation of cdj from the experimental value can be explained by the influence of side groups in the polydiacclylene polymer chain on the co frequency. 

Spontaneous Raman scattering always occurs when the laser excitation frequency is less than the frequency associated with an allowed electronic transition of the molecule. As the probe laser frequency approaches that of an electronic transition in the molecule, certain vibrational modes that couple strongly to the transition increase in intensity (pre-resonance) with respect to other Raman allowed modes of the molecule. When the excitation frequency coincides with the electronic transition frequency (resonance), a dramatic increase in vibrational band intensities is observed. This effect has been observed in many molecules and especially in polymer films, such as polydiacetylene, that consist of extended regions of electron delocalization owing to the presence of conjugated double and triple carbon-carbon bonds in the linear network (40)(41). 

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