Infrared and Raman studies of poly(p-phenylenevinylene) and its model compounds

The infrared and Raman spectra of poly(p-phenylenevinylene) (PPV) and its low molecular weight model compounds CH₃(C₆H₄CH=CH)_nC₆H ₄CH₃ (n = 1-3) have been studied, and the following points have been made clear. (1) The vinylene groups in PPV are slightly distorted from a planar trans form, so that the PPV molecule has approximate C_2h symmetry. (2) Extensive π-electron conjugation takes place between the phenylene and vinylene groups, and the degree of conjugation increases with increasing chain length. (3) The positions and relative intensities of the infrared and Raman bands of the model compounds change regularly as the chain length increases and rapidly converge on those of PPV, which are considered to be practically identical to those of an infinitely long chain. (4) The positions of the Raman bands of PPV slightly shift with the excitation wavelength. The 363.8-nm-excited Raman spectrum seems to arise from segments whose lengths are of the order of the model compound n = 3, whereas the Raman spectra excited at longer wavelengths correspond to longer segments which are dominant in quantity.