Polyacetylene is presented both as a prototype conducting polymer and as a prototype one-dimensional metal and semiconductor. The concepts of the physics of one-dimensional metals are demonstrated on idealized polyacetylene. A review of the chemical and physical properties of real polyacetylene is given and experimental results on electrical conductivity, magnetic behaviour, optical spectroscopy and photoconductivity are discussed in the context of soliton-like excitations.

Solitons in polyacetylene

Conjugated polymers such as polyacetylene form a group of model semiconductors in which the 'one-dimensional' character of the polymer strongly modifies the behaviour of charges added to the chains from that usually observed in three-dimensionally bonded semiconductors. First, the polymer chain can undergo local reorganisation of the pi -electron bonding in the vicinity of a charge added to it. This has the effect of localising the charge in a state that can be described as a soliton for the particular case of trans-polyacetylene, and more generally as a polaron. Secondly, the motion of charged excitations is highly anisotropic, with strong confinement to the chain through the 'polaronic' structural relaxation of the chain. This anisotropy strongly affects charge transport and charge separation following photo-excitation. The authors review some of the recent work in this area and present polarisation-dependent measurements of photoluminescence (PL) and photo-induced absorption (PA) on highly oriented films of polyacetylene and poly(phenylenevinylene). They find that charge separation, detected through PA from photogenerated solitons or bipolarons, is the result of inter-chain electron transfer, whereas intra-chain excitation leads to charge confinement, and decay by PL (for poly(phenylenevinylene)) and by non-radiative channels. They find furthermore that these non-radiative channels become more efficient as the length of the conjugated polymer chain increases, and consider that this occurs through rapid motion of the excited state to recombination centres. They have identified both monomolecular and bimolecular kinetics for these processes.

https://iopscience.iop.org/article/10.1088/0022-3727/20/11/005/pdf

Photo-excitation in conjugated polymers

A simple interpretation of the vibrational spectra of undoped, doped and photoexcited polyacetylene: Amplitude mode theory in the GF formalism

We have measured the solid state Raman scattering spectra of a homologous series of linear polyenes, with the number of alternated double bonds varying from 3 to 12. While we find a linear dependence of the Raman shifts of resonantly coupled modes with inverse conjugation length, we have also followed the suggestion of previous work in examining the inverse square product of the several Raman frequencies as a function of the logarithm of the measured energy gap of the molecule. This provides a linear relationship, as found for trans‐polyacetylene, a result which is qualitatively consistent with the amplitude mode model of Horovitz and co‐workers. We also find, consistent with previous work on polyacetylene, a monotonic decrease in the ratios of oscillator strengths of the two strongest bands with conjugation length, as recently predicted by a series of molecular dynamics calculations. Suggested interpretations of a number of qualitative observations, including splitting of modes for shorter conjugation le...

http://web.mit.edu/robertsilbey/research/papers/1991-2000/rsilbey_conjugation_length_dep_Raman_scattering_linear_polyenes.pdf

Conjugation length dependence of Raman scattering in a series of linear polyenes: Implications for polyacetylene

New classes of polymers and oligomers whose backbone consists of a network of delocalized π electrons have recently been synthesized and show electrical and non-linear optical properties of great interest for new technologies. The vibrational spectra of these materials show peculiar features which, if properly interpreted, can provide unique structural information. The interpretation of the spectra requires the development of new theoretical tools. The theoretical and experimental aspects of the problem are presented. It is also shown that the nuclear relaxation following the excitation of the electrons by an intense electromagnetic field can be described in terms of the vibrational modes and can be related to the nuclear contributions to the non-linear optical responses of the material. In this case Raman intensities play a determining role.

Vibrational Raman spectroscopy of polyconjugated organic oligomers and polymers

We observed soliton shape modes by photoinduced absorption in partially isomerized ${(\mathrm{CH})}_{x}$ and ${(\mathrm{CD})}_{x}$, coexisting with the soliton translational modes. The shape modes' ir activity, their dependence on the degree of isomerization, and the photoinduced spectra are explained in detail by a symmetry-breaking interaction which allows coupling of shape modes and translational modes. We also observed soliton shape-mode absorption in fully isomerized samples as phonon sidebands associated with optical transitions of neutral solitons above 1.5 eV.

Detection of soliton shape modes in polyacetylene.

Infrared active vibrations of photogenerated solitons in polyacetylene: Shape modes

Resonant Raman Scattering and doping induced IR absorption are explained in terms of amplitude modes of the 1-d gap. The RRS frequencies and relative intensities of the renormalized phonon lines at the various concentrations are readily obtained.

H. Fujimoto

Papers

Detection of soliton shape modes in polyacetylene.

Infrared active vibrations of photogenerated solitons in polyacetylene: Shape modes

Amplitude Modes in Mixed Polymers Trans [C2H2)y(C2D2) 1-y]x