Solvatochromism polyacetylenes

Champagne et al.206 have studied solvent effects, through a continuum model, on the a and response functions of polyacetylene chains in the TDHF approximation. They find large increases in the values which they relate to the solvatochromic shifts in the lowest optically allowed transition. Density functional theory has also been assessed207 in connection with the calculation of the same response functions, but has been found to be inadequate due to the inability of the exchange/correlation potentials to satisfactorily represent the effects of the ends of the polymer. Schmidt and Springborg208 have calculated the static hyperpolarizability of polyacetylene and polycarbonitrile in DFT in the presence of external fields. 

A group of new amphiphilic macromolecules comprised of hydrophobic polyacetylene backbone and hydrophilic pendant groups of naturally occurring species such as amino acids, saccharides, and nucleosides are synthesized. The polymers exhibit solvatochromism. The macromolecular chains show helical confommtions that depend on the molecular structures of the pendants, solvent, temperature, pH, and additives. 

The anq)hiphilic polyacetylene chains may take different conformations in differoit environments (e.g., in different solvents) and the different conformers may undergo different electronic transitions. The polymers exhibit interesting solvatochromism the absorption spectra of their solutions change with a change in the solvent. 

Chloroform is a good solvent to both the hydrophobic backbone and Ihe hydrophilic pendants. The polymer chains may be well solvated by the solvoit and take an extended planar conformation, in which the polyacetylene backbone is better conjugated. Toluene is, however, a poor solvent of the hydrophilic pendants. To minimize the exposure of their pendants to the un vorable hydrophobic solvent environment, the polymer chains may take a coiled non-planar conformation, in which the polyene bacldione may be less conjugated. Since the conformational change is induced by the noncovalent solvent-polymer interaction, it is easy to understand why the solvatochromic change is continuous and reversible. 

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