Knowledge Bank

The delocalized $\pi$-electron polymer, [Graphics], where R is $CH3-CH2-NH-COO-(CH2)4-$, prepared by solid state polymerization of the corresponding diacetylene monomer, undergoes a reversible thermochromic (green-red) phase transition in the $117\circ C\le T\le 137\circ C$ range. Such polydiacetylenes have extensive regions over which $\pi$-electron density is delocalized along the network (backbone) carbon chains. The positions of broad, incense absorptions in the visible spectra are indicative of the distributions in lengths of chain over which delocalization occurs. Raman spectra have been obtained as a function of temperature through the transition, using several $Kr+$ and $Ar+$ laser lines. The dramatic changes in frequency and intensity of the $\nu$(C$=$C) and $\nu$(C$=$C) bands, the most intense Raman spectral features, are due to resonance enhancement as a function of the delocalization length which in turn is a function of temperature. Thus the resonance Raman phenomena are due to selective interactions between $\pi$-electron transitions and intrachain C-C vibrations particular to that domain in the polymer.