Raman photoselection and conjugation-length dispersion in conjugated polymer solutions

Resonance Raman scattering (RRS) in conjugated polydiacetylene solutions is employed in order to determine the vibrational spectra of resonance-selected chromophores within a single conformationally disordered polymer chain. Although the polymer-chain length contains up to 1000 repeat units, the conjugation length (i.e., the length over which backbone planarity is maintained without interruption and which defines the chromophore) may be as small as three units. Variation of the solvent system can increase the conjugation length to essentially infinity. The absorption spectrum of the polymer in chloroform consists of an inhomogeneously broadened line shape with contributions from a distribution of individual chromophores. RRS can photoselect chromophores with specific transition energies and, therefore, with specific conjugation lengths. The delocalization of electron density along the conjugated unit, which decreases the absorption energy of the chromophore, also causes a decrease in frequency for those Raman modes which primarily correspond to multiple bond stretching, such as ${\ensuremath{\nu}}^{\mathrm{C}=\mathrm{C}}$. The change of ${\ensuremath{\nu}}^{\mathrm{C}=\mathrm{C}}$ and ${\ensuremath{\nu}}^{\mathrm{C}\ensuremath{\equiv}\mathrm{C}}$ as the incident laser energy is tuned through the absorption band is reported here. Far from the absorption band an average value for ${\ensuremath{\nu}}^{\mathrm{C}=\mathrm{C}}$ is obtained. As the incident energy approaches the absorption band, long-conjugation-length chromophores are photoselected and ${\ensuremath{\nu}}^{\mathrm{C}=\mathrm{C}}$ decreases markedly. At higher energies within the absorption band, shorter-conjugation-length chromophores are photoselected and ${\ensuremath{\nu}}^{\mathrm{C}=\mathrm{C}}$ increases, becoming larger than the average value of ${\ensuremath{\nu}}^{\mathrm{C}=\mathrm{C}}$. Similar results are obtained for ${\ensuremath{\nu}}^{\mathrm{C}\ensuremath{\equiv}\mathrm{C}}$. A model for the conjugation-length dispersion is presented and found to be in good agreement with absorption profiles, frequency shifts, and RRS intensities.