Electro-optical studies of a soluble conjugated polymer with particularly low intrachain disorder

In contrast with many soluble conjugated polymers, the absorption and electroabsorption spectra of the methyl-substituted ladder-type poly(para-phenylene) show particularly well-resolved excitonic transitions, whose linewidth approaches the narrow values previously only associated with single crystals or isolated chains of polydiacetylenes. At least two distinct vibrational modes couple strongly to the electronic transitions. The electroabsorption spectrum increases quadratically with the electric-field strength, and closely follows the first derivative of absorption throughout most of the visible and near-ultraviolet range, indicative of a quadratic Stark shift of the ${1B}_u$ singlet exciton. In the region 3.2–3.6 eV, the agreement between the electroabsorption spectrum and the first derivative is offset by a broader spectral feature, which does not correspond to any feature in the linear absorption spectrum. Two-photon fluorescence excitation spectroscopy identifies this state as a forbidden exciton ${(mA}_g),$ located 0.7 eV above the lowest singlet exciton ${(1B}_u).\mathrm{}$ A polarizability of 2060 ${\mathrm{\AA }}^3$ and a dipole moment of $7.17e\mathrm{\AA }\left(34\mathrm{}\mathrm{D}\right)$ is calculated for the ${1B}_u$ singlet exciton. The spectra show evidence of a small increase of the vibrational frequencies as the polarization of the π system increases.