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

In contrast with many soluble conjugated polymers, the absorption and elect roabsorption spectra of the methyl-substituted ladder-type poly(para-phenyl ene) show particularly well-resolved excitonic transitions, whose linewidth approaches the narrow values previously only associated with single crysta ls or isolated chains of polydiacetylenes. At feast two distinct vibrationa l modes couple strongly to the electronic transitions. The electroabsorptio n spectrum increases quadratically with the electric-held strength, and clo sely follows the first derivative of absorption throughout most of the visi ble and near-ultraviolet range, indicative of a quadratic Stark shift of th e 1B(u) singlet exciton. In the region 3.2-3.6 eV, the agreement between th e electroabsorption spectrum and the first derivative is offset by a broade r spectral feature, which does not correspond to any feature in the Linear absorption spectrum. Two-photon fluorescence excitation spectroscopy identi fies this state as a forbidden exciton (mA(g)), located 0.7 eV above the lo west singlet exciton (1B(u)). A polarizability of 2060 Angstrom(3) and a di pole moment of 7.17e Angstrom ( 34D) is calculated for the 1 B-u singlet ex citon. The spectra show evidence of a small increase of the vibrational fre quencies as the polarization of the pi system increases.