SPECTROSCOPIC STUDIES OF POLYDIACETYLENES - RAMAN EVIDENCE FOR SURFACE PHASES ON SINGLE-CRYSTALS

Approaches to the use of pi donor-acceptor interactions to modify the linear electronic spectrum of polydiacetylene (PDA) crystals are discussed. Raman spectra of the chemically similar polydiacetylene-1,6-(di-N-carbazolyl)-2,4-hexadiyne (PDA-DCH) and polydiacetylene-1, 1,6,6-tetraphenyl-2,4-hexadiynediamine (PDA-THD) were studied as a function of the excitation wavelength. Using 1064, 632.8, 514.5, 488.0 and 457.9 nm as excitation wavelengths, spectra are observed using light polarized in the chain axis direction. Spectra observed at 457.9 nm are clearly different to those obtained at the two longest wavelengths used, and it is deduced that these spectra are associated with surface phases of these PDA. The position of the dilute solution absorption maximum of a soluble PDA with p-chlorocinnamate side groups is dependent on the refractive index of the solvent, indicating the sensitivity of the conjugated backbone to the local solvent environment.