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Infrared absorbance (IR) spectra and Raman scattering spectra were measured as a function of pressure for molecular crystals of several 1,3-diketones, which are composed of hydrogen-bonded π -molecular chains. An unconventional proton-related band is commonly observed in the IR spectra around 1800-2000cm−1 in these compounds. We ascribed the observed band to proton vibration localized around kink-(or antikink-) solitons. The intensity of the band rapidly increases with application of hydrostatic pressure, suggesting increase in number of the solitons-like defects of the molecular chains via deformation of the double-well type proton potential.

Optical properties of new polydiacetylene (PDA) crystals substituted with fluorobenzene derivatives have been investigated by reflectance, Raman, and electroreflectance (ER) measurements. The results indicate that these fluorobenzene‐substituted PDAs have an intermediate optical character between those of the acetylene‐type and butatriene‐type PDA chains. One of these PDA crystals, DFMDP, has been found to show an exceptionally high crystalline quality, which has enabled us to make a detailed spectroscopic study. ER studies on this DFMDP crystal have revealed the charge–transfer(CT)‐type exciton lying at ∼2.7 eV on the higher‐energy side of the lowest 1Bu exciton at ∼2.2 eV. On the basis of experimental results, it is conjectured that two‐photon resonance with the CT exciton plays a major role in the third‐order nonlinear optical process in PDA chains.

Polydiacetylene crystals with different kinds of side chains are formed and show two characteristic nonlinear-optical responses: the photo-induced structure change and the very large third-order optical susceptibility. The former property may be used as optical memory and the latter as the optical information processor. These two aspects of polydiacetylene are discussed in this paper.

Infrared (IR) and visible absorbance spectra were measured as a function of hydrostatic pressure for five kinds of organic crystals composed of hydrogen‐bonded molecular chains. An unconventional vibrational band is observed in the IR spectra around 1800 cm− 1 commonly in these compounds and its intensity rapidly increases with pressure. We have ascribed the observed band to a proton vibration localized around kink‐type defects or solitons in the hydrogen‐bonded molecular chains. In accordance with the pressure‐induced growth of the IR band, a new electronic absorption band appears in the visible region below the ordinary molecular excitation, suggesting a change in the electronic structures within the molecules around the soliton.