Anomalous pressure effects on optical spectra of hydrogen-bonded molecular chain solids: Possible formation of kink solitons

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.