PHASE-TRANSITIONS OF ISOLATED INTRAMOLECULAR HYDROGEN-BONDED SYSTEMS,MEHPLN AND BRHPLN

Phase transitions of the new materials, MeHPLN(5-methyl-9-hydroxyphena lenone, C14H10O2) and BrHPLN(5-bromo-9-hydroxyphenalenone, C13H7BrO2) are studied by x-ray scattering experiments. The characteristic of the se systems is an isolated intramolecular hydrogen-bonded system. Large isotope effect on the phase transition temperature was found in BrHPL N while no effect was seen in MeHPLN. Furthermore, an intermediate pha se was found only in BrHPLN. The phase transition scheme in BrHPLN was reported to be normal, incommensurate and commensurate phases, and is revealed for MeHPLN in this paper to be normal-to-commensurate phase transition. The difference of the isotope effect is discussed based on structural view points.