Spectroscopic ellipsometry and fluorescence study of thermochromism in an ultrathin poly(diacetylene) film: Reversibility and transition kinetics

We have investigated the thermochromic transition of an ultrathin poly(diac etylene) film. The Langmuir film is composed of three layers of polymerized 10,12-pentacosadiynoic acid [CH3(CH2)(11)C=CC=C(CH2)(8)-COOH] (poly-PCDA) organized into crystalline domains on a silicon substrate. Spectroscopic el lipsometry and fluorescence intensity measurements are obtained with in sit u temperature control. Poly-PCDA films exhibit a reversible thermal transit ion between the initial blue form and an intermediate "purple" form that ex ists only at elevated temperature (between 303 and 333 K), followed by an i rreversible transition to the red form after annealing above 320 K. We prop ose that the purple form is thermally distorted blue poly-PCDA and may repr esent a transitional configuration in the irreversible conversion to red. T his hypothesis is supported by the appearance of unique features in the abs orption spectra for each form as derived from the ellipsometry measurements . Significant fluorescence emission occurs only with the red form and is re duced at elevated temperatures while the absorption remains unchanged. Redu ced emission is likely related to thermal fluctuations of the hydrocarbon s ide chains. Time-resolved fluorescence measurements of the irreversible tra nsition have been performed. Using a first-order kinetic analysis of these measurements, we deduce an energy barrier of 17.6 +/- 1.1 kcal mol(-1) betw een the blue and red forms.