EFFECTS OF MOLECULAR AREA ON THE POLYMERIZATION AND THERMOCHROMISM OF LANGMUIR-BLODGETT-FILMS OF CD2+ SALTS OF 5,7-DIACETYLENES STUDIED USING UV-VISIBLE SPECTROSCOPY

The first study of the polymerization and thermochromism in Langmuir-Blodgett films of the Cd2+ salts of 5,7-docosadiynoic acid (DCDA) and 5,7-tretracosadiynoic acid (TTCDA) formed at molecular areas of 22 and 26 Angstrom(2)/molecule is reported. The polymerization behavior and:subsequent thermochromism of the resulting polymers was followed using visible spectroscopy. The high-density (low molecular area) polymerized films resulted in visible absorption spectra which were distinctly different from the spectra resulting from the low-density (high molecular area) polymerized films. The temperature dependence of the visible absorption spectra of the two densities of films was distinctly different as well. In addition, the temperature dependence of the visible absorption spectra of the polymerized films could be qualitatively accounted for by a two-reaction scheme in which the blue (B) form of the polymer forms an intermediate purple (P) form in a reversible reaction. The purple form of the polymer is transformed to the red (R) form in an irreversible reaction. (B reversible arrow P, P --> R). Estimated activation barriers for the forward reaction B --> P were obtained from the temperature dependence of the visible absorption spectra using a normal preexponential factor of 10(12) s(-1). For LB films of DCDA these were E = 22.4 and 21.7 kcal/mol for the low- and high-density films, respectively. For LB films of TTCDA these were E = 23 and 21 kcal/mol for the low- and high-density films, respectively.