Synthesis and reversible thermochromism of azobenzene-containing polydiacetylenes

A series of diacetylenic monomers containing azobenzene chromophores with different terminal groups {CH₃-(CH₂)₁₁-CC-CC-(CH₂)₈-COO-(CH₂)₂-O-C₆H₄-NN-C₆H₄-R, where R = NO₂, CN, CH₃ and labeled as 3a, 3b and 3c, respectively} were synthesized and characterized by ¹H NMR. The photo-polymerization of spin-coated films of the monomers was carried out by irradiation at 254 nm under monitoring the optical absorption in the visible region and polymers (P3a-P3c) were obtained as blue films that were no longer soluble in common organic solvents. The thermochromic behavior of the polymers was evaluated with UV, Raman, thermogravimetric analysis, X-ray diffraction and differential scanning calorimetry analyses. It was found from results that all of the polymers were thermally stable and decomposed at temperatures as high as ∼300 °C. By tracing the treatment of samples with heating-cooling cycles, the temperature ranges between the low and the high critical temperature for the red-phase were obtained to be 60-160 °C, 57-162 °C and 50-154 °C for P3a, P3b and P3c, respectively, larger than those reported before for other PDAs. The relationship between characteristics above and the chemical structure of polymers was discussed.