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.