Synthesis of a diacetylene-containing polyisophthalate and its third-order nonlinear optical susceptibility

In order to obtain amorphous thin films of a polydiacetylene, a polyisophthalate containing a push–pull diacetylene chromophore was synthesized by the reaction of p-(N,N-diethanolamino)-p’-nitrodiphenylbutadiyne with isophthaloyl chloride in anhydrous 1-methyl-2-pyrrolidone. The polymer was soluble in organic solvents, such as chloroform, tetrahydrofuran, dimethylformamide, etc., and films with good optical quality were obtained by spin coating from its N-methyl-2-pyrrolidone solution. It was characterized by ¹H-NMR, IR, DSC, UV–Visible, X-ray, etc. The diacetylene groups underwent polymerization in the film when irradiated with UV light at 120 °C under Ar atmosphere, and it contained free radicals with a radical concentration of 10¹⁶ radicals/g. The third-order nonlinear optical susceptibility, χ⁽³⁾, of the polymer film measured by the Z-scan technique at 1064 nm was not noticeable, but when the film was heated and irradiated with UV light, a χ⁽³⁾ value of 6.4?×?10^?10 esu was obtained due to increase in π-conjugation by the formation of polydiacetylene in the side chain.