Second harmonic generation in polymers containing a new azo chromophore based on phenylnitrobenzoxazole

The synthesis of the new chromophores 2-4-[4-(N,N-dihydroxyethyl-amino)-phenylazo]-phenyl-6-nitrobenzoxazole (1) and 4-[4-(N,N-dihydroxyethylamino)-phenylazo]-benzoic acid (2) is described. Three homopolymers obtained by the polycondensation of 1 with propyloxy-terephthalic acid and 1 and 2 with 2,4-tolylenediisocianate as well as two copolymers obtained by polycondensation of appropriate mixtures of chromophores 1 and 2 with 2,4-tolylene-diisocianate were prepared and characterized. All as-prepared polymers but one (homopolyurethane of 1) were amorphous and exhibited good optical clarity, good thermal stability above the glass transition and solubility in organic solvents. The second-order nonlinear optical behavior of the amorphous polymers was investigated on spin-coated corona-poled films through second harmonic generation (SHG) experiments at 1064 nm. At this wavelength, resonance-enhanced values of the SHG coefficient d33 were found in the range between 6 and 34 pm/V where higher values were observed for polymers with a higher molar content of 1.