Different nonlinear optical performances of polymers containing benzimidazole chromophores

Two NLO azo-benzimidazolic chromophores, bis-ethanol, 2,2′-[[4-[[4-(1-ethyl-6-nitro-1H-benzimidazol-2-yl)phenyl]azo]phenyl]imino] (BZI1) and bis-ethanol, 2,2′-[[4-[[4-[(1E)-2-(1-ethyl-6-nitro-1H-benzimidazol-2-yl)ethenyl]phenyl]azo]phenyl]imino] (BZI2), properly functionalised for polycondensation reactions, have been used for the synthesis of new NLO polyurethanes and polyesters. The polymers have high thermal stability and Tg ranging between 146 and 173 °C. Thin films of polymers have been prepared and oriented by electrical poling. The resonance free NLO activity has been measured at a fundamental wavelength of 1368 nm by second harmonic generation (SHG) and a maximum d33 of 2.3 pm/V has been observed. Polymers containing the less active chromophore (BZI1, μβ = 950 × 10−48 esu) show higher d33 value than the analogous polymers containing BZI2 chromophore (μβ = 1400 × 10−48). A theoretical analysis suggests that this unexpected behaviour can be only partially attributed to thermodynamic effects, dynamical effects should also play a role.