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 T, ranging between 146 and 173 degrees 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 d(33) of 2.3 pm/V has been observed. Polymers containing the less active chromophore (BZI1, mu beta = 950 x 10(-48) esu) show higher d(33) value than the analogous polymers containing BZI2 chromophore (mu beta = 1400 x 10(-41)). A theoretical analysis suggests that this unexpected behaviour can be only partially attributed to thermodynamic effects, dynamical effects should also play a role. (c) 2007 Elsevier B.V. All rights reserved.