Third-order nonlinear optical susceptibilities of new copolymers containing alternate 3,4-dialkoxythiophene and (1,3,4-oxadiazolyl)pyridine moieties

A new series of conjugated copolymers (P1–P3) consisting of alternate 3,4-dialkoxythiophene and (1,3,4-oxadiazolyl)pyridine moieties have been synthesized using the precursor polyhydrazide route. They have been characterized by FTIR, 1H NMR spectral and elemental analyses. These copolymers possess well defined structure and exhibit good thermal stability with the onset decomposition temperature in nitrogen at around 300 °C. Their molecular weights were determined by gel permeation chromatography (GPC). The optical and charge-transporting properties of the copolymers were investigated by UV–visible spectroscopy, fluorescence emission spectroscopy and cyclic voltammetry. Their UV–visible absorption spectra showed a λmax at around 342 nm and displayed bluish-green fluorescence in solution state. The band gaps were found to be at about 2.55 eV for all the copolymers. The third-order nonlinear optical properties (NLO) of these copolymers were studied by Z-scan technique. The measurements were performed at 532 nm with 7 ns laser pulses using a Nd:YAG laser in solution form. The real part of χ(3) were estimated to be −0.881 × 10−12, −0.901 × 10−12 and −1.030 × 10−12 esu for P1, P2 and P3 respectively. The imaginary part of χ(3) for the copolymers P1, P2 and P3 were determined to be 0.192 × 10−12, 0.253 × 10−12 and 0.272 × 10−12 esu respectively. The copolymers exhibit strong reverse saturable absorption and good optical limiting behaviour at 532 nm.