Synthesis and Characterization of New Donor-Acceptor Type Conjugated Polymers for NLO Applications

Abstract

Over the last two decades, conjugated polymers have received significant, scientific and technological interest due to their broad range of applications. Among various conjugated polymers, donor-acceptor (D-A) type conjugated polymers are in the forefront of research efforts because of their easy processability, stability, and readiness to functionalize. The present research work was concentrated on design, synthesis and characterization of four new series (P1-P16) of D-A type conjugated polymers derived from 3,4-disubstituted thiophene. In the new polymers 3,4-dialkoxy/aryloxy thiophene and 1,3,4-oxadiazole were introduced as electron donor and electron acceptor segments respectively. Also, aromatic systems such as benzene, thiophene, pyridine, phenylenevinylene, EDOT, naphthalene and biphenyl groups were introduced as conjugated spacers in the polymer chain. It was expected that the new polymers would exhibit good electrochemical and optical properties. The new intermediates, monomers and four series of polymers P1-P16 were synthesized through multistep reactions. The structures of new intermediates and monomers were confirmed by FTIR, 1H NMR, 13C NMR spectral methods followed by elemental analyses. Further, the molecular structures of polymers were elucidated by FTIR, 1H NMR spectroscopy, GPC and elemental analysis. Their thermal stability was determined using TGA. The linear optical properties of polymers were investigated by UV-visible absorption and fluorescence spectroscopic studies. The electrochemical properties were determined using cyclic voltammetry (CV). Their bandgap was found to be in the range of 1.8-2.5 eV. Further, their nonlinear optical (NLO) properties were investigated by Z-scan and degenerate four wave mixing (DFWM) methods using 532 nm, 7 ns laser pulses. The effect of different substitution was discussed in detail with regard to electrochemical and optical properties. Polymers of series 1, 2 and 4 showed two-photon absorption (2PA) while polymers of series 3 displayed three-photon absorption (3PA). A further insight on the NLO behavior was provided by employing suitable theories and equations.