Third-order nonlinear optical susceptibilities of new copolymers containing alternate 3,4-dialkoxythiophene and (1,3,4-oxadiazolyl)pyridine moieties Pramod Kumar Hegde a , A. Vasudeva Adhikari a, * , M.G. Manjunatha a , P. Poornesh b , G. Umesh b a Organic Chemistry Division, Department of Chemistry, National Institute of Technology Karnataka, Surathkal, Mangalore-575025, India b Department of Physics, National Institute of Technology Karnataka, Surathkal, Mangalore-575025, India article info Article history: Received 30 August 2008 Received in revised form 1 November 2008 Accepted 19 November 2008 Available online 8 January 2009 PACS: 42.70.J 42.70.M 42.70.N Keywords: Conjugative polymers 3,4-Dialkoxythiophene 1,3,4-Oxadiazole Pyridine NLO properties Z-scan abstract 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, 1 H NMR spectral and elemental analyses. These copolymers possess well defined structure and exhibit good thermal stability with the onset decomposition temperature in nitro- gen 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 k max 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 per- formed at 532 nm with 7 ns laser pulses using a Nd:YAG laser in solution form. The real part of v (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 v (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. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Conjugative polymers have been the materials of interest for many researchers in the field of photonics and optoelectronics in the past several years due to their large third-order nonlinear re- sponse and related device application [1–3]. A strong delocalization of p-electrons along the polymer chain is known to be the reason for their third-order nonlinearity. These polymers offer good flexibil- ity at both the molecular and bulk levels for the structural modifica- tions that are necessary to optimize them for the practical use [4]. Various conjugated polymers, such as polyacetylene, polydiacety- lene, and poly(p-phenylene vinylene), have been studied extensively, and large values of third-order nonlinear optical susceptibility v (3) have been measured [4,5]. However, the application of these may face the problems in the areas of processing or environmental stability; as a result a number of other conjugated polymers have also been inves- tigated for their third-order NLO properties. Amongst the various polymers studied, polythiophenes are gaining considerable interest as materials for nonlinear optics be- cause of their large third-order nonlinear response, chemical sta- bility, and processability with suitable substituents [6,7]. Also, good film forming properties, solubility, and adequate mechanical properties made them better choice for device fabrication in com- parison with their inorganic counterparts. The good NLO response of these polymers is primarily determined by their molecular structure; hence one can employ molecular modelling to design new structures by introducing suitable substituents. As described by Fuks-Janczarek et al. [8], dipole moment of polythiophene plays an important role in the behavior of their observed UV–vis spectra. Based on theoretical molecular geometry and quantum chemical calculations, they observed that the first absorption maxima shows red shift during changing the number of dimers from n = 1 to 15. The shift of the absorption spectra may reflect a specific geometry of the thiophene group and hence affect the optical linearity and nonlinear susceptibilities. By considering all these aspects, few copolymers derived from thiophenes were synthesized and studied for third NLO properties [9–11]. According to authors, donor and acceptor groups along the polymer backbone would be a promising molecular design for enhancing the third-order NLO properties. In this context, it has been planned to incorporate additional electron withdrawing moiety in between 3,4-dialkoxy substituted thiophenyl oxadiazole systems in our synthetic design of new 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.11.015 * Corresponding author. Tel.: +91 8242474046; fax: +91 8242474033. E-mail addresses: avchem@nitk.ac.in, avadhikari123@yahoo.co.in (A.V. Adhi- kari). Optical Materials 31 (2009) 1000–1006 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat