Synthesis and characterization of [60]fullerene-based nonlinear optical polyacrylonitrile derivatives

Abstract

Nonlinear Optical (NLO) materials have gained increasing interest, particularly in the past decades, for their realized and potential applications in many fields of photonics, optical communication and laser technology. Along with metallophthalocyanines and mixed metal clusters [1], C60 and other fullerenes have emerged as promising candidate materials for the nonlinear optical applications owing to their intrinsic NLO properties which primarily originate from their largeπ electron conjugations and strong electron affinity [2]. However the ability to fabricate NLO devices based on C60 has been limited due to its poor processability and multiaddition of organic group [3]. Therefore, the main key to the development of C60-based NLO materials is to synthesize highly soluble C60 derivatives with excellent NLO properties under very mild conditions. Very recently, studies on the optical limiting (OL) property (one of the NLO responses) of simple blends of [60]fullerene and polymer (i.e., [60]fullerene-doped polymer) have grown rapidly. Although doping of C60 into polymer matrices may bring about enhancement of nonlinear response, there are, however, problems associated with this approach. These problems stem from the tendency for the dopant molecules to aggregate in the solid state, leading to difficulty in achieving homogeneous dispersions and ultimately phase separation at high loadings [4]. Polymeric modification of fullerene can overcome these problems. So far, only a few articles reported such work concerning C60 polymeric derivatives [5, 6]. For this reason, we synthesized the highly soluble [60]fullerene-based polyacrylonitrile (PAN) derivatives with different C60 contents using a previously reported one-pot experiment procedure [7, 8], and described initial results of an investigation into their third-order nonlinear optical response by optical-heterdyne-detected optical-Kerr-effect (OHDOKE) technique. The parent polymer PAN and the soluble C60acrylonitrile (C60-AN) copolymers with C60 wt % of ∼12% and ∼18% were synthesized as shown in Scheme 1 under a purified argon atmosphere. The resultant polymers were redissolved in N,N-dimethylformamide (DMF), filtered to remove any unreacted C60 and trace of cross-linking product (if any), and reprecipitated with methanol to give the powder product (the purification procedure was repeated twice). Its color is closely related to the fullerene content in the copolymer: the higher the content, the deeper the product’s color. These copolymer structures were characterized by a variety of physical methods, including ultraviolet-visible (UV-Vis) absorption, electron spin resonance (ESR), differential scanning calorimeter analysis (DSC), Fourier transform infra-red (FT-IR), scanning electron microscope (SEM) and X-ray powder diffraction (XRD). The UV-Vis absorption spectra in DMF of C60-AN copolymers differ greatly from that of pure PAN. For the former, its spectrum with only one sharp absorption peak at 295 nm(s) keeps essentially transparent at wavelengths longer than 400 nm, whereas in the copolymer with high C60 content, except for the peak at 295 nm, two new peaks at 257 nm (m, shoulder) and 264 nm(s), which are ascribed to the characteristic absorption for the covalent attachment of C60 to the PAN backbone, were observed. In contrast to the light-yellow exhibited by the pure PAN, the color of copolymers gradually changes from brown to black as the C60 content increases. Obviously, the covalent attachment of C60 to the PAN chains leads to the enhancement of the absorption degree at longer wavelengths. This implies that there should be somewhat electronic interaction between C60 and PAN chains. As discussed for the UV-Vis absorption spectra, there exists considerable difference in electronic structure