Influence of single-walled carbon nanotubes enriched in semiconducting and metallic tubes on the electropolymerization of tetrabromo ortho-xylene: Insights on the synthesis mechanism of poly(ortho-phenylenevinylene)

Highlights

• Spectroelectrochemical properties of poly(ortho-phenylenevinylene) (POPV) are shown.

• Covalently functionalization of S-SWNTs with POPV was proved by IR spectroscopy.

• The POPV chains length correspond to 3 and 7–10 repeating units.

• POPV luminescence quenching process is induced of SWNTs (14, 7), (12, 10) and (11, 9).

Abstract

New composite materials based on single-walled carbon nanotubes effectively separated into semiconducting (S-SWNTs) and metallic tubes (M-SWNTs) and poly(ortho-phenylenevinylene) (POPV) were synthesized by the electrochemical polymerization of the α,α,α′,α′-tetrabromo-o-xylene monomer onto the surface of Au supports covered with carbon nanotubes films. Using FTIR spectroscopy, an increase of the quinoid structure weight is highlighted when a charge transfer is induced to take place between the two constituents of the POPV/S-SWNT composite material. In addition, a down-shift of the Raman line belonging to POPV (from 530 cm⁻¹ to 515 cm⁻¹), which was accompanied by an increase in relative peak intensity, is reported as a result of steric hindrance effects induced by the covalent bonding of POPV onto the S-SWNT surface. The non-covalent functionalization of M-SWNTs with POPV is demonstrated to occur by Raman scattering and FTIR spectroscopy. The photoluminescence (PL) spectrum of POPV exhibits two bands at 2.82 and 2.25 eV, which were assigned to the electronic emission transitions of macromolecular chains having 3 and 7–10 repeating units, respectively. We demonstrate that POPV photoluminescence quenching in the presence of M + S-SWNTs, M-SWNTs and S-SWNTs is achieved by tubes with chirality (14, 7), (12, 10) and (11, 9), respectively.

Introduction

Poly(ortho-phenylenevinylene) (POPV) is a conjugated polymer (CP) that has been studied very little to date and is akin to poly(para-phenylenevinylene) (PPV). The first article that focused on POPV is dated 2001, and it was demonstrated that this polymer can be synthesized by the electrochemical polymerization of the α,α,α′,α′-tetrabromo-o-xylene monomer (TBOX), by using cyclic voltammetry [1]. Despite the progress that has been reported concerning the optical properties of PPV [2], in the case of POPV, the only information concerning the vibrational modes active in IR spectroscopy has been reported by Peres et al. [1]. To anticipate the potential applications of this CP, new optical properties of POPV, highlighted by Raman scattering, Fourier transform infrared (FTIR) and UV-VIS absorption spectroscopies and photoluminescence (PL), are reported in this work.

One application often reported in the case of PPV is that found in the composite materials field. A broad range of composites were synthesized in the last two decades, and a few examples worthy of mention are those based on PPV and single-walled carbon nanotubes (SWNTs) [3], graphene [4], fullerene [5], TiO₂ [6], ZnO [7], ZnS [8], and so on. The main optical properties of the PPV/SWNT composites that have been reported until now, regard: (i) The PL quenching effect of PPV induced by the addition of SWNTs, when the composites were synthesized by the annealing conversion method of a PPV precursor solution and by the electrochemical polymerization of the α,α,α′,α′-tetrabromo-p-xylene monomer [9], [10]. Recently, it was demonstrated that the PPV PL quenching effect was due to metallic nanotubes, a fact explained on the basis of the de-excitation pathways found in a diagram of the energy levels of the two constituents [11]. (ii) The non-covalent functionalization of carbon nanotubes with PPV molecules, which was evidenced by the decrease of the SWNTs’ radial breathing mode (RBM) intensity and accompanied by a decrease in the strength of an abnormal anti-Stokes Raman emission [11]. (iii) The infrared dichroism of films of PPV and its composites deposited on Ag and Au supports when a change of the orientation angle of the transition dipole moment vector for the IR absorption band at 835 cm⁻¹ was reported as a result of the π-π∗ interaction between the phenyl group of PPV and the sidewall of the carbon nanotubes [10]. (iv) An anti-Stokes PL, with an efficiency that increases, when the composites were deposited onto a rough Au surface [11]. (v) A decrease in the formation of PPV macromolecular chains (MCs) with lengths of 7–10 repeating units (RUs), which has been reported in both the case of the PPV non-covalently functionalized SWNTs and the PPV covalently functionalized SWNTs [10], [11]. As shown in Refs. [10], [11], the two composites were prepared by the annealing conversion method of a PPV precursor solution and the electrochemical polymerization of the α,α,α′,α′-tetrabromo-p-xylene monomer in the presence of SWNTs. To the best of our knowledge, no further paper has been published concerning the vibrational properties and photoluminescence of composites based on POPV and SWNTs, as either a mixture of semiconducting (66%) and metallic (33%) tubes ((M + S)-SWNTs) or as SWNTs effectively separated in semiconducting (S-SWNTs, 99%) or metallic (M-SWNTs, 98%) tubes. Several targets considered to be achieved in this manuscript are: (i) the establishment of the chemical mechanism that describes the electrochemical polymerization of TBOX in the presence of SWNTs and the presentation of experimental evidence for the non-covalent or covalent functionalization of SWNTs with POPV; (ii) an understanding of the charge transfer processes at the interface of the two constituents of the POPV/SWNTs composites; and (iii) evidencing the role played by the semiconducting and metallic SWNTs with respect to the POPV PL properties. The knowledge of the PL and photoconductivity properties of the POPV/M-SWNTs and POPV/S-SWNTs composites opens new opportunities for using these compounds as emissive layers in various light-emitting devices. Taking into account the studies, reported by Z. Wang et al., on carbon nanotubes non-covalent functionalized with conjugated polymers which have demonstrated an improve of non-linear optical response [12], the use of M-SWNTs non-covalent functionalized with POPV in the ultrafast optical switch devices field is anticipated in the next period. The detailed knowledge of electrochemical properties of the POPV/M-SWNTs and POPV/S-SWNTs composites will enable the development of new applications in the field of electrochemical sensors and energy storage (as active materials for electrodes of Li rechargeable batteries and supercapacitors).