Tuning the backbones and side chains of cationic meta-linked poly(phenylene ethynylene)s: Different conformational modes, tunable light emission, and helical wrapping of multi-walled carbon nanotubes

Abstract We have an ongoing interest in the design and control of helical conformation of water-soluble meta-linked poly(phenylene ethynylene)s (PPEs) and the further development of novel functional materials. Four cationic meta-linked PPEs (P1′-P4′) were synthesized to study the influence of differences in the backbone and side chain structure on their conformations. For P1′ and P4′ without side chains on the para -phenylene units, fluorescence spectroscopic investigations indicated obvious intramolecular helical folding, whereas for P2′ and P3′ with nonpolar and polar side chains on the para -phenylene units respectively, a significantly different conformational mode, namely, cofacial intermolecular aggregation was suggested. The side chains on the para -phenylene units of P2′ and P3′ may be located in the interior cavity of helix and induce steric effect on the formation of helix. However, the introduction of 2,1,3-benzothiadiazole (BT), a low energy gap unit, into the backbone of P4′ showed little influence on the formation of helix despite its larger and more rigid structure than the phenylene unit. Thus, the light emission of these polymers can be tuned in the range from blue, green to yellow with the changes of conformational modes. Moreover, the functionalization of multi-walled carbon nanotubes (MWCNTs) by P4′ in methanol and water, and by using its neutral precursory polymer P4 in tetrahydrofuran was explored through transmission electron microscopy and fluorescence spectroscopy. P4′ was directly observed to individualize MWCNT by forming a monolayer helical wrapping on the nanotube surface, which may be attributed to the backbone flexibility of meta-linked PPE and the strong π−π interactions between the PPE backbone and the CNT surface. Moreover, P4′ served as a better dispersing agent for MWCNT than P4, suggesting that the cationic side groups may act as solubilizing groups which also separated the individual nanotubes because of charge repulsion.