Synthesis of Fluorescent Conjugated Polymers for the Applications of Surface-Modified Surfactant and Chemosensor Materials

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A novel light-emitting monomer M1 and its side-chain polymer P1 containing three conjugated aromatic pendants, including one pyridyl terminus, were successfully synthesized via Wittig and Pd-catalyzed Heck coupling reactions. The fluorescence of polymer P1 was efficiently quenched upon addition of different metal ions due to the facile energy transfers from the pendent groups of polymer P1 to specific metal ions. Among these metal ions, P1 exhibited extraordinary sensory selectivities for Ni2+ and Cu2+ ions over the other metal ions due to the stronger binding capabilities of Ni2+ and Cu2+ ions with polymer P1. From the time-resolved fluorescence (TRF) signals in photoluminescence spectra, the emergences of τ1 decay components in polymer complexes (P1+Ni2+) and (P1+Cu2+) clearly indicated that their TRF traces consisted of two contributions, one from the complexes (τ1) and the other from free polymer P1 (τ2). Upon addition of Ni2+ and Cu2+ ions, polymer P1 showed faster decay time constants (τ1) of metal ion quenching on TRF signals (i.e., better quenching efficiencies on photoluminescence) than its monomer M1. Furthermore, the on-off-on fluorescent switching behavior by adding a tridentate ligand 1,1,4,7,7-pentamethyldiethylenetriamine (PMDTA) to the polymer complex (P1+Cu2+) for several successive cycles demonstrated a superior reusable chemosensor of P1 for further applications. Second, a series of fluorescent conjugated homopolymers (P1 and P4) and copolymers (P2 and P3) containing proton-acceptor monomer M1 (pyridyl receptor) and proton-donor monomer M2 (benzoic acid) were synthesized and developed to study their tunable chemosensor sensitivities for Ni2+ ions (via adjustable fluorescence energy transfers between M1 and M2 moieties with various degrees of H-bonded interactions). The amounts of available (i.e., non-H-bonded) pyridyl receptors can be adjusted by the copolymer design with different molar ratios of proton-acceptor (pyridyl) monomer M1 and proton-donor (benzoic acid) monomer M2, such as copolymers P2 and P3. The significant chleation interactions between Ni2+ ions and fluorescent homopolymers (P1 and P4) as well as copolymers (P2 and P3) can be distinguished by the distinct fluorescence quenching behaviors, which possess specific quenching constants (KSV) of P1 > P2> P3 > P4 according to their molar ratios of pyridyl receptors. However, a third component, i.e., cyclodextrin (CD), was introduced to cape benzoic acid moieties and thus to modify (i.e., reduce) the supramolecular (H-bonded) interactions between M1 and M2 moieties to different extents in copolymers P2 and P3. Accordingly, the amounts of available (i.e., non-H-bonded) pyridyl receptors in tunable chemosensor copolymers P2 and P3 can be regained due to the capture of benzoic acid (i.e., monomer M2) moieties by CDs, so the quenching constants (Ksv) of P2+CD, P3+CD, and blend(P1/P4)+CD were enlarged due to the release of H-bonded pyridyl receptors. Therefore, with the addition of CDs, the reductions of supramolecular (H-bonded) interactions between pyridyl receptor (proton-acceptor M1) and benzoic acid (proton-donor M2) moieties in copolymers promote fluorescence quenching efficiencies of Ni2+ ions in the fluorescence decay experiments, which facilitates the supramolecular side-chain copolymers as tunable chemosensor applications. Third, the first example of in situ synthesis of gold nanoparticles (AuNPs) from water-soluble conjugated-polyfluorene (NPF) containing pendent ammonium groups, where polymer NPF acted not only as aqueous surfactants but also as fluorescent nanoreactors, was demonstrated in this report. Well-dispersed nanocomposites (NPF-AuNPs) could be produced without any additional reducing agents. The photoluminescence (PL) emission intensities of nanocomposite (NPF-AuNPs) solutions were further quenched by increasing the concentrations of AuNPs, which could be explained by the energy transfer or electron transfer from the fluorescent polymer to the metallic nanoparticles. Due to the variations of protonated/deprotonated amine (i.e., ammonium/amine) groups on NPF after different pH treatments, the quenching extents of AuNPs in nanocomposite (NPF-AuNPs) solutions were directly affected by the adsorption and desorption behaviors of NPF on metallic surfaces. More aggregations of AuNPs were observed by TEM in the nanocomposite solution of pH 3, which were attributed to larger amounts of ammonium groups (changed from -NMe2 at pH 8.5 to -NMe2H+ at pH 3) on polymer side-chains adsorbed on the gold surfaces. In addition to PL and TEM experiments, distinct time-resolved fluorescence (TRF) signals of protonated/deprotonated NPF quenched by gold nanoparticles (AuNPs) under different pH conditions also confirmed similar interaction effects between NPF and AuNPs. Finally, two conjugated surfactants of monomer TF and polymer PTF with pendent thiol groups were synthesized for surface-modifications of gold nanorods GNRs. The gold nanorods CTAB-GNRs (without surface-modification) in aqueous solutions were also prepared, and the original CTAB surfactant could be replaced with TF and PTF due to their thiol-gold grafting reactions. Then, surface-modified gold nanorods TF-GNRs and PTF-GNRs in THF were obtained, respectively. Owing to consistent red-shifts in the absorption maxima of surface plasmon bands, both UV-visible spectra of TF-GNRs and PTF-GNRs provided the grafting evidence of TF and PTF to the surfaces of gold nanorods GNRs. Compared with TF-GNRs, the resulting PTF-GNRs show a better solubility and stability to against their aggregations of nanorods in THF due to the stronger longitudinal surface plasmon band. Moreover, both TEM and XPS analyses confirmed the existence of binding bonds between the sulfur groups of polymer PTF and the gold surface in surface-modified gold nanorods PTF-GNRs. Compared with polymer surfactant (PTF), the faster PL emission decay (with a shorter life-time of 0.42 ns) in surface-modified gold nanorods PTF-GNRs matches well with the PL quenching of surface-modified gold nanorods PTF-GNRs due to the energy transfer of polymer surfactant (PTF) to gold nanorods GNRs. Therefore, surface modifications of gold nanorods by grafting fluorene-based conjugated copolymers were verified by UV-visible, TEM, XPS, PL, life-time measurements in this study.