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Fluorescent metallacycle-cored polymers via covalent linkage and their use as contrast agents for cell imaging

Significance Fluorescent polymers play an important role in bioimaging due to their improved brightness, inertness to microenvironment, and good biocompatibility. In this article, we used tetraphenylene (TPE) derivatives that give strong fluorescence emission in an aggregated state as fluorophores and synthesized fluorescent polymers via the covalent linkage of TPE-based rhomboidal Pt(II) metallacycles. Due to the integration of covalent linkage-induced aggregation of the monomers, the aggregation-induced emission character of TPE derivatives together with Pt(II)-based metal−ligand interactions, these polymers exhibit enhanced emission compared with their corresponding precursors, making them applicable as excellent cell imaging agents. Considering the potential anticancer activity of rhomboidal Pt(II) metallacycles, these polymers may serve as theranostic agents for both bioimaging and cancer therapy. The covalent linkage of supramolecular monomers provides a powerful strategy for constructing dynamic polymeric materials whose properties can be readily tuned either by the selection of monomers or the choice of functional linkers. In this strategy, the stabilities of the supramolecular monomers and the reactions used to link the monomers are crucial because such monomers are normally dynamic and can disassemble during the linking process, leading to mixture of products. Therefore, although noncovalent interactions have been widely introduced into metallacycle structures to prepare metallosupramolecular polymers, metallacycle-cored polymers linked by covalent bonds have been rarely reported. Herein, we used the mild, highly efficient amidation reaction between alkylamine and N-hydroxysuccinimide-activated carboxylic acid to link the pendent amino functional groups of a rhomboidal metallacycle 10 to give metallacycle-cored polymers P1 and P2, which further yielded nanoparticles at low concentration and transformed into network structures as the concentration increased. Moreover, these polymers exhibited enhanced emission and showed better quantum yields than metallacycle 10 in methanol and methanol/water (1/9, vol/vol) due to the aggregation-induced emission properties of a tetraphenylethene-based pyridyl donor, which serves as a precursor for metallacycle 10. The fluorescence properties of these polymers were further used in cell imaging, and they showed a significant enrichment in lung cells after i.v. injection. Considering the anticancer activity of rhomboidal Pt(II) metallacycles, this type of fluorescent metallacycle-cored polymers can have potential applications toward lung cancer treatment.