ABSTRACT
A new discrete supramolecular metallacycle functionalized with an alkynylplatinum(II) bzimpy moiety was successfully prepared via coordination-driven self-assembly, and it displayed a reversible color change in the solid state between yellow and red, triggered by CH2Cl2 vapor or mechanical grinding. Notably, unlike many known vapochromic systems, the obtained vapochromic metallacycle exhibits ultra-stability, with the red color remaining unchanged in air for several months at room temperature or even under vacuum for >1 week. Further investigation revealed that the chair conformation of the metallacyclic scaffold, which was thought to prevent intermolecular steric repulsion between the alkyl chain and triethylphosphine, favored close molecular stacking through intermolecular Pt···Pt and π-π stacking interactions, thus allowing such vapochromic behavior with ultra-stability.
ABSTRACT
In recent past years, investigation of hierarchical self-assembly for constructing artificial functional materials has attracted considerable attention. Discrete metallacycles based on coordination bonds have proven to be valid scaffolds to fabricate various supramolecular polymers or smart soft matter through hierarchical self-assembly. Here, we present the first example of the hierarchical self-assembly of discrete metallacycles by taking advantage of the positive charges of the organoplatinum(II) metallacycle skeleton through multiple electrostatic interactions. Heparin, a sulfated glycosaminoglycan polymer that has been widely used as an anticoagulant drug, was selected to induce hierarchical self-assembly because of the existence of multiple negative charges. To investigate the hierarchical self-assembly process, an aggregation-induced emission (AIE) active moiety, tetra-phenylethylene (TPE), was introduced onto the metallacycle via coordination-driven self-assembly. Photophysical studies revealed that the addition of heparin to the tris-TPE metallacycles solution resulted in dramatic fluorescence enhancement, which supported the aggregation between metallacycle and heparin driven by multiple electrostatic interactions. Moreover, the entangled pearl-necklace networks were obtained through hierarchical self-assembly as detected by SEM, TEM, and LSCM experiments. In particular, single bead-like chains were observed in the AFM and TEM images, which provided direct, visual evidence for the aggregation of positively charged metallacycles and negatively charged heparin. More interestingly, further optical study demonstrated that this TPE-decorated metallacycle could function as a turn-on fluorescent probe for heparin detection with high sensitivity and selectivity. Thus, this research presents the first example of counter polyanion-induced hierarchical self-assembly of discrete metallacycles and provides a "proof-of-principle" method for heparin sensing and binding.