Clustering-Triggered Emission of Poly(N-hydroxysuccinimide Methacrylate)

Nonconventional luminogens without classic aromatic or conjugated structures are attracting increasing interests owing to their fundamental importance and promising applications in diverse areas. Many of them even exhibit unique aggregation-induced emission (ME) characteristics. The emission mechanism, however, remains under debate. Previously, we proposed the clustering-triggered emission (CTE) mechanism, namely the clustering of nonconventional chromophores and subsequent electron overlap to rationalize the emission behaviors of such luminogens. To further our understanding, herein, we designed and synthesized poly(N-hydroxysuccinimide methacrylate) (PNHSMA) without any aromatic structures, which was obtained by the radical polymerization of N-hydroxysuccinimide methacrylate (NHSMA) monomer in toluene at 65 degrees C utilizing azobisisobutyronitrile (AIBN) as initiator. And NHSMA was prepared through the elimination between N-hydroxysuccinimide (NHS) and methacryloyl chloride in the presence of triethylamine (Et3N). It is found that PNHSMA is virtually nonluminescent in dilute solutions (<= 0.4 mg.mL(-1)) even at 77 K, but gets emissive in concentrated solutions (e.g. 40 mg.mL(-1)) with photoluminescence (PL) maxima at 434 and 485 nm at room temperature. Moreover, its solid powders emit intense blue light with multiple PL peaks upon UV irradiation, indicating its AIE nature and the formation of varying emission species. Further PL measurement of PNHSMA in dimethylformide (DMF) and DMF/acetone (good solvent/nonsolvent) mixtures duly verifies its AIE feature. Meanwhile, NHSMA monomer shows similar emission behaviors to those of PNHSMA, demonstrating concentration enhanced emission and AIE characteristics. In light of above results, it is assumed that NHSMA and its polymeric counterpart PNHSMA may share the similar emission mechanism. Single crystal structure of NHSMA reveals the conjugation of imide group and 3D intermolecular interactions of C=O center dot center dot center dot C=O (n-pi, 3.072 angstrom), C=O center dot center dot center dot H-C (2.651, 2.642 angstrom) and C=O center dot center dot center dot C-H (3.099 angstrom). The emission of PNHSMA and NHSMA in concentrated solutions and solid states is thus ascribed to the clustering of imide and ester groups, which results in electronic interactions. The overlap of pi and lone pair (n) electrons among C=O, N and O units, together with effective intermolecular interactions in the solid powders extend the conjugation and rigidify the molecular conformations, thus leading to boosted emissions. Such CTE mechanism might be well extended to other nonconventional systems and should be inspiring for the rational design of novel luminogens.