Building Conceptionally New Clusteroluminescent Materials via Clusterization of Nonemissive Molecules

Luminescence, as a natural gift, plays an essential role in our colorful world. So far, it is believed that the development of highly efficient fluorophores relies on covalently electronic conjugation. The traditional photophysical mechanism based on through-bond conjugation seems to be perfect in explaining the luminescence of most conjugated molecules. However, some common materials without any aromatic conjugation, such as peptides, polysaccharides, and some artificial polymers, can emit visible and bright emission. Although this phenomenon has been observed for a long time, it has not attracted much attention as the underlying mechanisms were still unclear, and these materials were hard to utilize as luminescent materials due to their low efficiency. Recent works illustrate that the single-dispersed solutions of these (macro)molecules do not emit light under UV excitation, but their aggregates can emit bright visible light. This kind of unconventional emission is named "clusteroluminescence (CL)" and the luminogens with CL characteristics are termed "clusteroluminogens (CLgens)". Further experiments and theoretical calculations suggest that the through-space electronic interactions between the lone pairs of heteroatoms would form rigid cluster structures in the aggregate state, which ultimately emit light as a chromophore. Several approaches have been developed to manipulate the performance of CLgens with controllable CL. In terms of application, CLgens always show excellent biocompatibility, which may find applications as bioimaging or tracking probes. Meanwhile, as the inter- or intra-molecular distance is sensitive to mechanical force, smart mechanically responsive materials can be developed.