Clusterization-triggered emission: Uncommon luminescence from common materials

pi-Conjugated chromophores have been investigated for many years and successful theoretical models have been developed to explain their photophysical properties. However, materials have appeared sporadically that do not fit within these existing models. Some of these materials possess entirely nonconjugated structures based on saturated C-C, C-O or C-N bonds, but their aggregates or solid-state forms show bright visible emission. This phenomenon is termed as clusterization-triggered emission (CTE) and the materials possessing the property are labeled clusteroluminogens. In this review, we provide a brief summary of the recent development of clusteroluminogens. The materials are classified into three categories: polymers (natural and synthetic polymers), small molecules (with and without aromatic rings) and metal clusters. Possible luminescence mechanisms underpinning the different categories of clusteroluminogens are analyzed individually. Finally, we put forward a comprehensive theory of the through-space conjugation (TSC) for these chromophores. Based on the CTE effect and TSC theory, various applications have been envisioned, for example in the areas of process monitoring, structural visualization, sensors, and probes. It is anticipated that this new research direction will bring many breakthroughs, not only in the theoretical areas, but also in these advanced applications of light-emitting materials.