Unorthodox luminescent polymers without large π-conjugated structures, as promising soft luminescent materials, have recently received considerable attention owing to the availability of versatile preparation methods and their good processability. These polymers generally possess electron-rich moieties, with a diversity of intermolecular and intramolecular interactions through delocalized electrons. Therefore, polymerization itself can be an effective way to realize fluorescence emission. Here, polymerization-induced emission (PIE) is conceptualized as the process of transforming non-luminescent small molecules (i.e., monomers) into luminescent polymers. Correspondingly, PIE luminogens (PIEgens) with luminescent clusters, which are proposed as group aggregates with the minimum critical size for emission, require further investigation in order to understand the structure–emission relationships and related mechanisms. Promisingly, advanced polymerization methods offer unlimited opportunities to develop new soft luminescent polymers owing to the multi-level structures of polymers.
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