Contrary to the well-known aggregation caused quenching of chromophore, there are certain materials show no or weak emission at solution, but strong emission when aggregated, e.g. in solid films. The phenomenon is called aggregation induced emission (AIE).
It was first discovered by Prof. Ben Zhong Tang, and intrigued huge research interests afterward. The mechanism is so called “restriction of intramolecular motion” (Reference 1, 2). Once chromophores are excited to form excitons by protons or electrical fields, there are competing channels for excitons to relax to ground state: radiative emission (e.g. photoluminescence), non-radiative decay which also includes several processes. Therefore in AIE, scenario, since chromophores have large freedom to move in solutions the excited chromophore transfer its energy to the vibration/rotation motions through this non-radiative emission mechanism, instead of radiative emission. However in solid state (or aggregated state) the vibration/rotation is hindered; thus radiative emission becomes a dominate mechanism (as shown in the following figure) (Reference 3, 4).
