Reexamining the heavy-atom-effect: The universal heavy-atom-induced fluorescence enhancement principle for through-space conjugated AIEgens

Classic heavy-atom effect was reexamined from a completely fresh perspective of emerging AIEgens, and a universal heavy-atom-induced fluorescence enhancement principle suited for most AIEgens was proposed, which prompts the redefinition of heavy-atom effect depending on distinct conjugation patterns and also provides an effective strategy to achieve highly efficient AIEgens. • Photophysics of heavy-atom-substituted AIEgens disobeys classic heavy-atom effect. • Heavy-atom-induced fluorescence enhancement principle for AIEgens is revealed. • A novel and simple strategy is proposed to make AIEgens brighter significantly. • Bright AIEgens (nearly 100%) can be designed by introducing suitable heavy atoms. • Heavy-atom-induced restriction of intramolecular vibrations is well researched. The heavy-atom effect is a classic photophysical principle that dominates heavy-atom-induced fluorescence quenching and phosphorescence enhancement and has been universally suited for conventional luminogens with through-bond conjugation; however, its universality to emerging AIEgens (i.e., luminogens exhibiting aggregation-induced emission) with through-space conjugation is questionable. Herein, a series of tetraphenylethylene (TPE)-based AIEgens containing diverse non-metallic elements were designed by introducing each non-metallic element into the typical TPE skeleton, and their abnormal heavy-atom-induced fluorescence enhancement behaviors were examined against the classic heavy-atom effect. The underlying nature of heavy-atom-induced fluorescence enhancement was experimentally and theoretically explored. It was found that both insignificant heavy-atom-induced intersystem crossing processes (owing to the intrinsic through-space conjugation feature) and significant heavy-atom-induced restriction of the intramolecular vibrations cooperatively contribute to such an abnormal fluorescence enhancement. In addition, this principle was found to be universally applicable to TPE-based AIEgens containing other heavy atoms, which validates the universality of the heavy-atom-induced fluorescence enhancement principle for typical AIEgens with through-bond and through-space conjugation. In summary, this study re-examined the classic heavy-atom effect from a completely fresh perspective of emerging AIEgens and proposed a universal heavy-atom-induced fluorescence enhancement principle suited for most AIEgens.