Molecular mechanism of aggregation-induced emission

Deep understanding of the inherent luminescence mechanism is essential for the development of aggregation-induced emission(AIE)materials and applications.We first note that the intermolecular excitonic coupling is much weaker in strength than the intramolecular electron-vibration coupling for a majority of newly termed AIEgens,which leads to the emission peak position insensitive to excitonic coupling,hence the conventional excitonic model for J-aggregation cannot effectively explain their AIE phenomena.Then,using multiscale computational approach coupled with our self-developed thermal vibration correlation function rate formalism and transition-state theory,we quantitatively investigate the aggregation effect on both the radiative and the nonradiative decays of molecular excited states.For radiative decay processes,we propose that the lowest excited state could convert from a transition dipole-forbidden“dark”state to a dipole-allowed“bright”state upon aggregation.For the radiationless processes,we demonstrate the blockage of nonradiative decay via vibration relaxation(BNR-VR)in harmonic region or the removal of nonradiative decay via isomerization(RNR-ISO)or minimum energy crossing point(RNR-MECP)beyond harmonic region in a variety of AIE aggregates.Our theoretical work not only justifies a plethora of experimental results but also makes reliable predictions on molecular design and mechanism that can be experimentally verified.Looking forward,we believe this review will benefit the deep understanding about the universality of AIE phenomenon and further extending the scope of AIE systems with novel applications.