Molecular mechanism of aggregation‐induced emission

Abstract

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.

Optical Characteristics and Applications of AIE Racemic C6-Unsubstituted Tetrahydropyrimidines

“…Since the advantages of AIE fluorophores in wide application areas and the complex interactions in aggregates, the AIE mechanisms were deeply studied, and different mechanisms have been proposed (Chen et al, 2019;Peng and Shuai 2021;Rodrigues and Melo 2021), for example, restriction of intramolecular rotation (RIR) (Chen et al, 2003;Zeng et al, 2007), restriction of intermolecular motion (RIM) (Mei et al, 2015), minimum energy conical intersection (MECI) (Sasaki et al, 2016), suppression of Kasha's rule (Qian et al, 2017), excited-state double-bond torsion , etc. Although some debate still exists, restriction of intramolecular motions (RIM) has been generally accepted as central to the AIE working mechanism (Chen et al, 2019).…”

Progress on Exploring the Luminescent Properties of Organic Molecular Aggregates by Multiscale Modeling

“…Li and Blancafort et al put forward the restricted access to a conical interaction (RACI) mechanism based on the potential energy surface analysis ( Peng et al, 2016 ; Crespo-Otero et al, 2019 ). Shuai’s group also proposed the blockage of non-radiative decay via the minimum energy crossing point (MECP) away from the harmonic region in aggregates ( Ou et al, 2020 ; Peng and Shuai, 2021 ). In addition, other scenarios have also been declared, including excited-state intramolecular proton-transfer (ESIPT)–inspired solid state emitters ( Padalkar and Seki, 2016 ; Zhao J. et al, 2019 ), the restriction of the E/Z isomerization mechanism ( Chung et al, 2013 ), the blockage of access to the dark state with n → π* or σ→ π* in the aggregation phase ( Ma et al, 2016 ; Tu et al, 2019 ; Peng et al, 2021 ; Tu et al, 2021 ), halogen bonding interactions–induced effective phosphorescence ( Cai et al, 2018 ; Yang et al, 2020 ), the energy transfer–facilitated room temperature phosphorescence in a trace amount guest-doped host-matrix system ( Lei et al, 2020 ; Lei et al, 2021 ; Wang et al, 2021 ), and so on.…”

More is different: activating luminescence in aggregate via restriction of inter-molecular motion

“…As revealed by supramolecular chemistry and photoluminescence (PL) principles, [3] the molecular motions are capable of influencing the structures and the properties of luminogens, for example, aggregation-induced emission (AIE). [4] Usually, owing to free intra-molecular motion in the isolated state, luminogens with AIE characteristic (AIEgens) often show weak or no emission. On contrary, the highly emissive luminescence could be obtained in aggregates because of restriction of intramolecular motion (RIM) for single-component luminogens (Figures S1 and S2, Supporting Information).…”