Aggregation‐induced emission materials for nonlinear optics

Abstract

Aggregation‐induced emission (AIE) is a vital photophysical phenomenon that the luminogens in the concentrated or aggregated cases will engender the dramatically boosted emission in comparison with the dispersive states. Given this extraordinary emitting capacity exactly resolves the aggregation‐caused quenching (ACQ) situations residing in the traditional luminophores, the booming AIE luminogens have drawn tremendous interest owing to their advanced performances and colossal potential applications in various areas. Further exploitations of AIE molecules also drive the research interests in the midst of these AIE materials toward the nonlinear optical (NLO) regime. The combination of AIE and NLO effects have nurtured some unforeseen properties of AIE materials and extended their application spheres. Therefore, some NLO‐active AIE materials have been wielded in many crucial applications, for example, optical limiting, laser, bioimaging, and photodynamic therapy. Meanwhile, the impacts of aggregate on the NLO effect also deserve deep considerations and pursuits, and the modifications of aggregates promise an easy, efficient, and prompt avenue to tune the NLO properties of materials. The recent achievements and progress in the NLO properties of AIE materials have been summarized in this review. The second‐order and third‐order NLOs of the AIE materials have been introduced and their correlative applications have been discussed.

Aggregation-induced barrier to oxygen—a new AIE mechanism for metal clusters with phosphorescence

“…Aggregation-induced emission (AIE), a term coined by B.Z. Tang, has been used to explain various issues in which luminogens are dark in the aggregated state [ 9–11 ]. The ongoing development of luminescent materials has also promoted the elucidation of AIE mechanisms, such as restriction of intramolecular rotation (RIR), restriction of intramolecular vibration (RIV), excited-state intramolecular proton transfer (ESIPT) and clusterization-triggered emission (CTE) [ 12–16 ].…”

Two-Photon Absorption: An Open Door to the NIR-II Biological Window?

“…in this organised aggregated state causes a strong decrease in the probability of non-radiative decay, and therefore a strong increase in fluorescence (Figure 4A, left). The concept of "aggregation-induced emission" (AIE) was first described in 2001 by Tang and co-workers (Luo et al, 2001), and has since become a popular approach to the design of 1P and 2P theranostic agents (Zhu et al, 2018;Yang et al, 2020b;Lu et al, 2020;Han et al, 2021) and circumvent the limitations of standard NIR-dye design strategies. In contrast to traditional organic fluorophores, AIE luminogens (AIEgens) typically exhibit low fluorescence in dilute solutions, but both high Φ f and photostability in the aggregated state, which are key requirements for high-resolution imaging.…”

Aggregate Materials beyond AIEgens

“…Fox example, single peptide molecules usually lack of physiological function, while their assembled aggregates with hierarchical structures can generate specific physiological functions . Similarly, luminophores with planar structures generally show bright emission in the monodispersed state, while the emission of their aggregates is mostly weakened or even quenched, which has been known as aggregation-caused quenching (ACQ) (Figure a). − More interestingly, some propeller-shaped molecules show nonemission in their dilute solutions (single molecular state) but much enhanced emission when aggregated, which has been known as AIE (Figure b). , These phenomena generally suggest that in multibody systems the aggregation or assembling of molecules may change the characteristics of molecules either by losing their original functions (1 → 0) or even by endowing them with new properties or functions (0 → 1). Therefore, investigating the multibody system in the aggregate state, including structure, interaction, hierarchical structure, and morphology, and unveiling the influence of aggregation on molecular and aggregate properties and further attaining a deep understanding are essential for material design and development.…”