Principles of Aggregation-Induced Emission: Design of Deactivation Pathways for Advanced AIEgens and Applications

Abstract

Twenty years ago, the concept of aggregation-induced emission (AIE) was proposed, and this unique luminescent property has attracted scientific interest ever since. However, AIE denominates only the phenomenon, while the details of its underlying guiding principles remain to be elucidated. This minireview discusses the basic principles of AIE based on our previous mechanistic study of the photophysical behavior of 9,10-bis(N,N-dialkylamino)anthracene (BDAA) and the corresponding mechanistic analysis by quantum chemical calculations. BDAA comprises an anthracene core and small electron donors, which allows the quantum chemical aspects of AIE to be discussed. The key factor for AIE is the control over the non-radiative decay (deactivation) pathway, which can be visualized by considering the conical intersection (CI) on a potential energy surface. Controlling the conical intersection (CI) on the potential energy surface enables the separate formation of fluorescent (CI:high) and non-fluorescent (CI:low) molecules [control of conical intersection accessibility (CCIA)]. The novelty and originality of AIE in the field of photochemistry lies in the creation of functionality by design and in the active control over deactivation pathways. Moreover, we provide a new design strategy for AIE luminogens (AIEgens) and discuss selected examples.

Keywords: aggregation-induced emission; bis(dialkylamino)anthracene; control of conical intersection accessibility.

Figure 1

Typical AIEgens and schematic illustration of the RIR mechanism.

Figure 2

Selected representative AIEgens and a new class of AIEgens: 9,10‐Bis(N,N‐dialkylamino)anthracenes (BDAA) and their archetypical example 9,10‐bis(piperidyl)anthracene (BPA) with its crystallographic structure.

Figure 3

Schematic illustration of the conical intersection accessibility in dilute solution (left) and aggregates (right).

Figure 4

Jablonski diagram of a typical organic fluorophore.

Figure 5

Photographs of the fluorescence of BPAs in the solid state and in solution.

Figure 6

AIE behavior of a) 1,4‐BPA and b) 9,10‐BPA measured in THF–water. Fluorescence quantum yield (Φ _fl) values of their aggregates (diameter: ca. 100 nm; water fraction=90 vol. %) are noted for comparison. Adapted from ref. 21 with permission from the Royal Society of Chemistry ©2015.

Figure 7

Stokes shifts of 1,4‐BPA and 9,10‐BPA in polycrystalline samples (solid lines) and in toluene solution (dashed lines). Diffuse‐reflectance spectra were measured for polycrystalline samples dispersed in NaBr. Adapted from ref. 21 with permission from the Royal Society of Chemistry ©2015.

Figure 8

Viscosity‐sensitive fluorescence of 9,10‐bis(N,N‐dimethylamino)anthracene. a) Viscosity‐dependent fluorescence spectra and b) the corresponding double‐logarithmic plots. Adapted from ref. 22 with permission from the American Chemical Society ©2016.

Figure 9

Schematic illustration of the PES of 9,10‐bis(N,N‐dimethylamino)anthracene obtained from DFT/TD‐DFT calculations at the B3LYP/6‐31+G(d) level of theory. The ground state adopts two stable conformations (S0MIN) that are interconverted through the inversion of its NR₃ tetrahedra. After photoexcitation, each Franck–Condon (FC) state relaxes through planarization of one NR₃, leading to the conformation shown as S1MIN1. This “umbrella motion” occurs also at the other NR₃ tetrahedron under concomitant tilting of these planar trigonal planes, which gives another stable conformation (S1MIN2). These structural relaxations decrease the S₀–S₁ transition energy to the extent comparable with the experimental Stokes shifts. Any MECI was not sampled in this calculation and is hence not shown here.

Figure 10

a) Natural orbital (occupation number=0.745) at the MECI calculated for benzene; b) S₁/S₀ MECI of 9,10‐bis(N,N‐dimethylamino)anthracene and its Lewis structure. The S₀/S₁ MECI search was performed at the CASSCF(10e,8o)/6‐31G(d) level of theory. Adapted from ref. 22 with permission from the American Chemical Society ©2016.

Figure 11

Design of dialkylamino‐substituted PAHs as new AIEgens.

Figure 12

Comparison of the fluorescence quantum yields of 1‐dimethylaminonaphthalene, 1,4‐bis(dimethylamino)naphthalene, 1‐dimethylamino‐2,3‐dimethylnaphthalene, and 1,4‐bis(dimethylamino)‐2,3‐dimethyl)naphthalene together with photographic images of their fluorescence in solution or in the polycrystalline state. Adapted from ref. 22 with permission from the American Chemical Society ©2016.

Figure 13

a) Molecular structure of N,N‐dimethylamine‐substituted pyrene derivatives; b) X‐ray crystallographic structure of 4,5‐Py; c) S₁/S₀ MECI structure calculated at the CASSCF(12e,9o)/6‐31G(d) level of theory.

Figure 14

Schematic illustration of a typical potential energy surface (PES). Left: Low conical intersection (CI) causes competition between fluorescence and internal conversion through a CI. Middle: If a CI is sufficiently high, fluorescence becomes dominant. Right: In some cases, an ISC to the triplet state occurs, where phosphorescence ensures.