materials science, medical and biomedical science and shows promising applications in these fields. Noteworthy, as a general platform to study aggregate science, AIE keeps integrating with other research fields involving materials, biology, medicine, energy and environment, and injects new vitality into these fields. When more researchers are working on this field, more breakthroughs in both fundamental research and application are envisioned in the future.
This handbook is an essential reading for scientists and engineers who are designing optoelectronic materials and chemical/biomedical sensors. It is also a valuable reference book to academic researchers in materials science, physical and synthetic organic chemistry as well as physicists and biological chemists.
Preface to Volume 1: Fundamentals
Volume 1 surveys the breakthrough of aggregation‐induced emission (AIE) research area, focusing mainly on the fundamentals of various branched areas. In particularly, this volume presents the new properties that molecular ensembles bring to molecules and highlight the role of molecular aggregates in endowing or improving the performance of organic materials. The branches of AIE include crystallization‐induced emission (CIE), room temperature phosphorescence (RTP), aggregation‐induced delayed fluorescence (AIDF), anti‐Kasha transition (AKT), clusterization‐triggered emission (CTE), through space interaction (TSI), mechanoluminescence (ML), circularly polarized and others. We specifically focus on the new properties of materials endowed by molecular aggregates beyond the microscopic molecular level. We hope this volume will inspire more research into molecular ensembles at/beyond meso level and lead to the significant progresses in material science, biological science, etc.
Youhong Tang
Flinders University, Australia
Ben Zhong Tang
The Chinese University of Hong Kong, Shenzhen, China
1 The Mechanistic Understanding of the Importance of Molecular Motions to Aggregation‐induced Emission
Junkai Liu1 and Ben Zhong Tang1,2,3
1 Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction and Institute for Advanced Study, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
2 Shenzhen Institute of Aggregate Science and Technology, School of Science and Engineering, The Chinese University of Hong Kong, Shenzhen, 2001 Longxiang Boulevard, Longgang District, Shenzhen City, Guangdong 518172, China
3 State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, Center for Aggregation-Induced Emission, South China University of Technology, Guangzhou, China
1.1 Introduction
Molecular motions drive most natural processes, ranging from the formation and annihilation of astronomical objects to the metabolism of microbes. All kinds of intra‐ or intermolecular interactions induce molecular motions in specific forms, through which the energy is generated and transformed. Mechanistic understanding and manipulation of molecular motions can lead to the effective design of smart molecular systems to achieve programmed tasks, as well as dynamic control of multiple processes. Indeed, by employing the light as a stimulus, researchers have developed a variety of functional molecular machines (e.g. molecular motors, molecular switches, molecular shuttles, and supramolecular assemblies) to achieve effective control of catalysis activity and chirality transfer in various practical applications, in which the dynamic manipulation of diverse molecular motions is critical [1].
For organic luminescence processes, molecular motions also extensively affect the photophysical behaviors for organic luminophores and govern the excited‐state decay pathways, including vibrational relaxation, internal conversion, intersystem crossing, kinetic quenching, etc.[2]. Upon photoexcitation, the coupled nuclear and electronic motions will drive the excited molecules to evolve through radiative and nonradiative pathways [3]. However, traditional photophysical research usually focuses on the highly rigid and conjugated molecules and investigates the targets in the gas phase or solution, in which the importance of intramolecular motions to the luminescence is less considered, and the variation of molecular motions in the solid state is often ignored [4]. In 2001, the discovery and proposal of aggregation‐induced emission (AIE) triggered the research on molecular motions and photophysical studies in the aggregate or the solid state [5]. The luminogens with the AIE property (AIEgens) often show different scales of molecular motions in different phases. Vigorous intramolecular motions in the solution can nonradiatively dissipate excited‐state energy and always result in weak light emission, whereas such motions can be restricted in the solid state due to the environmental constraints so that the emission can be strongly enhanced [6].
The restriction of intramolecular motion (RIM) has been concluded as the most widely accepted and applicable mechanism for the AIE phenomenon through numerous experimental and theoretical exploration, and the strong electron‐vibration coupling (EVC) between the excited and the ground state has been revealed to be the quantum origin of the weak emission of AIEgens in the solution [3, 7]. Recently, the connotation of RIM has been further clarified with deeper mechanistic insights into various novel AIEgens. The nonadiabatic excited‐state molecular dynamics [8] and theoretical model based on multiple electronic states [9] have been employed to elucidate the dynamic evolution of excited states through molecular motions of tetraphenylethylene (TPE) derivatives and heteroatom‐containing AIEgens. Meanwhile, the aromaticity reversal in the excited state [10] and the anti‐Kasha emission from higher excited states [11] have provided another proof for the enhanced emission efficiency in the solid state for AIEgens. Apart from the critical effect on the nonradiative decay, molecular motions have also been verified to promote the electronic transition process [12, 13]. The excited‐state through‐space conjugation facilitated by excited‐state molecular motions has been proved to contribute to the enhanced visible light emission of a series of nonconjugated AIEgens [13].
In this chapter, we will take a journey of mechanistic studies for AIE from the general RIM to mechanisms developed recently and propose the perspective on the further exploration in the future.
1.2 Restriction of Intramolecular Motion
When we look into the AIE phenomenon, two essential questions arise: why do the AIEgens show none or weak emission in the solution? Why does the aggregation brighten the light emission of AIEgens? Enthusiastic efforts have been devoted to deciphering the AIE mechanism. Several possible mechanisms have been proposed, including intramolecular conformation planarization, J‐aggregate formation, E/Z isomerization, and excited‐state intramolecular proton transfer, but they are only applicable for specific molecular systems [6c]. A general working mechanism for AIE is highly desired for fundamental understanding and material designing.
Upon absorption of photons, the excited molecule will decay through radiative and nonradiative paths or the photochemical process [2]. Hence, AIEgens in the solution may mainly undergo nonradiative decay or photochemical reaction to dissipate the majority of energy. Once aggregation occurs, the nonradiative decay paths can be blocked, or the radiative paths can be facilitated so that the emission can be enhanced. As the structure determines the property, typical AIEgens own flexible structures, containing multiple rotor or vibrator substituents, like hexaphenylsilole (HPS) and TPE, which endow them with high flexibility and potential to consume energy through intensive intramolecular motion, whereas multiple intermolecular interactions exist in the aggregate state, which can serve as constraints to molecular motions detrimental to the emission
