Functional Scaffolds from AIE Building Blocks

Abstract

The functional design of molecular materials is an enduring pursuit. In some cases, molecular ensembles can exhibit new properties and functions that are not found in their molecular species. This evolutionary process occurs primarily at the mesoscale aggregate level, which motivates the design of materials outside of the classical “structure-property correlation” box. By reviewing scaffolding materials with precisely organized aggregation-induced emission (AIE) chromophores, we investigate the interplay of these building blocks in the evolving luminescence and other functions. They can work synergistically and collectively at the molecular-length scale, leading to stronger luminescence behaviors, evolution of new emission modes, and integration of versatility for functional applications at the macroscale level. Thus, this may open new doors to advanced luminescent materials and beyond, even giving materials science new perspectives on structural to functional design. In materials science, the whole is typically greater than the sum of the parts. This principle has been spotlighted in advanced molecular materials, where the assembly of fundamental building blocks at the molecular scale can lead to a hierarchy of materials with enhanced properties, sometimes with new functions at larger scales. One exciting example is the phenomenon of aggregation-induced emission (AIE). In contrast to a single free molecule that exhibits no or weak luminescence, aggregation of AIE fluorogens can produce a plethora of solid-state luminescent materials. In this Review, we explore the synergistically collective behaviors of multiple AIE units in ordered arrays by investigating scaffolding materials ranging from discrete cage compounds to infinite crystalline frameworks. As shown, AIE building blocks endow the materials with intriguing luminescence properties: intensive and tunable emission, unprecedented activities in specific emission modes (such as room temperature phosphorescence, multiphoton-excitable luminescence, coherent harmonic generation, circularly polarized luminescence, and lasing), stimulus-responsive luminescence for chemosensing and bioimaging, etc. In contrast, conventional building blocks frequently suffer from faint or extinguished emission in these scaffolding materials. Furthermore, the self-reporting luminescence properties endowed by built-in AIE building blocks were integrated into essential functions such as matter adsorption and diffusion, light harvesting, and photocatalysis. In addition to their unique configurations, processability, and facile derivatization chemistries, AIE-active building blocks in scaffolds act as the origin of a new phase of matter. This work helps to motivate the exploration of “advisable aggregation effects” in emerging functions of material systems. In materials science, the whole is typically greater than the sum of the parts. This principle has been spotlighted in advanced molecular materials, where the assembly of fundamental building blocks at the molecular scale can lead to a hierarchy of materials with enhanced properties, sometimes with new functions at larger scales. One exciting example is the phenomenon of aggregation-induced emission (AIE). In contrast to a single free molecule that exhibits no or weak luminescence, aggregation of AIE fluorogens can produce a plethora of solid-state luminescent materials. In this Review, we explore the synergistically collective behaviors of multiple AIE units in ordered arrays by investigating scaffolding materials ranging from discrete cage compounds to infinite crystalline frameworks. As shown, AIE building blocks endow the materials with intriguing luminescence properties: intensive and tunable emission, unprecedented activities in specific emission modes (such as room temperature phosphorescence, multiphoton-excitable luminescence, coherent harmonic generation, circularly polarized luminescence, and lasing), stimulus-responsive luminescence for chemosensing and bioimaging, etc. In contrast, conventional building blocks frequently suffer from faint or extinguished emission in these scaffolding materials. Furthermore, the self-reporting luminescence properties endowed by built-in AIE building blocks were integrated into essential functions such as matter adsorption and diffusion, light harvesting, and photocatalysis. In addition to their unique configurations, processability, and facile derivatization chemistries, AIE-active building blocks in scaffolds act as the origin of a new phase of matter. This work helps to motivate the exploration of “advisable aggregation effects” in emerging functions of material systems. The immense diversity of macroscopic materials—hard, soft, optical, etc.—is made up of about a hundred distinct kinds of atoms and, at a larger scale, basic lattices or molecular units.1Crabtree G. Sarrao J. Alivisatos P. Barletta W. Bates F. Brown G. French R. Greene L. Hemminger J. Kastner M. et al.From quanta to the continuum: opportunities for mesoscale science.http://science.energy.gov/∼/media/bes/pdf/reports/files/OFMS_rpt.pdfDate: 2012Google Scholar Taking molecular materials as an example, significant effort has been devoted to understanding the physical properties of macroscopic materials from the basic molecular units and, with hope, to guiding the de novo design of functional materials. This research philosophy seems to be not always workable. In certain cases, molecular materials exhibit new properties and functions that are not founded in their molecular species.1Crabtree G. Sarrao J. Alivisatos P. Barletta W. Bates F. Brown G. French R. Greene L. Hemminger J. Kastner M. et al.From quanta to the continuum: opportunities for mesoscale science.http://science.energy.gov/∼/media/bes/pdf/reports/files/OFMS_rpt.pdfDate: 2012Google Scholar, 2Anderson P.W. More is different.Science. 1972; 177: 393-396Crossref PubMed Google Scholar, 3Zhao Z. Zhang H. Lam J.W.Y. Tang B.Z. Aggregation-induced emission: new vistas at the aggregate level.Angew. Chem. Int. Ed. 2020; 59: 9888-9907Crossref PubMed Scopus (25) Google Scholar, 4Zhang H. Zhao Z. Turley A.T. Wang L. McGonigal P.R. Tu Y. Li Y. Wang Z. Kwok R.T.K. Lam J.W.Y. Tang B.Z. Aggregate science: from structure to properties.Adv. Mater. 2020; https://doi.org/10.1002/adma.2001457Crossref Google Scholar For instance, many aromatic hydrocarbon molecules emit strongly in the monodispersed state but faintly, or are even quenched, as solid powders, known as the aggregation-caused quenching (ACQ) phenomenon. In contrast, other molecules with aggregation-induced emission (AIE) attributes, named as AIEgens, show faint or no emission in the monodispersed state but much enhanced emission in aggregate states, from nanoparticles to bulk powders.5Mei J. Hong Y. Lam J.W.Y. Qin A. Tang Y. Tang B.Z. Aggregation-induced emission: the whole is more brilliant than the parts.Adv. Mater. 2014; 26: 5429-5479Crossref PubMed Scopus (1691) Google Scholar, 6Mei J. Leung N.L. Kwok R.T.K. Lam J.W.Y. Tang B.Z. Aggregation-induced emission: together we shine, united we soar!.Chem. Rev. 2015; 115: 11718-117940Crossref PubMed Scopus (3254) Google Scholar, 7Feng G. Kwok R.T.K. Tang B.Z. Liu B. Functionality and versatility of aggregation-induced emission luminogens.Appl. Phys. Rev. 2017; 4: 021307-021348Crossref Scopus (77) Google Scholar The enormous differences separating single molecules and macroscale materials appear to be irreconcilable, in terms of their size scale, complexity, and operating principles. From a material science perspective, the basic units are connected by a sequence of mesoscale architectures and phenomena that form, step by step, a staircase reaching from molecules to macroscopic materials.1Crabtree G. Sarrao J. Alivisatos P. Barletta W. Bates F. Brown G. French R. Greene L. Hemminger J. Kastner M. et al.From quanta to the continuum: opportunities for mesoscale science.http://science.energy.gov/∼/media/bes/pdf/reports/files/OFMS_rpt.pdfDate: 2012Google Scholar This evolutionary process of molecular material function can be experimentally observed, theoretically understood, and ultimately controlled.1Crabtree G. Sarrao J. Alivisatos P. Barletta W. Bates F. Brown G. French R. Greene L. Hemminger J. Kastner M. et al.From quanta to the continuum: opportunities for mesoscale science.http://science.energy.gov/∼/media/bes/pdf/reports/files/OFMS_rpt.pdfDate: 2012Google Scholar One example is the phenomenon of AIE, which describes an intriguing luminescence emergence process. The AIE phenomenon has a long discovery history, dating back to at least the 1850s, with work done on several platinum cyanides by Sir George Gabriel Stokes. However, only recent decades have witnessed a burst of research interest and scientific progress in this field, particularly on organic luminescent materials.8Luo J. Xie Z. Lam J.W. Cheng L. Chen H. Qiu C. Kwok H.S. Zhan X. Liu Y. Zhu D. Tang B.Z. Aggregation-induced emission of 1-methyl-1,2,3,4,5-pentaphenylsilole.Chem. Commun. (Camb.). 2001; 18: 1740-1741Crossref Scopus (4030) Google Scholar The restriction of intramolecular motions (RIM) is now accepted as a general working principle: the active intramolecular motions of AIEgens in excited states, for example, rotational phenyl ring dynamics in propeller-shaped tetraphenylethylene (TPE) derivatives,9Shustova N.B. Ong T.C. Cozzolino A.F. Michaelis V.K. Griffin R.G. Dincă M. Phenyl ring dynamics in a tetraphenylethylene-bridged metal-organic framework: implications for the mechanism of aggregation-induced emission.J. Am. Chem. Soc. 2012; 134: 15061-15070Crossref PubMed Scopus (260) Google Scholar are efficiently restricted upon aggregation to reduce the non-radiative decay of excitons and thereby enhance the photoluminescence (PL). On basis of the RIM principle, the concept of AIE phenomena is not strictly based on the aggregated state for emission, if the motions can be attenuated by other means, including covalent reactions, coordination interactions, supramolecular interactions, physical adsorption, and rigidification effects by environmental factors.10Shustova N.B. McCarthy B.D. Dincă M. Turn-on fluorescence in tetraphenylethylene-based metal-organic frameworks: an alternative to aggregation-induced emission.J. Am. Chem. Soc. 2011; 133: 20126-20129Crossref PubMed Scopus (425) Google Scholar, 11Qian H. Cousins M.E. Horak E.H. Wakefield A. Liptak M.D. Aprahamian I. Suppression of Kasha's rule as a mechanism for fluorescent molecular rotors and aggregation-induced emission.Nat. Chem. 2017; 9: 83-87Crossref PubMed Scopus (0) Google Scholar, 12Wu T. Huang J. Yan Y. Self-assembly of aggregation-induced-emission molecules.Chem. Asian J. 2019; 14: 730-750Crossref PubMed Scopus (21) Google Scholar, 13Wang D. Tang B.Z. AIE luminogens for activity-based sensing.Acc. Chem. Res. 2019; 52: 2559Crossref PubMed Scopus (58) Google Scholar The AIE wonderland has thereby greatly expanded.5Mei J. Hong Y. Lam J.W.Y. Qin A. Tang Y. Tang B.Z. Aggregation-induced emission: the whole is more brilliant than the parts.Adv. Mater. 2014; 26: 5429-5479Crossref PubMed Scopus (1691) Google Scholar, 6Mei J. Leung N.L. Kwok R.T.K. Lam J.W.Y. Tang B.Z. Aggregation-induced emission: together we shine, united we soar!.Chem. Rev. 2015; 115: 11718-117940Crossref PubMed Scopus (3254) Google Scholar, 7Feng G. Kwok R.T.K. Tang B.Z. Liu B. Functionality and versatility of aggregation-induced emission luminogens.Appl. Phys. Rev. 2017; 4: 021307-021348Crossref Scopus (77) Google Scholar Throughout the years, the AIE phenomenon has served as a reliable design principle for the creation of novel luminescent materials, establishing applications in sensing, imaging, lighting, optoelectronic devices, etc. For AIE materials, the structure and luminescence properties at the single-molecule level and condensed-materials level can be extensively characterized; but their correlations are comparatively less understood at the aggregate level. Going forward, the understanding of mesoscale aggregates, especially their transition from molecules to macroscale materials, is critically important, as it might ultimately lead to fundamental concepts and innovative solutions for functional materials and application technologies.14Cranford S.W. Chin S.Y. Sun J. The “What” and “Why” of materials.Matter. 2019; 1: 1-3Abstract Full Text Full Text PDF Google Scholar In molecular science, a “scaffold” is best understood as a molecular core to which functional groups are attached.15Hu Y. Stumpfe D. Bajorath J. Computational exploration of molecular scaffolds in medicinal chemistry.J. Med. Chem. 2016; 59: 4062-4076Crossref PubMed Scopus (48) Google Scholar Likewise, in material science, the scaffold concept has been used to represent the core architectures featuring well-defined structures for functionalization, including dendrimers, gels, micelles, frameworks, etc.16Beuerle F. Gole B. Covalent organic frameworks and cage compounds: design and applications of polymeric and discrete organic scaffolds.Angew. Chem. Int. Ed. 2018; 57: 4850-4878Crossref PubMed Scopus (181) Google Scholar,17Gu Y. Zhao J. Johnson J.A. Polymer networks: from plastics and gels to porous frameworks.Angew. Chem. Int. Ed. 2020; 59: 5022-5049Crossref PubMed Scopus (8) Google Scholar Here, we use “scaffold” to describe the aggregation structures of precisely ordered building blocks. In fact, a wide variety of scaffolds have been constructed. These studies are prone to focusing on microscopic packing structures, instead of macroscopic properties and functions straightforwardly from the perspective of function design. Even some property-oriented measurements, for example, gas adsorption tests, are ultimately used for structural elucidation. In contrast, much less exploration has been devoted to modulation of aggregation structures to tune macroscale material properties. Owing to well-defined and designable architectures, scaffolding materials can be one of the best modeling entities for aggregation structures, to study the structure-property-functionality relationships beyond the molecular-length scale. This line of research is becoming popular with the rise of interest in functional materials and corresponding aggregate science.3Zhao Z. Zhang H. Lam J.W.Y. Tang B.Z. Aggregation-induced emission: new vistas at the aggregate level.Angew. Chem. Int. Ed. 2020; 59: 9888-9907Crossref PubMed Scopus (25) Google Scholar,4Zhang H. Zhao Z. Turley A.T. Wang L. McGonigal P.R. Tu Y. Li Y. Wang Z. Kwok R.T.K. Lam J.W.Y. Tang B.Z. Aggregate science: from structure to properties.Adv. Mater. 2020; https://doi.org/10.1002/adma.2001457Crossref Google Scholar AIE-active chromophores can be a unique class of molecular building blocks for functional scaffold design and construction. The potential advantages and functionalities are summarized in Figure 1. First, their unique molecular structures (Figure 2) provide great freedom and rich variety in structural design at the three-dimensional (3D) level. Because of the twisted non-planar conformations of AIE building blocks, these materials would enjoy excellent processability benefiting chemical synthesis, scaffold assembly, and device fabrication. The scaffolds themselves would also have inherent conformational flexibility, deriving from the active dynamic intramolecular motions of building blocks. Altogether, these structural features offer rich opportunities to modulate the packing mode, pore size, wall affinity, and surface areas of the scaffolds microscopically, through the de novo design. Second, the resulting scaffolds are envisioned with prominent photo-functions, since optical properties appear to be particularly predetermined by the character of individual subunits and by the interplay between them. The building blocks can preserve the intrinsic AIE activities so that the luminescence properties of materials can be modulated by manipulating intramolecular motions. The scaffolds with built-in luminogenic components offer an ideal platform for specific sensing and imaging. By making use of the spatial isolation of building blocks, these materials can display guest-dependent emission. Moreover, AIE building blocks, such as TPE derivatives, have potent axial chirality and can impose supramolecular asymmetry, leading to circularly polarized luminescence (CPL) activities. Third, the photoactivities can be merged with intrinsic cavities and other void space for lasing, catalysis, guest adsorption, and transport. By synchronizing the responses of arrayed units, the scaffolds can behave as intriguing intelligent materials in response to external stimuli.Figure 2Strategies to Incorporate AIE Building Blocks into ScaffoldsShow full caption(A) Preformed AIEgen as building block.(B) Formation of AIE building blocks from pro-AIEgens via chemical reactions. Reproduced with permission from Haldar et al.27Haldar S. Chakraborty D. Roy B. Banappanavar G. Rinku K. Mullangi D. Hazra P. Kabra D. Vaidhyanathan R. Anthracene-resorcinol derived covalent organic framework as flexible white light emitter.J. Am. Chem. Soc. 2018; 140: 13367-13374Crossref PubMed Scopus (53) Google Scholar Copyright 2018, American Chemical Society. Reproduced with permission from Jin et al.28Jin E. Li J. Geng K. Jiang Q. Xu H. Xu Q. Jiang D. Designed synthesis of stable light-emitting two-dimensional sp2 carbon-conjugated covalent organic frameworks.Nat. Commun. 2018; 9: 4143-4152Crossref PubMed Scopus (73) Google Scholar Copyright 2018, American Chemical Society.(C) Assembly of scaffold structures from pro-AIEgens showing clusterization-triggered emission. Reproduced with permission from Zhang et al.4Zhang H. Zhao Z. Turley A.T. Wang L. McGonigal P.R. Tu Y. Li Y. Wang Z. Kwok R.T.K. Lam J.W.Y. Tang B.Z. Aggregate science: from structure to properties.Adv. Mater. 2020; https://doi.org/10.1002/adma.2001457Crossref Google Scholar Copyright 2020, WILEY-VCH Verlag GmbH & Co.View Large Image Figure ViewerDownload (PPT) (A) Preformed AIEgen as building block. (B) Formation of AIE building blocks from pro-AIEgens via chemical reactions. Reproduced with permission from Haldar et al.27Haldar S. Chakraborty D. Roy B. Banappanavar G. Rinku K. Mullangi D. Hazra P. Kabra D. Vaidhyanathan R. Anthracene-resorcinol derived covalent organic framework as flexible white light emitter.J. Am. Chem. Soc. 2018; 140: 13367-13374Crossref PubMed Scopus (53) Google Scholar Copyright 2018, American Chemical Society. Reproduced with permission from Jin et al.28Jin E. Li J. Geng K. Jiang Q. Xu H. Xu Q. Jiang D. Designed synthesis of stable light-emitting two-dimensional sp2 carbon-conjugated covalent organic frameworks.Nat. Commun. 2018; 9: 4143-4152Crossref PubMed Scopus (73) Google Scholar Copyright 2018, American Chemical Society. (C) Assembly of scaffold structures from pro-AIEgens showing clusterization-triggered emission. Reproduced with permission from Zhang et al.4Zhang H. Zhao Z. Turley A.T. Wang L. McGonigal P.R. Tu Y. Li Y. Wang Z. Kwok R.T.K. Lam J.W.Y. Tang B.Z. Aggregate science: from structure to properties.Adv. Mater. 2020; https://doi.org/10.1002/adma.2001457Crossref Google Scholar Copyright 2020, WILEY-VCH Verlag GmbH & Co. Since the seminal introduction of TPE ligands in metal-organic frameworks (MOFs),10Shustova N.B. McCarthy B.D. Dincă M. Turn-on fluorescence in tetraphenylethylene-based metal-organic frameworks: an alternative to aggregation-induced emission.J. Am. Chem. Soc. 2011; 133: 20126-20129Crossref PubMed Scopus (425) Google Scholar enormous efforts have been devoted to the assembly of AIE building blocks in precisely ordered scaffolds.18Dalapati S. Gu C. Jiang D. Luminescent porous polymers based on aggregation-induced mechanism: design, synthesis and functions.Small. 2016; 12: 6513-6527Crossref PubMed Scopus (55) Google Scholar, 19Ma L. Feng X. Wang S. Wang B. Recent advances in AIEgen-based luminescent metal–organic frameworks and covalent organic frameworks.Mater. Chem. Front. 2017; 1: 2474-2486Crossref Google Scholar, 20Gui B. Yu N. Meng Y. Hu F. Wang C. Immobilization of AIEgens into metal-organic frameworks: ligand design, emission behavior, and applications.J. Polym. Sci. A Polym. Chem. 2017; 55: 1809-1817Crossref Scopus (9) Google Scholar There has been a burst of related publications recently. It is the right time to summarize the achievements regarding the design, fabrication, and functional applications of AIE-based scaffolds. In this Review, representative examples are selected to unveil the structure-property correlation at the aggregate level. We focus on the evolving functions as a major part of the review. The design principles and advantages of scaffolding architectures are discussed. Finally, we conclude the review with insights into the challenges and prospects in this rapidly emerging research field. The scaffold is used to describe mesoscale aggregates with precisely ordered building blocks. We initially became interested in porous crystalline materials, particularly from discrete cage compounds to infinite framework series, including MOFs, covalent organic frameworks (COFs), hydrogen-bonded organic frameworks (HOFs), and supramolecular organic frameworks (SOFs).16Beuerle F. Gole B. Covalent organic frameworks and cage compounds: design and applications of polymeric and discrete organic scaffolds.Angew. Chem. Int. Ed. 2018; 57: 4850-4878Crossref PubMed Scopus (181) Google Scholar,21Tian J. Wang H. Zhang D. Liu Y. Li Z. Supramolecular organic frameworks (SOFs): homogeneous regular 2D and 3D pores in water.Natl. Sci. 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Rev. 2020; 49: 1144-1172Crossref PubMed Google Scholar The cages and frameworks are well-defined networks with precisely aligned units and inherent porosity. For the scaffolds, one distinct feature is that unique topology diagrams for structural guidance are available on the basis of reticular chemistry and other sequence-dependent precise synthesis. Topology diagrams can be used to direct the growth of the frameworks in a predictable manner and thus allow for the design of multiscale structures. Consequently, the geometry and dimensions of the organic building blocks and linkages should determine the structures of the resulting scaffolds. This structure designability is not easy to achieve in other scaffolding materials, such as gels, vesicles, micelles, etc. Therefore, these scaffolds offer a valuable platform for a systematic study of correlated AIE units. Figure 2 shows three main strategies to incorporate AIE building blocks into the construction of scaffolds. As indicated by their extension directions, these building blocks can be used as nodes and/or linkers. The first strategy is using preformed AIE building blocks in the construction process directly (Figure 2A). In the second strategy, AIE building blocks are formed from pro-AIEgens in situ to assemble scaffolds (Figure 2B). One example is the synthesis of IISREP-COF7 material (Figure 2Bi), in which the green-emissive anthracene amine units are joined with red/green-emissive resorcinol units through the Schiff condensation.27Haldar S. Chakraborty D. Roy B. Banappanavar G. Rinku K. Mullangi D. Hazra P. Kabra D. Vaidhyanathan R. Anthracene-resorcinol derived covalent organic framework as flexible white light emitter.J. Am. Chem. Soc. 2018; 140: 13367-13374Crossref PubMed Scopus (53) Google Scholar The imine linkage with the neighboring ortho-hydroxyl group is known for its ESIPT (excited state intramolecular proton transfer) and AIE features, thus providing the COF scaffold with strong emission. Similarly, condensations to form hydrazone, azine, imide, and squaraine (SQ) structures were also reported to incorporate potential AIE activities into structural units.5Mei J. Hong Y. Lam J.W.Y. Qin A. Tang Y. Tang B.Z. Aggregation-induced emission: the whole is more brilliant than the parts.Adv. Mater. 2014; 26: 5429-5479Crossref PubMed Scopus (1691) Google Scholar, 6Mei J. Leung N.L. Kwok R.T.K. Lam J.W.Y. Tang B.Z. Aggregation-induced emission: together we shine, united we soar!.Chem. Rev. 2015; 115: 11718-117940Crossref PubMed Scopus (3254) Google Scholar, 7Feng G. Kwok R.T.K. Tang B.Z. Liu B. Functionality and versatility of aggregation-induced emission luminogens.Appl. Phys. Rev. 2017; 4: 021307-021348Crossref Scopus (77) Google Scholar Another example is to form AIE-active cyanostilbene building blocks from the Knoevenagel condensation between aldehydes and benzylic nitriles (Figure 2Bii).28Jin E. Li J. Geng K. Jiang Q. Xu H. Xu Q. Jiang D. Designed synthesis of stable light-emitting two-dimensional sp2 carbon-conjugated covalent organic frameworks.Nat. Commun. 2018; 9: 4143-4152Crossref PubMed Scopus (73) Google Scholar As shown, these chemistries are particularly useful to transfer ACQ-active reaction precursors into AIE-active building blocks. A third strategy, which is relatively less explored, is based on the AIE-related clusterization-triggered emission (CTE) phenomenon (Figure 2C).29Zhang H.K. Zhao Z. McGonigal P.R. Ye R.Q. Liu S.J. Lam J.W.Y. Kwok R.T.K. Yuan W.Z. Xie J.P. Rogach A.L. Tang B.Z. Clusterization-triggered emission: Uncommon luminescence from common materials.Mater. Today. 2020; 32: 275-292Crossref Scopus (53) Google Scholar Upon close and ordered assembly of isolated cluster luminogens, new emissive species can be formed in situ, primarily ascribed to through-space interactions of units at excited states. Usually, the CTE shows color-tunable luminescence properties controlled by “clustering” size. The CTE activity is present in a range of special structures absent of the π conjugation, such as natural polymers and metal clusters (Figure 2C), therefore providing rich opportunities for novel scaffolds of this class. Figure 3 summarizes AIE building blocks used in the construction of various scaffolds. They are grouped according to the linking chemistry: coordinative ligands for metal-organic structures, covalently reactive linkers for organic porous materials, and multivalent H-bonding active linkers in HOFs. Among these, TPE derivatives were overwhelmingly used, likely due to easy chemistry modification and conformational features.30Feng H.T. Yuan Y.X. Xiong J.B. Zheng Y.S. Tang B.Z. Macrocycles and cages based on tetraphenylethylene with aggregation-induced emission effect.Chem. Soc. Rev. 2018; 47: 7452-7476Crossref PubMed Google Scholar Moreover, in 3D framework synthesis, the propeller-shaped TPE building blocks with good formability were reported to achieve a high crystallinity and well-defined facets, by forming periodic docking sites to direct the successive attachment of 2D COF layers.31Ascherl L. Sick T. Margraf J.T. Lapidus S.H. Calik M. Hettstedt C. Karaghiosoff K. Doblinger M. Clark T. Chapman K.W. et al.Molecular docking sites designed for the generation of highly crystalline covalent organic frameworks.Nat. Chem. 2016; 8: 310-316Crossref Scopus (224) Google Scholar The use of AIE building blocks is attractive to construct luminescent scaffolds. These scaffolds can show quite different luminescence properties, from being dark to showing intensive emission. The intensive AIE emission is mainly ascribed to (1) active RIM to enhance the radiative decay and (2) formation of twisted conformations to reduce the self-quenching caused by π-π interactions. This provides a guiding design for luminescent systems as well as strategies to modulate their luminescence. When interchromophore distances are large enough, the RIM is not fully activated by neighboring units, allowing for efficient vibrational energy dissipation. The chromophore building blocks with flexible conformations moreover favor co-facial stackings, leading to intermolecular energy migrations and non-radiative electronic transitions. In another extreme case, AIE building blocks can be actively rigidified by directional linking interactions and neighboring hinderance. The fixed yet twisted conformations of units would enhance their intermolecular separations, thereby reducing self-quenching. In addition, the luminescent units can adopt specific conformations that would otherwise be impossible, hence producing different emission and/or absorption energies in scaffolds. Figure 4 shows typical functional scaffolds with intensive and tunable emission properties. Coordination-driven assembly is an alternative way to evoke AIE emission in addition to molecular aggregation.10Shustova N.B. McCarthy B.D. Dincă M. Turn-on fluorescence in tetraphenylethylene-based metal-organic frameworks: an alternative to aggregation-induced emission.J. Am. Chem. Soc. 2011; 133: 20126-20129Crossref PubMed Scopus (425) Google Scholar Interestingly, it can relieve the PL quenching of certain transition metals and has led to a unique class of luminescent coordination complexes. The spontaneous formation of metal-liga