Aggregation-Induced Emission: Materials and Applications Volume 1

Chapter 11

Recent Progress in New AIE Structural Motifs

Downloaded by CORNELL UNIV on October 2, 2016 | http://pubs.acs.org Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1226.ch011

Rongrong Hu,*,1 Qiguang Zang,1 and Ben Zhong Tang*,2 1State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China 2Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China *E-mails: [email protected]. (R.H.); [email protected]. (B.Z.T.)

Aggregation-induced emission (AIE) is a fast-developing hot research area in recently years. AIE materials are fascinating because of their efficient aggregated state emission, strong photostability, structural and property diversity, etc. Besides the classical AIE luminogens such as tetraphenylethene and hexaphenylsilole, new AIE structural motifs have been updated rapidly. In this chapter, the development of AIE structural motifs in the past two years will be introduced with the emphasis on the structural design, the mechanistic understanding, and the luminescence behavior of new AIE systems.

1. Introduction Organic compounds with efficient solid/aggregated state emission are highly desirable because of their applications in optoelectronic devices, biological probes and chemosensors (1). There is one dilemma in traditional organic fluorescent dyes that many of them emit brightly in dilute solution, however, the fluorescence weakened or even disappeared in concentrated solutions or aggregated states in the practical conditions, which is known as aggregation-caused quenching (ACQ) problem (2). This ACQ effect has greatly limited the practical application of traditional fluorescent dyes. Aggregation-induced emission materials are hence developed to provide a perfect solution to the ACQ problem, in which the aggregation of the luminogen plays a constructive, instead of a destructive role in the light-emitting process. The typical phenomenon of AIE compound is that © 2016 American Chemical Society Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


when the molecules are dissolved in good solvent, the solution is non-emissive or faintly emissive; when poor solvent or nonsolvent was added into the solution, the molecules turn to form nanoaggregates which result in bright fluorescence (3). To elucidate the underlying mechanism of AIE phenomenon, the effect of structural planarity and rotatability, intramolecular restriction, intermolecular interaction, photochemical reaction on AIE behavior has been systematically studied, and various mechanisms such as conformational planarization, restriction of intramolecular rotation, J-aggregation, hydrophobic effect, isomerization, excited state intramolecular protonation transfer, and twisted intramolecular charge transfer have been investigated (4). It is now widely accepted that the restriction of intramolecular motion (RIM), including rotation and vibration, is the main reason for AIE effect and the structural rigidification (SR) plays an important role in AIE compounds (5). The intramolecular motions are active in solution, which serve as nonradiative relaxation channels for the excited state energy; on the other hand, such motions are restricted in the aggregated states due to the physical constraint, which blocks the non-radiative decay pathway and renders the compound emissive. Based on the design principle of AIE system, a large number of AIE compounds have been developed in the past decade with the typical examples shown in Figure 1, including tetraphenylethene (TPE), hexaphenylsilole (HPS), distyrylanthracene (DSA), and 10,10′,11,11′-tetrahydro-5,5′-bidibenzo[a,d][7]annulenylidene (THBA) (6). Among AIE compounds, there are different classes of luminogens such as pure hydrocarbons, heteroatom-containing molecules, hydrogen-bonding capable compounds, polymeric and organometallic compounds, molecules without orthodox chromophores, etc. The structure-property relationship related to π-conjugation, steric hindrance, electron donor/acceptor structure, substituents, molecular conformation and packing are summarized (7). Many traditional ACQ luminophores such as anthracene, carbazole, polycyclic aromatic hydrocarbons, pyran-containing dyes, difluoro-boradiazaindacenes, have also been successfully converted to AIE compounds by structural modification (8).

Figure 1. Chemical structures of classical AIE compounds. Because of the advantages of AIE compounds such as highly emissive aggregated state, strong photostability, good biocompatibility, “turn-on” sensing mode upon aggregation, large Stokes shifts, etc, they have been demonstrated to possess high performance in a series of high-tech applications such as electroluminescence devices for display and lighting, fluorescence chemosensors for the detection of important metal ions, gases, and small molecular analytes 194 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


which matters in water, air, or food, as well as biosensor and bioimaging for the detection of biologically important signal molecules or analysis of key bio-processes (9). Abundant reviews have published to introduce this young area, with the emphasis in different aspects of AIE materials such as molecular design, polymer materials, biological applications, etc. (10) In this chapter, the most up-to-date progress in the development of new AIE motifs which have not been summarized in the previous reviews will be introduced. New structures such as TPE-based symmetric star-shape compounds, macrocycles or metallacages, salicylaldehyde azines with excited state intramolecular proton transfer (ESIPT) attributes, heterocyclic AIE compounds, boron-containing AIE compounds, and AIE-active metal complexes will be discussed. AIE compounds with unique luminescence behaviors which reveal new mechanistic understanding involving intramolecular vibration or twisting, and AIE-active small molecules or polymers contain neither conventional fluorophore nor extensive conjugated structures will be introduced. AIE compounds with advanced properties such as near IR emission, delayed fluorescence, single molecular white light emission, and mechanoluminescence will also be presented.

2. AIE Compounds with New Structural Design 2.1. Tetraphenylethene Derivatives With the classical AIE building block, tetraphenylethene (TPE), a few clever design have been reported with interesting symmetric structures, while preserving the AIE nature of the TPE moiety. Previously, AIE compounds constructed by more than four TPE units as peripheries have barely been reported because of the difficulty in synthesizing such rigid molecules with high steric hindrance. Recently, Bhosale et al. has reported a rigid star-shaped compound 1 with six TPE moieties connected to the cyclohexanehexone core structure (Figure 2) (11). The fluorescence quantum yields of its THF solution and thin film are 0.08% and 54%, respectively. When adding water into its THF solution to cause aggregation, it possesses a dramatic fluorescence enhancement of 218-fold. The emission maximum of 1 in the aggregated thin film state is located at 485 nm, which is similar with the emission of TPE, indicating that such structural modification does not affect the emission of TPE units. Reversible piezofluorochromic behavior through grinding and solvent treatment have also been observed in the solid powder of 1. On the contrary, Zn(II) tetraazaporphyrin 2 with eight TPE moieties on the periphery does not show AIE characteristics (12). The emission maximum of its solution is located beyond 670 nm, independent with the excitation wavelength, which originates from the tetraazaporphyrin and the emission from TPE has been completely quenched. Theoretical calculation suggested that intramolecular charge transfer process might take place to cause the fluorescence quenching, because a significant mixing between the HOMO of the macrocycle core and a close-lying molecular orbitals of the periphery TPE is observed. 195 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 2. Chemical structures of TPE-based symmetric compounds 1-7.

196 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Compared with the tedious covalent synthetic procedures for the preparation of single entity with multiple AIEgens, coordination-driven self-assembly is a convenient alternative. Yan, Huang, and Stang et al. have reported a series of discrete multiple TPE-containing metallacycles 3-7 prepared based on supramolecular coordination complex platforms (Figure 2) (13). These multi-TPE discrete organoplatinum(II) metallacycles were prepared by the combination of 120° TPE-based dipyridyl ligand and 60°, 120°, and 180° organoplatinum(II) acceptors. Through the directional-bonding approach driven by metal-coordination, the corresponding metallacycles are produced in nearly quantitative yields. In each TPE unit, there are two unrestricted dangling phenyl rings which preserve the AIE feature in such metallacycles, and cause weak fluorescence in the solution. The dilute CH2Cl2 solutions of 3-7 displayed weak fluorescence with low quantum yield (ΦF) of 0.088% (3), 0.25% (4), 0.14% (5), 0.19% (6), and 0.15% (7), respectively. At CH2Cl2/hexane mixtures with 90% hexane content, the ΦF values of 3-7 increased to 1.83% (3), 6.90% (4), 2.82% (5), 10.2% (6), and 3.19% (7), respectively. The solid state quantum yield is generally higher than the nanoaggregates in CH2Cl2/hexane mixture. The same group also reported a highly emissive platinum metallacage 8a-b through coordination-driven self-assembly (14). The tetragonal prismatic discrete supramolecular coordination complexes are self-assembled by mixing a metal acceptor Pt(PEt3)2(OSO2CF3)2 with two organic ligands, a four pyridyl-containing TPE and a two benzene dicarboxylate species (Figure 3A). Such metallacages can dissolve well in CH2Cl2, showing bright-yellow fluorescence at about 555 nm with ΦF values of 23.2% (8a) and 10.8% (8b), respectively. When 90% hexane was added into the CH2Cl2 solution, the ΦF values increased to 60.6% (8a) and 51.3% (8b), accompanying with an obvious blue shift (Figure 3B). Interestingly, a rare white-light emission is observed from the THF solution of 8b, due to the mixed yellow and blue emission originated from the monomeric and aggregate ensembles of the metallacage, respectively.



Figure 3. (A) Chemical structures of tetragonal metallacages 8a-b. (B) The fluorescence photographs of 8b in CH2Cl2/hexane mixtures with different fractions of hexane upon UV irradiation. (C) Fluorescence emission spectra and (D) plots of maxima emission intensities and wavelength of 8b versus hexane fraction in CH2Cl2/hexane mixture. Adapted with permission from ref (14). Copyright 2015 Nature Publishing Group.



2.2. Salicylaldehyde Azine Derivatives The aggregation-induced emission characteristics of salicylaldehyde azine (SA) 9 was first reported by Tong et al. in 2009 (15). SAs are readily available through simple Schiff base chemistry, which possess ESIPT process and involve dual fluorescence originated from the keto and enol tautomers, respectively. The advantages of SAs are ease of modifications, free of fluorescence self-quenching, excellent light-up ratio and large Stokes shift. The intramolecular rotation of phenyl ring is locked by intramolecular hydrogen bonds, while the N-N bond is still rotatable, enabling AIE feature. Moreover, the phenolic hydroxyl group is crucial to the ESIPT process as well as the luminescence behavior, hence functionalization of the phenolic hydroxyl group and further deprotection of modified group can trigger the ESIPT process and further reflected on the change of the emission color and intensity. Most importantly, ESIPT process can extensively red shift the emission color, which might produce AIE dyes with long emission wavelength. Combining the advantages of AIE and ESIPT attributes of SAs, a series of ratiometric fluorescent pH probes 10a-c and 11a-b are developed by Xiang et al (Figure 4) (16). Different electron-acceptors such as NO2, F, and Cl, electrondonors such as OCH3, and NEt2, or π-extended naphthalene are installed on the phenyl rings of SAs to endow the compounds with strong blue, green, and red fluorescence in aqueous media and solid state. They possess tunable pKa values ranging from 7.5 to 13.3. The solid state fluorescence quantum efficiency can achieve 30%. The absorption and emission of SAs are highly dependent on the pH value of the medium, enabling their application as pH probes. For example, the protonated 10c emits at 565 nm at pH < 7.0, while the intact deprotonated 10c emits at 515 nm at pH > 11.0 (Figure 5). When the pH value of the solution increases from 7.0 to 11.0, the red emission at 565 nm reduces and the blue-green emission at 515 nm increases. The ratio of I515 to I565 shows adverse pH responses with I565, demonstrating a ratiometric detection of pH value by visual sensing of emission color change.

Figure 4. Chemical structures of AIE-active salicylaldehyde azine derivatives 9-12.



Figure 5. (A) Photographs of 10c in B-R buffer solution with different pH values (top panel: under daylight; bottom panel: under 360 nm UV irradiation). (B) PL spectra and (C) I565 and I515/I565 versus pH value of 10c in buffer solution with different pH values. Adapted with permission from ref (16). Copyright 2015 Royal Society of Chemistry. When the ESIPT process is blocked by chemical modification of the hydroxyl group, the salicylaldehyde azine dyes only show weak blue emission; however, upon deprotection of the hydroxyl group, ESIPT process is rebooted by forming the intramolecular hydrogen bonds, which generates strong fluorescence in the aggregated state. Through this design principle, multi-target light-up fluorescence probe can be developed. For example, Liu et al. has synthesized SA dyes 12a-c specially designed for the response of multiple targets including palladium cation, perborate anion, and UV irradiation (17). Upon addition of Pd(PPh3)4 into the aqueous solution of 12a, Pd0-catalyzed Tsuji-Trost reaction takes place which cleaves the allyl group and release the hydroxyl group to afford emission from ESIPT process; the sensing of perborate is realized by selective deprotection of aryl acetates in 12b under mild conditions to regenerate the hydroxyl group; the UV sensing is realized by photo-cleavage of the well-developed photocleavable protecting nitrobenzyl group of alcohols and amines in 12c. The protected salicylaldehyde azine dyes generally show weak blue emission when ESIPT process is inhibited, after deprotection, ESIPT process regenerate and result in strong yellow emission at about 550 nm. Tong et al. also use similar strategy to develop a fluorescent probe 12d for the turn-on detection of cysteine (Cys) (18). 200 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

The salicylaldehyde azine is decorated with one acryloyl group on one of the hydroxyl group, which is a well-known selective reaction group of Cys. After addition of Cys, the acrylate group is hydrolyzed and generate the intramolecular hydrogen bonding, enabling the ESIPT process and turning on the emission.


2.3. AIE-Active Heterocyclic Compounds Nowadays, aggregation-induced emission has been proved to be a generally applicable phenomenon to many known compounds, even without classical propeller-shaped structures. In particular, several heterocyclic compounds have demonstrated to be AIE-active recently (Figure 6). For example, Guo and Wong et al. have reported the AIE property of a long known compound, 2,2′:5′,2′′-terthiophene-5-carbaldehyde 13. In dilute THF solution, 13 emitted at 479 nm with a ΦF of 7.59%; when 70% water was added into the THF solution, the ΦF value was increased to 57.96% with a significant red-shift of the emission maximum of about 60 nm. Such bathochromic shift is attributed to typical twisted intramolecular charge transfer caused by the increased solvent polarity (19). It is suggested that the ordered nanoscale aggregates of 13 formed in THF/water mixtures are responsible for its AIE feature. Yang and Ma et al. have synthesized a new AIE heterocyclic 14 through facile one-pot tandem reaction. In THF solution, compound 14 emits at 400 nm with a ΦF value of 9.0%; when the fraction of water is raised, the fluorescence is first quenched, then regenerated a new emission peak at 450 nm and shown enhanced emission, as the molecules began to form sub-micron particles after the water fraction increased to 90% (20). This heterocyclic can undergo a ring-opening reaction in the presence of thiol nuclephile to enable selective cysteine and glutathione detection. Qian, Yi, and Huang et al. reported an AIE-active phenylbenzoxazole-based compound 15 with the locally excited state emission located at 343-351 nm and a twisted intramolecular charge transfer emission located at 470-525 nm, depending on the solvent polarity. The THF solution of 15 shows almost no emission (ΦF = 0.17%) upon UV irradiation, but exhibits strong blue emission in powder with a ΦF value of 8 ± 2% (21). The AIE feature of 15 is reported to arise from an emissive quasi-TICT excited state. The intramolecular rotation is more restricted with the increased order of molecular arrangements, which results in stronger quasi-TICT* emission. An AIE-active self-assembled organogel prepared from 5-(4-nonylphenyl)7-azaindole 16 was reported by García-Frutos et al. (22). The monomeric species of 16 in dilute cyclohexane solution possess an emission maximum at 349 nm. Increasing the solution concentration to 0.01 M can cause complete fluorescence quenching. When gel was formed in the concentrate solution at room temperature through the hydrophobic interaction of the long alkyl chain and the dual hydrogen bonds, blue emission was observed, red shifted compared with that of the dilute solution of 16. This is attributed to the formation of complexes with a more coplanar conformation which resulted the AIE effect. Similarly, Lin and Zhang et al. reported a rational design of organogelator 17, consisting of a coplanar nitrophenylfuran moiety, enabling π-π stacking interaction, in which the nitrophenyl group serves as a chromophore and the phenylfuran group serves as a 201 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


fluorophore to achieve dual-channel response (23). The aroylhydrazone linker is designed as the F- binding sites as well as the hydrogen bond self-assemble sites; a 3,4,5-tris(hexadecyloxy)phenyl tail is designed for good solubility in organic solvent as well as strong hydrophobic interaction which enables easy gelation and relative stable gelator. The hydrophobic interaction, π-π stacking interaction, and hydrogen bonding work together at elevated temperature to form stable organogels in various solutions at low critical gelation concentrations, accompanying with fluorescence enhancement at 524 nm. When F- was added into the organogel, the fluorescence was quenched and the appearance of the organogel turns from yellow to dark red, while the system remain to be an organogel. The emission and yellow color can be recovered by adding H+ into the gel.

Figure 6. Chemical structures of AIE-active heterocyclic compounds 13-20.

Qin and Tang et al. recently reported a new kind of AIEgen named tetraphenylpyrazine 18 which generally emit in the wavelength range of 390-460 nm. The tetraphenylpyrazine dyes possess good thermal stability, facile preparation, and tunable emission color by easy modulation of the structure (24). Chen and Xu et al. developed a new and efficient strategy, utilizing Cu/Pd-catalyzed isomerization/insertion/oxidative coupling cascade reaction of cyclopropene with internal alkynes to afford a large variety of cis-tetrasubstituted olefins as the single stereoisomer (25). These tetraarylethenes are proved to be AIE-active. For example, compound 19 emits at 500 nm with ΦF values of 0.47% and 9.69% in THF solution and solid state, respectively. Squarine or squarylium dyes with a unique four-member hydrocarbon ring are extensively studied as a red emissive compounds, however, their solid-state light emission has barely 202 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

reported. Ohsedo et al. have synthesized a series of asymmetric squarylium dyes such as compound 20 (26). The compound is non-emissive in DMF solution but show strong emission at 534 nm with a ΦF value of 36%.


2.4. AIE-Active Boron-Containing Compounds Besides heterocyclics containing common heteroatoms such as N, O, and S, organoboron compounds have also attracted much attention as efficient luminescent dyes with high aggregated state emission. One typical example is the previously reported AIEgen-modified BODIPY derivatives (27, 28). Recently, scientists have paid more attention of boron ketoiminate units with the boron atom coordinated by a nitrogen atom, an oxygen atom, and two fluorine atoms. Tanaka and Chujo et al. have done many work related to highly emissive organoboron compounds (Figure 7). For instance, they have reported a variety of boron ketoiminates 21a-e with a bithiophene bridge to connect two ketoiminate units and different substitution groups on the terminal phenyl ring (29). These compounds possess emission maxima located in the range of 617-671 nm with solid state emission quantum yield of up to 48%. The emission intensity of the boron ketoiminates are generally enhanced in the solid states compared with the solution states. Mechanofluorochromic effect is generally observed for 21a-e, which suggests a phase transition between the crystalline and amorphous states. The shift of emission wavelength before and after the mechanical stimuli is highly dependent on the substituents: the bulky substituents lead to a bathochromic-shift after grinding while the small substituents are opposite. As an important analogues of BODIPYs, pyridine-based organoboron compounds has proved to possess high fluorescence quantum yield and high stability in solution. In particular, pyridine-based organoborons with unsymmetrical structures generally possess moderate ΦF values in the solid state (30). Wang, Liu and He et al. have reported two pyridine-ketoiminate-boron-based luminophores 22a-b with propeller-shaped structures and AIE feature. Two fused rings were formed by the coordination of boron with the pyridyl nitrogen atom and the ketoiminate oxygen atom, two periphery phenyl rings were decorated to provide intramolecular rotation and hence enable AIE effect. 22a-b emit faintly in solvents with low viscosity and their emission could be enhanced by increasing the solvent viscosity or aggregation. In the solid state, 22a-b possess narrow emission bands with high quantum yields of 53% and 46%, respectively, probably due to the weak intermolecular interactions such as C-H···F and C-H···π which fixing the molecular conformations and restricting the intramolecular motions. Similarly, Shankarling et al. reported a series of new keto-enol tautomeric benzoxazolyl and benzothiazolyl-1,2-diaryl β-ketoiminate based organoboron complexes 23a-b (31). They barely emit in THF solution but show enhanced emission efficiency in the THF/water mixtures with 90% water content, and their emission maxima are 505 nm (23a) and 531 nm (23b).



Figure 7. Chemical structures of AIE-active boron-containing compounds 21-24. Furthermore, an AIE-active o-carborane 24, a polyhedral boron cluster compound with two adjacent carbon atoms in the cluster cage, is designed and synthesized as a stimuli-responsive compound by Morisaki and Chujo et al. (32) Anthracene was selected as the π-bridge to connect two o-carborane cages. The assemblies of the anthracene compounds can further tune the luminescence of the solid state. The THF solution, the THF/water mixture with 99% water content, and the crystal formed in CHCl3 of 24 show emission maxima at 648, 643, and 627 nm, respectively, with their emission efficiencies to be

Aggregation-Induced Emission: Materials and Applications Volume 1

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