Aggregation-Induced Emission - American Chemical Society

Chapter 10

AIE Active Heteroarylethylenes

Downloaded by KANSAS STATE UNIV on December 5, 2016 | http://pubs.acs.org Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1226.ch010

Moustafa T. Gabr and F. Christopher Pigge* Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States *E-mail: [email protected]

Fluorescent compounds and materials that exhibit the property of aggregation induced emission, or AIE, are emerging as valuable constituents of functional materials in chemical biology and materials science. Foremost among AIE active compounds are derivatives of tetraphenylethylene (TPE). Simple tetraphenylethylenes are synthetically accessible and have been incorporated into a multitude of “turn-on” fluorescence indicators for applications in analyte sensing/detection and bioimaging. Several structurally related analogues of tetraphenylethylene that possess one or more heteroarene rings in place of phenyl groups have also been reported, however heteroarylethylenes have not been extensively examined as fluorescent molecular building blocks. Consequently, the development of general synthetic routes leading to construction of diverse heteroaromatic tetraarylethylenes has been initiated, and a preliminary assessment of the AIE properties of several pyridine-based tetraarylethylenes has been performed. As neutral compounds these heteroarylethylenes display the expected AIE fluorescent behavior in solution and the solid state, but significant departures from conventional AIE activity are observed in heteroarylethylenes containing N-alkylpyridinium groups. The modular synthetic approaches to these heteroarylethylenes also present opportunities to tune the fluorescence response.

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


Introduction Aggregation induced emission (AIE) is a trait exhibited by select organic and organometallic molecules that distinguishes these materials from conventional fluorophores. Most fluorescent compounds are emissive in dilute solution and experience severely attenuated luminescence in concentrated solution, in poor solvents, and in the solid state due to aggregation-caused quenching. In contrast, AIE active compounds typically exhibit minimal fluorescence in dilute solution but become markedly more fluorescent as aggregates in solution (i.e., in the presence of poor solvents) and in the solid state. Thus, AIE active compounds exhibit “turn-on” fluorescence under conditions in which many other fluorophores experience fluorescence quenching. This property has led to the incorporation of AIE active compounds into a number of fluorescent sensors, bioimaging agents, and other functional materials (1–3). While several types of organic molecules have been found to display AIE activity, compounds based on the tetraphenylethylene framework (TPE, 1 in Figure 1) have been the most extensively investigated and utilized (4–7). The basis for the AIE properties of TPE is believed to reside in the rotational freedom of the phenyl rings with respect to the central alkene linkage (8). In dilute solution there is free rotation about the sp2-sp2 centers, and this provides an avenue for non-radiative relaxation of excited-state molecules. Aggregation of TPEs, however, impedes these rotations and opens up a fluorescent channel for energy dissipation from an excited state. Numerous functionalized derivatives of TPE have been prepared in order to facilitate incorporation of this scaffold into inter alia organic polymers and biomolecules with the aim of eliciting aggregation induced emission in the presence of targeted analytes or in response to specific chemical or biochemical events (e.g., protein – ligand interactions, enzymatic activity, etc) (4–7). A subset of TPE derivative that has proven to be especially useful is that in which one or more heterocyclic arenes (pyridine, in particular) have been attached to the periphery of the tetraphenylethylene framework. Examples of these TPEs are also shown in Figure 1 and include symmetrical tetrapyridyl tetraphenylethylenes (such as 2), divinylpyridyl and vinylpyridyl tetraphenylethylenes 3 and 4, and monopyridyl TPE 5. The Lewis basicity and metal ligating abilities of pyridine substituents in 2-5 in combination with the AIE activity of the TPE core have been utilized in a number of diverse supramolecular chemistry applications, including synthesis of metal-organic frameworks (9–11), construction of pH-sensitive fluorescence indicators as well as metal-ion selective fluorescence indicators (12–16), construction of mechanochromic switches (17, 18), and self-assembly of halogen bonded crystalline organic networks (19). Replacement of one (or more) of the phenyl groups in tetraphenylethylene with an aromatic heterocycle also provides access to heteroarylethylenes and serves as an alternative to grafting heterocyclic substituents onto the periphery of functionalized TPEs. Heteroarylethylenes may retain the AIE photophysical characteristics of their tetraphenyl analogues while exhibiting new properties inherent to the heterocyclic fragment (such as reactivity, Lewis basicity, metal ligating ability, redox activity, etc). In addition, tetraarylethylenes directly constructed from heterocyclic components will possess smaller molecular 176 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


footprints than TPEs decorated with heterocyclic units, a structural feature that may be important in certain biological applications. To date, relatively few examples of tetraarylethylenes (TAEs) containing heterocyclic rings have been reported. Some examples are shown in Figure 2 and include mono- and tetrapyridyl derivatives 6-7 (20, 21), the furan containing TAE 8 (22), a bis(pyrrole)-substituted derivative 9 (23), and the tetra(thiophene)ethylene 10 (24). To further expand the family of potentially AIE-active tetraarylethylenes to encompass those possessing core heteroaromatic rings, we have begun to develop general synthetic routes to mono-, bis-, tris-, and tetra(heteroaryl)ethylenes, and have targeted pyridine-containing systems for initial investigation (25). Preliminary examination of the fluorescence properties of pyridine-based TAEs reveal some similarities with their TPE counterparts, but some important differences as well.

Figure 1. Tetraphenylethylene (TPE) and some pyridine-substituted TPEs.

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


Figure 2. Tetraarylethylenes possessing one or more heteroaromatic core rings.

Discussion The most direct route for the synthesis of tetraphenyl- and substituted tetraarylethylenes is McMurry coupling of benzophenone derivatives. This approach is especially well-suited for the preparation of symmetrical tetraphenylethylenes (e.g., 11) as illustrated in Scheme 1A (26). Two problems emerge when using this method for the synthesis of unsymmetrical tetraarylethylenes. First, if the target TAE is derived from two different benzophenone precursors (Scheme 1B), then the product of the desired cross-McMurry coupling (12) will be produced as a mixture with symmetrical products (1 and 13) arising from self-condensation of the starting benzophenones (27). Second, if the target TAE is derived from a single unsymmetrical diaryl ketone, then McMurry coupling produces diastereomeric mixtures of E and Z alkene isomers (13, Scheme 1C) (28). Additionally, the reaction conditions required for McMurry couplings are not compatible with easily reduced functional groups or ring systems (29). Accordingly, we have opted to pursue more versatile and modular synthetic approaches to (hetero)tetraarylethylenes that do not involve McMurry couplings. In this way we hope to maximize synthetic flexibility and increase the scope of accessible heterocyclic TAEs. An alternative approach to tetraphenylethylenes that has proven utility in delivering unsymmetrical derivatives involves application of the Peterson olefination reaction between a silylated diphenylmethyl anion and a benzophenone (30). This approach works well provided the starting diphenylmethane is not substituted with electron donating groups, which have the effect of destabilizing the corresponding anions. We have adopted this method to successfully prepare mono-pyridyl tetraarylethylenes that incorporate either 2-, 3-, or 4-substituted pyridines. The synthesis of the triphenyl vinylpyridine (or TPVP) 16 is illustrated in Scheme 2 and proceeds in reasonable overall efficiency from lithium anion 14 and 4-pyridyl phenyl ketone 15. In similar fashion, the isomeric TPVP derivatives 17 and 18 have been prepared in comparable yields from 14 and 3-pyridyl phenyl ketone or 2-pyridyl phenyl ketone, respectively (25). 178 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 1. Advantages and drawbacks of McMurry coupling for synthesis of TPE derivatives. A) Convenient access to symmetrical TPEs. B) Product mixtures arising from cross-McMurry coupling of non-identical benzophenones. C) E/Z TPE diastereomers arising from McMurry coupling of an unsymmetrical benzophenone.

Scheme 2. Synthesis of TPVP derivatives 16-18. 179 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


In the solid state all three TPVPs (16-18) exhibit fluorescence emission upon irradiation with a hand-held UV lamp, a trait consistent with AIE. In solution, however, notable differences in the fluorescence profiles of these compounds emerge. While 16 exhibits the AIE effect in CH3CN/H2O mixture (i.e., minimal fluorescence emission in pure CH3CN that increases steadily upon addition of water as a poor solvent, Figure 3A), 17-18 each display a fluorescence signal that is relatively unaffected by aggregation (Figure 3B). Thus, incorporation of a single pyridine ring into the core tetraarylethylene framework significantly influences the AIE fluorescence response as a function of pyridine isomer. The origin of this behavior has not been definitively established, but it is speculated that reduced steric interactions resulting from replacement of an aryl C-H group with a N atom in 17 and 18 may allow for greater conjugation between the pyridine ring and the ethylene unit, in turn leading to fluorescence emission irrespective of aggregation state. The synthetic route used to prepare 16-18 works well for these relatively simple and unfunctionalized mono-pyridyl tetraarylethylenes, but lacks generality. To address this issue an alternative preparative sequence was developed based on double Suzuki coupling of 1,1-dibromo-2,2-diarylethylenes (31). Representative examples of this approach are illustrated in Scheme 3 (25). Olefination of 4-pyridyl phenyl ketone upon exposure to CBr4/PPh3 affords the corresponding geminal dibromoethene 19 in reasonable isolated yield. Compound 19 is an ideal substrate for double Suzuki coupling with various aryl boronic acids, leading to functionalized TPVPs such as 20 and 21. The use of heteroaryl boronic acids as Suzuki coupling partners also proceeds in straightforward fashion to provide tetraarylethylenes possessing three heteroarene components, such as 22-24. In each case the yields for the cross coupling step are good and the preliminary scope of this route as indicated by the results depicted in Scheme 3 is quite broad. Initial assessments of the fluorescence properties of 20-24 reveal the expected AIE activity. For example, fluorescence spectra of 20 taken in CH3CN and CH3CN/H2O mixtures are shown in Figure 4. In 100% CH3CN modest emission is observed at λ = 384 nm. Addition of water results in enhanced emission at this wavelength up to ~60% H2O content. Further addition of water elicits a slight red shift in the emission to ~ 404 nm accompanied by a decrease in emission intensity. Concomitantly, a new emission at lower energy (490 nm) becomes evident. This longer wavelength emission may be produced by protonation of the pyridine group at high water content, facilitated by conjugation of the pyridine ring to the electron-donating methoxyphenyl groups. Indeed, the fluorescence spectra of 20 taken in 1 M aqueous HCl exhibited a similar emission maximum at ~ 509 nm (dashed line in Figure 4). The fluorescence profile of 20 illustrates the tuning of AIE-based emission that may be possible in heteroaromatic tetraarylethylenes as a function of solvent.



Figure 3. A) Fluoresence spectra of 16 in CH3CN/H2O mixtures demonstrating AIE effect (λex = 305 nm, [16] = 10 μM). B) Plot illustrating difference in AIE effect in TPVPs 16-18. Reproduced with permission from reference (25). Copyright 2015 Royal Society of Chemistry.



Scheme 3. Synthesis of heteroarylethylenes via double Suzuki coupling.

Figure 4. AIE behavior of 20 in CH3CN/H2O mixtures and in 1 M aqueous HCl (λex = 328 nm, [20] = 10 μM). Adapted with permission from reference (25). Copyright 2015 Royal Society of Chemistry. 182 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


The remaining examples of heteroarylethylenes shown in Scheme 3 displayed conventional AIE effects. As a representative example, the fluorescence spectra of 23 taken in CH3CN and CH3CN/H2O mixtures are shown in Figure 5. Minimal change in the weak fluorescence at λem ~ 438 nm was observed upon addition of increasing amounts of water. In 90% H2O/CH3CN, however, an intense and slightly red-shifted emission appeared centered at ~ 490 nm, demonstrating the pronounced AIE behavior of this compound.

Figure 5. Fluoresence spectra of 23 in CH3CN/H2O mixtures demonstrating AIE effect (λex = 341 nm, [23] = 10 μM).

Variations on the synthetic scheme described above also allow for convenient access to 1,1-diaryl-2,2-di(heteroaryl)ethylenes, as well as tetra(heteroaryl)ethylenes. In each case double Suzuki reaction of gemdibromoethylene precursors is employed as the key transformation. Two specific examples are shown in Scheme 4 that illustrate the synthesis of a (dipyridyl)diphenylethylene and a tetrapyridylethylene. In the former, the known dibromoethylene 25 (32) is treated with 4-pyridine boronic acid to give 26 in 55% isolated yield. Similarly, treatment of dibromoethylene 27 (prepared from the corresponding dipyridyl ketone) with 3-pyridine boronic acid affords the symmetrical tetrapyridylethylene 28. The isomeric dipyridyland tetrapyridylethylenes 29 and 30 (Scheme 4) were prepared analogously. The modular nature of these synthetic routes and the availability of numerous heteroaryl boronic acids should facilitate construction of a diverse range of heteroarylethylenes with interesting and potentially tunable photophysical and redox properties. 183 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 4. Synthesis of dipyridyldiaryl- and tetrapyridylethylenes.

The presence of pyridine rings within the tetraarylethylene framework offers additional opportunities for synthetic manipulation. Indeed, physical and optical properties of pyridine-based tetraarylethylenes can be easily modified through simple N-alkylation of the pyridine nitrogen, and several of the tetraarylethylenes described above have been successfully converted to N-methyl pyridinium salts in good yield upon treatment with trimethyloxonium tetrafluoroborate ([(CH3)3O][BF4], Figure 6). Interestingly, the fluorescence properties of cationic pyridinium tetraarylethylenes differ significantly from those exhibited by their corresponding neutral precursors. For example, the mono-pyridinium tetraarylethylene 31 (obtained by alkylation of 16) proved to be soluble in CH2Cl2 and displays a UV absorption at ~ 360 nm. Excitation of a dilute CH2Cl2 solution of 31 at this wavelength elicited a fluorescence emission that could be readily discerned with the naked eye. Incremental addition of hexane (a poor solvent for 31) to this solution in order to induce aggregation, however, produced very little change in the fluorescence spectrum (Figure 7). Thus, conversion of AIE active 16 to the N-methyl tetrafluoroborate salt effectively “switched on” the fluorescence of this tetraarylethylene, irrespective of aggregation state.

Figure 6. Mono- and dicationic N-alkylpyridinium tetraarylethylenes.



A contrasting fluorescence profile was observed in the spectra of dipyridinium diphenyl salts derived from tetraarylethylenes such as 26. The dialkyl salt 32 was easily obtained by treatment of 26 with an excess of trimethyloxonium tetrafluoroborate. The neutral dipyridyl-ethylene 26 exhibits aggregation induced enhanced emission in CH3CN/H2O mixtures as shown in Figure 8A. The intensity of emission observed at ~ 370 nm in 100% CH3CN was found to increase incrementally upon addition of increasing amounts of H2O to induce aggregation. In contrast, the visible emission observed in the fluorescence spectrum of dimethyl pyridinium salt 32 in CH2Cl2 (a solvent in which 32 is readily soluble) was quenched upon aggregation induced by addition of hexane (Figure 8B). Thus 32 behaves more like a conventional organic fluorophore and exhibits aggregation caused quenching. This behavior is attributed to the presence of a second electron-withdrawing N-methyl pyridinium group within the tetraarylethylene scaffold, since 31, possessing a single pyridinium ring, maintains fluorescence under identical conditions. We speculate that the incorporation of a second electron deficient N-methylpyridinium group in 32 facilitates fluorescence quenching by electron transfer from the comparatively electron rich phenyl rings (33). It is notable, then, that at least for some pyridine-containing heteroarylethylenes the AIE-type emission typically observed in derivatives of this structural type can be significantly altered simply by pyridine N-alkylation.

Figure 7. Fluorescence spectra of pyridinium salt 31 in CH2Cl2 and CH2Cl2/hexane mixtures (λex = 363 nm, [31] = 10 μM). Reproduced with permission from reference (25). Copyright 2015 Royal Society of Chemistry.



Figure 8. A) Fluorescence spectra of 26 in CH3CN/H2O mixtures demonstrating AIE effect. B) Fluorescence spectra of dicationic tetraarylethylene 32 in CH2Cl2/hexane mixtures demonstrating ACQ effect. Reproduced with permission from reference (25). Copyright 2015 Royal Society of Chemistry.



The effect of linking two TPVP derivatives together in the form of alkyl bis(pyridinium) salts also reveals distinct fluorescence signatures as a function of alkyl chain length. The 4-pyridyl TPVP 16 reacted smoothly with 1,3-dichloropropane and 1,10-dibromodecane to give the corresponding bis(pyridinium) alkanes 33 and 34 (Figure 9A) as hexafluorophosphate salts after anion metathesis (25). Both 33 and 34 are soluble in CH2Cl2 (in line with other tetra(N-alkylpyridinium)(aryl)ethylenes) and both exhibit fluorescence emission when excited at their UV absorption maximum. Addition of hexane to solutions of propyl-linked pyridinium TPVP 33 produces a gradual decrease in emission intensity coupled with a slight blue shift in the emission maxima (Figure 9B). Such behavior is similar to what was observed in the fluorescence spectra of dipyridyl TAE 32 under identical conditions. Thus, the short hydrocarbon chain linking the two TPVP moieties results in a fluorescence profile similar to that encountered in a monomeric diphenyl-dipyridyl analogue. In contrast, connecting the TPVP frameworks with the longer 10-carbon alkyl chain (34) produces a bis(pyridinium) compound that displays visible fluorescence in dilute CH2Cl2 solution, and the intensity of the emission is not significantly diminished upon aggregation Figure 9C). This fluorescence response somewhat mirrors the behavior of N-methylpyridinium salt 31 and may indicate that the decane linker is sufficiently long such that the pyridinium TPVP groups retain the luminescent properties of the monomeric analogues.



Figure 9. A) Structure of linked TPVP salts 33 and 34. B) Fluorescence spectra of 33 in CH2Cl2/hexane mixtures ([33] = 10 μM, λex = 390 nm). C) Fluorescence spectra of 34 in CH2Cl2/hexane mixtures ([34] = 10 μM, λex = 369 nm). Figures 9B and 9C reproduced with permission from reference (25). Copyright 2015 Royal Society of Chemistry.



Summary and Conclusions Concise and modular synthetic routes to tetraarylethylenes that allow incorporation of one, two, three, or four heteroaromatic components have been developed. The key transformation enabling the success of this approach is the double Suzuki coupling between diaryl(heteroaryl)-gem-dibromoethylenes and aryl or heteroaryl boronic acids. Initial efforts have focused on the preparation of pyridine-based tetraarylethylenes, but the procedures developed in the course of this work should be broadly applicable to the synthesis of other heteroarylethylenes as well. The positioning of heterocyclic (pyridine) ring(s) within the tetraarylethylene scaffold offers opportunities for additional synthetic manipulation and a means to tune the AIE luminescence and redox properties of these materials. These features render (heteroaryl)ethylenes potentially valuable building blocks in supramolecular chemistry and chemical biology. Indeed, the use of AIE-active compounds as bio-signalling and bio-imaging agents is attracting increasing attention (1–7, 34, 35), and the availability of low molecular weight heteroaryl analogues should contribute to future advances in these areas. Heteroarene-based tetraarylethylenes may also serve as convenient platforms for construction of new multi-stimuli responsive photochromic systems (36) and redox-active supramolecular assemblies (37). The ability of tetraarylethylenes to undergo chemical or photo-induced oxidative cyclization is an additional intriguing feature of these compounds (38, 39). Examination of the photocyclization behavior of (hetero)tetraarylethylenes should be interesting in the context of developing dynamic photoresponsive switches and as synthetic routes to new redox-active heterophenanthrene analogues (40).

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