Luminescent Organoboron Element-Blocks Exhibiting AIE Properties

Chapter 9

Luminescent Organoboron Element-Blocks Exhibiting AIE Properties Downloaded by CORNELL UNIV on October 5, 2016 | http://pubs.acs.org Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1226.ch009

Kazuo Tanaka* and Yoshiki Chujo* Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University Katsura, Nishikyo-ku, Kyoto 615-8510, Japan *E-mails: [email protected] (K.T.); [email protected] (Y.C.)

Organoboron conjugated molecules including polymers have attracted attention as a key material in modern organic opto-electronic devices. However, most of emissive properties are often spoiled in the solid state by the aggregation-caused quenching. To solve this problem, one of feasible strategies should be to apply AIE properties to organoboron complexes for receiving solid-state emission. In this chapter, the recent progress of the AIE-inducible organoboron complexes and resulting AIE-active materials including polymers are presented. Initially, the transformation of commodity fluorescent organoboron dyes to the AIE-active molecules is explained. Based on this result, the conjugated polymers with AIE properties were obtained. The optical properties of conjugated polymers involving boron element are illustrated. Moreover, the applications of these AIE-active polymers for the film-type sensors are mentioned. Next, as another instance of the AIE-active “element-blocks” composed of organoboron molecules, the AIE behaviors observed in the carborane materials are presented. Unique solid-state emission of the carborane derivatives is demonstrated.

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

Downloaded by CORNELL UNIV on October 5, 2016 | http://pubs.acs.org Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1226.ch009

1. Introduction Conjugated molecules have been applied for fabricating advanced organic devices such as paper displays, flexible photovoltaic cells, and printing electronic devices, and organoboron complexes including polymers have attracted much attention as a key material for developing next-generation opto-electronic devices because of their unique functions (1). Most of organoboron molecules can be classified as three- or four-coordinated complexes. Boron belongs to the 13 group and has one less electron than carbon. By introducing the conjugation system with the three-coordinated boron atom, the conjugation can be extended via the boron due to the vacant p-orbital even in the polymers (2). As a result, strong emission was observed from the conjugated polymers including the three-coordinated boron in the main chains. In addition, because of strong electron affinity of three-coordinated boron, the conjugation systems can show the strong affinity toward the Lewis bases. Based on this fact, chemical sensors were constructed for anion sensing (3). As another instance, owing to the recent advance of organic synthetic chemistry, unstable electronic states such as anti-aromaticity were realized in the borafluorene strucutre with the three-coordinate state of boron (4). However, the synthesis and material usages are still challenging. In contrast to the three-coordinate boron, the organoboron complexes with the four-coordinate boron generally show higher stability. So far, various types of luminescent materials involving the four-coordinate boron have been reported. For example, boron dipyrromethene (BODIPY) derivatives are well known to have superior optical properties such as large light absorption and emission ability, sharp spectra, and high photo-stability and are used for a wide variety of applications in both material science and biotechnology (5–7). Therefore, the series of BODIPYs and their derivatives were synthesized for optically-functional materials such as solid-state emitters (8), light absorbers (9, 10), light-harvesting antennae (11–13) and white-light emitting materials (14). Furthermore, it was reported that the BODIPY derivative-containing conjugated polymers can work not only as efficient emitters in deep-red and near infrared regions (15–17) but also as an efficient electron-carrier materials (18). In particular, it was demonstrated that higher electron-carrier ability from the film samples composed of the BODIPY derivative-containing conjugated polymers was obtained than those of Alq3 crystal which is commonly used as an electron-carrier material in the conventional optical devices. Thus, the development of novel organoboron complexes and the material usages based on their electronic properties is a topic with high relevance. There is one critical issue to be solved for the application of organoboron complexes as emissive materials. In general, the strong emissions of conjugated molecules would be spoiled by condensation. This fact means that optical properties should be lost in the solid or film states. These behaviors are called concentration quenching or aggregation-caused quenching (ACQ). On the contrary, it has been reported that some of compounds showed stronger emission in the aggregation states. Tang et al. firstly showed unique behaviors with tetraphenyl-substituted silole compounds in 2001 (19). Only when the compounds were aggregated in the poor solvent, the bright emission can be observed. This phenomenon is called aggregation-induced emission (AIE). One 158 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


of strategies for overcoming the ACQ is to apply the solid-state emission of the AIE-active materials. In particular, since ACQ directly contributes to the decreases of the device efficiency in the EL devices, the development of new AIE-active molecules is still of great interest in the development of modern electric devices. In addition, the AIE-active BODIPYs have been also prepared (20–22). Thus, new AIE-gens and their working mechanisms are attracted much attention for receiving advanced optically-functional materials. We have recently proposed a new concept of “element-block” consisting of nanobuilding units or clusters of heteroatoms (20). Simply by introducing “element-blocks” composed of heteroatoms or organometallic complexes in the conjugated system and connecting with other functional units, it is expected that the series of functional emissive materials can be readily obtained. From this view point, we pay attention to the organoboron complexes as a versatile “element-block” and indeed received various types of advanced functional materials including polymers (8–18, 23–25). In this chapter, the recent progress of the development of the AIE-active materials based on organoboron-containing AIE-inducible “element-blocks” is presented. Initially, the transformation of commodity fluorescent organoboron dyes to the AIE-active molecules is described. Based on this result, the conjugated polymers with AIE properties were obtained. The optical properties of conjugated polymers involving boron element are explained. In addition, the applications of these AIE-active polymers for the film-type sensors are mentioned. Next, as another instance of the AIE-inducible “element-block” composed of organoboron molecules, the AIE behaviors observed in the carborane materials are illustrated. The material design and unique solid-state emission of the carborane derivatives are demonstrated.

2. Transformation of Conventional Fluorescent Dyes to AIE-Active Molecules 2.1. Design of Boron Ketoiminates Initially, rational design for the transformation of commodity fluorescent dyes to the AIE-active molecules based on boron diketonate is explained. Boron diketonates (Figure 1) having four-coordinated boron atom are one of the simple and stable emissive organoboron complexes. Some of boron diketonates showed unique optical properties such as phosphorescence, mechanochromic luminescence, and multi-emission (26). Therefore, boron diketonates are regarded as versatile “element-blocks” for constructing functional emissive materials. However, the critical ACQ was often observed. Emission quantum yields in the solid state were usually lower than those in the solution state. In addition, although various color of emissive materials can be prepared by introducing boron diketonate into the polymers, the strong emission can be obtained only in the diluted solution. The film samples usually showed slight emissions. Thus, to develop organic devices with boron diketonates, the establishment of design strategy for avoiding ACQ is strongly required. We aimed to realize the AIE behavior based on boron diketonate derivatives. 159 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 1. Chemical structures of organoboron complexes.

It was proposed that the intramolecular motion of the conjugated substituents plays significant roles in the AIE behavior according to the proposed mechanism in the previous reports (27–30). In the solution state, the excitation energy can be consumed by the intramolecular tumbling of the substituents directly tethered to the chromophore, resulting in the annihilation of the emission. On the other hand, these molecular motions would be inhibited in the aggregation state. Furthermore, the intermolecular electronic interaction can be also disturbed by these bulky groups in the condensed state. As a result, the ACQ can be efficiently avoided, leading to the solid-emissive property. In order to realize the same behavior in boron diketonate, we designed the boron ketoiminate structure in which one of oxygen atoms connecting to boron is replaced to nitrogen atom (Figure 1) (31). By replacing oxygen to nitrogen, the flexibility of the six-membered ring involving boron should be enhanced because of the relatively-weaker B−N bond than B−O bond. It was presumed that the excitation energy could be consumed by the intramolecular tumbling in the solution state, leading to emission quenching. Similarly to other AIE-active molecules, these quenching paths can be suppressed in the aggregation state. In addition, it was expected that the substituent group in the nitrogen atom can critically disturb the intermolecular interaction in the condensed state. To confirm the validity of this idea, the series of boron ketoiminates with various substituents were synthesized and compared the optical properties in the solution and aggregation states. Figure 2 shows the emission properties of ketoiminates and diketonate. Boron diketonate exhibited strong blue emission in THF (5.0×10–5 M, Φsolution = 0.91), and annihilation was observed in the aggregation state in the aggregation state (Φagg = 0.36). This is the typical ACQ behavior. On the other hand, ketoiminates showed very weak emissions in the THF solutions (5.0×10–5 M, Φsolution < 0.01). By adding the water content in the THF solutions, white turbidity appeared. Correspondingly, the significant emission was obtained. The aggregation states of ketoiminates in the mixed solvent of THF/H2O (1/9 (v/v)) showed drastic increases in their emission intensities (Φagg = 0.30–0.76). These results clearly indicate that ketoiminates are AIE-active molecules. In the frozen state in 2-methyl-THF solution at 77 K, the strong emission was recovered. In addition, by enhancing the viscosity of the solution with the addition of ethylene glycol, the emission enhancement was observed. These data indicate that the intramolecular motion should be responsible for the quenching in the solution state. 160 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 2. Comparison of optical properties of organoboron complexes.

The solid-state emission properties of boron ketoiminates were supported by the crystal packing structures. From the X-ray single crystal analyses, it was revealed that boron diketonate should have high planarity, while the distortion was induced in the complex originated from relatively-longer bond lengths of B−N in boron ketoiminates than that of the B−O bond. These data propose that the boron-chelating rings on ketoiminates should be more flexible than that on diketonate. It was suggested that the energy consumption should occur in the solution state. As a result, the slight emission was observed from the solutions of boron ketoiminates. On the other hand, molecular tumbling should be suppressed in the aggregation state. Moreover, because of the steric hindrance of the substituent at the nitrogen atom, the intermolecular interaction and stacking could be disturbed, resulting in the inhibition of the ACQ. Thus, the AIE behavior can be obtained as we expected. From the following researches with boron diiminates, the similar data were obtained from X-ray crystal analyses as well as from the optical measurements (32). Thereby, the above mechanism is the most likely explanation on the AIE behavior of boron ketoiminates and diiminates.

2.2. Mechanofluorochromism of Boron Ketoiminates It has been discovered that some of organometallic compounds such as gold (33, 34) and boron diketonates (35–41) showed chromic luminescence induced by the external physical stimuli such as scratching, crashing, and grinding with the sample powders or films. These chromic properties are called as mechanochromic luminescence or mechanofluorochromism (MFC), and these materials are paid attention as a platform for developing pressure sensors. The MFC behavior was observed from boron ketoiminate derivatives (Figure 3) (42). The triads composed of bithiophene and two boron ketoiminate connected at both ends were synthesized, and the triads also exhibited AIE properties. Moreover, the MFC behaviors were observed. From the structural and thermal analyses, it was shown that the MFC property of boron ketoiminates was derived from a phase transition between the crystalline and amorphous states. The degree of intermolecular interaction would be changed during the phase transition, leading to the luminescent chromism.

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


Figure 3. Optical properties of boron ketoiminates and plausible mechanism of their MFC behaviors.

Interestingly, depending on the functional groups at the end of the triads, the hypsochromic and bathochromic shifts of the emission bands were individually observed (Figure 3). In particular, the direction of the peak shifts of the emission bands was significantly depended on the size of the end group. From the complexes with relatively small end groups such as hydrogen, the AIE colors were red at the initial crystalline state. After grinding, hypsochromic changes were induced. On the other hand, the complexes with larger end groups such as iodo and trimethylsilyl groups presented the yellow AIE before grinding. After adding the mechanical stimuli, bathochromic shifts in the emission spectra were detected. It was proposed that the alteration of the degree of intermolecular interaction between the crystalline and amorphous states could induce MFC. Boron ketoiminates with small substituents formed relatively densely packed structures in crystalline states due to the small steric hindrance present. Because of the loose packing structure in the amorphous state, weak π−π intermolecular interactions relative to those in the crystal state should be induced. As a result, the emission band could be shifted to shorter wavelengths after transformation to 162 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


the amorphous state. In contrast, the dense packing structure could be disturbed by the large substituents in the crystalline states of boron ketoiminates having relatively-larger end groups, resulting in the formation of loose packing even in the crystalline state. By the external mechanical stress, the transition to random configuration would occur. The molecules can distribute freely and form the thermally-stable morphology. Finally, it was implied that stronger π−π intermolecular interactions relative to the crystalline samples could be obtained in the amorphous states. Chloro-, bromo- and iodo-substituted boron ketoiminates showed larger emission shifts than those of other materials. The intermolecular halogen bond could influence these large, anomalous emission shifts.

2.3. Sponge-Type Chemical Sensors for Protein Adhesion Utilizing Boron Ketoiminate-Modified Hydrogels New types of sensing materials can be produced utilizing boron ketoiminate-modified polymers. The sponge-like chemosensors for protein were achieved based on the unique optical properties from the aggregates of the boron ketoiminate-modifying hydrogels (Figure 4) (43). Initially, the modified hydrogels involving boron ketoiminate with poly(γ-glutamic acid) were synthesized, and the optical properties were evaluated. In the swollen state, the slight emission band with the peak around 500 nm was observed (Φ < 0.06). In contrast, the magnitude of the emission intensity was enhanced at least 3-folds after removing water (Φ > 0.15). These data indicate that the synthesized hydrogels have the AIE properties derived from the boron ketoiminate dye. It is likely that the molecular motion of the organoboron dye is highly disturbed in the dried state. Thereby, the emission was recovered. The excitation state should be quenched in the swollen hydrogels due to the vigorous intramolecular motions. In addition, these emission changes can be induced repeatedly by swelling and drying.

Figure 4. Plausible mechanism of the aggregation-induced blue-shifted emission in the modified hydrogels. 163 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Further unique optical properties were observed from the boron ketoiminate-modified hydrogels. The AIE color was changed by exposing the hydrogels to organic solvents for shrinking the hydrogels. Interestingly, after the treatments with organic solvents, the AIE color of the hydrogel was diverse. This unique behavior can be explained as aggregation-induced blue shift emission (44). It was proposed that not only the matrix but also the boron ketoiminate dye should be condensed in the shrunk hydrogels with the polar solvents (Figure 4). Because of the structural restriction, the relaxation in the excited state could proceed incompletely. Therefore, the emission band from the meta-stable energy levels was obtained. Thus, the aggregation-induced blue shift emission was observed. In contrast, in the shrunk hydrogels with low polar solvents such as THF in which boron ketoiminate is soluble, although the gel matrix should be condensed, the molecular motion should be partially maintained in the residual solvents. Thereby, the typical AIE properties could be detected without the peak shift of the emission band. Based on this chromism in the shrinking, the protein sensing was performed. The proteins with hydrophobic cavity can have high affinity toward the gel matrix via the van deer Waals hydrophobic interaction. When the protein adsorbs onto the pretreated hydrogels with organic solvents, the residual solvent molecules should be extruded. Subsequently, the aggregation-induced blue shift is induced, leading to the color change of the AIE-active hydrogels. To show the validity of this concept on the sponge-type chemosensors, the emission color changes were monitored with the series of the hydrogels containing various organic solvents. In the buffer, the solution containing bovine serum albumin (BSA) was added to respective organic solvent-retaining hydrogels. Then, the changes in the emission color were evaluated with the organic solvent-retaining hydrogels in the absence and presence of BSA (Figure 5). The color changes to green were clearly induced by the BSA addition with acetone, acetonitrile and DMF-retaining hydrogels. In the cases of acetonitrile- and acetone-retaining hydrogels, the peak shifts to the green regions were detected by 45 nm and 30 nm, respectively. Even with the hydrogel including DMF in which the solvent molecules can be captured inside the gel matrices tightly because of high polarity, the clear color change was observed (Δλmax = +14 nm). This result means that BSA should make strong interaction to the organic solvent-suspended hydrogels. Then, the organic solvents can be released from the hydrogels, leading to the color changes of AIE. It was indicated that the organic solvent-retaining hydrogels can detect the protein existence. The protein aggregation and cohesion are an important phenomenon in biology. The small molecule-based conventional probes for the protein aggregation would be adsorbed to the specific site on the protein such in the hydrophobic pockets. This means that the troublesome immobilization pretreatments to form the covalent bonds with the probe molecule at the preprogrammed position on the target protein should be required to monitor the cohesion of the whole protein. This sponge-type sensor should be feasible for precisely evaluating the cohesion ability of the protein from a whole molecule.



Figure 5. Schematic illustration of protein sensors utilizing boron organic solvents-retaining modified hydrogels with aggregation-induced blue-shifted emission properties.

2.4. AIE-Active Conjugated Polymers with Boron Ketoiminates Next, by employing boron ketoiminate structures, the main-chain type conjugated polymers were prepared for obtaining the AIE-active polymeric materials (Figure 6) (45). The alternating copolymers PF and PT were designed and synthesized via the Suzuki−Miyaura cross-coupling reaction with the fluorene and bithiophene comonomers, respectively. The optical properties of the synthesized compounds were evaluated. The boron ketoiminate monomer showed the emission band around 450 nm in the emission spectra. In contrast, PF and PT presented the emission bands with the peaks at 562 nm and 646 nm, respectively. These data mean that the electronic conjugation can be extended through the polymer main-chains. Furthermore, AIE properties were obtained from both polymers. In the solution state, PF and PT showed weak emissions (PF: Φsolution = 0.10, PT: Φsolution = 0.04). Much larger emission efficiencies were observed from the solid samples than those from the solutions (PF: Φfilm = 0.13, PT: Φfilm = 0.06). It should be mentioned that the AIE color was modulated by the type of the comonomer unit. This result indicates that the optical properties of the boron ketoiminate conjugated polymers can be easily tuned by selecting a comonomer unit. This tunability is feasible for improving the performance of organic electronic devices.



Figure 6. Chemical structures and optical properties of AIE-active conjugated polymers composed of boron ketoiminates.

Next, based on the AIE properties of boron ketoiminates, the solid-state emissive polymers were developed (46). According to the quantum calculation of molecular orbitals, it was suggested that highest occupied molecular orbitals (HOMOs) are localized around the boron atom in the ketoiminate skeleton. The robust conjugation through the polymer main-chains should be elongated in PF and PT. Then, the AIE behaviors can be observed from both polymers. In contrast, if the connection points with each monomer unit are changed to the ligand moiety, it was presumed that the main-chain conjugation should be isolated at the organoboron complex in the polymers. Then, each complex is aligned through the polymer main-chain as a tandem repeat of chromophores. As a result, the solid-state emission properties can be obtained. To examine the validity of this idea, the series of organoboron polymers were prepared, and their optical properties were evaluated (Figure 7). The synthesized polymers can show the strong emissions in the visible region from green to red. The polymers demonstrated strong emissions in the solution state (Φsolution = 0.46−0.80). Notably, the emission efficiencies were maintained at some degree even in the film state (Φfilm =0.13−0.38). These data represent that the solid-state emissive conjugated polymers can be obtained utilizing the AIE-active organoboron complexes.

Figure 7. Optical properties of solid-state emissive polymers.



2.5. Evolution of AIE Properties by Boron Diiminate To improve the AIE properties of organoboron conjugated polymers, further modification was introduced into the organoboron complexes. By replacing another oxygen atom to nitrogen in boron ketoiminate, it was presumed that larger changes in the emission properties between solution and solid states can be obtained. Based on this idea, boron diiminates were designed (Figure 1) (32). The second functional group can be introduced into the nitrogen atom in the ligand. Therefore, comparing to boron diketonates and boron ketoiminates, there is room in boron diiminates for functionalization. Because the inhibition of intermolecular interactions in solid state is very important to show strong solid-state emission, boron diiminates have a high potential as efficient AIE-active materials. The series of boron diiminates were prepared, and their AIE-properties were investigated with the similar methods to the previous organoboron complexes (Figure 8) (32, 47). Similarly to ketoiminates, diiminates also presented the AIE properties. The emission spectra showed slight luminescence in THF (Φ < 0.01), whereas the strong emissions were obtained by the aggregation formation. Interestingly, it was found that crystallization led to drastic increases in their emission intensities (M-H(Me): Φagg = 0.02, Φcr = 0.23, M-H: Φagg = 0.02, Φcr = 0.11). The emission efficiencies from the crystal samples were larger than those of amorphous samples. These results indicate that the synthesized boron diiminates can have not only AIE properties but also crystallization-induced emission (CIE) properties. Moreover, similarly to ketoiminates, the color tuning was accomplished by modulating the functional groups. It should be mentioned that the phenyl rings on the nitrogen atom were perpendicularly distributed toward the six-membered ring involving boron according to the X-ray structural analyses. Although the distorted structures are generally unfavorable to form the robust electronic interaction via π-conjugation, electronic properties can be modulated by the substituent effect at the end of complexes via the electronic conjugation including boron. It was demonstrated that these polymers can be used as an organic scintillator. By the X-ray irradiation, the radioluminescences were observed with the similar shapes with the corresponded photoluminescence (48). These data indicate that AIE-active materials are also versatile for the application to modern emissive materials.

Figure 8. Optical properties of boron diiminates.



The reversible control of the emission properties was accomplished based on the CIE property of boron diiminate (47). The crystal sample of M-H(Me) was heated over the melting temperature and then cooled immediately for preparing the amorphous sample. Corresponded to the optical measurements, the decrease of emission efficiencies from 0.11 to 0.02 was observed. By heating again and subsequently cooling to ambient temperature, the crystallization was induced. The recovery of the emission intensity was obtained. This change can be repeated many times without loss of emission properties. These data represent the solid-state emission can be modulated by the external stimuli. 2.6. Film-Type Chemical Sensors with Boron Diiminate-Containing Polymers The conjugated polymers including boron diiminates with various functional groups were prepared (Figure 9) (47, 49). The structure−property relationship was evaluated from the optical measurements. The significant emissions were hardly observed from the THF solutions of the polymers. The emission efficiencies were under detectable levels (Φsolution < 0.01). In contrast, bright emissions were obtained from the film samples. This result indicates that the clearer AIE behaviors were accomplished by employing the diiminate structure. Furthermore, the color tuning of AIE from green to red was achieved by the substituent effect at the side chains. It was demonstrated that the degree of the electronic coupling in the main-chain conjugation can be modulated by the functional groups in the boron diiminate conjugated polymers.

Figure 9. Optical properties of AIE-active conjugated polymers composed of boron diiminates. Next, based on the tunability of the electronic structures by the substituent effect, the film-type chemical sensors were demonstrated (47). The polymer containing dimethylamino groups showed red emission in the film state (λem = 628 nm). In contrast, the polymer having electron-withdrawing groups (nitro groups) showed the blue-shifted emission in the yellow region (λem = 575 nm). The fluorochromic sensor was constructed based on this color change with the dimethylamino-substituted polymer (Figure 10a). By exposing to the trifluoroacetic acid (TFA) and triethylamine (NEt3) vapors, the change of the emission properties was monitored with the film samples. Accordingly, the 168 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


AIE properties of boron diiminates-containing polymers can be dynamically changed. When the film was placed in a small container saturated with TFA vapors, the fumed film showed blue-shifted emission spectrum than those of the parent sample. Emission color was changed from red to yellow. Furthermore, the optical properties of the TFA-exposed film were converted back into the parent state again by exposing to NEt3 vapor. These data clearly show that the optical properties can be tuned reversibly. The strong donor-acceptor interaction between the amino group and the boron chelating ring could be suppressed by the protonation of the amino group. As a result, the color was changed.

Figure 10. Schematic illustration of film-type sensors (a) for acidic and basic vapors and (b) for hydrogen peroxide based on functional group-substituted boron diiminate conjugated polymers with AIE properties. 2.7. Film Sensors Based on Oxidation-Induced Emission Properties To extend the application of the stimuli-responsive AIE-active polymers to the ON/OFF sensors, another system was designed. The films with the AIE-active copolymers composed of fluorenes and the sulfide-substituted boron diiminates were prepared, and the changes in the optical properties by the oxidation were evaluated (Figure 10b) (49). Hydrogen peroxide (H2O2) is classified as reactive oxygen species and is endogenously produced in energy metabolism. The excessive H2O2 induces cell damage, leading to many pathological problems such as diabetes, cardiovascular diseases, cancer and so on. Thereby, the development of the facile detection techniques for H2O2 is still required. Before treatment, the film showed weak emission in the yellow region. The emission intensity at 550 nm from the film gradually increased during the incubation by soaking the film samples of the polymer in the solution containing H2O2. The quantum yields 169 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


were correspondingly enhanced up to 3-times larger by oxidation at the sulfide groups. From the 1H NMR analysis with the sample solution in chloroform, it was found that the oxidation at the sulfide group to yield sulfoxide proceeded. Charge transfer-emissive fluorescence should be enhanced between the fluorene units and the boron diiminates having the enhanced electron-withdrawing ability by transforming from the sulfide to the sulfoxide groups. These data indicate that the synthesized polymers have oxidation-induced emission properties and work as a film-type optical sensor with turn-on fluorescence. Hence, it was proposed that the AIE-active polymeric materials should be a versatile platform as a biosensor for longitudinal monitoring the bio-related reactions.

3. o-Carborane-Based AIE-Active Materials 3.1. AIE-Active Polymers Containing o-Carborane Next, as the second example of the AIE-inducible “element-block”, o-carborane derivatives are introduced. o-Carboranes are icosahedral clusters consisting of 10 boron and 2 carbon atoms with three-center two-electron bonds. Because of the three-dimensional electron delocalization, it is observed that o-carboranes exhibit high thermal and chemical stabilities. o-Carboranes have been used for various purposes such as heat-stable materials, boron-neutron capture therapy, and molecular machines. However, there are still limited numbers of researches on the synthesis of carborane-containing polymers and optical materials based on the electronic characteristics of carborane. Initially, the first example to offer the AIE-active polymers containing o-carborane is described. Next, the series of AIE-active and solid-emissive materials based on o-carborane are demonstrated. The alternating polymers were synthesized with o-carborane and p-phenyleneethynylene sequences, and their optical characteristics were examined (50). Accordingly, in the THF solutions, the polymers with electron-donating π-conjugated linkers exhibited almost no emission (Figure 11). By increasing the water content in the solution of the polymers to form the aggregation, the emission intensity ratios (I/I0) gradually increased. Finally, similarly to other AIE-active polymers, in the mixed solvent of THF/H2O = 1/99, these polymers exhibited strong emissions. These results clearly indicate that the o-carborane polymers can work as an AIE-active material. From the mechanistic studies of AIE from o-carborane, it was summarized that the quenching of the emission from the linker by o-carborane plays a critical role in the AIE behavior of o-carborane derivatives. o-Carborane can work as a strong electron-withdrawing unit because of the intrinsic properties of electron-deficient boron clusters. Therefore, the emission attributable to the intramolecular charge transfer in the conjugated system between o-carborane and the electron-donating linkers can be obtained. In the solution, the excitation energy can be consumed by the molecular motion at the C-C bond of o-carborane which is involved into the conjugated system. On the other hand, in the solid state, the quenching via the molecular motion can no longer take place. Finally, the AIE behaviors can be obtained from the o-carborane-containing conjugated molecules. 170 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 11. Chemical structures and intensity enhancements of o-carborane-containing polymers.

3.2. Stimuli-Responsive Emissive Gels Containing o-Carborane The stimuli-responsive reversible control of the AIE properties of the o-carborane-containing polymeric materials was also achieved by employing hydrogels (Figure 12). o-Carborane-based cross-linking reagent was designed to prepare the translucent hydrogels consisting of poly(γ-glutamic acid) (51). The luminescence intensity from the modified hydrogels was drastically influenced by the gel-shrinkage from the change of ionic strength. In the swollen state, weak emission was observed. By drying the hydrogel, the strong emission was obtained. Furthermore, these hydrogels showed effective reversible fluorescence switching between the swollen and dried states. It was likely that the molecular motion at the o-carborane cross-linkers should be suppressed in the dried state. Then, the significant emission can be observed from the material.

Figure 12. Schematic model of stimuli-responsive emissive hydrogels containing o-carborane cross-linker. 171 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


4. Conclusion In this chapter, the discovery of the AIE-inducible organoboron complexes and the resulting AIE-active molecules including polymers are described. By the introduction of AIE-inducible organoboron complexes or clusters into the conjugation system, AIE properties can be obtained from the materials. As a result, the series of AIE-active conjugated polymers, film-type chemical sensors, and bright solid-state emissive materials were obtained. These studies fundamentally represent the synergetic effects with heteroatom and conjugation system and present the significant characteristics which are unable to be obtained only with C, H, O, and N elements. The design strategies for the functional materials with“element-blocks“ described here are promised to be a valid concept not only for improving the efficiencies of the conventional devices but also for generating novel organic devices.

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Luminescent Organoboron Element-Blocks Exhibiting AIE Properties

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