DOI: 10.1002/chem.201500849
Full Paper
& Functional Organogels
Fluorescent Dendritic Organogels Based on 2-(2’-Hydroxyphenyl)benzoxazole: Emission Enhancement and Multiple Stimuli-Responsive Properties Hui Chen,[a, b] Yu Feng,*[b] Guo-Jun Deng,[a] Zhi-Xiong Liu,[b] Yan-Mei He,[b] and QingHua Fan*[b]
Chem. Eur. J. 2015, 21, 11018 – 11028
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Full Paper Abstract: A new highly efficient and versatile poly(benzyl ether) dendritic organogelator HPB-G1 with 2-(2’-hydroxyphenyl)benzoxazole (HPB) at the focal point has been designed and synthesized. HPB-G1 can form stable organogels toward various apolar and polar organic solvents. Further studies revealed that intermolecular multiple p–p stacking interactions are the main driving forces for the formation of the organogels. Notably, dendron HPB-G1 exhibited a signifi-
Introduction Supramolecular gels formed by the self-assembly of low-molecular-weight gelators (LMWGs) through weak intermolecular interactions, such as hydrogen bonding, van der waals interactions, and p–p stacking, are useful soft materials with diverse applications.[1] Due to the dynamic character of noncovalent bonding, the sol–gel transition for organogels is thermally reversible and can be further tuned by other physical and chemical stimuli. These external-stimuli-responsive organogels are highly desirable for advanced applications of sensors, actuators, drug delivery, catalysis, and molecular logic gates.[2] Despite the great progress made recently in this field, organogels with multiple stimuli-responsive properties are still rarely reported.[3] Excited-state intramolecular proton-transfer (ESIPT) luminescence has been a topic of interest for years because of their unique luminescent characteristics and potential applications.[4] The most remarkable photophysical property of ESIPT chromophores is the large Stokes shift with an almost complete lack of spectral overlap between absorption and emission (without self-absorption), thus making ESIPT molecules ideal candidates as chemosensors, molecular probes, luminescent materials, molecular logic gates, and so forth.[5] ESIPT chromophores are normally more stable in the enol and keto forms in the ground and excited states, respectively. Moreover, it is well known that most luminophores exhibit aggregation-caused quenching (ACQ) of emission. Since the first example reported by Tang and co-workers of silole derivatives with aggregation-induced emission (AIE) properties in 2001,[6] a number of luminogenic molecules with propeller-shaped structures have been found [a] H. Chen, Prof. Dr. G.-J. Deng Key Laboratory for Environmentally Friendly Chemistry and Application of Ministry of Education College of Chemistry, Xiangtan University Xiangtan 411105 (P.R. China) [b] H. Chen, Prof. Dr. Y. Feng, Z.-X. Liu, Y.-M. He, Prof. Dr. Q.-H. Fan Beijing National Laboratory for Molecular Sciences and CAS Key Laboratory of Molecular Recognition and Function Institute of Chemistry, Chinese Academy of Sciences (CAS) Beijing 100190 (P.R. China) Fax: (+ 86) 10-62554472 E-mail:
[email protected] [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201500849. Chem. Eur. J. 2015, 21, 11018 – 11028
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cantly enhanced emission in the gel state in contrast to weak emission in solution. Most interestingly, these dendritic organogels exhibited multiple stimuli-responsive behaviors upon exposure to environmental stimuli, including temperature, sonication, shear stress, and the presence of anions, metal cations, acids/bases, thus leading to reversible sol–gel phase transitions.
to show pronounced AIE or aggregation-induced enhanced emission (AIEE) characteristics.[7] Very interestingly, most ESIPTbased molecules, such as 2-(2’-hydroxyphenyl)benzoxazole (HPB) derivatives, have exhibited an unusual AIEE phenomenon in the aggregated or solid state.[8] Thus, it is desirable to design new fluorescent gelators based on ESIPT chromophores, which could provide an efficient and easy way for the fabrication of highly emissive nanostructures for applications in photoelectrical or laser nanodevices. However, only a limited number of examples of fluorescent gels based on an ESIPT chromophore have been reported to date.[9] Dendrimers and dendrons are highly branched macromolecules with well-defined molecular architectures and are ideal candidates for the construction of gel-phase materials due to the advantages of significant steric impact and the ability to form multiple noncovalent interactions.[10] To date, a number of physically thermoreversible gels based on dendrons or dendrimers with different chemical functionalities have been reported.[11] However, in a few cases, fluorescent dendritic gels with AIEE characteristics have been observed.[12] Surprisingly, there has been no report of fluorescent dendritic gels based on ESIPT chromophores. Following our continued pursuit of developing efficient functional dendritic organogels,[13] we report herein a new dendritic gelator HPB-G1 with the HPB unit at the focal point (Scheme 1). This molecular design was made on the basis of the following considerations: 1) HPB and its analogues, as typical ESIPT chromophores, have been widely used to fabricate luminescent materials due to their remarkably high intensity of fluorescence emission and large Stokes shift;[4] 2) the hydroxy groups in HPB can selectively interact with fluoride anions, which may bring about a gel-to-sol transition and color changes;[9b, 14] 3) the HPB unit can bind with metal cations, therefore disrupting the p–p interactions between the HPB units in the presence of metal cations, thus possibly bringing about the gel-to-sol transition;[15] 4) recent studies from our group have highlighted that peripherally dimethyl isophthalate (DMIP)-functionalized poly(benzyl ether) dendrons show unprecedented and highly efficient gelation ability.[13a,b] With these facts in mind, we have synthesized a new poly(benzyl ether) dendron with a HPB chromophore at the core. The HPB-G1 derivative shows outstanding gelation abilities and exhibits a significantly enhanced emission in the gel state relative to the solution state. Most importantly, the resulting fluorescent dendritic organogels exhibit a reversible sol–gel phase transition in response to external environmental
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Full Paper Table 1. Gelation properties of HPB-G1 in various organic solvents and a mixed solvent.
Entry
Solvent
State[a]
CGC [g L¢1][b]
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 15
toluene anisole cyanobenzene ethyl acetate acetone acetonitrile 2-methoxyethanol benzylalcohol 1-butanol methanol CCl4 1,2-dichloroethane dichloromethane DMSO THF THF/CCl4 (5:1)
G G G G G G G G G I G G S G G G
16.0 14.5 13.0 8.1 14.0 6.5 5.4 10.0 8.4 – 10.0 20.0 – 5.0 24.0 18.0
[a] G = stable gel, S = soluble (> 60 mg mL¢1), I = insoluble. [b] CGC is the critical gelation concentration and is the concentration at which gelation was observed to restrict the flow of the medium at 25 8C.
Scheme 1. Synthetic route to the dendritic gelator HPB-G1.
stimuli, including temperature, sonication, shear stress, and the presence of anions, metal cations, acids/bases. To the best of our knowledge, this report is the first of a fluorescent dendritic gel system based on ESIPT chromophores with multiple stimuli-responsive properties.
Morphologies of organogels
Results and Discussion Synthesis and characterization of the dendritic gelator HPBG1 According to our previous method,[16] the core HPB-functionalized poly(aryl ether) dendritic gelator HPB-G1 was synthesized by using the ether-synthesis reaction between the first-generation dendritic bromide with 2-(2’,4’-dihydroxyphenyl)benzoxazole in 78 % yield (the synthetic details and characterization data are provided in the Experimental Section). The purity and chemical structure of this compound was confirmed by 1H and 13 C NMR spectroscopic, ESI high-resolution (HR) mass-spectrometric, and elemental analysis. Gelation behaviors and sol–gel phase-transition temperature The gelation behavior of HPB-G1 in various organic solvents and solvent mixtures was investigated by using “stable to inversion of the test tube”. Dendron HPB-G1 can form gels in a wide variety of organic solvents, including 12 organic solvents and a solvent mixture (Table 1); for example, 5 mg of dendritic gelator HPB-G1 can cause the gelation of 1 mL of dimethyl sulfoxide (DMSO), thus suggesting that 2145 DMSO molecules are immobilized per molecule of dendron HPB-G1. Chem. Eur. J. 2015, 21, 11018 – 11028
To explore the thermal stability of the obtained gel-phase material further, the sol–gel phase-transition temperatures (Tgel) of the HPB-G1 gels formed from DMSO and 2-methoxyethanol at different concentrations were tested by using the tube-inversion method (Figure S1). Obviously, the Tgel value increased with an increase of the concentration of dendron HPB-G1, thus suggesting that the stability of the gel was enhanced with the increase of the concentration.
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The dendritic xerogels (air-dried gels) of HPB-G1 were investigated by using SEM and TEM analysis to gain visual images of the microscopic morphology of the organogels from various organic solvents (see Figure 1 and Figures S2 and S3 in the Supporting Information). Fiber- or ribbonlike morphologies in different organic solvents were observed (see Figure 1 and Figure S2 in the Supporting Information). It should be noted that the different morphologies of the xerogels might depend on the properties of the gelation solvents. For example, the xerogels in acetonitrile and anisole revealed a nanoscale fibrous structure with high aspect ratios, widths of 20–80 nm, and lengths of several micrometers; furthermore, this assembly created a closely packed three-dimensional (3D) fibrillar network structure by entanglement of such nanofibers, which trapped the solvent molecules into its interstices (Figure 1 A, B). However, the xerogels from ethyl acetate exhibited larger fibrillar structures with diameters of approximately 60–200 nm (Figure 1 C). Notably, the xerogel in 1,2-dichloroethane showed a structure with shorter and straight ribbons and large diameters of 100–300 nm (Figure 1 D). The morphological properties were further confirmed by means of TEM analysis (see Figure S3 in the Supporting Information). Small-angle X-ray scattering (SAXS) experiments were conducted on this gel system to obtain detailed structural insight
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Figure 3. Proposed packing model of HPB-G1 in the gel tissues.
Figure 1. SEM images of xerogel HPB-G1 from A) acetonitrile, B) anisole, C) ethyl acetate, and D) 1,2-dichloroethane.
into the assembled HPB-G1 in the gel phase. The freeze-dried xerogel of HPB-G1 from 1,2-dichloroethane (20 mg mL¢1) was subjected to SAXS analysis. An SAXS profile of the xerogel shows the appearance of strong peaks at q = 2.50 (110), 3.51 (200 and 020), 7.06 (400), 7.90 (420), and 17.72 nm¢1 (001), which are reasonably assigned to a columnar square-packing structure,[17] with cell parameters of a = b = 35.8 and c = 3.5 æ (Figure 2). Taking a density of d = 1.1 g cm¢3 into consideration, we can calculate Z = 2, that is, two molecules per unit cell. Therefore, a packing structure as shown in Figure 3 is proposed as the most likely aggregation model.
Driving-force study In our previous research, we demonstrated that the multiple strong p–p stacking interactions due to the peripheral DMIP
Figure 2. SAXS diagram of the freeze-dried xerogel of HPB-G1 from 1,2-dichloroethane (20 mg mL¢1). Chem. Eur. J. 2015, 21, 11018 – 11028
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motifs played a significant role in the formation of the self-assembled gel.[13a,b] In this study, the temperature-dependent (TD) and concentration-dependent (CD) 1H NMR spectra of HPB-G1 in CD3CN were examined to obtain further information about p–p stacking in the self-assembly (see Figure S4 in the Supporting Information). In the TD-NMR (1H) experiments, it is obvious that the resonance signals for the protons on the peripheral DMIP units, the internal benzyl rings, and the HPB unit were strikingly shifted downfield upon increasing the temperatures from 278 to 323 K (see Figure S4 b in the Supporting Information). This observation reflects the decreasing p–p stacking interactions upon heating. In addition, the results from CDNMR (1H) experiments also confirmed that the p–p interactions took part in the gel formation (see Figure S4 a in the Supporting Information). In addition, the p–p interactions were also evidenced by means of a SAXS study. A prominent reflection characteristic of a typical p–p stacking distance was observed in the wide-angle region at 3.5 æ of the SAXS patterns (Figure 2). Based on this experimental evidence and our previous study,[13a,b] the strong intermolecular multiple p–p stacking might be the key contributor in the formation of the self-assembled gel. Spectroscopic and AIEE investigations In recent years, functional fluorescent gels have gained much interest because of their potential applications for optoelectronic devices and light-harvesting materials.[18] Among hundreds of functional fluorescent organogels reported to date, only very few of them exhibited unique AIEE properties.[19] The UV/Vis and fluorescence spectral of HPB-G1 in CCl4 was recorded first (Figure 4). The lower absorption at l = 293 nm was attributed to coupling between the oxazole and the fused phenyl rings. Another intense absorption band at l = 338 nm corresponded to coupling between the benzoxazole and hydroxyphenyl rings.[9e] Moreover, the intense absorption bands at l = 315 and 322 nm were attributed to the dendritic unit. Interestingly, the maximum peak in the absorption spectra of HPB-G1 as a thin film is clearly redshifted relative to the spectra recorded of a dilute solution of HPB-G1, thus indicating that “J aggregates” from dendron HPB-G1 were formed in the aggregated state. In the fluorescence spectra, the emission
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Figure 4. A) Absorption (solid line) and fluorescence spectra excited at l = 321 nm (dashed line) of HPB-G1 at a concentration of 5.0 Õ 10¢5 m in CCl4.
maximum of HPB-G1 at l = 471 nm in CCl4 was due to the keto tautomer upon the ESIPT process and the small emission at the short wavelength of l = 364 nm was observed from the enol tautomer. These findings imply that HPB-G1 exists dominantly as the keto form in CCl4. Moreover, a large Stokes shift of Dl = 133 nm was observed in HPB-G1. Conventional luminescent molecules in the solid and/or aggregated state typically show fluorescence quenching. Very excitingly, the organogels of HPB-G1 exhibited significantly enhanced emission relative to the solution state. Dendron HPBG1 in solution was weakly luminescent under UV light, whereas its gel was highly fluorescent with a green emission (see Figure 5 A and Figure S5 in the Supporting Information). Such enhanced emission by aggregation was monitored by TD fluorescence spectra of HPB-G1 in CCl4 (Figure 5 B). The fluorescence intensity at l = 475 nm from the keto form of the HPB unit increased when cooling the hot solution to room temperature, thus showing that stronger emission intensity upon aggregation happened. Further evidence of AIEE was observed from solvent-titration experiments (Figure 5 C), in which the fluorescence intensity at l = 471 nm that resulted from the keto tautomer of HPB-G1 increased when the ratio of CCl4 (gelation media)/THF (good solvent) by volume increased in the binary solvent.
This AIEE phenomenon from HPB-G1 gels can be unambiguously explained based on above experimental evidence and previous studies of the AIEE phenomenon in HPB derivatives.[8, 9] The benzoxazole ring of HPB-G1 molecules in dilute solution could rotate freely around the single bonds and the radiative-decay channel would be effectively suppressed by this type of intramolecular torsion. While in the aggregate gel state, intramolecular rotation and torsion were greatly impeded and a more planar and conjugated conformation of the HPB units in HPB-G1 was induced due to strong intermolecular forces. Therefore, the ESIPT process in the HPB units from the hydroxy proton to the nitrogen atom (i.e., from the enol tautomer to the keto form) can be more easily developed (see Scheme S1 in the Supporting Information). Moreover, the nonradiative-decay channel was effectively restricted in the aggregate gel state, which in turn populated the irradiative state of the excited molecules and resulted in the remarkable fluorescence enhancement. Furthermore, the bulky dendritic units in HPB-G1 play an important role of favoring J-type aggregation in the aggregated state, which restricts the formation of the excimer complex.[7b] In addition, it should be noted that the keto tautomer of HPB units shows stronger p–p stacking interactions due to its more planar structure, and hence a higher tendency for enhanced gelation ability with the assistance of p–p interactions between the dendritic units. Consequently, the synergetic effects of restricting intramolecular rotation, ESIPT phenomenon, and J-aggregate formation are speculated to be responsible for the AIEE phenomenon of the HPB-G1 gels. Sol–gel transformations of dendritic organogels by anions With reference to previous studies, it is demonstrated that the hydroxy groups in HPB can selectively interact with anions.[14, 9b] Therefore, we reasoned that this dendritic gel might be selectively responsive to anions. Thus, the anion responsive properties of the gels of HPB-G1 toward a number of selected target anions (i.e., F¢ , Cl¢ , Br¢ , I¢ , and HSO4¢ as tetrabutylammonium salts) were studied in THF and DMSO gels (see Figure 6 A and Figure S9 in the Supporting Information). Upon the addition of two equivalents of the anion on top of the THF gel, a dramatic color change from a translucent colorless gel to a light-yellow
Figure 5. A) Photograph of HPB-G1 in the gel and solution states in CCl4 under UV-light illumination. B) Variable-cooling time fluorescence spectra of HPB-G1 in CCl4 (5 Õ 10¢5 m) upon cooling to room temperature from 76 8C. C) Fluorescence changes of HPB-G1 in mixed solvents with different solvent ratios (1.0 Õ 10¢5 m; lex = 321 nm). Chem. Eur. J. 2015, 21, 11018 – 11028
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Full Paper solution and a rapid gel-to-sol transition were observed, as expected, only in the case of the fluoride anion (Figure 6 A). Therefore, we believe that the HPB-G1 gel can be used as a selective naked-eye sensor system for fluoride anions. Very interestingly, the fluoride anion also caused a morphology change of the gel system (Figure 6 B, C). Relative to the entangled 3D network of the xerogel in THF, SEM images of the dried sol after addition of fluoride anions showed no sign of fibers. The gel, which was recovered by the addition of H2O, showed a similar fibrillar morphology again. Thus, the addition of fluoride anions disrupted the molecular packing of the dendritic gelator HPB-G1, which was followed by dissociation of the fibers and the 3D gel network. To understand the anion-responsive properties further, the UV/Vis and fluorescence spectra of dendron HPB-G1 in THF (5 Õ 10¢5 m) in the presence of 2.0 equivalents of various anions were studied. It is clearly shown in Figure 6 D that the absorption maxima were increased and significantly shifted to longer wavelengths only in the presence of fluoride ions. Furthermore, a noticeable spectral shift was only observed in the fluorescence spectrum upon the addition of fluoride ions and the fluorescence intensity increased (Figure 6 E). Moreover, the UV/Vis spectra of dendron HPB-G1 in THF (5 Õ 10¢5 m) in the presence of an increasing amount of fluoride ions were conducted. Upon Figure 6. A) Photographs of the HPB-G1 gel (THF, 25.0 mg mL¢1) upon the addition of 2.0 equiv of each anion. B) SEM image of the xerogel after drying from the gel state. C) SEM image of the aggregates in the solution state after the addition of F¢ ions. D) UV/Vis absorption and E) fluorescence changes of HPB-G1 in THF (5 Õ 10¢5 m) upon the addition of 2.0 equiv of different anions. Chem. Eur. J. 2015, 21, 11018 – 11028
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Full Paper an increase in the concentration of fluoride anions (0–5 equiv.), the maximum absorbance intensity gradually decreased and an absorbance at approximately l = 402 nm increased, thus indicating further that the addition of fluoride ions generates a new species (see Figure S8 A in the Supporting Information). In addition, the 2:1 stoichiometric ratio between the gelator HPB-G1 and fluoride anions was confirmed by a Job-plot analysis (see Figure S7 in the Supporting Information). Additional evidence for the anion-responsive mechanism was obtained from 1H NMR experiments in [D6]DMSO. The proton signal assigned to the hydroxy group in HPB disappeared completely as the concentrations of fluoride anions increased to 0.5 equivalents (see Figure S11 in the Supporting Information). This outcome implies that the hydroxy groups perhaps underwent a deprotonation reaction by the fluoride anions. By taking into account the above experimental evidence and previous studies,[9b, 14] the reason for the selective response of the dendritic organogel to fluoride anions can be explained: The proton of the hydroxy group in HPB was only abstracted from the gel by the fluoride anion due to the stronger basicity of this ion; subsequently, the solubility of gelator HPB-G1 is changed and the p–p interactions in the gel are weakened, thus leading to the gel-to-sol conversion. Thus, the hydroxy group in the dendron HPB-G1 is crucial to the gelation and the selective detection of anionic species.
Sol–gel transformations of dendritic organogels by cations We realized that the HPB unit can provide a coordination site for the formation of stable metal complexes.[15] The cation-responsive properties of HPB-G1 was studied by investigating the influence of different metal cations (Figure 7 A); for exam-
ple, the HPB-G1 gel gradually transforms into a fluid solution with a small amount of white precipitate in four days upon the addition of one equivalent of Zn(OAc)2. However, the gels were not affected after the addition of the same amount of other metal cations, such as Ca2 + , Cu2 + , Cd2 + , and Pb2 + . These results clearly demonstrate that these organogels have a selective-responsive nature to target metal cations. Moreover, the corresponding coordination compounds were disrupted by the addition of an equivalent of ethylenediaminetetraacetic acid (EDTA), followed by heating and cooling the mixture to room temperature, thus reproducing the gel. The regeneration of the gel state indicates the EDTA exhibited stronger coordinating ability than the HPB unit, thus leaving dendron HPB-G1 free for self-assembly. The SEM analysis revealed that the entangled 3D fibrillar network of dendron HPB-G1 was converted into a structure with shorter and straight ribbons with large diameters of approximately 1 mm after the addition of Zn2 + ions (Figure 7 B, C). The coordination interaction between Zn2 + ions and dendron HPB-G1 was also studied by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass-spectrometric analysis, the Zn2 + -ion coordination peaks for [HPB-G1·Zn] (m/ z 826.1) and [2HPB-G1·Zn + Na] (m/z 1609.8) were observed (see Figure S12 in the Supporting Information). In addition, the coordination of Zn2 + ions to the dendritic ligands was confirmed by means of 1H NMR spectroscopic analysis. The integral of the hydroxy proton in HPB in the 1H NMR spectrum decreased by upon the addition of Zn2 + ions (see Figure S13 in the Supporting Information). Moreover, the chemical shifts of the protons in the HPB moiety of HPB-G1 were shifted upfield, thus suggesting that dendron HPB-G1 was coordinated to the Zn2 + ion. On the basis of the above experimental evidence and a previous study,[20] the reason for the selective response to the Zn2 + ion among other metal cations was probably due to the higher reactivity of the HPB unit with Zn2 + ions relative to Ca2 + , Cu2 + , Cd2 + , and Pb2 + ions in the gel; furthermore, the coordination behavior of HPB with Zn2 + ions destroyed the p– p interactions between the HPB units, thus leading to the gelto-sol conversion.[15b] Sol–gel transformations of dendritic organogels by change in pH
Figure 7. A) Photographs of the HPB-G1 gel (DMSO, 9.4 mg mL¢1) upon the addition of 1 equiv of each cation. B) SEM image of the xerogel after drying from the gel state. C) SEM image of the aggregates from the solution state after the addition of Zn2 + ions. Chem. Eur. J. 2015, 21, 11018 – 11028
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Very interestingly, the resultant gel also can reversibly transition between the solution and gel states under acid/base control. For example, a hot solution of HPB-G1 in DMSO could not be changed into a stable gel in the presence of five equivalents NaOH. The further addition of five equivalents of a solution of HCl in DMSO to the system, followed by heating and cooling resulted in a gel again (see Figure S14 in the Supporting Information). Moreover, the gel-to-sol transition is accompanied by a color change from white to light yellow. 1 H NMR titration experiments were carried out for these pHstimulus systems in [D6]DMSO (see Figure S15 in the Supporting Information). After the addition of one equivalent of hydroxide anions, the OH signal disappeared, thus indicating the
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Full Paper possible reaction between the hydroxy proton in HPB and the hydroxide anion. Furthermore, SEM analysis and UV/Vis and fluorescence spectroscopic experiments also confirmed the pH-responsive phenomenon (see Figure S16 in the Supporting Information). Sol–gel transformations of organogels by mechanical stimuli: a rheological study More interestingly, these dendritic gels are sensitive to shear stress; for example, the gel of HPB-G1 in cyanobenzene was changed into a viscous solution by vigorous shaking and the gel could be regenerated after a while. The rheological property of the gel was investigated in detail (Figure 8). The storage and loss moduli (G’ and G’’, respectively) were monitored as a function of the angular frequency for three different concentrations of the gel (Figure 8 A). At low strain values (0.05 %), all three samples showed that the elastic-storage modulus G’ was greater than the loss modulus G’’ over the entire range of frequencies. Strain-amplitude sweeps were also studied (c = 36 mg mL¢1; Figure 8 B). The G’ and G’’ values rapidly deceased above the critical region (g = > 5 %), thus representing a partial breakup of the gel into a quasi-liquid state. Interestingly, this gel showed a very quick recovery of the mechanical strength after a large-amplitude oscillatory breakdown (Figure 8 C). Once a large strain was applied to the gel (g = 100 %), the G’ value decreased to approximately 150 Pa, thus resulting in a quasi-liquid state. However, the G’ value immediately recovered close to 6 Õ 104 Pa within 10 seconds when the amplitude oscillations changed to a small value (g = 0.05 %), thus showing a quasi-solid state. Notably, this thixotropic process could be repeated several times. Clearly, the dendritic gel exhibited a smart thixotropic property. Sol–gel transformations of organogels by ultrasound Ultrasound also was used as an external stimulus to trigger the phase transition of the gel.[13d, 21] In this case, HPB-G1 gels could be formed only upon ultrasonic treatment in some solvents, such as 2-methoxyethanol, ethyl acetate, anisole, and THF. For example, a hot solution of HPB-G1 in 2-methoxyethanol was cooled to room temperature and a partial gel instead of a stable gel was observed. Gelation was observed exclusively when ultrasound was used as an external stimulus (see Figure S17 a in the Supporting Information). The morphologies of the HPB-G1 gels before and after sonication were investigated by means of SEM analysis to understand the effects of ultrasonic treatment on the gelation process (see Figure S17 b, c in the Supporting Information). The prominent morphological changes from a foldlike structure into a 3D entangled network were responsible for the gelation process triggered by sonication. On the basis of these studies, these fluorescent dendritic organogels exhibit a reversible sol–gel phase transition in response to external environmental stimuli, including temperature, sonication, shear stress, and the presence of anions, metal cations, acids/bases, thus suggesting that such responChem. Eur. J. 2015, 21, 11018 – 11028
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Figure 8. Rheological data of the organogels prepared from HPB-G1 in cyanobenzene. A) Frequency sweeps (0.01–100 rad s¢1) of three samples of the organogels with different concentrations (c = 20, 28, and 36 mg mL¢1). B) Strain-amplitude sweeps and C) step-strain measurement of the organogel (c = 36 mg mL¢1). Step strain was carried out in a continuous measurement (four cycles).
sive fluorescent gel systems are highly desirable for the development of sensor devices, optoelectronic devices, and lightharvesting materials.
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Full Paper Conclusion We have designed and synthesized a highly efficient and versatile dendritic organogelator HPB-G1 based on a HPB-cored poly(aryl ether) dendron. The HPB-G1 dendron could form stable organogels toward various apolar and polar organic solvents. A preliminary study has revealed that multiple p–p stacking interactions promote the formation of stable organogels. Interestingly, significant fluorescence enhancement was observed after gelation, which could be ascribed to the formation of J aggregates and the ESIPT phenomenon in the gel state. More importantly, these organogels exhibite multistimuli-responsive behaviors toward multiple external stimuli, including temperature, sonication, shear stress, and the presence of anions, metal cations, acids/bases, thus leading to reversible sol–gel phase transitions (Figure 9). The further application of these organogels in the fabrication of photoelectrical nanodevices and switchable fluorescent sensors will be conducted our laboratory.
Figure 9. Reversible sol–gel phase transition of the HPB-G1 gel triggered by shear stress, sonication, temperature, and the presence of anions, metal ions, acids/bases.
Experimental Section General: All the starting materials were obtained from commercial suppliers and used as received. All the solvents were distilled over suitable drying agents. Moisture-sensitive reactions were performed in an atmosphere of dry argon. 1H and 13C NMR spectra were recorded on Bruker AMX 300 spectrometer (1H: 300, 13C: 75 MHz) or Bruker AMX-600 spectrometers (1H: 600, 13C: 150 MHz) at 298 K. Chemical shifts are reported in parts per million (ppm) relative to the internal standards, partially deuterated solvents, or TMS. HRMS (ESI) mass spectra were obtained on a Bruker APEX IV instrument. MALDI-TOF mass spectrometry was performed on a Bruker Biflex III MALDI-TOF spectrometer with a-cyano-4-hydroxylcinnamic acid (CCA) as the matrix. Elemental analyses were performed on a Carlo–Erba-1106 instrument. The SAXS measurements were performed on equipment with a SAXSess camera (AntonPaar, Graz, Austria), which was connected to an X-ray generator (Philips) operating at 40 kV and 50 Ma and employed CuKa radiation (l = 0.154 nm). Compounds 1,3,5-tris(bromomethyl) benzene[22] and 2-(2’,4’ dihydroxyphenyl)benzoxazole[23] were prepared according to previously reported synthetic procedures. Chem. Eur. J. 2015, 21, 11018 – 11028
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Synthesis of G1-CH2Br: A mixture of 1,3,5-tris(bromomethyl)benzene (2.41 g, 6.8 mmol), dimethyl 5-hydroxyisophthalate (2.14 g, 10.2 mmol), and potassium carbonate (K2CO3, 2.82 g, 20.5 mmol) in anhydrous DMF (30 mL) was stirred under nitrogen for 6 h at 65 8C. The reaction mixture was poured into water (100 mL) and stirred. The resulting precipitate was filtered, washed with water, and dissolved in CH2Cl2 (300 mL). The organic layer was washed with brine (50 mL), dried over Na2SO4, and concentrated under reduced pressure. The residue was purified by flash column chromatography to afford G1-CH2Br (1.4 g, 48 %) as a white solid. 1H NMR (300 MHz, CDCl3): d = 3.95 (s, 12 H; COOCH3), 4.53 (s, 2 H; ArCH2Br), 5.16 (s, 4 H; ArCH2O), 7.47 (s, 3 H; ArH), 7.84 (d, J = 1.5 Hz, 4 H; ArH), 8.31 ppm (t, J = 1.4 Hz, 2 H; ArH,); 13C NMR (75 MHz, CDCl3): d = 165.9, 158.5, 138.8, 137.4, 131.9, 127.8, 126.3, 123.4, 120.0, 69.7, 52.4, 32.8 ppm; HRMS-ESI: m/z calcd for C29H27BrO10Na: 637.06798 [M+ +Na] + ; found: 637.06769. Synthesis of HPB-G1: A mixture of 2-(2’,4’-dihydroxyphenyl)benzoxazole (123 mg, 0.54 mmol), G1-CH2Br (315 mg, 0.52 mmol), and potassium carbonate (K2CO3, 149 mg, 1.08 mmol) in anhydrous DMF (10 mL) was stirred under nitrogen for 6 h at 65 8C. The reaction mixture was poured into water (100 mL) and stirred. The resulting precipitate was filtered, washed with water, and dissolved in CH2Cl2 (300 mL). The organic layer was washed with brine (50 mL), dried over Na2SO4, and concentrated under reduced pressure. The residue was purified by flash column chromatography to afford HPB-G1 (320 mg, 78 %) as a white solid. 1H NMR (300 MHz, DMSO): d = 3.94 (s, 12 H; COOCH3), 5.17–5.20 (m, 6 H; ArCH2O), 6.65–6.71 (m, 2 H; ArH), 7.32–7.37 (m, 2 H; ArH), 7.52 (s, 3 H; ArH), 7.56–7.59 (m, 1 H; ArH), 7.68–7.71 (m, 1 H; ArH), 7.85 (d, J = 1.5 Hz, 4 H; ArH), 7.91–7.94 (m, 1 H; ArH), 8.30 ppm (t, J = 1.4 Hz, 2 H; ArH); 13 C NMR (75 MHz, CDCl3): d = 166.0, 163.1, 163.0, 160.7, 158.6, 149.0, 140.1, 137.5, 137.3, 131.9, 128.4, 126.2, 124.9, 123.4, 120.1, 118.8, 110.5, 108.2, 104.2, 102.3, 70.0, 69.7, 52.5 ppm; HRMS-ESI: m/z calcd for C42H34NO13 : 760.20247 [M¢H] + ; found: 760.20350; elemental analysis (%) calcd for C42H35NO3 : C 66.22, H 4.63, N 1.84; found: C 65.77, H 4.63, N 1.99. Gelation test: A weighed sample of the dendritic organogelator was mixed with a selected solvent (0.5 mL) in a septum-capped vial and heated in an oil bath until the solid dissolved. The sample vial was cooled to room temperature, and the aggregation state was assessed. If no flow was observed when inverting the vial, a stable gel had formed and was noted as gelation (G). If the clear solution (> 60 mg mL¢1) was retained, it was marked as soluble (S). If the compound was unable to dissolve, it was noted as insoluble (I). In some solvent systems (e.g., 2-methoxyethanol, ethyl acetate, anisole, and THF), short-term sonication (0.40 W cm¢2, 40 KHz, 1– 2 min) was needed at the beginning of the cooling process. Repeated heating and cooling confirmed the thermoreversibility of the gelation process. The CGC value of the organogelator was determined by measuring the minimum amount of gelator required for the formation of a stable gel at room temperature. UV/Vis absorption spectra: The UV/Vis absorption spectra were obtained on a Lambda 950 UV/VIS/NIR spectrophotometer within the range l = 200–600 nm. Solution spectra were recorded in solutions of THF or DMSO with concentrations of approximately 5 Õ 10¢5 m in quartz cells with a pathlength of 1 cm. The solid-film sample was prepared by drop-casting (3000 rpm) a highly concentrated solution of HPB-G1 in CH2Cl2 (ca. 0.1 mL, 20 mg mL¢1) onto a quartz plate (12 Õ 25 mm2, thickness = 1 mm) in air at room temperature, which was dried under vacuum before the measurement was taken. Fluorescence spectra: The fluorescence spectra were recorded on a Hitachi F-7000 fluorescence spectrophotometer within the range
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Full Paper l = 340–700 nm. The solution spectra were recorded in solutions of THF or DMSO with concentrations of approximately 5 Õ 10¢5 m in quartz cells with a pathlength of 1 cm. Rheological measurements: The rheological measurements were carried out with a stress-controlled rheometer (TA Instruments, ARG2) equipped with steel parallel-plate geometry (diameter = 40 mm). The gap distance was fixed at 750 mm. A solvent-trapping device was placed above the plate to avoid evaporation. The gel samples were mounted on the plate and allowed to stand for at least 5 min to facilitate the recovery of the structure. All the measurements were made at 15 8C. Strain sweep at a constant frequency (6.28 rad s¢1) was performed in the range 0.01–200 % to determine the linear viscoelastic region (LVER) of the gel sample. The frequency sweep was obtained at 0.1–100 rad s¢1 at a constant strain of 0.05 %, well within the linear regime determined by the strain sweep. A thixotropic study was conducted to examine the recovery behavior of dendritic organogels after the strain sweep. The recovery of the storage modulus of the destroyed gel was monitored at a constant frequency (6.28 rad s¢1) under a low strain (0.05 %) just after the strain-sweep progress. The storage G’ and loss G’’ moduli were recorded as functions of time in the recovery process. To investigate the recovery properties of the samples in response to applied shear forces, we used the following programmed procedure (the applied shear force is expressed in terms of strain: 1) The gel sample was allowed to stand for 5 min after mounting; 2) linear small-amplitude oscillations (g = 0.05 % and w = 1 Hz) were performed for 5 min to monitor the initial G’ and G’’ values; 3) nonlinear, large-amplitude oscillations (g = 100 %) were performed at the same frequency for 1.5 min to breakdown the gel structure; 4) linear small-amplitude oscillations (g = 0.05 %) were performed for 5 min at the same frequency to monitor the recovery of mechanical strength of the gels; 5) three further cycles of the breakdown and recovering processes, as described in (3) and (4), were performed. The measurements were carried out with a solvent trap to prevent drying the gel sample during the long-time measurement. The storage G’ and loss G’’ moduli were recorded as functions of time in both processes.
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