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Thermoresponsive Amphipathic Fluorescent Organic Liquid Takashi Takeda, Shunsuke Yamamoto, Masaya Mitsuishi, and Tomoyuki Akutagawa J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01131 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018
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The Journal of Physical Chemistry
Thermoresponsive Amphipathic Fluorescent Organic Liquid Takashi Takeda,* Shunsuke Yamamoto, Masaya Mitsuishi and Tomoyuki Akutagawa Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai, 980-8577, Japan. ABSTRACT: We herein report a thermoresponsive amphipathic fluorescent organic liquid, tetraethyleneglycol(TEG)ester-substituted tetraphenylethylene (TPE) 1, as a new class of functional organic materials. AIE fluorescence of 1 could be modulated by controlling the temperature. The introduction of hydrophilic PEG chains induced the formation of a uniform fluorescent colloidal supramolecular structure in H2O. This colloidal structure of 1 also showed temperaturedependent fluorescence due to its motility in the supramolecular structure and the collapse/precipitation of the colloid of 1.
Introduction Nonionic organic fluorescent liquids are rare category of compounds.1-3 Organic fluorescent liquids based on derivatives of oligo(p-phenylenevinylene),4 diphenylanthracene,5 pyrene,6,7 benzothiadiazoles,8 tetrazine,9 azaacene10 with branched alkyl chains have been reported. In these cases, the branched alkyl chains reduced molecular aggregation and increased motility in the condensed phase to lower the solidification temperature. These organic fluorescent liquids have been studied with regard to their unique luminescent properties, including color tuning by mixing with dopant molecules, and their potential application to organic electronics. The unique properties of organic fluorescent liquids could be investigated further by modification of the central electronic core and peripheral aliphatic chains. Aggregation-induced emission (AIE)11,12 has attracted considerable interest not only from a fundamental perspective but also with regard to practical applications, such as organic light-emitting diode devices. In contrast to typical emissive polyaromatic hydrocarbons (PAH), which lose fluorescence with an increase in concentration, AIE luminophores show increased fluorescence. Whereas PAH easily form excimers in a concentrated solution that lose photoexcitation energy through nonradiative processes, rotation of the substituents in AIE luminophores is restricted in the aggregate, which suppresses nonradiative deactivation to increase the fluorescence intensity. Considering this unique luminescent property, fluorescent organic liquids based on the AIE core should be interesting. Among AIE materials, including penta/hexaphenylsiloles,13,14 polyphenylenes,15 and arylenevinylenes,16-18 the tetraphenylethylene (TPE) core19-23 should be suitable for studying fluorescent organic liquids with an AIE mechanism thanks to its simple molecular structure and easy functionalization.
During the course of our studies on photophysical and ferroelectric properties of alkylamide-substituted aromatics,24,25 we synthesized TPE tetraamide 2 with strong aggregation ability. TPE tetraamide 2 showed three kinds of fluorescence based on hydrogen-bonded excimer.26 In addition, we found that the melting point of TPE tetraester 3 was much lower than that of 2. We expected that further modification of the terminal aliphatic group should enable the creation of an AIE-based organic fluorescent liquid. Especially, we desired to use different terminal groups from traditional hydrophobic branched alkyl chains, which would expand the potential of fluorescent organic liquids. In this work, we report a thermoresponsive amphipathic fluorescent organic liquid with TPE tetraethyleneglycol (TEG) ester (1).27,28 We demonstrate the utility of linear TEG units for preparing a hydrophilic organic fluorescent liquid. The high fluidity of PEG units30-35 and thermal stability enabled 1 to maintain its liquid state in a very broad temperature range of ca. -60 °C - > 300 °C. Thanks to the high motility of the AIE core of TPE,35 it was not only fluorescent in the neat state at room temperature, but was also thermoresponsive. The introduction of hydrophilic PEG units allowed us to observe unique aggregation behavior in aqueous medium, which led to the formation of a uniform colloidal structure with temperaturedependent fluorescence. Results and discussion TPE-TEGester 1 was prepared by the reaction of TPEtetracarboxylic acid36 with thionyl chloride followed by the corresponding TEG alcohol, as summarized in Figure 1a. Purification of the reaction mixture by silica gel column chromatography followed by recycling GPC gave pure 1 as a dark yellow viscous oil of 1•CHCl3 (Figure 1b).
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Figure 1. (a) Preparation of TPE-TEGester 1. (b) Photograph of neat 1. (c) DSC chart of 1.
Figure 1c and Figure S1 show TG and DSC curves of TPETEGester 1. In the TG analysis, a decrease in weight corresponding to the desorption of CHCl3 was observed between 25 °C - 300 °C, which demonstrates the high thermal stability of this compound. 1 maintained its liquid state even after the desorption of CHCl3. In the DSC measurement, the glass transition temperature Tg of 1 was observed at ca. -60 °C, and 1 showed a liquid state under ambient conditions. It is interesting to compare the melting points of 1-3. While these three compounds have similar molecular structures, their melting points are significantly different (1: -60 C; 2: 252-253 C26; 3: 62-63 C26). This difference could be accounted for by the degree of intermolecular noncovalent interactions. Strong intermolecular hydrogen bond increased the melting point of 2. Notably, Tg of 1 is ca. 120 °C lower than the solid-liquid phase transition temperature of 3 (62-63 °C)26, which has the same chain length, and comparable to that of a recently reported TPE-based liquid with branched alkyl chains (ca. -55 °C).27 The subtle different degree of van der Waals interaction of terminal alkyl chains or TEG chains induced huge difference of melting point between 1 and 3. 1 could maintain its liquid state due to the contributions of both the high motility of the twisted central TPE core29 and the high fluidity/motility of the TEG chain.30-35 The twisted arrangement of skeleton of TPE is not suitable to obtain intermolecular or other interactions to inhibit molecular motion. The rotation of phenyl rings of TPE is energetically easy process. Thus, TPE core of 1 should have high motility due to thermal excitation even in neat liquid state. The photophysical properties of 1 in solution were similar to those of the parent TPE and its derivatives. The absorption maxima of 1 in CHCl3 (301 and 335 nm, Figure 2a) were similar to those of 3 (300 and 335 nm), indicating the complete dissociation of 1 in organic solvents. While a small bathochromic shift (ca. 20 nm) was observed with neat 1 (Figure S2), no distinct difference in absorption was observed between neat 1 and 1 in solution. On the other
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hand, distinctly different fluorescence properties of 1 were observed depending on the state of 1, probably due to limited intermolecular interaction of 1 both in neat and in solution state. While TPE-TEGester was nonfluorescent in solution, it gave sharp fluorescence in the neat state upon irradiation with UV light (Figure 2c). Figure 2b shows the fluorescence spectrum of neat 1 under irradiation with 380 nm UV light. Distinct fluorescence with an emission maximum of 500 nm was observed, which is the typical AIE fluorescence of solid TPE and its derivatives. If we consider the high viscosity of 1, the molecular motion of the TPE core was suppressed even in the neat liquid state to induce its AIE emission. A similar discussion has recently been reported by Sada, Kokado and co-workers regarding TPE liquid with branched alkyl chains.27 However, our compound 1 showed a relatively low fluorescence quantum yield (f = 16.2%) compared with liquid TPE with aliphatic chains (f = 46-93%) and parent TPE [f = 50.3% in a mixed solvent of water/MeCN (9/1)]19. The high fluidity of PEG units induced the fluctuation of 1, including TPE core, in the neat liquid state to decrease the efficiency of fluorescence. (b)
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Figure 2. (a) UV-Vis spectra of 1 and 3 in CHCl3. (b) Fluores-3 cent spectra of 1, neat and in CHCl3 solution (1.01 10 M) at room temperature. (ex = 380 nm) (c) Photograph of fluorescence of 1 in CHCl3 solution (left) and neat (right) upon irradiation with 365 nm UV light.
If we consider the mechanism of AIE emission, modulation of the motility of the electronic core of 1 could induce a change in fluorescence. Thanks to the high thermal stability of the liquid phase of 1, we could modulate the motility of the electronic core by changing the temperature. Figure 3a shows temperature-dependent change in fluorescence of 1. Upon cooling of neat 1, a drastic increase in fluorescence intensity with a hypochromic shift (em = 517 nm at 32 °C to em = 466 nm at 196 °C) was observed. This temperature-dependent change in fluores-
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cence was observed reversibly and repeatedly. The change in fluorescence was observed at around Tg (ca. 60 °C, shown by the vertical gray broken line in Figure 3b). Moreover, 1 in a polystyrene matrix (Tg = 100 °C37) exhibited hypochromic fluorescence (em = 475 nm; Figure S3), while neat 1 exhibited fluorescence of em = 517 nm at the same temperature (25 °C). These results indicate that the emission wavelength of 1 is significantly dependent on the molecular motility. In fact, his drastic change in fluorescence was not observed with the parent solid TPE, in which the motility is highly restricted. It is interesting to compare the temperature-dependent fluorescence change of 1 with that of TPE based liquid crystal reported by Cho and co-workers.38 While TPE LC showed temperaturedependent change of fluorescent wavelength at above room temperature, liquid TPE-TEG ester 1 showed similar change at below temperature. This difference could be understood by the different degree of freedom of motility in the condensed phases. To further understand this quenching, the integrated emission intensity I was depicted in an Arrhenius plot (Figure 3c). The observed linear relationship indicates a thermal-activated emission process with activation energy EA = 0.82 kJ mol-1. This activation energy is understood to be the activation of a non-radiative quenching pathway, assuming a temperature-independent radiative transition rate constant. Due to this low activation energy, the quenching upon heating can be attributed to fluctuation of the TPE core, which gradually induced non-radiative deactivation. At high temperature, this non-radiative deactivation became dominant due to fast fluctuation/rotation of the TPE core to completely quench the fluorescence. Such a gradual change in fluorescence may be suitable for application as a temperature sensor. Figure 3b shows a plot of the fluorescence maximum of 1 against temperature, with which we could monitor temperatures below room temperature. We could also monitor the temperatures above room temperature by the fluorescence intensity. The fluorescence of neat 1 disappeared upon heating to 150 °C and recovered upon cooling to room temperature (Figure S4 and Movie S1).
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Figure 3. (a) Temperature-dependent fluorescence spectra of neat 1 (ex = 355 nm). (b) Plot of the emission peak wavelength λem against temperature. The vertical broken line indicates the glass transition temperature of 1. (c) Arrhenius plot of integrated emission intensity 1 at high temperature.
Due to its hydrophilic TEG chains, 1 could form a welldefined colloidal structure in water.39 Figure 4a shows a
photograph of 1 in H2O. Clear Tyndall scattering was observed upon irradiation with a laser pointer. This colloidal aqueous solution of 1 emitted vivid fluorescence, in contrast to an organic solution. Figure 4b shows the fluorescence spectra of the colloidal aqueous solution of 1. Clear fluorescence derived from AIE of the TPE core was observed. In water dispersions, the spectrum has a peak at around 500 nm, which corresponds to the emission peak of neat TPE-TEGester 1 at room temperature. This indicates that the molecular environment of 1 in water dispersions is similar to that in the neat liquid, and a certain degree of molecular motion of 1 occurs in the colloid (Figure 4c). (a)
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Figure 4. (a) Tyndall scattering and fluorescence (ex = 365 nm) of TPE-PEGester 1 in H2O. (b) Fluorescence spectrum of -4 1 in H2O (1.27 10 M, 25 °C). (c) Schematic illustration of the formation of colloid of 1 in H2O.
To further understand the properties of the molecule 1 and the structure of the colloidal structure, π-A isotherm and dynamic light scattering (DLS) measurements were performed in combination with other techniques. Figure 5a shows the π-A isotherm of Langmuir monolayer of 1 at the air–water interface to examine the amphiphilic property of the molecule. The isotherm indicates that the monolayer has transition through gas to solid phase, indicating the formation of monolayer at the air–water interface. From the fitting line shown as gray broken curves in the figure, the surface limiting areas S were obtained for liquid-expanded (LE) and solid (S) phases to be Sliq = 3.2 nm2 for LE and SSol = 0.92 nm2 for S phase, respectively. The SSol value corresponds to the projected area of TPE core modeled by the molecular dynamics calculation (0.90 nm2). This denotes the ordered molecular conformation in S phase: the TPE core and TEG chains are located at the air–water interface and in the water subphase, respectively. In the LE phase, larger Sliq value means that not only TPE core but also TEG chains are adsorbed on the air–water interface, suggesting the moderate hydrophilic nature of TEG chains. This analysis shows that TEG chains in the TPE-TEGester 1 behave as weak hydrophilic part and fully hydrated only at a highly compressed case. Figure 5b-d shows the size distribution of the aggregates
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The Journal of Physical Chemistry in water at various concentration of 1. All distributions have single peaks at around 150 nm except in the concentrated case (10-3 M). The AFM observation of the aggregate revealed the spherical shape (Figure S5). The concentration independent size for CAC). The intensity of the fluorescence decreased with a further increase in the concentration (1.3 10-3 M). If we consider the aggregation behavior of 1 in H2O, this change in fluorescence can be understood in terms of the formation of the colloidal structure. The decrease in the fluorescence intensity of 1 at higher concentration can be accounted for by the formation of secondary larger particles (Figure 5b). As the size of colloidal particle of 1 increases, not only the decrease of the fluorescent intensity but also blueshift of the fluorescence was observed. This tendency is quite similar to temperature-dependent fluorescence of 1 in water below clouding temperature (Figure S7-9). It is well known that the degree of hydrogen-bonding interaction between PEG chains and water molecules is weakened as temperature increases to lead decrease of the hydration capacity.43 If we consider these facts, we can propose a mechanism for this unique concentrationdependent fluorescence of 1 in water. As the concentration increased the colloidal structure of 1 turned larger with less hydration of water per molecule. As a result, the motility of TPE core changed to induce the decrease of the fluorescence intensity in addition to blue-shift of the fluorescence. Figure 6b and Figures S7-9 show the temperature-dependent fluorescence of 1 in H2O at various concentrations below and above the CAC. Typically, the fluorescence of an organic molecule in solution decreases with an increase in temperature as observed in the solution of 1 at low concentration (Figure S7). However, the fluorescence intensity of 1 in H2O above the CAC showed a different response (Figure 6b and Figures S8, 9). While the fluorescence intensity of the 1.3 10-4 M aqueous solution of 1 decreased at 40 °C, it increased above 40 °C as the solution turned cloudy (Figure 6b-d). Further heating (> 65 °C) of the solution of 1 again decreased the fluorescence intensity. If we consider the cloudiness of the solution and typical AIE, this unique multistep change in fluorescence could be accounted for by the collapse of the colloidal structure governed by lower critical solution temperature (LCST) mechanism, and the molecular motility in the aggregates. It is well known that the degree of hydrogen bond between PEG chain and water decreases as temperature increases, which leads to decrease of solubility of the PEGylated molecules/polymers.43 Heating of the colloidal solution of 1 induced not only an increase in molecular motility but also decrease the solubility of the colloidal structure to form a microsolid of 1. Due to the limited molecular motility of 1 in the solid state compared to that in the aggregate, the fluorescence intensity increased even upon heating the solution.
Interestingly, 1 showed concentration- and temperature-dependent fluorescence in H2O governed by formation/collapse of the colloidal structure and the molecular motility in the aggregates. Figure 6a shows the
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Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT This work was partly supported by JSPS KAKENHI Grant Number JP17K05769 and IMRAM project grant for intramural cooperative study of young scientists (FY 2017). (c)
REFERENCES
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Figure 6. (a) Concentration-dependent fluorescence spectra of 1 in H2O at 20 °C (ex = 380 nm). (b) Plot of the emission intensity (507 nm) and absorbance (600 nm) of 1 in H2O -4 (1.3 10 M) against temperature. (c) Temperature-4 dependent UV-Vis spectra of 1 in H2O (1.3 10 M). Absorbance at long-wavelength region observed at high temperature corresponds to scattering of solution. (d) Photograph of -4 1 in H2O (1.3 10 M) at various temperatures.
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Summary In summary, we have reported TPE-TEGester 1 as a thermoresponsive amphipathic fluorescent organic liquid. We demonstrated the utility of PEG chains for the creation of a functional organic liquid material. TPE-TEGester maintained a liquid state and exhibited fluorescence based on an AIE mechanism. Thanks to the motility of the scaffold of TPE even in the liquid neat state, its fluorescence could be modulated by changing its temperature. A hydrophilic TEG chain allowed us to create uniform nanodroplets in aqueous media. We believe that this amphipathic fluorescent organic liquid could lead to the development of a new field of material chemistry.
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ASSOCIATED CONTENT Supporting Information. Experimental section. TG curve and UV-Vis spectrum of neat 1. Fluorescence spectrum of 1 in a polystyrene matrix. Temperature-dependent on/off switching of fluorescence of neat 1. AFM images of aggregates of 1. Salt effect on the size distribution of aggregate 1. Tempera1 13 ture-dependent fluorescence spectra of 1 in H2O. H and C NMR spectra of 1 (PDF). Video of temperature dependent on/off switching of fluo-rescence of neat 1 (MPG). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION Corresponding Author *
[email protected].
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Ethyleneglycol functionalized TPE derivatives with terminal hydroxyl groups were also reported by Zheng et al. These materials were solid and the formation of colloidal structures was not reported. No temperature dependency of their fluorescence has been reported. See: Song, S.; Zheng, H.-F.; Li, D.-M.; Wang, J.-H.; Feng, H.-T.; Zhu, Z.-H.; Chen, Y.-C.; Zheng, Y.-S. Monomer Emission and Aggregate Emission of TPE Derivatives in the Presence of -Cyclodextrin. Org. Lett. 2014, 16, 2170-2173. Shustova, N. B.; Ong, T. –C.; Cozzolino, A. F.; Michaelis, V. K.; Griffin, R. G.; Dincă, M. Phenyl Ring Dynamics in a Tetraphenylethylene-Bridged Metal–organic Framework: Implications for the Mechanism of Aggregation-Induced Emission. J. Am. Chem. Soc. 2012, 134, 15061–15070 and references cited therein. Kawaguchi, S.; Imai, G.; Suzuki, J.; Miyahara, A.; Kitano, T.; Ito, K. Aqueous Solution Properties of Oligo- and Poly(ethylene oxide) by Static Light Scattering and Intrinsic Viscosity. Polymer 1997, 38, 2885-2891. Pike, E. R.; Pomeroy, W. R. M.; Vaughan, J. M. Measurement of Rayleigh Ratio for Several Pure Liquids Using a Laser and Monitored Photon Counting. J. Chem. Phys. 1975, 62, 3188-3192. Kinugasa, S.; Hayashi, H.; Hattori, S. Preparation and Characterization of Pure Oligoethylene Glycols I. Polym. J. 1990, 22, 1059-1064. Abe, A.; Tasaki, K.; Mark, J. E. Rotational Isomeric State Analysis of Poly(oxyethylene). Conformational Energies and the Random-Coil Configuration. Polym. J. 1985, 17, 883-893. Kugler, J.; Fischer, E. W.; Peuscher, M.; Eisenbach, C. D. Small Angle Neutron Scattering Studies of Poly(ethylene oxide) in the Melt. Makromol. Chem. 1983, 184, 2325-2334. Vennemann, N.; Lechner, M. D.; Oberthur, R. C. Thermodynamics and Conformation of Polyoxyethylene in Aqueous Solution under High Pressure: 1. Small-angle Neutron Scattering and Densitometric Measurements at Room Temperature. Polymer 1987, 28, 1738-1748. Shustova, N. B.; McCarthy, B. D. Dincă, M. Turn-On Fluorescence in Tetraphenylethylene-Based Metal– Organic Frameworks: An Alternative to AggregationInduced Emission. J. Am. Chem. Soc., 2011, 133, 2012620129. See also ref. 26. Polymer Handbook 2nd Edition; Brnsrup, J., Immergut, E. H., Eds.; John Wiley & Sons: New York, 1975. Bui, H. T.; Kim, J.; Kim, H.-J.; Cho, B.-K.; Cho, S. Advantages of Mobile Liquid-Crystal Phase of AIE Luminogens for Effective Solid-State Emission. J. Phys. Chem. C 2016, 120, 26695-26702. Similar studies on aggregation behavior in water with tetrakis(phenyltriazolyl)TPE with terminal aliphatic or TEG chains were reported by Cho and co-workers. No temperature dependency of their fluorescence in aqueous media has been reported. See references 40 and 41. Han, S.-B.; Kim, H.-J.; Jung, D.; Kim, J.; Cho, B.-K.; Cho, S. Polarity Effect of Exterior Chains on Self-Assembled Structure and Aggregation Mechanism of Tetraphenylethene Derivatives in THF/Water Mixtures. J. Phys. Chem. C 2015, 119, 16223-16229. Han, K.; Cho, B.-K. Chain-Dependent Emission Color Codes of Extended Tetraphenylethylene Derivatives: Discrimination between Water and Methanol. RSC Adv. 2015, 5, 9510-9517.
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Butt, H.-J.; Graf, K.; Kappl, M. Physics and Chemistry of Interfaces, 3rd Edition; Wiley-VCH, Weinheim, 2013. Ren, C.-I.; Nap, R. J.; Szleifer, I. The Role of Hydrogen Bonding in Tethered Polymer Layers. J. Phys. Chem. B 2008, 112, 16238-16248.
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TOC Graphic Thermoresponsive fluorescence in neat and water 150 °C
MeO(H2CH2CO)4OC
CO(OCH2CH2)4OMe
MeO(H2CH2CO)4OC
CO(OCH2CH2)4OMe
30 °C
Amphipathic fluorescent organic liquid
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