Ratiometric Indicator Based on Vibration-Induced Emission for in Situ

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A Ratiometric Indicator Based on Vibration-Induced Emission for In-Situ and Real-Time Monitoring of Gelation Processes Guangchen Sun, Haitao Zhou, Yang Liu, Yiru Li, Zhiyun Zhang, Ju Mei, and Jianhua Su ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06461 • Publication Date (Web): 17 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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ACS Applied Materials & Interfaces

A Ratiometric Indicator Based on Vibration-Induced Emission for In-Situ and Real-Time Monitoring of Gelation Processes Guangchen Sun, † Haitao Zhou, † Yang Liu, † Yiru Li, † Zhiyun Zhang,‡ Ju Mei,*,† and Jianhua Su*,† †

Key Laboratory for Advanced Materials, Institute of Fine Chemicals, School of Chemistry &

Molecular Engineering, East China University of Science & Technology, No. 130 Meilong Road, Shanghai 200237, China ‡

Department of Chemistry, National Taiwan University, Taipei, 10617 Taiwan, R.O.C.

KEYWORDS: fluorescent visualizer, ratiometric indicator, low molecular-weight gelator, vibration-induced emission (VIE), gelation process

ABSTRACT: Monitoring specific processes such as gelation in a ratiometric and visual manner is of scientific value and has practical implications, but remains challenging. Herein, an innovative fluorescent low-molecular-weight gelator (DPAC-CHOL) capable of revealing and self-revealing the gelation processes in situ and in real time via the ratiometric fluorescence change from orange-red to blue has been developed. By virtue of its vibration-induced emission (VIE) attribute, the gelation point, critical gelation concentration (CGC), and the internal

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stiffness of the gel networks of DPAC-CHOL and other gelation systems could be facilely evaluated in a ratiometric and naked-eye observable fashion. Noteworthily, the DPAC-CHOLdoped gelation system Ph-CHOL can quantitively identify the environmental temperature in a daily-concerned range (i.e. 20‒55 oC). This work not only provides a versatile advanced material but also opens up a new avenue for the investigation of gelation systems.

INTRODUCTION Low-molecular-weight gelators (LMWGs),1−4 which could form organogels or hydrogels through weak intermolecular interactions,5−14 have currently attracted considerable attention in various fields due to their potential applications in sensing,15−20 drug delivery,6,21,22 optoelectronics,23−25 chiral materials,26,27 multi-responsive materials,28−32 etc. While massive excellent LMWGs have been reported in recent years, most of the studies are merely concentrated on their final gel states or sol-gel transitions rather than the gelation processes. Undoubtedly, understanding gelation process can help researchers to cognize gelators more deeply and popularize their application in many fields.33−35 Nevertheless, it remains difficult to analyze the gelation process in a visible and direct manner. Hence, it is of paramount significance to develop convenient methods for gelation process research. Fluorescent species whose emission sensitively changes with the microenvironmental variation is probably an ideal choice for visualizing and monitoring of such processes.33−35 However, few of the common fluorescent dyes are qualified enough on account of their inconspicuous fluorescence color change from sol to gel state. Lately, a class of novel fluorogens showing the unique VIE effect has been reported by our group.36−41

These

disubstituted

hydrophenazine

derivatives

intrinsically display blue

fluorescence in the constrained state but abnormally exhibit orange-red fluorescence in the free


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state, undergoing an extremely distinct fluorescence color change that is visible by naked eyes. This fantastic phenomenon has been proven to be attributed to the intramolecular vibrations.36 The free intramolecular vibrations bring the hydrophenazine unit to a stable planar state, resulting in the orange-red emission (IOR). On the contrary, any means to restrict the intramolecular vibrations could keep the hydrophenazine unit taking a bent shape caused by the steric hinderance between the N,N'-disubstitutents thus giving rise to the blue fluorescence (IB) due to the smaller electronic conjugation. Such intriguing bent-to-planar vibrational motions of the hydrophenazine unit are just like butterflies fluttering their wings, so the VIE-active fluorogens are always presented in the form of butterflies. By principle, the intramolecular vibrations could be restricted by increasing the constraining degree of microenvironment, leading to the decrease of orange-red emission and the increase in blue fluorescence. Such a dramatic fluorescent ratiometric response (c.a. 150 nm) to environmental change makes VIE a feasible tool to visualize and decipher gelation processes. Gels are semisolid viscoelastic systems with continuous viscosity values from sol to gel state. The gelation process involves the self-assembly or co-assembly to give one-dimensional polymer-like fibers and the intertwinement of fibers to form a 3D architectures.42 Thus, constraint varying with the gelation process would be imposed on the intramolecular motions of the gelator molecules or the other species incoporated in the gelation system.26,43,44 Hence, it can be envisaged that the VIE effect could be fine-tuned by the gelation process, and in turn, the gelation process could be visualized and monitored by a VIE-active luminogen via ratiometric fluorescence response (Scheme 1). Herein, as a proof of concept, the adduct of the archetypal VIE-active fluorogen 9,14-diphenyl-9,14-dihydrodibenzo[a,c]phenazine (DPAC) and the widely-used gelating moiety cholesterol (CHOL), namely DPAC-CHOL, was designed and


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synthesized. In this adduct, DPAC acts as a fluorescent signaler plus an aromatic group, and CHOL serves as a steroidal group. These two parts are connected by a methyl (3-(ethylamino)-3oxopropyl)carbamate chain, which works as a spacer as well as hydrogen-bonding sites (Scheme 1). Three kinds of intermolecular interactions, including π-π interactions, intermolecular interactions and hydrogen bonding, are supposed to be involved in the gelation.43,44 To our delight, the value of IB/IOR increased as the gelation proceeded, suggesting that the gelator DPAC-CHOL is not only a self-indicator but also a ratiometric visualizer for a wide range of gelation systems. Furthermore, it has been revealed by the experimental results that the gelation effect is closely associated with the intramolecular vibrations and the VIE process of DPAC skeleton, offering an alternative scheme to the estimation of CGC, the detection of environmental temperature and the sensing of internal stiffness of the gel network. Scheme 1. Schematic illustration of the ratiometric visualization of gelation process with the proposed VIE-active indicator DPAC-CHOL.

RESULTS AND DISCUSSION


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The novel DPAC derivative DPAC-CHOL was facilely synthesized (Scheme S1, ESI) and fully characterized first (Figures S1‒S3, ESI). By directly cooling its hot solution to room temperature, the gelation ability of DPAC-CHOL was carefully assessed in nhexane/toluene, n-hexane/o-xylene and nujol/o-xylene mixtures with varying ratios in volume (Table S1). Gels can readily form in these solvent mixtures with appropriate volume ratios, suggesting that DPAC-CHOL is a gelator. Scanning electron microscope and transmission electron microscope analyses were carried out to gain further insights into the morphology, microstructures and internal structures of the gels formed by DPACCHOL. Xerogels attained from n-hexane/toluene or n-hexane/o-xylene displayed similar networks composed of intertwined fibres with the width ranging from 60 to 200 nm or from 120 to 800 nm (Figures S4 and S5), making it clear that DPAC-CHOL is a typical LMWG. The photophysical properties of DPAC-CHOL were investigated in a number of organic solvents ranging from nonpolar n-hexane to polar acetonitrile (Figure S6 and Table S2). The absorption maximum situated at around 350 nm, irrelevant to the solvent polarity. In the meantime, in each fluorescence spectrum, there showed a dominating orange-red emission band at around 595 nm and a much weaker blue emission which varied from 428 to 454 nm along with the increase in solvent polarity. It manifests that DPAC-CHOL inherits the VIE feature from the DPAC core. Given this, DPAC-CHOL is believed to hold a potential to self-indicate the gelation process via the ratiometric change in fluorescence (Scheme S2).


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Figure 1. (a) FL spectra of sol and gel formed by DPAC-CHOL (7 mg/mL, nujol/o-xylene = 8/2, v/v, λex = 365 nm). Inset: the photograph of a cured gel in the mode taken under 365 nm UV light at different temperatures (left half: 80 °C, sol; right half: room temperature, gel), which was cut out from the video demonstrating the gel to sol transition (Video 1). (b) Corresponding photographs of sol and gel taken under ambient light (left panel) and 365 nm UV irradiation (right panel). Fluorescence measurements of DPAC-CHOL in sol and gel states were therefore performed in the nujol/o-xylene system (7 mg/mL). Obviously, at the sol state, the fluorescence spectrum of DPAC-CHOL was dominated by the orange-red fluorescence peaked at 585 nm with a negligible blue emission band at around 420 nm (Figure 1a). When the sol was cooled to room temperature, however, the blue emission peak (λem = 455 nm) occupied the primary part of the spectrum. Accordingly, the sol of DPAC-CHOL was transparent and flowable under ambient light (Figure 1b), exhibiting orange-red fluorescence when observed with a handheld UV lamp (365 nm). In sharp contrast, when the above sol underwent the sol-gel transition and turned into gel, it became opaque and immobile, showing intense blue luminescence under UV irradiation. Similar phenomenon was also observed in the n-hexane/o-xylene system (Figure S7).


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Figure 2. (a) FL spectra of DPAC-CHOL recorded at different time during the gelation process; the sol with cuvette was cured at 20 °C (7 mg/mL, nujol/o-xylene = 8/2, v/v, λex = 365 nm). (b) Corresponding plot of I455/I585 versus time. (c) Evolution of elastic modulus (G′) and loss modulus (G″) of the gelation system with time; the sol was cured from 80 to 20 °C (7 mg/mL, nujol/o-xylene = 8/2, v/v). (d) Photographs of DPAC-CHOL taken during the gelation course under UV light (365 nm). Detailed information on the successive gelation process was acquired via the time-dependent fluorescence measurement. The fresh sample of DPAC-CHOL placed in the cuvette was first heated and stored at 80 °C under a water bath for 3 mins to get clear sol. Then the fluorescence tests were started as soon as the cuvette was rapidly put into the sample receptacle of the


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spectrofluorometer which had been pre-set at 20 °C. As time elapsed, the blue fluorescence increased while the orange-red emission decreased (Figures 2a and 2b). The Commission Internationale de L'Eclairage (CIE) coordinates were calculated to be (0.30, 0.31) at 90 s, very close to (0.33, 0.33) of pure white light. It is noteworthy that the ratio of fluorescence intensities at 455 and 585 nm (I455/I585) went through a very narrow induction period in the first 60 s, subsequently underwent a significant change in the time range of 60‒300 s, and finally arrived to the plateau region afterwards. It indicated that self-assembly and gelation mainly occurred in the initial 300 s. Rational understanding of the thermal gelation process was further assisted by the rheological experiment (Figures 2c and S8). The gelation point which was obtained in the rheological experiment and defined as the crossover of the G′ and G″ curves45 was approximately 80 s, which coincided well with the time point (90 s) estimated from the plot of I455/I585 versus time. It suggested that the gelation point could be indicated by the gelator DPAC-CHOL itself. Moreover, the gelation process could not only be revealed by the fluorescence spectra but also was able to be observed by the naked eyes. As can be seen from the photographs taken under UV illumination shown in Figure 2d, during the whole gelation process, the overall fluorescence color varied from moderate orange-red to white and finally to bright blue. In this manner, the insitu gelation self-indicating capacity of gelator DPAC-CHOL was fully demonstrated. As can be easily seen from Figures S9 and S10, distinct fluorescence spectra were recorded as the DPAC-CHOL concentration ranged from 1.0 to 7.0 mg/mL in nujol/o-xylene (8/2, v/v) or from 1.0 to 16.0 mg/mL in n-hexanel/o-xylene (6/4, v/v). The dramatic fluorescence change around CGC manifested that DPAC-CHOL is capable of estimating the CGC. In addition, DPAC-CHOL also holds a capability to probe the internal stiffness/rigidity of the gels formed by itself under different conditions. To be specific, by finely altering the volume ratio of nujol/o-


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xylene (20/5, 19/6, 18/7, 17/8, 16/9, 15/10), an array of color-tunable gels were obtained (Table S3). As the volume fraction of nujol decreased, there was a significant increase in the orange-red emission (Figure S11). The overall fluorescence color varied from blue via near white to orangered. In this sense, the subtle difference in the internal stiffness/rigidity could be sensitively reflected by its own ratiometric fluorescence change of the gelator DPAC-CHOL. Inspired by the fantastic self-indicating ability of the gelator DPAC-CHOL, the feasibility of exploiting DPAC-CHOL as a ratiometric visualizer to reveal the gelation processes of other gelators was further evaluated. It can envision that if there is sufficient interaction between the gelator and the fluorescent visualizer, different photophysical properties may be exhibited during the sol-gel transitions,33,34,46−49 owing to the variation in the rigidity degrees imposed on the visualizer molecules by the gelation system (Scheme 1). Given that VIE is just extremely sensitive to the change in the environment, the DPAC-CHOL was doped into three common gelating systems (Table S4) and utilized as a fluorescent probe. The gelator B (2-((9Z,12Z)octadeca-9,12-dienoyloxy)-3-(palmitoyloxy)propyl (2-(trimethylammonio)ethyl) phosphate) is commercially avaiable, and gelators A (tert-butyl (1,5-bis(octadecylamino)-1,5-dioxopentan-2yl)carbamate) and C (Ph-CHOL) were simply synthesized (ESI) and characterized (Figures S12−S14). Delightedly, even at a very low doping concentration, there was an obvious fluorescence color change from sol to gel state in all these three gelation systems doped by DPAC-CHOL (Figure 3). It implied that for one thing, DPAC-CHOL possess a potential to visualize the gelation process by virtue of the environmentally-sensitive VIE feature, and for another it is widely applicable for various gelating systems. Obviously, at sol state, orange-red fluorescence peaked at 585 nm was dominant in the fluorescence


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spectra of all the three systems, with a much weaker blue emission (Figure 3a), which were in line with the results obtained in the self-indicating experiments. When the gel formed at room temperature, however, the blue fluorescence peak (λem = 440 nm) became dominating. The remarkable fluorescence change displayed by the conversion from sol to gel state indicated that the intramolecular bent-to-planar vibrations of the DPAC moiety were greatly hampered due to the gelation effect. In view of its largest difference between sol and gel state, Ph-CHOL was selected as the model gelating system and used for the following studies. The little impact of different doping ratios of DPAC-CHOL exerted on the fluorescence spectra (Figure S15) of the DPAC-CHOL-doped gels and sols of PhCHOL suggested the high flexibility of DPAC-CHOL as a visualizer. Similarly, as depicted in Figure 3b, the sol-gel transition of Ph-CHOL in the mould is also palpably revealed by the distinct fluorescence colours between the sol and gel.

Figure 3. (a) Fluorescence spectra of the sol and gel of gelators A, B, and C doped by DPACCHOL (0.05 mg/mL), λex = 365 nm. (b) Photographs of the gelator system C (Ph-CHOL) doped by DPAC-CHOL in the mould taken under 365 nm UV light at different temperatures (left half: 80 °C, sol; right half: 20 °C, gel), which were cut out from the video demonstrating the sol-gel transition (Video 2).


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Figure 4. (a) Fluorescence spectra of the gelator C (Ph-CHOL) at 10 mg/mL doped by 0.05 mg/mL of DPAC-CHOL (nujol/o-xylene = 8/2, v/v, λex = 365 nm) which were recorded when the gel was heated from 20 to 65 °C. (b) Corresponding plot of I440/I585 versus temperature, where I440/I585 stands for the ratio of fluorescence intensities at 440 and 585 nm. The temperature-dependent fluorescence spectra shown in Figure 4 distinctly demonstrate the gelating visualization capability of DPAC-CHOL. The cured gel samples of DPAC-CHOLdoped Ph-CHOL prepared at 20 °C went through a stepwise heating process from 20 to 65 °C. Along with the increase of temperature, the orange-red light peaked at 585 nm gradually took over the leading position in the fluorescence spectra (Figure 4a). As displayed in Figure 4b, the ratio value of fluorescence intensities at 440 and 585 nm decreased linearly as temperature increased from 20 to 55 °C and remained as a constant of about 0.1 after 55 °C. It is because higher temperature not only activated the intramolecular motions of DPAC moieties but also weakened the intermolecular interactions of the gel, and ultimately led to the transition from gel to sol. In this case, the DPAC-CHOL molecules became freer as temperature rose, which increased the possibility of bent-to-planar conformation change and gave rise to the more intensified IOR/IB. In the meantime, the gel began to flow at 55 °C, which was defined as the gelto-sol temperature, i.e. the melting temperature of the gel (Tmelt).29 It means that the Tmelt of the


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gel formed by Ph-CHOL could be conveniently determined by DPAC-CHOL. Such a marked ratiometric fluorescence change clearly revealed the capability of DPAC-CHOL of reporting the reversible sol-gel transition of other gelation systems. Furthermore, it was notable that there was a linear relationship between I440/I585 and T in the range of 20−55 °C, which could be fitted as I440/I585 = −0.09151T+5.16753. The linear change of I440/I585 with T demonstrated that this thermosensitive gelation system doped by DPAC-CHOL could readily sense the environmental temperature in a daily concerned range. Analogous to the case when DPAC-CHOL itself functioned as a gelator and self-indicator, the dynamic gelation process of Ph-CHOL was vividly illustrated by the doped DPAC-CHOL (Figure 5). As time went by, the self-assembly of the gelator Ph-CHOL proceeded gradually and the gel subsequently formed, restraining the intramolecular vibrations of the DPAC moiety. Consequently, as can be seen from Figure 5a, the blue fluorescence increased dramatically while the orange-red emission had little changed. It is worth mentioning that the value of I440/I585 experienced a very small increase in the first 60 s, hereafter went through a significant change when the time elapsed from 60 to 240 s, and finally reached the maximum and kept almost unchanged after that point (Figure 5b). It suggested that the self-assembly and gelation of PhCHOL mainly took place in the first 240 s. As reported,26 at the initial cooling stage of the hot sol, driven by the super-saturation via stochastic nucleation, the aggregation of gelator molecules into 1-dimemensional (1D) assemblies arises. As the gelation proceeds, the junction zones and branching between the self-assembled fibre strands contribute to the rigidity of the microenvironment of gel matrixes and glue the 1D fibres into 3D networks spreading the whole system and entrapping the solvents and dopants. Ultimately, the rigid 3D gel network formed.


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Such a multi-step thermally activated gelation process could be clearly visualized by DPACCHOL.

Figure 5. (a) Fluorescence spectra of Ph-CHOL doped by 0.05 mg/mL of DPAC-CHOL, which were recorded at different time during the gelation process; the sol with cuvette was cured at 20 °C (Ph-CHOL: 10 mg/mL, nujol/o-xylene = 8/2, v/v, λex = 365 nm). (b) Corresponding plot of I440/I585 versus time. (c) Evolution of elastic modulus (G′) and loss modulus (G″) of Ph-CHOL with time; the sol was cured from 80 to 20 °C (10 mg/mL, nujol/o-xylene = 8/2, v/v). (d) Photographs of Ph-CHOL doped by DPAC-CHOL taken during the gelation process under UV light (365 nm).


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The rheological data obtained by the dynamic viscoelastic method shown in Figures 5c and S16 offered more proofs to the gelation visualizing capability of DPAC-CHOL. The storage modulus G′ represented the elastic ability and was essentially related to the formation of junction points in the gelating system. Meanwhile, the junction points formation vastly influenced the motions of the gelator chains, thus restricting the intramolecular vibrations of the entrapped DPAC-CHOL molecules as well as their fluorescence behaviors.33−35 That is why the gelation process could be revealed by the VIE-active indicator DPAC-CHOL. As time went on, the temperature of the gelating system decreased naturally and the G′ value underwent small fluctuations in the first 90 s in the G‒time curve, suggesting the initial stochastic nucleation stage of the gelating system. At the sol state, the G′ value was lower than the loss modulus G″ value. The faster increase of G′ as compared to that of G″ led to a crossover of these two curves, which was recognized as the gelation point. The muted growth of G′ after 150 s suggested that the formation of more junction points was impeded and the gelation process came close to the end. The gelation point obtained in the dynamic viscoelastic method was approximately 90 s, which matched well with the critical time point (90 s) estimated from the plot of I440/I585 versus time (Figure 5b), which was the starting point of dramatic fluorescence change. It implied that the gelation point of Ph-CHOL could be indicated by the doped DPAC-CHOL. Very similarly, the gelation process could also be witnessed by the naked eyes (Figure 5d). As the gelation progressed, the overall fluorescence changed from relatively weak orange-red to near white and finally to intense blue. In this way, the capacity of DPAC-CHOL to visualize the gelation in situ and in real time was vividly elaborated. In addition to the capability to identify the sol-gel transition, measure the environmental temperature and define the Tmelt, monitor the whole gelation process, and indicate the gelation


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point, DPAC-CHOL doped in the gelating system also holds the ability to reveal the internal stiffness/rigidity of the gels formed under different conditions. More specifically, by increasing the gelator concentration, the value of I440/I585 went through a sharp-to-muted growth (Figures 6a and 6b). It is unquestionable that higher concentration of the gelator means higher stiffness and more crowded microenvironment of the gel networks exerted on the doped DPAC-CHOL molecules. Obviously, the orange-red emission dominated the fluorescence spectra when the concentration of Ph-CHOL was below 2.0 mg/mL, at which point the gel could not form. However, a sharp increase in the blue fluorescence occurred with the I440/I585 value close to 1, when the Ph-CHOL concentration reached 3.0 mg/mL. Moreover, at a concentration higher than 3.0 mg/mL, Ph-CHOL could be readily gelated and the intramolecular vibrations of a larger portion of DPAC-CHOL molecules would be hindered, causing a further raise in the blue emission. The results implied that the CGC of Ph-CHOL in the mixture of nujol/o-xylene (8/2, v/v) should be approximately 3.0 mg/mL. Furthermore, altering of the volume ratio of nujol/oxylene (20/5, 19/6, 18/7, 17/8) gave rise to an array of color- tunable gels (Table S5). As the volume fraction of nujol decreased, there was a significant decrease in the blue fluorescence. The overall fluorescence color varied from blue via near white to orange-red (Figures 6c and 6d). Such a volume ratio-reliant fluorescence of the DPAC-CHOL-doped gels can be explained as below: The internal stiffness of the gel network should be increased with a higher fraction of nujol, allowing severer restriction to be applied on the intramolecular vibrations and consequently intensifying the blue fluorescence. Therefore, with the aid of the doped DPACCHOL, the subtle difference of the internal stiffness/rigidity could be sensitively reflected.


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Figure 6. (a) Fluorescence spectra of Ph-CHOL at various concentrations (mg/mL) doped by 0.05 mg/mL of DPAC-CHOL in the mixture of nujol/o-xylene (8/2, v/v), λex = 365 nm. (b) Corresponding fluorescence intensity ratio of 440 and 585 nm. (c) Fluorescence spectra of the gels form by Ph-CHOL (10 mg/mL) in the mixture of nujol/o-xylene with various volume ratios, where 0.05 mg/mL of DPAC-CHOL was doped into each gel. (d) Corresponding fluorescent photographs taken under UV light (365 nm). CONCLUSION In conclusion, by simply integrating the VIE-active DPAC core and the gelating module CHOL together, a versatile molecular system was successfully developed. The novel adduct DPACCHOL was featured with the VIE attribute as well as the gelating ability, and hence could play multiple roles as low-molecular-weight gelator, ratiometric self-indicator for its own gelation, and the fluorescent visualizer of the other gelation systems. With the aid of DPAC-CHOL, the sol-gel transition and the whole gelation process of itself and the other gelation systems could be


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monitored in an in-situ and real-time fashion by the ratiometric fluorescence change origiated from the VIE feature. Moreover, DPAC-CHOL could self-indicate/reveal the gelation point, the CGC of the gelators, and the internal stiffness of the gel networks. Very importantly, the DPACCHOL-doped Ph-CHOL gelation system has the ability to accurately sense the daily-concerned evironmental temperature. To the best of our knowledge, this is the first VIE-based gelator and fluorescent indicator which has a great potential to be applied for gelation process visualization. This work opens up a new avenue for the investigation of gelation systems. Further applications may be extended to the monitoring of the gelation processes of hydrogels, drug delivery and biological processes, as fluorogens featured with VIE characteristics could easily achieve distinct fluorescence color change, which could reflect the subtle variations in a ratiometric mode. ASSOCIATED CONTENT Supporting Information Experimental details including materials synthesis and characterization, the 1H NMR, 13C NMR and HRMS spectra, the experimental results for DPAC-CHOL when it acted as a self-indicator to monitor the gelation process of itself, the SEM and TEM images, the G" and G′ values of the gels on frequency sweep and strain sweep, and so on. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. (J. M.) Telephone: +86-021-64252758. *E-mail: [email protected]. (J. S.) Telephone: +86-021-64252258. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work is financially supported by the Programme of Introducing Talents of Discipline to Universities (B16017), NSFC/China (21604023, 21790361, 21421004) and the Fundamental Research Funds for the Central Universities (222201714011, 222201717003) and sponsored by the Shanghai Sailing Program (16YF1402200). REFERENCES (1) Raeburn, J.; Adams, D. J. Multicomponent Low Molecular Weight Gelators. Chem. Commun. 2015, 51, 5170−5180. (2) Babu, S. S.; Praveen, V. K.; Ajayaghosh, A. Functional π-Gelators and Their Applications. Chem. Rev. 2014, 114, 1973−2129. (3) Buerkle, L. E.; Rowan, S. J. Supramolecular Gels Formed from Multi-component Low Molecular Weight Species. Chem. Soc. Rev. 2012, 41, 6089−6102. (4) Dawn, A.; Shiraki, T.; Haraguchi, S.; Tamaru, S.; Shinkai, S. What Kind of “Soft Materials” Can We Design from Molecular Gels? Chem. Asian J. 2011, 6, 266−282. (5) Wu, Y.; Hirai, Y.; Tsunobuchi, Y.; Tokoro, H.; Eimura, H.; Yoshio, M.; Ohkoshi, S.-i.; Kato, T. Supramolecular Approach to the Formation of Magneto-active Physical Gels. Chem. Sci. 2012, 3, 3007−3010. (6) Hirst, A. R.; Miravet, J. F.; Escuder, B.; Noirez, L.; Castelletto, V.; Hamley, I. W.; Smith, D. K. Self−Assembly of Two−Component Gels: Stoichiometric Control and Component Selection. Chem. Eur. J. 2009, 15, 372−379. (7) Liu, C.; Yang, D.; Jin, Q.; Zhang, L.; Liu, M. A Chiroptical Logic Circuit Based on Self−Assembled Soft Materials Containing Amphiphilic Spiropyran. Adv. Mater. 2016, 28, 1644−1649.


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Table of Contents Graphic


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Ratiometric Indicator Based on Vibration-Induced Emission for in Situ

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