Tunable and Switchable Control of Luminescence ... - ACS Publications

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Tunable and Switchable Control of Luminescence through Multiple Physical Stimulations in Aggregation-Based Monocomponent Systems Xudong Yu,†,‡ Xiaoting Ge,‡ Haichuang Lan,† Yajuan Li,‡ Lijun Geng,‡ Xiaoli Zhen,‡ and Tao Yi*,† †

Department of Chemistry and Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, 220 Handan Road, Shanghai 200433, China ‡ College of Science and Hebei Research Center of Pharmaceutical and Chemical Engineering, Hebei University of Science and Technology, Yuhua Road 70, Shijiazhuang 050080, China S Supporting Information *

ABSTRACT: This report describes how the luminescence of naphthalimide could be tuned by various physical stimuli, including heat, sonication, and grinding. Herein, instant and switchable control of color and fluorescent emissions has been achieved by the sonicationtriggered gelation of an organic liquid with naphthalimide-based organogelators (N3−N7). Green emissive suspensions of the gelators in organic liquids are transformed into orange emissive gels upon brief irradiation with ultrasound with an emission wavelength red-shift of approximately 60 nm and fluorescence intensity quenching by a factor of 20, which can subsequently be reversed by heating. When sonication-triggered S-gels are evaporated to Sxerogels, the solid state xerogels (N3, N4, N6, N7) exhibit mechanochromism, the color of which changes from red to yellow and the emission color of which changes from orange to green with enhanced intensity by grinding. This mechanochromic property can be reversed through a regelation process. The mechanochromic character of the S-xerogel of N3 is thus applied to quantitatively sense the mechanical pressure range from 2 to 40 MPa through fluorescence changes, reflecting a new type of application for gelation assembly. The physical stimuli triggered fluorescence changes of these compounds strongly depend on the molecular structure and solvent. The results demonstrate that the different aggregation modes and long-range order arrangement of the molecules regulated by the stimulus may affect the internal charge transfer (ICT) process of the naphthalimide groups, resulting in the tunability of the photophysical properties of the gelators. This report provides a new strategy for tunable and switchable control of luminescence through nonchemical stimuli in aggregation-based monocomponent systems. KEYWORDS: mechanochemistry, soft material, luminescence, ultrasound, grind chromism

1. INTRODUCTION The control of the luminescence in emissive materials plays an important role in the field of color displays, light emission devices, and light-harvesting systems, as well as chemical and biological sensors, and it therefore has attracted considerable attention in recent decades.1−9 Luminescence control triggered by chemical stimuli such as the polarity of a solvent, pH, and host−guest interactions have been extensively studied.10−19 However, the systems whose luminescence can be controlled by physical stimuli such as grinding, shaking or sonication are still very limited. Those physical stimuli controlled luminescent systems are especially attractive because the response process is often instant, switchable, and remote and normally does not produce any byproducts during the switching process.20−24 It is well-known that the luminescence of a molecule in an aggregation state is related to the packing modes of the fluorophores. The different stacking modes of the fluorophores would cause significant variation of their photophysical properties. Previous studies have proposed that a physical stimulus (except light irradiation) may affect the molecular © XXXX American Chemical Society

aggregation modes via the adjustment of noncovalent intra- or intermolecular interactions such as π−π stacking, hydrogen bonding and hydrophobic interaction, thus possibly resulting in the modulation of the luminescence of a fluorescent material.5,25−32 Ultrasound has been regarded as an efficient stimulus to afford gel-like soft materials through the adjustment of the molecular configuration or aggregation mode.33−41 Therefore, it is reasonable that the luminescence color of a florescent molecule may be controlled by sonication in a gel system. However, so far, only a few examples of luminescent metallogels whose fluorescent emission could be switched in response to sonication and heat stimuli have been reported.20,32 One notable example is the Pt-based assembly switched on by sonication and heat with “off-on” emission changes.20 In addition to sonication, mechanical force, as another most reliable and fundamental power source, is also recognized to be Received: September 7, 2015 Accepted: October 13, 2015

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DOI: 10.1021/acsami.5b08402 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Scheme 1. Chemical Structures of Naphthalimide-Based Derivatives from N1 to N7

color change was observed due to the conformation change of the tBu3tpy platinum complex from perpendicular to more coplanar with the 1,8-naphthalimide ring, resulting in a more effective ICT process in the gel state.32 Even though the sonication-triggered fluorescence change on the wavelength was not so distinct in this system, it hints that the modulation of the ICT process of 1,8-naphthalimide by a self-assembly process may be a strategy to construct a sonication-triggered fluorescence switch. Inspired by this concept, here, by linking a cholesterol group to the 4-amino position of 1,8naphthalimide and adding an electron-donating alkyne or pyridyl to the imide side of naphthalimide, we report the switching and instant fluorescence color changes based on ultrasound-induced aggregation (N1−N7 in Scheme 1). Furthermore, the sonication-triggered aggregation in its solid state (xerogel) exhibited a mechanochromic property responsive to mild grinding or mechanical pressure, which was further used to sense pressures ranging from 2 to 40 MPa with high sensitivity. In those gel tissues, the polymorphism could be regulated in a programmed manner by physical stimuli such as sonication, heat, grinding, and pressure. To the best of our knowledge, such multiple physical stimuli triggering polymorphism in a monocomponent organogel system has not been previously reported.

able to change the mode of molecular packing and induce luminescence changes, which is known as mechanochromism. Mechanochromic luminescence was only observed in a few examples of large conjugated systems.2 Examples of ultrasoundcontrolled emissive systems with mechanochromic properties that can further respond to grinding and mechanical pressure have not been reported. 1, 8-Naphthalimide derivatives are important fluorophores that have been widely used as probes for ions and biomolecules.42−49 Among those naphthalimide derivatives, the 4-amino-1,8-naphthalimide group with a donor-π-acceptor (D-π-A) electronic structure is most commonly employed to construct probes in solution, polymeric networks, or hybrids because of its tunable fluorescent properties.50−52 Because of the existence of an internal charge transfer (ICT) excited state, 4-amino-1,8-naphthalimide is very sensitive to solvent and external stimuli such as ions, pH, small guest molecules or biomolecules, with an ICT or PET (photoinduced electron transfer) process resulting in luminescence changes.50 However, instant and remote control of the luminescence properties of naphthalimide-based compounds via nonchemical stimuli has continued to be a challenge and paid little attention on by coworkers. We have previously developed a series of 1,8-naphthalimide based gelators containing steroidal groups whose gelation process could be switched on by sonication.32,36,53−58 However, both in our work and the work of others, the sonicationinduced fluorescence wavelength change was not observed. When we induced a terpyridyl platinum to the 4 position of 1,8naphthalimide, an exciting sonication-triggered absorption

2. EXPERIMENTAL SECTION Materials. All starting materials were obtained from commercial suppliers and used without further purification. Cholesteryl chloroformate (99%) was obtained from Sigma-Aldrich. 6-Aminocaproic acid, 2-propynylamine, L-alanine, and allylamine were provided by Alfa B


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Figure 1. Absorption and fluorescent spectra of the solution, precipitate, S-gel and S-xerogel of N3 and N7. Absorption spectra of the solution (1 × 10−4 M) and S-gel (25 mg/mL, 3.6 × 10−2 M) of N3 in (a) ethyl acetate and (c) ethanol, and (e) N7 in methanol; fluorescence spectra of the diluted solution (S1, 1 × 10−4 M), concentrated solution (S2, 25 mg/mL), precipitate (25 mg/mL, obtained by H−C process), S-gel (25 mg/mL), and S-xerogel of N3 in (b) ethyl acetate and (d) ethanol, and (f) N7 in methanol. Insets in a, c, and e are photos of the sol (or suspension) and S-gel in the corresponding solvents under daylight; insets in b are photos of the sol and gel, in d are photos of precipitate and S-xerogel of N3 in dark under UV light (365 nm), and in f are photos of powder, S-xerogel from ethanol, and S-xerogel from methanol of N7 in daylight and in dark under 365 nm light (from left to right). Aesar. Propylamine (98.5%), 4-bromo-1,8-naphthalic anhydride (95%), aminocaproic acid, HOBt (N-hydroxybenzotriazole, 98%), EDC·HCl (1-ethyl-3-(3-dimethyllaminopropyl carbodiimide hydrochloride, 98%) were supplied from Shanghai Darui Fine Chemical Co., Ltd. Ethylenediamine, triethylamine, trichloromethane, n-propanol, ethanol, tetrahydrofuran, n-butyl alcohol, and other reagents were purchased from Tianjin Hengxing Chemical Preparation Co., Ltd. Techniques. FTIR spectra were recorded using an IRPRESTIGE21 spectrometer (Shimadzu). SEM images of the xerogels were obtained using SSX-550 (Shimadzu) and FE-SEM S-4800 (Hitachi) instruments. Samples were prepared by spinning the gels on glass slides and coating them with Au. NMR spectra were performed on a Bruker Advance DRX 400 spectrometer operating at 500 and 125 MHz for 1H NMR and 13C NMR spectroscopy, respectively. The high-resolution mass spectra (HR-MS) were measured on a Bruker Micro TOF II 10257 instrument. Fluorescence spectra were collected on an Edinburgh Instruments FLS-920 spectrometer with a Xe lamp as an excitation source. SAXS experiments were carried out on a Nanostar U SAXS system (Bruker) at room temperature. The X-ray diffraction pattern (XRD) was generated by using a Bruker AXS D8 instrument (Cu target; λ = 0.1542 nm) with a power of 40 kV and 50 mA. UV−vis absorption spectra were recorded on a UV−vis 2550 spectroscope (Shimadzu). Sonication treatment of sols was performed

in a KQ-500DB ultrasonic cleaner (maximum power, 100 W, 40 kHz, Kunshang Ultrasound Instrument Co., Ltd., China). The pressure sensing tests were carried out by a 769YP-15A powder compressing machine (Tianjin City Coant High-tech Company). Methods. Different Gelation Processes. The solid obtained from column chromatography on silica gel (200−300 mesh) was referred to as a powder. After a heating−cooling process, if the compound precipitates from the solution, it was called a precipitate. Therefore, a precipitate-to-gel transition needs heating; while a powder-to-gel transition was a direct process at room temperature without heating. The gelation process accelerated by sonication was called an S-gel. An S-xerogel was obtained after evaporating the S-gel under vacuum by freezing. Pressure Sensing Process. One hundred milligrams of S-xerogel was put into a mold and subjected to different pressures for approximately 2 min. Then, the fluorescence spectra were tested by an FLS-920 spectrometer.

3. RESULTS AND DISCUSSION 3.1. Gelation Properties of N1−N7. The synthesis and characterization of naphthalimide derivatives N1−N7 are described in the Supporting Information. All the compounds C


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Figure 2. Normalized fluorescence spectral change of N3 (25 mg/mL) in the reversible gelation process switched by alternating sonication and heating; (a) the sol−gel process in ethyl acetate with sonication time; (b) the reverse gel−sol process by heating; (c) the precipitate-gel process in ethanol with sonication time; and (d) the reverse gel-suspension process by heating. Insets in a and c are the changes of the emission intensity at 549 nm as a function of the sonication time; insets in b and d are the changes in the emission intensity at 527 nm as a function of temperature (°C).

and an emission peak at 504 nm (Figure 1a), which was attributed to the ICT transition of the 4-amino-1,8naphthalimide unit. Upon increasing the concentration to 25 mg/mL (3.6 × 10−2 M), the emission of the N3 sol exhibited a 23 nm red-shift, indicating the enhanced ICT with the aggregation of the molecules. After the in situ treatment of sonication, the absorption band was broadened with a shoulder at 494 nm in its S-gel, and a larger red shift from 527 to 575 nm was observed in the emission wavelength from sol to S-gel, indicating the formation of a J-type aggregate (Figure 1b). After evaporation of the gel to remove the solvent, the emission of the obtained S-xerogel continued red-shifting from 575 to 581 nm. Such a large red-shift of the emission from a dilute solution to the aggregated state (76 nm) has scarcely been reported in a 1, 8-naphthalimide fluorophore.12 In a protic solvent such as ethanol, N3 showed similar absorption spectral changes between the solution (1 × 10−4 M) and the S-gel with those in ethyl acetate (Figure 1c). When the concentration of N3 increased to the gelling concentration of 25 mg/mL, the green emissive solution (λem = 527 nm) changed to a suspension with a yellow-green emission (λem = 544 nm). When subject to sonication for 20−60 s, the yellow suspension of N3 was changed to a red gel, and the emission color gradually red-shifted from yellow-green (λem = 544 nm) to yellow then to orange (λem = 587 nm) and further redshifted to 598 nm in its xerogel after evaporation (Figure 1d). The absolute fluorescent quantum yields of N3 decreased from 60.3% in dilute solution to 9.2% in S-gel, with the lifetime shortened from 3.38 to 1.61 ns (Table S3). Similar sonication-triggered spectral changes were also observed in N6 and N7. N7 in a dilute solution of methanol (1 × 10−4 M) showed an absorption peak at 440 nm. After sonication, two new absorption shoulders at 494 and 525 nm appeared in the S-gel, with the color changing from yellow to red (Figure 1e). The emission wavelength of N7 shifted from 540 to 595 nm (55 nm red-shift) after 3 min sonication. The

have the same structure on the 4 position of the naphthalene, but different groups on the imide part. The gelation properties of those compounds were checked with or without sonication in 10 different organic solvents (Tables S1 and S2). All of compounds N1−N7 cannot gel in the tested solvents by a typical heating−cooling process. However, N3−N7 can gelate short chain alcohols, ethyl acetate or benzene at room temperature through the irradiation of ultrasound. For example, the precipitate or concentrated solution of N3 (25 mg/mL) could gel in ethanol or ethyl acetate, respectively, upon brief irradiation of ultrasound (40 Hz, approximately 60 s). The gelation of N4−N7 at the same concentration needs a longer sonication time. The sonication times for the formation of a stable gel are 10, 3, 2, and 1.5 min for N4 (35 mg/mL in propanol), N5 (25 mg/mL in ethanol or benzene), N6 (25 mg/mL), and N7 (precipitate, 25 mg/mL), respectively. Surprisingly, N7 in methanol or ethanol was able to undergo a direct powder-to-gel transformation at room temperature without a heating process, which was more meaningful for the construction of the gel.46 However, N1 and N2 cannot gelate any tested solvents, even with sonication. The above results reveal that compounds with a conjugated electron-donor group such as an alkyne or pyridyl linked to the imide side were prone to form gels when treated with ultrasound. 3.2. Switching of the Color and Fluorescence by Physical Stimuli. To our surprise, the sonication-triggered gels (S-gel) show observably different color and emission compared with their solution and solid powders (Figure S1). Particularly, the color of N3, N4, N6, and N7 changes from green or yellow to orange, with the fluorescence emission changing from green to yellow or orange in the sonicationtriggered gelation process, whereas the changes on both color and emission color are not so distinct in the gelation process of N5. The gelation processes were traced by absorption and fluorescence spectra at room temperature. N3 in ethyl acetate solution (1 × 10−4 M) gave a strong absorption band at 439 nm D


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ACS Applied Materials & Interfaces direct powder-to-gel transition process of N7 in ethanol and N6 in propanol without heating also displayed similar fluorescence changes (Figure 1f, Figure S2). On the contrary, with a longer linker between alkyne and naphthalimide, the emission wavelength of N5 blue-shifted by approximately 6 nm (from 531 to 525 nm) in the sonication-triggered sol−gel process (Figures S3 and S4). The above results suggested highly structure and aggregate-dependent luminescent properties and sonication-responsive behavior of those gelators. All of the color or emission changes could be reverted to the initial state by a heat stimulus without fatigue. Notably, both N3 and N7 showed regular fluorescence quenching in the gelation process via sonication (Figure 2a). Taking N3 as an example, with sonication for 5 s, the sol of N3 in ethyl acetate was first changed to a suspension with fluorescence enhancement and red-shifted from 527 to 549 nm. With the prolonging of the sonication time from 5 to 70 s, the suspension completely transformed to a gel, together with a regular fluorescence quenching by a factor of 20, and the maximum emission peak red-shifted from 549 to 575 nm. This process could be reversed by heating. When heating the S-gel of N3 from 25 to 90 °C, the gel changed to a suspension and the fluorescence intensity of N3 was enhanced by a factor of 127 (Figure 2b). Upon further increasing the temperature to 100 °C, the suspension changed to a full solution with the fluorescence intensity decreasing again. The precipitate-to-gel process of N3 in ethanol generated by sonication showed a direct fluorescence intensity decrease by a factor of 129 with a sonication time of 65 s, which was accompanied by a red-shift of the wavelength from 550 to 587 nm (Figure 2c). Using a heating stimulus, the reverse gel-to-suspension process could also be tracked by the fluorescence spectral change (Figure 2d). Such a reversible process could be repeated by several cycles, and the intensity of the S-gels could be efficiently repeated with a difference smaller than 4% (Figure S5). The sonication timedependent fluorescence spectrum of N7 in methanol in the direct powder to S-gel process is similar to that of N3 in ethanol (Figure S6). 3.3. Mechanisms for Ultrasound-Induced Emission Color Changes. Scanning electron microscopy (SEM) and transmission electron microscope (TEM) were used to study the microstructure transformation accompanied by emission property changes in the gelation process. The green emissive precipitate of N3 in ethanol showed a microsphere structure with diameters ranging from 1 to 3 μm (Figure 3a). TEM images showed that the spheres were not hollow (Figure S7). When treated with sonication for 20 s, the microspheres were first cut into nanospheres (200−800 nm) and then cross-linked as precursors of emerged fibers (Figure 3b). Upon further sonicating the sample for 40 s, spheres and short nanofibers coexisted to form a partial gel, displaying a yellow emission color (Figure 3c and Figure S8c). After 60 s sonication, a large area of densely entangled nanofibers with a diameter of approximately 10 nm formed with an orange emission color (Figure 3d, Figure S8d). The sonication-induced sol−gel transition of N3 in ethyl acetate (Figure 3e, f) and N5 (Figure S9) in ethanol displayed similar morphological changes from microspheres to nanofibers. On the other hand, the direct powder-to-gel transformation of N7 triggered by sonication showed a different morphology change traced by SEM images (Figure S10). The powder of N7 gave an irregular short rod structure (Figure S10a), which fused after sonication for 40 s (Figure S10b). Many pores appeared on the surface of the

Figure 3. SEM images of N3 assemblies. (a) Precipitate of N3 in ethanol; (b) treated with sonication for 20 s; (c) treated with sonication for 40 s; (d) treated with sonication for 60 s; (e) powder of N3 obtained by evaporation (25 mg/mL) from ethyl acetate; and (f) sonication for 60 s. Scale bars: a, 5 μm; b, 5 μm; c, 10 μm; d, 1 μm; e, 4 μm; f, 1 μm.

fused rod after sonication for 80 s, which could be attributed to the cavitation effect of ultrasound (Figure S10c),32−34 and finally, a gel formed with a multiporous structure and an orange emission after sonication for 2 min (Figure S10d). To gain a deeper insight into the emission behavior controlled by molecular aggregation, SAXS (small-angle X-ray scattering) experiments were performed to observe the different aggregation structures of the solid powders and Sgels. The SAXS pattern of the solid powder of N3 showed peaks at 3.4 and 5.0 nm from ethyl acetate and ethanol, respectively (Figure 4a, 4b), and that for the solid powder of N7 from methanol also gave a peak at 3.4 nm (Figure S11). The distance of 3.4 nm is close to the length of a single molecule of N3 or N7 in an extended configuration. After sonication, new peaks at 9.4 and 9.2 nm appeared in the N3 gels of ethyl acetate and ethanol, respectively, accompanied by a broad peak at approximately 5.2 nm. The XRD data of the Sxerogel of N3 from ethanol showed a series of intense and sharp peaks in the 2θ range of 5−30° (Figure 4c). In the gelation process of N7 in methanol, a new SAXS peak at 5.3 nm was also observed, except for the peak at 3.4 nm (Figure S11). The SAXS data indicated the change of the aggregation mode of both N3 and N7 after the ultrasound stimulus. IR spectra of the precipitate and the S-gel of N3 in ethanol were studied as shown in Figure S12, which were also carried out to check the ultrasound effect on the aggregates of N3 molecules. The IR band at 3366 cm−1, which belonged to the stretching vibration of N−H, moved to 3358 cm−1 in the gelation process, indicating the strengthened hydrogen bonding in the S-gel. The vibrations of CC at 2121 cm−1 and C−H at 3262 cm−1 of alkyne groups also shifted to 2134 and 3234 cm−1, respectively, E


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Figure 4. SAXS data of the solid (by evaporation) and S-xerogel of (a) N3 from ethyl acetate; (b) SAXS data of N3 precipitate, S-xerogel, and after grinding from ethanol; XRD data of (c) powder and S-xerogel of N3 from ethanol; and (d) S-xerogel after grinding for different times.

Figure 5. Emission color changes of S-xerogels of (a) N3 and (c) N7 by gradual grinding; (b) fluorescence spectral changes of S-xerogels of (b) N3 and (d) N7 upon grinding (λex = 365 nm).

indicating the existence of donor−acceptor interaction of alkyne and naphthalimide groups. 3.4. Grinding-Chromism. To our surprise, the sonicationtriggered xerogels (S-xerogels) of N3, N4, N6 and N7 exhibited grinding-induced emission changes. For example, the orange emissions of N3 and N4 xerogels could be changed to yellowgreen upon exposure to grinding, while N7 displayed an orange to green color change (Figure S13). An amazing result was that the mechanochromic luminescence of the S-xerogels was very sensitive to gentle grinding. When the S-xerogel of N3 in two quartz plates was subjected to the compression of grinding by only the fingers, it underwent orange to yellow and then to green emission color changes (Figure 5a). The gradual and regular blue shift from 598 to 553 nm was traced by the in situ fluorescent spectra (Figure 5b). The larger adjustment of the

emission color by grinding was also observed in the methanol S-xerogel of N7 from 598 to 538 nm (Figure 5c, d). Moreover, the grinding-induced color change could be reversed to the original S-xerogel state by the regelation process triggered by ultrasound followed by evaporation. XRD measurements were performed to better understand the interesting luminescent mechanochromism. The XRD pattern of the N3 S-xerogel from ethanol showed intense and sharp peaks in the 2θ range of 5− 30° (Figure 4c). After grinding, two new diffraction peaks at approximately 5.9 and 5.0 Å appeared, which were very close to that of the original powder without sonication, and the peak at 3.3 Å assigned to the d value of π−π stacking shifted to 3.4 Å (Figure 4d). All the peaks in XRD became broader and weaker after further grinding. The comparison of the SAXS patterns before and after grinding showed an obvious change of the F


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Figure 6. Schematic illustration of self-assembly of N3 and its response to multiple physical stimuli (A, aromatic; L, linker; S, steroidal).

Figure 7. (a) Fluorescence changes of the S-xerogel of N3 when exposed to increasing pressure; (b) reciprocal of fluorescence intensity of the Sxerogel of N3 at 576 nm when treated with increasing pressure, inset: a photo of the mode for the powder test.

with a longer flexible alkyl linker between the naphthalimide and the alkyne group, the intermolecular interaction between them would be drastically weakened in N5. Therefore, a very small variation in the emission wavelength was observable in the gelation process of N5. From the comparison of the gelation and photophysical properties of N3, N4, and N5, it was proposed that the chiral amino segment in N4 was not favorable for the gelation since N4 could only gelate propanol with a higher gelation concentration assisted by sonication (Table S3). The steric effect of the chiral amino segment may restrain the π···π stacking of naphthalimide for gelation. However, the photophysical change of N4 in the gelation process was similar to that of N3, indicating that the amide group did not affect the electron donor−acceptor interaction for the red-shifted emission. In contrast, with the longer linker (alkyl chain) between naphthalimide and alkyne groups, the solubility and gelation capability of N5 were improved, but the longer distance was not favorable for the intermolecular host− guest interaction between naphthalimide and alkyne groups. In one word, the gelation and photophysical properties of those compounds were affected by comprehensive factors including the multiple intermolecular interactions, steric effect and distance between acceptor and donor. The resulting S-xerogels of N3, N4, N6, and N7 were metastable, but very sensitive to environmental stimuli such as heat and grinding, even in the solid state. Those physical stimuli could reregulate the intermolecular π···π stacking of the molecules, resulting in the rearrangement of the aggregation back to the original stable state to some extent, accompanied by the reversibility of the fluorescence both in intensity and wavelength. 3.5. Pressure Sensor. Constructing pressure sensors by organic molecules have attracted increasing attention in mechanochemistry during the past decades.59−63 Recently, L.

molecular arrangement. Specifically, the strong peak of the Sxerogel at 9.2 nm disappeared, and the two broad bands at 3.4 and 5.0 nm shifted to 3.6 and 5.6 nm, respectively (Figure 4b). The above results suggest that when the mechanical stretching energy is delivered to the π conjugated system, disruption of the lattice occurs to promote an emission color transition. Because most of the grinding chromism phenomena were based on pure and large π conjugated derivatives, the grinding chromism in naphthalimide-based systems provides a new way for constructing physical-responsive materials with tunable photophysical properties. On the basis of the above results, our findings could be summarized as follows (Figure 6). N3 molecules were aggregated to nanosized spherical particles in solution with a yellow-green emission in a typical heating−cooling process. Sonication modulated the intermolecular interactions in the assembly of N3, especially the π···π interactions between naphthalimides and naphthalimide/alkyne group, resulting in a J-type aggregate in the S-gel with a densely entangled fibril network. The interaction between naphthalimide as an electron acceptor and alkyne (or pyridine as in N6 and N7) as an electron donor may promote a more efficient ICT process and thus cause the large red shift of the fluorescence in the gel state. A similar large red-shift of the emission in the aggregated state has been recently reported by us in a 4-ethynyl-1,8naphthalimide gelator.12 As a result, an output signal change of fluorescence intensity quenching and an emission color change from yellow-green to orange were observed with sonication time. This phenomenon was also clearly observed in the assemblies of N4, N6, and N7. However, N1 and N2 without electron donor groups on the imide part could not gelate solvents, even under sonication, indicating the importance of the conjugated electron-donor group. However, G



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Pan and co-workers reported an ultrasensitive pressure sensor based on a polymer thin film with a resistance signal output.64 However, few of these sensors could be responsive to pressure with a fluorescence signal output. The fluorescence technology always has the merits of high sensitivity and a fast time response for naked eye detection, as well as inexpensive installation. Therefore, the S-xerogel of N3 from ethanol was used to sense pressures from 2 to 40 MPa with a “turn on” fluorescence signal output using a powder compressing machine. As seen from Figure 7a, with the increasing pressure, the fluorescence intensity was regularly enhanced by a factor of 8.5 (λex = 469 nm, λem = 576 nm), which was accompanied by a 22 nm blue shift. Figure 7b showed the dependence of the emission intensity at 576 nm on the different pressure values. We observed that the two parameters (emission intensity and pressure) followed a simple linear equation from 2 to 40 MPa with a correlation coefficient of R2 = 0.99347.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b08402. Synthesis details and additional spectra (PDF)

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REFERENCES

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4. CONCLUSION This report describes the first reversible switching of fluorescent color changes by nonchemical stimuli with pure organic dyes on a monocomponent assembly platform. The gel tissues perform multiple chromic effects, including grinding-, sonication-, and thermal-dependent chromism, thus supplying a facile method for the adjustment of luminescent properties (such as intensity, color, and lifetime) by physical stimulation without chemical structural change. The grinding-induced mechanochromism is first found in the 4-amino-1,8-naphthalimide-based solid system due to aggregation changes, which supplies a new strategy for luminescence control. Additionally, tuning the ICT process of naphthalimide compounds, displaying a blue or red shift due to the arrangement of the aggregation mode, by physical stimuli in this work, is different from the literature reports that generally employ guest molecules to tune the ICT process and the emission properties. It was deduced that these physical stimuli could modulate the sequence or strength of the intermolecular interactions, resulting in different photophysical properties. At last, the ordered assembly could be used to sense lower pressures in the range of 2−40 MPa with a fluorescent signal output, which opens a new application of soft materials in physical sensors and soft optical devices.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+86) 21-55664621. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thanks for the financial support by NNSFC (21125104, 21401040, 21301047, 51373039), Xiaoli fund SW (2014PT76), Natural Science Foundation of Hebei Province (No.B2014208160, B2014208091), Specialized Research Fund for the Doctoral Program of Higher Education (20120071130008). H


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