General Platform for Remarkably Thermoresponsive Fluorescent

Department of Chemistry, Institute for Advanced Study, Division of Biomedical Engineering, State Key Laboratory of Molecular, Neuroscience and Institu...
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General Platform for Remarkably Thermoresponsive Fluorescent Polymers with Memory Function Guodong Liang,*,† Jialong Wu,† Haiyang Gao,† Qing Wu,† Jiang Lu,† Fangming Zhu,*,† and Ben Zhong Tang*,‡ †

DSAP, PCFM and GDHPPC Lab, School of Materials Science and Engineering, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China ‡ Department of Chemistry, Institute for Advanced Study, Division of Biomedical Engineering, State Key Laboratory of Molecular, Neuroscience and Institute of Molecular Functional Materials, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China S Supporting Information *

ABSTRACT: Memory polymers capable of remembering their shape or thermal history have attracted increasing interest due to their potential applications in smart and medical devices. Memory polymers established are mechanically based, which suffer from some inherent limitations such as low sensitivity and bulky size. Here, we develop a general platform for sensitive memory polymers. Incorporating crystallizable polymers with solid-state fluorescent dyes results in crystallizable fluorescent polymers. Such polymers show remarkably temperature-dependent fluorescence emission. Interestingly, fluorescence of the polymers shows a hysteresis between heating and subsequent cooling scans, which offers them a valuable thermally stimulated recording function. Both off−on and on−off recording functions can be achieved. Characters recorded on the polymer films can be erased and rewritten. Moreover, thermal history subjected to the polymers can be memorized and retrieved by measuring fluorescence intensity. With the merit of easy synthesis, recording function, remarkably thermoresponsive fluorescence with memory function, superior flexibility, and biocompatibility inherited from polymers, crystallizable fluorescent polymers offer a general platform for memory fluorescent polymers that are potentially useful for biosensing, recording materials, and smart devices. Alternatively, aggregation/dispersion of fluorophores of TFPs is affected by temperature variation, which in turn has an effect on fluorescence emission of TFPs.4 Fluorescent dyes are incorporated with thermoresponsive polymers such as poly(N-isopropylacrylamide) (PNIPAAm) to synthesize TFPs. Dispersion or aggregation of dyes is regulated by the thermoresponsive polymers. Tang and co-workers synthesized tetraphenylethene-functionalized PNIPAAm (PNIPAAmTPE), which showed a temperature-dependent fluorescence emission in aqueous media.4e Below the lower critical solution temperature (LCST) of PNIPAAm, PNIPAAm-TPE dispersed in aqueous media and emitted weakly, while PNIPAAm-TPE coagulated and radiated efficiently above the LCST. Although this strategy is useful to create TFPs, only in aqueous media the polymers are thermoresponsive because hydrogen-bonding and dipole−dipole interaction among polymer chains is related to hydration of polymers in water. In addition, doping polymers with fluorescent dyes were also used to fabricate TFPs.5

hermoresponsive fluorescent polymers (TFPs) featuring temperature-dependent fluorescence properties present a class of promising functional materials. Either fluorescence emission intensity or the wavelength of TFPs changes remarkably as a function of temperature, which offers them a broad range of applications in smart optical devices, biosensing, and so on.1 TFPs can be classified into two catalogs in terms of thermoresponsive mechanism. The conformation of fluorophores of TFPs is temperature dependent. π-Conjugated polymers such as polyacetylenes, polydiacetylenes, poly(phenylene vinylidenes), and polythiophenes, showing dynamic helical conformation, present a kind of typical TFP.2 The helical sense of the π-conjugated polymers is dependent on hydrogen bonds, steric hindrance, and so on among pendant units,3 which is thermally sensitive. Temperature fluctuation induces the variation of the helical sense of the polymers, πconjugation of polymer fluorophores, and then fluorescence emission. At high temperature, polymer backbones adopt a twisted conformation, resulting in fluorescence emission with short wavelength. Fluorescence emission red-shifts by 30 nm when temperature cools, at which polymer backbones adopt an extended conformation.

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© XXXX American Chemical Society

Received: June 13, 2016 Accepted: July 13, 2016

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DOI: 10.1021/acsmacrolett.6b00453 ACS Macro Lett. 2016, 5, 909−914

Letter

ACS Macro Letters Recently, memory polymers capable of memorizing their mechanical history (shape memory polymers) have attracted increasing interest due to their potential applications in smart and medical devices.6 More recently, temperature memory polymers remembering thermal history have also been achieved.7 Miscible PLLA/PMMA blends with broad glass transitions have been used as shape memory polymers at different stretching temperatures. It was found that the switch temperature was related to stretching temperatures, showing temperature memory effect.8 The memory polymers established are based on mechanical properties, which suffer from some inherent limitations such as low sensitivity, massive samples, bulky size, properly produced specimens, and so on. This strictly restricts their applications in engineering sectors. Developing new strategies for fabricating memory polymers is highly desirable. In contrast to mechanically based memory polymers, TFP is a promising candidate for developing memory polymers due to high sensitivity, ease of fabrication, and miniaturation. However, few TFPs mentioned above show thermostimulated memory function because their fluorescence is almost fully reversible when temperature recovers. Crystallization is a naturally occurring self-assembly process.9 Compared with organic small molecules, crystallization of polymers is very difficult.10 Macromolecules fold back and forth to form anisotropic lamellae with a thickness of approximately 10 nm.11 Crystallization of the polymer takes place at a temperature much lower than their melting temperature (over 20 °C). Such hysteresis of crystallization over melting of polymers offers them valuable opportunities for designing memory materials. Here, taking advantage of a broad supercooling temperature window of crystallizable polymers, we design a facile strategy for generation of memory fluorescent polymers (MFPs). Crystallizable polymers are incorporated with solid-state fluorescent dyes to create crystallizable fluorescent polymers. Such polymers show remarkably temperature-dependent fluorescence emission. Interestingly, fluorescence emission of the polymers shows a hysteresis between heating and subsequent cooling scans, which enables them valuable thermally stimulated recording function. Both on−off and off−on dual recording functions can be readily achieved with identical polymers. Characters recorded on the polymer films can be erased and rewritten. Moreover, thermal history subjected to the polymers can be memorized and retrieved by measuring fluorescence intensity. With the merit of recording function, remarkably thermoresponsive fluorescence emission with memory function, superior flexibility, and biocompatibility inherited from polymers, crystallizable fluorescent polymers offer a general platform for memory fluorescent polymers useful for biosensing, recording materials, and smart devices. As a demonstration-of-concept, poly(ε-caprolactone) was selected as a polymer matrix due to its strong crystallizability, mild melting, and crystallization temperatures, as well as designable chain structures (Scheme 1). A fluorescent dye of tetraphenylethene (TPE) showing intensive fluorescence emission in the solid state was used as report units.12 To avoid possible aggregation of TPE in polymers, TPE units were covalently bonded to polymers, which is preferential for superior thermal stability of ultimate fluorescent polymers. Poly(ε-caprolactone) terminated with tetraphenylethene (PCL-TPE) was synthesized through ring-opening polymerization using 2-(4-(triphenylvinyl)phenoxy)ethanol (1) as initiator. Detailed procedures for synthesis of PCL-TPE were described in the Supporting Information (SI). The number-

Scheme 1. Schematic Illustration of Switching Fluorescence of Polymers through Crystallization and Melting

average molecular weight of PCL-TPE was 11.7 kg/mol (denoted as PCL12k-TPE, the number of repeating units of caprolactone was 100, and weight percentage of TPE was 2.8%, Figures S1−S2, and Table S1, SI). PCL-TPE emitted efficiently at room temperature upon UV radiation but radiated weakly at 80 °C. To eliminate the effect of thermal history on fluorescence emission, PCL-TPE was subjected to identical thermal treatments (held at 120 °C for 5 min under N2, cooled to 25 °C for 10 min, and further cooled to −10 °C for 20 min) prior to fluorescence measurements. Fluorescence spectra of PCL-TPE at various temperatures in a heating scan are shown in Figure 1. PCL-TPE emitted intensive blue light with a wavelength of 480 nm at 0 °C (denoted as Sonset state). The fluorescence intensity decreased gradually with increasing temperature. Fluorescence intensity at 80 °C was only 1/60 of that at 0 °C (Figure 1c). Upon subsequent cooling, the same specimen from 80 to 0 °C, fluorescence intensity was enhanced. Fluorescence intensity at 0 °C (denoted as Smelt state) is half of that at Sonset. Annealing PCL-TPE at −10 °C for 20 min and 0 °C for 10 min resulted in a recovery to the Sonset state. It is interesting that a hysteresis loop was observed between heating and cooling scans (Figure 1c). Fluorescence intensity of PCL-TPE in the cooling scan was lower than that in a heating scan in a broad temperature range from 0 to 60 °C. For instance, the fluorescence intensity of PCL-TPE at 25 °C before melting (in heating scan) was much higher than that after melting (20.6 and 8.3, respectively). The fluorescence quantum yield of PCL-TPE at 25 °C before melting is 1.7-fold of that after melting (5.4% and 3.1%, respectively, Table S2, SI). The ratio of fluorescence intensity in a heating scan to that in a cooling scan (Iheating/Icooling), indicating fluorescence contrast, was summarized in Figure 1d. Iheating/Icooling had values over 2 in a broad temperature range from 0 to 55 °C. The maximum value of 5 was achieved at 40 °C, showing an optimized temperature existed for fluorescence contrast. To confirm the optimized temperature, PCL-TPE films were heated at 120 °C for 5 min to erase thermal history, followed by cooling to room temperature in 10 min. A piece of ice with size of approximately 5 mm (−10 °C) was mounted on polymer films to freeze the polymer films locally. Fluorescence of the polymer films contacting with ice turned on immediately under UV radiation (365 nm). The ice was allowed to melt in 5 min, followed by heating polymer films to 40 °C. Clear fluorescence from the water droplet was observed by the naked eye (Figure 1d). Similarly, digital photos were also taken at 25 and 60 °C, respectively, for the purpose of comparison. A digital photo of PCL-TPE at 40 °C showed clearer fluorescence contrast than those at 25 and 60 °C. 910

DOI: 10.1021/acsmacrolett.6b00453 ACS Macro Lett. 2016, 5, 909−914

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emission of TPE, which allows enhancing fluorescence intensity by increasing TPE concentration. To investigate the effect of TPE concentration, PCL-TPEs with various molecular weights were synthesized. Since TPE was the end group, PCL4k-TPE with the lowest molecular weight had the highest TPE concentration (Table S1, SI). PCL4k-TPE emitted the most brightly (Figure 2a). The fluorescence quantum yield of PCL-

Figure 1. Temperature dependence of fluorescence of PCL12k-TPE. (a) Fluorescence intensity decreased with increasing temperature from 0 to 80 °C. (b) Fluorescence intensity was restored partially upon cooling from 80 to 0 °C. (c) Fluorescence intensity (480 nm) for PCL12k-TPE (sphere) and the initiator (compound 1, SI) (square) in heating and cooling scans. The fluorescence intensity was normalized to that at 70 °C. The heating or cooling rate was 0.5 °C/min. Excitation: 350 nm. Inset showed fluorescence recovery from Smelt to Sonset by annealing at −10 °C for 20 min and 0 °C for 10 min. (d) Fluorescence contrast (Iheating/Icooling) for PCL-TPE (sphere) and the initiator (square). Inset showed photos at various temperatures (25, 40, and 60 °C (from left to right)).

Figure 2. (a) Fluorescence intensity of PCL-TPE with various molecular weights (from top to bottom: PCL4k-TPE, PCL8k-TPE, and PCL12k-TPE, respectively) as a function of temperature in heating and cooling scans. The inset shows a digital photo of PCL, PCL12k-TPE, PCL8k-TPE, and PCL4k-TPE (from left to right) under UV radiation (365 nm) at 25 °C. (b) Fluorescence contrast (Iheating/Icooling) as a function of temperature. Inset showed fluorescence photos of PCL4k-TPE (left) and PCL8k-TPE (right) at 40 °C. (c) Comparison between fluorescence and DSC results of PCL12k-TPE.

Reversibility of the fluorescence of PCL-TPE was evaluated. Switching at −10 °C, 0 °C, and 80 °C (Figure S3) was carried out as follows: holding samples at 80 °C for 5 min, cooling to 0 °C (Smelt), further cooling to −10 °C for 20 min, and restoring to 0 °C for 10 min (Sonset), followed by heating to 80 °C. This process was repeated 19 times. Fluorescence spectra of PCLTPE repeatedly switching among −10 °C (Sonset), 0 °C (Smelt), and 80 °C showed that fluorescence of PCL-TPE was reversible over 20 cycles in the temperature range investigated, likely due to excellent thermal stability of PCL-TPE (Figure S4, SI) and covalently bonding TPE with polymer backbones. Tuning fluorescence intensity of PCL-TPE is possible. TPE is a typical aggregation-induced emission (AIE) luminogen that emits intensively in the solid state and radiates weakly in the solution state.12 Aggregation or solidification favors the

TPE increased from 5.4% to 7.2% with decreasing molecular weight from PCL12k-TPE to PCL4k-TPE. Both PCL8k-TPE and PCL4k-TPE also showed thermoresponsive fluorescence (Figures S5 and S6, SI). Fluorescence contrast (Iheating/Icooling) had a value over 2 in a broad temperature range from 0 to 55 °C. An optimized temperature of 40 °C was also observed. Interestingly, the optimized temperature of PCL-TPE is independent of its molecular weight. This allows tuning its fluorescence intensity without altering operation temperature. 911

DOI: 10.1021/acsmacrolett.6b00453 ACS Macro Lett. 2016, 5, 909−914

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showed two distinct diffraction peaks at 21.3 and 23.6°, associated with (110) and (200) lattice planes of orthorhombic unit cells of PCL, respectively, verifying that PCL crystallized at low temperatures. With increasing temperature to 54 °C, the crystalline peaks disappeared. A sharp drop in crystallinity and crystallite size (L110 and L200) of PCL-TPE at 54 °C showed that PCL crystals were destroyed at this temperature. The melting temperature was close to that by DSC (52.5 °C). In a subsequent cooling scan, diffraction peaks of PCL were observed when temperature decreased to 48 °C, showing the formation of ordered structures. It was noted that the fluorescence intensity did not increase in the temperature range from 48 to 44 °C in a cooling scan (Figure 1c). This demonstrated that the formation of ordered structures at the initial stage of crystallization contributed little to fluorescence, possibly because the loosely packed ordered structures formed were not enough to restrict intramolecular motions of TPE. Moreover, XRD results (Figure S12, SI) revealed that crystallinity of PCL-TPE at the temperature ranging from 52 to 0 °C in the cooling scan was lower than that in the heating scan, consistent with fluorescence results. As mentioned above, PCL-TPEs showed remarkable thermoresponsive fluorescence and hysteresis between heating and cooling scans, which offered them valuable merits as thermally stimulated recording materials. To fulfill off−on recording function, PCL-TPE films were heated to 80 °C for 5 min, followed by cooling to 25 °C. An ice rod (−10 °C) was used to write characters on PCL-TPE films. The characters lighted up upon UV radiation (365 nm) (Figure 3a, upper-left). The characters written on PCL-TPE films could be erased. Heating PCL-TPE films to 80 °C erased the characters. Upon drawing with an ice rod, the patterns recorded appeared again (Figure 3a, bottom-left). On−off recording function could also be fulfilled. PCL-TPE films were first annealed at −10 °C for 20 min to light up films. Upon writing with a hot steel rod (80 °C), fluorescence of the written characters turns off under UV radiation (Figure 3a). Similarly, the written words could be erased by holding the PCL-TPE films at −10 °C for 20 min. The writing−erasing process could be repeated. Polarizing optical microscopy (POM) was further used to monitor the crystalline structures of PCL-TPE films (Figure 3b). Fan-like bright motifs were observed for the PCL-TPE films before melting, while isolated bright dots were observed for the films after melting. PCL-TPE was useful for memorizing thermal history. The PCL-TPE film frozen at −10 °C for 20 min was restored to 25 °C. The PCL-TPE film was then heated to a given temperature (historical temperature), followed by restoring to 25 °C. The fluorescence spectrum was scanned. Fluorescence intensity of PCL-TPE against historical temperatures was shown in Figure 3c and Figure S13 (SI). For the purpose of comparison, fluorescence intensity was normalized to that at 70 °C, and fluorescence intensity during heating and cooling scans (Figure 1c) was also presented. Fluorescence intensity of PCL-TPE decreased linearly with increasing historical temperature until 57 °C. Correlation between fluorescence intensity and historical temperatures (25−57 °C) demonstrated that it was possible to know which temperature the polymers had ever been heated to or retrieve the thermal history by measuring fluorescence intensity. In the range of 57−65 °C, an abnormal increase in fluorescence intensity was observed, possibly because annealing the polymers at these temperatures increases

To understand the thermoresponsive mechanism of PCLTPE, fluorescence of the initiator (compound 1, SI) at various temperatures was also investigated (Figure 1c and S7, SI). Fluorescence intensity of the initiator decreased by 3.6 times from 0 to 70 °C, by far lower than that of PCL-TPE (62 times for PCL12k-TPE). Moreover, no obvious hysteresis of fluorescence was observed for the initiator. This revealed that the unique thermoresponse of PCL-TPE was closely related to the polymer. Thermal transition of PCL-TPE was investigated using differential scanning calorimetry (DSC) (Figure 2c and Figure S8, SI). An endotherm appeared at 52.5 °C due to melting of PCL crystals in a heating scan, while in the cooling scan an exothermic peak was observed at 32 °C, ascribed to crystallization of PCL macromolecules. At low temperature PCL segments fold back and forth into lamellar crystals. Fluorescence molecules of TPE are excluded out of PCL crystals and solidified at rigid crystal surfaces (Scheme 1).13 Based on DSC results and the Gibbs−Thomson equation, the thickness of PCL crystals was calculated to be 9.1 nm for PCL12k-TPE. Coverage ratio of TPE on the PCL-TPE crystal surface was estimated to be 21% (Tables S3 and S4, SI). On the rigid surface of PCL-TPE lamellar crystals, intramolecular motions (such as rotation, bending, and vibration) of the phenyl rings of TPE are restricted,10 giving rise to intensive fluorescence emission of PCL-TPE at low temperatures. On the other hand, with increasing temperature, PCL crystals melt, and mobility of PCL segments is activated with increased free volume in polymers. Intramolecular motions of TPE are activated and annihilate the energy of its excited state, leading to weak fluorescence emission. Indeed, the lifetime of the excited state of PCL-TPE at 80 °C was much shorter than that at 0 °C (1.0 and 2.4 ns, respectively, for PCL12k-TPE, Figure S9 and Table S5). Moreover, the TPE moiety enriches or aggregates on the surface of PCL lamellar crystals at low temperatures rather than being dispersed throughout an amorphous polymer at high temperatures. Such concentration effect at various temperatures may also contribute to the thermoresponse of PCL-TPE. DSC results showed that crystallization of PCL-TPE seriously lagged behind its melting (over 20 °C, Figure 2c), indicating that crystalline structures of PCL-TPE at a given temperature were dependent on its thermal history (heating from 0 °C or cooling from 80 °C). Such variation in crystalline structures gave rise to different fluorescence intensity even at identical temperatures. Correspondingly, a fluorescence hysteresis of PCL-TPE between heating and cooling scans was observed. When comparing fluorescence and DSC results, it is interesting that maximum fluorescence contrast was located between crystallization and melting temperatures of PCL-TPE. This further revealed that the thermoresponse of PCL-TPE was closely related to crystallization and melting of polymers. It is noted that DSC peaks corresponding to melting and crystallization were clear and sharp, while the emission intensity changed gradually. A possible reason is that DSC detects heat variation corresponding to evolution of polymer crystals during crystallization or melting, while emission intensity is affected by a few factors including evolution of polymer crystals, viscosity, and concentration effect, and so on. This makes emission intensity varied in a broad temperature range. To monitor the evolution of crystalline structures of PCLTPE in heating and cooling scans, X-ray diffractometry (XRD) at various temperatures was carried out (Figure S10 and S11, SI). XRD spectra of PCL-TPE at low temperatures (e.g., 0 °C) 912

DOI: 10.1021/acsmacrolett.6b00453 ACS Macro Lett. 2016, 5, 909−914

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inherited from polymers, crystallizable fluorescent polymers offer a general platform for memory fluorescent polymers useful for biosensing, recording materials, and smart devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00453. Synthesis of PCL-TPE, DSC and XRD curves of PCLTPE (PDF) Video of heating and cooling a speciman from 0 °C to 80 °C (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support is from NSFC (21374136 and 21374136). REFERENCES

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Figure 3. Off−on and on−off recording and retrieving thermal history using PCL-TPE films. (a) Digital photos of off−on (left) and on−off (right) recording. (b) Polarizing optical microscopy (POM) images of PCL-TPE before (left) and after (right) melting. Retrieving thermal history above 25 °C (c) and below 25 °C (d).

their crystallinity (Figure S14 and S15, SI) and then enhances fluorescence emission. Retrieving the thermal history below 25 °C was also possible. A step increase in fluorescence intensity of PCL-TPE was observed when historical temperature was decreased to −5 °C (Figure 3d and Figure S16). Thus, it is possible to know whether the polymers have ever been frozen below −5 °C by detecting fluorescence emission. One possible reason is that at low temperatures polymer chains crystallize and form a rigid phase. Intramolecular motions of TPE embedded in polymers are prohibited, which gives rise to intensive fluorescence emission. In summary, we develop a facile strategy for generation of sensitive memory polymers. Incorporating crystallizable polymers with solid-state fluorescent dyes results in crystallizable fluorescent polymers. Such polymers show remarkable temperature-dependent fluorescence property. Interestingly, fluorescence of the polymers shows a hysteresis between heating and cooling scans, which offers them valuable thermally stimulated recording function. Both off−on and on−off recording function can be achieved. Moreover, thermal history subjected to the polymers can be memorized and retrieved by measuring fluorescence intensity. With the merit of easy synthesis, dual recording function, remarkably thermoresponsive fluorescence with memory function, superior flexibility, and biocompatibility 913

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