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Multiple luminescent switching of pyrenyl-substituted acylhydrazone derivative
Dyes and Pigments 152 (2018) 93–99
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Multiple luminescent switching of pyrenyl-substituted acylhydrazone derivative
T
Qing Chaia,b, Jue Weib, Binglian Baib,∗, Haitao Wanga, Min Lia,∗∗ a b
Key Laboratory for Automobile Materials (JLU), Ministry of Education, Jilin University, Changchun 130012, PR China College of Physics, Jilin University, Changchun 130012, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Aggregation-induced emission Xerogel Mechanofluorochromism Photo-fluorochromism
This report describes the multiple luminescent switching of pyrenyl-based acylhydrazone derivative PAH-8. Firstly, PAH-8 shows an uncommon aggregation-induced emission (AIE) phenomenon. Secondly, PAH-8 xerogel from DMSO shows mechanofluorochromic behavior, its maximum emission at 458 nm shifts to 514 nm upon grinding. Thirdly, PAH-8 precipitate from THF exhibits mechanofluorochromic behavior, whereas no mechanofluorochromic behavior was observed for the drop-casting film of PAH-8 solution in THF. In addition, the PAH8 xerogel exhibits the photo-fluorochromic behavior. The different luminescent property of PAH-8 is attributed to the switch of self-assembled structures.
1. Introduction Fluorescence of organic materials is often quenched in their solid states, which could limit the performance in optical devices. The aggregation-induced emission (AIE) could offer a potential solution for this challenge. Organic fluorescent materials possessing the AIE features have been investigated extensively [1–3]. Recently, numerous efforts have been devoted to the development of stimuli responsive organic fluorescent materials, whose fluorescent properties can be either switched on-off or tuned in the presence of external stimuli [3–7]. Of particular interest are organic molecules that can show a change in fluorescence color induced by external mechanical stimuli to the solid state owing to their fundamental importance and promising applications in sensors, memory devices, security ink [8–11]. Pyrene is a highly studied fluorophore for a variety of applications [12]. It was reported [13,14] that pyrenes exhibited not only monomeric fluorescence from their excited states, but also fluorescence from their excimers in the longer wavelength region formed in concentrated solutions and in the solid state due to extensive π-π stacking of their planar pyrene rings. Mechanochromic luminescent based on pyrene derivates have been reported and different mechanism of change in fluorescence color was proposed e.g., excimer-to-excimer transition [15–19], monomer-to-excimer transition [20], conformation transition [21] or change in the intermolecular packing and phase transition [22]. Recently, various novel mechanofluorochromic compounds were synthesized and thus broadened the field of mechanofluorochromic
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materials [23–25]. However, reports on mechanofluorochromic materials with photo responsive fluorescence switching properties are still limited. Jia et al. [26] reported that an anthracene derivative containing of aspartic acid and phenylalanine, showed force-induced reversible color changes and photochromic switching. The switchable emission colors were attributed to the different molecular packing modes and the light-induced [4 + 4] photocycloaddition reaction of the molecule. Wang et al. [27] reported that 3 (5)-(9-anthryl)pyrazole and its derivatives showed reversible piezo- and photochromic behaviors accompanied by emission color switching, and the photochromic behaviors was also due to the light-induced [4 + 4] photocycloaddition reaction. The hydrazone derivative [28,29] might show photo−responsive behavior due to the configurational isomerization of -C=N- bond upon UV irradiation [30,31]. Herein, we used pyrene and phenyl-acylhydrazone as the chromophores to construct a compound, which aimed to further understand the structure–property relationship by exploring the mechano- and photo-responsive properties of different chromophores. The obtained pyrene-based acylhydrazone derivative, 4-(3, 4-dioctyloxy) phenyl-1-pyrene acylhydrazone (PAH-8, Scheme 1) exhibits an uncommon AIE phenomenon involving a fluorescence color change as well as reversible mechano-response and irreversible photo-response between blue and green.
Corresponding author. Corresponding author. E-mail addresses: [email protected] (B. Bai), [email protected] (M. Li).
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https://doi.org/10.1016/j.dyepig.2018.01.051 Received 4 December 2017; Received in revised form 23 January 2018; Accepted 29 January 2018 0143-7208/ © 2018 Elsevier Ltd. All rights reserved.
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Scheme 1. The molecular structure of PAH-8.
2. Experimental section 2.1. Materials The compound 4-(3, 4-dioctyloxy) phenyl-1-pyrene acylhydrazone (PAH-8) was synthesized through 3,4-octyloxy-benzhydrazide and 1Pyrenecarboxaldehyde, the synthetic details were reported elsewhere [32]. And its chemical structure was confirmed by FT-IR, 1H NMR, 13C NMR, MS spectra and elemental analysis. 1 H NMR (300 MHz, DMSO‑d6), (ppm, from TMS): 11.90 (s, 1H), 9.52 (s, 1H), 8.89–8.79 (d, J = 9.6 Hz, 1H), 8.64–8.52 (d, J = 7.2 Hz, 1H), 8.49–8.32 (d, J = 7.7 Hz, 1H), 8.30–8.23 (q, J = 8.9 Hz, 2H), 8.18–8.08 (t, J = 7.7 Hz, 1H), 7.69–7.51 (m, 2 H), 7.17–7.09 (d, J = 8.5 Hz,1H), 4.09–4.05 (t, J = 6.3 Hz, 4H), 1.83–1.63 (m, 4H), 1.60–0.94 (m, 20H), 0.94–0.68 (d, 6H). 13 C NMR (75 MHz, chloroform-d), (ppm, from TMS): 152.55, 148.98, 132.48, 131.07, 130.37, 129.26, 128.34, 127.17, 126.10, 125.55, 125.00, 122.13, 112.34, 77.34, 76.91, 76.49, 76.49, 69.40, 69.07, 31.72, 29.19, 25.93, 22.56, 13.96. FT-IR (silicon wafer, cm−1): 3221, 3039, 2995, 2955, 2921, 2847, 1670, 1604, 1586, 1568, 1514, 1468, 1437, 1426, 1393, 1329,1306, 1272, 1221, 1187, 1157, 1133, 1095, 1074, 1022. Elemental analysis: calcd for C40H48N2O3: C, 79.43; H, 8.00; N, 4.63. Found: C, 79.47; H, 7.90; N, 4.64. MALDI-TOF MS: calcd for C40H48N2O3: 604.37, found: 605.10.
Fig. 1. Fluorescence spectra of PAH-8 in THF/water mixtures with different water fractions, excitation wavelength: 390 nm.
seen that PAH-8 showed two weak emissions at λmax = 409 nm and 431 nm in THF and emits a weak blue fluorescence under 365 nm UV illumination. The fluorescence intensity of PAH-8 in THF-water mixtures with less than 60% water (Fig. 2d-j) increased gradually with a slight red shift with the increase of water, and the quantum yields of PAH-8 in THF-water mixture (30% water) was only 0.6%. In contrast, the maximum fluorescence emission of PAH-8 in THF-water (70% water) located at λ = 437 nm and 459 nm, which shifted to 510 nm with a higher quantum yield (11%) for that in THF-water with much higher water content (70%–90% water) (Fig. 2a-c), and the luminescence color changed from blue to yellow-green. To understand the novel aggregation-induced characteristics of PAH-8, the UV-Vis spectra (Fig. S2) were recorded in THF-water mixtures with differing water content. In THF, the absorption spectrum of PAH-8 revealed two peaks at 370 nm and 398 nm (shoulder peak). The absorption spectra of PAH8 were broadened with a tail in the THF-water mixture with the increase of water content from 10% to 60%, indicative of the formation of nano-aggregate. Further increase of water content to 70% in the mixtures caused the red-shift of the absorption peak of PAH-8, which suggested the formation of another different aggregate. The formation of different aggregates of PAH-8 in THF-water mixtures were also confirmed by SEM observation (Fig. S3). Thus PAH-8 exhibited an uncommon AIE phenomenon involving shift of maximum emission which was attributed to different aggregate formation [21,33]. The change of emission color was so distinct that it could be distinguished by naked eye under 365 nm UV illumination.
2.2. Characterization 1
H NMR spectra using dimethyl sulfoxide-d6 as solvent and 13C NMR spectra using chloroform-d as solvent and tetramethylsilane (TMS) as an internal standard (δ = 0.00) were recorded with a Varian Unity 300 spectrometer at 300 MHz and 75 MHz, respectively. MS spectra were measured with AXIMA-CFR for MALDT-TOF. Field emission scanning electron microscopy (FE−SEM) images were taken with a JSM−6700F apparatus. X−Ray diffraction was carried out with a Bruker Avance D8 X-ray diffractometer. FT-IR spectra were recorded with a Perkin-Elmer spectrometer (Spectrum One B). The xerogels were obtained by freezing and pumping the organogel of PAH-8 for 12 h, and then the xerogels was cast onto silicon wafer for FT-IR measurement. UV lights were generated by a 500 W Xe lamp with a standard bandpass filter (UV: 280–375 nm), respectively. UV-vis absorption spectra were recorded on a Shimadzu UV-2550 spectrometer, and photoluminescence (PL) was measured on a Perkin-Elmer LS 55 spectrometer. Solid state PL efficiencies were measured using an integrating sphere (Fluorescence spectrophotometer SENS-900). Fluorescence Lifetime was performed using time-correlated single-photon counting (TCSPC) method and collected on an Edinburgh FLS980, with an Edinburgh EPL375 ps pulsed diode laser as the excitation source. Differential scanning calorimetry (DSC) curves were obtained on a TA Q20 DSC instrument before and after grinding. The rate of heating and cooling was 10 °C min−1.
4. Mechanochromic luminescent properties of PAH-8 4.1. Xerogels PAH-8 can form stable organogel in dimethyl sulfoxide (DMSO), the xerogels were obtained by freezing and pumping the organogel of PAH8 for 12 h. Fig. 3 showed photographs of PAH-8 xerogels upon 365 nm UV illumination, as shown in Fig. 3a, PAH-8 xerogel displayed a blue emission, while the ground xerogel showed (Fig. 3b) green emission. The blue emission could be recovered by annealing the ground xerogel at 120 °C for 30 min (Fig. 3c). These naked-eye-visible fluorescence color changes were recorded on a luminescence spectrophotometer. As shown in Fig. 4, the maximum emission of PAH-8 xerogel located at 458 nm (quantum yield ΦF = 0.077) with well-resolved vibronic emission band. While the ground xerogel of PAH-8 showed emission at 514 nm (quantum yield ΦF = 0.31), which returned to 457 nm upon being annealed at 120 °C for 30 min, indicating the reversible mechanochromic luminescent
3. AIE phenomenon of PAH-8 The fluorescence spectra of PAH-8 in tetrahydrofuran (THF)-water mixtures with different water fraction were measured (Fig. 1). It can be 94
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Fig. 2. Fluorescent images of PAH-8 in THF/water mixtures with different water fractions (a–j) under 365 nm UV illumination.
entangled network structure (Fig. 5a), whereas the irregular and rough aggregates were observed for the ground xerogel (Fig. 5b). Short ribbon fiber (the width is about 1 μm) was observed for the annealed ground xerogel (at 120 °C for 30 min). The hydrogen bonding interactions of PAH-8 were studied by FT-IR spectra (Fig. 6). The characteristic stretching vibration band of the amide N-H group was observed at 3203 cm−1, which became broad and shifted to higher wavenumber (3222 cm−1) after grinding, indicating the partial destruction of hydrogen bonds and the change of assembled structures. In xerogel, the relatively stronger intermolecular hydrogen bonding interaction immobilized the translational and rotational motion of molecules, thus conformational of pyrenyl moieties was restricted. The external force might alter the molecular interactions and facilitate the rearrangement of molecules. In order to further investigate the change of fluorescent color of PAH-8 under external force, X-ray diffraction (XRD) was performed. As shown in Fig. 7, five sharp peaks appear in the lower-angle region, and their d-spacings are 44.94 Å, 22.72 Å, 15.06 Å, 11.45 Å and 8.82 Å, illustrating a lamellar structure with 44.94 Å of interlayer distance in the xerogel phase. The ground xerogel showed weak diffraction peaks, and their full width at half maximum of diffraction peak broadened, and some of the diffraction peaks even disappeared, indicating its poor crystalline feature. As shown in Fig. 8, the thermal properties of the PAH-8 were studied by differential scanning calorimetry (DSC). The PAH-8 xerogel and the ground xerogel showed two endothermic peaks at 89 °C (ΔH = 23.72 J/g) and 164 °C (ΔH = 75.32 J/g) (xerogel), 77 °C (ΔH = 7.71 J/g) and 162 °C (ΔH = 65.66 J/g) (ground xerogel) in first heating, corresponding to crystalline transition and melting point, the higher temperature of crystalline transition and transition enthalpy of xerogel than that of ground xerogel indicated that the xerogel was thermodynamically more stable. There was an exothermic peak at 70 °C for ground xerogel during first heating, which further confirmed that the ground xerogel was metastable at room temperature. In addition, maybe due to the relatively strong intermolecular hydrogen bonding and π-π interactions, the DSC curves exhibited obvious glass transition at 107 °C (xerogel) and 110 °C (ground xerogel) in the first heating run, suggesting the supramolecular polymer characteristic [34].
Fig. 3. Photographs of emitting samples upon 365 nm UV illumination (a) PAH-8 xerogel, (b) ground xerogel and (c) after annealing treatment for (b).
Fig. 4. Fluorescence spectra of PAH-8 (a) xerogel, (b) ground xerogel, (c) after annealing treatment for (b).
characters, and the repeated grinding-annealing cyclic process was shown in Fig. S4. The fluorescence decay curve of PAH-8 xerogel and ground xerogel were measured by TCSPC system and all the data were fitted with deconvolution skill (Fig. S5, Table S1 and Table S2). The decay of the PAH-8 xerogel is bi-exponential fitting function and the average lifetime [τpw] is 1.53 ns, whereas the ground xerogel, a triexponential fitting function and the average lifetime [τpw] increased to 9.47 ns. The emission at 458 nm (xerogel) and 514 nm (ground xerogel) probably originate from two different pyrenyl chromophore excimer E1 (partially overlapping) and E2 (completely overlapping) [15]. It is considered that the partially overlapping excimer E1 exhibits a short lifetime and low quantum yield, and the sandwich packing excimer E2 exhibits a long lifetime and high quantum yield. The non radiation rate (Knf) of ground xerogel is 7.28 × 107 s−1, which is much lower than that of the xerogel (60.17 × 107 s−1) (Table S1), and the radiation rate (Kf) of ground xerogel remained almost unchanged compared to that of the xerogel. This result indicates that non radiation dissipation is the main reason for the low luminescence efficiency of the xerogel. However, the experimental results showed that the absorption spectra of ground xerogel showed a very small blue shift compared with that of xerogel (Fig. S6). Consequently, the red shifted emission in the ground xerogel probably was not all coming from the excimer emission, it might also partly ascribe to the physical processes such as change in the intermolecular packing and phase transition [22]. The aggregation morphology of PAH-8 was investigated by SEM, as shown in Fig. 5, The PAH-8 xerogel exhibited ribbon-like fibrous
4.2. Precipitate and the drop cast sample The photoluminescence property of PAH-8 also depended on the preparation of the sample (Fig. 9, Fig. S9). The PAH-8 precipitate from THF displayed a blue color fluorescent at 461 nm. The ground precipitate showed green intensified fluorescent emission at 514 nm. The blue emission could be recovered by annealing the ground precipitate at 120 °C for 30 min. Whereas the drop cast sample of hot solution of PAH-8 in THF did not exhibit mechanofluorochromism, and green fluorescence emission at 514 nm remained unchanged despite of grinding. The decay of the PAH-8 precipitate was detected and the result was fitted well by the single exponential (Fig. S10), and the fitting parameters were shown in Table S4. The photoluminescence decay curves of the ground precipitate and the drop cast samples were a triexponential fitting function and the average lifetime increases significantly. Table S3 showed that both the lifetime and quantum yield increased for the ground precipitate, whose average lifetime 95
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Fig. 5. SEM images of PAH-8 (a) xerogel, (b) ground xerogel and (c) after annealing treatment for (b).
Fig. 6. FT-IR spectra of PAH-8 (a) xerogel, (b) ground xerogel.
Fig. 8. DSC curves of PAH-8 (a) xerogel and (b) ground xerogel in the first heating run.
Fig. 9. Photographs of emitting samples from THF upon 365 nm UV illumination (a) PAH8 precipitate, (b) the drop cast sample, (c) ground precipitate, (d) after annealing treatment for (b) and (c).
were almost the same, suggesting that non radiation dissipation is the main reason for the low luminescence efficiency of PAH-8 precipitate from THF. The morphology also varied for PAH-8 precipitate and the drop cast sample. The ordered fibrous structure was observed in PAH-8 precipitate from THF (Fig. S11a), while cotton clusters disorderly aggregates were observed for the drop cast sample (Fig. S11b). In contrast, amorphous aggregates were observed in the ground precipitates (Fig. S11c). Further annealing either the ground precipitate or the drop cast sample resulted in short rod-like aggregates (Fig. S11d). Fig. 10 shows the FT-IR spectra of PAH-8 precipitate and the drop cast film from THF. The precipitate showed typical wavenumbers of N–H vibration at around 3164 cm−1 (Fig. 10a), whereas those of the drop cast film, at 3177 cm−1 and 3205 cm−1 (Fig. 10b), indicating that the intensity of the intermolecular hydrogen bonding in precipitate was
Fig. 7. XRD patterns of PAH-8 (a) xerogel, (b) ground xerogel.
(τpw = 9.53 ns) and quantum yield (ΦF = 0.31) of ground precipitate were higher than those of the precipitate (τpw = 0.56 ns, ΦF = 0.033), agreeing well with the documented pyrenyl excimers changed from E1 to E2 before and after grinding [15]. Whereas the photophysical property of the drop cast sample was similar to that of the ground precipitate, indicating a typical feature of the E2 excimer of pyrenyl groups. In addition, the non radiation rate (Knf) of the ground precipitate was 7.24 × 107s−1, which was lower than that of the drop cast sample (8.36 × 107s−1) (Table S3). Whereas the radiation rates (Kf) of PAH-8 in its the precipitate, ground precipitate or the drop cast film 96
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Fig. 10. FI-IR of PAH-8 from THF (a) precipitate, (b) drop cast sample, (c) ground precipitate, (d) after annealing treatment for (b) and (c).
Fig. 13. Fluorescence spectra of the PAH-8 xerogel from DMSO under UV irradiation for different time.
Fig. 11. XRD patterns of PAH-8 from THF (a)precipitate; (b) drop cast sample, (c) ground precipitate.
Fig. 14. SEM image of PAH-8 xerogel from DMSO under UV irradiation for 210 min.
Fig. 12. Fluorescent images of PAH-8 xerogel from DMSO (a) 0 min; (b) 5 min; (c) 48 min and (d) 210 min under UV irradiation.
stronger than that of the drop cast sample. Compared with that of precipitate, the N–H stretching of vibration band of the ground precipitate became weakened, broad and shifted to 3205 cm−1 (Fig. 10c), indicating that the hydrogen bonding interaction was somehow damaged by external force. The N–H stretching vibration band returned to the original wavenumbers of precipitate for both the annealed ground precipitate and the drop cast sample (Fig. 10d). The XRD patterns of precipitate (Fig. 11a) revealed a hexagonal columnar structure with the spacing of 22.65 Å (100), 13.62 Å (110) and 11.04 Å (200) in the low angle region with a reciprocal spacing ratio of 1: 1/√3: 1/2. The estimated all-trans molecular length of the most extended conformation of PAH-8 is 27.61 Å, obtained by the MM2
Fig. 15. XRD patterns of PAH-8 xerogel from DMSO (a) 0 min; (b) 210 min under UV irradiation.
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method, and the diameter of a column of the columnar phase is 26.15 Å. However, the finely ground precipitate (Fig. 11c) displayed a distinct scattering profile with the scattering vector ratio of 2: 1, verifying the transition from a hexagonal columnar pattern to a lamellar packing with a d-spacing of 42.32 Å. The drop cast sample (Fig. 11b) exhibited lamellar packing with the d-spacing identical to the ground precipitate (Fig. 11c). We proposed that PAH-8 molecules can not self-assemble to afford well-organized structure of the drop cast sample due to rapid volatilization of THF molecules. Therefore, the energy barrier of molecular motion lowered and the possibility of sandwich-type excimer formation increased. Based on the above data, the unusual stimuli-responsive behavior of PAH-8 could be understood. The PAH-8 molecules tended to self-assemble in xerogel and precipitate, strong intermolecular hydrogen bonding might force the pyrenyl groups to pack in a constricted environment, so the pyrenyl groups were partially overlapped showing blue emission. On the other hand, relatively weak intermolecular hydrogen bonding in the ground precipitate and the drop cast sample led to a sandwich-like arrangement showing green emission.
stimuli-responsive materials. Acknowledgments This work was supported by the Natural Science Foundation of Jilin Province (20170101112JC) and Project 985−Automotive Engineering of Jilin University. Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.dyepig.2018.01.051. References [1] Tomkeviciene A, Sutaite J, Volyniuk D, Kostiv N, Simkus G, Mimaite V, et al. Aggregation-induced emission enhancement in charge-transporting derivatives of carbazole and tetra(tri)phenylethylene. Dyes Pigments 2017;140:363–74. [2] Chen Z, Liang JH, Han X, Yin J, Yu GA, Liu SH. Fluorene-based novel highly emissive fluorescent molecules with aggregate fluorescence change or aggregationinduced emission enhancement characteristics. Dyes Pigments 2015;112:59–66. [3] Zhang XQ, Zhang XY, Tao L, Chi ZG, Xu JR, Wei Y. Aggregation induced emissionbased fluorescent nanoparticles: fabrication methodologies and biomedical applications. J Mater Chem B 2014;2:4398–414. [4] Xue PC, Yao BQ, Wang PP, Gong P, Zhang ZQ, Lu R. Strong fluorescent smart organogel as a dual sensing material for volatile acid and organic amine vapors. Chem Eur J 2015;21:17508–15. [5] Hagihara R, Harada N, Karasawa S, Koga N. Crystalline transformations of dinaphthyridinylamine derivatives with alteration of solid-state emission in response to external stimuli. CrystEngComm 2015;17:8825–34. [6] Yao X, Ru JX, Xu C, Liu YM, Dou W, Tang XL, et al. Multistimuli-responsive luminescence of naphthalazine based on aggregation-induced emission. ChemistryOpen 2015;4:478–82. [7] Xu YL, Li CT, Cao QY, Wang BY, Xie Y. A pyrenyl-appended organogel for fluorescence sensing of anions. Dyes Pigments 2017;139:681–7. [8] Roberts DRT, Holder SJ. Mechanochromic systems for the detection of stress, strain and deformation in polymeric materials. J Mater Chem 2011;21:8256–68. [9] Chi ZG, Zhang XQ, Xu BJ, Zhou X, Ma CP, Zhang Y, et al. Recent advances in organic mechanofluorochromic materials. Chem Soc Rev 2012;41:3878–96. [10] Ariga K, Mori T, Hill JP. Mechanical control of nanomaterials and nanosystems. Adv Mater 2012;24:158–76. [11] Ma ZY, Wang ZJ, Teng MJ, Xu ZJ, Jia XR. Mechanically induced multicolor change of luminescent materials. ChemPhysChem 2015;16:1811–28. [12] Figueira-Duarte TM, Mullen K. Pyrene-based materials for organic electronics. Chem Rev 2011;111:7260–314. [13] Winnik FM, Tamai N, Yonezawa J, Nishimura Y, Yamazaki I. Temperature-induced phase transition of pyrene-labeled (hydroxypropyl) cellulose in water: picosecond fluorescence studies. J Phys Chem 1992;96(4):1967–72. [14] Tsujii Y, Itoh T, Fukuda T, Miyamoto T, Ito S, Yamamoto M. Multilayer films of chromophoric cellulose octadecanoates studied by fluorescence spectroscopy. Langmuir 1992;8(3):936–41. [15] Teng MJ, Jia XR, Chen XF, Ma ZY Wei Y. Mechanochromic luminescent property of a polypeptide-based dendron. Chem Commun 2011;47:6078–80. [16] Ma ZY, Teng MJ, Wang ZJ, Jia XR. The mechanically induced color change from UV to visible region. Tetrahedron Lett 2013;54:6504–6. [17] Teng MJ, Jia XR, Yang S, Chen XF, Wei Y. Reversible tuning luminescent color and emission intensity: a dipeptide-based light-emitting material. Adv Mater 2012;24:1255–61. [18] Ma ZY, Teng MJ, Wang ZJ, Yang S, Jia XR. Mechanically induced multicolor switching based on a single organic molecule. Angew Chem Int Ed 2013;52:12268–72. [19] Ma ZY, Wang ZJ, Xu ZJ, Jia XR, Wei Y. Controllable multicolor switching of oligopeptide-based mechanochromic molecules: from gel phase to solid powder. J Mater Chem C 2015;3:3399–405. [20] Nagata E, Takeuchi S, Nakanishi T, Hasegawa Y, Mawatari Y, Nakano H. Mechanofluorochromism of 1-alkanoylaminopyrenes. ChemPhysChem 2015;16:3038–43. [21] Jadhav T, Dhokale B, Mobin SM, Misra R. Aggregation induced emission and mechanochromism in pyrenoimidazoles. J Mater Chem C 2015;3:9981–8. [22] Rao MR, Liao CW, Su WL, Sun SS. Quinoxaline based D–A–D molecules: high contrast reversible solid-state mechano- and thermo-responsive fluorescent materials. J Mater Chem C 2013;1:5491–501. [23] Zhao DB, Li GC, Wu D, Qin XR, Neuhaus P, Cheng YY, et al. Regiospecific N-heteroarylation of amidines for full-color-tunable boron difluoride dyes with mechanochromic luminescence. Angew Chem 2013;125:13921–5. [24] Matsunaga Y, Yang JS. Multicolor fluorescene writing based on host-guest interactions and force-induced fluorescence-color memory. Angew Chem 2015;127:8096–100. [25] Xu SD, Liu TT, Mu YX, Wang YF, Chi ZG, Lo CC, et al. An organic molecule with asymmetric structure exhibiting aggregation-induced emission, delayed fluorescence, and mechanoluminescence. Angew Chem Int Ed 2015;54:874–8.
5. Photo-fluorochromism properties PAH-8 xerogel exhibited photochromic luminescent behavior. Fig. 12 showed fluorescent images of the xerogel at different UV irradiated time. The PAH-8 xerogel emitted blue light, the UV-irradiated xerogel showed green emission and then non-emission with the increase of irradiation time. These naked-eye-visible fluorescence color changes were also recorded on a luminescence spectrophotometer. The PL spectra of PAH-8 were given in Fig. 13, it can be seen that the fluorescent emission intensity of xerogel at 457 nm decreased upon photoirradiation, and meanwhile, a wide shoulder peak at around 520 nm appeared. The irradiated xerogel showed fused fibers (Fig. 14) and the weakened diffraction peaks, indicating its poor crystalline feature (Fig. 15). Nandi et al. recently reported that the fluorescence intensity of the xerogel decreased upon photoirradiation because of the photoisomerization of imine (C=N) bond from the E form to the Z form [30]. Similar photoisomerization of imine (C=N) bond was confirmed in PAH-8 in our recent publication [32]. Considering the results above, we proposed a plausible mechanism. The decrease of fluorescent emission was due to the photoisomerization of partly imine (C=N) bond upon UV irradiation, which weakened the intermolecular interaction. Simultaneously, the energy barrier of molecular motion lowered, thus some of pyrenyl groups changed from partially overlapping excimer E1 in xerogel to sandwich packing excimer E2 upon UV irradiation. Meanwhile, the amount of Z form increased and the fluorescence almost completely disappeared with the prolonged of irradiation. In addition, less ordered structure for ground xerogel (as mentioned above) will be in favour of absorbing more the light energy that facilitated E−Z isomerization along the C=N bond [31]. So the fluorescent emission of the ground xerogel at 514 nm decreased more quickly than that of the xerogel upon photoirradiation (Fig. S15). 6. Conclusion In summary, PAH-8 showed an uncommon AIE phenomenon involving a fluorescence color change. PAH-8 xerogel exhibited favorable mechano-fluorochromic behaviors with naked-eye-visible fluorescence color changes, and the emission could be reversibly switched between blue and green color. Simultaneously, the photoluminescence property of PAH-8 could be tuned depending on the preparation conditions. The PAH-8 precipitate from THF displayed mechano-fluorochromic behavior, whereas the drop cast sample from THF did not exhibit mechanofluorochromic. In addition, the PAH-8 xerogel exhibited photo-fluorochromic behavior. As the PAH-8 could show mechano- and photofluorochromism behavior, it might be potential candidate for external 98
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Multiple luminescent switching of pyrenyl-substituted acylhydrazone derivative
T
Qing Chaia,b, Jue Weib, Binglian Baib,∗, Haitao Wanga, Min Lia,∗∗ a b
Key Laboratory for Automobile Materials (JLU), Ministry of Education, Jilin University, Changchun 130012, PR China College of Physics, Jilin University, Changchun 130012, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Aggregation-induced emission Xerogel Mechanofluorochromism Photo-fluorochromism
This report describes the multiple luminescent switching of pyrenyl-based acylhydrazone derivative PAH-8. Firstly, PAH-8 shows an uncommon aggregation-induced emission (AIE) phenomenon. Secondly, PAH-8 xerogel from DMSO shows mechanofluorochromic behavior, its maximum emission at 458 nm shifts to 514 nm upon grinding. Thirdly, PAH-8 precipitate from THF exhibits mechanofluorochromic behavior, whereas no mechanofluorochromic behavior was observed for the drop-casting film of PAH-8 solution in THF. In addition, the PAH8 xerogel exhibits the photo-fluorochromic behavior. The different luminescent property of PAH-8 is attributed to the switch of self-assembled structures.
1. Introduction Fluorescence of organic materials is often quenched in their solid states, which could limit the performance in optical devices. The aggregation-induced emission (AIE) could offer a potential solution for this challenge. Organic fluorescent materials possessing the AIE features have been investigated extensively [1–3]. Recently, numerous efforts have been devoted to the development of stimuli responsive organic fluorescent materials, whose fluorescent properties can be either switched on-off or tuned in the presence of external stimuli [3–7]. Of particular interest are organic molecules that can show a change in fluorescence color induced by external mechanical stimuli to the solid state owing to their fundamental importance and promising applications in sensors, memory devices, security ink [8–11]. Pyrene is a highly studied fluorophore for a variety of applications [12]. It was reported [13,14] that pyrenes exhibited not only monomeric fluorescence from their excited states, but also fluorescence from their excimers in the longer wavelength region formed in concentrated solutions and in the solid state due to extensive π-π stacking of their planar pyrene rings. Mechanochromic luminescent based on pyrene derivates have been reported and different mechanism of change in fluorescence color was proposed e.g., excimer-to-excimer transition [15–19], monomer-to-excimer transition [20], conformation transition [21] or change in the intermolecular packing and phase transition [22]. Recently, various novel mechanofluorochromic compounds were synthesized and thus broadened the field of mechanofluorochromic
∗
materials [23–25]. However, reports on mechanofluorochromic materials with photo responsive fluorescence switching properties are still limited. Jia et al. [26] reported that an anthracene derivative containing of aspartic acid and phenylalanine, showed force-induced reversible color changes and photochromic switching. The switchable emission colors were attributed to the different molecular packing modes and the light-induced [4 + 4] photocycloaddition reaction of the molecule. Wang et al. [27] reported that 3 (5)-(9-anthryl)pyrazole and its derivatives showed reversible piezo- and photochromic behaviors accompanied by emission color switching, and the photochromic behaviors was also due to the light-induced [4 + 4] photocycloaddition reaction. The hydrazone derivative [28,29] might show photo−responsive behavior due to the configurational isomerization of -C=N- bond upon UV irradiation [30,31]. Herein, we used pyrene and phenyl-acylhydrazone as the chromophores to construct a compound, which aimed to further understand the structure–property relationship by exploring the mechano- and photo-responsive properties of different chromophores. The obtained pyrene-based acylhydrazone derivative, 4-(3, 4-dioctyloxy) phenyl-1-pyrene acylhydrazone (PAH-8, Scheme 1) exhibits an uncommon AIE phenomenon involving a fluorescence color change as well as reversible mechano-response and irreversible photo-response between blue and green.
Corresponding author. Corresponding author. E-mail addresses: [email protected] (B. Bai), [email protected] (M. Li).
∗∗
https://doi.org/10.1016/j.dyepig.2018.01.051 Received 4 December 2017; Received in revised form 23 January 2018; Accepted 29 January 2018 0143-7208/ © 2018 Elsevier Ltd. All rights reserved.
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Scheme 1. The molecular structure of PAH-8.
2. Experimental section 2.1. Materials The compound 4-(3, 4-dioctyloxy) phenyl-1-pyrene acylhydrazone (PAH-8) was synthesized through 3,4-octyloxy-benzhydrazide and 1Pyrenecarboxaldehyde, the synthetic details were reported elsewhere [32]. And its chemical structure was confirmed by FT-IR, 1H NMR, 13C NMR, MS spectra and elemental analysis. 1 H NMR (300 MHz, DMSO‑d6), (ppm, from TMS): 11.90 (s, 1H), 9.52 (s, 1H), 8.89–8.79 (d, J = 9.6 Hz, 1H), 8.64–8.52 (d, J = 7.2 Hz, 1H), 8.49–8.32 (d, J = 7.7 Hz, 1H), 8.30–8.23 (q, J = 8.9 Hz, 2H), 8.18–8.08 (t, J = 7.7 Hz, 1H), 7.69–7.51 (m, 2 H), 7.17–7.09 (d, J = 8.5 Hz,1H), 4.09–4.05 (t, J = 6.3 Hz, 4H), 1.83–1.63 (m, 4H), 1.60–0.94 (m, 20H), 0.94–0.68 (d, 6H). 13 C NMR (75 MHz, chloroform-d), (ppm, from TMS): 152.55, 148.98, 132.48, 131.07, 130.37, 129.26, 128.34, 127.17, 126.10, 125.55, 125.00, 122.13, 112.34, 77.34, 76.91, 76.49, 76.49, 69.40, 69.07, 31.72, 29.19, 25.93, 22.56, 13.96. FT-IR (silicon wafer, cm−1): 3221, 3039, 2995, 2955, 2921, 2847, 1670, 1604, 1586, 1568, 1514, 1468, 1437, 1426, 1393, 1329,1306, 1272, 1221, 1187, 1157, 1133, 1095, 1074, 1022. Elemental analysis: calcd for C40H48N2O3: C, 79.43; H, 8.00; N, 4.63. Found: C, 79.47; H, 7.90; N, 4.64. MALDI-TOF MS: calcd for C40H48N2O3: 604.37, found: 605.10.
Fig. 1. Fluorescence spectra of PAH-8 in THF/water mixtures with different water fractions, excitation wavelength: 390 nm.
seen that PAH-8 showed two weak emissions at λmax = 409 nm and 431 nm in THF and emits a weak blue fluorescence under 365 nm UV illumination. The fluorescence intensity of PAH-8 in THF-water mixtures with less than 60% water (Fig. 2d-j) increased gradually with a slight red shift with the increase of water, and the quantum yields of PAH-8 in THF-water mixture (30% water) was only 0.6%. In contrast, the maximum fluorescence emission of PAH-8 in THF-water (70% water) located at λ = 437 nm and 459 nm, which shifted to 510 nm with a higher quantum yield (11%) for that in THF-water with much higher water content (70%–90% water) (Fig. 2a-c), and the luminescence color changed from blue to yellow-green. To understand the novel aggregation-induced characteristics of PAH-8, the UV-Vis spectra (Fig. S2) were recorded in THF-water mixtures with differing water content. In THF, the absorption spectrum of PAH-8 revealed two peaks at 370 nm and 398 nm (shoulder peak). The absorption spectra of PAH8 were broadened with a tail in the THF-water mixture with the increase of water content from 10% to 60%, indicative of the formation of nano-aggregate. Further increase of water content to 70% in the mixtures caused the red-shift of the absorption peak of PAH-8, which suggested the formation of another different aggregate. The formation of different aggregates of PAH-8 in THF-water mixtures were also confirmed by SEM observation (Fig. S3). Thus PAH-8 exhibited an uncommon AIE phenomenon involving shift of maximum emission which was attributed to different aggregate formation [21,33]. The change of emission color was so distinct that it could be distinguished by naked eye under 365 nm UV illumination.
2.2. Characterization 1
H NMR spectra using dimethyl sulfoxide-d6 as solvent and 13C NMR spectra using chloroform-d as solvent and tetramethylsilane (TMS) as an internal standard (δ = 0.00) were recorded with a Varian Unity 300 spectrometer at 300 MHz and 75 MHz, respectively. MS spectra were measured with AXIMA-CFR for MALDT-TOF. Field emission scanning electron microscopy (FE−SEM) images were taken with a JSM−6700F apparatus. X−Ray diffraction was carried out with a Bruker Avance D8 X-ray diffractometer. FT-IR spectra were recorded with a Perkin-Elmer spectrometer (Spectrum One B). The xerogels were obtained by freezing and pumping the organogel of PAH-8 for 12 h, and then the xerogels was cast onto silicon wafer for FT-IR measurement. UV lights were generated by a 500 W Xe lamp with a standard bandpass filter (UV: 280–375 nm), respectively. UV-vis absorption spectra were recorded on a Shimadzu UV-2550 spectrometer, and photoluminescence (PL) was measured on a Perkin-Elmer LS 55 spectrometer. Solid state PL efficiencies were measured using an integrating sphere (Fluorescence spectrophotometer SENS-900). Fluorescence Lifetime was performed using time-correlated single-photon counting (TCSPC) method and collected on an Edinburgh FLS980, with an Edinburgh EPL375 ps pulsed diode laser as the excitation source. Differential scanning calorimetry (DSC) curves were obtained on a TA Q20 DSC instrument before and after grinding. The rate of heating and cooling was 10 °C min−1.
4. Mechanochromic luminescent properties of PAH-8 4.1. Xerogels PAH-8 can form stable organogel in dimethyl sulfoxide (DMSO), the xerogels were obtained by freezing and pumping the organogel of PAH8 for 12 h. Fig. 3 showed photographs of PAH-8 xerogels upon 365 nm UV illumination, as shown in Fig. 3a, PAH-8 xerogel displayed a blue emission, while the ground xerogel showed (Fig. 3b) green emission. The blue emission could be recovered by annealing the ground xerogel at 120 °C for 30 min (Fig. 3c). These naked-eye-visible fluorescence color changes were recorded on a luminescence spectrophotometer. As shown in Fig. 4, the maximum emission of PAH-8 xerogel located at 458 nm (quantum yield ΦF = 0.077) with well-resolved vibronic emission band. While the ground xerogel of PAH-8 showed emission at 514 nm (quantum yield ΦF = 0.31), which returned to 457 nm upon being annealed at 120 °C for 30 min, indicating the reversible mechanochromic luminescent
3. AIE phenomenon of PAH-8 The fluorescence spectra of PAH-8 in tetrahydrofuran (THF)-water mixtures with different water fraction were measured (Fig. 1). It can be 94
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Fig. 2. Fluorescent images of PAH-8 in THF/water mixtures with different water fractions (a–j) under 365 nm UV illumination.
entangled network structure (Fig. 5a), whereas the irregular and rough aggregates were observed for the ground xerogel (Fig. 5b). Short ribbon fiber (the width is about 1 μm) was observed for the annealed ground xerogel (at 120 °C for 30 min). The hydrogen bonding interactions of PAH-8 were studied by FT-IR spectra (Fig. 6). The characteristic stretching vibration band of the amide N-H group was observed at 3203 cm−1, which became broad and shifted to higher wavenumber (3222 cm−1) after grinding, indicating the partial destruction of hydrogen bonds and the change of assembled structures. In xerogel, the relatively stronger intermolecular hydrogen bonding interaction immobilized the translational and rotational motion of molecules, thus conformational of pyrenyl moieties was restricted. The external force might alter the molecular interactions and facilitate the rearrangement of molecules. In order to further investigate the change of fluorescent color of PAH-8 under external force, X-ray diffraction (XRD) was performed. As shown in Fig. 7, five sharp peaks appear in the lower-angle region, and their d-spacings are 44.94 Å, 22.72 Å, 15.06 Å, 11.45 Å and 8.82 Å, illustrating a lamellar structure with 44.94 Å of interlayer distance in the xerogel phase. The ground xerogel showed weak diffraction peaks, and their full width at half maximum of diffraction peak broadened, and some of the diffraction peaks even disappeared, indicating its poor crystalline feature. As shown in Fig. 8, the thermal properties of the PAH-8 were studied by differential scanning calorimetry (DSC). The PAH-8 xerogel and the ground xerogel showed two endothermic peaks at 89 °C (ΔH = 23.72 J/g) and 164 °C (ΔH = 75.32 J/g) (xerogel), 77 °C (ΔH = 7.71 J/g) and 162 °C (ΔH = 65.66 J/g) (ground xerogel) in first heating, corresponding to crystalline transition and melting point, the higher temperature of crystalline transition and transition enthalpy of xerogel than that of ground xerogel indicated that the xerogel was thermodynamically more stable. There was an exothermic peak at 70 °C for ground xerogel during first heating, which further confirmed that the ground xerogel was metastable at room temperature. In addition, maybe due to the relatively strong intermolecular hydrogen bonding and π-π interactions, the DSC curves exhibited obvious glass transition at 107 °C (xerogel) and 110 °C (ground xerogel) in the first heating run, suggesting the supramolecular polymer characteristic [34].
Fig. 3. Photographs of emitting samples upon 365 nm UV illumination (a) PAH-8 xerogel, (b) ground xerogel and (c) after annealing treatment for (b).
Fig. 4. Fluorescence spectra of PAH-8 (a) xerogel, (b) ground xerogel, (c) after annealing treatment for (b).
characters, and the repeated grinding-annealing cyclic process was shown in Fig. S4. The fluorescence decay curve of PAH-8 xerogel and ground xerogel were measured by TCSPC system and all the data were fitted with deconvolution skill (Fig. S5, Table S1 and Table S2). The decay of the PAH-8 xerogel is bi-exponential fitting function and the average lifetime [τpw] is 1.53 ns, whereas the ground xerogel, a triexponential fitting function and the average lifetime [τpw] increased to 9.47 ns. The emission at 458 nm (xerogel) and 514 nm (ground xerogel) probably originate from two different pyrenyl chromophore excimer E1 (partially overlapping) and E2 (completely overlapping) [15]. It is considered that the partially overlapping excimer E1 exhibits a short lifetime and low quantum yield, and the sandwich packing excimer E2 exhibits a long lifetime and high quantum yield. The non radiation rate (Knf) of ground xerogel is 7.28 × 107 s−1, which is much lower than that of the xerogel (60.17 × 107 s−1) (Table S1), and the radiation rate (Kf) of ground xerogel remained almost unchanged compared to that of the xerogel. This result indicates that non radiation dissipation is the main reason for the low luminescence efficiency of the xerogel. However, the experimental results showed that the absorption spectra of ground xerogel showed a very small blue shift compared with that of xerogel (Fig. S6). Consequently, the red shifted emission in the ground xerogel probably was not all coming from the excimer emission, it might also partly ascribe to the physical processes such as change in the intermolecular packing and phase transition [22]. The aggregation morphology of PAH-8 was investigated by SEM, as shown in Fig. 5, The PAH-8 xerogel exhibited ribbon-like fibrous
4.2. Precipitate and the drop cast sample The photoluminescence property of PAH-8 also depended on the preparation of the sample (Fig. 9, Fig. S9). The PAH-8 precipitate from THF displayed a blue color fluorescent at 461 nm. The ground precipitate showed green intensified fluorescent emission at 514 nm. The blue emission could be recovered by annealing the ground precipitate at 120 °C for 30 min. Whereas the drop cast sample of hot solution of PAH-8 in THF did not exhibit mechanofluorochromism, and green fluorescence emission at 514 nm remained unchanged despite of grinding. The decay of the PAH-8 precipitate was detected and the result was fitted well by the single exponential (Fig. S10), and the fitting parameters were shown in Table S4. The photoluminescence decay curves of the ground precipitate and the drop cast samples were a triexponential fitting function and the average lifetime increases significantly. Table S3 showed that both the lifetime and quantum yield increased for the ground precipitate, whose average lifetime 95
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Fig. 5. SEM images of PAH-8 (a) xerogel, (b) ground xerogel and (c) after annealing treatment for (b).
Fig. 6. FT-IR spectra of PAH-8 (a) xerogel, (b) ground xerogel.
Fig. 8. DSC curves of PAH-8 (a) xerogel and (b) ground xerogel in the first heating run.
Fig. 9. Photographs of emitting samples from THF upon 365 nm UV illumination (a) PAH8 precipitate, (b) the drop cast sample, (c) ground precipitate, (d) after annealing treatment for (b) and (c).
were almost the same, suggesting that non radiation dissipation is the main reason for the low luminescence efficiency of PAH-8 precipitate from THF. The morphology also varied for PAH-8 precipitate and the drop cast sample. The ordered fibrous structure was observed in PAH-8 precipitate from THF (Fig. S11a), while cotton clusters disorderly aggregates were observed for the drop cast sample (Fig. S11b). In contrast, amorphous aggregates were observed in the ground precipitates (Fig. S11c). Further annealing either the ground precipitate or the drop cast sample resulted in short rod-like aggregates (Fig. S11d). Fig. 10 shows the FT-IR spectra of PAH-8 precipitate and the drop cast film from THF. The precipitate showed typical wavenumbers of N–H vibration at around 3164 cm−1 (Fig. 10a), whereas those of the drop cast film, at 3177 cm−1 and 3205 cm−1 (Fig. 10b), indicating that the intensity of the intermolecular hydrogen bonding in precipitate was
Fig. 7. XRD patterns of PAH-8 (a) xerogel, (b) ground xerogel.
(τpw = 9.53 ns) and quantum yield (ΦF = 0.31) of ground precipitate were higher than those of the precipitate (τpw = 0.56 ns, ΦF = 0.033), agreeing well with the documented pyrenyl excimers changed from E1 to E2 before and after grinding [15]. Whereas the photophysical property of the drop cast sample was similar to that of the ground precipitate, indicating a typical feature of the E2 excimer of pyrenyl groups. In addition, the non radiation rate (Knf) of the ground precipitate was 7.24 × 107s−1, which was lower than that of the drop cast sample (8.36 × 107s−1) (Table S3). Whereas the radiation rates (Kf) of PAH-8 in its the precipitate, ground precipitate or the drop cast film 96
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Fig. 10. FI-IR of PAH-8 from THF (a) precipitate, (b) drop cast sample, (c) ground precipitate, (d) after annealing treatment for (b) and (c).
Fig. 13. Fluorescence spectra of the PAH-8 xerogel from DMSO under UV irradiation for different time.
Fig. 11. XRD patterns of PAH-8 from THF (a)precipitate; (b) drop cast sample, (c) ground precipitate.
Fig. 14. SEM image of PAH-8 xerogel from DMSO under UV irradiation for 210 min.
Fig. 12. Fluorescent images of PAH-8 xerogel from DMSO (a) 0 min; (b) 5 min; (c) 48 min and (d) 210 min under UV irradiation.
stronger than that of the drop cast sample. Compared with that of precipitate, the N–H stretching of vibration band of the ground precipitate became weakened, broad and shifted to 3205 cm−1 (Fig. 10c), indicating that the hydrogen bonding interaction was somehow damaged by external force. The N–H stretching vibration band returned to the original wavenumbers of precipitate for both the annealed ground precipitate and the drop cast sample (Fig. 10d). The XRD patterns of precipitate (Fig. 11a) revealed a hexagonal columnar structure with the spacing of 22.65 Å (100), 13.62 Å (110) and 11.04 Å (200) in the low angle region with a reciprocal spacing ratio of 1: 1/√3: 1/2. The estimated all-trans molecular length of the most extended conformation of PAH-8 is 27.61 Å, obtained by the MM2
Fig. 15. XRD patterns of PAH-8 xerogel from DMSO (a) 0 min; (b) 210 min under UV irradiation.
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method, and the diameter of a column of the columnar phase is 26.15 Å. However, the finely ground precipitate (Fig. 11c) displayed a distinct scattering profile with the scattering vector ratio of 2: 1, verifying the transition from a hexagonal columnar pattern to a lamellar packing with a d-spacing of 42.32 Å. The drop cast sample (Fig. 11b) exhibited lamellar packing with the d-spacing identical to the ground precipitate (Fig. 11c). We proposed that PAH-8 molecules can not self-assemble to afford well-organized structure of the drop cast sample due to rapid volatilization of THF molecules. Therefore, the energy barrier of molecular motion lowered and the possibility of sandwich-type excimer formation increased. Based on the above data, the unusual stimuli-responsive behavior of PAH-8 could be understood. The PAH-8 molecules tended to self-assemble in xerogel and precipitate, strong intermolecular hydrogen bonding might force the pyrenyl groups to pack in a constricted environment, so the pyrenyl groups were partially overlapped showing blue emission. On the other hand, relatively weak intermolecular hydrogen bonding in the ground precipitate and the drop cast sample led to a sandwich-like arrangement showing green emission.
stimuli-responsive materials. Acknowledgments This work was supported by the Natural Science Foundation of Jilin Province (20170101112JC) and Project 985−Automotive Engineering of Jilin University. Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.dyepig.2018.01.051. References [1] Tomkeviciene A, Sutaite J, Volyniuk D, Kostiv N, Simkus G, Mimaite V, et al. Aggregation-induced emission enhancement in charge-transporting derivatives of carbazole and tetra(tri)phenylethylene. Dyes Pigments 2017;140:363–74. [2] Chen Z, Liang JH, Han X, Yin J, Yu GA, Liu SH. Fluorene-based novel highly emissive fluorescent molecules with aggregate fluorescence change or aggregationinduced emission enhancement characteristics. Dyes Pigments 2015;112:59–66. [3] Zhang XQ, Zhang XY, Tao L, Chi ZG, Xu JR, Wei Y. Aggregation induced emissionbased fluorescent nanoparticles: fabrication methodologies and biomedical applications. J Mater Chem B 2014;2:4398–414. [4] Xue PC, Yao BQ, Wang PP, Gong P, Zhang ZQ, Lu R. Strong fluorescent smart organogel as a dual sensing material for volatile acid and organic amine vapors. Chem Eur J 2015;21:17508–15. [5] Hagihara R, Harada N, Karasawa S, Koga N. Crystalline transformations of dinaphthyridinylamine derivatives with alteration of solid-state emission in response to external stimuli. CrystEngComm 2015;17:8825–34. [6] Yao X, Ru JX, Xu C, Liu YM, Dou W, Tang XL, et al. Multistimuli-responsive luminescence of naphthalazine based on aggregation-induced emission. ChemistryOpen 2015;4:478–82. [7] Xu YL, Li CT, Cao QY, Wang BY, Xie Y. A pyrenyl-appended organogel for fluorescence sensing of anions. Dyes Pigments 2017;139:681–7. [8] Roberts DRT, Holder SJ. Mechanochromic systems for the detection of stress, strain and deformation in polymeric materials. J Mater Chem 2011;21:8256–68. [9] Chi ZG, Zhang XQ, Xu BJ, Zhou X, Ma CP, Zhang Y, et al. Recent advances in organic mechanofluorochromic materials. Chem Soc Rev 2012;41:3878–96. [10] Ariga K, Mori T, Hill JP. Mechanical control of nanomaterials and nanosystems. Adv Mater 2012;24:158–76. [11] Ma ZY, Wang ZJ, Teng MJ, Xu ZJ, Jia XR. Mechanically induced multicolor change of luminescent materials. ChemPhysChem 2015;16:1811–28. [12] Figueira-Duarte TM, Mullen K. Pyrene-based materials for organic electronics. Chem Rev 2011;111:7260–314. [13] Winnik FM, Tamai N, Yonezawa J, Nishimura Y, Yamazaki I. Temperature-induced phase transition of pyrene-labeled (hydroxypropyl) cellulose in water: picosecond fluorescence studies. J Phys Chem 1992;96(4):1967–72. [14] Tsujii Y, Itoh T, Fukuda T, Miyamoto T, Ito S, Yamamoto M. Multilayer films of chromophoric cellulose octadecanoates studied by fluorescence spectroscopy. Langmuir 1992;8(3):936–41. [15] Teng MJ, Jia XR, Chen XF, Ma ZY Wei Y. Mechanochromic luminescent property of a polypeptide-based dendron. Chem Commun 2011;47:6078–80. [16] Ma ZY, Teng MJ, Wang ZJ, Jia XR. The mechanically induced color change from UV to visible region. Tetrahedron Lett 2013;54:6504–6. [17] Teng MJ, Jia XR, Yang S, Chen XF, Wei Y. Reversible tuning luminescent color and emission intensity: a dipeptide-based light-emitting material. Adv Mater 2012;24:1255–61. [18] Ma ZY, Teng MJ, Wang ZJ, Yang S, Jia XR. Mechanically induced multicolor switching based on a single organic molecule. Angew Chem Int Ed 2013;52:12268–72. [19] Ma ZY, Wang ZJ, Xu ZJ, Jia XR, Wei Y. Controllable multicolor switching of oligopeptide-based mechanochromic molecules: from gel phase to solid powder. J Mater Chem C 2015;3:3399–405. [20] Nagata E, Takeuchi S, Nakanishi T, Hasegawa Y, Mawatari Y, Nakano H. Mechanofluorochromism of 1-alkanoylaminopyrenes. ChemPhysChem 2015;16:3038–43. [21] Jadhav T, Dhokale B, Mobin SM, Misra R. Aggregation induced emission and mechanochromism in pyrenoimidazoles. J Mater Chem C 2015;3:9981–8. [22] Rao MR, Liao CW, Su WL, Sun SS. Quinoxaline based D–A–D molecules: high contrast reversible solid-state mechano- and thermo-responsive fluorescent materials. J Mater Chem C 2013;1:5491–501. [23] Zhao DB, Li GC, Wu D, Qin XR, Neuhaus P, Cheng YY, et al. Regiospecific N-heteroarylation of amidines for full-color-tunable boron difluoride dyes with mechanochromic luminescence. Angew Chem 2013;125:13921–5. [24] Matsunaga Y, Yang JS. Multicolor fluorescene writing based on host-guest interactions and force-induced fluorescence-color memory. Angew Chem 2015;127:8096–100. [25] Xu SD, Liu TT, Mu YX, Wang YF, Chi ZG, Lo CC, et al. An organic molecule with asymmetric structure exhibiting aggregation-induced emission, delayed fluorescence, and mechanoluminescence. Angew Chem Int Ed 2015;54:874–8.
5. Photo-fluorochromism properties PAH-8 xerogel exhibited photochromic luminescent behavior. Fig. 12 showed fluorescent images of the xerogel at different UV irradiated time. The PAH-8 xerogel emitted blue light, the UV-irradiated xerogel showed green emission and then non-emission with the increase of irradiation time. These naked-eye-visible fluorescence color changes were also recorded on a luminescence spectrophotometer. The PL spectra of PAH-8 were given in Fig. 13, it can be seen that the fluorescent emission intensity of xerogel at 457 nm decreased upon photoirradiation, and meanwhile, a wide shoulder peak at around 520 nm appeared. The irradiated xerogel showed fused fibers (Fig. 14) and the weakened diffraction peaks, indicating its poor crystalline feature (Fig. 15). Nandi et al. recently reported that the fluorescence intensity of the xerogel decreased upon photoirradiation because of the photoisomerization of imine (C=N) bond from the E form to the Z form [30]. Similar photoisomerization of imine (C=N) bond was confirmed in PAH-8 in our recent publication [32]. Considering the results above, we proposed a plausible mechanism. The decrease of fluorescent emission was due to the photoisomerization of partly imine (C=N) bond upon UV irradiation, which weakened the intermolecular interaction. Simultaneously, the energy barrier of molecular motion lowered, thus some of pyrenyl groups changed from partially overlapping excimer E1 in xerogel to sandwich packing excimer E2 upon UV irradiation. Meanwhile, the amount of Z form increased and the fluorescence almost completely disappeared with the prolonged of irradiation. In addition, less ordered structure for ground xerogel (as mentioned above) will be in favour of absorbing more the light energy that facilitated E−Z isomerization along the C=N bond [31]. So the fluorescent emission of the ground xerogel at 514 nm decreased more quickly than that of the xerogel upon photoirradiation (Fig. S15). 6. Conclusion In summary, PAH-8 showed an uncommon AIE phenomenon involving a fluorescence color change. PAH-8 xerogel exhibited favorable mechano-fluorochromic behaviors with naked-eye-visible fluorescence color changes, and the emission could be reversibly switched between blue and green color. Simultaneously, the photoluminescence property of PAH-8 could be tuned depending on the preparation conditions. The PAH-8 precipitate from THF displayed mechano-fluorochromic behavior, whereas the drop cast sample from THF did not exhibit mechanofluorochromic. In addition, the PAH-8 xerogel exhibited photo-fluorochromic behavior. As the PAH-8 could show mechano- and photofluorochromism behavior, it might be potential candidate for external 98
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