Multi-Stimuli-Responsive Fluorescence Switching ... - ACS Publications

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Multi-stimuli Responsive Fluorescence Switching from a Pyridine Functionalized Tetraphenylethene AIEgen Jianbo Xiong, Kai Wang, Zhao-Quan Yao, Bo Zou, Jialiang Xu, and Xian-He Bu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18718 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018

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ACS Applied Materials & Interfaces

Multi-stimuli Responsive Fluorescence Switching from a Pyridine Functionalized Tetraphenylethene AIEgen Jianbo Xionga, Kai Wangb, Zhaoquan Yaoa, Bo Zoub, Jialiang Xu,c,* and Xian-He Bua,d,* a

School of Materials Science and Engineering, National Institute for Advanced Materials,

Tianjin Key Laboratory of Metal and Molecule-Based Material Chemistry, Nankai University, Tianjin 300350, China. Email: [email protected]. b

c

State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China.

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China.

Email: [email protected]. d

State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, Nankai

University, Tianjin 300071, China.

ACS Paragon Plus Environment

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ABSTRACT：：The discovery of the striking aggregation-induced emission (AIE) phenomenon has opened a new avenue for smart light-emitting materials. Herein, a new AIE luminogen (AIEgen), 1,1,2,2-tetrakis(4-((E)-2-(pyridin-2-yl)vinyl)phenyl)ethene (TP2VPE), has been designed and synthesized by introducing the vinylpyridine motifs into the tetraphenylethene (TPE) backbone. The emission spectrum of the new obtained AIEgen crystalline material can not only be switched in response to mechanical grinding and hydrostatic compression, but also to the protonation effect with excellent reversibility and reproducibility. Single-crystal X-ray structural analysis disclosed the supramolecular porous channel structure, which provides shrinkable volume to maintain the fluorescence emission upon high pressure. Furthermore, protonation– deprotonation of the pyridine moieties in TP2VPE has a significant effect on the frontier molecular orbitals, as well as the very distinctive emission characteristics upon acid and base stimuli. The dual response performance and the ease of its preparation and renewal endow the material with potential applications in pressure and acid/alkali fluorescence sensing.

KEYWORDS：：AIEgen, mechanochromism, protonation effect, multi-stimuli fluorescence responsive, reversible


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INTRODUCTION Stimuli response fluorescence materials with their emission behaviour can be reversibly transform along with the external stimuli are of great importance for their wide application ranging from mechanosensors, security papers, to optical storage.1-4 Among them, organic multistimuli responsive fluorescent materials are of particular interest because of their structural tunability and functional controllability.5-9 However, owing to the lack of a clear guideline on the design strategy, reports on the multi-stimuli responsive fluorescent organics are relatively rare.8 Especially, most conventional organic dyes suffer from the aggregation-caused quench (ACQ) effect, which greatly limits the widespread applications of the organic responsive fluorescent materials in the solid states.10-13 To pursue the effective solid-state emission, tremendous endeavours have been devoted to prevent or alleviate the ACQ effect through elaborate chemical, physical, and engineering approaches, which have, however, obtained only limited success.14-18 In 2001, Tang et al.19 made an important breakthrough in the field by having discovered a striking phenomenon coined as the aggregation induced emission (AIE), referring to a photophysical phenomenon exactly opposite to ACQ, in which the luminogenic materials have a very low emissive quantum yield when molecularly dissolved in good solvents but become brightly luminescent upon aggregation in the solid state.11 The restriction of intramolecular motions (RIM) has been proposed to be the main mechanism of this striking AIE effect.10-13 Such a discovery has opened a door to libraries of brilliant organic AIEgen materials, which serve as the ideal building blocks for organic stimuli-responsive materials.20-23 Piezochromic luminescent materials, especially the high pressure sustainable piezochromic luminescent materials are becoming a hotspot over the past decades for their promising applications.24-25 However, only a limited number of organic luminogens have demonstrated the


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ability to reversibly respond to high external pressure (>10 Gpa) due to the possiblity of structual collapse and/or the accompanied irresversible phase transition upon compression.1,26-27 In order to realize the piezochromic luminescence, on one hand, a highly efficient emission in the solid state is the prerequisite.28 On the other hand, the luminescent molecules should contain a rigid structure to sustain the molecular structure on the extremely high pressure and a relatively loose packing mode to support enough space for the molecular rearrangements during the compress process.29-32. In addition, the supermolecular interactions such as π-π interaction, dipole-dipole interaction, and hydrogen bonding play important roles in the structural rearrangement during the external mechanical stimuli.33 AIE luminogens (AIEgens), in this context, are one of the most suitable kinds materials for these demands. Tetraphenylethene (TPE), a prototypical AIEgen with structure of the central olefin stator surrounded by four peripheral aromatic rotors (phenyl rings), has a highly twisted conformation stabilized by multiple aromatic C-H···π and C-H···C effects.36-37 These weak interactions have been demanstrated to be modulated by the external pressure with associated spectroscopic properties, especially the fluorescence emission.32 In this work, we have designed and synthesized a TPE based multi-responsive luminogen by introducing the 2-vinylpyridine moiety into the four arms of TPE matrix which demonstrates highly sensitive responses to multiple external stimuli such as pressure and acid/alkali (Scheme 1). The exceptionally high thermal stability and the ease of its preparation and renewal through rotary evaporation facilitated practical applications of this smart material.38


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Scheme 1. The schematic representation of the design strategy and the multi-responsive behaviors of the target molecule TP2VPE.

EXPEIMENTAL SECTIONS Materials and Methods. All reagents were commercially purchased and used without further purification. 1H and 13C NMR spectra were recorded in CDCl3 on a 400 MHz (Bruker ARX400) NMR spectrometer. HRMS data were recorded on a microTOF instrument using the ESI technique. Powder X-ray diffraction (PXRD) patterns were collected with Cu Kα radiation (λ = 1.54178 Å). The elemental analyses were carried out on a vario EL CUBE elemental analyzer. Above-room-temperature thermo gravimetric analysis (TGA) measurements were recorded using a TA-Instruments STD 2960 from 293 to 1000 K. Steady fluorescence spectra were collected on a Hitachi F-7000 fluorescence spectrophotometer. The UV-vis spectra were recorded using a SHIMADZU UV-3600 spectrophotometer. The emission quantum yields were measured by using an Edinburgh FLS980 fluorescence spectrophotometer under air at room temperature. Fluorescent images were obtained using an automated Leica DM5000B microscope equipped with 340-380 nm band pass (BP) excitation filter cube. The electronic structure in the ground and


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excited states were calculated at the level of B3LYP/6-31g** using the Gaussian 09 software package. Syntheses. The target compound TP2VPE was synthesized through a Mizoroki-Heck reaction3942

as shown in Scheme S1. Tetra(4-bromophenyl)ethylene43 (2 g, 3.09 mmol), 2-vinylpyridine

(1.5 g, 14.4 mmol), K3PO4 (4 g, 18.8 mmol), Pd(OAC)2 (0.12g, 0.53 mmol) and DMAC (60 mL) were placed in a 150 ml Schlenk flask. The mixture was heated at 110 oC for 48 h under nitrogen atmosphere. After being cooled down to room temperature, suspended into 300 mL of water, and extracted with dichloromethane (3 × 150 mL), the combined organic layers were dried over anhydrous magnesium sulfate and filtered. After removing the dichloromethane by rotary evaporation, the crude product was recrystallized from dichloromethane and methanol to afford TP2VPE as a yellow crystal solid (1.5 g, yield: 65%). Elemental Analysis Calcd (%) for C54H40N4 (744.9): C, 87.07; H, 5.41; N, 7.52. Found: C, 87.06; H, 5.47; N, 7.49. 1H NMR (400 MHz, CDCl3) δ 8.58 (d, J = 4.6 Hz, 4H), 7.64 (t, J = 7.7 Hz, 4H), 7.55 (d, J = 16.1 Hz, 4H), 7.39 – 7.33 (m, 12H), 7.17 – 7.03 (m, 16H).

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C NMR (100 MHz, CDCl3) δ (ppm): 155.56, 149.49,

143.75, 140.85, 136.66, 135.01, 132.50, 131.91, 127.69, 126.70, 122.09, 122.02. MS (ESI) m/z: [M + H]+ calcd for C54H40N4, 744.90; found, 745.33 (Figure S1-S3, Supporting Information). Single Crystal X-ray Crystallography studies. Single crystal X-ray diffraction data were collected on a SuperNova diffractometer with graphite monochromated Cu Kα radiation (λ = 1.54184 Å). The structure was solved with direct methods using the SHELXTL programs44 and refined with full-matrix least squares on F2.45 The corresponding CCDC reference number (CCDC: 1563280) and the crystal information file (CIF) data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.


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Diamond Anvil Cell Technique. High-pressure experiments were carried out using a diamond anvil cell (DAC). The culet diameter of the diamond anvils was 0.5 mm. T301 stainless steel gaskets were preindented to a thickness of 60 µm, and the center holes of 0.16 mm were drilled for the sample. The ruby chip was used for pressure determination using the standard ruby fluorescent technique. Silicone oil was used as the pressure-transmitting medium. The photoluminescence measurements under high pressure were performed on a QuantaMaster 40 spectrometer in the reflection mode. The 370 nm line of a xenon lamp with a power of 60 W was used as the excitation source. The in-situ UV-vis absorption measurements under high pressure were performed on an Ocean Optics QE65000 Scientific-grade spectrometer. The fluorescent images of the single crystals under 370 nm UV excitation were taken by putting the Sapphireanvil cell (SAC) containing the sample on a Nikon fluorescence microscope. All experiments were performed at room temperature. RESULTS AND DISCUSSION Structural Description. Recrystallization of TP2VPE from dichloromethane (as good solvent) and ethanol (as poor solvent) afforded single crystals suitable for X-ray diffraction analysis. Single crystal X-ray diffraction (XRD) analysis reveals that TP2VPE crystallizes in the P-1 space group (Table S1) and has a distinctive three-dimensional (3D) supramolecular framework. The crystallographic data suggests that TP2VPE has a highly twisted molecular conformation owing to the flexible connection bonds and the steric effects among phenyl plane. The torsion angles between the central olefin and the substituted phenyl planes are α1 = 44.87o, α2 = 41.82o, α3 = 37.83o, and α4 = 47.12o, respectively. The corresponding torsion angles between the outstretched vinyl bond and phenyl planes on the four extend orientations are β1 = 19.35o, β2 = 1.88o, β3 = 3.43o, β4 = 1.22o and γ1 = 9.91o, γ2 = 2.17o, γ3 = 1.29o, γ4 = 3.30o, respectively (Figure


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1). Phenyl planes on the three of the four arms are nearly coplanar to their corresponding phenyl groups with small torsion angles except the arm (α1β1γ1) has a most distorted geometry with a contorted phenyl plane rotation caused by the C-H···π and C-H···N effects (Figure 2b). There are two kinds of C-H···π interactions, C51-H51···Cg51 and C68-H68···Cg68 with distances of 3.25 Å and 3.46 Å, respectively (Figure 2b). A typically moderate hydrogen bonding interaction is also noticed with the distance of 3.46 Å (C43-N5), and C43-H43···N5 angle of 156.5o. An inside pore was formed by the packing of four separated TP2VPE molecules through the intermolecular interactions, and a long-range orderly arrayed channel is therefore generated (Figure 2a). The pore is 8.0 × 15.0 Å2 in size as suggested from the crystal data. In this way, a robust 3D supramolecular framework is formed. The solvent-accessible void space of TP2VPE is estimated to be 17.1% of the whole crystal volume by PLATON analysis (Figure 2a).46 The relatively loose structure together with the orderly araryed channel provided the shrinkable space for structural rearrangements upon high pressure compression.

Figure. 1 Molecular configuration of TP2VPE. Torsion angles: α1 = 44.87o, β1 = 19.35o, γ1 = 9.91o, α2 = 41.82o, β2 = 1.88o, γ2 = 2.17o, α3 = 37.83o, β3 = 3.43o, γ3 = 1.29o, α4 = 47.12o, β4 = 1.22o, γ4 = 3.30o.


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Figure. 2 a) The porous channel tube crystal structure. b) The intermolecular supramolecular interactions, including C–H···N hydrogen bond, C–H···π interactions and the long π···π distance.

Stabilities. The material is insoluble in water and some other polar organic solvents such as CH3OH, CH3COCH3 and CH3CN. After immersing TP2VPE in the aforementioned solvents for 48 h, the PXRD patterns of the recycled samples still match well with the simulated data from the CIF (Figure S4a), showing that the supramolecular framework is well maintained. The above results reveal the good stability of TP2VPE in multiple solvents.47-48 The PXRD pattern of sample reprepared from rotary evaporation was almost the same as the recrystallized sample, which made scale production possible for practical use. Apart from the solvent stability, the thermal stability of the TP2VPE framework has been revealed by the little obvious changes in the PXRD patterns even at 230 oC in an air atmosphere (Figure S4b). The thermal gravimetric analysis (TGA) shows no weightloss until 400 oC in air atmosphere (Figure S5), demonstrating the very good thermal stability of this organic framework. AIE Property. Fluorescent images of TP2VPE in the solvents with diverse H2O/THF gradients vividly suggest a typical AIE phenomenon. The fluorescence of TP2VPE was weak in THF solution, but was significantly enhanced as the water fraction (fw) increased gradually (Figure 3a). When fw reaches 60%, the FL intensity was enhanced by more than five hundred


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times (Figure 3c, 3d).10-13 The AIE mechanistic of TP2VPE is of the restriction of intramolecular rotation (RIR).3 The solely dispersed TP2VPE molecule on the good solvents can undergo intramolecular rotations and enhance the non-radiactive dissipation of energy during the molecular decay form the excited state to ground state; while in the aggregated state, the intramolecular rotations were hampered by the physical constraint. Simultaneously, the radiactionless relaxation were blocked and the radiactive decay path opened, these lead to a greatly enhancement of fluorescence emission.11 Additionally, the PL intensity decreased with the fw higher than 90%. The formation of less emissive amorphous nanoaggregates by random molecular packing might be the cause of emission decreasing in high concentration.49-50 The AIE enhancement nature can be further proved by the temperature-dependent photoluminescence. As shown in Figure 3e, the PL spectrum of TP2VPE solution in THF (0.2 mM) in a frozen glass state at liquid nitrogen shows obviously enhancement, hundreds-fold higher than that at ambient temperature, indicating the energy dissipation process is prevented at low temperature24-25. The absorption spectrum of the suspended aggregates is similar to that of the solution, with a minor red shift. The photoluminescence spectra, however, differ dramatically between solution and aggregtated states. The THF solution of TP2VPE emits in the yellow range with a maximum intensity at 584 nm, whereas the aggregates show a green emission with the spectrum centered at 530 nm (Figure 3b). Such a dramatic blue shift (54 nm) in emission but with a relatively small shift in the absorption spectrum (

Multi-Stimuli-Responsive Fluorescence Switching ... - ACS Publications

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