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Polymorphism-dependent aggregation-induced emission of pyrrolopyrrole-based derivative and its multi-stimuli response behaviors
Accepted Manuscript Polymorphism-dependent aggregation-induced emission of pyrrolopyrrole-based derivative and its multi-stimuli response behaviors Yingchun Ji, Zhe Peng, Bin Tong, Jianbing Shi, Junge Zhi, Yuping Dong PII:
S0143-7208(16)31168-8
DOI:
10.1016/j.dyepig.2016.12.061
Reference:
DYPI 5688
To appear in:
Dyes and Pigments
Received Date: 11 November 2016 Revised Date:
13 December 2016
Accepted Date: 24 December 2016
Please cite this article as: Ji Y, Peng Z, Tong B, Shi J, Zhi J, Dong Y, Polymorphism-dependent aggregation-induced emission of pyrrolopyrrole-based derivative and its multi-stimuli response behaviors, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2016.12.061. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical abstract
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Dyes and Pigments j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m/ l o c a t e / d y e p i g
Polymorphism-dependent aggregation-induced emission of pyrrolopyrrole-based
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derivative and its multi-stimuli response behaviors
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Yingchun Ji,a Zhe Peng,a Bin Tong,*,a Jianbing Shi,a Junge Zhi,b and Yuping Dong*,a
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a Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications, School of Materials Science and Engineering,
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Beijing Institute of Technology, 5 South Zhongguancun Street, Beijing, 100081, China
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b School of Chemistry, Beijing Institute of Technology, 5 South Zhongguancun Street, Beijing, 100081, China
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*To whom correspondence should be addressed: Phone: +86-10-6891-7390;
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E-mail: [email protected], [email protected].
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Abstract: Bis(4-cyanophenyl)-4,4'-(2,5-diphenylpyrrolo[3,2-b] pyrrole-1,-4-diyl) dibenzoate (DPPCN) was synthesized and exhibited
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aggregation-induced emission properties. DPPCN was cultivated in different solvents getting three kinds of crystals, A, B, and C.
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Crystals of A and B were respectively blue and cyan-blue emissive, while C was yellow-green emissive. They all displayed crystalline-
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induced emission behaviors. According to X-ray single crystal diffraction analysis, the intermolecular interactions account for restricted
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internal rotations, leading to fluorescence enhancement. The different kinds of co-effects of the solvent, intermolecular forces,
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molecular conformations and molecular packing modes endow DPPCN polymorphisms with multi-color emissions (from blue to yellow
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and up to 60 nm) and different fluorescence efficiencies. The fluorescence of A can be reversibly tuned by grinding and exposing to
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chloroform vapor with a 46 nm wavelength change. The crystal of C can blue shifted the fluorescence that matches with B upon fumed
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by chloroform for about 12 h and further blue shifted the fluorescence emission that matches with A when fumed by chloroform for
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about 24 h. Multi-color fluorescence tuning and switching is attributed to the nature of polymorphs, that is, a change of molecular
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conformation and intermolecular packing modes. The crystal of A also exhibited thermo-stimulus fluorescence switching behavior due
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to the co-crystallization with solvent chloroform. The properties of A show promising applications in temperature monitoring and
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volatile organic compound detecting devices.
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Keywords: 1. Polymorphism; 2. Aggregation-Induced Emission; 3. Temperature-responsive; 4. Mechanochromism.
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1.
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Introduction
Organic solid luminescent materials with stimuli-responsive emission properties considered as smart
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materials have drawn great attention over the past years because of their promising application in optical
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recording [1-3], mechano-sensors [4-7], fluorescent π-gelators [8], data storage [9, 10]. These fluorescent
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molecules usually show tunable and switchable optical properties achieved by either controlling molecular
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arrangement or packing modes under the external piezo- [11-14], vapo- [15-19], thermos-stimuli [20-23]
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conditions. It means the change of optical properties can be achieved by physical methods, which is prior to
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chemical synthesis technology.
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Polymorphism, a single molecule exhibiting different style crystals due to different molecular
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geometry or packing styles, are considered as the most promising approaches to provide the materials with
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desired luminescent properties. During the past few years, various compounds with polymorphism have
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been reported [24-31]. Yi et al. synthesized a imidazole-based molecule obtaining three polymorphs by a
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slow evaporation method, which exhibit different emissions [28]. Wang and co-workers reported a kind of
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D-π-A-π-D compound with thiazole group as acceptor and N, N-diphenylaniline group as donor, getting
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four kinds of organic crystals (A, B, C, and D) with green, green-yellow, yellow, and orange fluorescence
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[29]. Moreover, the comparison between the different polymorphs could provide an ideal approach for
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understanding the relationships between molecular packing structures and the solid-state properties based
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on the same kind of molecules [30, 31].
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However, most organic fluorescent materials encounter major bottleneck due to the low emission
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efficiency in aggregate states [32, 33]. The discovery of the aggregation-induced emission (AIE), developed
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by Professor Tang and co-workers studying the emission behavior of siloes in solution and aggregate state,
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offer a promising alternative method to exploit efficient organic solid luminescent materials [34-37]. AIE is
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ascribed to the restriction of intramolecular motation (RIM) as well as the inhibition of aggregation
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induced-planarization and formation of J-aggregates in some cases [38-42]. The RIM process includes the
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restriction of intramolecular rotations [38] and intramolecular vibrations [42]. According to the RIM
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hypothesis, the multiple aryl rotors or bendable vibrators in an isolated AIEgen molecule undergo dynamic
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intramolecular rotation or vibration in the solution state, which non-radiatively deactivates the excited state
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and renders AIEgen non-emissive. The constraint between molecules in the aggregate state will make the ACCEPTED MANUSCRIPT
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AIEgens structural rigidification, obstruct the intramolecular motation and activate the RIM process, which
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blocks these radiationless channels, directing relaxation of excitons through radiative channels, thereby
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making the AIEgen aggregates luminescent. Crystalline-induced emission (CIE) also discovered by
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Professor Tang and co-workers is another important phenomenon corresponding the enhanced luminescent
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intensity in crystalline states compared with amorphous states [43-45]. Despite many successful examples
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of solid-state emissive materials based on the AIE and CIE effect have been reported, it is still challenging
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to design novel core structures possessing polymorphism with tunable solid-state emission.
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In this paper, we have designed and synthesized a new AIE-active compound named bis(4-
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cyanophenyl)-4,-4'-(2,5-diphenylpyrrolo[3,2-b]pyrrole-1,4-diyl)dibenzoate (DPPCN) (Scheme1), which
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showed three types of polymorphism (A, B and C) with blue, cyan-blue, and yellow-green fluorescence.
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They all displayed CIE behaviors. The emission color of crystal of A can be reversibly tuned by grinding
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and exposing to chloroform vapor and show a good reversible mechano-florochromic process. Crystal of A
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also exhibited thermo-stimulus fluorescence switching behavior. Based on X-ray single crystal diffraction
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analysis, the structure-property relationships were investigated in detail.
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Materials and Methods
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2.1 General Information
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Methyl 4-aminobenzoate and 4-Hydroxybenzonitrile were purchased from Aladdin. N-(3-
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Dimethylaminopropyl)-N-ethylcarbodiimide hydrochloride and 4-dimethyl -aminopyridine were purchased
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from J&K Scientific. Unless special instructions, all other reagents and solvents were commercially
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purchased from Beijing Chemical Reagent Company without further purification.
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The UV-Vis spectra was recorded on a TU-1901 UV-Vis spectrophotometer (Beijing Purkinje General
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Instrument Co., Ltd.). Photoluminescence (PL) spectra was collected on a Hitachi F-7000 fluorescence
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spectrophotometer at room temperature. Infrared spectra was recorded on a Bruker Alpha spectrometer.
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The nuclear magnetic resonance (NMR) spectra was recorded on a Bruker AMX-400 spectrometer. Matrix-
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assisted laser desorption/ionization ACCEPTED time-of-flight MANUSCRIPT (MALDI-TOF) was performed by using α-cyano-4-
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hydroxy-cinnamic acid (CCA) as the matrix under the reflector mode for data acquisition. Powder X-ray
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diffraction (PXRD) was performed using monochromatized Cu-Kα (λ=1.54178 Å) incident radiation with a
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Shimadzu XRD-6000 instrument operating at 40 kV voltage and 50 mA current. Thermal gravimetric
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analysis (TGA) was performed on a Shimadzu TGA-50 thermal analyzer at a heating rate of 10 oC/min in
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nitrogen. Fluorescent image was recorded under an Olympus CKX41 phase contrast microscope (Olympus,
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Japan) and a Nikon A1 confocal laser scanning microscope (CLSM, Nikon Corporation, Japan) at an
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excitation wavelength of 405 nm. The crystallographic structures were analyzed with Rigaku CCD Saturn
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724+ X-ray single crystal diffractometer.
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CCDC1502160 (the crystal of A), 1500808 (the crystal of B), 1500809 (the crystal of C) contain the
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supplementary crystallographic data for this paper. These data can be obtained free of charge from The
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Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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2.2 Synthesis of the target compound
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The synthetic route of DPPCN is shown in Scheme S1 in the Supporting Information (SI). The
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intermediates of 1 and 2 was synthesized according to previous literatures [46-47]. DPPCN was
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synthesized and purified as follows: a mixture of 2 (20.0 mmol), 1-ethyl-(3-dimethylaminopropyl)
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carbodiimide hydrochloride (EDCI, 24.0 mmol) and N-[3-(dimethylamino) propyl]-2-methyl-2-
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propenamide (DMPAP, 24.0 mmol) and anhydrous dichloromethane (40 mL) was stirred for 1.5 h at 0oC
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(kept in ice-water), then 4-cyanophenol (50.0 mmol) was added into the mixture and stirred for 12 h at
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ambient temperature. The resulting mixtures were extracted with chloroform and washed with water
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successively. The organic extracts were dried over anhydrous MgSO4, evaporated and purified on a silica
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gel column using hexane and dichloromethane (1:3) as the eluent, and then recrystallized from
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dichloromethane and n-hexane. DPPCN was obtained in 50.3% yield after dried under vacuum for 24 h.
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17.93 Hz, 4H), 7.31 (m, 8H), 6.56 (s, 2H);
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136.19.5, 133.8, 132.9, 131.52, 131.20, 128.59.8, 128.40, 127.06, 125.29, 124.56, 122.9, 118.28, 109.91,
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H-NMR (400 MHz, CDCl3, δ): 8.21 (d, J = 8.26 Hz, 4H), 7.78 (d, J = 8.33 Hz, 4H), 7.42 (dd, J = 8.27, 13
C-NMR (100 MHz, CDCl3, δ) : 163.73, 154.20, 144.77,
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-1 97.01, 77.36, 77.05, 76.73. FTIR (cmACCEPTED ): 466, 570, 695, 747, 816, 1678, 2230, 2850, 3420. MS (MALDIMANUSCRIPT
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TOF, m/z): [M+.] calcd. for C46H28N4O4: 700.12; found 700.5.
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Scheme 1
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Results and Discussion
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3.1 Photophysical properties of the DPPCN
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To test the AIE characteristics of DPPCN, the fluorescence spectrum of DPPCN was investigated in
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THF / H2O mixtures with water fraction (fw) from 0 to 99%. As shown in Fig. 1a, the fluorescence intensity
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of DPPCN was very weak in THF, which was consistent with the lower fluorescence quantum yield (ΦF =
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0.03%) of DPPCN in THF. With the addition of water, the fluorescence emission curves of DPPCN
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changed occurring three stages. The fluorescence intensity of DPPCN barely changed with increasing fw
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from 0 to 60%, then increased rapidly with fw changing from 60 to 95%, reaching the maximum with 18-
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fold enhancement at fw = 95%, and finally slightly decreased with fw = 99%. The reason why the
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fluorescence intensity of DPPCN at fw = 99% slightly decreased was that with the “high” water content, the
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dye molecules may quickly agglomerate in a random way decreasing the interaction between molecules
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leading to the electronic excited energy exhausted by intramolecular rotations. So the less emissive
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particles were formed. Since water is a poor solvent for DPPCN, the molecules of DPPCN aggregated in
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the THF / H2O mixtures with higher water contents. The dynamic light scattering experimental results
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confirmed the formation of DPPCN aggregates with an average diameter around 278 nm obtained in the
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THF / H2O mixture with fw = 90% (Fig. S2b in SI). At the same time, the Mie scattering effect observed in
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the UV-Vis spectra (Fig. S3b in SI) revealed that the DPPCN aggregates were also formed with higher
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water content. These results indicated that DPPCN was AIE active [48-49].
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Figure 1
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3.2 Polymorphism
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As is well-known that AIE active molecules possess strong fluorescence in solid state (or crystalline ACCEPTED MANUSCRIPT
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state) with their unique molecular packing arrangements. We try to use different crystallization conditions
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to cultivate the crystals of DPPCN. Interestingly, solid DPPCN was obtained as yellow precipitates after
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the reaction, and subsequent recrystallization from trichloromethane and n-hexane gave mixture of cubic
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blue fluorescence crystal of A and needlelike cyan-blue fluorescence crystal of B. Besides, needlelike
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yellow-green fluorescence emission crystal of C was also obtained by recrystallization from
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dichloromethane and n-hexane under relatively dilute condition (Fig. 2a).
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Each crystal form of DPPCN exhibited different emission wavelength and fluorescence quantum yield
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(ΦF). Crystal of A exhibited more intensive emission at longer wavelength (λmax = 480 nm, ΦF = 27%) than
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B (λmax = 504 nm, ΦF = 20.7%) and C (λmax = 540 nm, ΦF = 10%) (Fig. 2b). The fluorescence quantum
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yields of DPPCN crystals were all higher than that of the amorphous powders of DPPCN (ΦF = 3.3%),
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indicating they had CIE properties. The molecules of DPPCN were loosely packed in the amorphous form,
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leading the internal rotations within DPPCN cannot be inhibited exhausting the excited energy. The reason
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why the ΦF of A was highest and exhibited bluest emission wavelength than others would be deciphered by
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later analysis of the X-ray crystallographic structures.
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Figure 2
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Fig.3a shows the single crystal structures of DPPCN. The crystal cells of A and B were monoclinic
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form with space group of P21/n and P21/c, respectively. It should be noted that polymorph C belongs to the
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triclinic space group P-1 consisting of two kinds of conformation molecules (C1 and C2) in an unit cell
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which differed in the bond angles (shown in Fig. 3a and Table S1 in SI). They all had four DPPCN
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molecules in a unit cell, but the crystal of A included extra eight chloroform molecules and the crystal of C
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included six CH2Cl2 molecules in a crystal cell, while the crystal of B had no solvent molecules in the
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crystalline lattice. The crystallographic data and structure refinement parameters of the three kinds of
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crystals are shown in Table S2 in SI.
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In the crystal structure of A, chloroform solventMANUSCRIPT formed C-H…Cl hydrogen bonds interactions (2.854, ACCEPTED
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2.856, 2.920, 2.905, 3.041 and 3.295 Å) and C≡N…H interactions (2.570 and 2.975 Å) to connect the upper
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and lower DPPCN molecules to generate molecular chains shown in Fig. S5 in SI. However, this kind of
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hydrogen bonds could not be found even though there were CH2Cl2 solvents existing in the crystal of C.
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Those can be further illustrated by thermogravimetric analysis (TGA) experiments. Note that the samples
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were dried under vacuum for 12 h, TGA curves (Fig. S6 in SI) showed that A lost about 23% in weight
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around 100 oC. The lost weight can be attributed to the loss of chloroform in the molecules. The mole ratio
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of DPPCN molecules to chloroform in the crystal of A was 1:2. Changing to molecular weight ratio, the
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weight ratio of chloroform occupying the mixture of DPPCN and chloroform was 25%, approximately near
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to 23% weight loss measured by TGA. Whereas the crystal of C did not lose any weight until 200 oC,
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indicating complete removal of the CH2Cl2 molecules from the crystal of C after drying under vacuum for
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12 h. These results indicated that chloroform involved in the crystalline structure of A while CH2Cl2 merely
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dissociated in the crystalline structure of C and did not form interactions with DPPCN molecules.
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It can be seen from Fig. 3c that the crystal of A co-crystallizing with the solvent induced most uniform
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distribution packing style decreasing the conjugation between molecules, and produced strong
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intermolecular interactions making the fluorescence highly confined inside the crystal, eventually endowed
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the crystal of A with a blue shift and high fluorescence quantum yield among these polymorphs.
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Owing to no solvent existing in crystal of B, we can be more objective to analysis the effect of
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molecular packing on AIE properties in the solid state. As can be seen from Table S1 in SI, the twist angle
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between central pyrrolopyrrole moiety and peripheral phenyl excessed 30 degrees making molecules in B
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adopt highly twisted conformations. The molecules were assembled in the crystal lattice by a zigzag layer
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packing (Fig. 3c), which rule out π…π stacking interactions, excluding the formation of H- or J-aggregates.
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Molecules within the layers interacted with each other by C-H…O H-bonding interaction (2.551, 2.424 and
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2.813 Å) and C≡N…H interactions (2.697 and 3.499 Å)) and C-H…π interactions with a distance of 2.769 Å
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and 2.804 Å (Fig. 3b). These intermolecular weak interactions could help to inhibit internal rotations and
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fix the molecular conformations, and thus benefit to the radiative relaxation as evidenced by the relatively
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high fluorescence quantum yields in the crystals. TheMANUSCRIPT similar interactions can be also found in the crystal of ACCEPTED
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A and C. Those can effectively explain why DPPCN showed AIE and CIE properties.
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Figure 3
188 There are two kinds of packing types in polymorph C (Fig. 3a). Type 1 was composed of the
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molecular conformation of C1, which was assembled like type B that the molecules looked like interlaced
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fingers, and were interacted by strong hydrogen bonding (C-H…O, 2.493 Å). Type 2 consisted of the
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molecular conformation of C2, whose neighboring molecules were oriented toward each other like two
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people opening arms to embrace each other. The molecules in type 2 were also connected by strong
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hydrogen bonding (C-H…O, 2.564 Å). The packing style in the crystal of C allows us to conclude that
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crystal growth from dilute solution was carried out by means of delicate adjustment of the chains in the
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corresponding layers. Both the two types of stacking modes in the crystal of C can effectively increase
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space utilization, thus leading to a more close molecular packing.
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What is more, partial pyrrolopyrrole core overlap (approximately 13.87%) in type 2 between the
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neighboring molecules was found due to molecular close packing, and the vertical distance between the
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neighboring pyrrolopyrrole planes was approximately 3.931 Å resulting in weak π-π interactions and part
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of the fluorescence quenching. So the crystal of C had a lowest fluorescence quantum yield compared to the
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others. The unique stacking way between neighboring molecules and partial overlap of the pyrrolopyrrole
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core in polymorph C may help to explain the relatively large red-shift of fluorescence emission compared
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with the other two kinds of polymorphs.
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To sum up, the effects of the solvent, intermolecular forces, molecular conformations and molecular
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packing modes endowed DPPCN crystals with multi-color emission with large emission color/wavelength
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changes (from blue to yellow and up to 60 nm) and different fluorescence efficiencies.
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3.4 Stimuli-responsive luminescence tuning and switching properties of DPPCN
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Different molecular stacking modes and intermolecular interactions in solid-state organic crystals,
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which influence emission behavior, are not only achieved by the process of growing crystals, but also by
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physical stimulus, such as grinding, heating, pressureMANUSCRIPT changing, and chemical vapor fuming [50-53]. So we ACCEPTED
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performed the experiments of stimuli-responsive luminescence properties of DPPCN. As shown in Fig. 4a,
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the emission of the as-prepared A can be changed to bright yellow by simply grinding with a mortar. Red-
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shift of 46 nm (from 480 to 526 nm) by grinding treatment showed the typical mechanochromic
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phenomenon. Meanwhile, the ground A returned to the original emission by chloroform vapor fuming for
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10 min at room temperature. So the reversible changes in emission color of DPPCN were easily achieved.
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Ten cycles of repeated grinding-fuming processes were shown in Fig. 4b, which manifested the good
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reversible mechano-florochromic processes.
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To know well the mechanism of this mechano-florochromism, the powder X-ray diffraction (PXRD)
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was carried out. The pristine A exhibited much sharp and intensive diffraction peaks and the ground
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samples showed dispersion peak, indicating the molecular stacking modes changing from ordered crystal to
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disordered amorphous state. The diffraction peaks of ground samples appeared again by chloroform vapor
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fuming, revealing that A recovered to the ordered crystalline state, but was slightly different with the PXRD
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of A. When grinding A, the disruption of hydrogen bonding would lead to the decrease of the steric
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hindrance, facilitating the molecular rotation, thereby making the molecular conformational planarization,
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which caused a bathochromic shift.
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When ground using a pestle, the crystals of B and C showed a shift of maximum fluorescent emission
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wavelength of 22 nm from 504 nm to 526 nm and 14 nm from 540 nm to 526 nm, respectively. When C
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was exposed to chloroform vapor fuming at room temperature for about 12 h, it induced a 34 nm blue-
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shifted emission to 506 nm, which was coincident with the emission of B. Interestingly, when further
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fumed for another 12 h, the fluorescence of C further blue-shifted emission to 480 nm that matched with
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the crystal of A (Fig. 5a). Such behaviors may be attributed to C adsorbing chloroform to change its original
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crystal packing way. As shown in the XRD spectra (Fig. 5d), the diffraction patterns of fumed samples are
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similar but not completely consistent with B and A respectively, suggesting the vapochromic behavior is
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actually a molecule-repacking process. As suggestedMANUSCRIPT by the PL spectra shown in Fig. 5e, we can switch the ACCEPTED
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emission of C from yellow to the other colors (blue and cyan-blue) by fuming.
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242 3.5 Temperature monitoring
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Besides mechano-stimulus fluorescence behavior, thermo-stimulus fluorescence switching properties
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were also characterized. Before we did the thermo-stimulus test, the samples were dried under vacuum for
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12 h treated the same way as we do TGA test. The crystals was gently crushed with a metal spoon, then the
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sample was uniformly spread on a quartz dish with a cover slip, and then carried out the fluorescence
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measurements. A exhibited thermo-stimulus luminescence switching properties. But both B and C had no
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thermo-stimulus luminescence switching properties. The fluorescence wavelength of A was 480 nm at
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room temperature and basically kept unchanged when the temperature was below 75 oC. Continuously
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increasing temperature, the emission wavelength changed drastically to about 507 nm with nearly 27 nm
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bathochromic-shift. The differential curve in Fig. 6a shows an inflection point at 91.37 oC, where the
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wavelength change rate is maximal. As the temperature was increased, the interactions between DPPCN
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with solvent molecules were progressively reduced, and eventually all chloroform molecules escaped from
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the crystal of A inducing the increasing of molecular planarization, thus the emission wavelength
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bathochromic-shifted. Even though the heated samples have similar emission with B, the diffraction
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patterns of heated samples for 12 min at 110 oC are different from B as shown in the XRD spectra (Fig. S9
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in SI), suggesting they have different molecule-packing styles.
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To further analyze the response speed and temperature sensitivity, time-dependent emission spectra
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were carried out. The wavelength at maximum emission intensity versus time at different heating
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temperatures was plotted in Fig. 6b. The wavelength at maximum emission intensity increased with
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prolonged heating time and reached a stable value within a period of 10 min. The wavelength red-shifted 12
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nm when heating at 90 oC and reached up to 25 nm bathochromic-shift when heating at 110 oC. The
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temperature selectivity and the quickly response make DPPCN a promising thermo-responsive material for ACCEPTED MANUSCRIPT
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using in temperature monitoring devices.
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Conclusion
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In summary, a new compound DPPCN with donor-acceptor structure has been designed and
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synthesized, which is AIE- and CIE-active and shows morphology-dependent fluorescence properties.
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Three kinds of single-crystalline structures were obtained with different color emissions. The three crystals
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showed mechano-chromic emissive behavior. After grinding, A and B showed a bathochromic shift of 46
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nm and 22 nm, respectively, while C exhibited a hypochromatic shift of 14 nm. Moreover, the emission
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color of polymorph A has been reversibly tuned by grinding and exposing to chloroform vapor. Polymorph
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A also exhibited thermo-stimulus fluorescence switching behavior. The emissive wavelength change rate
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reached a maximum value when the temperature was 91.37 oC. While the yellow emission C switched to
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the other colors (blue and cyan-blue) by exposing to chloroform vapor for 12 h and 24 h, respectively.
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Basing on XRD analysis of single-crystalline structures, multicolor fluorescence tuning and switching are
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attributed to the nature of polymorphs, that is, a change of intramolecular conformation and intermolecular
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packing modes. These results are highly valuable for the further design of relevant compounds with tunable
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solid-state emissions.
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Acknowledgments
We are grateful for the support from the National Basic Research Program of China (973 Program:
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2013CB834704), the National Natural Scientific Foundation of China (Grant Nos. 51328302, 21404010,
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51073026, 51061160500).
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References and notes
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[2] Yuan MS, Wang DE, Xue P, Wang W, Wang JC, Tu Q, et al. Fluorenone organic crystals: two-color luminescence ACCEPTED MANUSCRIPT
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switching and reversible phase transformations between π–π stacking-directed packing and hydrogen bond-directed
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packing. Chem. Mater. 2014;26:2467-77.
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[3] Li CY, Luo XL, Zhao WJ, Huang Z, Liu ZP, Tong B, et al. Switching the emission of di(4ethoxyphenyl)dibenzofulvene among multiple colors in the solid state. Sci. China Chem. 2013;56:1173-7. [4] Peng BY, Xu SD, Chi ZG, Zhang XQ, Yi Z, Xu JR. Piezochromic aggregation-induced emission materials. Prog.
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pyrrole derivatives. J. Phys. Chem. B 2010;114:16731-6. [50] Zhang ZY, Xu B, Su JH, Shen LP, Xie YS, Tian H. Color-tunable solid-state emission of 2,2'-biindenyl-based fluorophores. Angew. Chem. Int. Ed. 2011;50:11654-7.
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AEE material: polymorphism, defection and nanofabrication mediated fluorescence tuning. J. Mater. Chem. C
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2015;3:8381-8.
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Scheme 1. Molecular Structure of DPPCN.
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Figure 1. (a) Fluorescence spectra of DPPCN in THF / H2O mixtures with different fw; (b) Correlation between the net change in PL intensity [I/I0] with different water fractions in the THF / H2O mixtures. I0: PL intensity in pure THF.
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Figure 2. (a) Fluorescent images of crystal of A, B and C taken under UV irradiation on a fluorescence microscope; (b)
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Figure 3.(a) Crystal cells of A, B and C (C having two molecules (C1 and C2) in asymmetric unit, C1 is painted red and
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Figure 4. (a) Fluorescence spectra of A before grinding, after grinding and fuming; (b) Maximum emission wavelength change upon repeated fuming–grinding cycles; (c) PXRD patterns of A before grinding, after grinding and fuming.
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A fumed 24 h B fumed 12 h
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Figure 5. Fluorescent images of (a) crystal of C, (b) after fumed by chloroform for about 12 h and (c) after fumed by chloroform for about 24 h taken under UV irradiation on a fluorescence microscope ; (d) PXRD patterns in different
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Figure 6. (a) The maximum emission intensity (black) and its differential (red) of the crystal of A versus temperature (excitation wavelength: 330 nm, time interval for heating: 60 s); (b) Changes of time-dependent wavelength of the crystal A at 90 oC, 100 oC and 110 oC.
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Highlights
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A novel
pyrrolopyrrole derivative DPPCN exhibited
aggregation- and
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DPPCN showed three types of polymorphism with blue, cyan-blue and yellow-green emissive, respectively.
The structure-property relationships were investigated based on X-ray single
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crystal diffraction analysis.
The mechano- and thermo-stimulus fluorescence switching behaviors of the three
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crystalline-induced emission properties.
S0143-7208(16)31168-8
DOI:
10.1016/j.dyepig.2016.12.061
Reference:
DYPI 5688
To appear in:
Dyes and Pigments
Received Date: 11 November 2016 Revised Date:
13 December 2016
Accepted Date: 24 December 2016
Please cite this article as: Ji Y, Peng Z, Tong B, Shi J, Zhi J, Dong Y, Polymorphism-dependent aggregation-induced emission of pyrrolopyrrole-based derivative and its multi-stimuli response behaviors, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2016.12.061. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical abstract
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Dyes and Pigments j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m/ l o c a t e / d y e p i g
Polymorphism-dependent aggregation-induced emission of pyrrolopyrrole-based
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derivative and its multi-stimuli response behaviors
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Yingchun Ji,a Zhe Peng,a Bin Tong,*,a Jianbing Shi,a Junge Zhi,b and Yuping Dong*,a
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a Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications, School of Materials Science and Engineering,
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Beijing Institute of Technology, 5 South Zhongguancun Street, Beijing, 100081, China
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b School of Chemistry, Beijing Institute of Technology, 5 South Zhongguancun Street, Beijing, 100081, China
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*To whom correspondence should be addressed: Phone: +86-10-6891-7390;
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E-mail: [email protected], [email protected].
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Abstract: Bis(4-cyanophenyl)-4,4'-(2,5-diphenylpyrrolo[3,2-b] pyrrole-1,-4-diyl) dibenzoate (DPPCN) was synthesized and exhibited
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aggregation-induced emission properties. DPPCN was cultivated in different solvents getting three kinds of crystals, A, B, and C.
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Crystals of A and B were respectively blue and cyan-blue emissive, while C was yellow-green emissive. They all displayed crystalline-
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induced emission behaviors. According to X-ray single crystal diffraction analysis, the intermolecular interactions account for restricted
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internal rotations, leading to fluorescence enhancement. The different kinds of co-effects of the solvent, intermolecular forces,
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molecular conformations and molecular packing modes endow DPPCN polymorphisms with multi-color emissions (from blue to yellow
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and up to 60 nm) and different fluorescence efficiencies. The fluorescence of A can be reversibly tuned by grinding and exposing to
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chloroform vapor with a 46 nm wavelength change. The crystal of C can blue shifted the fluorescence that matches with B upon fumed
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by chloroform for about 12 h and further blue shifted the fluorescence emission that matches with A when fumed by chloroform for
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about 24 h. Multi-color fluorescence tuning and switching is attributed to the nature of polymorphs, that is, a change of molecular
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conformation and intermolecular packing modes. The crystal of A also exhibited thermo-stimulus fluorescence switching behavior due
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to the co-crystallization with solvent chloroform. The properties of A show promising applications in temperature monitoring and
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volatile organic compound detecting devices.
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Keywords: 1. Polymorphism; 2. Aggregation-Induced Emission; 3. Temperature-responsive; 4. Mechanochromism.
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1.
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Introduction
Organic solid luminescent materials with stimuli-responsive emission properties considered as smart
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materials have drawn great attention over the past years because of their promising application in optical
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recording [1-3], mechano-sensors [4-7], fluorescent π-gelators [8], data storage [9, 10]. These fluorescent
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molecules usually show tunable and switchable optical properties achieved by either controlling molecular
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arrangement or packing modes under the external piezo- [11-14], vapo- [15-19], thermos-stimuli [20-23]
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conditions. It means the change of optical properties can be achieved by physical methods, which is prior to
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chemical synthesis technology.
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Polymorphism, a single molecule exhibiting different style crystals due to different molecular
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geometry or packing styles, are considered as the most promising approaches to provide the materials with
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desired luminescent properties. During the past few years, various compounds with polymorphism have
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been reported [24-31]. Yi et al. synthesized a imidazole-based molecule obtaining three polymorphs by a
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slow evaporation method, which exhibit different emissions [28]. Wang and co-workers reported a kind of
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D-π-A-π-D compound with thiazole group as acceptor and N, N-diphenylaniline group as donor, getting
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four kinds of organic crystals (A, B, C, and D) with green, green-yellow, yellow, and orange fluorescence
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[29]. Moreover, the comparison between the different polymorphs could provide an ideal approach for
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understanding the relationships between molecular packing structures and the solid-state properties based
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on the same kind of molecules [30, 31].
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However, most organic fluorescent materials encounter major bottleneck due to the low emission
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efficiency in aggregate states [32, 33]. The discovery of the aggregation-induced emission (AIE), developed
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by Professor Tang and co-workers studying the emission behavior of siloes in solution and aggregate state,
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offer a promising alternative method to exploit efficient organic solid luminescent materials [34-37]. AIE is
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ascribed to the restriction of intramolecular motation (RIM) as well as the inhibition of aggregation
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induced-planarization and formation of J-aggregates in some cases [38-42]. The RIM process includes the
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restriction of intramolecular rotations [38] and intramolecular vibrations [42]. According to the RIM
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hypothesis, the multiple aryl rotors or bendable vibrators in an isolated AIEgen molecule undergo dynamic
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intramolecular rotation or vibration in the solution state, which non-radiatively deactivates the excited state
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and renders AIEgen non-emissive. The constraint between molecules in the aggregate state will make the ACCEPTED MANUSCRIPT
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AIEgens structural rigidification, obstruct the intramolecular motation and activate the RIM process, which
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blocks these radiationless channels, directing relaxation of excitons through radiative channels, thereby
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making the AIEgen aggregates luminescent. Crystalline-induced emission (CIE) also discovered by
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Professor Tang and co-workers is another important phenomenon corresponding the enhanced luminescent
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intensity in crystalline states compared with amorphous states [43-45]. Despite many successful examples
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of solid-state emissive materials based on the AIE and CIE effect have been reported, it is still challenging
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to design novel core structures possessing polymorphism with tunable solid-state emission.
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In this paper, we have designed and synthesized a new AIE-active compound named bis(4-
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cyanophenyl)-4,-4'-(2,5-diphenylpyrrolo[3,2-b]pyrrole-1,4-diyl)dibenzoate (DPPCN) (Scheme1), which
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showed three types of polymorphism (A, B and C) with blue, cyan-blue, and yellow-green fluorescence.
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They all displayed CIE behaviors. The emission color of crystal of A can be reversibly tuned by grinding
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and exposing to chloroform vapor and show a good reversible mechano-florochromic process. Crystal of A
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also exhibited thermo-stimulus fluorescence switching behavior. Based on X-ray single crystal diffraction
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analysis, the structure-property relationships were investigated in detail.
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Materials and Methods
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2.1 General Information
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Methyl 4-aminobenzoate and 4-Hydroxybenzonitrile were purchased from Aladdin. N-(3-
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Dimethylaminopropyl)-N-ethylcarbodiimide hydrochloride and 4-dimethyl -aminopyridine were purchased
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from J&K Scientific. Unless special instructions, all other reagents and solvents were commercially
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purchased from Beijing Chemical Reagent Company without further purification.
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The UV-Vis spectra was recorded on a TU-1901 UV-Vis spectrophotometer (Beijing Purkinje General
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Instrument Co., Ltd.). Photoluminescence (PL) spectra was collected on a Hitachi F-7000 fluorescence
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spectrophotometer at room temperature. Infrared spectra was recorded on a Bruker Alpha spectrometer.
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The nuclear magnetic resonance (NMR) spectra was recorded on a Bruker AMX-400 spectrometer. Matrix-
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assisted laser desorption/ionization ACCEPTED time-of-flight MANUSCRIPT (MALDI-TOF) was performed by using α-cyano-4-
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hydroxy-cinnamic acid (CCA) as the matrix under the reflector mode for data acquisition. Powder X-ray
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diffraction (PXRD) was performed using monochromatized Cu-Kα (λ=1.54178 Å) incident radiation with a
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Shimadzu XRD-6000 instrument operating at 40 kV voltage and 50 mA current. Thermal gravimetric
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analysis (TGA) was performed on a Shimadzu TGA-50 thermal analyzer at a heating rate of 10 oC/min in
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nitrogen. Fluorescent image was recorded under an Olympus CKX41 phase contrast microscope (Olympus,
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Japan) and a Nikon A1 confocal laser scanning microscope (CLSM, Nikon Corporation, Japan) at an
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excitation wavelength of 405 nm. The crystallographic structures were analyzed with Rigaku CCD Saturn
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724+ X-ray single crystal diffractometer.
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CCDC1502160 (the crystal of A), 1500808 (the crystal of B), 1500809 (the crystal of C) contain the
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supplementary crystallographic data for this paper. These data can be obtained free of charge from The
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Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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2.2 Synthesis of the target compound
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The synthetic route of DPPCN is shown in Scheme S1 in the Supporting Information (SI). The
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intermediates of 1 and 2 was synthesized according to previous literatures [46-47]. DPPCN was
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synthesized and purified as follows: a mixture of 2 (20.0 mmol), 1-ethyl-(3-dimethylaminopropyl)
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carbodiimide hydrochloride (EDCI, 24.0 mmol) and N-[3-(dimethylamino) propyl]-2-methyl-2-
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propenamide (DMPAP, 24.0 mmol) and anhydrous dichloromethane (40 mL) was stirred for 1.5 h at 0oC
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(kept in ice-water), then 4-cyanophenol (50.0 mmol) was added into the mixture and stirred for 12 h at
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ambient temperature. The resulting mixtures were extracted with chloroform and washed with water
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successively. The organic extracts were dried over anhydrous MgSO4, evaporated and purified on a silica
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gel column using hexane and dichloromethane (1:3) as the eluent, and then recrystallized from
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dichloromethane and n-hexane. DPPCN was obtained in 50.3% yield after dried under vacuum for 24 h.
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17.93 Hz, 4H), 7.31 (m, 8H), 6.56 (s, 2H);
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136.19.5, 133.8, 132.9, 131.52, 131.20, 128.59.8, 128.40, 127.06, 125.29, 124.56, 122.9, 118.28, 109.91,
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H-NMR (400 MHz, CDCl3, δ): 8.21 (d, J = 8.26 Hz, 4H), 7.78 (d, J = 8.33 Hz, 4H), 7.42 (dd, J = 8.27, 13
C-NMR (100 MHz, CDCl3, δ) : 163.73, 154.20, 144.77,
4
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-1 97.01, 77.36, 77.05, 76.73. FTIR (cmACCEPTED ): 466, 570, 695, 747, 816, 1678, 2230, 2850, 3420. MS (MALDIMANUSCRIPT
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TOF, m/z): [M+.] calcd. for C46H28N4O4: 700.12; found 700.5.
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Scheme 1
108 3.
Results and Discussion
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3.1 Photophysical properties of the DPPCN
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To test the AIE characteristics of DPPCN, the fluorescence spectrum of DPPCN was investigated in
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THF / H2O mixtures with water fraction (fw) from 0 to 99%. As shown in Fig. 1a, the fluorescence intensity
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of DPPCN was very weak in THF, which was consistent with the lower fluorescence quantum yield (ΦF =
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0.03%) of DPPCN in THF. With the addition of water, the fluorescence emission curves of DPPCN
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changed occurring three stages. The fluorescence intensity of DPPCN barely changed with increasing fw
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from 0 to 60%, then increased rapidly with fw changing from 60 to 95%, reaching the maximum with 18-
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fold enhancement at fw = 95%, and finally slightly decreased with fw = 99%. The reason why the
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fluorescence intensity of DPPCN at fw = 99% slightly decreased was that with the “high” water content, the
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dye molecules may quickly agglomerate in a random way decreasing the interaction between molecules
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leading to the electronic excited energy exhausted by intramolecular rotations. So the less emissive
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particles were formed. Since water is a poor solvent for DPPCN, the molecules of DPPCN aggregated in
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the THF / H2O mixtures with higher water contents. The dynamic light scattering experimental results
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confirmed the formation of DPPCN aggregates with an average diameter around 278 nm obtained in the
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THF / H2O mixture with fw = 90% (Fig. S2b in SI). At the same time, the Mie scattering effect observed in
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the UV-Vis spectra (Fig. S3b in SI) revealed that the DPPCN aggregates were also formed with higher
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water content. These results indicated that DPPCN was AIE active [48-49].
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Figure 1
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3.2 Polymorphism
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As is well-known that AIE active molecules possess strong fluorescence in solid state (or crystalline ACCEPTED MANUSCRIPT
132
state) with their unique molecular packing arrangements. We try to use different crystallization conditions
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to cultivate the crystals of DPPCN. Interestingly, solid DPPCN was obtained as yellow precipitates after
134
the reaction, and subsequent recrystallization from trichloromethane and n-hexane gave mixture of cubic
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blue fluorescence crystal of A and needlelike cyan-blue fluorescence crystal of B. Besides, needlelike
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yellow-green fluorescence emission crystal of C was also obtained by recrystallization from
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dichloromethane and n-hexane under relatively dilute condition (Fig. 2a).
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Each crystal form of DPPCN exhibited different emission wavelength and fluorescence quantum yield
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(ΦF). Crystal of A exhibited more intensive emission at longer wavelength (λmax = 480 nm, ΦF = 27%) than
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B (λmax = 504 nm, ΦF = 20.7%) and C (λmax = 540 nm, ΦF = 10%) (Fig. 2b). The fluorescence quantum
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yields of DPPCN crystals were all higher than that of the amorphous powders of DPPCN (ΦF = 3.3%),
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indicating they had CIE properties. The molecules of DPPCN were loosely packed in the amorphous form,
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leading the internal rotations within DPPCN cannot be inhibited exhausting the excited energy. The reason
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why the ΦF of A was highest and exhibited bluest emission wavelength than others would be deciphered by
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later analysis of the X-ray crystallographic structures.
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Figure 2
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Fig.3a shows the single crystal structures of DPPCN. The crystal cells of A and B were monoclinic
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form with space group of P21/n and P21/c, respectively. It should be noted that polymorph C belongs to the
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triclinic space group P-1 consisting of two kinds of conformation molecules (C1 and C2) in an unit cell
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which differed in the bond angles (shown in Fig. 3a and Table S1 in SI). They all had four DPPCN
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molecules in a unit cell, but the crystal of A included extra eight chloroform molecules and the crystal of C
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included six CH2Cl2 molecules in a crystal cell, while the crystal of B had no solvent molecules in the
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crystalline lattice. The crystallographic data and structure refinement parameters of the three kinds of
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crystals are shown in Table S2 in SI.
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In the crystal structure of A, chloroform solventMANUSCRIPT formed C-H…Cl hydrogen bonds interactions (2.854, ACCEPTED
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2.856, 2.920, 2.905, 3.041 and 3.295 Å) and C≡N…H interactions (2.570 and 2.975 Å) to connect the upper
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and lower DPPCN molecules to generate molecular chains shown in Fig. S5 in SI. However, this kind of
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hydrogen bonds could not be found even though there were CH2Cl2 solvents existing in the crystal of C.
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Those can be further illustrated by thermogravimetric analysis (TGA) experiments. Note that the samples
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were dried under vacuum for 12 h, TGA curves (Fig. S6 in SI) showed that A lost about 23% in weight
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around 100 oC. The lost weight can be attributed to the loss of chloroform in the molecules. The mole ratio
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of DPPCN molecules to chloroform in the crystal of A was 1:2. Changing to molecular weight ratio, the
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weight ratio of chloroform occupying the mixture of DPPCN and chloroform was 25%, approximately near
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to 23% weight loss measured by TGA. Whereas the crystal of C did not lose any weight until 200 oC,
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indicating complete removal of the CH2Cl2 molecules from the crystal of C after drying under vacuum for
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12 h. These results indicated that chloroform involved in the crystalline structure of A while CH2Cl2 merely
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dissociated in the crystalline structure of C and did not form interactions with DPPCN molecules.
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It can be seen from Fig. 3c that the crystal of A co-crystallizing with the solvent induced most uniform
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distribution packing style decreasing the conjugation between molecules, and produced strong
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intermolecular interactions making the fluorescence highly confined inside the crystal, eventually endowed
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the crystal of A with a blue shift and high fluorescence quantum yield among these polymorphs.
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Owing to no solvent existing in crystal of B, we can be more objective to analysis the effect of
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molecular packing on AIE properties in the solid state. As can be seen from Table S1 in SI, the twist angle
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between central pyrrolopyrrole moiety and peripheral phenyl excessed 30 degrees making molecules in B
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adopt highly twisted conformations. The molecules were assembled in the crystal lattice by a zigzag layer
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packing (Fig. 3c), which rule out π…π stacking interactions, excluding the formation of H- or J-aggregates.
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Molecules within the layers interacted with each other by C-H…O H-bonding interaction (2.551, 2.424 and
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2.813 Å) and C≡N…H interactions (2.697 and 3.499 Å)) and C-H…π interactions with a distance of 2.769 Å
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and 2.804 Å (Fig. 3b). These intermolecular weak interactions could help to inhibit internal rotations and
183
fix the molecular conformations, and thus benefit to the radiative relaxation as evidenced by the relatively
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high fluorescence quantum yields in the crystals. TheMANUSCRIPT similar interactions can be also found in the crystal of ACCEPTED
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A and C. Those can effectively explain why DPPCN showed AIE and CIE properties.
186 187
Figure 3
188 There are two kinds of packing types in polymorph C (Fig. 3a). Type 1 was composed of the
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molecular conformation of C1, which was assembled like type B that the molecules looked like interlaced
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fingers, and were interacted by strong hydrogen bonding (C-H…O, 2.493 Å). Type 2 consisted of the
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molecular conformation of C2, whose neighboring molecules were oriented toward each other like two
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people opening arms to embrace each other. The molecules in type 2 were also connected by strong
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hydrogen bonding (C-H…O, 2.564 Å). The packing style in the crystal of C allows us to conclude that
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crystal growth from dilute solution was carried out by means of delicate adjustment of the chains in the
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corresponding layers. Both the two types of stacking modes in the crystal of C can effectively increase
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space utilization, thus leading to a more close molecular packing.
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What is more, partial pyrrolopyrrole core overlap (approximately 13.87%) in type 2 between the
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neighboring molecules was found due to molecular close packing, and the vertical distance between the
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neighboring pyrrolopyrrole planes was approximately 3.931 Å resulting in weak π-π interactions and part
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of the fluorescence quenching. So the crystal of C had a lowest fluorescence quantum yield compared to the
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others. The unique stacking way between neighboring molecules and partial overlap of the pyrrolopyrrole
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core in polymorph C may help to explain the relatively large red-shift of fluorescence emission compared
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with the other two kinds of polymorphs.
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To sum up, the effects of the solvent, intermolecular forces, molecular conformations and molecular
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packing modes endowed DPPCN crystals with multi-color emission with large emission color/wavelength
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changes (from blue to yellow and up to 60 nm) and different fluorescence efficiencies.
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3.4 Stimuli-responsive luminescence tuning and switching properties of DPPCN
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Different molecular stacking modes and intermolecular interactions in solid-state organic crystals,
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which influence emission behavior, are not only achieved by the process of growing crystals, but also by
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physical stimulus, such as grinding, heating, pressureMANUSCRIPT changing, and chemical vapor fuming [50-53]. So we ACCEPTED
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performed the experiments of stimuli-responsive luminescence properties of DPPCN. As shown in Fig. 4a,
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the emission of the as-prepared A can be changed to bright yellow by simply grinding with a mortar. Red-
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shift of 46 nm (from 480 to 526 nm) by grinding treatment showed the typical mechanochromic
215
phenomenon. Meanwhile, the ground A returned to the original emission by chloroform vapor fuming for
216
10 min at room temperature. So the reversible changes in emission color of DPPCN were easily achieved.
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Ten cycles of repeated grinding-fuming processes were shown in Fig. 4b, which manifested the good
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reversible mechano-florochromic processes.
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To know well the mechanism of this mechano-florochromism, the powder X-ray diffraction (PXRD)
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was carried out. The pristine A exhibited much sharp and intensive diffraction peaks and the ground
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samples showed dispersion peak, indicating the molecular stacking modes changing from ordered crystal to
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disordered amorphous state. The diffraction peaks of ground samples appeared again by chloroform vapor
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fuming, revealing that A recovered to the ordered crystalline state, but was slightly different with the PXRD
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of A. When grinding A, the disruption of hydrogen bonding would lead to the decrease of the steric
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hindrance, facilitating the molecular rotation, thereby making the molecular conformational planarization,
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which caused a bathochromic shift.
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Figure 4
When ground using a pestle, the crystals of B and C showed a shift of maximum fluorescent emission
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wavelength of 22 nm from 504 nm to 526 nm and 14 nm from 540 nm to 526 nm, respectively. When C
232
was exposed to chloroform vapor fuming at room temperature for about 12 h, it induced a 34 nm blue-
233
shifted emission to 506 nm, which was coincident with the emission of B. Interestingly, when further
234
fumed for another 12 h, the fluorescence of C further blue-shifted emission to 480 nm that matched with
235
the crystal of A (Fig. 5a). Such behaviors may be attributed to C adsorbing chloroform to change its original
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crystal packing way. As shown in the XRD spectra (Fig. 5d), the diffraction patterns of fumed samples are
237
similar but not completely consistent with B and A respectively, suggesting the vapochromic behavior is
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actually a molecule-repacking process. As suggestedMANUSCRIPT by the PL spectra shown in Fig. 5e, we can switch the ACCEPTED
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emission of C from yellow to the other colors (blue and cyan-blue) by fuming.
240 241
Figure 5
242 3.5 Temperature monitoring
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Besides mechano-stimulus fluorescence behavior, thermo-stimulus fluorescence switching properties
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were also characterized. Before we did the thermo-stimulus test, the samples were dried under vacuum for
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12 h treated the same way as we do TGA test. The crystals was gently crushed with a metal spoon, then the
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sample was uniformly spread on a quartz dish with a cover slip, and then carried out the fluorescence
248
measurements. A exhibited thermo-stimulus luminescence switching properties. But both B and C had no
249
thermo-stimulus luminescence switching properties. The fluorescence wavelength of A was 480 nm at
250
room temperature and basically kept unchanged when the temperature was below 75 oC. Continuously
251
increasing temperature, the emission wavelength changed drastically to about 507 nm with nearly 27 nm
252
bathochromic-shift. The differential curve in Fig. 6a shows an inflection point at 91.37 oC, where the
253
wavelength change rate is maximal. As the temperature was increased, the interactions between DPPCN
254
with solvent molecules were progressively reduced, and eventually all chloroform molecules escaped from
255
the crystal of A inducing the increasing of molecular planarization, thus the emission wavelength
256
bathochromic-shifted. Even though the heated samples have similar emission with B, the diffraction
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patterns of heated samples for 12 min at 110 oC are different from B as shown in the XRD spectra (Fig. S9
258
in SI), suggesting they have different molecule-packing styles.
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To further analyze the response speed and temperature sensitivity, time-dependent emission spectra
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were carried out. The wavelength at maximum emission intensity versus time at different heating
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temperatures was plotted in Fig. 6b. The wavelength at maximum emission intensity increased with
262
prolonged heating time and reached a stable value within a period of 10 min. The wavelength red-shifted 12
263
nm when heating at 90 oC and reached up to 25 nm bathochromic-shift when heating at 110 oC. The
10
264
temperature selectivity and the quickly response make DPPCN a promising thermo-responsive material for ACCEPTED MANUSCRIPT
265
using in temperature monitoring devices.
266 267
Figure 6
268 4.
Conclusion
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In summary, a new compound DPPCN with donor-acceptor structure has been designed and
271
synthesized, which is AIE- and CIE-active and shows morphology-dependent fluorescence properties.
272
Three kinds of single-crystalline structures were obtained with different color emissions. The three crystals
273
showed mechano-chromic emissive behavior. After grinding, A and B showed a bathochromic shift of 46
274
nm and 22 nm, respectively, while C exhibited a hypochromatic shift of 14 nm. Moreover, the emission
275
color of polymorph A has been reversibly tuned by grinding and exposing to chloroform vapor. Polymorph
276
A also exhibited thermo-stimulus fluorescence switching behavior. The emissive wavelength change rate
277
reached a maximum value when the temperature was 91.37 oC. While the yellow emission C switched to
278
the other colors (blue and cyan-blue) by exposing to chloroform vapor for 12 h and 24 h, respectively.
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Basing on XRD analysis of single-crystalline structures, multicolor fluorescence tuning and switching are
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attributed to the nature of polymorphs, that is, a change of intramolecular conformation and intermolecular
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packing modes. These results are highly valuable for the further design of relevant compounds with tunable
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solid-state emissions.
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Acknowledgments
We are grateful for the support from the National Basic Research Program of China (973 Program:
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2013CB834704), the National Natural Scientific Foundation of China (Grant Nos. 51328302, 21404010,
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51073026, 51061160500).
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[47] Han T, Zhang YJ, Feng X, Lin ZG, Tong B, Shi JB, et al. Reversible and hydrogen bonding-assisted piezochromic luminescence for solid-state tetraaryl-buta-1,3-diene. Chem. Commun. 2013;49:7049-51. [48] Qi QK, Liu YF, Fang XF, Zhang YM, Chen P, Wang Y, et al. AIE (AIEE) and mechanofluorochromic performances of TPE-methoxylates: effects of single molecular conformations. RSC Adv. 2013;3:7996-8002. [49] Feng X, Tong B, Shen JB, Han TY, Chen L, et al. Aggregation-induced emission enhancement of aryl-substituted
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pyrrole derivatives. J. Phys. Chem. B 2010;114:16731-6. [50] Zhang ZY, Xu B, Su JH, Shen LP, Xie YS, Tian H. Color-tunable solid-state emission of 2,2'-biindenyl-based fluorophores. Angew. Chem. Int. Ed. 2011;50:11654-7.
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[41] Mei J, Hong Y, Lam J W, Qin AJ, Tang YH, Tang BZ. Aggregation-induced emission: the whole is more brilliant ACCEPTED MANUSCRIPT
[51] Teng MJ, Jia XR, Yang S, Chen XF, Wei Y. Reversible tuning luminescent color and emission intensity: a dipeptidebased light-emitting material. Adv. Mater. 2012;24:1255-61.
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[52] Imoto H, Kizaki K, Watase S, Matsukawa K, Naka K. Color Tuning of the aggregation-induced emission of maleimide dyes by molecular design and morphology control. Chem. Eur. J. 2015;21:12105-11.
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[53] Hariharan PS, Moon D, Anthony SP. Reversible fluorescence switching and topochemical conversion in an organic
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AEE material: polymorphism, defection and nanofabrication mediated fluorescence tuning. J. Mater. Chem. C
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2015;3:8381-8.
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20
5000
(b)
fw (%) 0 10 20 30 40 50 60 70 80 85 90 95 99
2000
I/I0
3000
15
10
M AN U
4000
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(a)
PL.Intensity (a.u.)
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Scheme 1. Molecular Structure of DPPCN.
5
1000
0
0 450
500
550
0
20
40
60
80
100
Water fraction (%)
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Wavelength (nm)
600
Figure 1. (a) Fluorescence spectra of DPPCN in THF / H2O mixtures with different fw; (b) Correlation between the net change in PL intensity [I/I0] with different water fractions in the THF / H2O mixtures. I0: PL intensity in pure THF.
AC C
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Concentration: 10 µM; λex = 330 nm.
1
(b)
480 nm 504 nm 526 nm 540 nm
A B C amorphous
0.8
0.6
0.4
0.2
0.0 450
500
550
600
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400
(c)
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Normalized Intensity (a.u.)
1.0
SC
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10
Wavelength (nm)
20
A B C amorphous
30
40
2θ (deg)
Figure 2. (a) Fluorescent images of crystal of A, B and C taken under UV irradiation on a fluorescence microscope; (b)
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Corresponding normalized fluorescence spectra of crystal of A, B and C; (c) PXRD patterns of three crystals and
AC C
amorphous state of DPPCN.
2
M AN U
SC
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Figure 3.(a) Crystal cells of A, B and C (C having two molecules (C1 and C2) in asymmetric unit, C1 is painted red and
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C2 is painted blue for clarity); (b) Illustration of H-bonding interactions (C-H…O, C≡ N…H) and C-H…π in the single
540
(a) 480 nm
526 nm
A ground fumed
0.6
0.4
0.2
(b)
(c)
Grinding
530
Wav elength (nm)
0.8
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Normalized Intensity (a.u.)
1.0
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crystal; (c) Crystal packing of the three crystals (A, B and C).
A ground fumed
520 510 500 490 480
Fuming 0.0 400
470 450
500
550
Wavelength (nm)
600
1
2
3
4
5
6
7
Repeat cycle
8
9
10
5
10
15
20
25
30
35
40
45
2θ (deg)
Figure 4. (a) Fluorescence spectra of A before grinding, after grinding and fuming; (b) Maximum emission wavelength change upon repeated fuming–grinding cycles; (c) PXRD patterns of A before grinding, after grinding and fuming.
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1.0
Normalized Intensity (a.u.)
A fumed 24 h B fumed 12 h
30
0.6
0.4
0.0
40
400
2θ (deg)
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20
C ground fumed 12 h fumed 24 h
0.8
0.2
C 10
(e)
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(d)
450
500
550
600
M AN U
Wavelength (nm)
Figure 5. Fluorescent images of (a) crystal of C, (b) after fumed by chloroform for about 12 h and (c) after fumed by chloroform for about 24 h taken under UV irradiation on a fluorescence microscope ; (d) PXRD patterns in different
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solid states; (e) Normalized fluorescence spectra in different solid states.
510
(a) 505
495 490
0.8 0.6 0.4
485
60
495
490 Temperature (oC) 110 100 90
485
0.2
480
480
40
(b)
500
Wavelength (nm)
1.0
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500
Deriv.Intentity
1.2
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Wavelength (nm)
505
1.4
0.0 80
100
0
2
o
Temperature ( C)
4
6
8
10
12
Time (min)
Figure 6. (a) The maximum emission intensity (black) and its differential (red) of the crystal of A versus temperature (excitation wavelength: 330 nm, time interval for heating: 60 s); (b) Changes of time-dependent wavelength of the crystal A at 90 oC, 100 oC and 110 oC.
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Highlights
•
A novel
pyrrolopyrrole derivative DPPCN exhibited
aggregation- and
•
DPPCN showed three types of polymorphism with blue, cyan-blue and yellow-green emissive, respectively.
The structure-property relationships were investigated based on X-ray single
SC
•
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crystal diffraction analysis.
The mechano- and thermo-stimulus fluorescence switching behaviors of the three
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crystals were investigated.
AC C
•
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crystalline-induced emission properties.