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Piezoluminescence and Liquid Crystallinity of 4,4’-(9,10-anthracenediyl)bispyridinium Salts Shigeo Kohmoto, Tomotaka Chuko, Shugo Hisamatsu, Yasuhiro Okuda, Hyuma Masu, Masahiro Takahashi, and Keiki Kishikawa Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b00028 • Publication Date (Web): 01 May 2015 Downloaded from http://pubs.acs.org on May 6, 2015

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Crystal Growth & Design

Piezoluminescence and Liquid Crystallinity of 4,4’(9,10-Anthracenediyl)bispyridinium Salts Shigeo Kohmoto,*† Tomotaka Chuko,† Shugo Hisamatsu,‡ Yasuhiro Okuda,‡ Hyuma Masu,†,§ Masahiro Takahashi,† and Keiki Kishikawa† †

Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

‡

Actinide Coordination Chemistry Group, Quantum Beam Science Directorate, Japan Atomic Energy Agency, Shirakatashirone 2-4, Tohkai-mura, Ibaraki 319-1195, Japan §

Center for Analytical Instrumentation, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

ABSTRACT: Piezoluminescence and liquid crystallinity of anthracene-based bispyridinium salts were investigated for stimulus-responsive luminescent organic crystals and luminescent ionic liquid crystals. The salts possess an anthracene moiety as a fluorophore in their center and the pyridiniums attached to the anthracene moiety are substituted with trialkoxybenzyl groups. Single crystals of the salts bearing two trimethoxybenzyl groups were obtained as solvates. Ethyl acetate, acetone, and dioxane solvates of the chlorides have almost the same crystal structures

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with 1D channels. Grinding of the solvated crystals caused the extrusion of the included solvent molecules, which resulted in the red shifts of their fluorescence in solid state. The dimethyl sulfoxide solvate of the hexafluorophosphate also showed piezoluminescene by grinding. Tris(octyloxy)benzyl and tris(dodecyloxy)benzyl derivatives exhibited rectangular columnar liquid crystals on heating for their bromide and tetrafluoroborate salts and on cooling for their hexafluorophosphate salts. Out of these pyridinium salts, hexafluorophosphates showed characteristic solvent-dependent fluorescence.

Introduction Piezoluminescence of organic crystals has been paid considerable attentions.1-6 It can potentially be applied to memory devices and sensory, motion, and security systems. Recently, a number of unique features of piezoluminescence have been reported such as high contrast,7-9 multicolor

swiching,10

full-color-tunability,11

hydrostatic-pressure-response,15

far-red

multi-responsive

luminescence,16

fluorescent

mesomorphic,17

and

probes,12-14 so

forth.

Piezoluminescence is also reported for cocrystals18 and organic salts.19-21 These stimuliresponsive phenomena are due to the change of the crystal packing environment. Modification of relative location of luminophoric moieties caused by stimuli, such as grinding, friction, and searing, is the reasons for these phenomena. The sliding of luminophoric π-planes caused by stimuli brings about the changes in the degree of π-overlapping, which results in the alteration of luminescence color. In some cases, fluorescence color changes are reversible. Heating or reincorporation of solvent molecules can regenerate the original crystal packing and thus can go back to the original color of luminescence. In the latter case, the changes of luminescence color occur in a sequence of piezo- and vapoluminescence.22-29 Comparing to a large number of


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examples of piezo- and vapoluminescence for metal organic complexes, the examples are relatively limited to metal-free organic compounds. In the course of our study on solvent polarity-dependent luminescence of anthracene-based organic salts,30 we found unique pyridinium salts which showed piezo- and vapoluminescence in their solid state responding to the inclusion and extrusion of solvated molecules. The immediate extrusion can be achieved by grinding the solvated crystals possessing channel structures and subsequent vapoluminescent behaviors were observed by the addition of solvent or by the exposure to solvent vapor. Herein, we report on the piezo- and vapoluminescent behaviors of novel pyridinium salts possessing an anthracene fluorophore together with their crystal structures and liquid crystallinity of their analogs substituted with long alkoxy chains.

Experimental Section Materials and Methods. All the reagents employed were commercially available and used as received without further purification. Melting points were determined on a Yanaco MP-S3 apparatus and were uncorrected. IR spectra were recorded on a PerkinElmer Spectrum Two. 1H NMR spectra were recorded on a Bruker DPX300 NMR spectrometer in CDCl3 with Me4Si as an internal standard. Fluorescence spectra were recorded on JASCO FP-750 spectrometer. Highresolution ESI mass spectra were measured with a Thermo Fisher Exactive mass spectrometer. X-ray diffraction (XRD) experiments were performed with CuKα radiation by using a Rigaku RINT 2200 diffractometer. Liquid crystalline properties were investigated by using Nikon ECLIPSE E400POL polarized optical microscopy. Differential scanning calorimetry (DSC) measurements were performed on a MAC Science DSC 3100S differential scanning calorimeter with heating rate of 3 °C min-1. Thermogravimetric analysis (TGA) was carried out on a


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RIGAKU TG8120 apparatus under flowing nitrogen with 5 °C min-1 ramp rate. Synthesis. 9,10-di(pyridin-4-yl)anthracene was prepared according to the literature.31 4,4'-(anthracene-9,10-diyl)bis(1-(3,4,5-trimethoxybenzyl)pyridin-1-ium) chloride (1a). A solution of 9,10-di(pyridine-4-yl)anthracene (0.100 g, 0.30 mmol) and 5-(chloromethyl)-1,2,3trimethoxybenzene (0.163 g, 0.75 mmol) in toluene (10 mL) was refluxed for 2 days under stirring. After removal of the solvent in vacuo, the residue was purified by column chromatography on silica gel (eluent: ethyl acetate followed by chloroform/methanol = 5/1) to give 0.163 g (71%) of 1a as a yellow solid. Mp 219 °C (dec.); 1H NMR (300 MHz, CDCl3) δ 3.89 (s, 6H), 3.96 (s, 12H), 6.40 (s, 4H), 7.15 (s, 4H), 7.24 (dd, J = 3.0, 6.0 Hz, 4H), 7.45 (dd, J = 6.0, 3.0 Hz, 4H), 7,91 (d, J = 6.0 Hz, 4H), 9.83 (d, J = 6.0 Hz, 4H); HRMS (ESI) calcd for C44H42O6N2 [M]2+ 347.1516, found 347.1511. 4,4'-(anthracene-9,10-diyl)bis(1-(3,4,5-trimethoxybenzyl)pyridin-1-ium) tetrafluoroborate (1b). A solution of 1a (0.060 g, 0.078 mmol) and ammonium tetrafluoroborate in dichloromethane (2 mL) and methanol (3 mL) was stirred for a day at ambient temperature. After removal of the solvent in vacuo, the residue was washed with chloroform to obtain 0.034 g (50%) of 1b as a yellow solid. Mp 274 °C (dec.); 1H NMR (300 MHz, DMSO-d6) δ 3.70 (s, 4H), 3.87 (s, 4H), 5.87 (s, 4H), 7.16 (s, 4H), 7.60 (m, 8H), 8.44 (d, J = 6.0 Hz, 4H), 9.43 (d, J = 6.0 Hz, 4H); HRMS (ESI) calcd for C44H42O6N2 [M]2+ 347.1516, found 347.1511. 4,4'-(anthracene-9,10-diyl)bis(1-(3,4,5-trimethoxybenzyl)pyridin-1-ium) hexafluorophosphate (1c). A solution of 1a (0.060 g, 0.078 mmol) and ammonium hexafluorophosphate in dichloromethane (2 mL) and methanol (3 mL) was stirred for a day at ambient temperature. After the removal of the solvent in vacuo, the residue was washed with


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chloroform to give 0.039 g (51%) of 1c as a yellow solid. Mp 296 °C (dec.); 1H NMR (300 MHz, DMSO-d6) δ 3.70 (s, 4H), 3.87 (s, 4H), 5.87 (s, 4H), 7.16 (s, 4H), 7.60 (m, 8H), 8.44 (d, J = 6.0 Hz, 4H), 9.43 (d, J = 6.0 Hz, 4H). 4,4'-(anthracene-9,10-diyl)bis(1-(3,4,5-tris(octyloxy)benzyl)pyridin-1-ium) bromide (2a). A solution of 9,10-di(pyridin-4-yl)anthracene (0.100 g, 0.30 mmol) and 5-(bromomethyl)-1,2,3tris(octyloxy)benzene (0.448 g, 0.75mmol) in toluene (10 mL) was stirred for two days under reflux. The solvent was removed in vacuo and the crude product was purified by silica gel column chromatography (eluent: ethyl acetate followed by chloroform/methanol = 5/1) to obtain 0.350 g (80 %) of 2a as an orange solid. Mp 228 °C (dec.); 1H NMR (300 MHz, CDCl3) δ 0.87 (t, J = 6.0 Hz, 18H), 1.29 (m, 48H), 1.49 (m, 12H), 1.76-1.85 (m, 12H), 3.98-4.08 (m, 12H), 6.30 (s, 4H), 6.99 (s, 4H),7.28 (m, 4H), 7.53 (dd, J = 6.0, 3.0 Hz, 4H), 7.84 (d, J = 6.0 Hz, 4H), 9.60 (d, J = 6.0 Hz, 4H); HRMS (ESI), calcd for C86H126O6N2 [M]2+ 641.4802, found 641.4800. 4,4'-(anthracene-9,10-diyl)bis(1-(3,4,5-tris(dodecyloxy)benzyl)pyridin-1-ium)

bromide

(3a). A solution of 9,10-di(pyridin-4-yl)anthracene (0.100 g, 0.30 mmol) and 5-(bromomethyl)1,2,3-tris(dodecyloxy)benzene (0.545 g, 0.75mmol) in toluene (15 mL) was stirred for two days under reflux. The solvent was removed in vacuo and the crude product was purified by silica gel column chromatography (eluent: ethyl acetate followed by chloroform/methanol = 5/1) to obtain 0.484 g (90 %) of 3a as an orange solid. Mp 235 °C (dec.); 1H NMR (300 MHz, CDCl3) δ 0.87 (t, J = 6.0 Hz, 18H), 1.26 (m, 96H), 1.50 (m, 12H), 1.80-1.83 (m, 12H), 3.97-4.06 (m, 12H), 6.31 (s, 4H), 6.99 (s, 4H), 7.26 (s, 4H), 7.52 (dd, J = 6.0, 3.0 Hz, 4H), 7.84 (d, J = 6.0 Hz, 4H), 9.60 (d, J = 6.0 Hz, 4H); HRMS (ESI), calcd for C110H175O6N2 [M]2+ 810.1720, found 810.1699. In a similar manner as for the preparation of 1b and 1c, tetrafluoroborates 2b and 3b, and hexafluorophosphates 2c and 3c were prepared from bromides 2a and 3a, respectively. They


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were purified by column chromatography on silica gel (eluent chloroform/methanol). 4,4'-(anthracene-9,10-diyl)bis(1-(3,4,5-tris(octyloxy)benzyl)pyridin-1-ium) tetrafluoroborate (2b). 96 % yield, an orange solid. Mp 218 °C (dec.); 1H NMR (300 MHz, CDCl3) δ 0.87 (t, 18H), 1.28 (m, 48H), 1.49 (m, 12H), 1.73-1.85 (m, 12H), 3.98-4.08 (m, 12H), 6.21 (s, 4H), 6.97 (s, 4H), 7.26 (m, 4H), 7.48 (m, 4H), 7.85 (d, J = 6.0 Hz, 4H), 9.47 (d, J = 6.0 Hz, 4H); HRMS (ESI), calcd for C86H126O6N2 [M]2+ 641.4802, found 641.4805. 4,4'-(anthracene-9,10-diyl)bis(1-(3,4,5-tris(octyloxy)benzyl)pyridin-1-ium) hexafluorophosphate (2c). 97 % yield, a yellow-green solid. Mp 216 °C; 1H NMR (300 MHz, CDCl3) δ 0.87 (t, J = 6.0 Hz, 18H), 1.28 (m, 48H), 1.51 (m, 12H), 1.75-1.85 (m, 12H), 4.00-4.09 (m, 12H), 5.86 (s, 4H), 6.90 (s, 4H), 7.27 (m, 4H), 7.29 (d, 4H), 7.76 (d, J = 6.0 Hz, 4H), 8.90 (d, J = 6.0 Hz, 4H); HRMS (ESI), calcd for C86H126O6N2 [M]2+ 641.4802, found 641.4808. 4,4'-(anthracene-9,10-diyl)bis(1-(3,4,5-tris(dodecyloxy)benzyl)pyridin-1-ium) tetrafluoroborate (3b) 96% yield, an orange solid. Mp 205 °C; 1H NMR (300 MHz, CDCl3) δ 0.87 (t, J = 6.0 Hz, 18H), 1.26 (m, 96H), 1.48 (m, 12H), 1.73-1.83 (m, 12H), 3.98-4.07 (m, 12H), 6.19 (s, 4H), 6.96 (s, 4H), 7.21 (m, 4H), 7.44 (m, 4H), 7.83 (d, J = 6.0 Hz 4H), 9.45 (d, J = 6.0 Hz, 4H); HRMS (ESI), calcd for C110H175O6N2 [M]2+ 810.1720, found 810.1702. 4,4'-(anthracene-9,10-diyl)bis(1-(3,4,5-tris(dodecyloxy)benzyl)-pyridin-1-ium) hexafluorophosphate (3c). 71% yield, an orange solid. Mp 220 °C; 1H NMR (300 MHz, CDCl3) δ 0.87 (m, 18H), 1.25 (m, 96H), 1.49 (m, 12H), 1.77-1.82 (m, 12H), 3.99-4.08 (m, 12H), 5.86 (s, 4H), 6.82 (s, 4H), 6.93 (s, 4H), 7.26 (m, 4H), 7.73 (d, J = 6.0 Hz, 4H), 8.90 (d, J = 6.0 Hz, 4H); HRMS (ESI), calcd for C110H175O6N2 [M]2+ 810.1720, found 810.1724. Recrystallization. Preparation of single crystals for X-ray structural analysis was carried out by recrystallization at room temperature using a vapor diffusion method. A vessel containing


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3~10 mg of sample dissolved in 3~6 mL of solvent was placed in a jar in which solvent to be permeated was added. After standing for several days, single crystals of compounds 1a and 1c were obtained as solvates, 1a•[EtOAc], 1a•[acetone], 1a•[dioxane] and 1c•[DMSO]2 from ethyl acetate, acetone, dioxane, and dimethyl sulfoxide, respectively. The ratios of the pyridinium salts and incorporated solvent molecules were measured by 1H NMR spectroscopy. X-ray Crystallography. X-ray diffraction data for the crystals were measured on Bruker APEXII CCD diffractometers. Data collections were carried out at low temperature by using liquid nitrogen. All structures were solved by direct methods SHELXS-9732 and the nonhydrogen atoms were refined anisotropically against F2, with full-matrix least squares methods SHELXL-97.32 All hydrogen atoms were positioned geometrically and refined as riding. Other details of refinements of the crystal structures are described in supporting information.

Results and Discussion Pyridinium salts 1 – 3 possessing an anthracene fluorophore in the center of the molecule were synthesized by Suzuki-Miyaura coupling of 9,10-dibromoanthracene with 4-(4,4,5,5-tetramethyl1,3,2-dioxaborolan-2-yl)pyridine followed by salt formations with the corresponding halides for 1a, 2a, and 3a. Tetrafluoroborates (1b, 2b, and 3b) and hexafluorophosphates (1c, 2c, and 3c) were prepared by counter-anion-exchange reactions of 1a, 2a, and 3a, respectively (Figure 1).

Figure 1. Chemical structures of anthracene-based pyridinium salts examined in this study.


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Crystal structures of 1a and 1c Single crystals were obtained for 1a and 1c by recrystallization and their single crystal X-ray analyses showed that both salts were solvates containing the solvents for recrystallization. However, a suitable single crystal of 1b for X-ray analysis could not be obtained. Recrystallization of 1a from ethyl acetate (EtOAc), acetone, and dioxane afforded the corresponding solvated crystals with a molar ratio of 1a to the incorporated solvent molecule of 1:1. Because of insolubility of 1c in most of organic solvents, its recrystallization was carried out in dimethyl sulfoxide (DMSO) with diffusion of water vapor. The DMSO solvated crystals of 1c were obtained with a molar ratio 1:2 (1c:DMSO). Crystallographic data for solvated crystals of 1a and 1c are presented in Table 1. Crystals of EtOAc, acetone, and dioxane solvates of 1a have the same space group C2/c and have almost the same unit cell dimension, which indicates that the way of inclusion of solvent molecules are identical in all solvates. The included EtOAc and acetone molecules are disordered in the channels of 1a. Figure 2a shows the 1D channels of the EtOAc solvate viewed along the crystallographic c axis in which included EtOAc molecules are omitted for clarity. The way of inclusion of solvent molecules is shown in Figures 2b–2e in which included solvent molecules are presented as a space-filling model colored orange. The channel has a slightly bent zigzag shape (Figure 2b). Included EtOAc molecules are inserted between anthracene moieties (Figure 2c). The C-H…O hydrogen bonding33-35 between the ester carbonyl oxygen atom and the nearest hydrogen atom of the pyridinium ring takes place with the C…O atomic distance of 3.34 Å. The distance between the two anthracene rings is 7.62 Å. The results indicate that an EtOAc molecule is closely packed between them considering from the thickness of the anthracene ring. The torsion angle between the anthracene ring and the pyridinium ring is 70.2° in 1a. Comparatively strong Cl-…H-C interaction36-43 plays a pivotal


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Table 1. Crystallographic data for the solvated crystals of 1a and 1c. solvated crystal

1a·EtOAc

1a·acetone

formula

C44H42N2O6, 2(Cl), C4H8O2,

C44H42N2O6,

crystal system

1a·dioxane

1c·(DMSO)2

C44H42N2O6,

C44H42N2O6, 2(F6P),

2(Cl), C3O

2(Cl), C4H8O2

2(C2H6OS)

Monoclinic

Monoclinic

Monoclinic

Monoclinic

space group

C2/c

C2/c

C2/c

P21/n

a (Å)

29.258(2)

29.465(2)

29.546(4)

8.2867(6)

b (Å)

8.1420(6)

8.1667(6)

8.2644(11)

15.1896(15)

c (Å)

22.6504(17)

22.2934(15)

22.337(3)

19.9602(17)

α (°)

90

90

90

90

β (°)

126.2930(10)

126.6470(10)

126.9476(17)

93.1960(10)

γ (°)

90

90

90

90

V (Å3)

4349.0(6)

4304.1(5)

4359.0(10)

2508.5(3)

Dc (Mg m-3)

1.304

1.262

1.301

11.511

Z

4

4

4

2

T (K)

173

173

173

173

R1, [I > 2σ(I)]

0.037

0.0422

0.0746

0.0515

wR2 [I > 2σ(I)]

0.1027

0.1188

0.2042

0.1389

a

a

The positions of hydrogen atoms included in the disordered acetone molecule were not calculated.

role in the creation of ladder-type structure. Chloride anions are located between the two neighboring pyridinium rings with multiple Cl-…H-C interactions (a – c) indicated in Figure 2c. Thus, chloride anions are acting as spacers to construct a ladder-type structure. A piling of ladders in a zigzag way creates channels. Ionic interactions play an important role as inter-ladder interactions. The chloride anion involved in the neighboring ladder is located on the pyridinium


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Figure 2. Channel structure of the solvates of 1a. (a) A top view of the channel viewed along the crystallographic c-axis in which included ethyl acetate molecules are omitted for clarity. (b) A channel including ethyl acetate molecules viewed along the crystallographic a-axis. Ladder-like structures created by multiple Cl-…H-C interactions for the crystals of (c) ethyl acetate, (d) acetone, and (e) dioxane solvates. Included molecules are presented by a space-filling model colored orange. Numbers in the figure indicate the distances between the chloride anion and the carbon atom in Cl-…H-C interactions.


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ring plane with a distance of 3.31 Å. Almost equivalent channel structures were created by the inclusion of acetone and dioxane molecules with 1a as shown in figures 2d and 2e, respectively. Nearly the same distance between the two anthracene rings as that of the EtOAc solvate, 7.55 Å, is observed for the acetone solvate. A slightly longer distance, 7.80 Å, is observed for the dioxane solvate. In contrast to 1a, a different crystal packing was observed for the DMSO solvate of 1c. Figure 3 shows the crystal structures of 1c•[DMSO]2. Pyridinium 1c is packed in an undulated way and included DMSO molecules are placed adjacent to hexafluorophosphate

Figure 3. Crystal structures of 1c•(DMSO)2. (a) A packing structure viewed along the crystallographic a-axis, (b) the arrangement of anthracene rings viewed along the crystallographic c-axis together with the space-filling model presentation of anthracene rings to indicate the degree of their overlapping, and (c) observed C-H…F and P-F/π interactions indicated blue and black dotted lines, respectively.


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anions (Figure 3a). Because of the large size of the hexafluorophosphate anion, it is difficult to create a ladder-like structure as observed in the case of 1a. Anthracene rings are aligned linearly. A space-filling presentation in Figure 3b shows a slight overlap of anthracene rings. The distance between the planes where the two neighboring anthracene rings are located is ca. 3.2 Å. Figure 3c represents C-H…F44-46 and anion/π (hexafluorophosphate/pyridinium)47-50 interactions. The C…F distances of 3.33 and 3.27 Å were observed for C(15)-H(15)…F(6) and C(20)H(20)…F(6), respectively. The hexafluorophosphate anion is located above the pyridinium ring plane in close proximity. The P-F/π interaction takes place with the centroid (the pyridinium ring) …F(3) distance of 3.48 Å.

Piezoluminescent behaviors of solvated crystals of 1a and 1c Piezoluminescent behaviors of solvated crystals were monitored by fluorescence spectroscopy. For the solvated crystals, 1a•EtOAc and 1a•dioxane, their solid-state fluorescence spectra were recorded in the following sequence; (1) before treatment, (2) after the first grinding, (3) after the subsequent addition of EtOAc, and then (4) after the following second grinding. Figures 4a and 4b show the fluorescence spectral changes before and after these sequential treatments for 1a•EtOAc and 1a•dioxane, respectively. Both solvated crystals show similar changes in fluorescence upon treatments. Pristine crystals of 1a•EtOAc and 1a•dioxane showed fluorescence with the similar emission maximum wavelength at 496 and 492 nm, respectively. After the first grinding, the color of fluorescence was changed to yellowish green. A red-shift of their fluorescence with emission maxima at 509 and 519 nm was observed for 1a•EtOAc and 1a•dioxane, respectively. The results indicated that the degree of overlapping of anthracene rings was changed. In the pristine solvated crystals of 1a, their fluorescence originates in the


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segregated arrangement of anthracene fluorophores. Because of extrusion of EtOAc or dioxane molecules by grinding, the molecules of 1a were supposed to be placed in close proximity to each other. The overlap of anthracene fluorophores turned into efficient. The tight stacking with increased π-π interactions caused by grinding is believed to bring about red-shift in fluorescence.51 Another possibility of the origin of red-shift is the generation of increased effective conjugation length by the conformational change of the twisted conformation of a fluorophore to the planarized one through mechanical stimuli.9 The tuning of fluorescence color of anthracene derivatives has been studied by modifying the degree of the overlapping of anthracene moieties in their crystal packing.52-57 An addition of EtOAc or dioxane to the ground

Figure 4. Fluorescence spectra of (a) 1a•EtOAc and (b) 1a•dioxane observed after sequential treatments from (1) to (4); (1) before grinding, (2) after the first grinding, (3) after the addition of solvent, and (d) after the second grinding. The inset figures show the changes of fluorescence maximum wavelength in the sequential treatments. The marks “G” and “A” indicate the grinding of the sample and the addition of solvent, respectively and the attached numbers to them correspond to the number of cycles. Excitation wavelength is 370 nm. The photographs below and above the inset figures indicate the fluorescence colors of the pristine crystals and the ground samples irradiated with 375 nm light, respectively.


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sample causes intrusion of EtOAc or dioxane molecules into the space between anthracene fluorophores. Accordingly, anthracene fluorophores are well segregated from each other, which results in a generation of the original emission color. The color of fluorescence of the sample changed to bluish green. The inset figure in Figure 4 presents the changes of fluorescence maximum wavelength in the repeated grinding-addition processes of the solvated crystals to show the switching of its fluorescence change. In order to confirm the extrusion of EtOAc molecules by grinding from the solvated crystals 1a•EtOAc, IR spectroscopic study was carried out (Figure 5). The measurements before grinding were carried out in two ways, ATR (attenuated total reflection) (Figure 5a) and KBr pellet

Figure 5. IR spectra (a ~ d), DSC charts (e and f), and TGA diagram (g) of 1a•EtOAc. (a) The crystalline sample before grinding (ATR measurement), (b) the crystalline sample before grinding (KBr pellet), (c) after ground at room temperature (KBr pellet), (d) heated at 180°C for 15 min (KBr pellet), (e) the second heating, (f) the first heating, and (g) TGA diagram. The inset photographs in (f) show the changes of the surface of the crystal during heating irradiated with 375 nm UV light.


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(Figure 5b) methods. The KBr pellet was prepared by pressing (1,000kg/cm2) the fine crystals of 1a•EtOAc placed between two KBr plates. In both cases, the stretching vibration corresponding to the ester carbonyl was observed at 1720 cm-1. It is noteworthy to mention that the press did not change the fluorescence color. Figure 5c shows IR spectrum of 1a•EtOAc after grinding. It is obvious that the band corresponding to the ester carbonyl disappeared completely. Similarly, an extrusion of EtOAc molecules was observed by heating at 180 °C for 15 min. (Figure 5d). Figures 5e and 5f show the DSC charts of the second and the first heating of 1a•EtOAc, respectively. Extrusion of EtOAc molecules started at around 140 °C and they are completely liberated at 200 °C (Figure 5f). Cracks were observed on the surface of the crystal due to the extrusion of EtOAc molecules (the inserted photograph at 180 °C in Figure 5f). No peak was observed in the second heating (Figure 5e). The results indicate that the stress by grinding is required to extrude EtOAc molecules and for subsequent sliding of the molecule to furnish the efficient overlapping of anthracene moieties leading to red-shifted emission. To confirm further the liberation of ethyl acetate and also the thermal stability of the solvate, TGA (thermogravimetric analysis) of 1a•EtOAc was carried out (Figure 5g). The loss of mass by 180 °C was 12.7 % which corresponded almost to the liberation of one molecule of ethyl acetate (10.3%) from the solvate. The results accord with the DSC measurements. After the emission of the solvated ethyl acetate, continuous heating caused degradation of 1a in two steps. Due to unstable nature of the acetone solvate, grinding experiments for it were difficult to carry out. The DMSO solvate, 1c•(DMSO)2, also exhibited piezoluminescent behavior. The inset photographs in Figure 6 shows the fluorescence microscope photographs of its crystals before (A) and after grinding (B). Simple crush of the crystals did not cause the change of fluorescence color. However, when the grinding was carried out on the crushed crystals of 1c•(DMSO)2 by


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Figure 6. Fluorescence spectral changes of 1c•(DMSO)2 after the sequential treatments from (1) to (4); (1) before grinding and (2) after the first grinding, (3) the addition of acetone, and (4) the second grinding. The inset photographs (A) and (B) show the color of fluorescence before and after partial grinding, respectively. Excitation wavelength: 370 nm.

spatula, the color of fluorescence became yellowish green. The photograph (B) shows the fluorescence color change from bluish green to yellowish green occurred on the area where grinding was carried out. The color of fluorescence remained unchanged in the area without grinding. Fluorescence spectra in Figure 6 show the spectral changes of 1c•(DMSO)2 after treatments. A powdery sample of 1c before recrystallization without containing DMSO molecules showed fluorescence with an emission maximum at 482 nm similar to that of DMSO solvated crystals. Grinding of the crystalline sample of 1c•(DMSO)2 caused red-shift of its fluorescence. Possibly, grinding could bring about sliding of the molecules to enhance the overlap of anthracene moieties with respect to each other, which gave red-shifted emission. The addition of a drop of acetone to the ground crystals resulted in a blue-shift of its fluorescence.


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The subsequent second grinding afforded almost the same fluorescence spectrum as that after the first grinding.

Liquid crystallinity of 2 and 3 Imidazolium and pyridinium derivatives are often utilized as ionic liquid crystals.58-62 Ionic interactions play pivotal roles in assembling to create liquid crystalline superstructures. Pyridinium salts 2 and 3 possessing long alkoxy chains showed columnar phases. Their phase transition behaviors were examined with POM (polarized optical microscopy) and phase structures were characterized by powder XRD (X-ray diffraction). Table 2 shows their phase Table 2. Phase transition temperatures of anthracene pyridinium salts. Salt

Phase transition temperature (°C)

2a

Cr

2b

Cr

197 181

G

3a

Cr

3b

Cr Cr

3c

G

Col

228 218

decomp. decomp.

216

Cr 2c

Col

Col 203 185

Col Col

Iso 151 235 205

decomp. decomp.

220 Col

Iso 148

Cr: crystal, Col: columnar, Iso: isotropic phase, decomp.: decomposition


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Figure 7. POM photographs (x 500) of 2a (a), 2b (b), 2c (c), 3a (d), 3b (e), and 3c (f) at 206, 189, 230, and 198 ºC for 2a, 2b, 3a, and 3b on heating respectively, and at 40 and 130 ºC for 2c and 3c on cooling, respectively.

transition temperatures observed by POM. The salts with bromide (2a and 3a) and tetrafluoroborate (2b and 3b) as counter anions showed columnar phases on heating. Further heating of these salts resulted in decomposition before phase transition to isotropic liquids. In contrast, hexafluorophosphate (2c and 3c) were stable upon heating and melted to isotropic liquids without showing liquid crystallinity. However, it exhibited columnar phases on cooling from the corresponding isotropic liquids. Both of them showed similar phase transition temperatures at around 150 ºC. On further cooling, they were transferred to a glassy state. The phase transitions occurred gradually and it was difficult to determine their clear phase transition temperatures. Figure 7 shows their POM microphotographs. Except 2c, they showed marble-like textures, while batonnets were observed for 2c. The XRD pattern of 3a showed three distinctive reflection peaks with d spacings of 31.8 Å, 24.8 Å, and 16.5 Å (Figure 8). It was difficult to consider that its liquid crystalline phase was a hexagonal columnar phase. The relation between the d-spaceings of (100) and (110) should be 1:1/√3 in a hexagonal columnar phase. The first two d-spaceings of 3a did not satisfy this


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relation. The reciprocals of d-spaceings were plotted on the 2D rectangular reciprocal lattice. The peaks were assigned as (200), (110), and (400) of a rectangular columnar phase, respectively. A broad halo centered on 25° of 2θ indicates the molten alkyl chains. Lattice parameters a and b were determined to be 68.4 and 28.5 Å, respectively. A number of molecules in a unit cell (Z) were calculated to be 2.3 with an assumption that the density of the salt was 1.0 and the layer distance between the discs was 3.5 Å which was the thickness of anthracene ring. Therefore, one molecule of 3a affords approximately one disc. The XRD patterns of other salts are shown in Figures S5–S9 in Supporting Information. Their observed d-spacings and lattice parameters are summarized in Table S5 in Supporting Information. The lattice constants of the hexafluorophosphate 3c are slightly larger than those of the corresponding bromide and tetrafluoroborate. The analyses of their XRD data except 2c showed that their phases were also rectangular columnar phases and similarly, a disc consisted of approximately one molecule. Because of the lack of the (110) peak in the XRD pattern of 2c (Figure S7), its liquid crystalline phase could not be determined as a rectangular columnar phase. From its texture observed by POM and its XRD pattern in which (100), (200) and (300) peaks were observed, its mesophase could possibly be assigned as a lamellar phase. The core parts of these liquid crystalline

Figure 8. XRD pattern of 3a at 210 °C.


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molecules are not flat deduced from the crystal structures of 1a and 1c, which is usually unsuitable for discotic liquid crystals. It is noteworthy to mention that even such non-flat molecules can exhibit liquid crystallinity. Possibly, the molecules become flatter at the temperature exhibiting liquid crystal phase because of the molecular motion by heating.

Fluorescence properties in solution In our previous study on fluorescent amphiphilic anthracene-base imidazole salts,30 we observed solvent-polarity-dependent fluorescence. Monomer or excimer-like emission was selectively observed depending on the choice of the solvent polarity. The ratio of these two emissions was controllable by mixing the solvents of two distinctive polarities. We are curious about fluorescence properties of the present anthracene-based pyridinium salts whether they show solvent-polarity-dependent fluorescence similar to the previous example or not. Their fluorescence spectra were measured in hexane, THF, dichloromethane, and ethanol (Figure 9). Bromides and tetrafluoroborates (2a, 2b, 3a, and 3b) have similar emission maxima in the range from 510 to 530 nm. Solid-state fluorescence of these salts is almost the same as in

Figure 9. Fluorescence spectra of (a) 2c and (b) 3c. Concentration: 1.0×10-4M; excitation wavelength: 405 nm.


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solution (Figure S10 in supporting information). Distinctive difference was observed for the fluorescence spectra of hexafluorophosphates 2c and 3c. In both cases, the longest fluorescence maximum wavelengths (541 – 544 nm) were observed in dichloromethane and in THF. In the case of 2c, the color of fluorescence is changed from blue (hexane) to green (EtOH) and then to yellow (THF, dichloromethane). Fluorescences of 3c in hexane, ethanol, and solid state are almost identical with emission maxima at 490 nm. However, bathochromic shifts of ca. 50 nm were observed in THF and in dichloromethane. The hexafluorophosphate anion have a closer contact with the positively charged pyridinium than other counter anions, which is indicated by the observation of high-field-shift of the protons in the vicinity of the hexafluorophsphate anion in 2c and 3c. 1H NMR spectra of them in CDCl3 showed that the ortho-protons of the pyridinium appeared at δ 8.9 in 2c and 3c while those of 2a, 2b, 3a, and 3b appeared at δ 9.5 – 9.6. The chemical shifts of benzylic protons of 2c and 3c are nearly the same and appeared at δ 5.9. In contrast, those of 2a, 2b, 3a, and 3b had lower shift values at δ 6.2 – 6.3. A counter anion should be settled in close vicinity to the corresponding cation. Because of the large size of hexafluorophosphate anion, it is probable that the ortho-protons of pyridinium and also the benzylic protons of 2c and 3c are located in the proximity of the anion. Therefore, these protons could be affected by shielding with the anion. Studies of ion solvation and ion association of ntetrabutylammonium hexafluorophosphate showed that relatively large association constants were observed in THF and in dichloromethane.63 Deducing from this observation, the pyridinium moieties of 2c and 3c have close contact with a hexafluorophosphate anion in THF and in dichloromethane. Possibly, this could effect on the fluorescence of them. Fluorescence lifetimes of all the salts examined in all the solvents employed (2~6 ns) indicate that emission of these salts originate in their monomers. The present results are different from


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Figure 10. Solvent-polarity dependent fluorescence spectra of 2c (a) and 3c (b) in varying ratios of hexane/THF. Concentration: 1.0×10-4 M; excitation wavelength: 405 nm. Inset photographs (irradiated with 375 nm uv light) show the changes of fluorescence color.

Figure 11. Fluorescence spectra of 3c in the solid state (a solid line) and in the film derived from the rapid cooling of its liquid crystalline state (a dashed line), excitation wavelength: 405 nm. Inset photographs show the color of fluorescence (a) in the solid state and (b) in the film irradiated with 375 nm uv light.

our previous findings of the similar fluorescent amphiphiles which showed both monomer and excimer-like emission. In those cases, the switching from monomer to excimer-like emission


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originated in the aggregation of the amphiphiles. In the present system, the pyridinuim moiety is directly connected to the anthracene ring which causes non-planarity of the anthracene and pyridinium rings. This non-planarity of the fluorophoric part could possibly avoid the generation of excimer. The ratiometric emission response was observed when fluorescence was measured in the mixed solvent system of hexane/THF. Figures 10a and 10b show fluorescence spectral change of 2c and 3c, respectively. Color of fluorescence changed from blue, green and then to yellow with increasing ratio of THF. As described in the section of liquid crystallinity, it is interesting that the twisted π-system like present molecules can exhibit liquid crystallinity to show columnar phases. Possibly, the molecules become slightly planer at the temperature exhibiting liquid crystallinity. This was confirmed by the comparison of the fluorescence spectra of their solid state and the film prepared by the rapid cooling of their liquid crystalline state. Figure 11 shows the fluorescence spectra of 3c in solid state and the film derived from its liquid crystalline state. Red shift of 21 nm at emission maximum wavelength was observed for the film. Similarly prepared film of 2c showed 9 nm of red-shift in its fluorescence.

Conclusion We have prepared piezoluminescent solvated crystals of 4,4’-(9,10-anthracenediyl)bispyridinium salts. Crystal structure analysis of the bispyridinium chlorides showed that they had channel structures in which fluorophoric anthracene moieties were segregated by the solvated molecules included in the channel. The Cl-…H-C interactions are responsible for the creation of the channel structures. Elimination of solvated molecules is triggered by grinding to bring about rearrangement of crystal structures for red-shifted emission. The emission color change was reversible. Vapoluminescent behaviors were observed to reproduce the original color of


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fluorescence before grinding. The DMSO solvate of the corresponding hexafluorophosphate also showed stimuli-responsive luminescent behaviors. The bispyridinium salts possessing long alkoxy chains exhibited liquid crystallinity. Rectangular columnar phases were observed. A film prepared by the rapid cooling of their liquid crystals showed fluorescence with longer emission wavelength than those of the corresponding solids. The present findings demonstrate that flexible crystal structures of organic salts bearing aromatic π-planes are useful for stimuli-responsive materials.

ASSOCIATED CONTENT Supporting Information. Tables of distances and angles of intermolecular hydrogen bonds, ORTEP diagrams, X-ray crystallographic information files (CIF) of compounds, XRD patterns and lattice constants of compounds 2 and 3, and fluorescence spectra of 2a, 2b, 3a, and 3b are available free of charge via the internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *(S.K.) E-mail: [email protected]. Tel: +81-43-290-3420 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENT This research is supported in part by a Grant-in-Aid for Scientific Research from Japan Society for the Promotion of Science (JSPS) (No. 2541008). SH is grateful for JSPS Research Fellowships for Young Scientists. REFERENCES 1) Sagara, Y.; Kato, T. Nature Chem. 2009, 1, 605–610. 2) Pucci, A.; Bizzarri, R.; Ruggeri, G. Soft Matter 2011, 7, 3689–3700. 3) Chi, Z.; Zhang, X.; Xu, B.; Zhou, X.; Ma, C.; Zhang, Y.; Liu, S.; Xu, J. Chem. Soc. Rev. 2012, 41, 3878–3896. 4) Ariga, K.; Mori, T.; Hill, J. P. Adv. Mater. 2012, 24, 158–176. 5) Zhang, X.; Chi, Z.; Zhang, Y.; Liu, S.; Xu, J. Mater. Chem. C 2013, 1, 3376–3390. 6) Varughese, S. J. Mater. Chem. C 2014, 2, 3499–3516. 7) Luo, J.; Li, L.-Y.; Song, Y.; Pei, J. Chem. Eur. J. 2011, 17, 10515–10519. 8) Yuan, W. Z.; Tan, Y.; Gong, Y.; Lu, P.; Lam, J. W. Y.; Shen, X. Y.; Feng, C.; Sung, H. H.Y.; Lu, Y.; Williams, I. D.; Sun, J. Z.; Zhang, Y.; Tang, B. Z. Adv. Mater. 2013, 25, 2837–2843. 9) Gong, Y.; Tan, Y.; Liu, J.; Lu, P.; Feng, C.; Yuan, W. Z.; Lu, Y.; Sun, J. Z.; He, G.; Zhang, Y. Chem. Commun. 2013, 49, 4009–4011.


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For Table of Contents Use Only

Piezoluminescence and Liquid Crystallinity of 4,4’-(9,10anthracenediyl)bispyridinium Salts

Shigeo Kohmoto,*† Tomotaka Chuko,† Shugo Hisamatsu,‡ Yasuhiro Okuda,‡ Hyuma Masu,†,§ Masahiro Takahashi,† and Keiki Kishikawa† †

Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

‡

Actinide Coordination Chemistry Group, Quantum Beam Science Directorate, Japan Atomic Energy Agency, Shirakatashirone 2-4, Tohkai-mura, Ibaraki 319-1195, Japan §

Center for Analytical Instrumentation, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

Synopsis: Piezochromic behaviors of pyridinium salts bearing an anthracene fluorophore were examined. Grinding of their solvated crystals resulted in the fluorescence color change from blue to green. The salts substituted with long alkyl chains exhibited monotropic rectangular columnar liquid crystalline phases.


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bispyridinium Salts - ACS Publications - American Chemical Society

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