Fluorine-containing bistolanes as light-emitting liquid crystalline molecules

Shigeyuki Yamada *a, Kazuya Miyano a, Tsutomu Konno a, Tomohiro Agou b, Toshio Kubota b and Takuya Hosokai c
aFaculty of Molecular Chemistry and Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan. E-mail: syamada@kit.ac.jp
bDepartment of Materials Science, Ibaraki University, 4-12-1 Nakanarusawa, Hitachi, 316-8511, Japan
cNational Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba 305-8568, Japan

Received 6th June 2017 , Accepted 24th June 2017

First published on 26th June 2017


We synthesised a series of dissymmetric bistolane derivatives and evaluated their liquid-crystalline (LC) and photoluminescence properties in detail. In measuring LC behaviours, rational structural design based on the dissymmetric molecular structure and electron-density distribution facilitated the production of the LC phase with a wide temperature range (up to 97 °C). In addition, dissymmetric bistolane derivatives were shown to strongly emit blue-photoluminescence in dilute solution and in crystalline states. It was found that dissymmetric bistolanes possess emissive features in even the LC phase and photoluminescence behaviours such as emission intensity and colour were sensitively switched depending on the molecular aggregate structure caused by applying a thermal stimulus.


Introduction

In recent years, enormous attention has been paid to the development of new light-emitting liquid crystalline (LELC) materials1 that possess both light-emitting and liquid-crystalline (LC) properties as next-generation display devices because such LELC materials are among the most promising candidates to overcome potent disadvantages of conventional LC display devices such as limited brightness and low energy efficiency.2 Nevertheless, the development of luminous molecules in the LC phase is one of the most strenuous tasks since conventional luminophores smoothly quench in the condensed phase, e.g., crystal and LC phases, due to intermolecular energy transfer3 (called the aggregation-caused quenching (ACQ) effect).4 Therefore, extensive efforts are devoted to developing a new class of luminophores that are strongly emissive in condensed phases.

Several intriguing phenomena reported by Tang and co-workers, called aggregation-induced emission enhancement (AIEE)5 or crystallisation-induced emission enhancement (CIEE),6 can show enhanced emission in the solid state by restricting intramolecular motion due to aggregation. Although these discoveries open avenues to new luminous molecules in crystalline phases, it is still desirable to develop new molecular systems with luminescent characteristics in mesophases.

To identify a new class of luminescent molecules in mesophases, we designed a research strategy to provide LC properties to potent luminophores. Among a number of luminophores, in this study, we selected the bistolane framework as a luminous core structure because of the linear molecular shape and expanded π-conjugation structure.7,8 Although there are several reports on bistolane compounds possessing either LC behaviour7 or photoluminescence in solution,8 little attention has been paid to bistolane derivatives with both luminous and LC properties. In addition to the bistolane framework, we focussed on fluorine-containing organic molecules with unique character9 as the fluorine atom has the highest electronegativity and small atomic radius, and forms C–F bonds with a large bond energy. The fact that rod-shaped organic molecules with fluorine or fluorine-containing substituents are used in LC display devices as the guest LC content encouraged us that the present molecular design should be effective.10

Therefore, in this study, we synthesised a series of bistolane derivatives without and with fluorine-substituents and evaluated their LC and photophysical properties in detail (Fig. 1). In addition, we tried to elucidate the relationship between their molecular structure and physical properties.


image file: c7ob01369h-f1.tif
Fig. 1 Structural motif for bistolane derivatives.

Experimental

General

1H- and 13C-NMR spectra were obtained with a Bruker AVANCE III 400 NMR spectrometer (1H: 400 MHz and 13C: 100 MHz) in chloroform-d (CDCl3) solution and the chemical shifts are reported in parts per million (ppm) using the residual proton in the NMR solvent. 19F-NMR (376 MHz) spectra were obtained with a Bruker AVANCE III 400 NMR spectrometer in CDCl3 solution with CFCl3 (δF = 0 ppm) as an internal standard. Infrared spectra (IR) were recorded in a KBr method with a JASCO FT/IR-4100 type A spectrometer; all spectra were reported in wavenumber (cm−1). High resolution mass spectra (HRMS) were recorded on a JEOL JMS-700MS spectrometer using fast atom bombardment (FAB) methods. Elemental analyses were conducted with a Yanaco CHN CORDER MT-5 instrument. All reactions were carried out using dried glassware with a magnetic stirrer bar. All chemicals were of reagent grade and where necessary were purified in the usual manner prior to use. Column chromatography was carried out on silica gel (Wakogel® 60N, 38–100 μm) and TLC analysis was performed on silica gel TLC plates (Merck, Silica gel 60F254).

Synthesis

Prototypical bistolane 1 was purchased from Wako Pure Chemical Industries, Ltd and the bistolane derivatives 2 were synthesised from (4-alkoxyphenyl)acetylene 3 with easy three-step manipulations based on (i) the Pd(0)-catalyzed Sonogashira cross-coupling reaction with 1-bromo-4-[2-(trimethylsilyl)ethynyl]benzene (4), (ii) the deprotection of the TMS group, and (iii) the Sonogashira cross-coupling reaction with the corresponding aromatic bromide (Scheme 1). A typical synthetic procedure for the final step to prepare 2 is described below. The molecular structures of bistolanes 2 obtained were characterised by multiple spectroscopic analyses, e.g. NMR, IR, and HRMS, and the purity was proved by elemental analysis to be sufficient to evaluate their phase transition and photoluminescence behaviours.
image file: c7ob01369h-s1.tif
Scheme 1 Synthetic route for bistolane derivatives 2. Reagents and conditions: (i) 3 (1.5 equiv.), 4 (1.0 equiv.), Cl2Pd(PPh3)2 (5 mol%), PPh3 (5 mol%), CuI (10 mol%), Et3N, reflux, 15 h; (ii) 5 (1.0 equiv.), K2CO3 (1.5 equiv.), MeOH, rt, 15 h; (iii) 6 (1.0 equiv.), ArBr (1.5 equiv.), Cl2Pd(PPh3)2 (5 mol%), PPh3 (5 mol%), CuI (10 mol%), Et3N, reflux, 15 h.

Typical procedure for the synthesis of 1-(2-phenylethynyl)-4-[4-methoxyphenylethynyl]benzene (2aA)

To a mixture of Cl2Pd(PPh3)2 (0.053 g, 0.075 mmol), PPh3 (22 mg, 0.075 mmol), 4-methoxyphenylehynylphenyl acetylene (6aA) (0.23 g, 1.0 mmol), bromobenzene (0.24 g, 1.5 mmol), and CuI (28 mg, 0.15 mmol) was added Et3N (20 mL) at 100 °C (bath temp.) under argon. The reaction mixture was stirred at that temperature for 15 h. After the solvent was removed using a rotary evaporator, the crude product was extracted with AcOEt and washed with saturated aqueous NH4Cl solution (three times) and brine (once). The organic layer was dried over anhydrous Na2SO4, filtered, and concentrated in vacuo. The resultant was subjected to silica-gel column chromatography (eluent: hexane/CH2Cl2 = 2/1) to provide the title compound (0.12 g, 0.40 mmol) in 40% yield as a white solid, which was further purified by recrystallisation from a mixed solvent of CH2Cl2 and methanol.

1-(2-Phenylethynyl)-4-[2-(4-methoxyphenyl)ethynyl]benzene (2aA)

Yield: 40% (colourless crystal); m.p.: 171 °C determined by DSC; 1H-NMR (CDCl3): δ 3.84 (s, 3H), 6.89 (d, J = 8.9 Hz, 2H), 7.33–7.38 (m, 3H), 7.45–7.47 (m, 1H), 7.48–7.50 (m, 5H), 7.51–7.53 (m, 2H); 13C-NMR (CDCl3): δ 55.3, 87.9, 89.2, 91.0, 91.3, 114.0, 115.1, 122.7, 123.1, 123.4, 128.3, 128.4, 131.3, 131.5, 131.6, 133.1, 159.8; IR (KBr): ν 3020, 2941, 2842, 2200, 1605, 1518, 1285, 1247, 1180, 1030, 838 cm−1; HRMS (FAB+) m/z [M+] calcd for C23H16O: 308.1201; found: 308.1202; anal. calcd for C23H16O: C, 89.58; H, 5.23. Found: C, 89.22; H, 5.36.

1-[2-[4-(Trifluoromethyl)phenyl]ethynyl]-4-[2-(4-methoxyphenyl)-ethynyl]benzene (2aB)

Yield: 39% (colourless crystal); m.p. 235 °C determined by DSC; 1H-NMR (CDCl3): δ 3.84 (s, 3H), 6.89 (d, J = 8.8 Hz, 2H), 7.48 (d, J = 8.8 Hz, 2H), 7.51 (s, 4H), 7.60–7.63 (m, 4H); 13C-NMR (CDCl3): δ 55.3, 87.7, 89.5, 91.5, 91.7, 114.1, 115.0, 121.9, 124.1, 125.3 (q, J = 3.9 Hz), 126.9, 130.0 (q, J = 32.4 Hz), 131.4, 131.6, 131.8, 133.1, 159.9, one carbon of 1JC–F could not be detected due to low solubility in CDCl3; 19F NMR (CDCl3): δ −63.30; IR (KBr): ν 2963, 2843, 2210, 1926, 1608, 1520, 1407, 1328, 1251, 1125, 1066, 836 cm−1; HRMS (FAB+) m/z [M+] calcd for C24H15F3O: 376.1075; found: 376.1075; anal. calcd for C24H15F3O: C, 76.59; H, 4.02. Found: C, 76.60; H, 4.27.

1-[2-(2,3,4,5,6-Pentafluorophenyl)ethynyl]-4-[2-(4-methoxyphen-yl)ethynyl]benzene (2aC)

Yield: 62% (colourless crystal); m.p.: 149 °C determined by DSC; 1H-NMR (CDCl3): δ 3.83 (s, 3H), 6.89 (d, J = 8.8 Hz, 2H), 7.49 (d, J = 8.8 Hz, 2H), 7.51 (d, J = 8.6 Hz, 2H), 7.55 (d, J = 8.6 Hz, 2H); 19F-NMR (CDCl3): δ −136.4 to −136.5 (m, 2F), −152.97 (t, J = 21.2 Hz, 1F), −162.1 to −162.3 (m, 2F); IR (KBr): ν 3019, 2940, 2843, 2211, 1922, 1597, 1496, 1252, 1172, 1028, 985, 836 cm−1; HRMS (FAB+) m/z [M+] calcd for C23H11F5O: 376.1075; found: 376.1075; anal. calcd for C23H11F5O: C, 69.35; H, 2.78. Found: C, 69.57; H, 2.86. The 13C spectra cannot be obtained due to extremely low solubility in deuterated solvent.

1-[2-(4-Ethoxyphenyl)ethynyl]-4-[2-(2,3,4,5,6-pentafluorophenyl)-ethynyl]benzene (2bC)

Yield: 61% (colourless crystal); m.p.: 143 °C determined by DSC; 1H-NMR (CDCl3): δ 1.43 (t, J = 7.0 Hz, 3H), 4.06 (q, J = 7.0 Hz, 2H), 6.88 (d, J = 8.9 Hz, 2H), 7.46 (d, J = 8.9 Hz, 2H), 7.48–7.52 (m, 4H); 13C-NMR (CDCl3): δ 14.7, 63.7, 74.4, 87.5, 92.3, 100.2 (t, J = 16.9 Hz), 101.2, 114.6, 114.7, 120.7, 125.1, 131.4, 131.8, 133.2, 137.7 (dm, J = 253.9 Hz), 141.5 (dm, J = 256.8 Hz), 147.1 (dm, J = 252.5 Hz), 159.3; 19F-NMR (CDCl3): δ −136.4 to −136.5 (m, 2F), −152.99 (t, J = 20.7 Hz, 1F), −162.1 to −162.4 (m, 2F); IR (KBr): ν 3018, 2982, 2203, 1595, 1523, 1497, 1285, 1247, 1046, 992, 837 cm−1; HRMS (FAB+) m/z [M+] calcd for C24H13F5O: 412.0887; found: 412.0890; anal. calcd for C24H13F5O: C, 69.91; H, 3.18. Found: C, 69.54; H, 3.55.

1-[2-(2,3,4,5,6-Pentafluorophenyl)ethynyl]-4-[2-(4-n-propylphen-yl)ethynyl]benzene (2cC)

Yield: 25% (colourless crystal); m.p.: 144 °C determined by DSC; 1H-NMR (CDCl3): δ 1.05 (t, J = 7.2 Hz, 3H), 1.82 (qt, J = 7.2, 6.8 Hz, 2H), 3.94 (t, J = 6.8 Hz, 2H), 6.88 (d, J = 8.8 Hz, 2H), 7.46 (d, J = 8.8 Hz, 2H), 7.49–7.56 (m, 4H); 13C NMR (CDCl3): δ 10.5, 22.5, 69.6, 74.5, 87.5, 92.4, 100.2 (t, J = 16.7 Hz), 101.2, 114.6, 120.7, 125.1, 131.4, 131.8, 133.1, 136.3–139.0 (dm, J = 251.8 Hz, 1C), 140.0–142.8 (dm, J = 256.1 Hz, 1C), 145.8–148.4 (dm, J = 248.9 Hz, 1C), 159.5, one sp2-carbon signal attached to sp-carbon in the C6H4 moiety is overlapped with a signal at 114.6 ppm; 19F-NMR (CDCl3): δ −136.4 to −136.5 (m, 2F), −152.99 (t, J = 20.7 Hz, 1F), −162.1 to −162.4 (m, 2F); IR (KBr): ν 3019, 2966, 2210, 1597, 1526, 1496, 1251, 1113, 984, 838 cm−1; HRMS (FAB+) m/z [M+] calcd for C25H15F5O: 426.1043; found: 426.1035; anal. calcd for C25H15F5O: C, 70.42; H, 3.55. Found: C, 70.01; H, 3.95.

1-[2-(4-n-Butoxyphenyl)ethynyl]-4-[2-(2,3,4,5,6-pentafluorophen-yl)ethynyl]benzene (2dC)

Yield: 16% (pale yellow crystal); m.p.: 122 °C determined by DSC; 1H NMR (CDCl3): δ 0.98 (t, J = 7.2 Hz, 3H), 1.50 (sext., J = 7.2 Hz, 2H), 1.74–1.82 (m, 2H), 3.99 (t, J = 6.4 Hz, 2H), 6.88 (d, J = 8.4 Hz, 2H), 7.46 (d, J = 8.4 Hz, 2H), 7.50–7.56 (m, 4H); 13C-NMR (CDCl3): δ 13.8, 19.2, 31.2, 67.8, 74.5, 87.5, 92.4, 100.2 (t, J = 17.5 Hz), 101.4, 114.58, 114.6, 120.7, 125.1, 131.4, 131.8, 133.1, 136.2–139.0 (dm, J = 248.1 Hz, 1C), 140.2–143.0 (dm, J = 262.0 Hz, 1C), 145.6–148.4 (dm, J = 256.9 Hz, 1C), 159.6; 19F-NMR (CDCl3): δ −136.3 to −136.5 (m, 2F), −152.99 (t, J = 20.3 Hz, 1F), −162.1 to −162.3 (m, 2F); IR (KBr): ν 3016, 2964, 2863, 2204, 1595, 1523, 1498, 1246, 1137, 992, 960, 831 cm−1; HRMS (FAB+) m/z [M+] calcd for C26H17F5O: 440.1200; found: 440.1199; anal. calcd for C26H17F5O: C, 70.91; H, 3.89. Found: C, 70.59; H, 4.30.

1-[2-(2,3,4,5,6-Pentafluorophenyl)ethynyl]-4-[2-(4-pentyloxyphen-yl)ethynyl]benzene (2eC)

Yield: 20% (pale yellow crystal); m.p.: 128 °C determined by DSC; 1H-NMR (CDCl3): δ 0.94 (t, J = 6.8 Hz, 3H), 1.38–1.48 (m, 4H), 1.76–1.83 (m, 2H), 3.96 (t, J = 6.6 Hz, 2H), 6.88 (d, J = 8.6 Hz, 2H), 7.46 (d, J = 8.6 Hz, 2H), 7.51 (d, J = 8.0 Hz, 2H), 7.54 (d, J = 8.0 Hz, 2H); 13C-NMR (CDCl3): δ 14.0, 22.4, 28.2, 28.9, 68.1, 74.5, 87.5, 92.4, 100.2 (t, J = 18.3 Hz), 101.3, 114.5, 114.6, 120.7, 125.1, 131.4, 131.8, 133.1, 136.2–139.1 (dm, J = 250.1 Hz, 1C), 140.0–142.9 (dm, J = 256.7 Hz, 1C), 145.6–148.5 (dm, J = 249.4 Hz, 1C), 159.6; 19F-NMR (CDCl3): δ −136.4 to −136.5 (m, 2F), −153.00 (t, J = 20.6 Hz, 1F), −162.2 to −162.3 (m, 2F); IR (KBr): ν 3019, 2933, 2862, 2204, 1596, 1497, 1286, 1248, 992, 833 cm−1; HRMS (FAB+) m/z [M+] calcd for C27H19F5O: 454.1356; found: 454.1363; anal. calcd for C27H19F5O: C, 71.36; H, 4.21. Found: C, 71.25; H, 4.65.

1-[2-(2,3,4,5,6-Pentafluorophenyl)ethynyl]-4-[2-(4-hexyloxyphen-yl)ethynyl]benzene (2fC)

Yield: 34% (white solid); m.p.: 120 °C determined by DSC; 1H-NMR (CDCl3): δ 0.90–0.94 (m, 3H), 1.32–1.37 (m, 4H), 1.43–1.48 (m, 2H), 1.75–1.83 (m, 2H), 3.96 (t, J = 6.6 Hz, 2H), 6.88 (d, J = 8.9 Hz, 2H), 7.46 (d, J = 8.9 Hz, 2H), 7.50–7.56 (m, 4H); 13C-NMR (CDCl3): δ 14.0, 22.6, 25.7, 29.1, 31.5, 68.1, 74.5 (q, J = 3.7 Hz), 87.5, 92.4, 100.2 (td, J = 18.4, 3.7 Hz), 101.2 (q, J = 2.9 Hz), 114.5, 114.6, 120.7, 125.1, 131.4, 131.8, 133.1, 136.2–139.0 (dm, J = 245.7 Hz, 1C), 140.0–143.0 (dm, J = 256.8 Hz, 1C), 145.6–148.5 (dm, J = 253.0 Hz, 1C), 159.5; 19F-NMR (CDCl3): δ −136.4 to −136.5 (m, 2F), −153.00 (t, J = 20.7 Hz, 1F), −162.3 to −162.4 (m, 2F); IR (KBr): ν 3019, 2960, 2848, 2205, 1523, 1496, 1245, 992, 832 cm−1; HRMS (FAB+) m/z [M+] calcd for C28H21F5O: 468.1513; found: 468.1513; anal. calcd for C28H21F5O: C, 71.79; H, 4.52. Found: C, 72.03; H, 4.73.

X-ray crystallography

Single crystals of the bistolanes 2aC and 2eC were obtained by slow evaporation from a mixed solvent system (dichloromethane/methanol = 1/1) and mounted on a glass fibre. All measurements for 2aC were made on a diffractometer with filtered Mo-Kα radiation (λ = 0.71075 Å) and a rotating anode generator using a VariMax with PILATUS/DW (Rigaku); all calculations were performed using the CrystalStructure crystallographic software package. The structure was solved by direct methods and expanded using Fourier techniques. On the other hand, the omega scanning technique was used to collect the reflection data of 2eC using a Bruker D8 goniometer with monochromatised Mo-Kα radiation; the initial structure of the unit cell was determined by direct methods using APEX2. The structural model was refined by a full-matrix least-squares method using SHELXL-2014/6.11 All calculations were performed using the SHELXL program. The crystal data for bistolanes 2aC and 2eC are summarised in the ESI and the indexed data have been deposited in the Cambridge Crystallographic Data Centre (CCDC) database (CCDC 1552781 for 2aC and 1552782 for 2eC).12

Phase transition properties

The phase transition properties of bistolanes were observed by polarizing optical microscopy (POM) using an Olympus BX51 microscope equipped with a temperature-controlled stage (Instec HCS302 microscope, hot and cold stages, and mK1000 temperature controller). The thermodynamic parameters were determined using a DSC (SII X-DSC7000) at heating and cooling rates of 5.0 °C min−1. At least three scans were conducted to confirm reproducibility.

Photophysical properties

UV-Vis absorption spectra were recorded using a JASCO V-500 absorption spectrometer. Steady-state photoluminescence spectra were obtained using a Hitachi F-7000 or JASCO FP-8500 fluorescence spectrometer. Photoluminescence quantum yields were estimated using a calibrated integrating sphere system (Hitachi or JASCO). Transient photoluminescence measurements were conducted using a Fluorocube fluorescence lifetime system (HORIBA).

Computation

All computations were performed with density functional theory (DFT) using the Gaussian 09 (rev. D.01) package.13 Geometry optimisations were executed with the B3LYP hybrid functional and Dunning's correlation consistent basis set cc-pVDZ.14 The stationary points were characterised by frequency calculations to confirm that the minimum energy structures had no imaginary frequencies.

Results and discussion

Crystal structures of bistolane derivatives 2aC and 2eC

Among the bistolane derivatives 2 synthesised in this study, fluorinated bistolane 2aC (with a methoxy group) and 2eC (carrying a pentoxy chain) furnished single crystals suitable for X-ray crystallographic analysis after dual purification by silica-gel column chromatography and recrystallisation from the dichloromethane/methanol system.

Bistolane 2aC was found to crystallise in the P21/n monoclinic space group and 2eC afforded crystals in the C1c1 monoclinic space group. The unit cells of 2aC and 2eC contained four molecules each.

Viewing the crystal structures of 2aC and 2eC from the top (Fig. 2a and d) and side (Fig. 2b and e), the three aromatic rings formed a coplanar structure in the crystal lattice, though the three aromatic rings generally may be in rapid equilibrium between coplanar and twisted conformations through free rotation of alkyne–aryl single bonds.8a In sharp contrast, the two bistolanes provided significantly different packing structures, as shown in Fig. 2c. 2aC (with a methoxy substituent) had an anti-parallel dimer structure located in the centre that was π-stacked (π⋯π distance: 343 pm on average) between electron-deficient pentafluorophenyl and relatively electron-abundant neighbouring aromatic rings. 2ac underwent further stabilisation to form crystals through rigid CH/π interactions (CH⋯π distance: 295 pm on average) with two other molecules. The shortest contacts of both π/π and CH/π interactions were almost equal to the sum of van der Waals (vdW) radii (carbon: 170 pm and hydrogen: 120 pm),15 indicating that both interactions are attributable to crystal formation. On the other hand, as shown in Fig. 2f, 2eC (with a pentoxy chain) preferentially formed a parallel dimer structure, which was a distinctly different structure from that of 2aC. On carefully analysing the packing structures of 2eC, a large stabilisation effect was observed involving the π/π interaction (π⋯π: 322 pm on average), CH/π interaction (CH⋯π: 291 pm on average), and van der Waals interaction between the C5H11 chains (HalkylHalkyl: 313 pm on average).


image file: c7ob01369h-f2.tif
Fig. 2 Crystal structures of 2aC and 2eC. (a), (d) Top view; (b), (e) side view; (c), (f) packing structures. Colour legend: C, grey; H, white; O, red; F, light green.

Phase transition properties

The phase transition properties of bistolanes 2aA–2fC were evaluated by polarising optical microscopy (POM) and differential scanning calorimetry (DSC). The phase sequences and transition temperatures observed are summarised in Table 1. The phase transition properties of a prototypical 1 are also shown for comparison.
Table 1 Thermodynamic behaviours of the bistolane derivatives
Bistolane Phase transition behavioura,b [°C]
a Abbreviations: Cr, crystalline; SmA, smectic A; N, nematic; Iso, isotropic. b Data obtained from DSC scans at a rate of 5.0 °C min−1. Phase transition temperatures were determined from the second cycles as the onset of the endothermic signals.
1 Heating Cr 183 Iso
Cooling Cr 173 Iso
2aA Heating Cr 171 N 181 Iso
Cooling Cr 154 N 181 Iso
2aB Heating Cr 235 N 253 Iso
Cooling Cr 228 N 253 Iso
2aC Heating Cr 149 N 225 Iso
Cooling Cr 140 N 225 Iso
2bC Heating Cr 143 N 238 Iso
Cooling Cr 140 N 239 Iso
2cC Heating Cr 144 N 220 Iso
Cooling Cr 123 N 220 Iso
2dC Heating Cr 122 SmA 134 N 215 Iso
Cooling Cr 117 SmA 136 N 216 Iso
2eC Heating Cr 128 SmA 143 N 201 Iso
Cooling Cr 96 SmA 144 N 202 Iso
2fC Heating Cr 120 SmA 152 N 197 Iso
Cooling Cr 90 SmA 153 N 197 Iso


Prototypical bistolane 1 (without any flexible moieties) exhibited a direct phase transition from crystal (Cr) to the isotropic (Iso) phase, indicating that 1 did not show an LC phase.16 In sharp contrast, bistolane 2aA with a dissymmetric structure displayed a bright-field image with fluidity under POM observation at 171–181 °C, which means that 2aA possesses LC characteristics. Judging from the four-brush schlieren textured image in the LC phase (Fig. S2 in ESI), it was concluded that the LC phase of 2aA was a nematic (N) phase. It is reasonably explained that the distinct phase transition behaviour between 2aA and 1 may be attributed to the enhancement of the flexibility and a dissymmetric molecular structure by introducing a methoxy substituent at the molecular edge. Further substitution of a CF3 group into another molecular edge, viz.2aB, also showed the N LC phase at 235–253 °C, and the N LC region was found to shift to higher temperature. Significant increases in phase-transition temperatures of Cr → N (Tm, melting temperature) and N → Iso (Tc, clearing temperature) can be attributed to the increase of molecular rigidity triggered by the extension of π-conjugation through the electron-density distribution.

Subsequently, we demonstrated the phase transition properties of a series of bistolane derivatives (2aC–2fC) with a pentafluorophenyl ring as the electron-deficient aromatic ring since it was reported that laterally substituted fluorine atoms effectively decrease the phase transition temperature (Tm) of Cr → LC due to the atomic radius 1.2 times larger than that of hydrogen.17 The phase transition behaviour of 2aC revealed that Tm and Tc for 2aC were 149 °C and 225 °C, respectively (LC temperature range (ΔTLC): 76 °C). Consequently, the electron-deficient characteristic and steric effect of the lateral fluorine substituents of the pentafluorophenyl moiety both proved to successfully effect the significant expansion of ΔTLC.

As summarised in Table 1, other dissymmetric bistolanes 2bC–2fC with various lengths of the alkoxy chain also exhibited LC phases, but different LC behaviours depending on the alkoxy chain length. Comparing the phase sequences observed in 2aC–2fC (1 ≤ n ≤ 6), only the N LC phase was seen in 2aC–2cC (with a shorter alkoxy chain; n ≤ 3), whereas 2dC–2fC (carrying a longer alkoxy chain; n ≥ 4) was found to provide two types of LC phases: we observed the phase sequence Cr → LC1 → LC2 → Iso. The POM measurements supported the determination of the LC1 and LC2 phases; a fan-shaped texture image was observed in the LC1 phase and a four-brushed schlieren texture image was obtained in the LC2 phase. Judging from the optical textures, LC1 and LC2 can be characterised as smectic (Sm) and N phases, respectively (Fig. S3). Additionally, it was found that shear stress to the fan-shaped texture in the Sm phase caused significant changes in the POM images from the bright-field to the dark-field image. This phenomenon can be usually observed when the molecular alignment switches from homogenous to homeotropic; texture switching behaviour is a typical observation in the SmA phase.18 Considering the value of transition enthalpy (ΔH) (Table S2 in ESI), the ΔH values in SmA/N phase transitions of 2dC–2fC were 0.30, 0.69 and 0.86 kJ mol−1, respectively. It was reported that the ΔH in SmA ⇄ N was 0.05–1.10 kJ mol−1, whereas ca. 2.1 kJ mol−1 of ΔH was required in the phase transition of SmB ⇄ N.19 Judging from ΔH in the Sm/N phase transition, the obtained Sm phase in this study can be safely characterized as the SmA phase, not the SmB phase as well. As a consequence, the phase transition properties of 2dC–2fC (with a longer alkoxy chain) showed distinct properties from that of 2aC–2cC (with a short flexible chain).

To understand the effect of the alkoxy chain length among 2aC–2fC on the phase transition behaviours, the phase transition temperatures were plotted as shown in Fig. 3.


image file: c7ob01369h-f3.tif
Fig. 3 Phase transition behaviours of 2aC–2fC depending on the chain length (CnH2n+1).

Comparing Tc (N → Iso), the highest Tc (238 °C) was observed in 2bC and the Tc values tended to gradually reduce with extension of the alkoxy chain length. It can be rationally explained by thermal activation of the molecular motion of the alkoxy chain to induce smooth destruction of the molecular aggregate. Similarly, Tm (Cr → LC) was observed to reduce with a longer alkoxy chain length. The opposite tendency was observed in SmA → N phase transitions: increasing alkoxy chain length led to higher phase transition temperature due to the increased stability of layer structures through larger van der Waals forces.

Photophysical behaviour

As mentioned above, bistolane derivatives are known to possess intriguing photoluminescence behaviours due to their extended π-conjugation.8 Therefore, our interest was directed toward the investigation of the photophysical properties of dissymmetric bistolane derivatives 2.

Initially, we obtained the absorption and photoluminescence spectra for 2 in dilute dichloromethane solution. Fig. 4a and b show absorption and emission spectra, respectively, and the photophysical data obtained are summarised in Table 2.


image file: c7ob01369h-f4.tif
Fig. 4 (a) Absorption spectra in dilute solution (CH2Cl2, 10−5 mol L−1), (b) corrected photoluminescence spectra in CH2Cl2 solution (10−6 mol L−1), and (c) HOMO/LUMO and the corresponding energies.
Table 2 Molecular orbital energies obtained from DFT calculation and the photophysical data
Compound HOMO/LUMO (ΔEg)a [eV] λ abs[thin space (1/6-em)]b [nm] (ε × 10−3 [L mol−1 cm−1]) Emission in the solution statec Emission in the crystalline state
λ em [nm] Φ em λ em [nm] Φ em
a Estimated from DFT calculation using the Gaussian 09 program package and the computations were performed at the B3LYP/cc-pVDZ level of theory. b Measured in CH2Cl2 solution in 10−5 mol L−1 concentration. c Measured in 10−6 mol L−1 solution in CH2Cl2.
1 −5.62/−1.96 (3.66) 320 (55.6), 340 (34.7) 348, 368 0.86 444
2aA −5.39/−1.82 (3.57) 325 (51.5), 345 (34.9) 377 0.85 449 0.88
2aB −5.56/−2.12 (3.44) 330 (55.1), 350 (38.8) 394 0.77 420 0.72
2aC −5.62/−2.18 (3.44) 332 (54.2) 403 0.79 419 0.66
2bC −5.60/−2.17 (3.43) 331 (50.3) 411 0.81 458 0.47
2cC −5.60/−2.17 (3.43) 332 (53.6) 409 0.69 420 0.68
2dC −5.59/−2.17 (3.43) 332 (53.0) 409 0.88 455 0.53
2eC −5.59/−2.17 (3.42) 333 (50.1) 405 0.70 450 0.28
2fC −5.59/−2.17 (3.42) 333 (51.7) 407 0.76 450 0.47


It was found that all molecules, 1 and 2, showed absorption bands only in the UV region (200–400 nm), indicating that the bistolanes showed transparency over the entire visible-light region, which is an important feature for practical application to light-emitting devices. Prototype 1 showed absorption maxima at 320 nm and 340 nm, while the absorption bands and absorption edges at the lowest energies for dissymmetric bistolanes (e.g.2aA, 2aB and 2aC) gradually shifted to longer wavelengths (Fig. 4a) due to the change in electron-distribution caused by electronic characteristics of the substituents.

To understand the long-wavelength shift in 2, calculations were carried out using density functional theory (DFT) with the B3LYP functional and the cc-pVDZ basis set. Fig. 4c shows frontier molecular orbitals and orbital energies for selected examples. In dissymmetric bistolanes 2 with electron-abundant and -deficient aromatic rings, e.g.2aB and 2aC, the HOMO is mainly localised over the alkoxy-substituted benzene ring and the LUMO is mainly delocalised over the electron-deficient aromatic ring. Hence, the HOMO → LUMO electronic transition in 2 is associated with π–π* and/or intramolecular charge transfer (ICT). The calculated HOMO–LUMO energy gaps (ΔEg) were 3.66 eV for 1, 3.57 eV for 2aA, 3.44 eV for 2aB and 2aC, and 3.42 eV for 2eC; the sequence was in good agreement with the sequence of the absorption edge, concluding that the wavelength shift of absorption spectra can be attributed to a change of the electronic structure caused by the electronic features of substituents.

Subsequently, when measuring the photoluminescence of 1 and 2 in dilute solution, prototype bistolane 1 showed a sharp emission signal at 348 nm with a slightly broad emission band at 368 nm, which stemmed from the vibrational structure, whereas other bistolane derivatives 2 exhibited a single photoluminescence band with an emission maximum at 377–411 nm.20 In fact, 1 in dilute solution was visible to purple photoluminescence under UV irradiation (λex = 365 nm) and 2 was observed by the naked eyes to emit blue photoluminescence (Φem = 0.69–0.88) (Fig. S4).

The spectral shift to a long wavelength was also observed with a similar tendency to the absorption behaviours. Comparing spectral shapes, the dissymmetric bistolane 2 formed a single emission band unlike prototype 1, which is because the photoemission in 2 stemmed from multiple transition processes like π–π* and ICT due to the large electron-distribution. In addition, by demonstrating transient photoluminescence measurements, we measured the time constants (τ) for the selected examples, viz.2aA, 2aB, and 2aC; their τ values were in the range of 1.1–1.3 ns, concluding that the strong photoluminescence observed in dilute solution is fluorescence (Fig. S6 and Table S3). Owing to the similar molecular and electronic structures among 2aC–2fC, it can be suggested that the photoluminescence from other dissymmetric bistolanes (2bC–2fC) in dilute solution might also be fluorescence.

Our next interest was in the photoluminescence properties of dissymmetric bistolane derivatives 2 in condensed phases, since the development of new luminescent materials in condensed phases is still challenging, as mentioned before. Therefore, we examined the photoluminescence properties of bistolanes 2 in crystalline and LC phases. Fig. 5a shows the emission spectra of dissymmetric bistolanes 2, which were prepared from a recrystallisation process, as well as prototypical 1; the photophysical data are also listed in Table 2.


image file: c7ob01369h-f5.tif
Fig. 5 (a) Corrected photoluminescence spectra of the bistolane derivatives 1 and 2 in the crystal after recrystallisation; inset: CIE colour diagram obtained from the corresponding spectrum. (b) Photographs of dissymmetric bistolanes 2 in the crystal irradiated by using a UV lamp (λex = 365 nm).

All molecules were observed to emit photoluminescence with a single emission maximum at 420 nm or 450 nm, whose emission colour was visible in the deep-blue range when irradiated with UV light (Fig. 5b). Comparing the photoluminescence spectra of 1, 2aA, and 2aB–C, those without any fluorine-substituent (1 and 2aA) emitted photoluminescence with emission maximum around 450 nm, whereas 2aB–C (with a fluorine-containing electron-deficient aromatic ring) exhibited photoluminescence with an emission band around 420 nm. As described before, in dilute solution, 2aB–C with low ΔEg emitted photoluminescence at a long wavelength, which was the opposite tendency to the luminescence behaviour in the crystal. Photoluminescence in dilute solution (10−6 mol L−1) is generally considered a radiative S1 → S0 transition involving a sole molecule, not a molecular aggregate, while crystals can be considered as molecular aggregates orderly fixed by plural intermolecular interactions. To understand the opposite luminescence behaviours in solution and in crystal phases, it is crucial to consider the molecular aggregates in the crystalline state. Therefore, taking the crystal structures of 1 and 2aC into consideration, 1 was reported to form herringbone-type packing structures via CH/π interactions (Fig. S1),21 while 2aC constructed molecular aggregates via both π/π and CH/π intermolecular interactions. Face-to-face packing, e.g. π/π interactions, can promote intermolecular charge transfer to induce a smooth S1 → S0 transition, therefore suggesting that 2aC results in blue photoluminescence at shorter wavelengths.

In the bistolanes 2aC–2fC with a pentafluorophenyl moiety, preliminary DFT calculations led us to anticipate that all moieties may show the same photoluminescence properties including the spectral shape and emission wavelength. However, we observed intriguing photoluminescence characteristics that showed different emission bands depending on the alkoxy chain length. Thus, 2aC (methoxy group) and 2cC (propoxy group) emitted deep-blue photoluminescence with a sharp emission band at ca. 420 nm, whereas the other bistolanes ethoxy-substituted 2bC, and butoxy-, pentoxy-, and hexoxy-substituted 2dC–2fC were found to emit light-blue photoluminescence with a broad emission band at 450–458 nm. As discussed above, the photoluminescence properties in the crystal are strongly affected by the molecular aggregates involving intermolecular interactions. As described in Fig. 2c and f, both 2aC (λem = 420 nm) and 2eC (λem = 450 nm) formed molecular aggregates in their crystalline states, but the packing structures obtained were distinctly different from each other: the former constructed a dimer structure stabilised by π/π and CH/π interactions aligned in an anti-parallel direction; the latter formed dimers involving π/π, CH/π and van der Waals interactions in a parallel direction. From the crystal structures of 2aC and 2eC, a slight difference in the closest interatomic distances between two molecules and intermolecular interactions was observed. In fact, the closest interatomic distances appearing for the π/π interaction were 343 pm for 2aC and 322 pm for 2eC, and the distances of the CH/π interaction were 295 and 291 pm for 2aC and 2eC, respectively. From the distances of the shortest interatomic contact, it was obvious that 2eC (with a long pentoxy chain) showed a stronger π/π interaction between molecules than 2aC (with a short methoxy group), leading to a longer wavelength shift of the emission band.22 In addition, comparing the difference of emission maxima in solution and in crystalline states (Δλem(solution–crystal)) for 2aC and 2eC, as shown in Table 2, the former 2aC was only 16 nm, whereas in the latter, the larger long-wavelength shift (45 nm) was observed in accordance with the variation in the molecular circumstance. These results imply that the strengths of intermolecular interactions worked in the crystal are different in both molecules: in 2aC, the crystal lattice was constructed with weak intermolecular π/π interactions triggered by CH/π interactions, resulting in a similar emission in the short-wavelength region to the luminescence in solution, whereas 2eC formed a crystal with stronger π/π intermolecular interactions without any disturbance. In our hypothesis based on the experimental facts, we consider that the strength of intermolecular π/π interaction worked in the crystal is a potent factor to induce a slight longer-wavelength shift of emission maxima in 2eC, rather than 2aC, though it is still unclear due to the complexity of the photochemistry in the excited states. Table 2 also revealed that 2cC emitted photoluminescence at a shorter wavelength at 420 nm, the Δλem(solution–crystal) was only 11 nm, which was the same tendency as 2aC. Based on the above arguments, emission of 2cC in the crystal is likely to appear from similar situations to 2aC, which may form anti-parallel dimer structures with weak intermolecular π/π interactions.

Carrying out the transient emission measurements of 2aA, 2aB, and 2aC in the crystalline phase, τ values were found to be 2.47–3.19 ns (Fig. S6 and Table S3), which indicates that the blue photoluminescence observed in the crystal was fluorescence, similar to the photoluminescence in solution. The values of kr and knr calculated from the τ value observed in the crystal were 108 s−1 and 107–108 s−1 (Table S3), respectively, from which molecular aggregates in the crystal contributed to suppress non-radiative quenching processes, leading to strong photoluminescence in the crystalline phase.

Finally, our interests were directed toward the photoluminescence behaviour of the dissymmetric bistolanes 2 in LC phases. To evaluate the temperature dependence of photoluminescence, we measured the photoluminescence of bistolanes 2aC and 2eC as selected examples with varying measurement temperature. The measurement was conducted in two heating and cooling cycles (30 ↔ 160 °C);23 the emission spectra obtained during the first cooling and the second heating processes are shown in Fig. 6a (2aC) and Fig. 6b (2eC).


image file: c7ob01369h-f6.tif
Fig. 6 (a) Temperature dependence of the photoluminescence behaviour of 2aC (λex = 365 nm). Spectrum in the N phase (blue-dashed thick line) at 160 °C after the 1st heating, spectrum in the Cr phase (red-solid line) at 30 °C after the 1st cooling, spectra in the Cr phase (red-dashed thin line) in the 2nd heating, spectra in the N phase (blue-dashed thin line) in the 2nd heating and spectrum in the N phase (blue-solid line) at 160 °C in the 2nd heating process; inset: plots of emission intensity at λmax during 1st cooling (blue square) and 2nd heating (red circle) processes. (b) Temperature-dependence of the photoluminescence behaviour of 2eC (λex = 340 nm). Spectrum in the N phase (blue-dashed thick line) at 160 °C after the 1st heating, spectrum in the Cr phase (red-solid line) at 30 °C after the 1st cooling, spectra in the Cr phase (red-dashed thin line) in the 2nd heating, spectrum in the SmA phase (green-solid line) at 140 °C in 2nd heating, spectra in the SmA phase (green-dashed thin line), spectra in the N phase (blue-dashed thin line) and spectrum in the N phase (blue-solid line) at 160 °C in the 2nd heating process; inset: emission spectra of 2eC in Cr at 30 °C (red-solid line) after the 1st cooling, SmA phase at 140 °C (green-solid line) in the 2nd heating, spectrum of 2aC in Cr (red-dashed line) and spectrum of 2eC in Cr (green-dashed line) prepared by recrystallisation. (c) CIE colour diagram of 2eC at 30 °C (Cr), 140 °C (SmA) and 160 °C (N) during the 2nd heating process.

2aC was shown to emit deep-blue photoluminescence even at 160 °C (N phase, blue-dashed line in Fig. 6a). When reducing the measurement temperature, it was found that the photoluminescence intensity gradually increased ∼2.7 times higher while maintaining the spectral shape as at 30 °C (Cr phase after heating/cooling processes, red-solid line). Continuous heating to 160 °C to induce a phase-transition from the Cr to N phase (blue-solid line) resulted in restoring the emission intensity to the initial intensity, i.e., 2aC was proved to possess reversible photoluminescence characteristics during the Cr ↔ N phase transition (Fig. 6a (inset)). The depression of emission intensity by heating is a usual phenomenon because the thermal energy activates molecular motion to facilitate the non-radiative transition.

Similarly, we analysed the photoluminescence properties of 2eC with varying temperature (30 ↔ 160 °C). As shown in Fig. 6b, the emission intensity, spectral shape, and emission maxima were found to switch depending on the measurement temperature. In fact, 2eC in the N phase at 160 °C showed a broad emission band with an emission maximum at 430 nm (blue-dashed thick line), whereas the emission band in the Cr phase at 30 °C after the 1st cooling caused an enhancement of emission intensity by a factor of four and a short wavelength shift of the emission maximum by 30 nm (red-solid line). Continuously, raising the temperature to 140 °C caused a phase transition from Cr to SmA, at which point the emission band became slightly broadened and shifted to 450 nm (green-solid line). By heating up to 160 °C again, the emission intensity smoothly diminished to revert to the initial intensity and the emission maximum shifted back to 430 nm (blue-solid line). These observations clearly indicate that the emission behaviour in bistolane 2eC can be switched depending on the changes in the condensed phases caused by a thermal stimulus and the emission colour is controllable by changing the aggregated structure (Fig. 6c).

As shown in Fig. 6b (inset), moreover, unique phenomena were observed, as follows:

(i) Comparing the spectra of 2eC in crystals after recrystallisation (green-dashed line) and after heating/cooling processes (red-solid line), a slight wavelength-shift by 30 nm was observed, meaning that 2eC forms polymorphs depending on the crystallisation process.

(ii) The spectrum of 2eC in the Cr phase after recrystallisation (green-dashed line) was closely in agreement with that in the SmA phase in the 2nd heating (green-solid line), which indicates that one polymorph obtained by recrystallisation forms similar molecular aggregation to that in the SmA phase.

(iii) The spectrum of 2eC in the Cr phase after heating/cooling processes (red-solid line) was almost consistent with the spectrum of 2aC in the Cr phase after recrystallisation (red-dashed line), indicating that crystal 2eC re-constructs the crystal structure similar to 2aA after thermal transitions.

Resulting from the temperature-dependent photoluminescence spectra in 2eC, we hypothesised that the photoluminescence behaviour of 2eC switched sensitively depending on the molecular aggregates caused by applying a thermal stimulus: molecular aggregates with anti-parallel dimer structures caused by weak intermolecular π/π interactions, like 2aC (shown in Fig. 2c), display deep-blue photoluminescence emitted at 420 nm, whereas packing structures with strong intermolecular π/π interactions induced by the formation of parallel dimer structures, like 2eC (shown in Fig. 2f), may lead to light-blue photoluminescence emitted at 450 nm, though the molecular mechanism is still unclear. As a consequence, dissymmetric liquid-crystalline bistolane derivatives 2 can precisely switch photoluminescence behaviours, e.g. the emission intensity and emission colour, by changing the aggregated structure through heat-induced phase transitions, which would be a powerful tool in photoluminescence sensing materials.

Conclusions

In this study, we synthesised dissymmetric bistolane compounds with alkoxy-substituted electron-abundant aromatic and/or fluorine-containing electron-deficient aromatic rings in the same molecule and evaluated phase transitions and photoluminescence properties. All dissymmetric bistolanes were found to appear in liquid-crystalline phases and underwent significant stabilisation of the LC phase by incorporating fluorine-containing electron-deficient aromatic rings to induce an electron-density distribution. In the photoluminescence measurements, the dissymmetric bistolane derivatives were shown to intensively emit blue-photoluminescence in dilute solution and crystalline phases. It is noteworthy that the luminescence properties can be controlled depending on the changes in the electronic structure and molecular aggregate structure. In addition, we observed that 2 exhibited remarkable photoluminescence properties that emitted light even in LC phases. Thus, with photoluminescence measurement under heating/cooling, it was proved that the emission intensity and colour changed depending on the phase, based on molecular aggregated structures. These observations indicate that the luminous bistolanes would be promising photoluminescence switching materials, e.g. luminescence sensors, to easily control the molecular alignment through a thermal stimulus. These would contribute to developing novel luminous materials in LC phases. Further studies to determine the mechanisms for LC and photoluminescence behaviours and to develop new molecules with multiple functionalities such as LC, photoluminescence, etc., are currently underway in our laboratory.

Acknowledgements

This work was partially supported by The Kyoto Technoscience Center and the Hitachi Metals · Materials Science Foundation. We thank Prof. Osamu Tsutsumi at Ritsumeikan Univ. and Profs. Kensuke Naka and Hiroaki Imoto at the Kyoto Institute of Technology for kindly performing physical property measurements.

Notes and references

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  22. For description of the emission wavelength shift induced by the strength of π/π interactions, but the opposite phenomenon from our molecules, see: T. Han, X. Gu, J. W. Y. Lam, A. C. S. Leung, R. T. K. Kwok, T. Han, B. Tong, J. Shi, Y. Dong and B. Z. Tang, J. Mater. Chem. C, 2016, 4, 10430 RSC .
  23. Phase transition sequences of 2aC and 2eC in the 1st heating process are as follows: Cr 149 N 226 Iso for 2aC and Cr 128 SmA 143 N 201 Iso for 2eC.

Footnote

Electronic supplementary information (ESI) available: NMR spectra for dissymmetric bistolanes 2, crystal structures, POM images, DSC curves, photoluminescence spectra and Cartesian coordinates. CCDC 1552781 and 1552782. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7ob01369h

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