Synthesis and study of hybrid hydrogen-bonded bent-core liquid crystal complexes containing C₆₀- and Si-based proton donors

Abstract

Novel hydrogen-bonded (H-bonded) C₆₀- and Si-based bent-core liquid crystal (LC) complexes containing a bent-shaped proton acceptor and C₆₀- and Si-based single/double armed proton donors were synthesized and mixed to produce hybrid H-bonded LC complexes. The SmCP phases could be introduced in the hybrid LC complexes with a very high C₆₀-based moiety up to 90 mol%. Among our H-bonded LC complexes, the hybrid LC complexes with low amounts of the C₆₀-based moiety (≦24 mol%) possessed the highest spontaneous polarization value (ca. 494 nC cm⁻²), the widest mesophasic range (40.8 °C) and the lowest saturated electric field (8.8 V_pp μm⁻¹). Under electric fields of modified triangle waveforms, both series of hybrid H-bonded LC complexes displayed an anti-ferroelectric (AF) to a ferroelectric (FE) polar switching as the C₆₀-based moiety increased up to 24–50 mol%. Therefore, the hybrid H-bonded LC complexes containing a broad molar ratio (0–90 mol%) of C₆₀-based moiety revealed a tunable route for the electro-optical applications of C₆₀-based H-bonded liquid crystals.

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Introduction

Buckyball (also named as buckminsterfullerene or [60]fullerene) is a structure of truncated icosahedron with formula C₆₀, which was first synthesized by Kroto et al. in 1985.¹ The buckyball structure displays various special properties, such as observable wave-particle duality,² superconductivity,^3–6 non-linear optical properties⁷ and high electronic affinity.^8,9 Moreover, with appropriate surface modification, its derivatives showed extensive applications, e.g., hydration,^10–12 hydrogenation,^13,14 halogenation,¹⁴ oxygenation,^15,16 cycloaddition,^17–19 free radical reaction,²⁰ photoreaction,^21,22 endohedral metallofullerene,^23,24 catalyst,²⁵ biomedical sensor/therapy^26,27 and electron acceptor.^28,29 The arrangements of charge transporting materials (e.g., buckyballs) were drawn much attention recently due to the improving of charge transfer in molecular electronics.^30–36 By attaching to mesogens, [60]fullerene could be aligned via liquid crystalline mesophases, and the methods were luxuriant (including some cost-effective ways) to enhance the arrangements of buckyballs.^31,37–50 With the advantages of easy-manipulating and flexible ratio-control (also reported as an effective conduit for charge transfer),⁵¹ hydrogen-bonds (H-bonds) were our aim to be introduced to the C₆₀-based liquid crystal (LC) systems.⁵² The electro-optical properties of fullerene-based LCs have been determined in the smectic⁵³ and columnar⁴⁹ phases accompanied with the molecular arrangements as well as the orientations of π-conjugated cores in mesophases to provide essential information for practical appearances.^54a–c However, the electro-optical performance of C₆₀-based FLCs are rare,^54d especially for those with H-bonds. Therefore, the combination of H-bonding, bent-core LCs, and fullerenes in this study are required for further investigations on their mesomorphic and electro-optical aspects. However, to our best knowledge, no hybrid H-bonded C₆₀-based liquid crystals have been synthesized up to date, which may be resulted from the weak H-bonding between proton donors and acceptors as well the strong aggregation of C₆₀ nanoparticles (NPs) to disturb the arrangements of mesogens.⁵⁵ Therefore, an efficient blending method was applied to induce and stabilize the mesophases of H-bonded C₆₀-based LCs in our research. Moreover, LCs with ferro- and anti-ferroelectricities were utilized to reinforce the control of the applied electric fields as well as strengthen the photovoltaic effect on charge transporting materials.^56–60 According to the previous research of Ros et al.,³⁷ covalent-bonded bent-core LCs based on [60]fullerene displayed broad ranges of SmCP phases and novel physical properties. Hence, as shown in Fig. 1 and 2, various H-bonded C₆₀-based bent-core complexes, i.e., FIA100 and FIIA100 (analogue IIA100 without C₆₀ as a comparison), were synthesized and investigated in this study, where a bent-shaped proton acceptor NBF14 and the corresponding proton donors FIA, FIIA and IIA were prepared. In addition, the previously reported H-bonded Si-based bent-core complex SiA100 containing the proton donor SiA was utilized as a LC host to mix with Au-based covalent-bonded bent-core dopant in order to facilitate the smectic alignment of Au-nanoparticles by electric fields.⁶¹ Furthermore, the Si-based bent-core LC complex SiA100 in this survey was also mingled with both H-bonded C₆₀-based bent-core complexes FIA100 and FIIA100 to induce and even extend the mesophasic ranges of the hybrid H-bonded bent-core LCs.

Fig. 1 Molecular structures of proton donors FIA, FIIA, SiA and IIA as well as a proton acceptor NBF14.

Fig. 2 (a) H-bonded bent-core complexes FIA100, FIIA100, SiA100 and IIA100; (b) hybrid LC complexes FIA50 and FIIA50 containing 50% molar ratio of SiA100 and 50% molar ratio of FIA100 and FIIA100, respectively.

Notably, with a minor molar ratio of SiA100 (i.e., 10 mol%), the hybrid H-bonded bent-core LC complexes containing both Si-based SiA100 and C₆₀-based FIA100 (or FIIA100) were able to display SmCP phases, and the electro-optical as well as mesophasic properties could be manipulated and optimized via the blending ratio of the Si- and C₆₀-based complexes, which could be more easily controlled than the covalent-bonded C₆₀-based LC systems.

Experimental

Synthesis

Synthetic steps of components FIA, FIIA, IIA, SiA and NBF14 are shown in Scheme S1 in the ESI.† Chemical characterization data of FIA, FIIA, IIA, SiA and NBF14 are provided below:

4-((12-(4-(1-Methylfulleropyrrolidin-2-yl)phenoxy)dodecyl)oxy) benzoic acid, (FIA). ¹H NMR (300 MHz, CDCl₃) δ (ppm): 8.00 (d, J = 9.0 Hz, 2H), 7.40 (m, 4H), 6.88 (d, 2H), 4.96 (d, J = 9.0 Hz, 1H), 4.87 (s, 1H), 4.22 (d, J = 9.0 Hz, 1H), 4.01–3.92 (m, 4H), 2.78 (s, 3H), 1.75 (m, 4H), 1.36–1.23 (m, 16H). ¹³C NMR (75 MHz, CDCl₃) δ (ppm) = 26.26, 29.38, 29.84, 30.60, 34.49, 40.29, 66.61, 68.23, 68.48, 70.22, 114.37, 114.81, 122.50, 124.73, 128.35, 128.95, 136.08, 139.83, 140.16, 140.41, 141.79, 141.94, 142.08, 142.25, 143.4, 144.65, 144.87, 144.97, 145.39, 145.52, 145.59, 145.76, 146.03, 146.38, 146.46, 146.55, 147.07, 147.33, 147.55, 159.44, 163.32, 166.49. Anal. calcd for C₈₈H₃₉NO₄: C, 90.01; H, 3.35; N, 1.19. Found: C, 90.58; H, 2.86; N, 0.99. MS (FAB−) m/z: 1173.3 (exact mass); found, 1174.6.

4,4′-(((Malonylbis(oxy))bis(dodecane-12,1-diyl))bis(oxy))(1,2-methanofullerene C₆₀)-61,61-dibenzoic acid, (FIIA). ¹H NMR (300 MHz, CDCl₃) δ (ppm): 7.96 (d, J = 9.0 Hz, 4H), 6.90 (d, J = 8.8 Hz, 4H), 4.47 (m, J = 6.6 Hz, 4H), 4.01 (t, J = 6.6 Hz, 4H), 1.89 (m, 8H), 1.43–1.29 (m, 32H). ¹³C NMR (75 MHz, CDCl₃) δ (ppm) = 26.01, 28.52, 29.13, 29.21, 29.40, 29.58, 67.31, 68.18, 114.05, 122.30, 131.55, 138.50, 138.59, 138.67, 139.12, 139.13, 139.18, 140.13, 140.34, 141.02, 141.20, 141.54, 141.84, 142.05, 142.32, 142.51, 142.85, 143.02, 143.36, 143.57, 144.01, 144.34, 144.58, 145.09, 145.11, 145.28, 145.37, 145.54, 146.02, 146.15, 146.45, 146.93, 147.21, 147.68, 162.93, 166.89. Anal. calcd for C₁₀₁H₅₈O₁₀: C, 84.74; H, 4.08. Found: C, 85.33; H, 4.55. MS (FAB+) m/z: 1430.4 (exact mass); found, 1431.1.

4,4′-(((Malonylbis(oxy))bis(dodecane-12,1-diyl))bis(oxy)) dibenzoic acid, (IIA). ¹H NMR (300 MHz, DMSO-d₆) δ (ppm) = 7.85 (d, J = 9.0 Hz, 4H), 6.97 (d, J = 8.7 Hz, 4H), 4.05–3.97 (m, 8H), 3.46 (s, 2H), 2.11–2.00 (m, 8H). 1.80–1.50 (m, 8H), 1.45–1.30 (m, 24H). Anal. calcd for C₄₁H₆₀O₁₀: C, 69.07; H, 8.48. Found: C, 68.77; H, 8.41.

4-(Dimethylethylsiloxyl) undecyloxybenzoic acid, (SiA). The H-donor SiA was synthesized and reported in the reference,^{61 1}H NMR (300 MHz, CDC_l3) δ (ppm): 8.05 (d, J = 9.0 Hz, 2H), 6.95 (d, J = 9.0 Hz, 2H), 4.05 (t, J = 6.6 Hz, 2H), 1.81 (m, 2H),1.52–1.32 (m, 16H), 0.92 (t, J = 8.1 Hz, 3H), 0.48 (t, J = 7.8 Hz, 4H), 0.01 (s, 6H). Anal. calcd for C₂₂H₃₈O₃Si: C, 69.79; H, 10.12. Found: C, 69.62; H, 10.05. MS (⁺) m/z: 378.26 (exact mass); found, 378 (M⁺).

3′-(4-(2,3-Difluoro-4-(tetradecyloxy)benzoyloxy)benzoyloxy) biphenyl-4-yl isonicotinate, (NBF14). ¹H NMR (300 MHz, CDCl₃) δ (ppm): 8.87 (d, 2H), 8.29 (d, J = 8.3 Hz, 2H), 8.02 (d, J = 8.4 Hz, 2H), 7.86 (m, 1H), 7.67 (d, J = 8.2 Hz, 2H), 7.54 (d, J = 4.7 Hz, 2H), 7.46 (s, 1H), 7.42 (d, J = 8.5 Hz, 2H), 7.31 (d, J = 8.7 Hz, 2H), 7.23–7.19 (m, 1H), 6.89–6.82 (m, 1H), 4.10 (t, J = 6.3 Hz, 2H), 1.80–1.75 (m, 2H), 1.43–1.28 (m, 22H), 0.86 (t, J = 6.3 Hz, 3H). Anal. calcd for C₄₆H₄₇F₂NO₇: C, 72.33; H, 6.20; N, 1.83. Found: C, 72.15; H, 6.42; N, 1.83.

Sample preparation

The H-bonded complexes were prepared by dissolving proton acceptor NBF14 and proton donors FIA, FIIA, SiA or IIA with equimolar amounts of pyridyl and benzoic acid groups in anhydrous tetrahydrofuran (THF). After each LC mixture was slowly evaporated at room temperature, the THF was completely removed in vacuum for 24 h, and then the hybrid LC complexes were obtained.

Characterization

¹H and ¹³C NMR spectra were verified with Varian Unity 300 MHz spectrometer. Mass spectra and elemental analyses were determined with Micromass TRIO-2000 GC-MS and Perkin-Elmer 240C elemental analyzer, respectively. MALDI-TOF measurements were carried out on AutoFlex III MALDI mass spectrometer (Bruker Daltonics, Breman, Germany). Fourier transform infrared spectra (FTIR) were recorded on a Perkin-Elmer Spectrum 100 Series with pressed KBr pellets. Mesophasic patterns, enthalpies of mesophase transitions and X-ray diffraction (XRD) were measured with polarizing optical microscope (POM, Leica DMLP; equipped with a hot stage, Linkam TMS-94/LTS350), differential scanning calorimeter (DSC, Perkin Elmer Diamond; rate 5 °C min⁻¹ of heating or cooling) and synchrotron X-ray radiation (at beamlines BL13A1, BL17A1 and BL01C2 at National Synchrotron Radiation Research Center, NSRRC, Taiwan), respectively. Typical X-ray scattering patterns were obtained with detectors (marCCD165 or mar345 image plates) for 1 to 30 seconds. The scattering angles were calibrated with two standard samples of silver behenate and silicon with beam diameter 0.5 mm for the 10 keV beam (λ = 0.124 nm⁻¹). The electro-optical properties were measured with prepared samples injected in commercially non-rubbed indium-tin-oxide (ITO) cells (mesophase state; thickness 9 μm; active area 0.25 cm²). With a digital oscilloscope (Tektronix TDS-3012B) connected to a high-power amplifier (Gwinstek) and a function generator (Tektronix AFG 3021), measurements of spontaneous polarization were made with a modified triangular-wave method at a frequency of 50 Hz (see Fig. S1 in the ESI†).⁶²

Results and discussion

Molecular structures and phase behavior of synthesized complexes

As shown in Fig. 2a, single-armed H-bonded bent-core complexes FIA100 and SiA100 (ref. 61) are composed of proton donor FIA (or SiA) and acceptor NBF14 with 1 : 1 molar ratio; while double-armed H-bonded bent-core complexes FIIA100 and IIA100 consist of proton donor FIIA (or IIA) and acceptor NBF14 with 1 : 2 molar ratio. The existence of H-bonded in the mesophases can be proved by temperature-various FTIR spectroscopy (see the ESI†). Among these H-bonded bent-core complexes, upon cooling only SiA100 and IIA100 processed the smectic CP (SmCP) and smectic C (SmC) phases, respectively. However, the C₆₀-based H-bonded bent-core complexes FIA100 and FIIA100 did not display any mesophases in contrast to their C₆₀-based covalent-bonded analogues,³⁷ because the mesophases were hindered by the stronger aggregation tendency of C₆₀ (ref. 55) with weaker H-bonds in the H-bonded complexes (i.e., FIA100 and FIIA100). Therefore, SiA100 was introduced to the H-bonded C₆₀-based bent-core complexes (FIA100 and FIIA100) to extend the mesophasic ranges of the hybrid H-bonded bent-core complexes. As illustrated in Fig. 2b, binary complex FIA50 was prepared from FIA, SiA and NBF14 with 1 : 1 : 2 mole ratio and binary complex FIIA50 was produced by FIIA, SiA and NBF14 with 1 : 2 : 4 mole ratio. In general, binary complexes FIAx and FIIAx were denoted as x% molar ratio of C₆₀-based bent-core proton donor (i.e., x = 4, 24, 50, 76, 90 and 100). For example, since a double-armed proton donor FIIA bearing two –COOH functional groups would be H-bonded with two pyridyl moieties of proton acceptor NBF14 (see Fig. 2b and Table S1 of the ESI†). In addition, the relationship for x values and weight percents (wt%) of C₆₀-based bent-core complexes are also presented in Table S1† in order to compare with our previous research.⁶¹ As shown in Table 1 and Fig. 3, the broadest mesophasic ranges (upon cooling) of binary complexes FIAx and FIIAx were both extended from 33.4 °C (SiA100) to 40.8 (at x = 24) and 37.6 °C (at x = 50), respectively. The POM images of typical broken-fan textures of the SmCP phase for x = 24 were shown in Fig. S2 of the ESI.†

Table 1 Phase transition temperatures and enthalpies of synthesized compounds and hybrid complexes

Compounds and complexes	Phase transition^a temperature/°C [enthalpy/J/g] heating (top)/cooling (bottom)
a The phase transitions were measured by DSC at the 2^nd heating and 1^st cooling scans with a cooling rate of 5 °C min^−1 with peak onset, Cr = crystal solid; SmC = smectic C phase; SmCPF = ferroelectric smectic C phase; SmCPA = anti-ferroelectric smectic C phase; Iso = isotropic phase.b Mesophases obtained by POM and XRD measurements (the data of DSC and POM were furnished in Fig. S3 and S4, respectively).
FIA100	Cr 89.3[21.0] Iso, Iso 72.3[−18.8] Cr
SiA100	Cr 97.5[17.1] SmCPA 116.4[23.6] Iso, Iso 124.0[−26.5] SmCPA 90.6[−13.8] Cr
FIIA100	Cr 87.0[25.6] Iso, Iso 82.8[−23.6] Cr
IIA100	Cr 115.5[53.2] Iso, Iso 127.0^b SmC 126.0^b Cr
FIA4	Cr 97.1[69.2] Iso, Iso 117.5[−30.9] SmCPA 87.7[−19.0] Cr
FIA24	Cr 95.3[36.7] Iso, Iso 120.1[−16.8] SmCPA 79.3[−14.4] Cr
FIA50	Cr 90.5[33.5] Iso, Iso 106.8[−9.1] SmCPF 72.8[−15.0] Cr
FIA76	Cr 87.0[26.9] Iso, Iso 106.1[−0.2] SmCPF 73.4[−24.9] Cr
FIA90	Cr 87.0 [29.5] Iso, Iso 80.4[−1.7] SmCPF 69.0[−20.7] Cr
FIIA4	Cr 98.5[31.8] Iso, Iso 121.8[−17.7] SmCPA 88.7[−11.8] Cr
FIIA24	Cr 98.7[16.8] SmCPA 117.0[14.1] Iso, Iso 123.5[−17.0] SmCPA 86.8[−17.9] Cr
FIIA50	Cr 97.5 [36.2] Iso, Iso 115.4[−11.3] SmCPF 77.8[−20.1] Cr
FIIA76	Cr 91.5[33.1] Iso, Iso 100.5[−3.2] SmCPF 78.9[−22.8] Cr
FIIA90	Cr 90.1[31.1] Iso, Iso 88.4[−1.5] SmCPF 77.6[−24.0] Cr

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Fig. 3 (a) Mesophasic behaviors and (b) mesophasic ranges of FIAx (black), FIIAx (white) and SiA100 (gray).

The single-armed binary complexes FIAx displayed lower crystallization temperatures (SmCP-Cr) than their double-armed analogues FIIAx, which could be attributed to the colligative property of C₆₀-based complexes. Notably, although most binary complexes possessed monotropic mesophase, only binary complexes FIA90 and FIIA90 containing a very high content of C₆₀-based H-bonded bent-core complexes demonstrated narrow mesophasic temperature ranges 11.4 and 10.8 °C, respectively (see Fig. 3b). Importantly, higher uptaken loads of C₆₀-based component can be blended into the binary complexes FIAx and FIIAx (i.e., x = 50, 76 and 90 without phase separation) than our previous Au-based covalent-bonded bent-core dopant (phase separation occurred at larger than 20 wt%) in SiA100. Therefore, an effective hybrid method for high contents of H-bonded C₆₀-based bent-core complexes was introduced in this study. In addition, by increasing the content of C₆₀-based moiety to moderate values, the polarity of the SmCP phase varied sequentially from anti-ferroelectricity to ferroelectricity in the binary complexes FIAx and FIIAx (at x = 24–50, see Fig. 3, which was also discussed in the section “Electro-Optical Properties”).

Molecular structures based on X-ray analysis

In order to decipher the molecular structures of the C₆₀-based bent-core complexes, FIAx and FIIAx were probed with powder XRD measurements (see Table S2 of the ESI†). The C₆₀-based bent-core complexes FIAx and FIIAx possessed one set of sharp peaks in small angle and broad diffuse scattering signals in wide angle regions indicating a smectic lamellar order of uniformly mixed systems coupled with similar orders of lateral intermolecular distances, respectively (e.g., x = 50 shown in Fig. 4a–c). Sharp peaks indexed as (01) in the small-angle region at the associated d-spacing values of d₁ with 46–50 Å for hybrid complexes FIAx and FIIAx, which were shorter than the calculated molecular lengths of FIA100, FIIA100 and SiA100 (MM+ method, 65–76 Å) indicating the existence of tilted angles examined in bent-core hybrid complexes (see Table S2†). Furthermore, no interdigitated structures are revealed due to the observation of one set ordered diffraction pattern only, and the d-spacing values of FIAx and FIIAx in Fig. 4c reflected a sequentially descending intensity from (01) to (02) and (03), which indicated simple monolayer organizations instead of C₆₀ aggregation-induced bilayer structures with the strongest intensity indexed at (02).^37,63,64 Both hybrid complexes FIAx and FIIAx possessed similar mesophases of SmCP because of the monolayer structure, rather than a bilayer structure induced by the competitions between Si-based nanosegregations and H-bonds.

Fig. 4 Proposed molecular orders and XRD patterns of (a) FIA50 and (b) FIIA50 as well as (c) both 1-D XRD integrations; (d) the d-spacing values of FIAx (black), FIIAx (white) and SiA100 (gray) at T = T_Iso–SmCP − 10 °C upon cooling.

The d-spacing values of hybrid complexes FIAx and FIIAx with same x values were almost the same, except for x = 90 with the largest d-spacing difference possibly due to different measured temperatures (see Fig. 4d and Table 1). Therefore, inferred from the monolayer structures characterized by the XRD measurements of FIAx and FIIAx, the decreased d-spacing value of SmCP with increasing C₆₀ molar ratio might be mainly attributed to the reduced measured temperature in each hybrid complex, which also suggested that no NP-induced nanosegregations occurred in FIAx and FIIAx.

Electro-optical properties

The electro-optical properties were investigated from the saturated spontaneous polarization (P_s) values obtained via non-rubbed ITO-sandwiched cells to avoid the influence of polyimide layers to ferroelectricity.⁶⁵ Under electric fields of modified triangle waveforms, both FIAx and FIIAx displayed an anti-ferroelectric (AF) to a ferroelectric (FE) polar switching as C₆₀ increased:⁶¹ the responsive current signals at the zero-voltage position declined and even vanished as the molar ratio of C₆₀-based moiety with x ≧ 50 (see the orange rectangle area marked in Fig. 5a and b), indicating a ferroelectric polar switching, where the SmCP range was maximized at x = 24 (see Table 1). According to the previous research,^65,66 bent-core complexes processed anti-ferroelectric polar switching behavior due to the easy-stacking of bent-shaped molecules with anti-polar order from neighboring layers, but the anti-polar packing could be diminished via NPs aggregated at the smectic layer interfaces. Hence, the switching from anti-ferroelectric to ferroelectric polar order along with C₆₀ ratio can be realized from the aggregation of NPs at the interfaces of smectic layers. The trends of P_s values and the saturated P_s values at certain electric fields of hybrid complexes FIAx and FIIAx are described in Fig. S7.† As shown in Fig. 5c, as the C₆₀-based moiety increased, both P_s values of hybrid complexes FIAx and FIIAx increased to be saturated at minor C₆₀ doped ranges of x = 4–24, which are higher than P_s (356 nC cm⁻²) of SiA100 (at x = 0) and reached the largest P_s values of 494 and 490 nC cm⁻² (both at x = 4) for FIAx and FIIAx series, respectively. In addition, the P_s values of FIAx and FIIAx dropped continuously to become smaller at higher C₆₀ doped ranges of x = 50–90. Simlarly, in contrast to FIIAx with the same x values, FIAx (at x ≧ 24) possessed lower P_s values due to higher C₆₀-based moieties. The minor doping effect of C₆₀-based moiety on promoted P_s values could be attributed to the reinforced separation of induced dipoles by the NP dopants under electric fields.⁶⁷ The saturated electric field (E_sat) was defined as the electric field at 90% of saturated P_s values (see Fig. S3 of the ESI†), and the lowest E_sat values of FIAx and FIIAx series were obtained at 8.8 and 12.0 V_pp μm⁻¹ for FIA4 and FIIA24, respectively (see Fig. 5d), which were possibly due to the easy driving tendencies of the decreased packing order in the mesophases with minor C₆₀-based contents.⁶⁸ By further increasing the ratio of C₆₀-based moiety, the field induced dipole reorientation was restricted by larger viscosities of the mesophases with higher C₆₀-based contents and led to low P_s values and high E_sat values.⁶⁹ Notably, the saturated P_s value of SiA100 decayed to half values for FIAx and FIIAx series at relatively high ratios of C₆₀-based moiety, i.e., FIAx at x ≧ 76 (i.e., 84 wt%, see Table S1†) and FIIAx at x ≧ 90 (i.e., 92 wt%), which happened in our previous report on Au-based LC composite at 5 wt% of NPs (a relatively low ratio of surface-modified Au NPs).⁶¹ Moreover, in contrast to a low wt% (5 wt%) of surface-modified Au NPs in the previous Au-based LC composite,⁶¹ the AF-FE switching behavior happened at a higher content of surface-modified C₆₀ (x = 50, i.e., 63 wt% and 56 wt% for FIAx and FIIAx, respectively) in both hybrid LC complexes. Finally, unlike the phase separation induced in the previous Au-based LC composite containing ∼20 wt% content of surface-modified Au NPs, much higher contents of surface-modified C₆₀ still could be miscible in both FIAx and FIIAx series at x = 90 (i.e., 94 and 92 wt%, respectively), which could be useful to utilize hybrid LC complexes with high contents of surface-modified C₆₀. Therefore, the anti-ferroelectric (AF) to a ferroelectric (FE) polar switching behavior as well as optimized P_s and E_sat values could be introduced by adjusting the contents of surface-modified C₆₀ in the hybrid LC complexes.

Fig. 5 The saturated P_s values under modified triangular waveforms of (a) FIAx and (b) FIIAx under SmCP phases; (c) saturated P_s and (d) E_sat values of FIAx (black), FIIAx (white) and SiA100 (gray) upon cooling. Blue dashed lines indicated applied electric field (33.3 V μm⁻¹ at 50 Hz), and the orange rectangular was marked as the evidence for AF-FE switching depending on C₆₀ molar ratio for FIA4 and FIIA24, respectively (the detailed switching current responses and POM images to the applied triangular waveform in the mesomorphic phases of FIA24 (SmCP_A) and FIA76 (SmCP_F) are illustrated in Fig. S5 and S6,† respectively).

Conclusion

Single- and double-armed C₆₀-based H-bonded bent-core complexes (FIA100 and FIIA100) were mixed with Si-based H-bonded bent-core LC SiA100 successfully to induce ferroelectric and anti-ferroelectric mesophases, where the hybrid C₆₀-based H-bonded LC complexes are synthesized and reported for the first time. Adjusting various molar ratios of surface-modified C₆₀ in the hybrid H-bonded bent-core LC complexes, the transition temperatures and ranges of the SmCP phases could be reduced and extended, respectively. Compared with Au-based LC composites, much higher contents of surface-modified C₆₀ could be miscible in both hybrid LC complexes FIAx and FIIAx series at x = 90 (i.e., 94 and 92 wt%, respectively). The anti-ferroelectric (AF) to a ferroelectric (FE) polar switching behavior occurred in the hybrid LC complexes with moderate molar ratios of C₆₀-based moiety. The electro-optical properties, such as P_s and E_sat values, could be manipulated and optimized via the blending ratio of the Si- and C₆₀-based complexes, which could be more easily controlled than the covalent-bonded C₆₀-based LC systems.

Acknowledgements

The financial support of this project is provided by the Ministry of Science and Technology (MOST) in Taiwan through MOST 103-2113-M-009-018-MY3 and 103-2221-E-009-215-MY3. We also thank Drs Ming-Tao Lee and Hwo-Shuenn Sheu for the assistance of measuring XRD at 13A1 and 01C2 in National Synchrotron Radiation Research Center (NSRRC) in Taiwan. Prof. Yu-Chie Chen and Ms Fang-Yin Kuo at Department of Applied Chemistry, National Chiao Tung University, are also acknowledged for their supports of the Mass measurements.

Notes and references

1. H. W. Kroto, J. R. Heath, S. C. Obrien, R. F. Curl and R. E. Smalley, Nature, 1985, 318, 162–163.
2. M. Arndt, O. Nairz, J. Vos-Andreae, C. Keller, G. van der Zouw and A. Zeilinger, Nature, 1999, 401, 680–682.
3. O. Gunnarsson, Rev. Mod. Phys., 1997, 69, 575–606.
4. R. C. Haddon, A. F. Hebard, M. J. Rosseinsky, D. W. Murphy, S. J. Duclos, K. B. Lyons, B. Miller, J. M. Rosamilia, R. M. Fleming, A. R. Kortan, S. H. Glarum, A. V. Makhija, A. J. Muller, R. H. Eick, S. M. Zahurak, R. Tycko, G. Dabbagh and F. A. Thiel, Nature, 1991, 350, 320–322.
5. A. F. Hebard, M. J. Rosseinsky, R. C. Haddon, D. W. Murphy, S. H. Glarum, T. T. M. Palstra, A. P. Ramirez and A. R. Kortan, Nature, 1991, 350, 600–601.
6. J. E. Han, O. Gunnarsson and V. H. Crespi, Phys. Rev. Lett., 2003, 90, 167006.
7. W. J. Blau, H. J. Byrne, D. J. Cardin, T. J. Dennis, J. P. Hare, H. W. Kroto, R. Taylor and D. R. M. Walton, Phys. Rev. Lett., 1991, 67, 1423–1425.
8. R. Mitsumoto, T. Araki, E. Ito, Y. Ouchi, K. Seki, K. Kikuchi, Y. Achiba, H. Kurosaki, T. Sonoda, H. Kobayashi, O. V. Boltalina, V. K. Pavlovich, L. N. Sidorov, Y. Hattori, N. Liu, S. Yajima, S. Kawasaki, F. Okino and H. Touhara, J. Phys. Chem. A, 1998, 102, 552–560.
9. Z. Xiao, D. He, C. Zuo, L. Gan and L. Ding, RSC Adv., 2014, 4, 24029–24031.
10. G. V. Andrievsky, V. K. Klochkov, A. B. Bordyuh and G. I. Dovbeshko, Chem. Phys. Lett., 2002, 364, 8–17.
11. G. Andrievsky, V. Klochkov and L. Derevyanchenko, Fullerenes, Nanotubes, Carbon Nanostruct., 2005, 13, 363–376.
12. G. V. Andrievsky, V. I. Bruskov, A. A. Tykhomyrov and S. V. Gudkov, Free Radical Biol. Med., 2009, 47, 786–793.
13. Y. F. Zhao, Y. H. Kim, A. C. Dillon, M. J. Heben and S. B. Zhang, Phys. Rev. Lett., 2005, 94, 155504.
14. S. H. Wang and S. A. Jansen, J. Phys. Chem., 1995, 99, 8556–8561.
15. R. Eigler, F. W. Heinemann and A. Hirsch, Chem. Commun., 2014, 50, 2021–2023.
16. N. C. Maliszewskyj, P. A. Heiney, D. R. Jones, R. M. Strongin, M. A. Cichy and A. B. Smith, Langmuir, 1993, 9, 1439–1441.
17. M. Prato, T. Suzuki, H. Foroudian, Q. Li, K. Khemani, F. Wudl, J. Leonetti, R. D. Little, T. White, B. Rickborn, S. Yamago and E. Nakamura, J. Am. Chem. Soc., 1993, 115, 1594–1595.
18. I. Fernández, M. Solà and F. M. Bickelhaupt, Chem.–Eur. J., 2013, 19, 7416–7422.
19. M. S. Markoulides, G. I. Ioannou, M. J. Manos and N. Chronakis, RSC Adv., 2013, 3, 4750–4756.
20. D. M. Guldi, H. Hungerbuhler, E. Janata and K. D. Asmus, J. Phys. Chem., 1993, 97, 11258–11264.
21. V. Bandi, M. E. El-Khouly, K. Ohkubo, V. N. Nesterov, M. E. Zandler, S. Fukuzumi and F. D'Souza, Chem.–Eur. J., 2013, 19, 7221–7230.
22. A. Kremer, E. Bietlot, A. Zanelli, J. M. Malicka, N. Armaroli and D. Bonifazi, Chem.–Eur. J., 2015, 21, 1108–1117.
23. B. Elliott, K. Yang, A. M. Rao, H. D. Arman, W. T. Pennington and L. Echegoyen, Chem. Commun., 2007, 2083–2085,  10.1039/b700320j.
24. A. Rodriguez-Fortea, A. L. Balch and J. M. Poblet, Chem. Soc. Rev., 2011, 40, 3551–3563.
25. V. Campisciano, V. La Parola, L. F. Liotta, F. Giacalone and M. Gruttadauria, Chem.–Eur. J., 2015, 21, 3327–3334.
26. L. Chao, L. Lu, H. Yang, Y. Zhu, Y. Li, Q. Wang, X. Yu, S. Jiang and Y.-H. Chen, PLoS One, 2013, 8, e66156.
27. L. Wang, X. Zhu, X. Tang, C. Wu, Z. Zhou, C. Sun, S.-L. Deng, H. Ai and J. Gao, Chem. Commun., 2015, 51, 4390–4393.
28. S.-H. Chan, C.-S. Lai, H.-L. Chen, C. Ting and C.-P. Chen, Macromolecules, 2011, 44, 8886–8891.
29. M.-E. Ragoussi and T. Torres, Chem. Commun., 2015, 51, 3957–3972.
30. R. J. Bushby, I. W. Hamley, Q. Y. Liu, O. R. Lozman and J. E. Lydon, J. Mater. Chem., 2005, 15, 4429–4434.
31. T. Chuard and R. Deschenaux, J. Mater. Chem., 2002, 12, 1944–1951.
32. W. L. Ma, C. Y. Yang, X. Gong, K. Lee and A. J. Heeger, Adv. Funct. Mater., 2005, 15, 1617–1622.
33. X. N. Yang, J. Loos, S. C. Veenstra, W. J. H. Verhees, M. M. Wienk, J. M. Kroon, M. A. J. Michels and R. A. J. Janssen, Nano Lett., 2005, 5, 579–583.
34. T. Wang, X. Liao, J. Wang, C. Wang, M.-Y. Chan and V. W.-W. Yam, Chem. Commun., 2013, 49, 9923–9925.
35. J. Kelber, M.-F. Achard, B. Garreau-de Bonneval and H. Bock, Chem.–Eur. J., 2011, 17, 8145–8155.
36. X. Chen, L. Chen and Y. Chen, RSC Adv., 2014, 4, 3627–3632.
37. J. Vergara, J. Barbera, J. Luis Serrano, M. Blanca Ros, N. Sebastian, R. de la Fuente, D. O. Lopez, G. Fernandez, L. Sanchez and N. Martin, Angew. Chem., Int. Ed., 2011, 50, 12523–12528.
38. T. N. Y. Hoang, D. Pociecha, M. Salamonczyk, E. Gorecka and R. Deschenaux, Soft Matter, 2011, 7, 4948–4953.
39. H. Mamlouk, B. Heinrich, C. Bourgogne, B. Donnio, D. Guillon and D. Felder-Flesch, J. Mater. Chem., 2007, 17, 2199–2205.
40. S. Campidelli, T. Brandmueller, A. Hirsch, I. M. Saez, J. W. Goodby and R. Deschenaux, Chem. Commun., 2006, 4282–4284,  10.1039/b610350b.
41. S. Campidelli, P. Bourgun, B. Guintchin, J. Furrer, H. Stoeckli-Evans, I. M. Saez, J. W. Goodby and R. Deschenaux, J. Am. Chem. Soc., 2010, 132, 3574–3581.
42. E. Allard, F. Oswald, B. Donnio, D. Guillon, J. L. Delgado, F. Langa and R. Deschenaux, Org. Lett., 2005, 7, 383–386.
43. J. Lenoble, S. Campidelli, N. Maringa, B. Donnio, D. Guillon, N. Yevlampieva and R. Deschenaux, J. Am. Chem. Soc., 2007, 129, 9941–9952.
44. C.-Z. Li, Y. Matsuo and E. Nakamura, J. Am. Chem. Soc., 2009, 131, 17058–17059.
45. T. T. Nguyen, S. Albert, T. L. A. Nguyen and R. Deschenaux, RSC Adv., 2015, 5, 27224–27228.
46. T. Kato, T. Yasuda, Y. Kamikawa and M. Yoshio, Chem. Commun., 2009, 729–739.
47. M. Lehmann and M. Hügel, Angew. Chem., Int. Ed., 2015, 54, 4110–4114.
48. X. Zhang, C. H. Hsu, X. Ren, Y. Gu, B. Song, H. J. Sun, S. Yang, E. Chen, Y. Tu and X. Li, Angew. Chem., Int. Ed., 2015, 54, 114–117.
49. M. Sawamura, K. Kawai, Y. Matsuo, K. Kanie, T. Kato and E. Nakamura, Nature, 2002, 419, 702–705.
50. Y. Matsuo, A. Muramatsu, Y. Kamikawa, T. Kato and E. Nakamura, J. Am. Chem. Soc., 2006, 128, 9586–9587.
51. H. N. Ghosh, K. Adamczyk, S. Verma, J. Dreyer and E. T. J. Nibbering, Chem.–Eur. J., 2012, 18, 4930–4937.
52. (a) V. Vagenende, T.-J. Ching, R.-J. Chua, N. Thirumoorthi and P. Gagnon, ACS Appl. Mater. Interfaces, 2013, 5, 4472–4478; (b) M. Wei, M. Zhan, D. Yu, H. Xie, M. He, K. Yang and Y. Wang, ACS Appl. Mater. Interfaces, 2015, 7, 2585–2596.
53. (a) D. Felder-Flesch, L. Rupnicki, C. Bourgogne, B. Donnio and D. Guillon, J. Mater. Chem., 2006, 16, 304–309; (b) W.-S. Li, Y. Yamamoto, T. Fukushima, A. Saeki, S. Seki, S. Tagawa, H. Masunaga, S. Sasaki, M. Takata and T. Aida, J. Am. Chem. Soc., 2008, 130, 8886–8887.
54. (a) P. H. J. Kouwer and G. H. Mehl, J. Mater. Chem., 2009, 19, 1564–1575; (b) R. Deschenaux, B. Donnio and D. Guillon, New J. Chem., 2007, 31, 1064–1073; (c) S. D. Peroukidis, A. G. Vanakaras and D. J. Photinos, J. Phys. Chem. B, 2008, 112, 12761–12767; (d) J. Vergara, J. Barberá, J. L. Serrano, M. B. Ros, N. Sebastián, R. de la Fuente, D. O. López, G. Fernández, L. Sánchez and N. Martín, Angew. Chem., 2011, 123, 12731–12736.
55. (a) T. Michinobu, T. Nakanishi, J. P. Hill, M. Funahashi and K. Ariga, J. Am. Chem. Soc., 2006, 128, 10384–10385; (b) L.-Y. Wang, I. H. Chiang, P.-J. Yang, W.-S. Li, I. T. Chao and H.-C. Lin, J. Phys. Chem. B, 2009, 113, 14648–14660.
56. S. Kimura, S. Nishiyama, Y. Ouchi, H. Takezoe and A. Fukuda, Jpn. J. Appl. Phys., 1987, 26, L255–L257.
57. H. Okada, T. Sakurai, T. Katoh, M. Watanabe, H. Onnagawa, N. Nakatani and K. Miyashita, Jpn. J. Appl. Phys., 1993, 32, 4339–4343.
58. H. Okada, K. Kurabayashi, Y. Kidoh, H. Onnagawa and K. Miyashita, Jpn. J. Appl. Phys., 1994, 33, 4666–4672.
59. S. Essid, M. Manai, A. Gharbi, J. P. Marcerou and J. C. Rouillon, Liq. Cryst., 2005, 32, 307–313.
60. N. Podoliak, O. Buchnev, M. Herrington, E. Mavrona, M. Kaczmarek, A. G. Kanaras, E. Stratakis, J.-F. Blach, J.-F. Henninot and M. Warenghem, RSC Adv., 2014, 4, 46068–46074.
61. W.-H. Chen, Y.-T. Chang, J.-J. Lee, W.-T. Chuang and H.-C. Lin, Chem.–Eur. J., 2011, 17, 13182–13187.
62. K. Miyasato, S. Abe, H. Takezoe, A. Fukuda and E. Kuze, Jpn. J. Appl. Phys., 1983, 22, L661–L663.
63. S. Campidelli, R. Deschenaux, J. F. Eckert, D. Guillon and J. F. Nierengarten, Chem. Commun., 2002, 656–657,  10.1039/b111595b.
64. C. Keith, R. A. Reddy, A. Hauser, U. Baumeister and C. Tschierske, J. Am. Chem. Soc., 2006, 128, 3051–3066.
65. C. Keith, R. A. Reddy, M. Prehm, U. Baumeister, H. Kresse, J. L. Chao, H. Hahn, H. Lang and C. Tschierske, Chem.–Eur. J., 2007, 13, 2556–2577.
66. C. L. Folcia, J. Ortega, J. Etxebarria, S. Rodriguez-Conde, G. Sanz-Enguita, K. Geese, C. Tschierske, V. Ponsinet, P. Barois, R. Pindak, L. Pan, Z. Q. Liu, B. K. McCoy and C. C. Huang, Soft Matter, 2014, 10, 196–205.
67. D. P. Singh, S. K. Gupta and R. Manohar, Adv. Condens. Matter Phys., 2013, 250301,  DOI:10.1155/2013/250301.
68. T. Zhang, C. Zhong and J. Xu, Jpn. J. Appl. Phys., 2009, 48, 055002.
69. O. Francescangeli, V. Stanic, S. I. Torgova, A. Strigazzi, N. Scaramuzza, C. Ferrero, I. P. Dolbnya, T. M. Weiss, R. Berardi, L. Muccioli, S. Orlandi and C. Zannoni, Adv. Funct. Mater., 2009, 19, 2592–2600.

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Footnote

† Electronic supplementary information (ESI) available: The details of synthetic steps and the elemental characterization of components FIA, FIIA, IIA, SiA and NBF14; the setup of P_s value measurement; the blending ratios of hybrid complexes; POM images of FIA24 and FIIA24; crystallographic parameters of synthesized compounds and hybrid complexes; P_s values vs. applied electric fields of FIAx and FIIAx. See DOI: 10.1039/c5ra11186b

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