Novel V- and Y-Shaped Light-Emitting Liquid Crystals with Pentafluorinated Bistolane-Based Luminophores.

Herein, we describe the synthesis of novel light-emitting liquid-crystalline (LC) compounds bearing pentafluorinated bistolane-based luminophores with a V- or a Y-shaped molecular geometry and the evaluation of their LC and photophysical characteristics. The V- or Y-shaped compounds exhibited a unique LC phase and showed photoluminescence (PL) behavior under various circumstances, such as in dilute solution or in the solid state. Notably, PL characteristics were observed even under high-temperature conditions with a crystal (Cr) to LC phase transition, although the PL efficiency (Φ PL) was gradually reduced because of thermal molecular motion. Interestingly, Φ PL was found to be completely recovered through the LC → Cr phase transition during the cooling process; the PL characteristics of the V- or Y-shaped compounds were sensitively changed by external thermal stress, giving these compounds the ability to act as thermoresponsive PL sensing materials.

Introduction

Solid-state light-emitting materials[1] have attracted enormous attention for use as a source of illumination in organic light-emitting diodes[2] or polymer light-emitting diodes.[3] However, most organic luminescent compounds do not emit luminescence in aggregated states, such as the solid state, because of the aggregation-caused quenching effect,[4,5] even though intensive emission has been observed in the dilute solution state. Therefore, the exploitation of light-emitting materials in the solid state has become increasingly demanded.

Because of the seminal findings of Tang, that is, aggregation-induced emission[6] and crystallization-induced emission (CIE),[7] the exploitation of such solid-state light-emitting compounds has accelerated, and a number of luminous compounds, including solid-state luminophores, have been successfully produced thus far. From the extensive studies on solid-state luminescent compounds, two important factors in controlling the luminescence color have emerged: (i) the molecular aggregation in the condensed phase[8] and (ii) the electron-density distribution of the whole compound.[9] Our group produced nonliquid-crystalline tolane and liquid-crystalline (LC) bistolane derivatives (1, Figure A) with a pentafluorinated aromatic scaffold and revealed that the emission behavior in the crystal dramatically changed depending on the length of the alkoxy chain substituted at the molecular terminal.[10] Additionally, modulating the electron-density distribution on the pentafluorinated bistolane derivatives by substituting a more electron-donating amino group for the terminal alkoxy substituent was found to cause a significant photoluminescence (PL) color change from deep blue to green (2, Figure A).[11] The former results made us envision that various PL behaviors, such as PL intensity and color, could be obtained if the molecular aggregation of 1 could be successfully controlled by the application of an external stimulus.

Figure 1

(A) Chemical structure of the previously reported pentafluorinated bistolane derivatives. (B) Conceptual illustration of the molecular design for light-emitting liquid crystals. (C,D) Schematic illustration of the novel molecular designs with V- or Y-shaped geometry.

The use of organic LC molecules to precisely control molecular aggregation is attractive because the aggregated structures of such compounds can change reversibly via thermal phase-transition between the crystal (Cr) and isotropic liquid (Iso) in response to external stimuli, for example, heat or electric fields. Therefore, organic luminescent compounds possessing LC characteristics, known as light-emitting liquid crystals, have received considerable interest. Light-emitting liquid crystals show intriguing optical characteristics in various molecular arrangements formed through phase transition by applying external heat or an electrical stimulus.[12,13] Among such light-emitting liquid crystals, our LC bistolane derivative (1) demonstrated to show intriguing switching of their PL behavior through the phase transition between the Cr ⇄ LC phases.[10b,14] The temperature-dependent PL switching was likely attributable to a dynamic change in molecular aggregation via a thermal phase transition. The pentafluorinated bistolane derivatives are light-emitting liquid crystals, that is, thermoresponsive PL compounds, which could become promising PL-sensing materials, such as fluorescent thermometers.

On the basis of these accumulated results, as shown in Figure B, molecular structures possessing both LC and PL characteristics can be easily designed by combining the following elements: (i) an extended π-conjugated structure for the light-emitting characteristics, (ii) an alkoxy-substituent at the longitudinal molecular terminal for LC properties, and (iii) a fluorinated aromatic moiety to determine the electron-density distribution of the molecule. To develop novel light-emitting liquid crystals, we then directed our attention toward dramatically changing the molecular geometry from a linear to a V- or Y-shaped geometry (Figure C,D) because bent molecules are known to form unique LC phases[15] and to show PL characteristics in solution or in the solid state.[16] In this article, the synthesis of V- and Y-shaped organic compounds bearing a pentafluorinated bistolane fragment as a luminophore and the evaluation of their LC and photophysical properties are disclosed in detail.

Results and Discussion

Synthesis

Our target structures for this study were the V-shaped molecules V-3 with an isophthalate-based core[17] and the Y-shaped analogues Y-4 with a 1,3,5-benzenetricarboxylate-based core,[18] both of which contain pentafluorinated bistolane-based luminophores on their arms. The V-shaped V-3 and Y-shaped Y-4 were prepared from readily accessible 4-[3-(methoxymethoxy)propyloxy]phenylacetylene (5a) or 4-[6-(methoxymethoxy)hexyloxy]phenylacetylene (5b), respectively, according to the procedure shown in Scheme .

Scheme 1

Synthetic Procedure for the Novel V-Shaped Molecules V-3 and Y-Shaped Molecules Y-4

Thus, 5a, which contained a propylene linker and was readily prepared from 4-bromophenol, participated in a Sonogashira cross-coupling reaction with 1-bromo-4-[2-(trimethylsilyl)ethyn-1-yl]benzene under the influence of a catalytic amount of Cl2Pd(PPh3)2, PPh3, and CuI in Et3N at 100 °C for 20 h to give the corresponding coupling product 6a in 59% yield, which subsequently underwent C–Si bond cleavage under basic conditions to provide the corresponding terminal acetylene 7a in 66% yield. 7a was subjected to the Sonogashira cross-coupling conditions described above with iodopentafluorobenzene followed by immediate deprotection of the acetal moiety under acidic conditions to obtain the corresponding pentafluorinated bistolane 2a with a 3-hydroxypropoxy substituent with a 59% two-step yield. Similarly, pentafluorinated bistolane 2b, which contained a 6-hydroxyhexyloxy-substituent, was successfully obtained in four reaction steps in a similar efficiency. Finally, the condensation of isophthaloyl chloride with the bistolane-luminophore-bearing alcohol 2a or 2b in the presence of a catalytic amount of N,N-dimethylaminopyridine (DMAP) and an excess amount of Et3N in CH2Cl2 at room temperature for 14 h gave rise to the corresponding V-shaped compounds V-3a and V-3b in 23 and 79% yield, respectively, after double purification by silica gel column chromatography, followed by recrystallization using a 1:1 ratio mixed solvent system (good solvent: CH2Cl2, poor solvent: MeOH). Under similar condensation conditions, the use of 1,3,5-benzenetricarbonyl trichloride rather than isophthaloyl chloride allowed the formation of the corresponding Y-shaped analogues Y-4a and Y-4b in 19% yields, respectively, just after the double purification technique. The chemical structures of the four compounds were fully characterized by 1H, 13C, and 19F nuclear magnetic resonance (NMR), infrared (IR) spectroscopy, and high-resolution mass spectrometry (HRMS). The 1H and 13C spectra confirmed that the compounds were sufficiently pure for evaluation of their LC and photophysical properties.

Phase-Transition Behavior

First, the phase-transition behavior of the novel V-shaped compounds V-3 and Y-shaped compounds Y-4 bearing pentafluorinated bistolane luminophores was evaluated using a polarizing optical microscope (POM) and differential scanning calorimeter (DSC). The phase sequences, temperatures, and optical textural images observed are listed in Table .

Table 1

Phase-Transition Behavior of the V-Shaped V-3a and V-3b and Y-Shaped Y-4a and Y-4b

Abbreviations. Cr: crystalline, NX: unusual nematic, N: nematic, Iso: isotropic phases.

Unless otherwise noted, phase-transition temperature was determined using DSC measurements (5.0 °C min–1, N2 atmosphere) of the second cycle.

Polarized optical micrographic images observed during the second cooling process.

DSC measurement was performed at a scan rate of 10 °C min–1 under an N2 atmosphere.

Phase and transition temperature were determined using POM because of a less exothermal heat value.

Not determined because of a low heat quantity.

POM observation clearly revealed that all of the V- and Y-shaped compounds bearing pentafluorinated bistolane luminophores were enantiotropic liquid crystals that showed an LC mesophase between the Cr and Iso phases during both the heating and cooling processes. Careful observation of the phase transitions of all samples during the second cooling process revealed that four-brushed schlieren or droplet textures, which are typical for nematic (N) LCs, were observed at temperatures slightly lower than the clearing temperature (Tc), indicating that all of the V-3 and Y-4 compounds exhibited an N LC mesophase just before the Iso phase. For the propylene-linker compound V-3a, gradual cooling from the N phase caused direct crystallization at 160 °C. In contrast, the other compounds, that is, V-3b, Y-4a, and Y-4b, exhibited smecticlike textures in the POM images with a fluidity intermediate between that of the N and Cr phases, in which the bright-viewing textures changed dramatically to the dark-viewing field after adding shear stress. To further characterize the LC phase, X-ray diffraction (XRD) measurements were carried out at various temperatures during the second cooling cycle for the three compounds V-3b, Y-4a, and Y-4b. Figure shows the XRD patterns obtained for the hexylene-linker compounds V-3b and Y-4b at 20 °C intervals from 200 to 60 °C during the second cooling process.

Figure 2

XRD patterns of (A) V-3b and (B) Y-4b measured at various temperatures between 200 and 60 °C during the second cooling process. Gray: crystalline, green: NX, blue: N, and red: Iso phase.

The XRD measurements showed that a significantly different diffraction pattern was observed below 100 °C; only halo peaks were found for both compounds V-3b and Y-4b above 100 °C, whereas well-defined diffractions were observed when the sample was cooled below 100 °C. As noted previously, optical textures observed under the POM suggested a smectic-like structure, whereas the XRD measurements did not exhibit any diffraction signal in the small-angle region at all, strongly indicating a mesophase with a nematic character without any long-range positional order.[19,20] Judging from the results of the XRD and POM measurements, the LC phases observed in V-3b, Y-4a, and Y-4b can be assigned as an unusual N phase, denoted as an NX phase, rather than a smectic phase.

Comparing the phase-transition temperatures of the samples summarized in Table , the molecular geometry and length of the flexible unit clearly influenced the stability of the Cr and LC phases. The melting temperature (Tm), which is defined as the temperature at which the phase transition from the Cr to the LC phase occurs, indicated that V-3 had a more stable Cr phase than Y-4. In addition, the compounds with propylene linkers, V-3a and Y-4a, possessed much higher Tm values than the corresponding hexylene-linker compounds V-3b and Y-4b. The Tc values showed the opposite trend, with compounds Y-4 having higher temperatures for the phase transition from the N to the Iso phase compared to the corresponding V-3 compounds. Moreover, the flexible chain length seemed to affect the stability of the LC phases in comparison with the corresponding short-linker compounds V-3a and Y-4a; compounds V-3b and Y-4b with long flexible units exhibited phase transition from the N to the Iso phase at temperatures approximately 40 °C lower because of the destruction of the directionally ordered structures by the dynamic molecular motion. Considering the relationship between the molecular geometry, the chain length of the flexible unit, and the phase-transition temperature, the stability of the Cr phase can be attributed to the V-shaped molecular geometry owing to its more symmetric structure, whereas greater LC phase stability can be obtained with the Y-shaped molecular geometry; LC phases were observed over a broad temperature range for the Y-shaped compounds. In terms of the length of the flexible chain, compounds with short flexible units showed enhanced stability in both the Cr and LC phases owing to their relatively rigid structures, which favored the maintenance of molecular order in the aggregated state. Thus, it can be concluded that not only the molecular geometry but also the length of flexible unit are important criteria to control the phase-transition characteristics of LC compounds.

UV–Vis Absorption and PL Behavior in Solution

Subsequently, the photophysical properties of V-3a, V-3b, Y-4a, and Y-4b in dilute CH2Cl2 solution were determined. The absorption and PL properties of CH2Cl2 solutions of these compounds were measured using a UV–visible spectrometer and fluorometer. Figure shows the absorption and PL spectra of the present compounds, photographs of their PL in CH2Cl2 solution, and the PL colors plotted on a Commission Internationale de l’Eclairage (CIE) diagram. For comparison, the absorption and PL spectra of the linear analogues 1a and 1b are also included in Figure A,B. The photophysical data obtained are summarized in Table .

Figure 3

(A,B) Absorption (10–5 mol L–1 in CH2Cl2 solution) and PL spectra (10–6 mol L–1 in CH2Cl2 solution) for linear 1, V-shaped V-3, and Y-shaped Y-4. Excitation wavelength (λex): 330 nm for 1a, V-3a, and Y-4a; 333 nm for 1b; and 334 nm for V-3b and Y-4b. (C) Photographs of the PL in CH2Cl2 solution (10–6 mol L–1) under UV light (λex = 365 nm). (D) CIE diagram for the PL colors observed in solution.

Table 2

Photophysical Data of Novel Luminophores, V-3 and Y-4

molecule	λabs [nm] (ε [103 L mol–1 cm–1])a	λPL [nm]b (ΦPL)c	CIE coordinate [x, y]	Stokes’ shift [cm−1]
V-3a	330 (94.2) in CH2Cl2	406 (0.99)	[0.155, 0.041]	5672
	329 (39.7) in AcOEt	407	[0.155, 0.044]	5825
	332 (23.8) in acetone	428	[0.152, 0.074]	6756
	332 (53.9) in DMF	440	[0.154, 0.106]	7393
	338 (4.2) in DMSO	442	[0.152, 0.113]	6962
	328 (17.0) in MeCN	432	[0.153, 0.083]	7340
V-3b	334 (104.1) in CH2Cl2	411 (0.81)	[0.156, 0.046]	5609
Y-4a	331 (93.4) in CH2Cl2	407 (0.95)	[0.156, 0.051]	5641
	329 (93.4) in AcOEt	407	[0.155, 0.054]	5825
	335 (92.4) in acetone	426	[0.153, 0.066]	6377
	334 (158.8) in DMF	438	[0.154, 0.108]	7109
	333 (75.9) in DMSO	443	[0.154, 0.126]	7457
	324 (117.0) in MeCN	430	[0.154, 0.075]	7608
Y-4b	334 (153.0) in CH2Cl2	410 (0.78)	[0.156, 0.053]	5550

Concentration of the solution: 1.0 × 10–5 mol L–1.

Concentration of the solution: 1.0 × 10–6 mol L–1.

Absolute quantum yield was measured by using a calibrated integrating sphere system.

As shown in Figure A, in CH2Cl2 solution (ca. 10–5 mol L–1), the propylene-linker compounds V-3a and Y-4a absorbed UV light with a single absorption band, of which the maximum wavelengths were 330 and 331 nm; this absorption involved electronic transition from the ground to the excited states. The exciton was observed to be immediately deactivated with light-emission at similar maximum wavelengths (V-3a: λPL = 406 nm; ΦPL = 0.99, Y-4a: λPL = 407 nm; ΦPL = 0.95) with high efficiency. Similarly, the hexylene-linker compounds V-3b and Y-4b also showed effective absorption characteristics at a maximum absorption wavelength (λabs) of 334 nm and intense PL with maximum λPL values of ca. 410 nm and a high ΦPL (Figure B). The λabs, λPL, and spectral shapes of the V- and Y-shaped compounds were almost identical to those of the previously reported linear analogues 1a and 1b,[10b] indicating that the presence of the isophthalate or 1,3,5-benzenetricarboxylate central core in compounds V-3 and Y-4 had no effect on their absorption and PL properties in dilute solution. As can be seen in Figure C, the PL of V-3 and Y-4 in CH2Cl2 solution was observed to be an intense deep blue by the naked eye; their PL colors were quantitatively visualized using the CIE diagram (Figure D). Moreover, the difference between λabs and λPL on the energy scale, which is called the Stokes’ shift,[21] was relatively large, ranging from 5550 to 5672 cm–1, which may be because of the excited state relaxation from locally excited to intramolecular charge transfer (ICT) states caused by stabilization through the solvent reorientation.

To confirm that the PL of the present V- and Y-shaped compounds originated from the ICT state, we evaluated the solvatochromic PL properties of the propylene-linker compounds V-3a and Y-4a as a selected example; their PL behavior was measured in five more solutions of ethyl acetate (AcOEt), acetone, N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), and acetonitrile (MeCN), which have different polarities, and the parameter ET(30) was used as a quantitative measure of solvent polarity.[22] The PL spectra of these compounds in different solvents and the CIE diagram of the PL color are shown in Figure A,C.

Figure 4

(A,C) Normalized PL spectra of V-3a and Y-4a in AcOEt (blue), CH2Cl2 (light blue), acetone (purple), DMF (yellow), DMSO (red), and MeCN (green). Values in parentheses are the ET(30) value, which is an indicator of solvent polarity. Inset: CIE diagram calculated from the PL spectra. (B,D) Lippert–Mataga plot calculated from the physical properties of the solvent and the dipole moment change (μe – μg) between ground state and excited state derived from the Lippert–Mataga plot.

As shown in Figure A,C, both V-3a and Y-4a showed PL in a more polar-solvent acetone (ET(30) = 42.2), DMF (ET(30) = 43.2), DMSO (ET(30) = 45.1), and MeCN (ET(30) = 45.6), and their λPL values in these solvents were expectedly shifted approximately 20–30 nm toward longer wavelengths in comparison with their λPL values in CH2Cl2 (ET(30) = 40.7), whereas the less polar solvent, AcOEt (ET(30) = 38.1), resulted in a short-wavelength shift of ca. 20 nm. As a result of this emission wavelength shift, a slight PL color change was observed depending on the solvent polarity (inset of Figure A,C). To gain additional experimental evidence for the PL from an ICT state, the Stokes’ shift (νabs – νPL) was plotted as a function of the orientation polarizability (Δf), which is known as a Lippert–Mataga plot,[23] to estimate the dipole moment (μe) in the excited state for V-3a and Y-4a. As shown in Figure B,D, the Stokes’ shift (νabs – νPL) was found to exhibit a relatively linear relationship to the Δf of the solvent by means of linear approximation with a least-square method. The difference in the dipole moment (μe – μg) between the ground state and the excited state, which could be calculated from the slope of the linear approximation, was found to be 24.3 D for V-3a and 26.7 D for Y-4a, respectively. On the basis of the ground-state dipole moment (μg) calculated using a density functional theory (DFT) in Gaussian 09 (Figures S35 and S36),[24,25] both compounds clearly possessed a large dipole moment in the excited state: μe = 28.2 D for V-3a and 31.3 D for Y-4a. As a consequence, it can safely be concluded that the PL behavior of the V- and Y-shaped compounds occurred through radiative deactivation from an ICT state.[26]

PL Behavior in the Solid State

To investigate the PL behavior of the present V- and Y-shaped compounds in condensed phases, their PL spectra were measured in the solid state after recrystallizing the compounds from a CH2Cl2/MeOH (1:1) mixed solvent system. Figure shows the PL spectra, photographs of the compounds under UV irradiation, and the CIE diagram of the PL colors. The photophysical data are tabulated in Table .

Figure 5

(A) Normalized PL spectra of linear 1, V-3, and Y-4 in the solid state obtained after recrystallization. (B) Photographs of the PL in the solid state under UV irradiation (λex = 365 nm). (C) CIE diagram of the PL color observed in the solid state.

Table 3

Photophysical Data Observed in the Solid State for V-3 and Y-4 after Recrystallization

molecule	λPL (nm) [λex (nm)]	ΦPLa	CIE coordinates [x, y]
V-3a	447 (355)	0.17	[0.163, 0.138]
V-3b	433 (355)	0.18	[0.175, 0.141]
Y-4a	432 (350)	0.19	[0.182, 0.159]
Y-4b	459 (365)	0.07	[0.185, 0.202]

Absolute quantum yield was measured using a calibrated integrating sphere system.

To our delight, all of the compounds with V- and Y-shaped molecular geometry exhibited PL behavior, even in the solid state, with moderate PL efficiency (ΦPL). The PL spectra were found to be completely different from those in dilute solution; not only were the spectral bands significantly broadened, but also the λPL values in the solid state showed long-wavelength shifts of 10–50 nm compared to those in the solution state (Figure A). The broadening of the PL band in the solid state is a typical phenomenon related to various intermolecular interactions formed in condensed phases, which can be clearly understood from the dramatic difference of the excitation spectra for the solid and solution states (Figure S33). The very different excitation spectra in the solid state strongly suggest the existence of multiple molecular aggregates with various intermolecular interactions in the excited states. As shown in Figure B, all of the compounds exhibited light-blue PL visible to the naked eye under UV irradiation (λex = 365 nm); the PL color is quantitatively plotted in the CIE diagram (Figure C). Moreover, compared to the solid-state PL spectra of the linear propoxy-containing 1a, the PL spectra of both propylene-linker compounds V-3a and Y-4a were slightly shifted toward the long-wavelength region, whereas the PL band for the hexylene-linker compounds V-3b or Y-4b showed a shift toward shorter wavelength or remained almost the same in comparison with their linear analogue 1b, which contains a hexyloxy flexible chain. However, the specific origin of the shifts in the emission maxima remains unclear, and we are not able to propose a reasonable explanation at present.

Temperature-Dependent PL Behavior in LC Phases

As mentioned previously, the compounds V-3 and Y-4 were found to be thermotropic LC compounds that exhibited LC phases, specifically, NX and/or N phases, between the Cr and Iso phases, which indicates that the formation of the LC phase may cause switching behavior of the PL induced by a dramatic change in the molecular aggregation. To investigate the influence of thermal stress on the PL, we subsequently focused on temperature-dependent PL behaviors through the Cr ⇄ LC phase transition. The PL behavior of the hexylene-linker compounds V-3b and Y-4b was evaluated as a selected example using an absolute quantum yield spectrometer with a temperature-control unit, and the PL spectra for temperatures of 45–180 °C during the second cooling process are shown in Figure .

Figure 6

Temperature-dependent PL behavior of (A) V-3b and (B) Y-4b during the second cooling process. Gray: crystalline, green: NX, blue: N, and red: Iso phase.

For V-3b, during the first heating–cooling cycle between 45 and 180 °C, the emission maximum wavelength (λPL) in the Cr phase shifted to 444 nm (a long-wavelength shift of 11 nm, Figure S34, and Table S2); this shift may have been induced by the reformation of molecular aggregation via the fluidic N phase at 180 °C. In the subsequent second heating from 45 to 180 °C, λPL was observed to shift toward the shorter wavelength region by 14 nm, resulting in a λPL of 430 nm at 180 °C, and the PL efficiency (ΦPL) also dropped from 0.18 at 45 °C (Cr) to 0.09 at 180 °C (N). Interestingly, as shown in Figure A, during the second cooling from 180 to 45 °C, the λPL value completely recovered to 444 nm and was accompanied by the recovery of the PL efficiency (ΦPL = 0.18). In the case of Y-4b, similarly, the λPL and ΦPL values were observed to change with the temperature from 415 nm (ΦPL = 0.011) at 180 °C to 455 nm (ΦPL = 0.043) at 45 °C (Figure B and Table S3). The observed temperature-dependent PL behavior suggested that the phase transitions among the Cr → NX → N phases induced the appearance of fluidity with directional anisotropy and dramatically altered intermolecular interactions, resulting in changes in the PL properties, for example, λPL and ΦPL. Notably, a significant decrease in ΦPL during the heating process is a typical phenomenon because of the significant promotion of nonradiative deactivation through the dynamic molecular motions induced by thermal energy.

In terms of practical application to thermosensing materials, for example, fluorescence thermometers,[27] at this stage, it would be still challenging to apply the compounds V-3b and Y-4b in fluorescence thermometers because of their insufficient PL efficiencies and narrow PL wavelength shifts. However, the fact that the PL properties of V-3b and Y-4b did not change significantly during the repeated heating–cooling processes can confidently be attributed to their LC characteristics, which allow their aggregated structures to be precisely tuned depending on the thermal stimulus, which represents a significant advantage for reusable fluorescence thermometers.

Conclusions

Novel V- and Y-shaped organic compounds bearing two or three pentafluorinated bistolane-based luminophores connected to the core by propylene or hexylene linkers were successfully synthesized via a facile four-step manipulation from readily accessible 4-[3-(methoxymethoxy)propyloxy]phenylacetylene or 4-[6-(methoxymethoxy)hexyloxy]phenylacetylene. POM and DSC measurements demonstrated that the present novel V- and Y-shaped compounds displayed LC characteristics, which brought about the formation of a distinct LC phase owing to their unique molecular geometry in comparison with the previously reported linear analogue. The LC properties, for example, Tm, Tc, and the temperature range for the LC phase, changed depending on the molecular geometry as well as the length of flexible chain moiety, which could allow control of the LC behavior by means of a precise tuning of the primary molecular structure. In addition, both the V- and Y-shaped compounds with the pentafluorinated bistolane-based luminophore showed PL properties under various conditions, such as in solution and in the solid state. The PL behavior, for example, λPL and ΦPL, was very similar for all of the compounds, with a slight difference in the λPL being observed with changing molecular geometry or the length of the flexible unit, which is likely because of the dramatic change in the molecular aggregated structures in the solid state. Notably, these compounds were found to emit PL even in LC phases, with decreasing emission efficiency (ΦPL) at higher temperature, which clearly demonstrates that the present V- and Y-shaped luminophores function as light-emitting liquid crystals. Further improvement of the thermal sensitivity of PL characteristics of the bistolane-type luminophores, including ΦPL and the maximum wavelength shift, could lead to practical utilization as useful thermosensing materials, such as fluorescence thermometers.

Methods

General

1H and 13C NMR spectra were obtained using AVANCE III 400 NMR spectrometer (1H: 400 MHz and 13C: 100 MHz) in chloroform-d (CDCl3) (Bruker, Germany), and the chemical shifts are reported in parts per million based on the residual proton signal of the NMR solvent. 19F NMR (376 MHz) spectra were obtained using AVANCE III 400 NMR spectrometer in CDCl3 with CFCl3 (δF = 0 ppm) as an internal standard (Bruker, Germany). Infrared spectra (IR) were recorded using the KBr method with FT/IR-4100 typeA spectrometer (JASCO, Japan); all spectra are reported in wavenumbers (cm–1). High-resolution mass spectra (HRMS) were recorded on JMS-700MS spectrometer (JEOL, Japan) using the fast-atom bombardment (FAB) method.

Materials

Starting materials for the preparation of 1 were obtained from the following commercial source; 4-bromophenol, 3-bromopropan-1-ol, 6-bromohexan-1-ol, chloromethyl methyl ether, triethylamine, and pentafluoroiodobenzene were purchased from FUJIFILM Wako Pure Chemical Corporation. Trimethylsilylacetylene was obtained from Apollo Scientific Ltd. A palladium catalyst, that is, Cl2Pd(PPh3)2, was purchased from Aldrich Fine Chemicals. PPh3 and CuI were also purchased from FUJIFILM Wako Pure Chemical Corporation. Column chromatography was carried out on silica gel (Wakogel 60N, 38–100 μm), and thin-layer chromatographic (TLC) analysis was performed on silica gel TLC plates (Merck, Silica gel 60F254). All reactions were carried out using dried glassware and magnetic stirrer bars. Typical synthetic procedures for V-3a and Y-4a and spectral data for the V-3 and Y-4 are described below. In the Supporting Information are summarized all synthetic procedures and spectral data for other compounds.

Typical Procedure for the Preparation of Bis[3-[4-(4-(2,3,4,5,6-pentafluorophenyl)-1-ethynyl)phenylethynyl]phenoxypropyl] Isophthalate (V-3a)

In a 100 mL two-necked round-bottomed flask was placed 2,3,4,5,6-pentafluoro-1-[2-[4-(2-(4-(3-hydroxypropoxy)phenyl)ethyn-1-yl)phenyl]ethyn-1-yl]benzene (2a, 0.49 g, 1.1 mmol), isophthaloyl chloride (0.10 g, 0.52 mmol), and CH2Cl2 (48 mL). To the solution was added slowly Et3N (1.3 mL, 9.4 mmol) at 0 °C, followed by the addition of DMAP (12 mg, 98 μmol) in one portion. The whole was stirred at room temperature. After 14 h, the resultant was poured into a saturated aqueous NH4Cl solution. The crude product was then extracted with AcOEt (three times), and the combined organic layer was washed with brine, dried over anhydrous Na2SO4, filtered, and the solvent was removed using a rotary evaporator. The resulting residue was subjected to column chromatography to obtain the title compound V-3a in 23% (0.12 g, 0.11 mmol) as a white solid.

Bis[3-[4-(4-(2,3,4,5,6-Pentafluorophenyl)-1-ethynyl)phenylethynyl]phenoxypropyl] Isophthalate (V-3a)

Yield: 23% (white solid); mp: 185 °C determined by DSC; 1H NMR (CDCl3): δ 2.29 (quin, J = 6.0 Hz, 4H, CH2CH2CH2 × 2), 4.16 (t, J = 6.0 Hz, 4H, CH2O × 2), 4.57 (t, J = 6.0 Hz, 4H, CH2O × 2), 6.89 (d, J = 8.4 Hz, 4H, Ar-H), 7.43–7.58 (m, 14H, Ar-H), 8.24 (dd, J = 7.6, 1.6 Hz, 2H, Ar-H), 8.68 (t, J = 1.2 Hz, 1H, Ar-H); 13C NMR (CDCl3): δ 28.7, 62.2, 64.5, 87.7, 92.1, 100.0–102.0 (m, 1C), 114.6, 115.1, 120.8, 125.0, 128.7, 130.7, 131.4, 131.8, 133.2, 133.9, 159.1, 165.6, other six carbon signals including C6F5 moiety could not be detected because of the lack of solubility in the lock solvent; 19F NMR (CDCl3): δ −136.47 (dd, J = 21.4, 6.8 Hz, 2F, Ar-F), −152.98 (t, J = 20.3 Hz, 1F, Ar-F), −162.26 (ddd, J = 21.4, 20.3, 6.8 Hz, 2F, Ar-F); IR (KBr) ν: 2965, 2206, 1718, 1598, 1524, 1497, 1250, 1093 cm–1; HRMS (FAB) m/z: [M]+ calcd for C58H32O6F10, 1014.2039; found, 1014.2070.

Bis[6-[4-(4-(2,3,4,5,6-Pentafluorophenyl)-1-ethynyl)phenylethynyl]phenoxyhexyl] Isophthalate (V-3b)

Yield: 79% (white solid); mp 121 °C determined by DSC; 1H NMR (CDCl3): δ 1.46–1.62 (m, 8H, CH2CH2 × 2), 1.84 (quin, J = 6.4 Hz, 8H, CH2CH2O × 2), 3.98 (t, J = 6.4 Hz, 4H, CH2O × 2), 4.37 (t, J = 6.4 Hz, 4H, CH2O × 2), 6.86 (d, J = 8.8 Hz, 4H, Ar-H), 7.45 (d, J = 8.8 Hz, 4H, Ar-H), 7.50 (ABq, J = 8.4 Hz, 4H, Ar-H), 7.54 (ABq, J = 8.4 Hz, 4H, Ar-H), 8.23 (dd, J = 7.6, 1.6 Hz, 2H, Ar-H), 8.69 (t, J = 1.6 Hz, 1H, Ar-H), one 1H signal at 5-position of isophthalate is 7.48–7.56 ppm, which is overlapped with other aromatic signals; 13C NMR (CDCl3): δ 25.76, 25.83, 28.6, 29.1, 65.3, 67.8, 74.6, 87.6, 92.3, 100.2 (ddd, J = 18.3, 14.7, 2.9 Hz), 101.2 (d, J = 2.9 Hz), 114.6, 114.7, 120.7, 125.1, 128.6, 130.6, 130.9, 131.4, 131.8, 133.2, 133.7, 136.0–139.4 (dm, J = 248.6 Hz, C–F), 139.7–143.4 (dm, J = 244.2 Hz, C–F), 145.8–148.8 (dm, J = 246.4 Hz, C–F), 159.4, 165.8; 19F NMR (CDCl3): δ −136.47 (dd, J = 21.8, 6.8 Hz, 2F, Ar-F), −152.98 (t, J = 20.3 Hz, 1F, Ar-F), −162.25 (ddd, J = 21.8, 20.3, 6.8 Hz, 2F, Ar-F); IR (KBr) ν: 2936, 2851, 2207, 1721, 1598, 1524, 1498, 1243, 1109 cm–1; HRMS (FAB) m/z: [M]+ calcd for C64H44O6F10, 1098.2978; found, 1098.2977.

Typical Procedure for the Preparation of tris[3-[4-(4-(2,3,4,5,6-Pentafluorophenyl)-1-ethynyl)phenylethynyl]phenoxypropyl] 1,3,5-Benzenetricarboxylate (Y-4a)

In a 100 mL two-necked round-bottomed flask was placed freshly prepared 2a (0.42 g, 0.95 mmol), 1,3,5-benzenetricarbonyl trichloride (0.080 g, 0.30 mmol), and CH2Cl2 (30 mL). To the solution was added slowly Et3N (0.75 mL, 5.4 mmol) at 0 °C, followed by the addition of DMAP (10 μg, 0.082 mmol) in one portion. The whole mixture was stirred at room temperature. After 22 h, the resulting solution was poured into a saturated aqueous NH4Cl solution. The crude product was then extracted with AcOEt (three times), and the combined organic layer was washed with brine, dried over anhydrous Na2SO4, filtered, and the solvent was removed using a rotary evaporator. The resulting residue was subjected to column chromatography to obtain the title compound Y-4a in 19% (0.083 g, 0.056 mmol) as a white solid.

Tris[3-[4-(4-(2,3,4,5,6-Pentafluorophenyl)-1-ethynyl)phenylethynyl]phenoxypropyl] 1,3,5-Benzenetricarboxylate (Y-4a)

Yield: 19% (white solid); mp: 175 °C determined by DSC; 1H NMR (CDCl3): δ 2.30 (quin, J = 6.0 Hz, 6H, CH2CH2CH2 × 3), 4.15 (t, J = 6.0 Hz, 6H, CH2O × 3), 4.60 (t, J = 6.0 Hz, 6H, CH2O × 3), 6.88 (d, J = 8.8 Hz, 6H, Ar-H), 7.45 (d, J = 8.8 Hz, 6H, Ar-H), 7.48 (ABq, J = 8.4 Hz, 6H, Ar-H), 7.52 (ABq, J = 8.4 Hz, 6H, Ar-H), 8.85 (s, 1H, Ar-H); 13C NMR (CDCl3): δ 28.6, 62.7, 64.5, 74.6 (q, J = 2.9 Hz), 87.7, 92.1, 100.2 (ddd, J = 20.5, 15.3, 2.2 Hz), 101.2 (d, J = 3.6 Hz), 114.6, 115.2, 120.8, 125.0, 131.3, 131.4, 131.8, 133.2, 134.6, 136.1–139.4 (dm, J = 254.5 Hz, C–F), 140.0–143.6 (dm, J = 251.6 Hz, C–F), 145.6–148.8 (dm, J = 245.8 Hz, C–F), 159.0, 164.8; 19F NMR (CDCl3): δ −136.47 (dd, J = 21.8, 6.8 Hz, 2F, Ar-F), −152.94 (t, J = 21.8 Hz, 1F, Ar-F), −162.24 (ddd, J = 21.8, 21.8, 6.8 Hz, 2F, Ar-F); IR (KBr) ν: 2963, 2361, 2211, 1734, 1597, 1525, 1498, 1240, 1047 cm–1; HRMS (FAB) m/z: [M]+ calcd for C84H46O9F15, 1482.2824; found, 1482.2824.

Tris[6-[4-(4-(2,3,4,5,6-Pentafluorophenyl)-1-ethynyl)phenylethynyl]phenoxyhexyl] 1,3,5-Benzenetricarboxylate (Y-4b)

Yield: 19% (yellow solid); mp: 110 °C determined by DSC; 1H NMR (CDCl3): δ 1.42–1.68 (m, 12H, CH2CH2 × 3), 1.71–1.92 (m, 12H, CH2CH2CH2CH2 × 3), 3.96 (t, J = 5.6 Hz, 6H, CH2O × 3), 4.40 (t, J = 6.0 Hz, 6H, CH2O × 3), 6.84 (d, J = 8.4 Hz, 6H, Ar-H), 7.43 (d, J = 8.4 Hz, 6H, Ar-H), 7.44–7.58 (m, 12H, Ar-H), 8.85 (s, 3H, Ar-H); 13C NMR (CDCl3): δ 25.7, 25.8, 28.5, 29.0, 65.6, 67.8, 74.5 (d, J = 2.9 Hz), 87.5, 92.2, 100.1 (ddd, J = 18.3, 14.7, 3.6 Hz), 101.2 (d, J = 2.2 Hz), 114.5, 114.6, 120.6, 125.0, 131.3, 131.4, 131.7, 133.1, 134.3, 136.0–139.2 (dm, J = 253.0 Hz, C–F), 139.8–143.1 (dm, J = 256.6 Hz, C–F), 145.5–148.8 (dm, J = 256.7 Hz, C–F), 159.4, 165.0; 19F NMR (CDCl3): δ −136.50 (dd, J = 20.3, 5.3 Hz, 2F, Ar-F), −153.0 (t, J = 20.3 Hz, 1F, Ar-F), −162.26 (ddd, J = 20.3, 20.3, 5.3 Hz, 2F, Ar-F); IR (KBr) ν: 2041, 2860, 2360, 2211, 1730, 1524, 1498, 1244, 1109 cm–1; HRMS (FAB) m/z: [M]+ calcd for C93H63O9F15: 1608.4233; found: 1608.4222.

X-ray Diffractometry

Powder XRD measurement was performed using a Rigaku SmartLab (Japan) with Cu Kα radiation (λ = 1.5418 Å) at 20 °C intervals from 200 to 60 °C during the second cooling process.

Photophysical Properties

UV–vis absorption spectra were recorded using a JASCO V-500 absorption spectrometer (JASCO, Japan). Steady-state PL spectra were obtained using a Hitachi F-7000 (Hitachi, Japan) or a JASCO FP-8500 fluorescence spectrometer (JASCO, Japan). PL quantum yields (PLQYs) were estimated using a calibrated integrating sphere system attached to a JASCO FP-8500 fluorometer. PLQYs during the heating/cooling processes were measured by using a Quantaurus-QY spectrometer (Hamamatsu Photonics K.K., Japan).

Thermal Properties

Phase-transition properties were observed by POM using an Olympus BX53 microscope (Olympus, Japan) equipped with a cooling and heating stage (Linkam Scientific Instruments, 10002L, UK). The thermodynamic behavior of each compound was determined using DSC (SHIMADZU DSC-60 Plus, Shimadzu, Japan) at heating and cooling rates of 5.0 or 10 °C min–1 for the V-shaped 3a and 3b and Y-shaped 4a and 4b.

Computation

All computations were performed using DFT with the Gaussian 09 (rev. C.01) program package,[24] and the geometric optimizations were executed using the CAM-B3LYP hybrid functional[25] and the 6-31G(d) basis set. Vertical excitations were also calculated using a time-dependent DFT method at the same level of theory.