Innovative molecular design of bridged biphenyls for calamitic nematic liquid crystals with extensive π-conjugated mesogens

Abstract

To develop advanced materials based on calamitic nematic liquid crystals, it is essential to design functional optoelectronic mesogens that can form nematic phases at low temperatures. This study proposes a new molecular design strategy for low-temperature nematic liquid crystals using large π-conjugated mesogens with optical/electrical functions. Bridged biphenyls were synthesized by bridging the two phenyl rings with propylene. This bridging structure reduced the molecular planarity and prevented the molecules from aligning neatly in one direction, resulting in lowering the temperature range of the nematic phases. Terphenyl and phenyltolane derivatives exhibited supercooled nematic phases at room temperature, while quarterphenyl and bis(phenylethynyl)-biphenyl derivates exhibited nematic phases below 100 °C. The proposed design is more effective for rigid mesogens compared to conventional calamitic nematic liquid crystal design.

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Introduction

Since their discovery, calamitic nematic liquid crystals (NLCs) have been intensively investigated owing to their unique properties, including electrical, magnetic, and optical anisotropy, as well as high fluidity.¹ In recent years, new nematic phases as the chiral nematic (cholesteric),^2–6 twist-bend,^7–10 biaxial,¹¹ blue,^12–14 splay nematic phases,¹⁵ and ferroelectric structures¹⁶ have been investigated. In addition, there has been the development of unique liquid crystal molecules which have a nematic phase.^17–28 From a materials viewpoint, calamitic NLCs are used in liquid crystal displays^29,30 and other functional materials, such as optical films^31–34 and distributed-feedback lasers.^35–39 These applications exploit the anisotropy and fluidity of liquid crystals. Additionally, molecules with optical/electrical functions within the liquid crystals are also gaining significant attention, leading to the development of calamitic NLCs with large π-conjugated systems.^40–45 More sophisticated materials and devices, such as dye-doped high-speed fluorescent switching devices^46–51 and liquid-crystalline organic semiconductors,^52–55 are currently being studied. In general, the nematic phase temperature range of π-conjugated liquid crystals with optical/electrical functions is often too high owing to the high melting point of the mesogen itself and strong intermolecular interactions resulting from their rigid skeletons and functional groups.^56–58 Extending the hydrocarbon chains in the tail to lower this temperature range tends to promote the formation of smectic phases.^59,60

The most common approach to addressing this issue is to introduce fluorine into the lateral position of mesogens.⁶¹ This method has been effective, as some terphenyl and phenyltolane molecules form a nematic phase below 100 °C due to fluorine incorporation.^23,40 However, introducing fluorine can sometimes alter the electronic states of the original molecules, significantly changing their optical and electrical properties.^24,61–63 Other molecular design strategies include introducing a substituent larger than fluorine and using branch chains in the tail.^64–67 Nevertheless, a new molecular design strategy that can be applied to various mesogens with optical and electrical functions is required for further development of calamitic NLCs.

Very recently, we reported π-extended bridged stilbenes that exhibited nematic phases at near room temperature with luminescence properties.⁶⁸ Bridged stilbenes have the 7-membered ring structure that connects the vinyl group and the phenyl ring of stilbenes with a propylene group, referred to as the ‘bridging structure.’ The flexible bridging structure promoted the intramolecular rotation and the bending motion of the π-conjugated skeletons, enabling the formation of various conformations. This increased the phase-transition entropies and contributed to the lowering of the temperature range of the nematic phase.

In this study, we synthesized bridged biphenyls by introducing a bridging structure to biphenyls, which are more common mesogens compared to stilbenes (Fig. 1).

Fig. 1 The conceptual image of the bridged stilbene and the bridged biphenyl.

Additionally, we developed biphenyl, terphenyl, quarterphenyl, phenyltolane, and bis(phenylethynyl)-biphenyl-type molecules. Terphenyl- and phenyltolane-type molecules that formed a nematic phase, even at room temperature, were obtained. Additionally, we achieved nematic phases below 100 °C with quarterphenyl and bis(phenylethynyl)-biphenyl molecules. The effect of the bridging structure in the bridged biphenyl also investigated and found to differ from that in the bridged stilbene. Despite this difference, the molecular design strategy of introducing a bridging structure proved superior to previous strategies in terms of lowering the nematic phase temperature range and applicability to large π-conjugated molecules.

Results and discussion

Synthesis and characterization

We synthesized biphenyl, terphenyl, quarterphenyl, phenyltolane, and bis(phenylethynyl)-biphenyl derivatives containing bridged biphenyl by introducing alkyl cyclic structure into biphenyl. For comparison, we synthesized a terphenyl derivative with two methyl groups introduced into the mesogen. The chemical structures of the molecules used in this study are shown in Fig. 2. The synthesis procedures for seven-membered bridged biphenyl, Bc5CT[7], and B5CT[7] are illustrated in Scheme 1, and those for the other final products are shown in Schemes S3–S5 (ESI†).

Fig. 2 The molecules list of this research.

Scheme 1 Synthesis of Bc5CT[7] and B5CT[7].

Seven-membered bridged biphenyl was synthesized from 2-formylphenylboronic acid (1) and 2′-bromoacetophenone (2) using Suzuki–Miyaura cross-coupling reaction, intermolecular aldol condensation and reduction by triethylsilane in a trifluoro acetate solution.⁶⁹ The cyano group was introduced through the formylation of the bromo group, followed by treatment with iodine and ammonia.⁷⁰ The alkoxy group was formed by first converting the bromo group to a boronate, then reducing it with m-chloroperoxybenzoic acid, and finally performing a Williamson ether reaction. Alkylation of the bromo group was achieved using a B-alkyl Suzuki–Miyaura cross-coupling reaction with 9-BBN.⁷¹ Terphenyl and quarterphenyl derivatives were synthesized from the corresponding bridged biphenyl derivatives and arylboronic acids via Suzuki–Miyaura cross-coupling.^72–74 Phenyltolane and bis(phenylethynyl)biphenyl derivatives were synthesized from the corresponding bridged biphenyl derivatives and 4-ethynyl aryl via Sonogashira cross-coupling.²⁴

The synthesized compounds were identified using ¹H-NMR, ¹³C-NMR, and HR-EI spectroscopies. FT-IR spectroscopy was used for molecules with cyano groups.

Phase transition behaviours

The phase transition behaviors of the synthesized compounds were evaluated using DSC and POM. Using POM, we observed the phase transition of the compounds during cooling and heating at a rate of 10 °C min⁻¹ for a molten sample sandwiched between untreated glass plates. The phase transition temperatures and enthalpies of synthesized molecules, 5CB, and 5CT during the second heating/cooling process are listed in Table 1. The bar graphs of the phase transition temperatures and ranges are shown in Fig. 3. The DSC thermograms are provided in Fig. 4a, 5a, and ESI.†

Table 1 Phase transition behaviours of the compounds, determined by DSC measurement at a rate of 10 °C min⁻¹ upon 2nd heating and cooling

Entry	Phase transition temperature [°C] (enthalpy [kJ mol^−1])
	Heating	Cooling
a There were no peak in DSC thermogram and no phase transition in POM observation at a rate of 10 °C min^−1, but B8CB[7] and B7OCB[7] showed a smectic C and a crystalline phase at room temperature after a few days in POM observation, respectively. On the other hand, B5CT[7] exhibited an isotropic–nematic phase transition after a few minutes and a gradual nematic–crystalline phase transition after a few more days at room temperature. b Phase transition temperatures were determined by POM.
5CB	N 35.8 (0.6) Iso	Iso 33.8 (−0.6) N
B5CB[6]	SmC 5.6 (0.4) Iso 31.5 (—) Cry 43.4 (—) Iso	Iso 3.7 (−0.4) SmC
B5CB[7]	—	—
B8CB[7]	—	—^a
B7OCB[7]	—	—^a
5CT	Cry1 105.6 (0.8) Cry2 130.9 (10.6) N 238.2 (1.2) Iso	Iso 236.3 (−1.1) N 126.8, 124.2 (−10.0) Cry1
DMc5CT	Cry1 −5.9 (−0.5) Cry2 84.0 (23.7) Iso	Iso 9.3 (−15.4) Cry1
Bc5CT[6]	N 43.6, 59.0 (−18.6) Cry 113.5 (28.3) N 208.4 (1.0) Iso	Iso 206.6 (−0.9) N
Bc5CT[7]	Cry1 36.0 (−7.7) Cry2 67.6 (−3.2) Cry3 120.7 (28.7) Iso	Iso 83.3 (−1.4) N 52.9 (−3.8) Cry1
B5CT[7]	—	—^a
B5T[7]	N 10.9 (0.25) Iso	Iso 8.9 (−0.2) N
B5(5O)T[7]	N 18.1 (−15.8) Cry 50.2 (21.5) N 59.7, 61.6 (2.2) Iso	Iso 57.6 (−0.5) N
B5Q[7]	Cry 142.6 (32.2) N 178.4 (1.0) Iso	Iso 176.3 (−1.1) N 82.4 (−26.3) Cry
B5OQ[7]	Cry 164.6 (40.9) N 229.2 (1.3) Iso	Iso 227.3 (−1.3) N 136.4 (−41.1) Cry
B5PhT[7]	N 49.2 (0.9) Iso	Iso 47.0 (−1.0) N
B5(5O)PhT[7]	N 36.2 (−23.2) Cry 78.9 (31.7) N 94.6 (1.4) Iso	Iso 92.5 (−2.0) N
BC5PhT[7]	Cry 132.2 (25.9) Iso	Iso 130.7 (−1.2) N 115.6 (−22.9) Cry
B5CPhT[7]	N 101.1 (0.3), 104.9 (0.6) Iso	Iso 102.4 (0.9) N
B5BPhEB[7]	Cry 120 N 240 Iso	Iso 237 N 52 Cry
B5OBPhEB[7]	Cry 150 N 260 Iso	Iso 254 N113 Cry

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Fig. 3 Schematic diagram showing the phase transition temperatures and ranges of the compounds, except for B5CB[7], B8CB[7], B7OCB[7], and B5CT[7]; upper bar: upon heating, lower bar: upon cooling.

Fig. 4 (a) DSC thermograms of DMc5CT at a rate of 10 °C min⁻¹ upon 2nd heating and cooling, and POM images of DMc5CT at (b) 0 °C during cooling and (c) room temperature left for a few days.

Fig. 5 (a) DSC thermograms of Bc5CT[7] at a rate of 10 °C min⁻¹ upon 2nd heating and cooling, and POM images of Bc5CT[7] at (b) 40 °C during heating, (c) 70 °C during heating, (d) 70 °C during cooling, and (e) 40 °C during cooling.

Phase transition behaviours of biphenyl derivatives. B5CB[6] exhibited endothermic peaks at 5.6 and 43.5 °C, as well as an exothermic peak at 31.5 °C in the heating DSC thermogram (Fig. S2, ESI†). The peaks at 31.5 °C and 43.5 °C overlapped, preventing the calculations of their ΔH values. In contrast, in the cooling DSC thermogram, only one exothermic peak at 3.7 °C was observed. When the sample of B5CB[6] was rapidly cooled from 100 to 0 °C by an ice water bath, a schlieren-like texture was observed by POM. But the proper POM image could not be taken because the texture was changed quickly after removal from the bath. The WAXD measurement of B5CB[6] could also not be performed due to having no appropriate equipment. We considered this phase to be smectic C because of its no fluidity, even though the ΔH were very small (= 0.4 kJ mol⁻¹). The crystallization continued when the sample was left at room temperature (Fig. S3, ESI†).

None of the seven-membered bridged biphenyl derivatives exhibited phase transitions in the DSC thermograms (Fig. S4, S5, and S7, ESI†). B5CB[7] maintained a dark field, B8CB[7] formed a schlieren texture (Fig. S6, ESI†), and B7OCB[7] crystallized when each prepared slide was left at room temperature for a few days (Fig. S8, ESI†). The schlieren texture of B8CB[7] exhibited no fluidity; thus, this phase is smectic C.

Bridged biphenyl derivatives did not show a nematic phase, unlike 5CB. These results were attributed to multiple factors, such as the steric hindrance of the bridging moiety, decrease in the planarity of the biphenyl, and absence of a biaryl rotational axis.

Terphenyl derivatives: DMc5CT, Bc5CT[6], Bc5CT[7], and B5CT[7]. DMc5CT did not form a liquid crystalline phase. The DSC thermograms of DMc5CT exhibited one endothermic peak and one exothermic peak during heating (Fig. 4a). Additionally, one endothermic peak was observed during cooling. In POM, DMc5CT exhibited only a crystalline phase (Fig. 4b and c). The large steric hindrance and torsion of the mesogen by the methyl groups inhibit the formation of liquid crystalline phases.

Bc5CT[6] formed an enantiotropic nematic phase. The DSC thermograms exhibited two exothermic peaks at 43.6 and 59.0 °C and two endothermic peaks at 113.5 and 208.4 °C during heating, while only one exothermic peak was observed at 206.6 °C during cooling (Fig. S10, ESI†). POM observations revealed that Bc5CT[6] exists in a nematic phase (Fig. S11b, ESI†). A gradual nematic-to-crystalline phase transition was observed at 40 °C (Fig. S11c, ESI†). The crystallization rate of Bc5CT[6] was lower than that of 5CT. In particular, the lower temperature limit of the nematic phase during cooling was significantly reduced.

In contrast, Bc5CT[7] formed a monotropic nematic phase, showing two exothermic peaks and one endothermic peak during heating and two exothermic peaks during cooling in the DSC thermograms (Fig. 5a). In POM, a nematic phase was observed between the two exothermic peaks during cooling (Fig. 5d). Moreover, unlike Bc5CT[6], Bc5CT[7] exists in at least three different crystalline phases (Fig. 5b, c and e). The isotropic-to-nematic and the nematic-to-crystalline phase transition temperatures of Bc5CT[7] are below at 153.0 °C and 71.5 °C compared to those of 5CT, respectively.

B5CT[7], with a different bridge position from Bc5CT[7], exhibited no phase transition peak during heating at a rate of 10 °C min⁻¹ (Fig. S12, ESI†). When the prepared slide of the once-melted B5CT[7] was left at room temperature for a few minutes, the isotropic–nematic phase transition proceeded, and the nematic–crystalline phase transition developed gradually after a few more days (Fig. S13, ESI†). This indicates that steric hindrance of the bridging moiety inhibit crystallization. The crystallized B5CT[7] transitioned to an isotropic phase at 86.7 °C upon heating at a rate of 10 °C min⁻¹ in POM.

The diversity of crystalline systems due to the bridging structure and the associated significant reduction in the phase transition temperatures occurred, which is also common to the stilbene and distyrylbenzene dyes we have previously reported.^68,75–78 This result suggests that the molecular design strategy of the seven-membered bridging is effective for the development of calamitic liquid crystals with a nematic phase near room temperature. The effect of the bridging structure in the bridged biphenyl is discussed later.

B5T[7] and B5(5O)T[7]. B5T[7] exhibited an enantiotropic nematic phase. One endothermic and exothermic peaks below room temperature were observed in the heating and cooling DSC thermograms, respectively (Fig. S14, ESI†). Immediate POM observations revealed a nematic phase after cooling the prepared slide to 0 °C (Fig. S15, ESI†).

B5(5O)T[7] also exhibited an enantiotropic nematic phase, with one exothermic peak and three endothermic peaks during heating. Moreover, it exhibited one exothermic peak during cooling in the DSC thermograms (Fig. S16, ESI†) and a schlieren texture (Fig. S17b and c, ESI†). B5(5O)T[7] maintained the nematic phase after cooling to 0 °C at a rate of 10 °C min⁻¹ and underwent cold crystallization upon heating to 20 °C, as indicated by a broad peak, indicating the remarkable effect of the seven-membered bridging structure. Prepared slide of B5(5O)T[7] transitioned from the nematic to the crystalline phase when left at room temperature for a few minutes (Fig. S17a, ESI†). There were two exothermic peaks near the nematic to isotropic phase transition during heating, indicating that a nematic–nematic phase transition would occur. However, we could not observe any event in POM and could not conduct the WAXD measurement in this temperature range as it was only 1.9 °C. Therefore, this is not discussed in detail.

Regardless of the substituents, the seven-membered bridging structure contributed significantly to the reduction in the temperature range of the nematic phase. The results of applying this molecular design strategy to even larger π-conjugated molecules are presented below.

Quarterphenyl derivatives: B5Q[7] and B5OQ[7]. Both B5Q[7] and B5OQ[7] are enantiotropic nematic liquid crystals, and their DSC thermograms exhibited two endothermic and exothermic peaks during heating and cooling, respectively (Fig. S18 and S20, ESI†). Moreover, they exhibited schlieren textures in the temperature range between these peaks (Fig. 6a and Fig. S21, ESI†). The temperature ranges of the nematic phase of B5Q[7] were 142.6–178.4 °C and 176.3–82.4 °C in heating and cooling, and those of B5OQ[7] were 164.6–229.2 °C and 227.3–136.7 °C, respectively. B5OQ[7] has a higher phase transition temperature than B5Q[7] because of the increased intermolecular interactions with the O atoms.

Fig. 6 POM images of (a) B5Q[7] at 100 °C during cooling, (b) B5PhT[7] at room temperature, and (c) B5BPhEBT[7] at 80 °C during cooling.

Phenyltolane derivatives. B5PhT[7] is a nematic liquid crystal. The DSC measurements revealed only one peak during heating and cooling (Fig. S22, ESI†). Additionally, in the POM observations, a schlieren texture was evident at room temperature (Fig. 6b). At a cooling rate of 10 °C min⁻¹, B5PhT[7] did not crystallize, and the nematic phase was retained even after the prepared slide used for POM observation was left at room temperature for several weeks. Such a thermodynamically stable nematic phase at room temperature from a single compound is quite rare.

B5(5O)PhT[7] exhibited an enantiotropic nematic phase with phase transition behavior similar to B5(5O)T[7] (Fig. S23, ESI†). The phase transition temperatures of B5(5O)PhT[7] increased by 18–35 °C compared to those of B5(5O)T[7]. In the POM observations, a schlieren texture was observed, and the nematic-to-isotropic phase transition of B5(5O)PhT[7] occurred over a period of 15 minutes at room temperature (Fig. S24, ESI†).

BC5PhT[7] formed a monotropic nematic phase, and its DSC thermograms exhibited an endothermic peak at 133 °C during heating. Additionally, two exothermic peaks at 131 and 116 °C during cooling were observed (Fig. S25, ESI†). Under POM, BC5PhT[7] exhibited a schlieren texture between the two exothermic peak temperatures (Fig. S26, ESI†).

On the other hand, B5CPhT[7] exhibited a nematic phase; the nematic ⇄ isotropic phase transition temperatures during heating and cooling were 105 and 103 °C, respectively (Fig. S27, ESI†). Moreover, the nematic-to-nematic phase transition was also observed on heating in the DSC chart. However, this is not discussed in detail for the same reason as for B5(5O)T[7]. While B5CPhT[7] did not show any crystallization peak in the DSC measurement despite cooling to 0 °C, the prepared B5CPhT[7] crystallized when left at room temperature for a few minutes (Fig. S28, ESI†). Although the crystallization temperature was not determined, this crystalline phase melted at 100.0 °C and transitioned to the nematic phase.

Unlike B5CPhT[7], where the bridge is located near the pentyl group, BC5PhT[7], where the bridge is situated near the cyano group, crystallized on a time scale of 10 °C min⁻¹. This trend was also observed for terphenyl derivatives. This can be attributed to the significantly increased flexibility of the molecules due to the proximity of the pentyl chains and bridging structures. The cause of this phenomenon remains unclear, and the molecular alignments were not determined. This will be explored in future investigations.

Bis(phenylethynyl)-biphenyl derivatives. The phase transition temperatures of the bis(phenylethynyl)-biphenyl derivatives were determined by POM, because their prepared samples discoloured from colourless to brown when heated on a hot-stage just below their clearing points. DSC measurements were not conducted due to suspected sample decomposition.

The bis(phenylethynyl)-biphenyl derivatives transitioned from colorless to brown upon heating to their isotropic phases. In particular, B5BPhEB[7] exhibited an enantiotropic nematic phase, displaying a schlieren texture at 120–240 °C and 237–52 °C during heating and cooling, respectively (Fig. 6c and Fig. S29, ESI†). B5OBPhEB[7] also exhibited an enantiotropic nematic phase at 150–260 °C and 254–113 °C during heating and cooling, respectively (Fig. S30, ESI†).

The use of the seven-membered bridging molecular design strategy has enabled these large π-conjugated molecules to successfully form nematic phases at temperatures below 100 °C, and some terphenyl and phenyltolane derivatives exhibited nematic phases at room temperature. Even in very rigid skeletons such as quaterphenyl and bis(phenylethynyl)-biphenyl, this strategy was effective in lowering the nematic phase temperatures.

Birefringence

To investigate the effect of the bridge structure on the birefringence, we measured the temperature dependence of birefringence (Δn) for 5CT and Bc5CT[7] in the nematic phases, using the method described in our previous report.^79–81 However, Bc5CT[7] crystallized during the measurement process, thus we conducted measurements for quarterphenyl and bis(phenylethynyl)-biphenyl derivatives, whose mesogens consist of large π-conjugated skeletons.

This study followed the general practice of indexing the temperature dependence properties in nematic phases to the reduced temperature (T_IN − T), where T_IN is the isotropic-to-nematic phase transition temperature, and T is the measurement temperature. Fig. 7a compares the dependence of the Δn at 550 nm on T_IN − T for 5CT, B5Q[7], B5OQ[7], B5BPhEB[7], and B5OBPhEB[7]. Quarterphenyl and bis(phenylethynyl)-biphenyl derivatives with the bridging structure exhibited larger Δn than that of 5CT at the same T_IN − T value, despite 5CT having a cyano group at the end. Among the same mesogens, B5OQ[7] and B5OBPhEB[7] exhibited larger Δn than that of B5Q[7] and B5BPhEB[7], respectively, owing to the polarizability of terminal groups in the following order: alkoxy group > alkyl group. In addition, these results were applied to the empirical Haller's equation⁸² for further detailed analysis in nematic phases. The empirical Haller's equation is as follows;

S = Δn/Δn₀

Δn = Δn₀ (1 − T/T_IN)^β

where S is the order parameter, Δn₀ is Δn at S = 1, and β is a constant. The values of S and Δn at T_IN − T = 15 °C, as well as Δn₀ and β are summarized in Table 2, and the T_IN − T dependance of S is shown in Fig. 7b. The trend of Δn₀ was similar to that of the Δn; however, the Δn₀ value of B5OQ[7] was smaller than that of B5Q[7]. Focusing on S and β, the values of 5CT and B5Q[7] were nearly identical. In contrast, the S values of B5OQ[7], B5BPhEB[7], and B5OBPhEB[7] were very large (>0.70), and their β values were very small (<0.10); typical calamitic NLCs have β values of approximately 0.20. B5OQ[7], B5BPhEB[7], and B5OBPhEB[7] exhibited a sudden increase of Δn values when the temperature difference T_IN − T was approximately 60 °C, while B5BPhEB[7] and B5OBPhEB[7] exhibited no Δn increase when the T_IN − T was less than 30 °C. This indicates that the Δn₀ values of B5OQ[7], B5BPhEB[7], and B5OBPhEB[7] may not be accurate. Conversely, those of 5CT and B5Q[7] were accurately measured. Considering the facts that the Δn value of B5Q[7] was larger than that of 5CT at each T_IN − T, and the S value of B5Q[7] was almost the same as that of 5CT, the bridging structure does not seem to have a significant effect on the birefringence and the order parameter in nematic phases.

Fig. 7 (a) Δn and (b) ΔS of 5CT (●), B5Q[7] (▲), B5OQ[7] (△), B5BPhEB[7] (■), and B5OBPhEB[7] (□) at each T_IN − T.

Table 2 Experimental Δn values, as well as extrapolated parameters for 5CT, B5Q[7], B5OQ[7], B5BPhEB[7], and B5OBPhEB[7] at T_IN − T = 15 °C

Compound	Δn^a	Δn0^b	S	β
a At TIN − T = 15 °C. b Extrapolated by fitting to Haller's equation.
5CT	0.15	0.26	0.54	0.17
B5Q[7]	0.20	0.36	0.54	0.18
B5OQ[7]	0.26	0.31	0.83	0.05
B5BPhEB[7]	0.29	0.38	0.75	0.08
B5OBPhEB[7]	0.34	0.42	0.80	0.06

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Photophysical properties

The photophysical properties of B5PhT[7] and B5(5O)PhT[7], which exhibit nematic phases at room temperature, were measured in dilute solution, their solid states, and nematic phases (Table 3). Their absorption and fluorescence spectra are depicted in Fig. 8.

Table 3 Photophysical properties of B5PhT[7] and B5(5O)PhT[7]

Entry	State	ε [M^−1 cm^−1]	λ abs [nm]	λ fl [nm]
a Excited at 304 nm for B5PhT[7]. b Excited at 309 nm for B5(5O)PhT[7].
B5PhT[7]	THF	47 000	304 (max), 321	340, 354 (max)
	Solid	—	—	365 (max), 408
	Nematic phase	—	—	353, 406 (max)
B5(5O)PhT[7]	THF	44 000	309 (max), 326	346 (shoulder), 359
	Solid			380, 394 (shoulder)
	Nematic phase			375

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Fig. 8 Absorption (dashed line) and fluorescence (solid line) spectra of (a) B5PhT[7] and (b) B5(5O)PhT[7].

The absorption spectrum of B5PhT[7] in THF solution exhibits a vibrational structure with peaks (the absorption wavelength, λ_abs) at 304 nm (max) and 321 nm (Fig. 8a). As indicated in a previous report,⁸³ the peaks are derived from the S₀ → S₁ transition. The molar absorption coefficient (ε) was 47 000 M⁻¹ cm⁻¹. The fluorescence spectrum of B5PhT[7] in THF also shows a vibrational structure, and the fluorescence wavelengths (λ_fl) are 346 nm and 359 nm (max). The λ_fl of B5PhT[7] in the solid state are 365 nm and 408 nm, and the longer λ_fl is red-shifted by 50 nm compared to that in THF. The λ_fl values in the nematic phase are almost identical to those in the solid state, 363 nm and 406 nm. On the other hand, the maximum λ_fl values in these states are different, which are 365 nm and 406 nm in the solid and nematic phases, respectively.

Both of the absorption/fluorescence spectra of B5(5O)PhT[7] in THF solution exhibit a vibrational structure similar to those of B5PhT[7] (Fig. 8b). The λ_abs values are 304 nm and 321 nm (max), and λ_fl values are 346 nm (shoulder) and 359 nm, all of which are red-shifted by 5 nm compared to those of B5PhT[7]. This can be attributed to the narrowing of the HOMO–LUMO gap by the electro-donating alkoxy group. The fluorescence spectrum in the solid state has a maximum peak at 380 nm and shoulder at 394 nm and that in the nematic phase only shows one peak at 375 nm, which is different from that of B5PhT[7]. The increase in the electronic molecular interaction owing to the alkoxy group is likely to have contributed to this result.

B5PhT[7] exhibits different fluorescence behavior in THF solution, in both the solid and nematic phases. To conduct detailed investigations, we first measured the absorption/fluorescence spectrum of the polymethacrylate (PMMA, M_n = 1 000 000) cast film doped with 0.1 wt% B5PhT[7]; however, they are almost the same as those in THF solution (Fig. S36, ESI†). Subsequently, a B5PhT[7] agglomeration experiment in a THF/water system was performed, and the aggregation of B5PhT[7] could be observed in ≤30% THF suspension. The absorption spectra in ≤30% THF suspension exhibit the decrease in ε and the slight red-shifted longer-wavelength peak (Fig. 9a). The fluorescence spectra in 20% and 30% THF solution also changed slightly, and in THF 10% solution, a fluorescence peak appears at 394 nm, with a spectrum similar to a superposition of those in the THF solution and nematic phase (Fig. 9b). These results suggest that the fluorescence peak near 394 nm was caused by intermolecular interactions. Finally, the fluorescence lifetimes of B5PhT[7] in the THF solution, solid state, and nematic phase were measured. Unfortunately, the lifetime in the THF solution could not be measured because it was very short (<1.0 ns) and we did not have the proper wavelength cutting filter or LED laser. However, one of the components in the solid and nematic phase exhibited long lifetimes (solid: 6.8 ns, nematic phase: 12.8 ns), complementing the presence of the fluorescence peak resulting from intermolecular interactions (Table S1 and Fig. S37, ESI†). A single-crystal X-ray structure analysis is effective for further investigations of the fluorescence behavior in the solid and nematic phases, but the single crystal of B5PhT[7] was not obtained.

Fig. 9 (a) Absorption and (b) fluorescence spectra of B5CT[7] in THF/water mixed solvent at 1.0 × 10⁻⁵ M.

B5PhT[7] and B5(5O)PhT[7] show blue luminescence in THF, in their solid and nematic phases. They can be used as liquid crystalline and even optical materials. Furthermore, fluorescent liquid crystals show the potential to be applied for more advanced optoelectronics, such as electric-field-responsive fluorescent switch devices.^68,84,85

Structural analysis

A single-crystal X-ray analysis of Bc5CT[7] was conducted (Fig. 10). Bc5CT[7] had two atropisomers, and it formed a γ structure. The torsional angle of the biaryl axis at the bridging part in both conformations was 46.8°, and the intermolecular aromatic carbon distance was 3.53 Å. In this single crystal, the molecular long axis was oriented in two different directions, forming an angle of about 60°. The single-crystal X-ray structure of 5CT was previously reported.⁸⁶ In the single crystal, the molecule was determined to have only one conformation, and the molecular long axis was aligned in one direction (Fig. S38, ESI†). The torsional angle of the biaryl axis and intermolecular aromatic carbon distance of 5CT were 40.9° and 3.63 Å, respectively.

Fig. 10 (a) Molecular conformations and (b) the crystal structure of Bc5CT[7] (ESI†).

To investigate the conformation of the bridged biphenyl in more detail, the optimized structure of BT[7] was calculated using density functional theory (DFT) at the B3LYP/6-311G(d) level⁸⁷ (Fig. 11).

Fig. 11 (a) The chemical structure of BT[7], the conformations of (b) Opt-BT[7]-I and (c) Opt-BT[c]-II, and E_bar of (d) φ₁ and (e) φ₂.

BT[7] showed two optimized structures (Fig. 11b and c). The torsional angle of φ₁ of Opt-BT[7]-I and Opt-BT[7]-II were 47.8° and 47.3°, respectively, which were 6.6° and 6.1°, respectively, larger than that of biphenyl (φ₁ = 41.2°). These structures were in an atropoisomeric relationship, in which the positions of C_α and C_β relative to the paper plane were reversed, consistent with the results of single-crystal Bc5CT[7].

Calculations of Opt-BT[7]-I with φ₁ varying in 5°-steps showed that the rotational energy barrier (E_bar) significantly changed around φ₁ = 30° (Fig. 11d). Calculations performed for every 1° φ₁ between 25–35° showed a sudden change at 28–29° (Fig. S43, ESI†). The maximum E_bar was 117.8 kJ mol⁻¹, which was much larger than that of biphenyl (9.7 kJ mol⁻¹). The scan calculation had two local minimum values at −50° and 45°, where the structures were almost identical to those of Opt-Bt[7]-I and Opt-BT[7]-II, respectively (Fig. S41, ESI†). The results of a set of calculations for varying φ₂ are shown in Fig. 11e, and they exhibit a significant change of E_bar and two local minimum values, like φ₁. The maximum E_bar of φ₂ was about half that of φ₁. Moreover, E_bar did not change much around φ₂ = −20–30°. In this angular range, the change of φ₁ was also suppressed (Fig. S45, ESI†). The bridging structure caused this behavior, resulting in the E_bar remaining almost constant. Conversely, the structures at each local minimum value were close to those of Opt-BT[7]-I and Opt-BT[7]-II (Fig. S42, ESI†). In these calculations, there were only two stable conformations of the seven-membered bridged biphenyl. This was different from the case of the bridged stilbenes, which had various conformations.⁶⁸ The biaryl axis of the bridged biphenyl was more twisted compared to that of biphenyl. Considering the result of the single-crystal X-ray analysis (ESI†), the decrease in the intermolecular interaction due to the low molecular planarity and difficulty in aligning the molecular long axis in one direction contribute to the lower phase transition temperatures of bridged biphenyl derivatives. This mechanism is different from that of bridged stilbene derivatives, where the increase in the entropy from forming various conformations contributes significantly.

Conclusions

We synthesized biphenyl, terphenyl, quarterphenyl, phenyltolane, and bis(phenylethynyl)-biphenyl derivatives, incorporating a seven-membered bridged biphenyl structure in the mesogens. Except for the biphenyl-type compounds, all compounds formed nematic phases. The nematic phase temperature range of Bc5CT[7] was approximately 100 °C lower than that of 5CT, and it exhibited a nematic phase even at room temperature. In contrast, DMc5CT exhibited no liquid crystalline phase. Moreover, some terphenyl/phenyltolane skeleton molecules exhibited nematic phases at or below room temperature. B5Q[7] and B5BPhEB[7], whose mesogens are quarterphenyl and bis(phenylethynyl)-biphenyl, respectively, exhibited nematic phases even below 100 °C. These results indicate that the molecular design strategy of seven-membered bridging can potentially solve a long-standing challenge in the field of calamitic NLC research, which continues from Vorländer's research.⁸⁸ In this study, we have focused on the phase transition behavior of calamitic molecules with only one bridging introduced. On the other hand, those of calamitic molecules with multiple bridging introduced and the effect of introducing the bridging structure into biphenyl on the nematic phases remain unexplored. We will investigate these unknowns in the future. In addition, functional materials, such as liquid crystalline organic semiconductors and advanced materials,^89–100 will be developed using this molecular design strategy.

Data availability

The data supporting this article have been included as part of the ESI.† Crystallographic data for Bc5CT[7] has been deposited at the CCDC under 2410034 and can be obtained from https://www.ccdc.cam.ac.uk/.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank Yuki Sawatari (Department of Chemical Science and Engineering, Institute of Science Tokyo) for the birefringence measurements. We also thank Masato Koizumi (Materials Analysis Division, Institute of Science Tokyo) for the HRMS measurements. This division is independent of our laboratory to ensure fairness. YS thanks JSPS Research Fellowships for Young Scientists. This project was supported in part by MEXT/JSPS KAKENHI grants 24KJ1084 (YS), 23H02036 (GK), Iketani Science and Technology Foundation (YS), Murata Science and Education Foundation (GK). Tokyo Institute of Technology merged with Tokyo Medical and Dental University to form Institute of Science Tokyo (Science Tokyo) on October 1, 2024.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: DSC charts, POM figures, absorption and fluorescence spectra, synthetic procedures, NMR and mass charts, crystallographic analysis. CCDC 2410034. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qm01116c

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