A chiral luminescent liquid crystal with a tolane unit

ABSTRACT We have synthesised a chiral liquid crystal with a tolane unit that exhibits intense fluorescence in both solution and the solid states. The liquid crystal can form the enantiotropic twist-grain boundary A (TGBA*) and Blue II phases, and the helical pitch of the TGBA* phase decreases with an increasing temperature. Graphical Abstract

Exploration of new luminophores exhibiting high efficiency in the condensed phases is thus highly desirable for the development of luminescent liquid crystals.
In 2001, Tang and co-workers discovered a novel phenomenon of aggregation-induced emission (AIE): a silole derivative with a propeller-shaped structure is non-emissive when molecularly dissolved but emits efficiently upon aggregation. [8] Since then, several types of AIE-active luminogens have been synthesised and applied in diverse fields, such as organic light-emitting diodes, chemo-/biosensors and in cell imaging. [9] Thanks to their high solid-state fluorescence efficiency, AIE-active luminogens are also ideal building blocks for the construction of luminescent liquid crystals. In recent years, exploratory work has been conducted. [10][11][12] Zhao et al. synthesised a fluorescent liquid crystal with enhanced emission through combining the AIE-active tetraphenylethylene core and the mesogens. [13] Chen et al. reported a calamitic liquid crystal based on a tolane derivative exhibiting the AIE property. [14] However, chiral luminescent liquid crystals are rarely reported.
In our previous work, we found that a tolane derivative with electron donor and acceptor units emitted intensively in solution and aggregate states, showing intramolecular charge transfer and AIE characteristics. [15] Considering that tolane derivatives may show both liquid crystal behaviour and efficient luminescence in the condensed phase, herein we designed and synthesised a tolane derivative with a chiral substituent (1TC8*). This compound is highly emissive in the solution, liquid crystal and solid state. It can form the enantiotropic twist-grain boundary A (TGB A *), Blue II and monotropic crystalline E (CrE) phases. Furthermore, the helical pitch of its TGBA* phase decreases with an increasing temperature.
The molecular structure and phase transition temperatures of 1TC8* are shown in Chart 1. It was prepared according to the previously reported methods. [16,17] The detailed synthetic procedures are described in the supplementary information. Its chemical structure was characterised by fourier transform infrared spectroscopy, nuclear magnetic resonance and elemental analysis, from which satisfactory data were obtained (see the supplementary information for details).
The photoluminescence (PL) spectra of 1TC8* were recorded both in THF solution and as a thin film at room temperature. The thin film was prepared by the evaporation of its 1,2-dichloroethane solution with a concentration of 0.01 mol/L on a quartz plate. As shown in Figure 1, 1TC8* exhibits intense blue fluorescence in both solution and solid states. The fluorescence quantum yield of 1TC8* is 7.2% in its THF solution. Its emission maxima in solution and film states are 390 and 398 nm, respectively. The emission of the compound is red-shifted in the film state, suggesting that the molecular packing forms in the solid state. Moreover, it also emits intensively in the liquid crystalline state (shown in Figure S1).
We then investigated the phase transition behaviours using polarised optical micrography (POM) and differential scanning calorimetry (DSC). Figure 2 shows the POM images of 1TC8* at 130°C and 143°C in the cooling process. The focal-conic ( Figure 2(a)) and platelet (Figure 2(b)) textures were observed, respectively. The optical textures are distinctive of enantiotropic TGB A * and Blue II phases, respectively. [18,19] Additionally, a monotropic CrE phase was identified during the cooling process. Furthermore, a small peak at 142.5°C was observed in the DSC curves, which was ascribed to the phase transition from the blue phase state to the isotropic liquid ( Figure S2). The measured phasetransition temperature is in good agreement with the value of 143°C recorded by using POM. The sample was also characterised by X-ray diffraction. Unfortunately, no reflection peaks were obtained in the mesophase, which may be due to its good fluidity in the liquid crystal state.
Although the cholesteric liquid crystal phases have been extensively investigated using circular dichroism (CD), [20,21] the TGB A * and blue phases have not. [22] For a better understanding of the chiral organisation of  the molecules in the condensed states, CD spectra of 1TC8* were performed using a cell with a path length of 0.01 mm. Since the large CD signals were dependent on temperature, they should originate from the macroscopic helical structures. During the heating process, 1TC8* was CD silent at the temperatures of 25°C and 35°C (Figure 3(a)). Although 1TC8* was still in crystalline phase at the temperatures of 45°C and 55°C, positive CD signals were observed at approximately 360 nm and negative ones were observed at 297 nm, which were proposed to originate from the chiral stacking of the tolane groups. Though the bulky crystalline structure was maintained, the molecules at the surface of quartz might start to move. Moreover, negative CD signals were also identified at approximately 548 nm at 65°C and 75°C, which were proposed to associate with the chiral lamellar organisation of the molecules. These results suggest that the crystalline structure of 1TC8* was partially destroyed. When the temperature was increased to 85°C, a strong negative CD signal at 517 nm appeared, indicating a chiral lamellar structure (Figure 3(b)). Since the signal was negative, the molecules should stack in a left-handed sense. Both the intensity and the maximum wavelength of the CD signals decreased with increasing temperature from 85°C to 142°C. The CD signals at 297-363 nm originated from the chiral stacking of the tolane groups. The sample formed the Blue phase II liquid crystal at 143°C. Two negative CD signals were identified at 490 nm and 394 nm, demonstrating a double-twist arrangement. [23] Because these two signals were negative, the molecules should stack in a lefthanded sense with different helical pitches along two axes. Moreover, these results can also illustrate the structural colour of the blue phase. [24] No CD signals were found when the sample formed the isotropic state at 150°C.
During the cooling process, two negative CD signals were observed at 498 nm and 394 nm at 143°C, owing to the double twist arrangement (Figure 3(c)). The signal intensity at 498 nm was higher than that observed during the heating process, indicating the formation of a more ordered structure. The sample formed the TGB A * phase at 142°C and one negative CD signal was identified at 400 nm. Then, the maximum wavelength was redshifted as the temperature was decreased from 142°C to 65°C, which was driven by the increase of helical pitch. [25] Meanwhile, the colour of the sample changed gradually from blue to orange (Figure 4). Because the sample recrystallised at 38.9°C, no CD signals were observed at 35°C.
The single crystal of 1TC8* was grown in methanol by a slow evaporation method. Figure 5 shows the packing of the molecules. 1TC8* forms an orthorhombic crystal and P2 1 2 1 2 1 space group with cell parameters of a = 8.0145 (15) Å, b = 10.4214 (19) Å, c = 60.923 (11) Å, α = β = γ = 90°. The cell volume is 5088.4 (16) Å 3 . We also identified associations among the alkoxy chains. The crystal structure exhibits π-π interactions between the centre phenyl rings of molecules 2 and 1A (operator A: 1-X, 0.5 + Y, 0.5-Z). The corresponding centroidcentroid distance is 3.697 Å, and the dihedral angle between phenyl planes is 0.26°. Besides this, there are also CH⋅⋅⋅π interactions, as shown in Figure 5. The corresponding H to phenyl plane distances are 2.537, 2.597, 3.014, 2.729, and 3.089 Å, respectively. Because no strong chiral stackings of benzene rings are identified in Figure 5, no induced CD signals were identified in the crystalline phase.
In summary, we have synthesised a tolane-based chiral compound, which emits intensively in the solution, liquid crystalline and solid states. The compound shows the enantiotropic TGB A *, Blue II and   monotropic CrE phases in a wide temperature range. The helical pitch of the TGB A * phase decreased with increasing temperature. The colour is also tunable by changing temperature. Such properties make 1TC8* a promising candidate for organic electronic applications. Furthermore, the CD characterisation gives us a better understanding of the TGB A * phase and the double-twist arrangement of the Blue II phase.

Disclosure statement
No potential conflict of interest was reported by the authors.