Columnar liquid-crystalline assemblies of X-shaped pyrene–oligothiophene conjugates: photoconductivities and mechanochromic functions

Abstract

π-Conjugated liquid crystals have great potential as organic semiconductors owing to their smooth thin-film forming and flexible properties, as well as charge carrier transport ability. In the present study, we have designed and synthesised X-shaped columnar liquid crystals with pyrene as the central core, which are fourfold conjugated with bithiophene moieties, and tethered with eight or twelve alkoxy chains at their extremities. These X-shaped molecules show hexagonal, tetragonal, and rectangular columnar liquid-crystalline (LC) phases over a wide range of temperature. The extended π-conjugation has led to the realisation of a moderately low bandgap. The hole mobilities of both molecules in their various LC states are revealed to be in the order of 10⁻⁴ to 10⁻⁵ cm² V⁻¹ s⁻¹ using the time-of-flight (TOF) photoconductivity measurements. In addition, the mechano-responsive photoluminescent change has been observed for the columnar phase of the X-shaped molecule with eight alkoxy chains. The possible π-stacking configurations are discussed. The introduction of a gelator, N,N′-bis(lauroyl)hydrazine, into the X-shaped molecules leads to the formation of a LC gel that produces a change in the measured hole mobility. The presented strategy could lead to the design of new solution-processable, low bandgap semiconducting materials for optoelectronic devices.

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Introduction

π-Conjugated liquid crystals have attracted considerable attention because of their ease of molecular alignment and anisotropic charge carrier transport ability.^1–4 Since the early reports of carrier transport of triphenylenes in the columnar phases by Boden, Haarar, and Ringsdorf et al.,^5,6 a variety of columnar liquid crystals that show charge carriers transport have been reported.^7–30 Polycyclic aromatic hydrocarbon derivatives such as perylene, pyrene, and other heteroaromatic molecules including phthalocyanine and fused thiophene have come under intense scrutiny because of their potential to be employed as mesogenic cores. These columnar liquid-crystalline (LC) π-conjugated molecules have found applications in organic optoelectronic devices such as organic light emitting diodes (OLEDs),^31,32 organic field-effect transistors (OFETs),^33–35 and organic photovoltaics (OPVs).^36–39 As such, π-conjugated columnar liquid crystals are emerging as promising semiconducting soft materials due to their abilities to form one-dimensional π–π columnar stacks that allow high charge carrier transport and their capabilities to form smooth thin films with a uniaxially oriented columnar domain, which are essential for optimal device performance.

Oligothiophene derivatives are promising for applications in electronics because of their charge transport abilities and fluorescence properties. Smectic and nematic LC oligothiophenes are intensively studied,^40–48 however the examples of the columnar LC assembly of oligothiophene are limited. We previously reported on the first example of columnar LC polycatenar oligothiophenes (Scheme 2) with electronic and photonic functions,^49,50 and helical stacking of propeller-shaped oligothiophene-triazine conjugates in columnar LC phases that results in ambipolar charge transport.⁵¹ In addition, we developed mechanochromic liquid crystals containing π-conjugated chromophores such as pyrene and oligothiophene as their central cores.^52–56 The luminescent colours vary by mechanical stimuli, which are attributed to the change in the LC molecular assembled structures between the columnar and cubic LC phases. However, LC molecules that exhibit both charge transport properties and mechanochromic functions have not been explored. If these properties could be realised, a new class of supramolecular optoelectronic materials could be created with possible applications to displays, memory devices, and sensors.^57–60 We envisaged that the combination of rod-shaped oligothiophenes with a disc-like pyrene would produce a multi-functional neoteric columnar liquid crystal that could exhibit both charge carrier transport and stimuli-responsive photoluminescent properties.

Herein, we report X-shaped molecules 1a and 1b (Fig. 1a) consisting of intercrossing polycatenar oligothiophenes through a central pyrene core (see ESI,† Fig. S1 for molecular modelling). The X-shaped molecules can form hole-transporting columnar LC phases via one-dimensional stacks (Fig. 1b). The molecular stacks are also tunable by the application of mechanical shear force, inducing a change in photoluminescence colour.

Fig. 1 The molecular structure of the X-shaped molecules with a π-conjugated core (a), and the one-dimensional π–π columnar stacks that facilitate charge transport (b).

Results and discussion

Synthesis and characterisation

The X-shaped pyrene–oligothiophene conjugates 1a and 1b were synthesised according to the synthetic route shown in Scheme 1 (see ESI,† Section S1 for detailed experimental procedures). The Suzuki–Miyaura cross-coupling reaction of alkoxy bromobenzene 2a with 2,2′-bithiophene-5-boronic acid pinacol ester yielded 3a, which is then followed by an iridium-catalysed borylation reaction⁶¹ with bis(pinacolato)diboron to afford the corresponding pinacol ester 4a. The fourfold Suzuki–Miyaura cross-coupling reaction of 4a with 1,3,6,8-tetrabromopyrene afforded the desired X-shaped molecule 1a. Compound 1b was obtained by the identical synthetic pathway. Both X-shaped molecules 1a and 1b were characterised and verified by ¹H and ¹³C NMR spectroscopy, MALDI-TOF mass spectrometry, and elemental analyses.

Scheme 1 The synthetic scheme of the X-shaped pyrene-bithiophene conjugates 1a and 1b.

Scheme 2 The polycatenar oligothiophene derivative 5 previously reported.^49,50

Compounds 1a and 1b are highly soluble in organic solvents like hexane, toluene, dichloromethane, chloroform, and THF. Both compounds exhibit a high thermal stability of above 300 °C under air (see ESI,† Fig. S2). As such, these molecules could be processed and annealed under air without extensive oxidative decomposition.

Liquid-crystalline properties

The thermotropic LC properties and X-ray diffraction (XRD) data of the X-shaped molecules 1a and 1b are summarised in Table 1 (see ESI,† Fig. S3 for DSC thermograms). The polarising optical microscopic (POM) textures and XRD patterns are shown in Fig. 2 (see ESI,† Fig. S4 for additional POM images and XRD patterns). Compound 1a with 8 alkoxy chains at the peripheries exhibits enantiotropic hexagonal columnar phases (Col_h). Observations under the POM show that 1a exhibits a fan-shaped texture with a large domain size when cooled from the isotropic melt at a rate of less than 1 °C min⁻¹ (Fig. 2a). The wide angle X-ray diffraction (WAXD) pattern at 125 °C shows two peaks at 40.7 (100) and 20.0 Å (200) (Fig. 2b). In addition, the two- dimensional small angle X-ray diffraction (SAXD) pattern at the same temperature reveals diffraction spots with a six-fold symmetry at the (100) plane (Fig. 2c), indicating a Col_h phase, with an intercolumnar distance of 47.0 Å (see ESI,† Fig. S5a and b). When 1a is cooled to below 117 °C, at which the DSC chart on a cooling process exhibits an exothermic peak, no visible change in the texture is seen (see ESI,† Fig. S4a). At 100 °C, the WAXD pattern of 1a shows three diffraction peaks at 43.2 (100), 21.2 (200), and 13.8 Å (300) (see ESI,† Fig. S4b) with a lattice parameter a of 49.9 Å. Furthermore, the two-dimensional SAXD pattern of 1a at 100 °C also exhibits a hexagonal symmetry at the (100) plane (see ESI,† Fig. S4c). These results suggest a transition from the Col_h2 to a new Col_h1 phase. The larger intercolumnar distance can be attributed to a change in the cofacial stacking configuration (vide infra), as well as more extended and rigid alkyl chains of the Col_h1 phase as compared to that of the Col_h2 phase. A transition to a glassy state is seen when 1a is cooled to below 20 °C.

Table 1 Phase transition thermal properties and XRD data for 1a and 1b

Compound	Phase transition behaviour^a			XRD data
				T/°C	Lattice parameters/Å
a Transition temperatures (°C) and transition enthalpies (kJ mol^−1) in parentheses, determined by DSC on cooling at a rate of 10 K min^−1. G, glassy; Iso, isotropic; Colh, hexagonal columnar phase; Colt, tetragonal columnar phase; Colr, rectangular columnar phase. Subscript numbers 1 and 2 indicate first and second columnar phases, respectively.
1a	G 20 Colh1	117 Colh2	132 Iso	125	a = 47.0
		(26)	(1)	100	a = 49.9
1b	Colr1 10 Colr2	100 Colt	135 Iso	125	a = 40.3
	(43)	(12)	(6)	90	a = 75.8 b = 52.0
				−5	a = 68.8 b = 47.9

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Fig. 2 POM photographs (left panels) and WAXD patterns (right panels). Compound 1a in the Col_h2 phase at 125 °C (a and b) with its SAXD pattern at the same temperature (c). Compound 1b in the Col_r2 phase at 90 °C (d and e).

Compound 1b with 12 alkoxy chains at the peripheries exhibits a similar isotropic transition temperature to 1a upon heating. Cooling from the isotropic melt reveals a polydomain texture of 1b at 125 °C (see ESI,† Fig. S4d). The WAXD and SAXD patterns of compound 1b at 125 °C show diffraction peaks at 40.3 (100), 29.3 (110), 19.6 (200), and 14.9 Å (300) (see ESI,† Fig. S4e and f), which indicate a tetragonal columnar (Col_t) phase (see ESI,† Fig. S5c). The texture of 1b (Fig. 2d) remains relatively unchanged upon cooling to below 100 °C as compared to the texture observed at 125 °C. However, the WAXD pattern is significantly different with four distinct peaks at 42.9, 37.9, 20.7, and 18.4 Å (Fig. 2e) observed at 90 °C, which correspond to the diffractions from the (110), (200), (220), and (320) planes of a rectangular columnar phase (Col_r) with a P2₁/a symmetry (see ESI,† Fig. S6a and S7). Using the Debye Scherer method, the lattice parameters a and b are evaluated to be 75.8 and 52.0 Å, respectively (see ESI,† Fig. S6a). Further cooling of 1b to below 10 °C induced a transition to a new Col_r phase (Col_r2 to Col_r1 phase), with the lattice parameters a and b of 68.8 and 47.9 Å, respectively (see ESI,† Fig. S4g, h and S6b).

Hole carrier transport

The time-of-flight (TOF) photoconductivity method was used to examine the hole mobilities of compounds 1a and 1b. The samples form LC domains oriented parallel to the surface of the indium-tin-oxide (ITO) cells with a sample thickness of 9 μm by slow cooling from isotropic states to room temperature.

Compounds 1a and 1b illuminated by a pulsed YAG laser at 355 nm under applied electric fields at room temperature show non-dispersive transient photocurrent curves of hole carriers (see ESI,† Fig. S8). The transit time is taken at the inflection point of the photocurrent curves. The hole mobilities measured are nearly independent of the applied voltage, which is the characteristic feature for charge carrier transport of semiconductors.⁴⁰

Temperature dependence of the hole carrier mobilities of 1a and 1b is shown in Fig. 3. Compound 1a in the isotropic state shows hole mobilities in the order of 10⁻⁵ cm² V⁻¹ s⁻¹. The transition from the isotropic state to the Col_h2 phase induces an increase in the hole mobility. The maximum value in the Col_h2 phase of 1a at 117 °C is 4.4 × 10⁻⁴ cm² V⁻¹ s⁻¹. This increment in the hole mobility can be attributed to the facilitation of charge carrier transport due to the formation of the one-dimensional π–π columnar stacks. In the Col_h1 phase of 1a, the hole mobilities slightly decrease with decreasing temperature.

Fig. 3 The logarithmic plot of hole mobilities of 1a (■) and 1b (○) as a function of temperature on cooling. Dotted lines denote phase transition temperatures.

As for compound 1b, an abrupt increase of the hole mobilities is observed at the Iso to Col_t transition. The maximum value of 2.3 × 10⁻⁴ cm² V⁻¹ s⁻¹ is observed at 130 °C in the Col_t phase. The hole mobilities of 1b in the Col_t phase decrease by approximately one order of magnitude with decreasing temperature. In the Col_r2 phase, no significant change in the hole mobilities is observed with varying temperature.

Mechanochromic properties

The stimuli-reponsive properties of the X-shaped molecules 1a and 1b were explored. Under the irradiation of UV light, the luminescence colour of compound 1a changes from red to orange when a mechanical shear force is applied on the sample in the Col_h1 phase at room temperature (Fig. 4), forming a new disordered Col phase (vide infra). The red luminescence of the initial state (Col_h1 phase) of 1a is recovered by heating the sheared sample to the isotropic state and then cooling to room temperature. In contrast, compound 1b shows no apparent difference in luminescence colour before or after shearing (see ESI,† Fig. S9).

Fig. 4 The photograph of 1a at room temperature on a quartz plate under illumination of UV light (365 nm) for the original Col_h1 sample (left half) and the sheared sample (right half).

To clarify the photophysical properties of 1a before and after mechanical shearing, UV-vis absorption (Fig. 5a, left) and photoluminescence spectra (Fig. 5a, right) of 1a have been measured on quartz substrates at ambient temperature. In addition, in the isotropic liquid phase at 150 °C and the Col_h2 phase at 125 °C of 1a, the UV-vis absorption and emission spectra (Fig. 5b) have also been taken for comparison with those of the shear-induced phase.

Fig. 5 The normalized UV-vis absorbance (left panels) and emission (right panels) spectra (excitation wavelength of 420 nm) of the effect of shearing on 1a at room temperature (a), and at its various LC phases.

The absorption spectra of 1a (Fig. 5a, left) show three absorption peaks (Fig. 5, left). The two absorption peaks with higher energy wavelengths at around 364 and 414 nm can be ascribed to the π → π* transition from the oligothiophene-conjugated pyrene core, while the absorption peak with lower energy wavelength at around 505 nm is probably due to n → π* from the sulfur heteroatom of the thiophene moiety.⁶² No significant differences in the absorption spectra before and after shearing are observed (Fig. 5a, left), although the absorption edge of the shear-induced phase is slightly blue-shifted to that of the Col_h1 phase. In contrast, the maximum wavelength of the emission attributable to the S₁ → S_o relaxation in the shear-induced phase (λ_max = 623 nm) is blue-shifted by 30 nm relative to that in the Col_h1 phase (Fig. 5a, right) while maintaining the absolute fluorescent quantum yield of 2% (see ESI,† Fig. S10). Moreover, the emission spectrum in the shear-induced phase at 25 °C (Fig. 5a, right) is found to be close to those in the Col_h2 phase (λ_max = 611 nm) at 125 °C and in the isotropic liquid phase (λ_max = 604 nm) at 150 °C, as shown in Fig. 5b, right. From these results, we consider that the application of shear force to the Col_h1 phase of 1a may induce disorder in the columnar stacking structure of the X-shaped molecule.

Further investigation of the shear-induced LC phase of 1a was performed. The thermal properties of the original LC sample and the sheared sample of 1a were characterised by DSC heating traces (Fig. 6a). It is observed that the sheared sample exhibits two distinct peaks at 132 and 136 °C, with transition enthalpies of 14 and 1 kJ mol⁻¹, respectively. The total transition enthalpy from the shear-induced Col_h phase to the isotropic state is considerably lower than the transition enthalpy from the Col_h1 phase to the isotropic state of another sample of 1a that is not subject to shearing (30 kJ mol⁻¹). This result suggests that shearing of the LC sample causes a disorder in the molecular arrangement. We suspect that the broad peak at 132 °C in the DSC trace of the shear-induced phase is due to the inherently incomplete shearing process.

Fig. 6 The DSC traces of a sample of 1a that is subject to shearing, and another sample that is cooled from an isotropic state to form the Col_h2 phase (a). The heating rate is 10 K min⁻¹ with transition enthalpies (kJ mol⁻¹) shown in parentheses. The WAXD pattern of the sheared-induced Col phase of 1a (b) at room temperature.

Furthermore, WAXD pattern of the sheared sample (Fig. 6b) shows only a broad peak at 47.7 Å (100), suggesting a new shear-induced Col phase. Assuming that the hexagonal lattice is maintained, the lattice parameter a of this disordered Col_h phase is calculated to be 55.5 Å, which is noticeably larger than that of Col_h1 and Col_h2 phases as shown in Table 1.

From these observations, we propose that in the Col_h1 phase, cofacial columnar stacks are formed with the individual X-shaped molecules of 1a rotated with an angle of about 15°, forming a helical packing structure (Fig. 7). This assembly was found to exhibit higher stabilisation energy.⁶³ The blue-shift in the emission spectrum for the transition from the Col_h1 phase to the Col_h2 phase can be explained by the decreased interchromophoric interactions,⁶⁴ which is evident by another blue-shift for the transition to the isotropic state. Based on these results, it can be deduced that 1a in the Col_h2 phase presumably adopts a less cofacial stacking configuration with the X-shaped molecules rotated at a larger acute angle. (Fig. 7)

Fig. 7 The proposed transition scheme and intermolecular stacking arrangement of 1a in the Col_h1 phase, Col_h2 phase and the shear-induced Col phase. The alkyl chains are omitted for clarity.

It is proposed that the mechanochromic response exhibited for compound 1a can be explained in a similar way. The mechanical shearing force on 1a disrupts the ordered arrangement of the aromatic cores, causing the sheared sample to adopt a new disordered Col phase, and resulting in lesser intermolecular π–π interactions. Consequently, a visible blue-shift in the fluorescence spectra is observed.

As for compound 1b, shearing causes no observable change in the fluorescent properties. This can be attributed to the more rigid Col_r phase as compared to the Col_h phase.

LC gel properties

Incorporation of self-assembled fibres into the π-conjugated LC molecules is one approach to increase the hole mobility.^65–71 We intend to investigate the influence on photoconductivities by the introduction of fibrous networks into the X-shaped liquid crystals. In our previous studies, triphenylene-based liquid crystals were found to form LC gels with gelator 6⁷¹ (Scheme 3). However in the case of compound 1a, the introduction of gelator 6 forms a macroscopically phase-separated mixture under the POM and scanning electron microscope (SEM) observations (see ESI,† Fig. S11). Compound 7 was thus employed in our study as it was known to exhibit good gelation properties in common organic solvents.⁷²

Scheme 3 The molecular structures of the gelator candidates employed.

The formation of a LC gel is achieved for compound 1a with 3 weight% of gelator 7. Upon cooling the mixture from the isotropic states, fibrous aggregates of 7 are first formed in the liquid of 1a. Upon subsequent cooling, the orientation of the LC domains of 1a along the fibrous network is observed under POM (Fig. 8a). The xerogel also reveals the fibrous network under SEM (Fig. 8b). The fibrous network is also observed by the introduction of gelator 7 into compound 1b (see ESI,† Fig. S12).

Fig. 8 The images of the LC gel of liquid crystal 1a and gelator 7 upon cooling from the isotropic state to 125 °C under POM (a), and the fibrous structure of the xerogel under SEM (b).

The hole mobilities of LC gels of 1a and 1b with 3 weight% of gelator 7 are measured at varying temperatures and compared to their pure counterparts. For the LC gel of 1a, consistently higher hole mobility was observed as compared to pure 1a (see ESI,† Fig. S13). At 117 °C, the LC gel shows a maximum hole mobility of 8.4 × 10⁻⁴ cm² V⁻¹ s⁻¹, almost double of that of pure 1a. In contrast, hole mobility of the LC gel of 1b shows no significant difference as compared to pure 1b (see ESI,† Fig. S14). In our previous studies, a significant increase in hole mobility was observed for the LC gels of triphenylene molecules, which could be attributed to the alignment of the liquid crystal on the fibrous structure that presumably suppresses the fluctuation of the molecular assembly of LC molecules.^68,71 The observations in this study suggest that the gelation exerted a smaller effect on the fluctuation of the X-shaped molecule 1b as compared to 1a, suggesting a more rigid stacking structure in the Col_t and Col_r phases of 1b as compared to the Col_h phases of 1a.

Conclusion

We have demonstrated that pyrene–oligothiophene based X-shaped molecules of 1a and 1b are capable of forming Col_h and Col_r phases over a wide range of temperature, including room temperature. TOF photoconductivity measurements reveal that the Col_h and Col_r phases of 1a and 1b show modest hole carrier transport in the order of 10⁻⁴ to 10⁻⁵ cm² V⁻¹ s⁻¹. Compound 1a exhibits mechano-sensitive photoluminescence properties, and the possible intermolecular configurations in different columnar phases have been elucidated. The introduction of a fibrous network also causes a small increase in hole mobility for 1a. This simple strategy for the preparation of new and multi-functional liquid-crystalline pyrene–oligothiophene based X-shaped molecules would potentially contribute to the growing field of organic electronics.

Acknowledgements

This study was partially supported by Grant-in-Aid for Scientific Research A (KAKENHI 23245030 for T. K.) and Scientific Research on Innovative Area (Stimuli-responsive Chemical Species, KAKENHI 15H00921 for M. Y.) from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). K. P. G. is grateful for the support by MEXT on his research and pursue of graduate degrees in the University of Tokyo.

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Footnote

† Electronic supplementary information (ESI) available: Experimental, molecular modelling, thermogravimetric curves, DSC traces, POM images and XRD patterns, lattice analyses, transient photocurrent curves, UV-vis and fluorescence spectra, POM/SEM images and hole mobilities of LC gels. See DOI: 10.1039/c6tc00808a

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