Thermally induced bilayered crystals in a solution-processed polycrystalline thin film of phenylterthiophene-based monoalkyl smectic liquid crystals and their effect on FET mobility

Herein, a series of asymmetric monoalkyl terthiophene derivatives, 5-phenyl-500-alkyl-2,20:50,200terthiophene (Ph-(Tp)3-Cn, 6 # n # 18, where n represents the number of carbons in the alkyl group), were synthesized to investigate the formation of monolayered crystals derived from the smectic phase and their phase transition to bilayered crystals, leading to a significant increase of FET mobility when applied to organic field-effect transistors. All compounds showed a highly ordered smectic phase, i.e., SmE phase, and gave a monolayered crystal when spin-coated at SmE temperature. It was found that phase transition from monolayered to bilayered crystals was induced by thermal annealing when the alkyl side chain length n $ 10, and OFET mobility in FETs fabricated with polycrystalline thin films was dramatically enhanced up to one order of magnitude when the bilayered crystals were formed. This result proves that this phase transition is not limited to a particular type of smectic liquid crystals consisting of an aromatic fused ring system reported in benzothienobenzothiphene (BTBT) as a core derivative and can be extended to a variety of core systems, e.g., oligomer type of smectic liquid crystals. These findingsmay provide a new strategy for molecular design to boost mobility in soluble OFETmaterials.


Introduction
Organic eld-effect transistors (OFETs) have attracted signicant attention in recent decades owing to increasing interest in printed electronics, which have a high potential in the manufacture of exible organic electronic circuits on plastic substrates. 1 Among various requirements of the OFET material, the carrier mobility, which is the most important property of OFET materials in determining the availability of the devices in practical applications, has been improved beyond a few cm 2 V À1 s À1 as a result of continuous efforts of materials scientists. 2,3 In recent years, liquid crystal materials have attracted extensive attention as OFET materials since they provide liquid crystallinity for device processes due to high solubility in organic solvents, easy fabrication of uniform and at polycrystalline lms via liquid crystalline phase, high mobility with a small variation, 4 and enhanced thermal stability of the lms featured by solid-like nature of highly ordered smectic phases. 4a,5-7 More recently, an asymmetric monoalkyl smectic liquid crystalline organic semiconductor, 7-decyl-2-phenyl- [1] benzothieno [3,2-b]benzothiophene, Ph-BTBT-10, whose chemical structure is shown in Fig. 1, has been proposed for a solution processable OFET material and its OFET performance has been reported. 2f Its most interesting result was the dramatic increase of FET mobility in polycrystalline lms by about one order of magnitude up to 14.7 cm 2 V À1 s À1 as a result of thermally induced crystal-to-crystal phase transition from the asfabricated monolayered crystal derived from the SmE phase to bilayered crystal aer thermal annealing. Therefore, if the enhancement of FET mobility accompanied with the crystal-tocrystal phase transition is not a particular case of Ph-BTBT-10, this procedure, that is, the fabrication of a uniform lm via a liquid crystalline phase 4a and successive thermal annealing would provide us with a powerful technique to reconcile lm uniformity and high mobility in OFETs at the same time.
In this study, we rst investigated the mesomorphic properties of all compounds by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD). Then, the crystal structures of polycrystalline thin lms fabricated via spin-coating at a temperature of smectic phase was studied by XRD under different thermal annealing (TA) processes. Moreover, FET performances in OFETs fabricated with these polycrystalline thin lms were explored under different TA process.
We have described these experimental results and discussed the important factors responsible for the crystal-to-crystal phase transition, reason for the FET mobility increase aer the phase transition, and the benets of the present molecular design for OFET materials.

Synthesis and characterization
A series of phenylterthiophene-based monoalkyl liquid crystalline organic semiconductors, Ph-(Tp) 3 -Cn, were successfully synthesized according to Scheme 1. In the synthesis, alkyl thiophene boronic acid (2), 8,9 which was obtained using thiophene as the starting material through two-step lithiation reactions, and Ph-BTP-Br (6) prepared from 2-bromothiophene were coupled by a microwave-assisted Suzuki-coupling (the details of the synthesis can be found in ESI †). 10 All compounds were light yellow solids, and their chemical structures were conrmed by 1 H-NMR spectra and high-resolution mass spectrometry (HRMS).

Mesomorphic behaviours
The mesophase behaviours of Ph-(Tp) 3 -Cn were investigated by DSC, POM, and variable temperature power XRD. All compounds showed a single liquid crystal phase. The phase transition temperature and enthalpy data are summarized in Table 1. Fig. 2 shows the DSC curves of a typical compound, Ph-(Tp) 3 -C18 at a scan rate of 10 C min À1 . In the curves, two obvious endothermic phase transition peaks at 116.5 C and 161.0 C were observed during the second heating run. This indicates that Ph-(Tp) 3 -C18 exhibits a mesophase. In addition, in the mesophase, a typical mosaic texture is observed in a POM image at 130 C (Fig. 3a), which oen appears in the texture of highly ordered phases obtained from an isotropic phase without lowordered liquid crystalline phases such as SmA and SmC phases, 11 indicating a highly ordered smectic phase, e.g., SmE, SmH or SmK. Furthermore, the appearance of cracks in the POM texture at 30 C ( Fig. 3b) suggested that the low temperature phase was a crystal phase, which also distinguished the mesophase from the crystalline phase. 12 Moreover, an additional Scheme 1 Synthesis route of Ph-(Tp) 3 -Cn.  This journal is © The Royal Society of Chemistry 2017 small exothermic broad peak at 92.9 C, as shown in the inset of Fig. 2, was observed during the second heating run. Because of the heating run of DSC, this exothermic peak is attributed to phase transition for more stable ordered crystal phase as compared to the crystal phase at room temperature. The mesophase was studied in detail by the variable temperature power XRD. Fig. 4 shows the typical XRD patterns of Ph-(Tp) 3 -C18 at 150 C on heating (red line). In the small angle area (2q ¼ 2-5 ), the d spacing of the rst reection (d ¼ 39.1Å) was almost equal to a molecular length of Ph-(Tp) 3 -18 calculated by MOPAC (d cal. ¼ 39.7Å). Moreover, in the wideangle region (2q ¼ 15-30 ), a set of three sharp peaks for d ¼ 4.6Å, 4.0Å, and 3.5Å was the signature of the SmE phase. 7b,13 Thus, the mesophase was assigned to be the SmE phase. Furthermore, as temperature decreased, e.g., at 30 C, multiple peaks appeared around the wide-angle region, which were attributed to the additional reection in the crystal phase, which further supported the present identication ( Fig. 4, black line).
Similarly, DSC, POM, and XRD results of other compounds were obtained, as shown in ESI Fig. S1-S3. † Their mesophase properties are summarized in Table 1. Fig. 5 shows a summary of phase transition behaviours of various Ph-(Tp) 3 -Cn as a function of temperature. As shown in Fig. 5, all compounds exhibited a highly ordered liquid crystal phase, i.e., SmE phase, and the mesophase ranges and isotropic points were decreased slightly as the alkyl side chain lengths increased. Furthermore, it should be noted that all the studied Ph-(Tp) 3 -Cn exhibited an additional crystal phase (Cr2) except for Ph-(Tp) 3 -C14, which exhibited another crystal phase (Cr3).

Effect of TA on the crystal structure in polycrystalline thin lms
To investigate the effects of TA on the crystal structure of Ph-(Tp) 3 -Cn crystalline lms derived from the SmE phase, the lms were fabricated by spin coating their 0.75% p-xylene solution on glass substrates at an elevated temperature for each SmE phase. The morphology in the resulting lms was very uniform and at as expected from the previous results due to smectic liquid crystallinity. 2f,4a,6c The crystal structure in the lms before and aer TA was studied by the variable temperature power XRD in the small angle region (2q ¼ 1-5 ). Fig. 6 shows the XRD patterns of Ph-(Tp) 3 -C18 as a function of TA temperature in the crystal state. The black curve is the    The thin film XRD pattern of Ph-(Tp) 3 -C18 obtained before and after TA at different temperatures in the crystal state (TA at each temperature for 5 min, then cooling to room temperature).
XRD pattern obtained before TA, i.e., as-coated lm, and the other curves are the XRD patterns obtained aer TA at different temperatures for 5 min.
The XRD peaks at around 2q ¼ 2.42 in Fig. 6 (in ESI Fig. S4 †) were assigned to the d-spacing of a layer thickness of a monolayer consisting of Ph-(Tp) 3 -C18 molecules, which was determined to be 36.5Å, i.e., for Cr1, indicating that crystals consisted of molecular layers of Ph-(Tp) 3 -C18, whose thickness was shorter than the molecular length of Ph-(Tp) 3 -C18 calculated by MOPAC, i.e., d cal. ¼ 39.7Å. The peak position and its intensity for these peaks remained almost unchanged until TA at 90 C. A new peak at around 2q ¼ 1.23 in Fig. 6, a red curve as shown in ESI Fig. S4, † corresponding to the d spacing of z71.8Å, appeared aer TA at 90 C. This d-spacing was almost double (z1.97) that of the layer thickness in the crystals before TA; this indicated that the crystalline thin lms were transformed into bilayered crystals, i.e., Cr2, aer TA. The intensity of this new peak increased with an increase in the TA temperature and saturated aer TA at 100 C; this suggested that the bilayer crystal was more thermally stable than the monolayered crystal fabricated via the SmE phase.
Judging from the XRD patterns in ESI Fig. S5, † all the crystalline thin lms of Ph-(Tp) 3 -Cn fabricated by spin-coating at a temperature of SmE phase yielded monolayered crystals when alkyl side chain lengths were n $ 10. Moreover, the lms are transformed into bilayered crystals aer TA at a temperature around their crystal temperature; this is in accordance with the previous results of Ph-BTBT-C10. 2f The present results can be summarized as follows: the asspin coated crystalline lms obtained from the SmE phase are monolayered (Cr1) and can be transformed into bilayered crystals (Cr2) aer TA at around the crystallization temperatures when alkyl side chain length is longer than C10, as shown in Table 2.
Although XRD results obtained from the lms of Ph-(Tp) 3 -Cn before and aer TA indicated the existence of a monolayered and bilayered lamellar molecular packing in the spin-coated crystalline thin lms, in fact, it was hard to obtain the exact images of molecular packing in the resulting lms by AFM and STM techniques because of the vulnerable nature of the present lms and the difficulty in analysing the XRD data for the lms polycrystallized. 14 The exploration of the exact molecular packing of the lms will be conducted in future.
Considering the unique asymmetrical molecular structures of monoalkyl Ph-(Tp) 3 -Cn which are similar with the biological membranes 15 and ionic liquid crystals, 16 a possible molecular packing is speculated, as shown in Fig. 7. According to recent studies on the SmE phase, 17 the molecular alignment in the SmE phase is characterized by a microphase-separated structure of core moieties and side chains, as illustrated in Fig. 7, and closely packed herringbone molecular arrangement in a molecular plane. In the fabrication of the lms by spin-coating a solution at the SmE temperature, the lms are subjected to fast cooling from the SmE temperature to the crystallization temperature, i.e., the Cr1 state. Therefore, it is quite plausible that the monolayered structure and herringbone packing of molecules in the SmE lm can be held in the resulting crystalline lms (C1) without any destructive change of the molecular alignment, whereas alkyl side chains are frozen and tilted towards the substrate, 17 i.e., layer plane, in which the Ph-(Tp) 3 moieties sit perpendicular due to the same herringbone molecular alignment in the crystal.
Subsequently, the resulting monolayered crystal (Cr1 state) is transformed into the bilayered crystal (Cr2 state) by successive TA. 2f, 16,18 Thus, lm uniformity and morphology of the bilayered crystal can be inherited from the SmE lm basically. This is quite benecial for polycrystalline thin lms with good surface morphology and area uniformity.

Effect of TA on the OFET performance
As discussed in the previous section, TA causes the crystal-tocrystal phase transition from the monolayered crystal (Cr1) to the bilayered crystal (Cr2), 2f as is the case of Ph-BTBT-10, in which the resulting bilayered crystals have a head-to-head arrangement in a molecular layer and afford enhanced mobility in OFET devices. 2f To examine the generality of this phenomenon, we fabricated bottom gate top contact OFET devices with polycrystalline thin lms of all compounds aer spin-coating their 0.75% p-xylene solution on SiO 2 /Si substrates in each SmE temperature and cooling the resulting lms at  room temperatures. Au electrodes of 50 nm thickness were deposited onto the resulting lms through shadow masks. The OFET performance was investigated before and aer TA. Fig. 8(a) shows the mobility of OFETs fabricated with Ph-(Tp) 3 -C18 as a function of TA temperatures in the crystal state, which has been estimated from transfer characteristics of the devices in the linear region at a room temperature aer TA at a given temperature for 5 min. The FET mobility started to increase at around 80 C, where the phase transition to Cr2 started, and then dropped at around 110 C, which coincided with the phase transition behaviour of Ph-(Tp) 3 -C18 elucidated by DSC measurement at a slow scanning rate of 1 C min À1 (ESI Fig. S6 †) and XRD results shown in Fig. 6. The typical output and transfer characteristics of Ph-(Tp) 3 -C18 in the monolayered crystal (Cr1) and the bilayered crystal (Cr2), that is, before and aer TA, are shown in Fig. 8(b)-(d). It is clear that the FET mobility of $4.6 Â 10 À3 cm 2 V À1 s À1 for Cr1 is increased by more than one order of magnitude up to 0.1 cm 2 V À1 s À1 in Cr2 aer TA.
The OFETs fabricated with other Ph-(Tp) 3 -Cn followed this trend although the degree of enhancement of the mobility depended on alkyl chain lengths, as shown in Fig. 9: OFETs fabricated with the Cr1 crystal of Ph-(Tp) 3 -Cn showed the mobility of around 5 Â 10 À3 cm 2 V À1 s À1 irrespective of alkyl chain lengths, Cn; aer TA at the Cr2 phase for 5 min, the mobility was increased by a factor of 2-20. In the present study, the FET performance of these devices has not been optimized yet, but it is still available to nd a general trend of the effect of TA on the device performance. In fact, as the alkyl side chain lengths increased (except for n ¼ 10), the FET mobility aer TA increased up to 0.1 cm 2 V À1 s À1 . Moreover, their OFET performance is summarized in detail in ESI Table S1 and Fig. S7. † There are several possible factors, such as improvement of lm crystallinity and electrical contact with electrode materials of Au and the enhanced transfer integrals among the molecules in the bilayered crystal, contributing to the enhancement of mobility aer TA. However, judging from the FET mobility and the formation of the bilayered crystals as a function of TA temperature, as shown in Fig. 6 and 8(a), respectively, it is very clear that the formation of the bilayered crystal and the enhancement of FET mobility occur at around 80 C abruptly, whereas they hardly occur at a temperature range lower than 80 C. This trend basically followed the previously reported case of Ph-BTBT-10. Therefore, we have concluded that the major contribution to the enhancement of FET mobility in OFETs fabricated with Ph-(Tp) 3 -C18 aer TA is attributed to the crystal transition from Cr1 to Cr2 induced by thermal annealing, rather than improvement of lm crystallinity and/or the electrical contact with Au electrodes, both of which may occur in thermal annealing at the same time. In fact, the exact reason why the bilayered crystal gives higher mobility is not clear although we can deduce that Cr1 is less ordered as compared to Cr2 because Cr1 inherits the disordered nature of the SmE phase, and the bilayered crystal has two conduction channels in the vicinity of the gate insulator, whose trap states are easily lled to reduce the harmful effect on the source-drain current.
Recently, vacuum evaporated polycrystalline lms of asymmetric BTBT derivatives, such as BTBT-Tp-Cn 19 and BTBT-Ph-12, 20 and solution-grown single crystals of BTT-Tp-12 21 were applied to fabricate processed OFETs. Judging from our previous and present results on the monoalkyl smectic liquid crystals, these materials would also be available for solutionprocessed bilayered crystalline thin lms if their liquid crystallinity is utilized to fabricate a lm.
The results obtained from Ph-BTBT-10 and Ph-(Tp) 3 -Cn lead to a possible conclusion that a monolayered crystal can be easily formed with a SmE lm of mono-alkylated smectic liquid Fig. 8 (a) OFET mobility of Ph-(Tp) 3 -C18 as a function of TA temperatures in the crystal state; (b) typical OFET transfer characteristics of Ph-(Tp) 3 -C18 before and after TA at the Cr2 phase (100 C); output characteristics of Ph-(Tp) 3 -C18 (c) as spin-coated, (d) after TA at Cr2 phase (100 C, 5 min). Fig. 9 OFET mobility of Ph-(Tp) 3 -Cn before and after TA (TA) at Cr2 phase (all data was obtained from the transfer curves in liner area at crystals, irrespective of the core structure, e.g., Ph-BTBT and Ph-(Tp) 3 , transformed into the bilayered crystals with the aid of thermal annealing, and the resulting bilayered crystals exhibit enhanced mobility in OFETs, if applied. Therefore, this crystalto-crystal phase transition accompanied by enhancement of FET mobility observed in Ph-BTBT-10 is not a particular case of Ph-BTBT-10, having an extended aromatic fused ring system, but probably common in the monoalkyl smectic liquid crystals with an extended p-conjugate core and an alkyl chain long enough to stabilize the microphase-separated structure of core and side chain moieties. In addition, it is worth noticing that the formation of the bilayered crystals via the SmE phase allows us to prepare a uniform and at thin lm of bilayered crystals suitable for OFET applications by solution processes.
From a molecular design point of view, the present ndings suggest a new possible strategy of molecular design for high quality OFET materials even when a p-conjugate moiety for charge carrier transport is kept unchanged: mono-alkylated OFET materials provide us with feasibility of fabricating bilayered polycrystalline lms, leading to enhanced mobility in OFETs; for example, C8-BTBT, i.e., 2,7-dioctyl-BTBT, cannot form the bilayered crystal because of its symmetric chemical structure, but Ph-BTBT derivatives afford the bilayered crystal with higher mobility as reported. Furthermore, this type of materials has another benet of higher solubility in solvents, for example, compared with the dimer type of OFET materials with the same core size, dialkyl dimer type of OFET materials, such as Cn-(Tp) 3 -phenylene-phenylene-(Tp) 3 -Cn, hardly dissolve in organic solvents, whereas monoalkyl OFET materials with the monomer core structure, e.g., Ph-(Tp) 3 -Cn, show good solubility although they can give a quasi-dimer type of core structure of ((Tp) 3 -Ph-Ph-(Tp) 3 ) in the bilayered crystal.

Conclusions
A series of asymmetric monoalkyl smectic liquid crystals, Ph-(Tp) 3 -Cn, was synthesized and the formation of monolayered crystals and the effect of thermal annealing on their transformation to the bilayered crystals and OFET performance fabricated with these polycrystalline lms were investigated. It was found that the monolayered crystals of Ph-(Tp) 3 -Cn were easily formed by spin-coating its solution at a SmE temperature, the crystal-to-crystal transition from monolayered to bilayered crystals was induced thermally when the alkyl side chain length n $ 10, and OFET mobility in FETs fabricated with the polycrystalline thin lms was dramatically enhanced up to one order of magnitude when the bilayered crystals were formed.
The present result indicates that this thermally-induced crystal-to-crystal phase transition and simultaneous enhancement of FET mobility in OFETs fabricated with the monolayered crystals is not specic in a particular material of Ph-BTBT derivatives, but is common in monoalkyl smectic liquid crystals with an extended p-conjugate moiety; this indicates the availability of a new strategy of molecular design featuring the bilayered crystals fabricated with monoalkyl smectic liquid crystals for boosting mobility in soluble OFETs materials.

General
All commercially available reagents and solvents were used as received from Aldrich Chemical, Tokyo Chemical Industry, Wako Pure Chemical Industries and Kanto Chemical Co., Inc. unless otherwise noted.
1 H-NMR spectra were obtained using Bruker NMR spectrometers (DMX 400 MHz), and chemical shis are given in parts per million (d) and referenced from tetramethylsilane (TMS). Multiplicities of the peaks are given as s ¼ singlet, d ¼ doublet, t ¼ triplet, and m ¼ multiplet. High resolution mass spectrum (HRMS) was obtained using a double-focusing magnetic sector mass spectrometer, JEOL JMS-700.
Microwave-assisted reactions were carried out in a Milestone START S microwave synthesis system. Differential scanning calorimetry (DSC) was carried out using the Shimadzu instruments' DSC-60. Polarizing optical microscopy was carried out using a Nikon OPTIPHOT2-POL microscope equipped with a Mettler Toledo FP82HT hot stage. 1D WXRD studies were conducted using a Rigaku RAD-2X diffractometer with CuKa radiation.
Bulk lm samples for XRD measurements were prepared by melting the powder of the sample from Iso to room temperature on a glass slide substrate with a polyimide cover lm before measurement, and the polyimide cover lms were removed and thin lm samples were prepared by spin coating at 3000 rpm for 30 s at a SmE temperature, which was similar with OFET thin lm fabrication except for the use of glass slide as substrates.

Device fabrication and characterization
Top contact bottom gate OFET devices were fabricated using SiO 2 (300 nm)/p + -Si substrates. Polycrystalline thin lms of all compounds were fabricated by spin coating at 3000 rpm for 30 s at a SmE temperature, for which the solution was preheated. The solution concentration was 0.75 wt% p-xylene. The detail of spin coating procedure can be found in our previously reported studies. 2f,4a Aer spin coating, 50 nm thick Au electrodes were deposited on the thin lms under 2 Â 10 À6 Torr through shadow masks. The channel length (L)/width (W) was 100/ 500 mm.
The OFET performance was characterized by two source measurement units (8252, ADCMT) under ambient conditions at room temperature. The mobility (m) was calculated by plotting the square root of the source-drain current (I ds ) versus gate voltage (V g ) in the linear region using the equation I ds ¼ C i (W/L) m(V g À V th )V ds , where C i is capacitance of gate insulator and V th is threshold voltage obtained from the linear region (V ds ¼ À50 V).

Conflicts of interest
There are no conicts to declare.