Synthesis and physical properties of brominated hexacene and hole-transfer properties of thin-film transistors

A halide-substituted higher acene, 2-bromohexacene, and its precursor with a carbonyl bridge moiety were synthesized. The precursor was synthesized through 7 steps in a total yield of 2.5%. The structure of precursor and thermally converted 2-bromohexacene were characterized by solid state NMR, IR, and absorption spectra, as well as by DFT computation analysis. It exhibited high stability in the solid state over 3 months, therefore can be utilized in the fabrication of opto-electronic devices. The organic thin-film transistors (OFETs) were fabricated by using 2-bromohexacene and parent hexacene through vaccum deposition method. The best film mobility of 2-bromohexacene was observed at 0.83 cm2 V−1 s−1 with an on/off ratio of 5.0 × 104 and a threshold of −52 V, while the best film mobility of hexacene was observed at 0.076 cm2 V−1 s−1 with an on/off ratio of 2.4 × 102 and a threshold of −21 V. AFM measurement of 2-bromohexacene showed smooth film formation. The averaged mobility of 2-bromohexacene is 8 fold higher than the non-substituted hexacene.


Introduction
Acenes are amongst the most representative hydrocarbons for analysing the physical properties of polycyclic hydrocarbon materials. 1 Along with the increase in the number of aromatic benzene rings, acenes exhibit a reduction of both the HOMO-LUMO gap and the reorganisation energy. 2 The chemistry of acenes higher than pentacene, particularly their open-shell characteristics 3 and high charge transport properties, has attracted considerable attentions. Hence, these compounds and their analogs 4 such thieonoacene based semiconductor 5 are suitable for use in organic electronic devices such as organic eld-effect transistors. [6][7][8] The extended p-conjugation of higher acenes also induce an interesting phenomenon of singlet ssion that can be used on light harvesting. 9 Bulky substituents can enhance the thermal and photo-stability of acenes by lowering the radical characteristic in the ground state. 10 The isolation of higher acenes, from hexacene to nonacene and derivatives, has been achieved by applying this strategy. The modication of physical properties of acenes in the solid state requires a crystal engineering approach; however, their isolation steps are either difficult or tedious in order to obtain qualied structures due to their high thermal and light sensitivity in solutions. 11,12 To overcome the difficulty, stable precursors of acenes are prepared rst, which can then be converted to the corresponding acenes quantitatively in demand through either a thermal or a photo-driven process. 13,14 The synthesis of nonacene derivatives has been achieved by this approach utilising a diketone precursor through photo-induced transformation. 15 Recently, the dimer structure of heptacene was converted to heptacene via a thermal retro-cyclization reaction, therefore showing its feasibility for further processes. 16 The precursor method can be used to produce higher acenes in large quantity that is required to become usable materials. Acenes have certain valuable potential applications, such as organic semi-conductors, 17 singlet ssion materials, 18,19 and organic biradical sources. 15 Previously, our group has developed the method of producing higher acene molecules from either monoketone precursors 20 or from diethylketomalonate precursors. 21 Both types of precursor can be cleanly converted to hexacene either thermally or photo-chemically. In addition, our group has generated halide-substituted tetracene 22 and pentacene 23 from their corresponding monoketone precursors. They were used successfully as the semiconductors in electronic devices. In these devices, single crystal bromopentacene device exhibits a signicant superior hole mobility (>5 cm 2 V À1 s À1 ) to the parent pentacene (1.4 cm 2 V À1 s À1 ). Such a high performance is comparable with other related materials, such as triisopropylsilylethynylpentacene (>1 cm 2 V À1 s À1 ), [24][25][26] 27,28 and the crystal of tetracene analogous of rubrene (>18 cm 2 V À1 s À1 ). 29,30 In previous reports, the charge mobility of hexacene and derivatives were measured in the devices either made with single crystals, 21 or with crystalline thin lms prepared through solution method. 22,31 However, the physical properties and transistor characteristics of hexacene thin lm that is prepared by vacuum deposition method have not yet been reported.
It is believed that the bromine substituent can provide a suitable size to improve crystal packing. Judged by the past high-performance of brominated tetracene and pentacene, it is therefore in demand to explore the possibility of brominated analogue of hexacene. In this regard, the brominated analogue of hexacene, i.e., 2-bromohexacene (1a), is synthesized and its charge-transport property is examined. Precursor 2a is cleanly converted to 1a by thermal decomposition, and 1a exhibits high thermal stability in the dark over 90 days (Fig. 1). This is the rst example of the charge-transport property of a stable vacuumdeposited thin-lm of hexacene and halogenated hexacene for electronic devices.

Synthesis
The synthesis of 2a is shown in Scheme 1. Dimethylfulvene derivative 3 and benzoquinone 4 were treated with s-tetrazine, affording diketone 5 as an endo-exo mixture, then the double bond of 5 was reduced using zinc in acetic acid, affording adduct 6. The aldol reaction of 6 with dialdehyde 7 afforded diketone 8 as an endo compound in 17% yield (from 3, three steps). Pure endo-8 was crystallized probably due to steric inuence by the bromo-substituent. It was reduced by NaBH 4 , and further treatment with POCl 3 /pyridine afforded aromatic compound 9 in 48% yield (from 8, two steps). The exo-double bond was treated with OsO 4 to give a diol, followed by treatment with PhI(OAc) 2 to give desired 2a in 31% yield. The total yield was 2.5% in 7 steps.
The absorption spectrum of 2a in 1,2,4-trichlorobenzene exhibited characteristic 1 A-1 L a transitions of the anthracene chromophore at 353, 371, 391 and 408 nm, with vibrionic progressions, which were red-shied from the peaks of parent 2b at 350, 368 and 391 nm in the 1,2,4-trichlorobenzene solution ( Fig. S1 †). When the solution of 2a was heated at 230 C, the solution changed from colorless to pale-green, which exhibited characteristic acene vibration absorption bands at 573, 623 and 679 nm. However, this colour changed to yellow within a few minutes owing to the dimerization or oxidation of 1a.

Physical properties
The absorption maximum in the visible range was observed at 679 nm, whereas parent hexacene 1b exhibited the peak maximum at 675 nm in 1,2,4-trichlorobenzene (Fig. S1 †). The peaks were red-shied from those of parent hexacene 1b by 4 nm, indicating the reduction of HOMO-LUMO gap of 1a by the bromo substituent. This phenomenon has similarly been observed in the pairs of tetracene (473 nm in THF) and 2-bromotetracene (477 nm in THF), 22 as well as pentacene (575 nm in THF) and 2-bromopentacene (578 nm in THF). 23 To compare with the reported hexacene analogues, the peak maximum was shown to be red-shied from that of pentaceno [2,3-b]thiophene (640 nm in o-DCB) 30 due to the increase of aromaticity. In comparison with other substituent group effect, however, it showed a blue shi from those of tricyclohexylsilylethynyl-octauorohexacene (725 nm), 31 tri-tert-butylsilylethynyhexacene (738 nm), 32,33 and trialkylsilylethynyl-azahexacene (825-842 nm in hexane) 34 due to the electron-donating effect of the trialkylsilylethynyl acetylene units.
In the thermal gravimetric analysis (TGA) prole of 2a, the rst weight loss (8.5%, calcd 6.5%) was observed at approximately 200 C to generate 1a. The thermal weight loss prole did not change up to 360 C. Then it was followed by the second weight loss caused by the vaporisation as well as the decomposition of 1a at temperatures greater than 400 C (Fig. 2a). Decarbonylation at 200 C was conrmed by infrared (IR) spectroscopy, which revealed the disappearance of the characteristic C]O stretching band at 1786 cm À1 aer heating at 230 C (Fig. 2b). The high-resolution FAB-MS spectrum revealed In contrast, aer heating at 230 C under nitrogen, a broad band was observed at 500-900 nm, and the peaks at 360-400 nm, which are characteristic of the anthracene moiety, disappeared.
These structures were different from that of 1a in solution. Film 1a exhibited characteristic peaks at 833 and 776 and at 708 and 661 nm. These peaks exhibited the appearance of Davydov splitting effect. 20,21,35 The same pattern appeared in other related acene structures in the solid state, including parent 1b, at 840 nm, 765 nm, 708 nm and 654 nm. 20 The rst bands at 833 and 775 nm corresponded to the 0-0 band, and those at 708 and 654 nm corresponded to the 0-1 band (Fig. 2c). The high thermal stability of 1b in the solid state was conrmed by solid-state NMR that maintained invariant over 1 month. 13 Comparing it with heptacene under a similar situation, the latter dimerised slightly aer 1 month. 16 To verify the thermal stability of 1a, 13 C CP-MAS NMR spectra were recorded for monitoring the variation of the carbon skeleton. Compound 2a exhibited three main peaks at 193.2 (bridge position of C]O), 137-114 (aromatic carbon atoms) and 54.4 ppm (bridgehead tertiary carbon atom). Aer the conversion to 1a, the spectrum exhibited aromatic carbon peaks at 125.7 and 122.9 ppm only, indicating a quantitative transformation. Aer maintaining 1a for 90 days in the dark under air atmosphere, no changes were observed in the CP-MAS spectrum, indicative of the high thermal stability of 1a (Fig. 2d). This high thermal stability can be compared with the reported property of hexacene (>1 month in dark) 20 and tricyclohexylsilylethynylhexacene (several month). 33 The lm of 1a was grown by vacuum sublimation and exhibited a structure similar to the lm in Fig. 2c (Fig. S3 †). The ionisation potential (E ip ) and electron affinity (E ea ) of the lm 1a were À5.24 and À3.30 eV, respectively, while those of 1b were À4.81 and À2.70 eV, respectively (Fig. S4 †). The E ip of thin lm 1b (À4.81 eV) was consistent with that of the crystalline powder reported previously (À4.96 eV). 14 The energy gap of 1a (1.94 eV) was less than that of 1b (2.11 eV). Theoretical computation results (DFT, B3LYP/6-31G(d) level) revealed the HOMO and LUMO of 1b to be À4.68 and À2.90 eV, respectively, while the corresponding values for 1a were À4.81 eV and À3.04 eV, respectively. The HOMO and LUMO were lowered by bromination compared with those of 1a, indicative of the electronwithdrawing effect by the bromo substituent. The HOMO-LUMO gap of 1a was 1.77 eV, whereas that of 1b was 1.78 eV, supporting the experimental results (Table 1).

Charge transport properties
The properties of organic eld-effect transistors (OFETs) made with the lms of 1a-b were examined. The OFET devices were fabricated by vacuum sublimation of 1a-b under a pressure of 8 Â 10 À6 torr to deposit the thin lms on an HMDS/SiO 2 /Si substrate, followed by the deposition of gold electrodes on the top of the lms. The lm thickness of 1a-b was 60 nm. The channel dimension of the source/drain electrodes was 45 Â 2000 mm. The output parameters were measured on a selected lm across a source-drain channel, followed by the plot of drain current (I D ) versus drain source voltage (V DS ) at various gate voltages (V G ). The corresponding transfer characteristics were plotted for log(I D ) versus V G at a V DS of À100 V and I D versus V DS  in the saturation mode. The eld-effect hole mobility of 1a was measured, and the mobility values ranged from 0.21 to 0.83 cm 2 V À1 s À1 with threshold voltages of À50 to À69.3 V. The averaged performance of six independent devices was 0.52 cm 2 V À1 s À1 and a threshold of À56.3 V. The best mobility of bromohexacene 1a was observed at 0.83 cm 2 V À1 s À1 with an on/off ratio of 5.0 Â 10 4 and a threshold of À52 V (Fig. 3a-b). To compare the hole mobility, parent 1b was also tested. The mobility of 1b in the saturation mode ranged from 0.072 to 0.076 cm 2 V À1 s À1 with a threshold voltage ranging from À19 to À22 V. The averaged performance of six independent devices was 0.074 cm 2 V À1 s À1 and a threshold of À20.7 V. The best lm mobility of 1b was observed at 0.076 cm 2 V À1 s À1 with an on/off ratio of 2.4 Â 10 2 and a threshold of À21 V (Fig. 3c-d). Previously, the hole mobility of hexacene 1b has been reported in the single-crystal phase and in the spin-coated thin-lm phase by the precursor method. The best hole-transfer mobility by spincoated 1b was 0.035 cm 2 V À1 s À1 , with a similar surface treatment on SiO 2 /Si substrate. Although our fabrication conditions were not fully optimised, the mobility of the vacuum-deposited lm was greater than that of the spin-coated one. It indicates that better crystalline lms of 1b were formed by thermal deposition. The mobility of lm 1a exhibited a larger range of randomness compared to 1b. However, a higher hole-transfer efficiency of 1a ranging 7-to 10-folds compared with that of 1b was observed in all tested devices. This result indicated that the packing moiety and/or lm morphology possessing a better charge-transfer pathway may account for the mobility. Previously, a single-crystal bromopentacene was found to exhibit a 4fold faster hole-transfer speed than that of non-substituted pentacene, while their reorganization energies were estimated to be 102 meV and 95 meV (B3LYP/6-31Gd level), respectively. 23 DFT computations revealed that the reorganisation energy between the radical cation and ground state of 1a was 85 meV (B3LYP/6-31Gd level). This value was quite close to that of 1b (79 meV), suggesting a similar energy loss during structure reorganization in the hexacenes 1a and 1b.

TEM, XRD, and AFM measurements of hexacene lms
To investigate the morphology of thin-lm, we performed a surface analysis on the lms 1a and 1b. Fig. 4 showed crosssection of transmission electron microscope (TEM) image of ca. 60 nm deposited lms 1a and 1b on HMDS/SiO 2 /Si surface. The ion milling method allows us to examine a cross-section of substrate at the interface of 1a-b and HMDS/SiO 2 /Si. The TEM image show a good continuous growth of 1a and 1b lms on HMDS/SiO 2 /Si surface, suggesting both lms were deposited uniformly on the substrate. The parent structure of hexacene 1b exhibited a herringbone arrangement, where the face-to-edge stacking arrangement between adjacent molecules avoided the progress of dimerization and led to a high thermal stability in the solid state. 14 The vacuum-deposited lm of 1b exhibited out-of-plane X-ray diffraction (XRD) peaks along (00l) direction and in-plane XRD along (hk0) direction (Fig. S4 †).
These patterns indicated that the molecules in the deposited lm are oriented vertical to the surface along their long axis. In addition, this lm orientation was consistent with the reported out-of-plane pattern of heat-converted lm 1b from corresponding precursor compound. 21 The (001) peak was observed at 4.84 corresponding to an interplanar distance of 18.3Å. This value can be compared with the d-spacing in the single crystal of 1b, which has been estimated to be 16.4Å. It indicates that the molecules in the crystalline lm 1b are tilted on the surface of HMDS/SiO 2 /Si substrate. The out-of-plane and in-plane XRD peaks of vacuum-deposited lm 1a exhibited (00l) and (hk0) patterns, indicating that molecules in lm 1a is oriented along the long axis normal to the surface (Fig. S4 †). Although lm 1a exhibited weaker XRD diffraction peaks compared with that of 1b, the 2q angle of 1a observed at a smaller angle of 4.43 on the HMDS/SiO 2 /Si substrate. The d-spacing of bromohexacene molecules is estimated to be 20.0Å. The larger d-spacing revealed that the molecules interact through the a-b axis.
To further study of nding the difference of mobility between 1a and 1b, the atomic force microscope (AFM) analysis was investigated. The lms 1b revealed a high surface roughness of 12.01 nm (Fig. 5). In contrast, lm 1a revealed a lower roughness of 6.48 nm. This smoother  surface in 1a achieved a small energy loss during the transport of holes between the source and drain.

General information
The 1 H and 13 C NMR spectra were recorded in solutions on a BrukerAV600 (600 MHz) spectrometer. The 1 H and the 13 C NMR chemical shis were reported as d values (ppm) relative to external Me 4 Si. The coupling constants (J) were given in hertz. High resolution FAB mass spectra were recorded on a JMS-700 MStation spectrometer. FAB MS spectra were measured with 3-nitrobenzyl alcohol (NBA) as the matrix. Analytical thin layer chromatography (TLC) was performed on silica gel 60 F 254 Merck. Column chromatography was performed on KANT-OSi60N (neutral). Absorption and reectance spectra were recorded on a SHIMADZU UV-3600. IR spectra were performed by SHIMADZU IRPrestige-21 spectrophotometer. AFM measurements were tested by SHIMADZU SPM-9700. The elemental analyses were recorded on a Yanaco CHN recorder MT-6. THF was distilled from sodium benzophenon ketyl. Toluene was distilled from CaH 2 . Other solvents and reagents were of reagent quality, purchased commercially, and used without further purication.

13 C CP/MAS NMR
The 13 C CP/MAS NMR spectra were acquired with a Bruker Avance 400 MHz NMR spectrometer, equipped with a 4 mm double resonance probe operating at the 1 H and the 13 C Larmor frequencies of 400.13 and 100.63 MHz, respectively. For the polarization transfer, the contact-time was set to 1.75 ms. During data acquisition, 1 H decoupling by spinal-64 was applied. Powdered samples were packed in 4 mm zirconium oxide MAS rotors with Kel-F cap. A sample spinning frequency of 12 kHz was used and regulated by a spinning controller within AE1 Hz. All CP-MAS experiments were carried out at ambient temperature. The 13 C NMR chemical shis are referenced to the methyl signal (¼17.36 ppm) of hexamethylbenzene, which was used as an external standard. All measurements were performed in air, and the sample tube was kept in the dark between measurements.

FET measurement
The electrical measurements were carried out in vacuum using a semiconductor parameter analyzer (B1500A, Keysight). The saturation mobility (m sat ) was extracted from the slope of the square root of the drain current plot vs. V G from eqn (1).
where I D,sat is the drain-to-source saturated current; W/L is the channel width to length ratio; C i is the capacitance of the insulator per unit area, and the V G and V T are gate voltage and threshold voltage, respectively. A heavily doped silicon (Si) wafer was used for a back gate electrode, which was covered with a 300 nm-thick thermally grown SiO 2 (C i ¼ 10.2 nF cm À2 ) as the gate insulator. Channel length (L) and width (W) were 2000 mm and 45 mm.

TEM measurement
The organic lms were vacuum sublimation of 1a-b under a pressure of 8 Â 10 À6 torr to deposit the thin lms for 350 nm on a HMDS/SiO 2 /Si substrate. Cross-sectional TEM samples for all of the lms were prepared by mechanical thinning and ion milling. The transmission electron microscope used in this study was a JEOL JEM-2100F, which was operated at an accelerating voltage of 200 kV.
The mixture of 5 (163 mg, 0.519 mmol) and zinc (600 mg) and glacial acetic acid (50 mL) was sonicated for 30 min at room temperature. Aer reaction, the suspension was ltered and the solution was evaporated to give the crude product. Silica gel chromatography of the crude product with CH 2 Cl 2 and treatment with iced MeOH afforded dione 6 (114 mg). The mixture was used for next step without further purication.
A mixture of diketone 6 (114 mg, 0.361 mmol) and 4-bromophthalaldehyde 7 (76.1 mg, 0.361 mmol) was dissolved in EtOH (50 mL). The EtOH solution was bubbled by nitrogen gas for 20 min to remove oxygen. To the mixture was added 10 wt% KOH/EtOH solution (2-3 drops) in the nitrogen atmosphere and stirred 72 h at room temperature under nitrogen gas. The mixture gradually became dark, and pale-yellow powder precipitated. Aer reaction, the precipitate was ltered and washed with EtOH and hexane to afford endo-8 (86.2 mg, 17% in three steps). Pale yellow powder (EtOH).   19.7, 51.3, 52.9, 117.9, 119.0, 124.1, 125.6, 127.5, 127.9, 128.6, 131.4, 131.9, 132.2, 132.8, 132.9, 133.5, 136.0, 142.3, 143.7, 197.0, 197.8. 3.5.2. Synthesis of (6S,15R)-10-bromo-17-(propan-2ylidene)-6,15-dihydro-6,15-methanohexacene 9. To a solution of dione 8 (220 mg, 0.446 mmol) in MeOH (50 mL) and THF (50 mL) in an ice bath was added NaBH 4 (74 mg, 1.65 mmol). Aer 2 h, the reaction mixture was quenched with water. The aqueous solution containing precipitates was extracted with CH 2 Cl 2 . The organic layer was washed with water, and dried over anhydrous MgSO 4 . Removal solvent gave the diol 9 (212 mg) as yellow solids. This crude compound was subjected to the next step without further purication. To a mixture of diol 9 and dried pyridine (10 mL) was added dropwise POCl 3 (0.9 mL) at 0 C. The resulting mixture was stirred at room temperature for 72 h, then at 80 C for another 30 min. The mixture was poured into ice water and was extracted with CH 2 Cl 2 . The organic layer was washed successfully with 3 N HCl and brine, then was dried over MgSO 4 . The crude product was puried by a silica gel chromatograph eluted with hexane/CH 2 Cl 2 (4 : 1) to give the aromatized compound 9 (98.8 mg, 48%) as pale yellow powder. Physical data of 9: pale yellow powder (EtOH 3.5.3. Synthesis of (6S,15R)-10-bromo-6,15-dihydro-6,15methanohexacen-17-one 2a. A mixture of olen 9 (200 mg, 0.433 mmol) and N-methylmorpholine N-oxide (NMO) (50% in H 2 O, 1.5 mL) in a mixed solvent of acetone (50 mL) and H 2 O (1.5 mL) was stirred at room temperature until 9 was dissolved completely. To the solution was added a few drops of OsO 4 (4% H 2 O soln). The reaction was monitored by TLC until completion, then the mixture was quenched with 15% aqueous Na 2 S 2 O 4 . The aqueous solution was extracted with EtOAc, dried over Na 2 SO 4 , and evaporated. The product was puried by a silica gel chromatograph eluted with CH 2 Cl 2 to give diol 10 (79 mg), which was directly used in the next step. The diol 10 (79 mg) and PhI(OAc) 2 (120 mg) in benzene (100 mL) was stirred at 60 C for 12 h. Aer reaction, the mixture was cooled in an ice bath, while white precipitates formed. The solids were collected by suction ltration to give compound 2a

Conclusions
A novel hexacene precursor was successfully synthesised, which can be quantitatively converted at around 200 C to the corresponding 2-bromohexacene. It exhibited high thermal stability over 3 months in the dark. The bromine atom affected the hexacene crystal packing and decreased the HOMO-LUMO energy gap. The thin-lm of 1a was fabricated by using both spin-coating and vacuum sublimation methods, and both lms exhibited exciton coupling, indicative the presence of herringbone arrangement in the polycrystalline lm. The lm of 1a exhibited a more efficient hole-transport property compared with that of parent 1b. Hence, lm 1a exhibits a higher hole mobility of 0.83 cm 2 V À1 s À1 than that of 1b (0.074 cm 2 V À1 s À1 ). Although these hole mobility were lower than that of single crystal hexacene (4.28 cm 2 V À1 s À1 ), 26 it was comparable with the reported value of solution-processed single crystal tricyclohex-ylsilylethynyloctauorohexacene (0.1 cm 2 V À1 s À1 ). 31 To the best of our knowledge, this is the rst study on the charge-transport property of a stable vacuum-deposited thin-lm of hexacene for electronic device. Currently, other derivatives of halogenated hexacene are prepared, and their properties related to optoelectronic devices are being examined. The results will be reported in due course.
Author's contributions MW designed and performed the experiments and theoretical calculations; TM, CTC, MS and SSS were synthesized, measured and analysed the materials and physical properties; TM and CA designed the devices and analysed the data; JM measured and analysed the TEM; MW and TI were measured and analysed the NMR data; MW, TM and TJC were co-wrote the manuscript; all authors gave nal approval for publication.

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