Zigzag‐Elongated Fused π‐Electronic Core: A Molecular Design Strategy to Maximize Charge‐Carrier Mobility

Abstract Printed and flexible electronics requires solution‐processable organic semiconductors with a carrier mobility (μ) of ≈10 cm2 V−1 s−1 as well as high chemical and thermal durability. In this study, chryseno[2,1‐b:8,7‐b′]dithiophene (ChDT) and its derivatives, which have a zigzag‐elongated fused π‐electronic core (π‐core) and a peculiar highest occupied molecular orbital (HOMO) configuration, are reported as materials with conceptually new semiconducting π‐cores. ChDT and its derivatives are prepared by a versatile synthetic procedure. A comprehensive investigation reveals that the ChDT π‐core exhibits increasing structural stability in the bulk crystal phase, and that it is unaffected by a variation of the transfer integral, induced by the perpetual molecular motion of organic materials owing to the combination of its molecular shape and its particular HOMO configuration. Notably, ChDT derivatives exhibit excellent chemical and thermal stability, high charge‐carrier mobility under ambient conditions (μ ≤ 10 cm2 V−1 s−1), and a crystal phase that is highly stable, even at temperatures above 250 °C.


Preparation of zinc chloride solution
An oven-dried Schlenk tube was charged with zinc chloride (479.5 mg, 3.50 mmol), and the tube was heated under evacuation until zinc chloride completely melted. After cooled to room temperature, THF (3.5 mL) was added to give 1.0 M zinc chloride solution in THF.

General for Synthesis and Characterization
All the reactions were carried out under an atmosphere of argon. Air-or moisture-sensitive liquids and solutions were transferred via a syringe or a Teflon cannula. Analytical thin-layer chromatography (TLC) was performed on glass plates with 0.25 mm 230-400 mesh silica gel containing a fluorescent indicator (Merck Silica gel 60 F254). TLC plates were visualized by exposure to ultraviolet lamp (254 nm and 365 nm) and by dipping with 10% phosphomolybdic acid in ethanol and heating on a hot plate. Flash column chromatography was performed on Kanto silica gel 60. Open column chromatography was performed on . All NMR spectra were recorded on a ECS400 spectrometer.
Chemical shifts are reported in parts per million (ppm,  scale) from residual protons in the deuterated solvent for 1 H NMR ( 7.26 ppm for chloroform, 5.93 ppm for 1,1,2,2tetrachloroehtane (TCE)) and from the solvent carbon for 13 C NMR ( 77.16 ppm for chloroform, 74.00 ppm for 1,1,2,2-tetrachloroehtane). The data were presented in the following format: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, m = multiplet), coupling constant in Hertz (Hz), signal area integration in natural numbers, assignment (italic). Mass spectra were measured on a BRUKER compact-TKP2 mass spectrometer. Melting points and elemental analysis were collected on a Mettler Toledo MP70 Melting Point System and J-Science Lab JM10 MICRO CORDER, respectively.

Chryseno[2,1-b:8,7-b']dithiophene (ChDT)
To a colorless solution of 3 (1.03 g, 3.02 mmol, 1.00 mol amt.) in DMF (30 mL) was added PtCl 2 (166 mg, 0.624 mmol, 0.21 mol amt.) at room temperature, and the resulting brown suspension was kept stirring at 80 °C for 16 h. After the reaction mixture was cooled to room temperature, the brown solid was filtered. The solid was dissolved with oDCB (ca. 1.5 L, 120 °C) and the resulting solution was passed through a short pad of silica gel. The resulting solution was condensed in vacuo and the solid was recrystallized from the solution to afford the titled compound as a yellow solid (593 mg, 58% yield). 1  Found: C 77.25; H 3.55.

Cores
The Kohn-Sham energy levels of all of compounds in this work were calculated at the B3LYP/6-311G(d) level of theory with the SPARTAN 14 package, Wavefunction Inc. The reported promising semiconducting π-electronic cores including pentacene,  Figure S1. derivatives (calculated at the B3LYP/6-311Gd level).

HOMO Energy Levels of ChDT Derivatives
The HOMO level of ChDT core (E HOMO = -5.68 eV) is significantly higher than that of DNT-W core (E HOMO = -5.86 eV). The introduction of an electron-donating alkyl group into the ChDT core leads to an effective rise of the HOMO level (R = Me, E HOMO = -5.53 eV) because the HOMO coefficient of ChDT core at -position of terminal thiophene unit exists.
Furthermore, the introduction of a thienyl group as a linker also lead a rise of HOMO level (R = Thienyl, E HOMO = -5.44 eV).

Solubility Test
To a weighed sample of around 1 mg was added 50 L of toluene, repeatedly. The resulting suspension was shaken and sonicated at 60 °C. The total amount of solvent (mL) was converted into solubility in wt%. The results are summarized in Table S1. Table S1. Solubility test of decyl-substituted ChDT derivatives in toluene at 60 °C, together with reported C 10 -DNTT and C 10 -DNBDT-NW for comparison.

Photoelectron yield spectroscopy (PYS) was performed on a Sumitomo Heavy
Industries Advanced Machinery PYS-202. For PYS measurement, thin films (ca. 100 nm) of all ChDT derivatives were thermally evaporated on ITO coated quartz substrates and measurements were performed in vacuum. The photoelectron yield spectra are depicted in Figure S3.

Chemical Stability Test
To evaluate the chemical stability of ChDT derivatives, time-dependent UV-vis absorption spectra were carried out for a period of 14 days. It was found that the spectra of πextended derivative, C 10 -Th-ChDT do not change over time, indicating that ChDT derivatives in this work have high chemical stability. Representative data for C 10 -Th-ChDT are shown in Figure S4.

Thermogravimetric Analysis (TGA)
TGA measurement was carried out with a Rigaku Thermo Plus EVO II TG 8121.
Sample was placed in aluminum pan and heated at the rate of 5 ºC/min, under N 2 purge at a flow rate of 100 mL/min. Al 2 O 3 was used as reference material. Prior to further purification to prepare device grade compounds, their thermal properties were investigated by TGA in the range of room temperature to 500 °C (See Figure S5). No thermal decomposition was observed in all of ChDT derivatives in that thermal range, implying that they can be purified by thermal sublimation. Figure S5. TGA plots of ChDT derivatives in the range of room temperature to 500 °C in a flow of nitrogen gas (scan rate: 5 °C/min, N 2 purge: 100 mL/min).

Differential Scanning Calorimetry (DSC)
DSC measurement was carried out with a Rigaku Thermo Plus EVO IIDSC 8231.
Sample was placed in aluminum pan and heated at the rate of 5 ºC/min, under N 2 purge at a flow rate of 100 mL/min. Al 2 O 3 was used as reference material. The DSC measurements could lead to phase-transition data for the endothermic process (either from solid to liquid or from solid to liquid crystal).

Single-Crystal Analyses
It is crucially important to estimate the potential of the semiconductors by examining not only their molecular structure but also their packing structure in the solid state. Single crystals were obtained by means of either physical vapor transport (PVT) technique or recrystallization from certain organic solvents. That of ChDT was grown by PVT (315 °C and 220 °C in argon flow of 40 ccm). Those of C 10 -ChDT and C 10 -Th-ChDT were grown by gradual diffusion of isopropanol into toluene solution. Single-crystal diffraction data were collected on a Rigaku R-AXIS RAPID II imaging plate diffractometer with CuK radiation for ChDT, C 10 -ChDT and C 10 -Th-ChDT. The molecular structures (ellipsoid type) in the front and side views, the intermolecular interactions and short contacts, and packing structures of ChDT and C 10 -ChDT are shown in Figure S7-12.

Transfer Integral and Band Calculations
Based on their packing structures, the transfer integral (t) values of the HOMOs between neighboring molecules were estimated by density functional theory at the PBEPBE/6-31G(d) level, as illustrated in Figure S13. Figure S14 shows transfer integral of C 10 -Th-ChDT packing structure depending on the displacement from its original packing structures in column direction, indicating that C 10 -Th-ChDT is less susceptible to the displacement parameter in the direction of molecular longitudinal axis.
To further understand the carrier transporting capabilities in the bulk state, their electronic band structure were also calculated at the same level as the intermolecular electronic couplings using the periodic boundary condition at the PBEPBE/6-31G(d) level.
Electronic band structure calculations were conducted based on the packing structure by way

Calculations for Amplitude of Translational Motions
The amplitude of translational motion of a molecule in the aggregated structure can be estimated as follows. First, the total energy of the cluster as shown in Figure S18a) for the experimental structure is calculated using the density functional theory at the B3LYP/6-31G(d) level with the van der Waals correction in the framework of DFT-D approach. Then, change in total energy is computed under a rigid molecule approximation as a function of displacement of the molecule surrounded by the black square in Figure S18a). The displacements along the column, transverse, and out-of-plane directions are investigated here.
As shown in Figure S18b), the amplitude of translational motion at the temperature T = 300K is obtained as the displacement where the total energy is increased up to k B T = 25.7 meV. The calculated amplitudes are summarized in Table S3.

OFET Device Fabrication and Evaluation Procedure
Organic field-effect transistors based on organic semiconducting crystals were fabricated with the lamination method for ChDT and the edge-casting method for C 10 -ChDT and C 10 -Th-ChDT. The surfaces of a heavily-doped silicon wafers with thermally-oxidized SiO 2 (500 nm) were pretreated with self-assembled monolayers (SAMs) prior to each method; heptadecafluorodecyltrimethoxysilan ( and Au (50-80 nm) were thermally deposited through a shadow mask to construct bottomgate-top-contact architecture. Finally, crystals were shaped into rectangular channels by laser etching to determine channel length and width for correct evaluation of mobility. In the device, SiO 2 layer acts as a gate insulator, doped silicon as a gate electrode, and Au as contact electrodes (source and drain electrode). The layer of F 4 -TCNQ, electron-accepting material, was introduced between organic semiconductor and contacts to reduce parasitic contact resistance. Figure S19. Illustration of the edge-casting method.
Transistor characterizations were carried out using Keithley 4200 semiconductor parameter analyzer. Transistor characteristics for ChDT and C 10 -ChDT are shown in Figure   S20 and Figure S21, respectively. After checking small hysteresis in transfer curve and saturation of drain current in output characteristics, field-effect mobilities ( FET ) in the saturation regime were evaluated using the following the equation: Where C i is the capacitance of gate insulator and V th is the threshold voltage. The highest mobility values are summarized in main text.

Atomic Force Microscopy
To reveal the surface morphorogy of solution-processed single-crystalline film for C 10 -ChDT and C 10 -Th-ChDT, atomic force microscopy (AFM) were carried out with Shimadzu SPM-9700HT. Since the device channels are formed as mono-domain signle crystal film, crystal steps were observed around channels. The AFM images are shown in Figure S22 for C 10 -ChDT and Figure S23 for C 10 -Th-ChDT crystalline films. The films have molecularlyflat terraces with one-or two-molecular steps. The heights correspond to each single-crystal structural data (Table S2), which is consistent with XRD experiments ( Figure S24).
Moreover, for C 10 -ChDT crystal, some cores can be clearly observed on the terrace. Figure S22. AFM images of C 10 -ChDT solution-processed crystalline film. Figure S23. AFM images of C 10 -Th-ChDT solution-processed crystalline film.

X-ray-diffraction Measurements for Solution-crystallized Thin Film
In order to determine the crystal directions in the actual devices, X-ray-diffraction measurements were carried out with transmission X-ray. Diffraction data collected on an imaging plate are shown in Figure S24 for C 10 -Th-ChDT. Under the assumption that the crystal structure of the crystalline films was the same as the structures unveiled by the singlecrystal structure analysis, the Laue spots would be assigned as described in the figures. In the C 10 -Th-ChDT device, the a axis is almost perpendicular to and the bc place is parallel to the substrate surface. The direction of the crystal growth and the channel direction are almost the c axis, which means carriers transport along the direction of smaller effective mass (m* || /m 0 = 1.06). Figure S24. a) In-plane and b) out-of-plane Laue spots on an imaging plate for edge-casted C 10 -Th-ChDT crystalline film with crystal structures.