Mixed-Orbital Charge Transport in N-Shaped Benzene- and Pyrazine-Fused Organic Semiconductors

The hole-carrier transport of organic semiconductors is widely known to occur via intermolecular orbital overlaps of the highest occupied molecular orbitals (HOMO), though the effect of other occupied molecular orbitals on charge transport is rarely investigated. In this work, we first demonstrate evidence of a mixed-orbital charge transport concept in the high-performance N-shaped decyl-dinaphtho[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (C10–DNBDT–NW), where electronic couplings of the second HOMO (SHOMO) and third HOMO (THOMO) also contribute to the charge transport. We then present the molecular design of an N-shaped bis(naphtho[2′,3′:4,5]thieno)[2,3-b:2′,3′-e]pyrazine (BNTP) π-electron system to induce more pronounced mixed-orbital charge transport by incorporating the pyrazine moiety. An effective synthetic strategy for the pyrazine-fused extended π-electron system is developed. With substituent engineering, the favorable two-dimensional herringbone assembly can be obtained with BNTP, and the decylphenyl-substituted BNTP (C10Ph–BNTP) demonstrates large electronic couplings involving the HOMO, SHOMO, and THOMO in the herringbone assembly. C10Ph–BNTP further shows enhanced mixed-orbital charge transport when the electronic couplings of all three occupied molecular orbitals are taken into consideration, which results in a high hole mobility up to 9.6 cm2 V–1 s–1 in single-crystal thin-film organic field-effect transistors. The present study provides insights into the contribution of HOMO, SHOMO, and THOMO to the mixed-orbital charge transport of organic semiconductors.


Materials and General Characterization Methods
Reagents and solvents were purchased from Tokyo Chemical Industry Co., Ltd, Sigma Aldrich, Wako Chemicals, and Kanto Chemicals. Anhydrous solvents were purified by the solvent purification system prior to use. All reactions were carried out under an atmosphere of argon.
Analytical thin-layer chromatography (TLC) was performed on glass plates with silica gel containing fluorescent indicator (Merck TLC Silica gel 60 F254, 1 mm). Column chromatography was performed on Kanto silica gel 60. All NMR spectra were recorded on JEOL ECS400 spectrometer. Chemical shifts were reported in parts per million (ppm, d scale) from residual protons in the deuterated solvent for 1 H NMR (5.93 ppm for 1,1,2,2tetrachloroethane-d2 (TCE-d2) and 7.26 ppm for chloroform-d (CDCl3)) and 13 C NMR (73.78 ppm for 1,1,2,2-tetrachloroethane-d2 (TCE-d2) and 77.16 ppm for chloroform-d (CDCl3)). The data were presented in the following format: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, quint = quintet, dd = doublet of a doublet, m = multiplet), coupling constant in Hertz (Hz), signal area integration in natural numbers. Time-of-flight high-resolution mass (TOF-MS) spectrometry measurements were measured on a BRUKER compact-TKP2 mass spectrometer with the atmospheric pressure chemical ionization (APCI) method. Elemental analysis measurements were carried out on a JScience Lab JM10 CHN analyzer at the Comprehensive Analysis Center, the Institute of Scientific and Industrial Research, Osaka University.

Theoretical Calculations
The initial molecular structures were constructed in the GaussView 5.0 Software and optimized at the B3LYP/6-311G(d) level of theory. Kohn-Sham energy levels of the compounds in this work were calculated at the same level with the Gaussian09 package. 1 Based on the packing structure, transfer integral (t) values of the HOMOs between neighboring molecules were estimated by the dimer method 2 with Density Functional Theory, and the electronic band structure using the periodic boundary were both calculated at the PBEPBE/6-31G(d) level of theory. Effective mass of holes were calculated as along respective directions. Figure S1. Effective masses and band structures of C10-DNBDT-NW from plane-wave and tight-binding approximations (colored shades illustrate the band widths).

Preparation of tBuZn(TMP)Li
anhydrous THF (25.0 mL) under argon and the mixture was cooled to -78 ˚C. Then, as prepared

Thin-Film UV-vis Spectroscopy
The UV-vis absorption spectra were recorded with a Jasco V-670 spectrometer. Thin film (ca. 100 nm) of C10-BNTP was thermally evaporated on a synthetic quartz substrate. Figure S18. UV-vis absorbance of 100 nm-thick C10-BNTP vacuum-deposited film.

X-Ray Crystallography
Single-crystal X-ray diffraction data were collected on a Rigaku R-AXIS RAPID II imaging plate diffractometer with CuKα radiation (λ = 1.54187 Å) at room temperature.
The surfaces of a heavily-doped silicon wafers with thermally-oxidized SiO2 were pretreated with the parylene diX-SR (thickness of 28 nm) insulating layer and b-phenylethyl where ID is the drain current, W the channel width, μ the mobility in the saturation regime, Ci the gate capacitance per unit area, L the channel length, VG the gate voltage, and Vth the threshold voltage.    shows the output characteristic of single-crystalline C10Ph-BNTP OFET measured before the transfer curve measurement. a The data are taken in our previous report. 3