Electronic Supplementary Information for N-Alkyl functionalized barbituric and thiobarbituric acid bithiophene derivatives for vacuum deposited n-channel OFETs

A family of barbituric and thiobarbituric acid end capped small molecule semiconductors were synthesized, characterized and shown to exhibit n-channel organic thin film transistor properties. By changing the N-alkyl substituent from methyl to ethyl, a dramatic increase in electron mobilities was observed with values nearing 0.3 cm2 V−1 s−1.


Computational analysis
Quantum chemistry calculations have been carried using density functional theory (DFT) as implemented in Gaussian 09 suite of programs. 1The B3LYP exchange-correlation functional 2 was used in most computations.The geometry of the gas phase target compounds was optimized at the B3LYP/6-31+G(d) spin-restricted level of theory.Vibrational frequency calculations were performed to verify that all the optimized geometries correspond to minima on the potential energy surfaces.Single point energy calculations were then conducted employing the optimized geometries and a larger 6-311+G(d,p) basis set.The calculated energies of the frontier molecular orbitals (MOs), the highest occupied molecular orbital Table S1: Frontier molecular orbital plots for n-type derivatives 7-14.

Experimental
2,2'-Bithiophene-5,5'-dicarbaldehyde 5 3 and thieno [3,2-b]thiophene-2,5dicarbaldehyde 6 4 were made following literature procedures and 1,3-dimethylbarbituric acid 1 and 1,3-diethyl-2-thiobarbituric acid 4 were purchased from Sigma-Aldrich. 1 H (400 MHz) spectra were recorded on a Bruker Av400 spectrometer using CDCl 3 .Chemical shifts (δ) are reported in ppm.Melting points were determined from differential scanning calorimetry (DSC).DSC was performed at a rate of 10°C/minute under nitrogen on a Mettler Toledo DSC821.Thermo gravimetric analysis (TGA) was run on a Mettler Toledo TGA/SDTA854 at a rate of 10°C/minute under nitrogen.High resolution (HR) mass spectra were measured on a ThermoQuest MAT95XL mass spectrometer using an ionization energy of 70eV.Thin film X-ray diffraction (XRD) patterns were recorded on evaporated thin films on ODTS treated SiO 2 substrates using a Bruker D8 Advance X-ray Diffractometer using CuKα radiation (40kV, 40mA) equipped with LynxEye silicon strip detector.Scanning electron microscopy (SEM) was performed on evaporated thin films of 7-14 on ODTS-treated SiO 2 substrates that were mounted on an aluminium stub with double-sided conductive carbon tape that had been iridium coated using a Polaron SC5750 sputter coater.SEM was carried out with a Philips XL30 field emission scanning electron microscope (FESEM) for imaging.

Synthesis
1,3-Diethylbarbituric acid 2. The procedure of Brooker and co-workers was followed with modifications. 5A solution of sodium ethoxide was prepared by the addition of sodium metal (1.10 g, 48.0 mmol) to dry ethanol (50 mL) under a nitrogen atmosphere.To this solution was added diethyl malonate (8.01 g, 50.0 mmol) and the reaction mixture was stirred for 5 min.
1,3-Dimethyl-2-thiobarbituric acid 3.The procedure of Brooker and co-workers was followed with modifications. 5A solution of sodium ethoxide was prepared by the addition of sodium metal (1.10 g, 48.0 mmol) to dry ethanol (50 mL) under a nitrogen atmosphere.To this solution was added diethyl malonate (7.68 g, 48.0 mmol) and the reaction mixture was stirred for 5 min.1,3-Dimethylthiourea (2.50 g, 24.0 mmol) was then added and the mixture refluxed for 8 hrs.To a stirred solution of thieno[3,2-b]thiophene-2,5-dicarbaldehyde 6 (295 mg, 1.50 mmol) of dry t-butanol (75 mL) under nitrogen was added 1,3-diethylbarbituric acid 2 (645 mg, 3.50 mmol) followed by 3 drops of piperidine.The solution was refluxed for 6 hrs then allowed to cool to RT to afford a solid.The precipitate was collected, washed with ethanol until the filtrate was colorless and then dried under high vacuum.

Differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA)
DSC was performed on a Mettler Toledo DSC821 instrument with a heating rate of 10 ºC per minute from 30-600 ºC under nitrogen.TGA was performed on a Mettler Toledo TGA/SDTA851 instrument with a heating rate of 10ºC per minute from 30-600 ºC under nitrogen.R 1 = 0.0480 wR2 = 0.0994

OFET device fabrication and testing.
Discrete Bottom Gate -Top Contact transistors (BG/TC) were fabricated on an n-doped (N ~ 3 × 10 17 cm -3 ) silicon wafer on which a thermally grown SiO 2 layer (230 nm) acts as the gate dielectric.Substrates were first cleaned with acetone, 2-propanol and then UV ozone treated.
Octadecyltrichlorosilane (ODTS) surface treatment was applied by soaking freshly cleaned substrates in a 2 mM solution of octadecyltrichlorosilane in anhydrous cyclohexane for 16 hrs in a dessicator and then drying in a stream of nitrogen.All cleaning and treatments were carried out in air.After substrate cleaning and surface treatment, all OFETs and thin films were prepared in a glovebox under an inert atmosphere.A layer of the active organic material was deposited by thermal evaporation using an Evovac-800 system (Angstrom Engineering).
Materials were deposited at a rate of 0.03 nm/s and film thicknesses (~40 nm) were measured using a Dektak 6M surface profilometer.Compounds 7-14 were deposited on substrates held at a temperature of 25 °C.The top contacts of the OFETs were fabricated by deposition of parallel Source-Drain gold electrodes by thermal evaporation through a shadow mask.OFET testing was performed under an inert atmosphere (N 2 ) in a glovebox using an Agilent Technologies B1500A Semiconductor parameter analyser.The carrier mobility values presented in this work were averaged from measurements done on several devices.From the respective slopes, ∂√I D /∂V G , and the device parameters: channel length L=60 µm, channel width W=2000 µm (W/L = 33) and the capacitance per unit area, C i = 10 nF/cm 2 , the field effect electron mobility; µ e , was calculated using:

X-ray photoelectron spectroscopy (XPS) data
X-ray photoelectron spectroscopy (XPS) analysis was performed using an AXIS Ultra DLD spectrometer (Kratos Analytical Inc., Manchester, UK) with a monochromated Al K α source at a power of 112 W (8 kV × 14 mA), a hemispherical analyser operating in the fixed analyser transmission mode and the standard aperture (analysis area: 0.3 mm × 0.7 mm) The total pressure in the main vacuum chamber during analysis was typically 10

Thin film scanning electron microscopy (SEM) images
Films of 7-14 were soft and unsuitable for direct contact imaging using AFM surface morphological studies of Substrates were sputter coated with a thin layer of iridium prior to imaging with a Polaron SC5750 sputter coater (~200 Å).The films were imaged in a Philips XL30 Field Emission Scanning Electron Microscope (FESEM) at a magnification of 20,000x and an accelerating voltage of 4 kV.

Thin film scanning electron microscopy (SEM) images
were soft and unsuitable for direct contact imaging using AFM surface morphological studies of our films using scanning electronic microscopy (SEM).
Substrates were sputter coated with a thin layer of iridium prior to imaging with a Polaron

Thin film scanning electron microscopy (SEM) images
were soft and unsuitable for direct contact imaging using AFM.We performed ng electronic microscopy (SEM).
Substrates were sputter coated with a thin layer of iridium prior to imaging with a Polaron SC5750 sputter coater (~200 Å).The films were imaged in a Philips XL30 Field Emission roscope (FESEM) at a magnification of 20,000x and an accelerating 14 (bottom row, left

(
HOMO) and the lowest unoccupied molecular orbital (LUMO) are shown in the Tablebelow.
Elemental analysis was performed by the Campbell Microanalytical Laboratory at University of Otago, Dunedin, New Zealand.Infrared (IR) spectra were measured using a Thermo Scientific Nicolet™ 6700 FT-IR spectrometer on solid powders.UV/Visible (UV-Vis) spectra were measured on thin films fabricated by vacuum deposition of materials onto cleaned glass substrates.UV-Vis was measured using a Varian Cary 5E UV/VIS/NIR Spectrometer.UV-Vis maximum absorption onset was used to determine the optical bandgap.The ionization potential (IP) was determined by ultraviolet photoelectron spectroscopy (UPS) on thin films fabricated by vacuum deposition of materials onto cleaned ITO/glass substrates.The ionization potential is an approximation of the E HOMO .Electron affinity (EA) was determined by using the difference between the thin film UV-Vis determined bandgap and the IP as determined from UPS.

Fig S20 .
Fig S20.SEM images of compounds to right).Scale on images indicates 500 nm.
SC5750 sputter coater (~200 Å).The films were imaged in a Philips XL30 Field Emission roscope (FESEM) at a magnification of 20,000x and an accelerating SEM images of compounds 7-10 (top row, left to right) and 11-14 to right).Scale on images indicates 500 nm.

Table S2 .
Crystallographic data for 7. of 8 were grown by sublimation and the data is summarised in the table below.
CCDC 964325 contains the supplementary crystallographic data for this paper.This data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Table S4 .
Ionization potential (IP) values derived from UPS measurements.The ionization potential is an approximation of the E HOMO .

Table S5 .
-8mbar.Survey spectra were acquired at a pass energy of 160 eV.To obtain more detailed information about chemical structure, oxidation states etc., high resolution spectra were recorded from individual peaks at 40 eV pass energy (yielding a typical peak width for polymers of 1.0 eV).Data processing was performed using CasaXPS processing software version 2.3.15(Casa Software Ltd., Teignmouth, UK).All elements present were identified from survey spectra.Survey data measured by XPS (atomic percentage, %) of bithiophenes.
The atomic concentrations of the detected elements were calculated using integral peak intensities and the sensitivity factors supplied by the manufacturer.Binding energies were referenced to the aliphatic hydrocarbon peak at 285.0 eV.The accuracy associated with quantitative XPS is ca.10% -15%.Precision (ie.reproducibility) depends on the signal/noise ratio but is usually much better than 5%.The latter is relevant when comparing similar samples.