Pyrrolopyrrole-Based Aza-BODIPY Small Molecules for Organic Field-Effect Transistors

Diketopyrrolopyrrole (DPP), due to its good planarity, π-conjugate structure, thermal stability, and structural modifiability, has received much attention from the scientific community as an excellent semiconductor material for its applications in the field of optoelectronics, such as organic solar cells, organic photovoltaics, and organic field effect transistors. In this study, a new small molecule, pyrrolopyrrole aza-BODIPY (PPAB), based on the thiophene-substituted DPP structure was developed using the Schiff-base formation reaction of DPP and heteroaromatic amines. Absorption spectroscopy, electrochemistry, X-ray diffraction, molecular theoretical simulation calculation were performed, and organic field-effect transistor properties based on PPAB were investigated. It was found that PPAB exhibits a broad absorption range in the visible and near-infrared regions, which is attributed to its long-range conjugate structure. In addition, it is worth noting that PPAB has multiple F atoms resulting in the low LUMO level, which is conducive to the injection and transportation of charge carriers between the semiconductor layer and the electrode. Meanwhile, its hole carrier mobility is up to 1.3 × 10−3 cm2 V−1 s−1 due to its large conjugate structure, good intramolecular charge transfer effect, and high degree of coplanarity. In this study, a new chromophore with electron-deficient ability for designing high-performance semiconductors was successfully synthesized.


Materials
All reagents were purchased from commercial sources and used without further purification unless otherwise noted. All the solvents needed in the experiment were dried and distilled by metal sodium and then used fresh, and all reaction processes are protected by nitrogen.

(2) 3,6-Di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione
Sodium (1.1g, 48 mmol) and trace ferric chloride are mixed after weighing and added to t-amyl alcohol solution (150 ml). About 1 hour and after the sodium is completely dissolved, 2-cyanothiophene (5 ml, 53.3 mmol) was dripped slowly for 1 hour. Another 1 hour, diisopropylsuccinate (4.4 mL, 22 mmol) was added dropwise to the reaction mixture during 1.5 hours. And then the reaction mixture was refluxed about 6 h. After that, 35% hydrochloric acid and methanol were added, stirred for 0.5 hours, filtered, and washed with a large amount of water and methanol, and dried to get the dark red precipitate 4.9 g, yield about 74%., The product is used in the next step without further characterization as the result of poor solubility.

Structure and molecular weight characterization
NMR spectra were obtained using a Mercury 500 spectrometer.

Thin film structures characterization
A 3 kW thin film X-ray diffraction (XRD) experiment was carried out on a powder Xray diffractometer (Inca energy, Oxford Instruments). The film was prepared by drop coating of PPAB solution (5 mg / ml chloroform solution).

UV/vis absorption spectra of monomer and polymers
UV/vis absorption spectra were recorded using a dual-beam grating Hitachi U-4100 absorption spectrometer. The solution UV/Vis absorption spectra of PPAB were recorded in chloroform at a standard concentration (0.1 mg/ml). The thin-film UV/Vis absorption spectra were measured for a spin-coated thin film. (2 mg/ml PPAB in chloroform on a quartz glass substrate, rotary speed: 1500 r/min)