In recent years, organic electronics have become very attractive due to their various advantages such as high flexibility, easy designability, low fabrication cost, easy processing and large-scale fabrication . Especially in display technology, organic-based materials have found use in many applications such as OLEDs, micro-LEDs, LCDs, lasers, and photodiodes by applying thin film methods and solution processes . The performance of organic electronics is based on the active layer composition as well as the fabrication methods and processing parameters. The organic active layers are composed of various aromatic π-conjugated small molecules/polymers including thiophene, anthracene, carbazole, and triphenylamine .

Thienothiophenes are two annulated thiophene rings having four isomers, among which the most widely used isomer is thieno[3,2-b]thiophene (TT) . These compounds are electron-rich, flat and electron-delocalized systems, properties that make them promising materials for the construction of conjugated energy-based semiconductors for OLEDs , perovskite solar cells , organic field-effect transistors (OFETs) , capacitors , hybrid films , and photosensitizers . Another important π-conjugated unit is triphenylamine (TPA), having an ionization potential of 6.80 eV, which is lower compared to many other organic cores, thus providing a strong electron-donating ability for organic electronic applications . Dimesitylboron (DMB), with its unoccupied p-orbital, is an electron-acceptor organoboron compound used in several donor–acceptor systems to provide the system with pull–push interaction .

In this work, we have designed and synthesized a D–π–A model pull–push fluorophore, DMB-TT-TPA (8), having TPA and DMB units as donor and acceptor units that were linked through a 4-MeOPh-substituted TT core as a π-spacer. The photophysical properties of the fluorophore were investigated by spectroscopic methods. Moreover, DMB-TT-TPA (8) was fabricated as an emitter for an organic light-emitting diode through a solution process. DMB-TT-TPA (8) displayed excellent performance in both device application and photophysical properties, i.e., a maximum solution fluorescence quantum yield of 86% in THF, maximum solid-state fluorescence quantum yield of 41%, maximum current efficiency of 10.6 cd/A, and maximum power efficiency of 6.70 lm/W.

Scheme 1: Synthesis of DMB-TT-TPA (8).

Table 1: Photophysical data of DMB-TT-TPA (8) .

aAbsorption and fluorescence maxima in THF. bStokes shift (cm−1) Δν = 1/λmax − 1/λem. cEoptic from the onset of the absorption spectrum. dSolid-state quantum yield. eSolution-state quantum yield in THF.

Figure 2: (a) Current efficiency–luminance, (b) current efficiency–voltage, (c) luminance–voltage, and (d) current density–voltage characteristics of DMB-TT-TPA (8).

Figure 3: (a) Power efficiency–luminance, (b) external quantum efficiency–luminescence, (c) electroluminescence–wavelength characteristics of DMB-TT-TPA (8).

Table 2: Photophysical data of DMB-TT-TPA (8).

aTurn-on voltage, recorded at the luminance of 1 cd·m2. bMaximum current efficiency. cMaximum luminance. eMaximum electroluminescence wavelength. dMaximum current efficiency. eMaximum external quantum efficiency. fMaximum power efficiency. gChromaticity coordinates according to the CIE 1931 diagram.

Figure 4: Thermal gravimetric analyses (TGA) of DMB-TT-TPA (8).

Figure 5: HOMO and LUMO diagrams calculated at the B3LYP/6-31G (d,p) level of theory.

Table 3: The HOMO and LUMO energy levels and absorption values calculated by TD-B3LYP/6-31G (d,p) level of theory.

A small fluorophore molecule, DMB-TT-TPA (8), containing dimesitylboron as an acceptor and triphenylamine as a donor linked through a thieno[3,2-b]thiophene core having a 4-MeOPh group, was designed as a D–π–A model and synthesized in 85% yield. Its photophysical properties were investigated by UV–vis and fluorescence spectroscopy. The obtained experimental results were found to be in good agreement with computational investigations. An OLED fabrication, where DMB-TT-TPA (8) was employed as an emitter, showed a maximum luminescence efficiency of 752 cd/m2, a maximum external quantum efficiency of 4.61%, a maximum power efficiency of 6.70 lm/W, and a maximum current efficiency of 10.6 cd/A on 2.9 V turn on voltage with CIE coordinates of 0.16 and 0.51 at λEL = 512 nm. The OLED, optical and thermal properties indicated that the composition of thienothiophene, triphenylamine, and boron is a highly suitable combination for fluorescent organic electronics in display technology.

1H and 13C NMR spectra were recorded on a Varian model NMR spectrometer (500 and 126 MHz) and chemical shift values are reported in ppm downfield from tetramethylsilane (TMS). UV–vis absorption spectra were obtained using a HITACHI U-0080D spectrophotometer. Fluorescence spectra were recorded on a HITACHI F-4500 fluorescence spectrophotometer. Time-resolved fluorescence studies were performed on a Horiba, FL3-2IHR fluorescence spectrophotometer. Solid-state and solution-state quantum yields were measured using a Hamamatsu Quantaurus-QY Absolute PL Quantum Yield Spectrometer. Thermal gravimetric analysis (TGA) was performed on a PerkinElmer Diamond TA/TGA with a heating rate of 10 °C min−1 under nitrogen flow.

3-Bromothiophene (97%, Across), 2-bromo-4′-methoxyacetophenone (97% Merck), N-bromosuccinimide (Sigma-Aldrich), polyphosphoric acid (PPA, 115% H3PO4 basis, Sigma-Aldrich), n-butyllithium (2.5 M in hexanes, Sigma-Aldrich), sodium sulfate (Merck), 4-bromotriphenylamine (Sigma-Aldrich), dimesitylboronfluoride (90%, Sigma-Aldrich), 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (Sigma-Aldrich), tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4, 99%, Sigma-Aldrich), were used as received. Diethyl ether and THF were dried over metallic sodium. Dimethylformamide (HPLC grade) was stored over activated molecular sieves (4 Å). Dichloromethane (Aldrich), methanol (Merck), and sodium carbonate (Merck) were used as received. Compounds 2–6 were synthesized following our previous reports . The characterization data of 7 and 8 are compatible with the published data in ref. .

Synthesized as described in . To a mixture of thienothiophene 4 (250 mg, 0.770 mmol) and borolane 6 (320 mg, 0.845 mmol) dissolved in THF (25 mL) and degassed for 45 min with N2 was added K2CO3 (2.5 mL, 2.5 M) and Pd(PPh3)4 (0.077 mmol). The mixture was then saturated with N2, the reaction flask sealed and the mixture stirred at 75 °C for 48 h. Afterwards, the reaction mixture was filtered through celite eluting with CH2Cl2, extracted with CH2Cl2/water, and the organic phase was washed with sodium carbonate solution (10%) and water, dried over sodium sulfate, filtered, and the solvent was evaporated under reduced pressure. The crude product was purified by column chromatography eluting with n-hexane/CH2Cl2 4:1 to obtain the title compound 7 (300 mg, 81%) as a white powder. Mp 141–142 °C; 1H NMR (500 MHz, CDCl3) δ 7.42 (d, J = 8.8 Hz, 2H), 7.35 (d, J = 5.2 Hz, 1H), 7.28 (t, J = 8.7 Hz, 5H), 7.20 (d, J = 8.7 Hz, 2H), 7.13 (d, J = 7.6 Hz, 4H), 7.05 (t, J = 7.3 Hz, 2H), 6.95 (d, J = 8.7 Hz, 2H), 6.92 (d, J = 8.8 Hz, 2H), 3.86 (s, 3H); 13C NMR (126 MHz, CDCl3) δ 158.89, 147.36, 147.17, 142.04, 139.51, 135.73, 130.12, 129.87, 129.29, 128.34, 127.96, 125.86, 124.80, 123.24, 122.53, 119.81, 114.14, 55.22.

Synthesized as described in . To a solution of compound 7 (200 mg, 0.410 mmol) in dry THF (50 mL) was added tert-butyllithium (0.3 mL, 1.7 M, 0.490 mmol) dropwise at −78 °C under a nitrogen atmosphere over a period of 45 min. Then, dimesitylborofluoride (130 mg, 0.490 mmol) was added rapidly. The mixture was further stirred at −78 °C for 1 h, then, allowed to warm to room temperature and stirring was continued overnight. The solution was extracted with dichloromethane, and the organic layer was washed with Na2CO3 solution (10%) and water. The organic layer was dried over Na2SO4, filtered and the solvent was evaporated under reduced pressure. The crude product was purified by flash column chromatography eluting with a mixture of n-hexane/CH2Cl2 3:1 and then crystallized from ethanol to give the title compound DMB-TT-TPA (8) as a yellow powder in 85% yield (256 mg). Mp 165–166 °C; 1H NMR (500 MHz, CDCl3) δ 7.59 (s, 1H), 7.39 (d, J = 8.8 Hz, 2H), 7.28 (d, J = 7.8 Hz, 4H), 7.19 (d, J = 8.7 Hz, 2H), 7.12 (d, J = 7.6 Hz, 4H), 7.05 (t, J = 7.3 Hz, 2H), 6.93 (d, J = 8.7 Hz, 2H), 6.87 (d, J = 8.8 Hz, 2H), 6.84 (s, 4H), 3.82 (s, 3H), 2.32 (s, 6H), 2.17 (s, 12H); 13C NMR (126 MHz, CDCl3) δ 158.92, 153.46, 151.26, 147.60, 147.20, 143.95, 141.05, 140.90, 138.50, 137.96, 132.59, 130.25, 129.86, 129.49, 129.33, 128.14, 127.85, 127.57, 125.01, 123.45, 122.08, 114.12, 55.23, 23.54, 21.22.