Solution-Processable Indenofluorenes on Polymer Brush Interlayer: Remarkable N-Channel Field-Effect Transistor Characteristics under Ambient Conditions

The development of solution-processable n-type molecular semiconductors that exhibit high electron mobility (μe ≥ 0.5 cm2/(V·s)) under ambient conditions, along with high current modulation (Ion/Ioff ≥ 106–107) and near-zero turn on voltage (Von) characteristics, has lagged behind that of other semiconductors in organic field-effect transistors (OFETs). Here, we report the design, synthesis, physicochemical and optoelectronic characterizations, and OFET performances of a library of solution-processable, low-LUMO (−4.20 eV) 2,2′-(2,8-bis(3-alkylthiophen-2-yl)indeno[1,2-b]fluorene-6,12-diylidene)dimalononitrile small molecules, β,β′-Cn-TIFDMTs, having varied alkyl chain lengths (n = 8, 12, 16). An intriguing correlation is identified between the solid–isotropic liquid transition enthalpies and the solubilities, indicating that cohesive energetics, which are tuned by alkyl chains, play a pivotal role in determining solubility. The semiconductors were spin-coated under ambient conditions on densely packed (grafting densities of 0.19–0.45 chains/nm2) ultrathin (∼3.6–6.6 nm) polystyrene-brush surfaces. It is demonstrated that, on this polymer interlayer, thermally induced dispersive interactions occurring over a large number of methylene units between flexible alkyl chains (i.e., zipper effect) are critical to achieve a favorable thin-film crystallization with a proper microstructure and morphology for efficient charge transport. While C8 and C16 chains show a minimal zipper effect upon thermal annealing, C12 chains undergo an extended interdigitation involving ∼6 methylene units. This results in the formation of large crystallites having lamellar stacking ((100) coherence length ∼30 nm) in the out-of-plane direction and highly favorable in-plane π-interactions in a slipped-stacked arrangement. Uninterrupted microstructural integrity (i.e., no face-on (010)-oriented crystallites) was found to be critical to achieving high mobilities. The excellent crystallinity of the C12-substituted semiconductor thin film was also evident in the observed crystal lattice vibrations (phonons) at 58 cm–1 in low-frequency Raman scattering. Two-dimensional micrometer-sized (∼1–3 μm), sharp-edged plate-like grains lying parallel with the substrate plane were observed. OFETs fabricated by the current small molecules showed excellent n-channel behavior in ambient with μe values reaching ∼0.9 cm2/(V·s), Ion/Ioff ∼ 107–108, and Von ≈ 0 V. Our study not only demonstrates one of the highest performing n-channel OFET devices reported under ambient conditions via solution processing but also elucidates significant relationships among chemical structures, molecular properties, self-assembly from solution into a thin film, and semiconducting thin-film properties. The design rationales presented herein may open up new avenues for the development of high-electron-mobility novel electron-deficient indenofluorene and short-axis substituted donor–acceptor π-architectures via alkyl chain engineering and interface engineering.


Synthesis of 3-octylthiophene:
To a solution of 1-bromooctane (2.95 mL, 16.99 mmol) in anhydrous THF (32.0 mL) magnesium (0.49 g, 20.08 mmol) and iodine (0.12 g, 0.46 mmol) were added under nitrogen and this mixture was stirred at 85 °C for 3 hours.The resulting Grignard reagent was allowed to cool down to room temperature.Then, the Grignard reagent was added slowly to a solution of 3-bromothiophene (1.45 mL, 15.45 mmol) and NiCl2(dppp) (0.12 g, 0.23 mmol)in anhydrous THF (12.0 mL) at 0 °C under nitrogen.The reaction mixture was allowed to warm to room temperature overnight.The resulting reaction mixture was quenched with water and extracted with dichloromethane.The organic phase was washed with water, dried over Na2SO4, filtered, and evaporated to dryness to give a crude product, which was purified by column chromatography on silica gel using hexanes as the eluent.Pure product was afforded as colorless oil (2.31 g, 76% yield). 1 H NMR (CDCl3): δ 7.25 (s, 1H), 6.94 (d, 2H, J = 8.0 Hz), 2.62 (t, 2H, J = 12.0 Hz), 1.28 (d, 12H, J = 16.0Hz), 0.88 (t, 3H, J = 12.0 Hz) ppm.(2.20 g, 12.33 mmol) was added under nitrogen.The mixture was stirred at room temperature overnight.The resulting reaction mixture was quenched with water and extracted with dichloromethane.The organic phase was washed with water, dried over Na2SO4, filtered, and evaporated to dryness to give a crude product, which was purified by column chromatography on silica gel using hexanes as the eluent.Pure product was afforded as light yellow oil (1.94 g, 59% yield). 1

Synthesis of 2-trimethyltin-3-octylthiophene (1):
To a solution of 2-bromo-3octylthiophene (1.94 g, 6.93 mmol) in anhydrous THF (50 ml) at -78 °C n-butyllithium (2.5 M in n-hexane) (2.91 ml, 7.28 mmol) was added dropwise under nitrogen.The mixture was stirred at −78 °C for 1 hour.Then, trimethyltinchloride (Sn(CH3)3CI) (1.52 g, 7.62 mmol)) was added at −78 °C, and the resulting reaction mixture was allowed to warm to room temperature overnight.The reaction mixture was quenched with water, and the product was extracted with hexanes.The organic phase was washed with water, dried over Na2SO4, filtered, and evaporated to dryness to give a crude product.The crude product was used for the next step without any further purification.The pure product was obtained as pale orange oil (2.41 g, 96% yield). 1

Synthesis of 3-dodecylthiophene:
To a solution of 1-bromooctane (4.08 mL, 16.99 mmol) in anhydrous THF (30.0 mL) magnesium (0.49 g, 20.08 mmol) and iodine (0.12 g, 0.46 mmol) were added under nitrogen and this mixture was stirred at 85 °C for 3 hours.The resulting Grignard reagent was allowed to cool down to room temperature.Then, the Grignard reagent was added slowly to a solution of 3-bromothiophene (1.45 mL, 15.45 mmol) and NiCl2(dppp) (0.12 g, 0.23 mmol) in anhydrous THF (12.0 mL) at 0 °C under nitrogen.The reaction mixture was allowed to warm to room temperature overnight.The resulting reaction mixture was quenched with water and extracted with dichloromethane.The organic phase was washed with water, dried over Na2SO4, filtered, and evaporated to dryness to give a crude product, which was purified by column chromatography on silica gel using hexanes as the eluent.Pure product was afforded as colorless oil (2.80 g, 72% yield). 1     dryness.Then, the collected product was washed with methanol and filtered to collect the crude product, which was purified by column chromatography on silica gel using CHCl3/hexanes (9:1 (v/v)) as the eluent.Finally, the product was washed with methanol and filtered to afford the final product as a purple solid (0.28 g, 45% yield).m.p.: 152-153 °C.
The reaction mixture was allowed to warm to room temperature overnight.The resulting reaction mixture was quenched with water and extracted with dichloromethane.The organic phase was washed with water, dried over Na2SO4, filtered, and evaporated to dryness to give a crude product, which was purified by column chromatography on silica gel using hexanes as the eluent.Pure product was afforded as colorless oil (3.04 g, 64% yield). 1 H NMR (CDCl3): Figure S2. 1 H NMR spectrum of 4,4''-dibromo-2,2''-methoxycarbonyl-[1,1';4',1'']terphenyl in CDCl3 at room temperature.CDCl3 and H2O peaks in NMR solvent are denoted by asterisks.

Figure S4. 1 H
Figure S4. 1 H NMR spectrum of 3-octylthiophene in CDCl3 at room temperature.CDCl3 and H2O peaks in NMR solvent are denoted by asterisks.

Figure S6. 1 H
Figure S6. 1 H NMR spectrum of 1 in CDCl3 at room temperature.CDCl3 and H2O peaks in NMR solvent are denoted by asterisks.

Figure S11. 1 H
Figure S11. 1 H NMR spectrum of 3-dodecylthiophene in CDCl3 at room temperature.CDCl3 and H2O peaks in NMR solvent are denoted by asterisks.

Figure S12. 1 H
Figure S12. 1 H NMR spectrum of 2-bromo-3-dodecylthiophene in CDCl3 at room temperature.CDCl3 and H2O peaks in NMR solvent are denoted by asterisks.Synthesis of 2-trimethyltin-3-dodecylthiophene (2):To a solution of 2-bromo-3dodecylthiophene (1.66 g, 5.01 mmol) in anhydrous THF (45 ml) at -78 °C n-butyllithium (2.5 M in n-hexane) (2.10 ml, 5.26 mmol) was added dropwise under nitrogen.The mixture was stirred at −78 °C for 1 hour.Then, trimethyltinchloride (Sn(CH3)3CI) (1.10 g, 5.51 mmol)) was added at −78 °C, and the resulting reaction mixture was allowed to warm to room temperature overnight.The reaction mixture was quenched with water, and the product was extracted with hexanes.The organic phase was washed with water, dried over Na2SO4, filtered, and evaporated to dryness to give a crude product.The crude product was used for the next step without any further purification.The pure product was obtained as pale orange

Figure S18. 1 H
Figure S18. 1 H NMR spectrum of 3-hexadecylthiophene in CDCl3 at room temperature.CDCl3 and H2O peaks in NMR solvent are denoted by asterisks.

Figure S20. 1 H
Figure S20. 1 H NMR spectrum of 3 in CDCl3 at room temperature.CDCl3 and H2O peaks in NMR solvent are denoted by asterisks.

Table S2 .
Thermodynamic quantities associated with phase transitions of the β