Design, synthesis, and characterization of α,ω-disubstituted indeno[1,2-b]fluorene-6,12-dione-thiophene molecular semiconductors. Enhancement of ambipolar charge transport through synthetic tailoring of alkyl substituents

Abstract

A series of indeno[1,2-b]fluorene-6,12-dione-thiophene derivatives with hydrocarbon substituents at α,ω-positions as side groups have been designed and synthesized. The new compounds were fully characterized by ¹H/¹³C NMR, mass spectrometry, cyclic voltammetry, UV-vis absorption spectroscopy, differential scanning calorimetry, thermogravimetric analysis, and melting point measurements. The solid state structure of the indeno[1,2-b]fluorene-6,12-dione acceptor core has been identified based on single-crystal X-ray diffraction (XRD). The structural and electronic properties were also studied using density functional theory calculations, which were found to be in excellent agreement with the experimental findings and provided further insight. The detailed effects of alkyl chain size and orientation on the optoelectronic properties, intermolecular cohesive forces, thin-film microstructures, and charge transport performance of the new semiconductors were investigated. Two of the new solution-processable semiconductors, 2EH-TIFDKT and 2OD-TIFDKT, were deposited as thin-films via solution-shearing, drop-casting, and droplet-pinned crystallization methods, and their morphologies and microstructures were investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM). The solution-processed thin-film transistors based on 2EH-TIFDKT and 2OD-TIFDKT showed ambipolar device operations with electron and hole mobilities as high as 0.12 cm² V⁻¹ s⁻¹ and 0.02 cm² V⁻¹ s⁻¹, respectively, with I_on/I_off ratios of 10⁵ to 10⁶. Here, we demonstrate that rational repositioning of the β-substituents to molecular termini greatly benefits the π-core planarity while maintaining a good solubility, and results in favorable structural and optoelectronic characteristics for more efficient charge-transport in the solid-state. The ambipolar charge carrier mobilities were increased by two–three orders of magnitude in the new indeno[1,2-b]fluorene-6,12-dione-thiophene core on account of the rational side-chain engineering.

---

Introduction

π-Conjugated small molecules constitute a highly investigated class of semiconductor materials that have attracted widespread scientific and technological interest during the last few decades to realize low-cost, mechanically flexible, large-area, and printed optoelectronics.^1–3 Today, numerous commercial products, such as organic light-emitting diode (OLED) based displays, employ functional small molecules as their electro-active layers.⁴ Compared to larger π-systems such as polymers and dendrimers, π-conjugated small molecules are advantageous in terms of facile synthesis, higher purity levels, good solubility, enhanced thin-film crystallinity, and relatively small batch-to-batch variations.⁵ To this end, various molecular design strategies have been extensively explored to optimize their physicochemical and optoelectronic characteristics, and to meet the specific measures of a particular application.^6,7 As a result of these studies, impressive performances have been realized with small molecules in organic thin-film transistors (OTFTs), organic photovoltaics (OPVs), OLEDs, and recently with organic light-emitting transistors (OLETs).⁸ One of these design approaches is to manipulate the effective π-conjugation of the molecules with the goal of achieving a favorable balance between solution-processability, solid-state packing, HOMO–LUMO energetics, and optical band gaps.⁹ This has been particularly important in the design of low band-gap ambipolar semiconductors, which are crucial for the development of single-component OLETs and integrated microelectronic organic logic circuits, and as well as fundamental understanding of hole vs. electron transport in molecular solids.¹⁰ The rationale for obtaining a low band-gap molecular semiconductor relies on building a small energetic separation between HOMO and LUMO, which leads to simultaneous injections of both holes and electrons with low energy barriers.¹¹ The most practical technique to achieve low band-gap and to enhance π-conjugation is to build donor–acceptor molecular architectures, and to minimize the inter-ring torsions between π-electron deficient (acceptor) and π-electron rich (donor) moieties.¹²

In a previous work, Marks and Facchetti et al. have reported a new family of indenofluorene and bisindenofluorene-based ladder-type semiconductors with good charge-transport properties and ambient-stability.¹³ One of these semiconductors is a solution-processable ambipolar small molecule, β-DD-TIFDKT (Fig. 1), which has a β-substituted donor–acceptor–donor type of molecular architecture with thiophene (donor) and indeno[1,2-b]fluorene-6,12-dione (acceptor) π-cores.¹⁴ However, preliminary solution-based electron and hole mobilities were quite poor (∼10⁻⁴ cm² V⁻¹ s⁻¹). Therefore, in this study, we aim to further optimize the chemical structure of this π-core via tailoring alkyl substitutions. From a molecular design standpoint, the earlier semiconductor displays large inter-ring dihedral angles of ∼47° between indeno[1,2-b]fluorene-6,12-dione core and the terminal thiophene units in its energy-minimized geometry due to the presence of β-substituents (vide infra). These twists undoubtedly contribute to the solubility of this large π-system, and enable the solvent-based chromatographic purification and device processing. However, we speculate that they also deteriorate intra-/intermolecular π-conjugation, resulting in the observed poor charge-transport. The repositioning of the β-substituents to molecular termini (α,ω-substitutions) is expected to greatly benefit the π-core planarity of the semiconductor backbone as a result of reduced steric interactions. However, it is known that the solubility of π-conjugated systems decrease with enhancing π-conjugation and planarization as a result of better solid-state packing. Therefore, rational alkyl chain engineering is required to establish a delicate balance of good solution processability, effective π-conjugation, and favorable solid-state packing/charge-transport. Previously, similar side chain engineering approaches have been successfully employed to tune the optoelectronic and charge-transport properties of solution-processable semiconductor polymers.¹⁵

Fig. 1 Chemical structures of β-DD-TIFDKT,¹⁴ and DD-TIFDKT, 2EH-TIFDKT, and 2OD-TIFDKT developed in this study.

Here, we report on the design, synthesis, and characterization of three new α,ω-disubstituted molecules (DD-TIFDKT, 2EH-TIFDKT, and 2OD-TIFDKT) based on indeno[1,2-b]fluorene-6,12-dione and thiophene building blocks. Based on the solubility of these molecules, 2EH-TIFDKT and 2OD-TIFDKT are characterized by optical absorption spectroscopy, cyclic voltammetry, differential scanning calorimetry, and thermogravimetric analysis. Corresponding thin-films are deposited via various solution processing methods including solution-shearing (SS), droplet-pinned crystallization (DPC), and drop-casting (DC), and they are studied by X-ray diffraction, atomic force microscopy (AFM), and field-effect transistor measurements. Single-crystal analysis of the intermediate compound 2 reveals crucial structural features of the 2,8-dibromo-indeno[1,2-b]fluorene-6,12-dione acceptor core. In our attempts to develop new solution-processable semiconductors, we first synthesized DD-TIFDKT with linear –n-C₁₂H₂₅ chains at α,ω-positions, which resulted in complete insolubility. Therefore, our choice was to incorporate two different swallow tail alkyl chains of 2-ethylhexyl (2-EH) and 2-octyldodecyl (2-OD). The presence of swallow tails, which enhances the degrees of rotational freedom in the molecular backbone, would endow the new semiconductors with good solubility in common organic solvents. To this end, α,ω-disubstituted indeno[1,2-b]fluorene-6,12-dione-thiophene semiconductor was first synthesized with 2-octyldodecyl chains (2OD-TIFDKT), which has yielded good solubility, but also highly unbalanced electron and hole mobilities of up to 0.04 cm² V⁻¹ s⁻¹ and ∼3 × 10⁻⁴ cm² V⁻¹ s⁻¹, respectively. Then, 2EH-TIFDKT was developed with reduced insulating alkyl chain (2-ethylhexyl) density, which yields good solution processability and relatively more balanced and higher charge carrier mobilities. Solution-processed OTFTs with 2EH-TIFDKT exhibit electron and hole mobilities of up to 0.12 cm² V⁻¹ s⁻¹ and 0.02 cm² V⁻¹ s⁻¹, respectively with on/off ratios of 10⁵ to 10⁶. This indicates an improvement of >100× in the ambipolar charge carrier mobilities compared to those of parent β-substituted semiconductor (β-DD-TIFDKT) as a result of superior structural and electronic properties.

Experimental

Materials and methods

All reagents were purchased from commercial sources and used without further purification unless otherwise noted. Conventional Schlenk techniques were used, and reactions were carried out under N₂ unless otherwise noted. NMR spectra were recorded on a Bruker 400 spectrometer (¹H, 400 MHz; ¹³C, 100 MHz). Elemental analyses were performed on a LecoTruspec Micro model instrument. MALDI-TOF was performed on a Bruker Microflex LT MALDI-TOF-MS Instrument. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements were performed on Perkin Elmer Diamond model instruments under nitrogen at a heating rate of 10 °C min⁻¹. UV-vis absorption measurements were performed on a Shimadzu, UV-1800 UV-vis Spectrophotometer. Electrochemistry was performed on a C3 Cell Stand electrochemical station equipped with BAS Epsilon software (Bioanalytical Systems, Inc., Lafayette, IN). Prior to the synthesis, the optimization of the molecular geometries and total energy calculations were carried out using density functional theory (DFT) at the B3LYP/6-31G** level by using Gaussian 09.¹⁶

Crystal structure determination

The intensity data for IFDK-BR2 (2) were collected on an Bruker APEX II QUAZAR three-circle diffractometer using monochromatized Mo Kα X-radiation (λ = 0.71073 Å). Indexing was performed using APEX2 [Bruker (2014) APEX2, version 2014.11-0, Bruker AXS Inc., Madison, Wisconsin, USA]. Data integration and reduction were carried out with SAINT [Bruker (2013) SAINT, version V8.34A, Bruker AXS Inc., Madison, Wisconsin, USA]. Absorption correction was performed by multi-scan method implemented in SADABS [Bruker (2014) SADABS, version 2014/5, Bruker AXS Inc., Madison, Wisconsin, USA]. The structures were solved and refined using the Bruker SHELXTL Software Package [Bruker (2010) SHELXTL, version 6.14, Bruker AXS Inc., Madison, Wisconsin, USA]. All non-hydrogen atoms were refined anisotropically using all reflections with I > 2σ(I). The C-bound H atoms were positioned geometrically and refined using a riding mode. Crystallographic data and refinement details of the data collection are summarized in Table S2.† The final geometrical calculations and the molecular drawings were carried out with Platon (version 1.17)¹⁷ and Mercury CSD (version 3.5.1)¹⁸ programs.

Synthesis and characterization

2-Dodecyl bromide and 2-ethylhexyl bromide were purchased from commercial sources, and 2-octyldodecyl bromide is synthesized from 2-octyl-1-dodecanol in accordance with the following procedure:

Synthesis of 2-octyldodecyl bromide. A mixture of 2-octyl-1-dodecanol (9.0 g, 30 mmol) and triphenylphosphine (11.4 g, 120 mmol) was dissolved in 400 mL THF under ambient conditions. Bromine (18.0 g, 120 mmol) was added slowly to this mixture, and the resulting reaction solution was stirred at room temperature for 3 h. Then, 6 mL of methanol was added and the solvent was removed on the rotary evaporator. The residue is suspended in hexane, and the insoluble part was removed by filtration. Then, the resulting filtrate was concentrated on the rotary evaporator to give a crude oil, which was purified by column chromatography on silica gel using hexane as the eluent to give the pure product as a colorless oil (10.61 g, 97.5%). ¹H NMR (400 MHz, CDCl₃): δ 3.44 (2H, d, J = 4.8 Hz), 1.58 (1H, m), 1.27 (32H, m), 0.88 (6H, t, J = 6.7 Hz).

Synthesis of 4,4′′-dibromo-2,2′′-methoxycarbonyl-[1,1′;4′,1′′]terphenyl (1). A mixture of 1,4-benzenediboronic acid bis(pinacol) ester (1.0 g, 3.03 mmol), methyl 2-iodo-5-bromobenzoate (2.262 g, 6.64 mmol), and Aliquat 336 (0.347 g, 0.86 mmol) was suspended in 20 mL of dry toluene under nitrogen. Then, tetrakis(triphenylphosphine)palladium (0.21 g, 0.18 mmol) and 1 M aqueous sodium carbonate solution (1.28 g in 12.2 mL of water), which was already deaerated for 2 h, were added under N₂. The mixture was heated at reflux for 2 days under nitrogen. The mixture was then cooled to room temperature and quenched with water. The reaction mixture was extracted with hexanes, and the organic phase was washed with water, dried over Na₂SO₄, filtered, and evaporated to dryness to give the crude product. The crude was then purified by column chromatography on silica gel using chloroform as the eluent to give the pure product as a white solid (1.316 g, 86.0% yield). ¹H NMR (400 MHz, CDCl₃): δ 3.71 (s, 6H), 7.32 (m, 6H), 7.67 (dd, 2H, J = 8.0 Hz and J = 2.4 Hz), 8.00 (d, 2H, 2.4 Hz).

Synthesis of 2,8-dibromo-indeno[1,2-b]fluorene-6,12-dione (2). Compound 1 (0.61 g, 1.20 mmol) was added to 60.0 mL of 80% H₂SO₄ (prepared from 12 mL of H₂O and 48 mL of concentrated (98%) H₂SO₄), and the reaction mixture was heated with stirring at 120 °C for 17 h. Then, the reaction mixture was poured into ice and stirred for 15 min to give a dark red solid, which was collected by filtration. The crude product was then washed with water, saturated sodium hydrogen carbonate (NaHCO₃) solution, and methanol, respectively. The crude product was then purified by thermal gradient sublimation under high vacuum (2 × 10⁻⁵ Torr) to afford the pure product as a cherry red crystalline solid (0.395 g, 74.0% yield). Note that during the sublimation, single-crystals of this compound were also obtained. Mp > 390 °C. Anal. calcd for C₂₀H₈O₂Br₂: C, 54.58; H, 1.83. Found: C, 54.70; H, 1.96.

Synthesis of 2,8-bis(5-dodecylthien-2-yl)indeno[1,2-b]fluorene-6,12-dione (DD-TIFDKT). The reagents 2,8-dibromo-indeno[1,2-b]fluorene-6,12-dione (2) (0.763 g, 1.73 mmol), 2-trimethylstannyl-5-dodecylthiophene (4) (1.58 g, 3.81 mmol), and Pd(PPh₃)₂Cl₂ (0.202 g, 0.29 mmol) in anhydrous DMF (80 mL) were heated at 125 °C under nitrogen for 24 h. Then, the reaction mixture was cooled down to room temperature and evaporated to dryness. The crude product was filtered by using methanol, and then washed with methanol, acetone, and hexanes to give a dark crude solid (1.89 g, 87.5% crude yield). The crude product was insoluble in common organic solvents, and, thus, thermal gradient sublimation was performed under high vacuum (2 × 10⁻⁵ Torr), which resulted in complete decomposition of the crude material.

Synthesis of 2,8-bis(5-(2-ethylhexyl)thien-2-yl)indeno[1,2-b]fluorene-6,12-dione (2EH-TIFDKT). The reagent 2,8-dibromo-indeno[1,2-b]fluorene-6,12-dione (2) (0.500 g, 1.136 mmol), 2-trimethylstannyl-5-(2-ethylhexyl)thiophene (6) (0.82 g, 2.272 mmol), and Pd(PPh₃)₂Cl₂ (79.4 mg, 0.113 mmol) in anhydrous DMF (65 mL) were heated at 125 °C under nitrogen for 18 h. Then, the reaction mixture was cooled down to room temperature and evaporated to dryness. The crude product was filtered by using methanol, and then washed with methanol, acetone, and hexanes to give a dark crude solid. The crude product was purified by column chromatography on silica gel with CHCl₃/hexanes (9 : 1) as the eluent to give final product as a dark green solid (0.37 g, 48.5% yield). Mp 292–293 °C. ¹H NMR (400 MHz, CDCl₃): δ 0.92 (t, 6H, J = 7.2 Hz), 1.26–1.40 (m, 9H), 2.74 (d, 2H, J = 6.4 Hz), 6.71 (d, 1H, J = 3.6 Hz), 7.15 (d, 1H, J = 3.6 Hz), 7.40 (d, 1H, J = 7.6 Hz), 7.63 (m, 2H), 7.75 (s, 1H). ¹³C NMR (100 MHz, CDCl₃): δ 10.9, 14.2, 23.0, 25.5, 28.9, 32.4, 34.2, 41.4, 115.8, 120.9, 121.0, 123.6, 126.4, 131.4, 134.6, 136.2, 139.4, 140.2, 141.3, 145.5, 192.6. IR (KBr): 1710 cm⁻¹ (C=O stretching). MS (MALDI-TOF) m/z (M⁺): calcd for C₄₄H₄₆O₂S₂: 670, found: 671 [M + H]⁺. Anal. calcd for C₄₄H₄₆O₂S₂: C, 78.76; H, 6.91, found: C, 78.43; H, 6.98.

Synthesis of 2,8-bis(5-(2-octyldodecyl)thien-2-yl)indeno[1,2-b]fluorene-6,12-dione (2OD-TIFDKT). The reagent 2,8-dibromo-indeno[1,2-b]fluorene-6,12-dione (2) (0.682 g, 1.55 mmol), 2-trimethylstannyl-5-(2-octyldodecyl)thiophene (8) (1.80 g, 3.41 mmol), and Pd(PPh₃)₂Cl₂ (0.109 g, 0.155 mmol) in anhydrous DMF (75 mL) were heated at 125 °C under nitrogen for 24 h. Then, the reaction mixture was cooled down to room temperature and evaporated to dryness. The crude product was filtered by using methanol, and then washed with methanol, acetone, and hexanes to give a dark crude solid. The crude product was purified by column chromatography on silica gel with CHCl₃/hexanes (7 : 3) as the eluent to give final product as a dark green solid (0.531 g, 34.0% yield). Mp 135–136 °C. ¹H NMR (400 MHz, CDCl₃): δ 0.90 (m, 6H), 1.28–1.31 (m, 33H), 2.74 (d, 2H, J = 6.4 Hz), 6.68 (d, 1H, J = 3.2 Hz), 7.12 (d, 1H, J = 3.6 Hz), 7.38 (d, 1H, J = 7.6 Hz), 7.60 (m, 2H), 7.70 (s, 1H). ¹³C NMR (100 MHz, CDCl₃): δ 14.1, 22.7, 26.6, 29.3, 29.4, 29.7, 29.8, 29.9, 30.0, 31.9, 33.2, 34.7, 40.0, 115.7, 120.8, 120.9, 123.5, 126.4, 131.3, 134.6, 136.1, 139.4, 140.2, 141.2, 145.4, 145.5, 192.5. IR (KBr): 1710 cm⁻¹ (C=O stretching). MS (MALDI-TOF) m/z (M⁺): calcd for C₆₈H₉₄O₂S₂: 1008, found: 1009 [M + H]⁺. Anal. calcd for C₆₈H₉₄O₂S₂: C, 81.06; H, 9.40, found: C, 81.31; H, 9.52.

Synthesis of 2-dodecylthiophene (3). To a solution of thiophene (2.0 g, 23.76 mmol) in THF (24 mL) at −78 °C was added 9.98 mL (24.95 mmol) of n-butyllithium (2.5 M in n-hexane) dropwise under nitrogen. The resulting mixture was stirred at −78 °C for 30 min and at room temperature for an additional 1 h. Then, 1-bromododecane (6.52 g, 26.15 mmol) was added to this mixture slowly at −78 °C. The resulting reaction mixture was stirred at room temperature for 1 h, and then heated to reflux for 12 h. The reaction was quenched with water, and the product was extracted with chloroform. The organic phase was washed twice with water, dried over Na₂SO₄, filtered, and evaporated to dryness to yield a crude product, which was purified by vacuum distillation to give the pure product as a colorless oil (5.57 g, 93.0%). ¹H NMR (400 MHz, CDCl₃): δ 0.90 (t, 3H, J = 6.8 Hz), 1.27–1.32 (m, 18H), 1.71 (m, 2H), 2.83 (t, 2H, J = 7.6 Hz), 6.79 (d, 1H, J = 3.6 Hz), 6.93 (dd, 1H, J = 5.0 Hz, J = 3.6 Hz), 7.11 (d, 1H, J = 5.0 Hz).

Synthesis of 2-dodecyl-5-trimethylstannylthiophene (4). To a solution of 2-dodecylthiophene (3) (2.0 g, 7.92 mmol) in THF (30 mL) at −78 °C was added 3.33 mL (8.32 mmol) of n-butyllithium (2.5 M in n-hexane) under nitrogen. The mixture was stirred at −78 °C for 30 min and at room temperature for 1 h. Then, trimethyltinchloride (1.74 g, 8.71 mmol) was added slowly at −78 °C, and the resulting reaction mixture was allowed to warm to room temperature and stirred at room temperature overnight. The reaction was quenched with water, and the product was extracted with hexane. The organic phase was washed with water, dried over Na₂SO₄, filtered, and evaporated to dryness to give the pure product as a colorless oil (2.96 g, 90.0%). ¹H NMR (CDCl₃, 400 MHz): δ 0.35 (s, 9H), 0.90 (t, 3H, J = 6.8), 1.27–1.36 (m, 20H), 2.87 (t, 2H, J = 7.6 Hz), 6.92 (d, 1H, J = 3.2 Hz), 7.02 (d, 1H, J = 3.2 Hz).

Synthesis of 2-(2-ethylhexyl)thiophene (5). To a solution of thiophene (3.0 g, 35.65 mmol) in THF (35 mL) at −78 °C was added 15.0 mL (37.44 mmol) of n-butyllithium (2.5 M in n-hexane) dropwise under nitrogen. The resulting mixture was stirred at −78 °C for 30 min and at room temperature for an additional 1 h. Then, 2-ethylhexylbromide (7.58 g, 0.039 mol) was added to this mixture slowly at −78 °C. The resulting reaction mixture was stirred at room temperature for 1 h, and then heated to reflux for 12 h. The reaction was quenched with water, and the product was extracted with chloroform. The organic phase was washed twice with water, dried over Na₂SO₄, filtered, and evaporated to dryness to yield a crude product, which was purified by vacuum distillation to give the pure product as a colorless oil (3.58 g, 51.0%). ¹H NMR (400 MHz, CDCl₃): δ 0.91 (m, 6H), 1.29–1.32 (m, 9H), 2.77 (d, 2H, J = 6.8 Hz), 6.77 (d, 1H, J = 3.6 Hz), 6.93 (dd, 1H, J = 5.2 Hz, J = 3.6 Hz), 7.13 (d, 1H, J = 5.2 Hz).

Synthesis of 2-(2-ethylhexyl)-5-trimethylstannylthiophene (6). To a solution of 2-(2-ethylhexyl)thiophene (5) (1.5 g, 7.64 mmol) in THF (30 mL) at −78 °C was added 3.2 mL (8.02 mmol) of n-butyllithium (2.5 M in n-hexane) under nitrogen. The mixture was stirred at −78 °C for 30 min and at room temperature for 1 h. Then, trimethyltinchloride (1.67 g, 8.4 mmol) was added slowly at −78 °C, and the resulting reaction mixture was allowed to warm to room temperature and stirred at room temperature overnight. The reaction was quenched with water, and the product was extracted with hexane. The organic phase was washed with water, dried over Na₂SO₄, filtered, and evaporated to dryness to give the pure product as a colorless oil (2.37 g, 86.5%). ¹H NMR (CDCl₃, 400 MHz): δ 0.35 (s, 9H), 0.90 (m, 6H), 1.25–1.35 (m, 9H), 2.81 (d, 2H, J = 6.4 Hz), 6.89 (d, 1H, J = 3.2 Hz), 7.01 (d, 1H, J = 3.2 Hz).

Synthesis of 2-(2-octyldodecyl)thiophene (7). To a solution of thiophene (0.671 g, 7.98 mmol) in THF (10 mL) at −78 °C was added 3.35 mL (8.38 mmol) of n-butyllithium (2.5 M in n-hexane) dropwise under nitrogen. The resulting mixture was stirred at −78 °C for 30 min and at room temperature for an additional 1 h. Then, 1-bromo-2-octyldodecane (3.05 g, 8.78 mmol) was added to this mixture slowly at −78 °C. The resulting reaction mixture was stirred at room temperature for 1 h and then heated to reflux for 12 h. The reaction was quenched with water, and the product was extracted with chloroform. The organic phase was washed twice with water, dried over Na₂SO₄, filtered, and evaporated to dryness to yield a crude product, which was purified by column chromatography on silica gel using hexane as the eluent to give the pure product as a colorless oil (1.51 g, 52.0%). ¹H NMR (400 MHz, CDCl₃): δ 0.89 (m, 6H), 1.27–1.33 (m, 33H), 2.76 (d, 2H, J = 6.8 Hz), 6.76 (d, 1H, J = 3.4 Hz), 6.92 (dd, 1H, J = 5.2 Hz, J = 3.4 Hz), 7.13 (d, 1H, J = 5.2 Hz).

Synthesis of 2-(2-octyldodecyl)-5-trimethylstannylthiophene (8). To a solution of 2-(2-octyldodecyl)thiophene (7) (1.4 g, 3.84 mmol) in THF (30 mL) at −78 °C was added 1.61 mL (4.03 mmol) of n-butyllithium (2.5 M in n-hexane) under nitrogen. The mixture was stirred at −78 °C for 30 min and at room temperature for 1 h. Then, trimethyltinchloride (0.841 g, 4.22 mmol) was added slowly at −78 °C, and the resulting reaction mixture was allowed to warm to room temperature and stirred at room temperature overnight. The reaction 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 the pure product as a colorless oil (1.94 g, 95.9%). ¹H NMR (CDCl₃, 400 MHz): δ 0.35 (s, 9H), 0.91 (m, 6H), 1.25–1.33 (m, 33H), 2.80 (d, 2H, J = 6.0 Hz), 6.88 (d, 1H, J = 3.2 Hz), 7.02 (d, 1H, J = 3.2 Hz).

Device fabrication and characterization

For the fabrication of top-contact/bottom-gate organic thin film transistors (OTFTs), highly n-doped (100) silicon wafers (resistivity < 0.005 Ω cm) with a 300 nm thermally grown oxide gate dielectric film were used as device substrates. The Si/SiO₂ substrates were washed via sonication in acetone for 10 min, followed by oxygen plasma for 5 min (18 W). For the formation of PS-brush layer, hydroxyl end-functionalized polystyrenes (M_n = 1.73, 10.0, and 28.6 kg mol⁻¹, Polymer Source) were spin-coated onto the Si/SiO₂ substrate from 0.5 wt% toluene solutions, then the substrates were heated at 170° for 48 h under vacuum to allow the hydroxyl end groups of the PS chains to react with silanol groups on the Si/SiO₂ substrate.¹⁹ The PS brush-treated substrates were rinsed with toluene to remove unreacted PS chains and annealed at 100 °C for 24 h under vacuum. The water contact angle of the PS brush-treated substrates was ∼74°. For the OTS treatment, a 0.1% solution of octadecyltrimethoxysilane in trichloroethylene was spin-coated onto the Si/SiO₂ substrate, and placed in the Schlenk line to react with ammonia vapor for overnight. Unreacted silanes were removed by sonication in toluene for 2 min, followed by rinsing with toluene, acetone and isopropyl alcohol, and drying under a stream of nitrogen. The water contact angle of the OTS-treated substrates was ∼96°. Semiconducting layers (2EH-TIFDKT and 2OD-TIFDKT) were formed via three different solution-processing methods – solution-shearing (SS), droplet-pinned crystallization (DPC), and drop-casting (DC). For all solution processes, PS brush-treated substrates were employed. For the optimization of film-forming process, various solvents including toluene, chlorobenzene, chloroform, 1,2-dichlorobenzene, p-xylene, tetralin, 1,2,4-trichlorobenzene with various concentrations (0.5–2 mg mL⁻¹) were employed. For the DC process, the solutions were drop-cast onto the substrates at preset temperature in solvent-vapor saturated environment, and the substrates were annealed in vacuum oven at 110 °C overnight. For the DPC process, the solutions were dropped onto the substrates (1 × 1 cm²) and a piece of silicon wafer (0.4 × 0.4 cm², pinner) was placed on the substrate to pin the solution droplet. The silicon substrate with the droplet was placed in a Petri dish sealed with parafilm on a hot plate at 30 °C until the solvent was evaporated. For the SS process, a few drops of semiconductor solution were cast onto a heated substrate (1 × 2 cm²), and the substrate was covered with a dewetting OTS-modified top substrate. The top substrate was then translated by an electrically-controlled syringe pump at a constant rate relative to the bottom substrate, gradually uncovering the sandwiched solution, which quickly evaporated leaving behind a polycrystalline thin film seeding from the shearing substrate frontier. Solvent evaporation was controlled by different deposition temperature (50–80% of the solvent boiling point in centigrade) and different solution shearing speed (0.1–18 mm min⁻¹). The solution-sheared substrates were placed in a vacuum oven at 90 °C overnight to remove the residual solvent. Film thicknesses were characterized by profilometer (DEKTAK-XT, Brucker) as 40–90 nm (SS), 100–200 nm (DPC), and 25–70 nm (DC), respectively. Au layers (40 nm) were thermally evaporated through a shadow mask to define source and drain contacts with various channel lengths (L; 100 and 50 μm) and widths (W; 1000 and 500 μm). The electronic characteristics of OTFTs were measured using a Keithley 4200-SCS. Carrier mobilities (μ) were calculated in the saturation regime by the formula, μ_sat = (2I_DSL)/[WC_i(V_G − V_T)²], where I_DS is the source–drain current, L is the channel length, W is the channel width, C_i is the areal capacitance of the gate dielectric (C_i = 11.4 nF cm⁻²), V_G is the gate voltage, and V_T is the threshold voltage. The microstructure and surface morphology of thin-films were characterized using an X-ray diffractometer (XRD, SmartLab, Rigaku) and an atomic force microscope (AFM, NX10, Park Systems), respectively.

Results and discussion

Computational modeling, synthesis and characterization

Prior to the synthesis, the optimization of the molecular geometries and total energy calculations were carried out using density functional theory (DFT) at the B3LYP/6-31G** level. The molecular structure of the previously developed β-DD-TIFDKT is illustrated with the model compound M1, and M2 and M3 illustrates DD-TIFDKT and 2EH-TIFDKT/2OD-TIFDKT, respectively (Fig. 2). While M1 and M2 are modeled with n-butyl chains, M3 is modeled with isobutyl (–CH₂CH(CH₃)₂) chains to demonstrate the structural/electronic differences between linear and swallow-tail alkyl chain systems, respectively. The optimized molecular geometries, HOMO–LUMO energy levels, and pictorial representations are shown in Fig. 2. For the indeno[1,2-b]fluorene-6,12-dione core, good agreement is found between the calculated gas-phase and X-ray crystallographic geometries (vide infra) with perfect core planarity, although the DFT calculations generally lead to optimized geometries with slightly less bond-length alternations (i.e., more delocalization) for the fused carbon–carbon backbones. In the optimized geometries, relocating alkyl substituents from β-positions to the molecular termini (α,ω-locations) induces a planarization between indeno[1,2-b]fluorene-6,12-dione and thiophene units, and the inter-ring torsional angle is found to reduce from ∼47° to ∼23° (Fig. 2b). This planarization enhances the electronic communication between donor (thiophene) and acceptor (indeno[1,2-b]fluorene-6,12-dione) moieties, which leads to enhanced π-delocalization along the molecular backbone, lower HOMO–LUMO gaps (Δ = 0.1–0.2 eV), and possibly enhanced intermolecular π–π interactions. All these advantages are expected to improve ambipolar charge carrier mobility of α,ω-disubstituted semiconductors compared to that of β-disubstituted semiconductors.

Fig. 2 (a) Chemical structures of the model compounds M1, M2, and M3. (b) Optimized molecular geometries showing inter-ring dihedral angles. (c) Computed HOMO and LUMO energy levels, and topographical representations (DFT, B3LYP/6-31G**).

The theoretical HOMO/LUMO energies were found to be −5.66/−2.91 eV for M1, −5.42/−2.85 eV for M2, and −5.40/−2.89 eV for M3. The LUMO electron density distributions of M1, M2, and M3 are mostly identical, and mainly delocalized on the acceptor indeno[1,2-b]fluorene-6,12-dione core. Therefore, LUMO energy level shows minimal changes (<0.06 eV) upon alkyl chain tailoring, which is mainly attributed to the differences in the inductive effects of alkyl substituents. However, the HOMO energy level is found to increase by 0.2–0.3 eV since the HOMO electron density is delocalized over the entire π-backbone, and π-electron-rich thiophene moiety is getting more conjugated (enhancing π-delocalization) with the whole π-core in the new α,ω-disubstituted structures. The theoretical HOMO and LUMO energies for M2 and M3 are in the range of those calculated for previously reported p-channel and n-channel semiconductors, respectively, indicating that, from a molecular orbital energetic perspective, the new molecules should be able to transport both holes and electrons.¹⁴ In the new α,ω-disubstituted indeno[1,2-b]fluorene-6,12-dione-thiophene molecular semiconductors, HOMO–LUMO energy gaps are found to decrease by 0.1–0.3 eV compared to the β-substituted core, which may energetically further facilitate ambipolar charge transport. On the other hand, although symmetric carbonyl functionalization results in nearly zero total molecular dipole moment, each carbonyl site induces local dipoles of 3.31 D, which are effective to facilitate dipole–dipole interactions in the solid-state. This is evident in the single-crystal structure of the indeno[1,2-b]fluorene-6,12-dione core (vide infra, Fig. 6d and e). In conclusion, for these new molecular systems, these theoretical results are in excellent agreement with the electrochemical, optical, and single-crystal characterizations (vide infra), and should favor efficient charge transport characteristics.

In the synthesis of α,ω-disubstituted thiophene-indeno[1,2-b]fluorene-6,12-dione molecules, DD-TIFDKT, 2EH-TIFDKT and 2-OD-TIFDKT, acid-catalyzed intramolecular Friedel–Crafts acylation reaction is used as the key step for the formation of carbonyl functionalized ladder-type acceptor core. As shown in Scheme 1, Suzuki coupling of 1,4-benzenediboronic acid dipinacol ester with methyl 5-bromo-2-iodobenzoate yields terphenyl intermediate 1 in 86% yield, which undergoes a double intramolecular Friedel–Crafts acylation reaction in 80% H₂SO₄ at 120 °C to yield 2 in 74% yield. On the other hand, alkyl-substituted thiophene reagents 3, 5, and 7 are synthesized from thiophene via lithiations followed by alkylbromide substitutions in 51–93% yields. It is noteworthy that for these reactions, swallow-tails give lower substitution yields compared to linear alkyl chains as a result of steric effects. The corresponding trimethylstannyl-functionalized thiophene reagents 4, 6, and 8 are synthesized from 3, 5, and 7, respectively, by lithiation/stannylation reactions in 86–96% yields.

Scheme 1 Synthesis of α,ω-disubstituted indeno[1,2-b]fluorene-6,12-dione-thiophene molecular semiconductors DD-TIFDKT, 2EH-TIFDKT and 2-OD-TIFDKT.

The new semiconductors are synthesized by Stille cross-coupling reactions of dibromo compound 2 with distannylated derivatives 4 (for DD-TIFDKT), 6 (for 2-EH-TIFDKT), and 8 (for 2-OD-TIFDKT) in DMF using Pd(PPh₃)₂Cl₂ catalyst. 2EH-TIFDK and 2OD-TIFDKT are found to be freely soluble in common organic solvents (CHCl₃, CH₂Cl₂, THF, toluene), which allows the convenient purification by flash column chromatography to yield pure semiconductors in 34–48% yields. However, DD-TIFDKT exhibits extremely low solubility, which prevents its solution-based purification. This indicates that enhanced core-planarity significantly lowers the solubility and it cannot be compensated by linear –n-C₁₂H₂₅ chains. It is noteworthy that linear –n-C₁₂H₂₅ chain substituted fused p-type semiconductors such as [1]benzothieno[3,2-b][1]benzothiophene (BTBT) have been previously reported to be highly soluble.^8a This indicates that in the current DD-TIFDKT molecular core, larger local dipole moments, enhanced molecular donor–acceptor characteristics, and improved molecular planarity plays a key role to facilitate intermolecular interactions, which significantly lowers the solubility. The purification of DD-TIFDKT was attempted via gradient sublimation under high vacuum (2 × 10⁻⁵ Torr), however it resulted in complete decomposition of the material with no observable pure product. This indicates that the current TIFDKT core may not be ideal for vacuum-based thermal evaporation methods, which is consistent with the OTFT inactivity and the formation of decomposed products after physical vapor deposition (vide infra). The Stille cross-coupling reaction was also performed with Pd(PPh₃)₄/toluene (catalyst/solvent) system, which yielded the desired products in similar yields (42–45%). This is in sharp contrast to our previous report, in which using Pd(PPh₃)₄ catalyst did not yield any product for sterically-encumbered Stille coupling of 2,8-dibromo-indeno[1,2-b]fluorene-6,12-dione (2) with 3-dodecyl-2-trimethylstannylthiophene reagent. This suggests that for the present sterically less-demanding coupling reaction, higher turnover frequencies of more coordinatively unsaturated Pd(PPh₃)₂Cl₂ catalyst is not essential.¹⁴ For this particular Stille coupling, the similar yields obtained for 2EH-TIFDK and 2OD-TIFDKT indicates that the reaction is not particularly sensitive to alkyl substituent encumbrance on the thiophene reagent. The chemical structures and purities of 2EH-TIFDK and 2OD-TIFDKT were characterized by ¹H and ¹³C NMR (Fig. S1–S3, S5–S7†), elemental analysis, IR (Fig. S8†), and mass spectroscopy (MALDI-TOF) (Fig. S4 and S7†). The good solubility of these molecules is a consequence of the swallow-tail alkyl substituent and offers a significant advantage in OTFTs for the deposition of semiconductor layers from solution. As shown in Fig. 3, in the ¹H NMR spectra of 2EH-TIFDKT and 2OD-TIFDKT, the chemical shifts of the aromatic protons are found to significantly shift upfield (Δδ = 0.15–0.20 ppm) with increasing concentration (0.5 mg mL⁻¹ → 20 mg mL⁻¹), which indicates a shielding effect as a result of staggered molecular stacking in solution via π–π interactions. Based on the chemical shifts, both indeno[1,2-b]fluorene-6,12-dione and thiophene units appear to be involved in the molecular stack formation in solution. In addition, due to a greater number of local electronic environments, the aromatic peaks are broadened with concentration, which further supports this stacking model. Similar self-assembly characteristics in solution have been observed in the literature for a number of π-conjugated molecules,^9c,20 and it offers key advantages to efficient charger-carrier transport.

Fig. 3 The concentration-dependent ¹H NMR spectra of 2EH-TIFDKT (a) and 2OD-TIFDKT (b) in CDCl₃.

Thermal properties

The thermal properties of the present compounds, 2EH-TIFDKT and 2OD-TIFDKT were characterized by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and conventional melting point measurements. Thermal scans are shown in Fig. 4 and data are presented in Table 1. A 5% mass loss in TGA scan is defined as the thermolysis threshold. For the present molecules, since the molecular architecture of the π-backbone (thiophene-indeno[1,2-b]fluorene-6,12-dione-thiophene) remains the same, alkyl substituents are found to have significant effects on thermal phase transitions by influencing two intermolecular forces: π–π stacking interactions between molecular backbones, and van der Waals' interactions between alkyl chains.

Fig. 4 Thermogravimetric analysis (TGA) (a), and differential scanning calorimetry (DSC) of the compounds 2EH-TIFDKT (b) and 2OD-TIFDKT (c) at temperature ramps of 10 °C min⁻¹ under N₂.

Table 1 Summary of thermal, optical absorption, and electrochemical properties of compounds 2EH-TIFDKT, 2OD-TIFDKT, and β-DD-TIFDKT ¹⁴ and corresponding estimated frontier molecular orbital energies

Compounds	Tmp (°C)	TDSC^a (°C) heating (cooling)	TTGA^b (°C)	Ered^1/2^c (V)	ELUMO^d (eV)	EHOMO^e (eV)	λ^sol.abs (nm) (Eg (eV))^f	λ^filmabs (nm) (Eg (eV))^g
a From DSC scans under nitrogen at a scan rate of 10 °C min^−1.b Onset decomposition temperature measured by TGA under nitrogen.c 0.1 M Bu4N^+PF6^− in THF at a scan rate of 50 mV s^−1.d Estimated from the equation ELUMO = −4.40 eV − Ered^1/2.e EHOMO is calculated from Eg = ELUMO − EHOMO.f From optical absorption in THF, optical band gap is estimated from the low energy band edge of the UV-vis absorption spectrum.g From optical absorption as spin-coated thin film on glass, optical band gap is estimated from the low-energy band edge of the UV-vis absorption spectrum.
2EH-TIFDKT	292	250, 252, 295 (239, 288)	401	−0.75 (vs. Ag/AgCl)	−3.65	−5.60	383, 556 (1.95)	371, 630 (1.79)
2OD-TIFDKT	135	130, 138 (110, 131)	415	−0.77 (vs. Ag/AgCl)	−3.63	−5.57	384, 560 (1.94)	406, 648 (1.76)
β-DD-TIFDKT^14	152	155 (128)	420	−0.74 (vs. SCE)	−3.70	−5.75	377, 525 (2.05)	365, 594 (1.89)

---

2EH-TIFDKT and 2OD-TIFDKT exhibit impressive thermal stability with thermolysis onset temperatures of 401 °C and 415 °C, respectively, which enables the semiconductor deposition over a broad range of processing temperature (Fig. 4a). Both compounds leave 20–40% nonvolatile residues, indicating some degree of chemical decomposition upon heating to >500 °C. Therefore, semiconductors thin-films fabricated by thermal evaporation technique may be contaminated by decomposition products, which is consistent with the observed OTFT inactivity (vide infra). On the other hand, differential scanning calorimetry (DSC) measurements indicate endotherms at 295 °C and 138 °C for 2EH-TIFDKT and 2OD-TIFDKT, respectively, which corresponds to the melting points (T_mp = 292–293 °C for 2EH-TIFDKT and T_mp = 135–136 °C for 2OD-TIFDKT) observed in conventional melting point measurements (Fig. 4b and c). The corresponding exothermic crystallization transitions are observed at 288 °C for 2EH-TIFDKT and at 131 °C for 2OD-TIFDKT in the reverse scans. In addition, for both compounds endothermic thermal transitions are observed prior to major melting process, which may be attributed to transition from solid state to a liquid crystalline mesophase as a result of lipophilic alkyl chain melting. The observed melting point for 2EH-TIFDK is much higher (ΔT_mp = 140 °C) than that of the corresponding beta-substituted structure, β-DD-TIFDKT (ΔT_mp = 152–153 °C). Considering the similar π-core sizes and relatively small changes in molecular weights, the observed large melting point increase indicates more effective solid-state packing, which is most likely due to enhanced intermolecular cohesive forces through dipole–dipole, donor–acceptor, and π–π interactions as a result of enhanced molecular backbone planarity. The single-crystal obtained for the intermediate precursor 2 shows that indeno[1,2-b]fluorene-6,12-dione cores form favorable π–π interactions (3.43 Å) and close dipole–dipole interactions (–C=O/–C=O) with a parallel-alignment (vide infra). On the other hand, when the lipophilic alkyl chains are changed to longer 2-octyldodecyl chains, the melting point significantly decreases to T_mp = 135–136 °C. This is ∼17 °C lower that of β-DD-TIFDKT, which indicates that the structural planarity gained by α,ω-substitution is overweighed by the presence of bulky swallow tail alkyl chains, which deteriorates the intermolecular interactions and results in poor solid-state packing. Based on these thermal studies, 2-ethylhexyl substitution might reflect the ideal balance between good solid-state packing and high solubility, which may lead to the highest OTFT performance from solution processing. This is in line with the OTFT results (vide infra).

Optical and electrochemical properties

The UV-vis absorption spectra of the present compounds in THF and as thin-films are shown in Fig. 5a and c, and optical data are collected in Table 1. The absorption spectra of 2EH-TIFDKT and 2OD-TIFDKT exhibit three maxima, two of them located below 400 nm, and the third one at 556–560 nm. The higher energy maxima (383 nm for 2EH-TIFDKT and 384 nm for 2OD-TIFDKT) correspond to the π–π* transitions of the indenofluorene-thiophene backbone, whereas the weaker absorptions at lower energies (556 nm for 2EH-TIFDKT and 560 nm for 2OD-TIFDKT) are attributed to the symmetry forbidden n–π* transition of the carbonyl group. The optical band gaps are estimated from the low-energy absorption edge as 1.95 eV (2EH-TIFDKT) and 1.94 eV (2OD-TIFDKT). These gaps are ∼0.1 eV smaller than that of β-DD-TIFDKT (2.05 eV), and the observed absorption maxima are red-shifted by 5–35 nm compared to that of β-DD-TIFDKT (377 nm/525 nm in THF). These changes are indicative of enhanced π-conjugation of the molecular backbone as a result of the planarization of the π-core upon relocating alkyl substituents. For both compounds, significant bathochromic shifts (Δλ = 80–90 nm) are observed in the solid-state UV-vis absorption spectra, most likely due to molecular planarization and enhanced π–π stacking/donor–acceptor interactions when going from dilute solution to solid-state.

Fig. 5 (a) Optical absorption in THF solution. (b) Cyclic voltammograms in THF (0.1 M Bu₄N⁺PF₆⁻, scan rate = 100 mV s⁻¹). (c) Optical absorption as thin-films. (d) Experimentally derived HOMO–LUMO energy levels for 2EH-TIFDKT and 2OD-TIFDKT. Note that the energy level values for β-DD-TIFDKT is taken from literature¹⁴ measured under the same experimental conditions.

The redox properties of the present compounds were investigated by cyclic voltammetry in solution. The cyclic voltammograms are shown in Fig. 5b, and electrochemical data are summarized in Table 1. The measurements were performed in THF by using Pt as the working and counter electrodes, and Ag/AgCl (3 M NaCl) as the reference electrode. Both compounds, 2EH-TIFDKT and 2OD-TIFDKT exhibit two reversible reduction peaks with the first half-wave potentials (E_red^1/2) located at 0.75 V (vs. Ag/AgCl) and 0.77 V (vs. Ag/AgCl), respectively, which are assigned as reduction of the diketone to the quinonoidal dianion. The reversibility of both reductions demonstrates the good redox stability of these new molecular diketone cores. The LUMO energies are estimated as −3.65 eV and −3.63 eV for 2EH-TIFDKT and 2OD-TIFDKT, respectively, using the vacuum level energy of the Ag/AgCl as −4.40 eV. The corresponding HOMO energies of −5.60 eV and −5.57 eV, respectively, are calculated from the optical band gaps. The optical and electrochemical characterizations confirm the theoretically estimated increases in the LUMO (0.05–0.07 eV) and HOMO energy levels (0.15–0.17 eV) after alkyl chain tailoring. From a molecular orbital energetic standpoint, the observed energies for HOMO and LUMO are highly favorable to achieve simultaneous hole and electron transport in these semiconductor thin-films.

Single-crystal structure

Since diffraction-quality single-crystals of the semiconductors, 2EH-TIFDKT and 2OD-TIFDKT, were not obtained, we focused on growing the crystals of the intermediate compound 2 to better understand the structural features of the indeno[1,2-b]fluorene-6,12-dione acceptor unit. Single crystals of 2 were grown using physical vapor transport in a thermal gradient under high vacuum (1 × 10⁻⁵ Torr), and its solid state structure was confirmed by single-crystal X-ray analysis (Fig. 6a).

Fig. 6 (a) ORTEP drawings of the crystal structure (50% probability level). (b) Perspective view of 2D network via –CH⋯Br (blue dashed lines) and –CH⋯O (red dashed lines) contacts. (c) Perspective view of brick-wall packing arrangement with an interplanar distance of 3.430 Å. (d) and (e) Representations of pairs of indeno[1,2-b]fluorene-6,12-dione molecules arranged in a slipped π-stacked fashion (the red, grey, brown, and white coloured atoms represent O, C, Br, and H, respectively).

The conjugated backbone of indeno[1,2-b]fluorene-6,12-dione is found to lie across an inversion center with a substantially planar molecular configuration with negligible interplanar twists. This is consistent with the computational optimization results, and it is similar to the previously reported fluorinated indeno[1,2-b]fluorene-6,12-dione derivative.²¹ The carbonyl (–C=O) functionalities are found to stay completely within the molecular plane, which indicates a highly favorable structural conformation for effective π-conjugation between these functionalities and the indeno[1,2-b]fluorene-6,12-dione core. For compound 2, short –CH⋯O (2.59 Å) and –CH⋯Br (3.03 Å) contacts are found to be effective between adjacent molecules (8 interactions per molecule; Fig. 6b), which results in the formation of continuous graphene-like two-dimensional π-layers. These interactions take place between benzene hydrogens, which are adjacent to the bromo functionality, and carbonyl oxygen and bromo substituents, respectively, which are ∼0.1 Å smaller than the corresponding van der Waals distances (r_vdw(O) + r_vdw(H) = 2.72 Å and r_vdw(Br) + r_vdw(H) = 3.05 Å).²² These 2D layers are packed in a brick-wall packing arrangement (Fig. 6c), which allows π–π-stacking interactions with a favorable interplanar distance of 3.43 Å. In addition, based on the molecular alignments between the layers, strong local dipoles of –C=O groups are found to align parallel to each other with slight slipping (slippage distance d = 1.772 Å, slippage angle β = 27.1°) (Fig. 6d and e). This indicates the presence of dipole–dipole interactions, which may overcome the dominating CH/π interactions (edge-to-face) in the classical herringbone arrangement, and results in the current highly-favorable slipped-cofacial arrangement. Cofacial π-stacked packing motif has been previously shown in high-mobility “TIPS-pentacene” and “TES-ADT” semiconductors.²³ It's noteworthy that although the solid-state packing of 2 does not resemble those of the final semiconductors, it still reveals key structural features of this acceptor core. The favorable structural properties of the indeno[1,2-b]fluorene-6,12-dione core such as short π–π stacking distances, close dipole–dipole interactions, core planarity, cofacial molecular arrangement, and lamellar structure formation hold great promise for their incorporation into larger molecular semiconductors for efficient charge-transport in the solid-state.

Thin-film morphology and microstructure

Microstructural order in the new solution-processed semiconductor thin- films was assayed by θ–2θ X-ray diffraction (XRD) scans. As shown in Fig. 7, S9–S12,† all the semiconductor films exhibit highly crystalline patterns with diffraction peaks up to ninth-order, indicating a high degree of solid state ordering. Based on the highest mobility thin-films achieved via solution-shearing of 2EH-TIFDKT and 2OD-TIFDKT, the major primary diffraction peaks (100) were observed at 2θ = 3.52° and 2θ = 2.92°, respectively, corresponding to d-spacings of 25.1 Å and 30.2 Å (Fig. 7). Although both molecules share exactly the same π-core, thin-films based on 2OD-TIFDKT show larger d-spacing compared to that of 2EH-TIFDKT as a result of the presence of longer alkyl substituents (2-octyldodecyl vs. 2-ethyhexyl). This also indicates that alkyl chains are somewhat aligned along the substrate normal and they are effective in determining the out-of-plane d-spacings. These d-spacings are considerably smaller than the computed molecular lengths for both compounds (30.5 Å for 2EH-TIFDKT and 41.7 Å for 2OD-TIFDKT), and longer than the length of T-IFDK-T π-core (19.1 Å). This indicates that alkyl chains are interdigitated, and π-cores and/or alkyl chains may be tilted from the substrate normal. Although the tilting angles and the extent of interdigitation together determine the d-spacing, based on the good mobilities observed, it's very likely that the molecules are adopting a mostly face-on π-orientation in the out-of-plane microstructure with significant alkyl chain interdigitation. This enables the formation of a “layer-by-layer” packing motif consisting of alternately stacked π-cores and intercalated aliphatic chains (Fig. S13 and S14†), which could explain the good mobilities observed. Surprisingly, a second set of reflections up to the seventh-order is observed in the film of 2EH-TIFDKT, which points to the presence of a secondary crystalline phase. For 2EH-TIFDKT-based films, although the diffraction angles remains the same, the relative intensity of the two crystalline phases change with the deposition technique (Fig. S9 and S10†), and in all cases the lower angle diffraction peaks are found to be the dominant phase. The primary diffraction peak for the secondary phase was at 2θ = 4.07° (d-spacing = 21.7 Å) corresponding to a likely a more tilted molecular orientation on the surface. Although the presence of two crystalline phases is not unusual for indenofluorene-based thin-films,¹⁴ it is quite surprising that the coexistence of two crystalline phases still leads to good charge carrier mobilities (vide infra).

Fig. 7 θ–2θ X-ray diffraction (XRD) scans and AFM topographic images of films fabricated by solution shearing of 2EH-TIFDKT (a and b) and 2OD-TIFDKT (c and d). Scale bars denote 2 μm.

AFM characterizations of solution-sheared 2EH-TIFDKT and 2OD-TIFDKT thin-films reveals relatively homogeneous morphologies with a surface roughness of <4.28 nm for 10.0 μm × 10.0 μm scan area. For 2EH-TIFDKT, two dimensional platelet grains (∼0.5–2.0 μm sizes) grown via layer-by-layer mode are observed. Specifically, grains in solution-sheared thin-films of 2EH-TIFDKT show preferred growth direction on a locale scale forming microscale rods with evident terraced islands. The step heights are ∼2.5 nm corresponding to the d-spacings obtained from the XRD characterizations. On the other hand, solution-sheared films of 2OD-TIFDKT exhibit totally different film morphologies with uniform and highly-interconnected isotropic spherulites, which are ∼50–200 nm in diameter (Fig. 7b and d).

Fig. 8 (a) N-type transfer curve, (b) N-type output curve, (c) P-type transfer curve, and (d) P-type output curve of the device with 2EH-TIFDKT.

Thin-film transistor device characterization

Top-contact/bottom-gate organic thin film transistors (OTFTs) were fabricated with thin-films of 2EH-TIFDKT and 2OD-TIFDKT as semiconducting layers via different solution processing methods (solution-shearing (SS), droplet-pinned crystallization (DPC), and drop-casting (DC)) on PS brush-coated Si/SiO₂ substrates.²⁴ For solution process, various conditions for the formation of thin-films including solvents (toluene, chlorobenzene, chloroform, p-xylene, 1,2-dichlorobenzene, tetralin, and 1,2,4-trichlorobenzene), concentration of solution, temperature of substrate, and shearing speed were optimized.²⁵ Au electrodes were deposited through a shadow mask to define source and drain. Table 2 summarizes the OTFT data, including charge carrier mobility, current on/off ratio, and threshold voltage. As expected from aforementioned physicochemical characterizations, physical vapor deposited films of 2EH-TIFDKT and 2OD-TIFDKT were inactive in OTFT devices, which can be attributed to the thermal decomposition of the materials during film deposition. However, solution-processed films of both 2EH-TIFDKT and 2OD-TIFDKT exhibit ambipolar behaviors as a result of their reduced band gap and energetically accessible HOMO and LUMO energy levels. It's noteworthy that for the present molecular design, the LUMO energies achieved are not low enough (should be <−4.1 eV) to give ambient-stable electron-transport, and therefore electron mobilities were only observed under vacuum (Table S1†). However, electron-transporting high mobility semiconductors with relatively high LUMO energies (>−3.7 eV) are very crucial to the development of green and blue emitting OLET devices.^8d As expected from film morphologies and crystallinities, solution-sheared films of 2EH-TIFDKT showed the best semiconductor performance with electron and hole mobilities of up to 0.12 cm² V⁻¹ s⁻¹ and 0.02 cm² V⁻¹ s⁻¹, respectively. Among different solution processing methods applied, SS afforded the highest device performance, compared to DC and DPC. As shown in Table 2, electron and hole mobilities obtained for thin-films formed via DPC and DC methods (∼10⁻³ to 10⁻⁴ cm² V⁻¹ s⁻¹​) were around two orders of magnitude lower than that of SS method. Films of 2OD-TIFDKT also exhibited ambipolar characteristics with electron mobility as high as 0.04 cm² V⁻¹ s⁻¹ and hole mobility as high as 3.3 × 10⁻⁴ cm² V⁻¹ s⁻¹. For the present semiconductors, the electrical performance of OTFTs is well-correlated with the thin-film microstructures and surface morphologies (Fig. 8).

Table 2 Electrical performance of OTFTs based on indeno[1,2-b]fluorene-6,12-dione-thiophene derivatives developed in this study^a

Material	Method	N-Channel			P-Channel
		μe (cm^2 V^−1 s^−1)	VT (V)	Ion/Ioff	μh (cm^2 V^−1 s^−1)	VT (V)	Ion/Ioff
a Devices were measured under vacuum.
2EH-TIFDKT	DC	1.6 × 10^−3	53	2.8 × 10^6	8.2 × 10^−4	−22	1.5 × 10^2
	DPC	7.7 × 10^−4	46	3.1 × 10^2	2.5 × 10^−4	−4.0	5.3 × 10^4
	SS	0.12	57	8.5 × 10^5	0.02	−62	2.1 × 10^2
2OD-TIFDKT	DC	1.5 × 10^−4	47	6.7 × 10^5	5.3 × 10^−7	−54	5.9 × 10^3
	DPC	0.013	26	1.2 × 10^−4	2.7 × 10^−4	−35	4.2 × 10^4
	SS	0.04	10	2.2 × 10^5	3.3 × 10^−4	−45	1.2 × 10^5

---

Conclusions

In summary, a series of new indeno[1,2-b]fluorene-6,12-dione-thiophene small molecules, DD-TIFDKT, 2EH-TIFDKT, and 2OD-TIFDKT, has been designed, synthesized and fully characterized. These new semiconductors consist of highly π-conjugated donor–acceptor molecular architectures based on indeno[1,2-b]fluorene-6,12-dione acceptor unit and thiophene donor units, which yields band gaps of 1.7–1.8 eV. The semiconductor structures are α,ω-end-functionalized with linear –n-C₁₂H₂₅ chains or swallow-tail 2-ethylhexyl-/2-octyldodecyl chains. While the linear –n-C₁₂H₂₅ chains impart good solubility in β-substituted semiconductor core, they don't provide the desired solubility in α,ω-end-functionalized core. However, 2-ethylhexyl and 2-octyldodecyl chains are found to yield entirely solution-processable molecular systems. The detailed study on the effects of alkyl chain size and orientation on the optoelectronic properties, intermolecular cohesive forces, thin-film microstructures, and charge transport performances of the new semiconductors reveal crucial structure–property–function relationships. Density functional theory (DFT) calculations are found to be in excellent agreement with the observed electronic structure and physicochemical trends associated with alkyl chain engineering, and provide further insight. In the rational design of the new molecules, the repositioning of the insulating β-substituents to molecular termini is found to significantly enhance the π-core planarity while maintaining a good solubility. 2-Ethylhexyl substitution is shown to provide the finest balance of good solubility, favorable physicochemical/optoelectronic properties, effective solid-state packing and high electrical performance. The solution-processed OTFT devices of the current semiconductors, 2EH-TIFDKT and 2OD-TIFDKT, exhibit excellent ambipolar behavior with carrier mobilities of 0.04–0.12 cm² V⁻¹ s⁻¹ and 0.0003–0.02 cm² V⁻¹ s⁻¹ for electrons and holes, respectively, and I_on/I_off ratios of 10⁵ to 10⁶. Specifically, solution-sheared thin-films of 2EH-TIFDKT show ambipolar device operation with electron and hole mobilities of 0.12 cm² V⁻¹ s⁻¹ and 0.02 cm² V⁻¹ s⁻¹, respectively, with I_on/I_off ratios of 10⁵ to 10⁶, which indicates two–three orders of carrier mobility enhancement compared to those of solution-processed β-substituted counterparts. The findings presented here suggest that through computational modeling guided rational design and synthetic tailoring of insulating alkyl substituents, electron/hole transport characteristics of molecular semiconductors can be significantly enhanced while still maintaining a favorable solution-processability. We believe that our results will provide key structural/electronic information and additional motivation to investigate and optimize molecular semiconductor structures for high-performance organic opto-electronic devices.

Acknowledgements

H. U. acknowledges support from The Science Academy, Young Scientist Award (BAGEP) and Turkish Academy of Sciences, The Young Scientists Award Program (TUBA-GEBİP). H. U. acknowledges support from AGU-BAP (FOA-2015-24) and TUBITAK 113C021. C. K. acknowledges support from Basic Science Research Program through the National Research Foundation of Korea (NRF) (NRF-2014R1A1A1A05002158) and from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Science, ICT & Future Planning (Code No. 2013M3A6A5073175).

References

1. (a) H. Sirringhaus, Adv. Mater., 2014, 26, 1319–1335; (b) J. E. Anthony, Chem. Rev., 2006, 106, 5028–5048; (c) A. Pierre, M. Sadeghi, M. M. Payne, A. Facchetti, J. E. Anthony and A. C. Arias, Adv. Mater., 2014, 26, 5722–5727; (d) L. Basirio, P. Cosseddu, B. Fraboni and A. Bonfiglio, Thin Solid Films, 2011, 520, 1291–1294; (e) K. C. Dickey, S. Subramanian, J. E. Anthony, L.-H. Han, S. Chen and Y.-L. Loo, Appl. Phys. Lett., 2007, 90, 244103; (f) R. Capelli, S. Toffanin, G. Generali, H. Usta, A. Facchetti and M. Muccini, Nat. Mater., 2010, 9, 496–503; (g) P. Y. Huang, L.-H. Chen, C. Kim, H.-C. Chang, Y.-J. Liang, C.-Y. Feng, C.-M. Yeh, J.-C. Ho, C.-C. Lee and M. C. Chen, ACS Appl. Mater. Interfaces, 2012, 4, 6992–6998.
2. (a) A. Broggi, I. Tomasi, L. Bianchi, A. Marrocchi and L. Vaccaro, ChemPlusChem, 2014, 79, 2–24; (b) F. Silvestri, A. Marrocchi, M. Seri, C. Kim, T. J. Marks, A. Facchetti and A. Taticchi, J. Am. Chem. Soc., 2010, 132, 6108–6123; (c) A. Marrocchi, F. Silvestri, M. Seri, A. Facchetti, A. Taticchia and T. J. Marks, Chem. Commun., 2009, 1380–1382; (d) B. Vercelli, M. Pasini, A. Berlin, J. Casado, J. T. L. Navarrete, R. P. Ortiz and G. Zotti, J. Phys. Chem. C, 2014, 118, 3984–3993; (e) J. Casado, R. P. Ortiz and J. T. L. Navarrete, Chem. Soc. Rev., 2012, 41, 5672–5686; (f) M.-C. Chen, Y.-J. Chiang, C. Kim, Y.-J. Guo, S.-Y. Chen, Y.-J. Liang, Y.-W. Huang, T.-S. Hu, G.-H. Lee, A. Facchetti and T. J. Marks, Chem. Commun., 2009, 1846–1848; (g) L.-H. Chen, T.-S. Hu, P.-Y. Huang, C. Kim, C.-H. Yang, J.-J. Wang, J.-Y. Yan, J.-C. Ho, C.-C. Lee and M.-C. Chen, ChemPhysChem, 2013, 14, 2772–2776.
3. (a) H. Lin, S. Chen, Z. Li, J. Y. L. Lai, G. Yang, T. McAfee, K. Jiang, Y. Li, Y. Liu, H. Hu, J. Zhao, W. Ma, H. Ade and H. Yan, Adv. Mater., 2015, 27, 7299–7304; (b) Y. Liu, J. Y. Lin Lai, S. Chen, Y. Li, K. Jiang, J. Zhao, Z. Li, H. Hu, T. Ma, H. Lin, J. Liu, J. Zhang, F. Huang, D. Yuc and H. Yan, J. Mater. Chem. A, 2015, 3, 13632–13636; (c) A. K. Palai, S. Kim, H. Shim, S. Cho, A. Kumar, J. Kwon, S.-U. Park and S. Pyo, RSC Adv., 2014, 4, 41476–41482; (d) J. Kim, H. M. Ko, N. Cho, S. Paek, J. K. Lee and J. Ko, RSC Adv., 2012, 2, 2692–2695.
4. (a) M. Cai, T. Xiao, E. Hellerich, Y. Chen, R. Shinar and J. Shinar, Adv. Mater., 2011, 23, 3590–3596; (b) Y. Kajiyama, K. Kajiyama and H. Aziz, Opt. Express, 2015, 23, 16650–16661.
5. (a) H. E. Katz, Z. Bao and S. L. Gilat, Acc. Chem. Res., 2001, 34, 359–369; (b) M. Ozdemir, S. Genc, R. Ozdemir, Y. Altintas, M. Citir, U. Sen, E. Mutlugun and H. Usta, Synth. Met., 2015, 210, 192–200.
6. W. Wu, Y. Liu and D. Zhu, Chem. Soc. Rev., 2010, 39, 1489–1502.
7. (a) M.-C. C. S. Vegiraju, C.-M. Huang, P.-Y. Huang, K. Prabakara, S. L. Yau, W.-C. Chen, W.-T. Peng, I. Chao, C. Kim and Y.-T. Tao, J. Mater. Chem. C, 2014, 2, 8892–8902; (b) M. Melucci, M. Zambianchi, L. Favaretto, M. Durso, C. Bettini, A. Zanelli, I. Manet, M. Gazzano, L. Maini, D. Gentili, F. Gallino, M. Muccini, M. Cavallini and S. Toffanin, J. Mater. Chem. C, 2015, 3, 121–131.
8. (a) K. Niimi, S. Shinamura, I. Osaka, E. Miyazaki and K. Takimiya, J. Am. Chem. Soc., 2011, 133, 8732–8739; (b) P. Kumaresan, S. Vegiraju, Y. Ezhumalai, S. L. Yau, C. Kim, W.-H. Lee and M.-C. Chen, Polymers, 2014, 6, 2645–2669; (c) J. Li, F. Yan, J. Gao, P. Li, W.-W. Xiong, Y. Zhao, X. W. Sun and Q. Zhang, Dyes Pigm., 2015, 112, 93–98; (d) H. Usta, W. C. Sheets, M. Denti, G. Generali, R. Capelli, S. Lu, X. Yu, M. Muccini and A. Facchetti, Chem. Mater., 2014, 26, 6542–6556.
9. (a) D. T. Chase, A. G. Fix, S. J. Kang, B. D. Rose, C. D. Weber, Y. Zhong, L. N. Zakharov, M. C. Lonergan, C. Nuckolls and M. M. Haley, J. Am. Chem. Soc., 2012, 134, 10349–10352; (b) J. Mei, Y. Diao, A. L. Appleton, L. Fang and Z. Bao, J. Am. Chem. Soc., 2013, 135, 6724–6746; (c) H. Usta, C. Newman, Z. Chen and A. Facchetti, Adv. Mater., 2012, 24, 3678–3684.
10. (a) A. J. Kronemeijer, E. Gili, M. Shahid, J. Rivna, A. Salleo, M. Heeney and H. Sirringhaus, Adv. Mater., 2012, 24, 1558; (b) S. Fabiano, H. Usta, R. Forchheimer, X. Crispin, A. Facchetti and M. Berggren, Adv. Mater., 2014, 26, 7438–7443; (c) A. C. Arias, J. D. MacKenzie, I. McCulloch, J. Rivnay and A. Salleo, Chem. Rev., 2010, 110, 3; (d) D. Khim, K.-J. Baeg, M. Caironi, C. Liu, Y. Xu, D.-Y. Kim and Y.-Y. Noh, Adv. Funct. Mater., 2014, 24, 6245–6245; (e) K.-J. Baeg, M. Caironi and Y.-Y. Noh, Adv. Mater., 2013, 25, 4210–4244.
11. A. Riano, P. Mayorga Burrezo, M. J. Mancheno, A. Timalsina, J. Smith, A. Facchetti, T. J. Marks, J. T. Lopez Navarrete, J. L. Segura, J. Casadob and R. Ponce Ortiz, J. Mater. Chem. C, 2014, 2, 6376–6386.
12. (a) R. P. Ortiz, H. Yan, A. Facchetti and T. J. Marks, Materials, 2010, 3, 1533–1558; (b) H. Usta, M. D. Yilmaz, A.-J. Avestro, D. Boudinet, M. Denti, W. Zhao, J. F. Stoddart and A. Facchetti, Adv. Mater., 2013, 25, 4327; (c) P. Sonar, S. P. Singh, Y. Li, Z.-E. Ooi, T.-j. Ha, I. Wong, M. S. Soh and A. Dodabalapur, Energy Environ. Sci., 2011, 4, 2288–2296.
13. H. Usta, A. Facchetti and T. J. Marks, J. Am. Chem. Soc., 2008, 130, 8580.
14. H. Usta, C. Risko, Z. Wang, H. Huang, M. K. Deliomeroglu, A. Zhukhovitskiy, A. Facchetti and T. J. Marks, J. Am. Chem. Soc., 2009, 131, 5586–5608.
15. (a) J. Mei and Z. Bao, Chem. Mater., 2014, 26, 604–615; (b) J. Min, Y. N. Luponosov, A. Gerl, M. S. Polinskaya, S. M. Peregudova, P. V. Dmitryakov, A. V. Bakirov, M. A. Shcherbina, S. N. Chvalun, S. Grigorian, N. Kaush-Busies, S. A. Ponomarenko, T. Ameri and C. J. Brabec, Adv. Energy Mater., 2014, 4, 1301234; (c) A.-R. Han, G. K. Dutta, J. Lee, H. R. Lee, S. M. Lee, H. Ahn, T. J. Shin, J. H. Oh and C. Yang, Adv. Funct. Mater., 2015, 25, 247–254; (d) J. Lee, A.-R. Han, H. Yu, T. J. Shin, C. Yang and J. H. Oh, J. Am. Chem. Soc., 2013, 135, 9540–9547.
16. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian 09, Revision C.01, Gaussian, Inc., Wallingford CT, 2010.
17. A. L. Spek, Acta Crystallogr., 2009, D65, 148–155.
18. C. F. Macrae, P. R. Edgington, P. McCabe, E. Pidcock, G. P. Shields, R. Taylor, M. Towler and J. van de Streek, J. Appl. Crystallogr., 2006, 39, 453–457.
19. S. H. Park, H. S. Lee, J. D. Kim, D. W. Breiby, E. Kim, Y. D. Park, D. Y. Ryu, D. R. Lee and J. H. Cho, J. Mater. Chem., 2011, 21, 15580.
20. J. Peng, F. Zhai, X. Guo, X. Jiang and Y. Ma, RSC Adv., 2014, 4, 13078–13084.
21. T. Nakagawa, D. Kumaki, J. Nishida, S. Tokito and Y. Yamashita, Chem. Mater., 2008, 20, 2615–2617.
22. A. Bondi, J. Phys. Chem., 1964, 68, 441–451.
23. J. E. Anthony, D. L. Eaton and S. R. Parkin, Org. Lett., 2002, 4, 15–18.
24. (a) H. Li, B. C.-K. Tee, J. J. Cha, Y. Cui, J. W. Chung, S. Y. Lee and Z. Bao, J. Am. Chem. Soc., 2012, 134, 2760–2765; (b) G. Giri, E. Verploegen, S. C. B. Mannsfeld, S. Atahan-Evrenk, D. H. Kim, S. Y. Lee, H. A. Becerril, A. Aspuru-Guzik, M. F. Toney and Z. Bao, Nature, 2011, 480, 504–508; (c) K. C. Dickey, J. E. Anthony and Y.-L. Loo, Adv. Mater., 2006, 18, 1721–1726; (d) H. A. Becerril, M. E. Roberts, Z. Liu, J. Rocklin and Z. Bao, Adv. Mater., 2008, 20, 2588–2594.
25. (a) Z. Liu, H. A. Becerril, M. E. Roberts, Y. Nishi and Z. Bao, IEEE Trans. Electron Devices, 2009, 56, 176–185; (b) C. S. Kim, S. Lee, E. D. Gomez, J. E. Anthony and Y.-L. Loo, Appl. Phys. Lett., 2008, 93, 103302; (c) J.-F. Chang, B. Sun, D. W. Breiby, M. M. Nielsen, T. I. Sölling, M. Giles, I. McCulloch and H. Sirringhaus, Chem. Mater., 2004, 16, 4772–4776.

---

Footnote

† Electronic supplementary information (ESI) available: ¹H and ¹³C NMR spectra (Fig. S1–S3, S5 and S6), MALDI-TOF spectra (Fig. S4 and S7), FTIR spectra (Fig. S8) of compounds 1, 2EH-TIFDKT, and 2OD-TIFDKT. θ–2θ X-ray diffraction (XRD) scans of films based on drop-casted/droplet-pinned crystallized 2EH-TIFDKT and 2OD-TIFDKT (Fig. S9–S12). The computed molecular dimensions and thin-film phase packing motif for 2EH-TIFDKT and 2OD-TIFDKT (Fig. S13 and S14). Tables S1 and S2. CCDC 1420110. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra22359h

---