N-Unsubstituted thienoisoindigo (TIIG) and its diphenyl derivative (dph-TIIG) are synthesized by using a tert-butoxy carbonyl (t-Boc) group as a protecting group. TIIG has a stacking structure analogous to isoindigo, and dph-TIIG is a hybrid of brickwork and herringbone structures. These compounds exhibit ambipolar transistor properties and particularly dph-TIIG shows the maximum hole and electron mobilities greater than 0.1 cm2 V−1 s−1.

Six conjugated polymers based on thienoisoindigo (TII) and thiophene-flanked diketopyrrolopyrrole (TDPP) units bearing either branched-alkyl or siloxane-terminated alkyl solubilizing groups have been synthesized. We report the impact of backbone selection and side chain engineering on the optical, electrochemical, and solid state packing, as well as carrier transport properties, for this set of polymers. The optical bandgap of the polymers is shown to systematically decrease on TII unit incorporation into the polymer chain. TII homopolymers absorb near infrared wavelengths and have extremely narrow optical band gaps below 0.60 eV, mainly as a result of the contribution of the flat and rigid π-frameworks corroborated with a strong quinoidal character of the TII unit. The theoretical calculations and Raman spectra measurements well demonstrate that the presence of a TII unit contributes to π-delocalization inducted by quinoidal resonance. Our X-ray analyses indicate that the TII unit prefers face-on lamellar orientations compared with the TDPP unit, and the largely separated branching point over the undecyl spacer of siloxane-terminated chains reinforces π–π interchain interactions in a crystalline film.

•Isoindigo shows ambipolar transistor properties.•Isoindigo has a variety of crystal structures.•Isoindigo derivatives whose HOMO levels are lower than −5.8 eV do not show hole transport.Structural and transistor properties of isoindigo derivatives are investigated. The unsubstituted isoindigo affords two polymorphs in addition to the reported brickwork structure; one has a stacking structure analogous to indigo, and another consists of nonplanar molecules. The unsubstituted isoindigo exhibits ambipolar transistor properties with the hole and electron mobilities more than 0.01 cm2/Vs, and 6.6′-diphenylisoindigo shows ambipolar transistor properties with the hole/electron mobilities of 0.037/0.027 cm2/Vs. Isoindigo derivatives with electron withdrawing groups show only electron transport, indicating that the lower limit of the HOMO level showing the hole transport is −5.7 eV.

A novel planar π-conjugated small molecule, benzothienoisoindigo (BTII), in which additional benzene rings are fused with the thieoisoindigo (TII) unit, has been designed and synthesized. We report the impact of the planar π-framework and π-conjugation length on the carrier transport properties using three sets of molecules, BTII, isoindigo (II) and TII, bearing the same hexyl-side chain. The absorption spectra are remarkably red-shifted in the order of II < TII < BTII along with the enhanced molar extinction coefficient in the low-energy region, leading to the reduced bandgap. The single-crystal structure analyses revealed that all molecules have a planar backbone, and II and BTII are packed into a slipped columnar structure showing highly one-dimensional π–π interactions, while TII did not form, any noticeable intermolecular overlaps. The carrier transport properties were investigated in field-effect transistors (FETs). All molecules exhibited typical ambipolar properties. Among them, BTII showed the highest FET p-dominant ambipolar performance with the hole mobility of 0.095 cm2 V−1 s−1 and electron mobility of 5.8 × 10−3 cm2 V−1 s−1 on the tetratetracontane (TTC)-modified substrate and p-type performance with the hole mobility of 0.18 cm2 V−1 s−1 on the octadecyltrimethoxysilane (OTMS)-modified substrate. The microstructure of thin films was characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements. These results indicated that smooth and densely packed nanorod-like crystalline grains are formed by extension of the π-conjugation in BTII. Due to the π-extension of planar organic semiconductors, the novel BTII unit can be extended for the rational design of high performance FET materials.

The influence of molecular planarity on field-effect-transistor and photovoltaic cell performance in thienoisoindigo derivatives has been studied. Thienoisoindigo derivatives end-capped with benzothiophene TII(SB)2 and benzofuran TII(OB)2 together with benzothiophene-capped isoindigo II(SB)2 are prepared, and their electronic properties are investigated. The crystal structures of TII(SB)2 and TII(OB)2 are determined by single-crystal X-ray structure analyses. The redox and optical measurements as well as the molecular orbital calculation indicate that thienoisoindigo-based molecules TII(SB)2 and TII(OB)2 have higher HOMO levels and smaller band gaps than II(SB)2. The single-crystal structure analysis reveals that TII(SB)2 and TII(OB)2 have flat form, agreeing well with the structure optimized by the density functional theory (DFT) calculation, and TII(SB)2 and TII(OB)2 form slipped one-dimensional stacks with the alkyl chains extending out of the molecular plane. As an active layer of organic field-effect transistors, TII(SB)2 and TII(OB)2 show one order of magnitude larger p-type carrier mobility than that of II(SB)2. It is noted that TII(SB)2 and TII(OB)2 fabricated on a tetratetracontane (TTC) modified substrate show balanced ambipolar properties (μh ≈ μe ≈ 10−2 cm2 V−1 s−1), where the carrier balance comes from well delocalized frontier molecular orbitals (FMOs). The photovoltaic properties of TII(SB)2, TII(OB)2, and II(SB)2 are investigated in bulk heterojunction devices using PC71BM. The devices show a photovoltaic efficiency up to 2.4% for TII(OB)2 and 1.4% for TII(SB)2. The device performance is closely associated with the flat structure of the thienoisoindigo unit, which effectively minimizes the steric interference of the benzothiophene and benzofuran units to facilitate the slipped co-facial π–π stacking.

N-Unsubstituted thienoisoindigo (TIIG) and its diphenyl derivative (dph-TIIG) are synthesized by using a tert-butoxy carbonyl (t-Boc) group as a protecting group. TIIG has a stacking structure analogous to isoindigo, and dph-TIIG is a hybrid of brickwork and herringbone structures. These compounds exhibit ambipolar transistor properties and particularly dph-TIIG shows the maximum hole and electron mobilities greater than 0.1 cm2 V−1 s−1.

The influence of molecular planarity on field-effect-transistor and photovoltaic cell performance in thienoisoindigo derivatives has been studied. Thienoisoindigo derivatives end-capped with benzothiophene TII(SB)2 and benzofuran TII(OB)2 together with benzothiophene-capped isoindigo II(SB)2 are prepared, and their electronic properties are investigated. The crystal structures of TII(SB)2 and TII(OB)2 are determined by single-crystal X-ray structure analyses. The redox and optical measurements as well as the molecular orbital calculation indicate that thienoisoindigo-based molecules TII(SB)2 and TII(OB)2 have higher HOMO levels and smaller band gaps than II(SB)2. The single-crystal structure analysis reveals that TII(SB)2 and TII(OB)2 have flat form, agreeing well with the structure optimized by the density functional theory (DFT) calculation, and TII(SB)2 and TII(OB)2 form slipped one-dimensional stacks with the alkyl chains extending out of the molecular plane. As an active layer of organic field-effect transistors, TII(SB)2 and TII(OB)2 show one order of magnitude larger p-type carrier mobility than that of II(SB)2. It is noted that TII(SB)2 and TII(OB)2 fabricated on a tetratetracontane (TTC) modified substrate show balanced ambipolar properties (μh ≈ μe ≈ 10−2 cm2 V−1 s−1), where the carrier balance comes from well delocalized frontier molecular orbitals (FMOs). The photovoltaic properties of TII(SB)2, TII(OB)2, and II(SB)2 are investigated in bulk heterojunction devices using PC71BM. The devices show a photovoltaic efficiency up to 2.4% for TII(OB)2 and 1.4% for TII(SB)2. The device performance is closely associated with the flat structure of the thienoisoindigo unit, which effectively minimizes the steric interference of the benzothiophene and benzofuran units to facilitate the slipped co-facial π–π stacking.