Quinoidal propylenedioxythiophene dimers for air-stable n-type semiconductors: achieving crystallinity and solution processability

Abstract

Rapid expansion of the Internet of Things has fueled a growing need for environment-friendly organic semiconductor materials, circumventing the use of rare metals. A key area of focus is the development of n-type semiconductors that, similar to their well-established p-type counterparts, exhibit air stability and solution processability. Synthesizing high-performance, air-stable n-type semiconductor thin films requires a delicate balance between a π-conjugated framework, which facilitates efficient intermolecular interactions for charge transport, and the incorporation of side chains to enhance solubility for energy-efficient solution processing. This study departs from traditional small molecule and polymer approaches by exploring oligomer backbones, which offer greater structural design flexibility, to create novel n-type semiconductor materials. Specifically, inspired by the doped poly(3,4-ethylenedioxythiophene) family, we introduced a dicyanomethylene end-capped quinoidal (q) structure into propylenedioxythiophene (P) oligomers to induce n-type semiconducting behavior. We synthesized the shortest dimers, q2P and q2P^Hex, by incorporating dimethyl and dihexyl groups as the side chains, respectively. The differing side chains influenced both solubility and crystal structure, leading to the formation of two types of crystalline thin films with effective intermolecular interactions. Field-effect transistor characterization of these thin films demonstrated stable operation in air. Notably, q2P^Hex, with its higher crystallinity, exhibited a mobility 1000 times greater than that of q2P. These results demonstrate the successful achievement of n-type semiconductor characteristics with an excellent balance of crystallinity, solution processability, and air stability.

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Introduction

Organic electronics, such as organic field-effect transistors (OFETs),^1–5 memory devices,^6,7 organic solar cells,⁸ and sensors,^9,10 based on organic semiconductors have been rapidly developing.¹¹ In organic semiconductors used for OFETs, p-type organic semiconductors, which transport holes as carriers, have been developed and have potential practical applications owing to their high crystallinity and good solution processability.^12–14 However, n-type organic semiconductors, which transport electrons, must overcome air stability limitations, and their highest mobility values are approximately one-third those of p-type organic semiconductors.¹⁵ Ambipolar organic semiconductors, which transport both holes and electrons, also lack sufficient air stability and carrier mobility.^16–18 Thus, the practical application of n-type and ambipolar organic semiconductors, which are necessary for the further development of organic electronics, is challenging, and further development is desirable.

In addition to possessing appropriate highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels, depending on the p-, n-, and ambipolar types, organic semiconductors must maintain high crystallinity and effective intermolecular interactions with few defects in the thin film state during film formation. Furthermore, from recent energy-consumption reduction perspective, solubility is important to enable solution processing. Polymers such as doped poly(3,4-ethylenedioxythiophene) (PEDOT)¹⁹ tend to have excellent film formability, conductivity, and thermal stability. However, structural disorder occurs due to variations in the molecular weight distribution. Therefore, the challenge in elucidating precise structure–property relationships hampers the development of molecular design guidelines for high carrier transport properties.^19–24

In contrast, for small molecules, although vacuum deposition method is often used to fabricate devices, a shift to solution-coating device-fabrication process is desired from an energy cost perspective. However, the solution-coating device-fabrication process face challenges such as film uniformity and solution applicability.²⁵

Oligomer materials, which are positioned between small molecules and polymers, have attracted much attention.^26–35 Oligomers are single-molecular-weight molecules consisting of a repeating structure of building blocks, and their electronic structure can be designed by precisely adjusting the type, number (oligomer length), and arrangement (sequence) of the building blocks. The flexible design of the molecular structure provides the solubility required for the solution-coating process and enables the adjustment of the stacking arrangement through spatial control. In particular, doped ethylenedioxythiophene (EDOT)-based oligomers, discrete models for doped PEDOT, have a highly planar molecular structure, facilitating the formation of densely stacked molecular structures such as π–π stacking, which is advantageous for obtaining efficient intermolecular interactions. Furthermore, EDOT-based oligomers have the synthetic advantage of avoiding cis/trans conformational mixing, which is a problem with bare thiophene-based materials.^36–50 Recently, we developed EDOT-based oligomers and observed a relationship between the conduction properties and the crystal structure of their radical cation salts.^26–32 In these studies, we demonstrated that EDOT-based oligomers have high crystallinity in both neutral and radical cation salt forms and that the stacking arrangement can be controlled through the arrangement structure of their monomer units.^29,30 In terms of optimizing carrier transport properties, undoped (neutral) oligomers show potential for structural controllability. Furthermore, as oligomers are located between small molecules and polymers, they exhibit the potential of both semiconducting small molecule and polymer materials; that is, oligomer semiconductors enable intermolecular carrier transport similar to small molecular conductors or hopping-modelled polymer conductors and enable intramolecular transport similar to polymer materials.⁵¹ Therefore, the relationship between conjugation length and transport properties by systematically varying the oligomer length and other structural parameters of oligomer semiconductors may help elucidate the optimal molecular structures for high-performance organic semiconductors.

However, as neutral EDOT-based oligomers with suitable structures for such molecular stacking are generally electron-rich, their HOMO levels increase with elongated oligomer-length, making them susceptible to oxidation and unstable.^52–54 Although, variety in electronic structure and carrier transport properties associated with oligomer-length elongation are of interest, the evaluation is still limited with only optical and electrochemical properties being evaluated in solution.^35,54 Therefore, by focusing on an electron-deficient quinoidal structure, we expected that EDOT-based oligomers could be realized as n-type organic semiconductors and their oligomer-length elongation effects could be systematically investigated. Following the molecular structure of tetracyanoquinodimethane (TCNQ),⁵⁵ a typical example of an electron-acceptor, EDOT-based oligomers are converted to the electron-deficient quinoidal structure by introducing dicyanomethylene groups at both ends of the molecules, which allows for the systematic investigation of the changes in the electronic structure associated with oligomer-length elongation. To the best of our knowledge, no EDOT-based oligomers have been reported to be applied as n-type organic semiconductors yet. Thus, their applicability to devices requiring solution processability and air stability has not been clarified. In particular, as quinoidal neutral oligomers consisting of a rigid π-conjugated core with dicyanomethylene substituents exhibit poor solubility,^39,56 ensuring solubility is important in both the synthesis and solution coating processes. In addition to oligomer-length elongation and the introduction of dicyanomethylene substituents, side chains are needed to improve solubility; however, the introduction of such side chains may interfere with intermolecular interactions. Therefore, the key is a molecular design of suitable side chains for quinoidal oligomers.

In this study, for application to air-stable n-type and ambipolar organic semiconductors, we designed quinoidal EDOT-based oligomers (Fig. 1) and predicted their electronic structure change with oligomer-length elongation via a theoretical study. As the shortest dimer models, we synthesized and investigated their physical properties, crystal structures, and carrier transport properties. To analyze the influence of side chains on the molecular stacking structure^57–59 of dimers in detail, we introduced dimethyl and di-n-hexyl substituents as side chains and established a molecular design guideline suitable for quinoidal EDOT-based oligomers. This design, which combines crystallinity and solubility, is expected to contribute significantly to the development of high-performance and solution processable n-type organic semiconductors.

Fig. 1 Molecular structures and design strategies employed in this study.

Results and discussion

Molecular design and density functional theory (DFT) calculation

First, the electronic structures of quinoidal EDOT-based oligomers were investigated. A thiophene unit (P) with a propylenedioxy substituent was employed as a monomer unit, to which alkyl chains can be easily introduced. The change in electronic structure with oligomer-length elongation was systematically investigated through DFT calculations using Gaussian 16, RB3LYP/6-31G+(d)⁶⁰ (qnP; Fig. 2). Consequently, the shortest dimer (q2P) exhibited a low LUMO level, indicating strong electron acceptivity. Air stable n-type properties can be obtained under atmospheric conditions when the LUMO level is in an energy region below approximately −4.0 eV.⁶¹ The LUMO level of q2P was observed in this region, indicating that q2P exhibits air-stable n-type semiconductor properties. Interestingly, these deep LUMO levels were maintained with little effect on oligomer-length elongation, which was derived from the distribution of the LUMOs delocalized over the entire molecules. These LUMOs had nodes on the bonds connecting each unit in the oligomer-length elongation direction, and the conjugated system expansion by oligomer-length elongation was accompanied by an increase in the number of nodes.^63–65 This result indicates that qnP can be utilized as electron-transporting materials. However, as the oligomer length was elongated, the levels of HOMO, also delocalized over the entire molecules, increased significantly to a region that does not compete with water-oxidation reactions.⁶² This indicates that qnP (n = 3–5) can also exhibit hole-transporting properties and can help realize air-stable ambipolar organic semiconductors.

Fig. 2 (a) and (b) Optimized molecular structures of qnP (n = 2–5) and q2P^Hexvia DFT calculations using Gaussian 16, RB3LYP/6-31G+(d): (a) side view and (b) top view. (c) Calculated LUMO and HOMO orbitals with energy levels. Ball and stick drawings are shown. Yellow: S, red: O, grey: C, blue: N, white: H. Blue blurred line represents the LUMO level limitation, below which levels are required for achieving air-stable n-type organic semiconductors.⁶¹ Red blurred line represents the HOMO level limitation, above which levels are required for achieving air-stable p-type organic semiconductors.⁶² When both the HOMO and LUMO are in these regions (HOMO > approximately −5.6 eV and LUMO < approximately −4.0 eV), air-stable ambipolar characteristics can be observed.^16,62

Based on these results, from the qnPs that exhibited almost the identical LUMO levels, we synthesized the shortest oligomers, dimers (i.e., compounds q2P and q2P^Hex), and investigated their electronic structure and carrier transport properties. This is because these compounds are considered to have high air stability and are synthetically accessible. Here, in addition to dimethyl substituents (i.e., q2P) with relatively low steric bulk as side chains, di-n-hexyl substituents were introduced to improve solubility (i.e., q2P^Hex). The HOMO and LUMO levels of q2P^Hex were slightly higher than those of q2P due to the higher electron-donating ability of the n-hexyl substituent compared to the methyl substituent (Fig. 2). We elucidate the effect of the side chains on the crystal structure and explore a design that achieves a balance between solubility and crystallinity, which is required for thin-film semiconductor materials.

Synthesis

Syntheses of q2P and q2P^Hex are shown in Scheme 1. Compounds 1^35,66–70 were used for the bromination with N-bromosuccinimide (NBS) to afford dibromo compounds 2. The quinoidal dimers (q2P and q2P^Hex) were synthesized via a reaction with malononitrile using sodium hydride and palladium catalysts followed by air oxidation. Both dimers were stable in air and had solubility that enabled purification with organic solvents (chloroform, chlorobenzene, and dichloromethane; see ESI†). In particular, q2P^Hex had higher solubility than q2P.

Scheme 1 Syntheses of q2P and q2P^Hex. dppf = 1,1′-bis(diphenylphosphino)ferrocene.

Electrochemical properties

The electronic structures of the synthesized dimers were experimentally elucidated from electrochemical measurements. The electron-accepting properties of q2P and q2P^Hex predicted through DFT calculations was confirmed via cyclic voltammetry (CV) measurements in a dichloromethane solution (Fig. S1 (ESI†) and Table 1). The first reduction potentials (E₁) of q2P and q2P^Hex were −0.800 and −0.818 V (vs. Fc/Fc⁺), respectively. The LUMO levels of q2P and q2P^Hex estimated from E₁ were −4.00 and −3.98 eV. These results are consistent with the DFT calculations. Additionally, the second reduction potentials (E₂) of q2P and q2P^Hex were −0.922 and −0.932 V (vs. Fc/Fc⁺), respectively. The first- and second-reduction waves were reversible and reproducible, indicating that q2P and q2P^Hex have high stability in the reduction reaction in the solution state, applicable for n-type organic semiconductors.

Table 1 Optical data, reduction potentials, and HOMO/LUMO levels of q2P and q2P^Hex

Compound	λ edge (nm)	Optical gap (exp.)^a (eV)	Optical gap (cal.)^b (eV)	E 1 (V)	E 2 (V)	LUMO (exp.)^d (eV)	LUMO (cal.)^b (eV)	HOMO (exp.)^e (eV)	HOMO (cal.)^b (eV)
a Estimated from λedge. b DFT calculation. c Half-wave potential. d Estimated from E1 based on the HOMO level (−4.8 eV) of ferrocene. e Estimated from the experimental LUMO level and optical gap.
q2P	626	1.98	2.14	−0.800	−0.922	−4.00	−4.05	−5.98	−6.19
q2P^Hex	626	1.98	2.12	−0.818	−0.932	−3.98	−3.97	−5.96	−6.09

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Optical properties

Although the LUMO levels of q2P and q2P^Hex could be estimated experimentally via CV measurements, the HOMO levels could not be determined due to CV measurement limitations, considering the potential window of the solvent. Therefore, the HOMO levels were estimated indirectly from the HOMO–LUMO gaps calculated from the ultraviolet-visible (UV-vis) absorption spectra of the dichloromethane solution. Both q2P and q2P^Hex had similar spectra with a strong absorption band at approximately 550 nm (Fig. S2, ESI†). These bands can be attributed to the HOMO–LUMO transition as predicted from the time-dependent DFT (TD-DFT) calculations (Table S1, ESI†). Furthermore, split bands were observed primarily due to the vibronic structures, implying that these compounds have a rigid π-skeleton.^52,53,71 Absorption edges (λ_edge) of these bands were 626 nm, and the optical gaps estimated from λ_edge were 1.98 eV. The HOMO levels of q2P and q2P^Hex calculated from these optical gaps and the LUMO levels determined via CV measurements were estimated to be −5.98 and −5.96 eV, respectively, demonstrating good agreement with HOMO–LUMO gaps and HOMO levels obtained from DFT calculations (Table 1). The good agreement between the DFT calculations and the experimental results of these dimers suggests that oligomers above the trimer predicted by the DFT calculations can realize ambipolar semiconductor properties. Experimental investigation of the electronic structures of molecules using a combination of CV and UV-vis absorption will be useful for oligomer oligomer-length elongation in the future.

Crystal structure analysis and transfer integral calculations

Encouraged by the demonstrated desirable electronic and physical properties of q2P and q2P^Hex for semiconductor materials obtained via CV and UV-vis measurements, we investigated their stacking arrangements in single crystals required for carrier transport. To address the structural insights in their packing modes, good quality single crystals of q2P and q2P^Hex were obtained via recrystallization by employing the slow cooling method of chlorobenzene solution and liquid–liquid diffusion method of dichloromethane/methanol, respectively (see ESI†). The crystal structures and crystallographic data of q2P and q2P^Hex are presented in Fig. 3, Fig. S3 and Table S2 (ESI†). The crystallographically independent molecules were a half of q2P and a half of q2P^Hex for q2P and q2P^Hex, respectively. In the crystal structures, the core systems containing bithiophene, oxygen atoms in the 3,4-propylenedioxy group, and dicyanomethylene groups were almost in the same plane, forming π-core systems, consistent with the frontier orbital shapes delocalized over the molecules (Fig. 2). The intramolecular S⋯O distance between each P unit and C⋯O distance between the P unit and the dicyanomethylene substituent were below the sum of van der Waals radii (S⋯O: 3.3 Å, C⋯O: 3.2 Å).⁷² Such intramolecular atomic interactions, called conformational lock, provide rigidity and high planarity to the π-core and are beneficial for effective intermolecular interactions.^{42–46,49–53} In contrast, the effect of the 3,4-propylenedioxy substituents, 3 and 4 positions of the thiophene ring, including the side chains was prominent in these crystal structures. A remarkable difference was observed in the conformation of the seven-membered ring. While the seven-membered ring of q2P formed a twist conformation, the seven-membered ring of q2P^Hex formed a chair conformation. Owing to this chair conformation of q2P^Hex, one of the two hexyl substituents was aligned in the same plane of the π-core and the other was aligned perpendicular to the π-core. The hexyl chain in the same plane as the π-core was disordered by the three terminal carbon atoms. In contrast, the hexyl chain perpendicular to the π-core exhibited an intermolecular interaction with the hexyl chain of the molecule in the adjacent layer, possibly inducing the fastener effect.⁷³ The π–π distance was 3.55 Å for q2P with the twisted conformation and was slightly longer than that of q2P^Hex with the chair conformation (3.49 Å). With regard to the π-stacking arrangement, both molecules were stacked avoiding the bulky side chain and slipped along the long-molecular direction by 5.44 and 4.56 Å for q2P and q2P^Hex, respectively (Fig. 3e, f and Fig. S3, ESI†).

Fig. 3 (a) and (b) Molecular structures in capped sticks drawings, (c)–(f) relationships between neighboring molecules, and (g) and (h) crystal structures of q2P and q2P^Hex. ORTEP drawings (50% thermal ellipsoid) are shown for (c), (d) and (h) the top side molecules, (e) and (f) the upper side molecules, and (g) the center column. Capped sticks drawings are shown for (c), (d) and (h) the bottom side molecules, (e) and (f) the lower side molecules, and (g) the side columns. Yellow: S, red: O, grey: C, pale blue: N. Hydrogen atoms are omitted for clarity. The orange, red, and pale blue dotted lines represent intramolecular S⋯O, C⋯O, and intermolecular C⋯N contacts, respectively. (e) and (f) The π–π distances of q2P and q2P^Hex are 3.55 and 3.49 Å, respectively, which were measured between the mean planes of 24 atoms involved in two thiophene rings, two dicyanomethylene substituents, and four oxygen atoms. The molecules are slipped along the long-molecular direction by 5.44 and 4.56 Å for q2P and q2P^Hex, respectively. (g) The zig-zag angle between two neighboring q2P molecules. The transfer integrals t_b, t_p1, and t_p2 are estimated to be 77, 7.1, and 7.1 meV for q2P, respectively. The transfer integral t_a is estimated to be 54 meV for q2P^Hex, while the transverse integrals are less than 1 meV.

These differences in the molecular structure and displacement affected the packing arrangement. In q2P, the side chains interlocked to fill the space when packing densely. Additionally, C⋯N contacts (3.19 Å) shorter than the sum of van der Waals radii (C⋯N: 3.3 Å)⁷² were found on the carbon and nitrogen atoms at the dicyanomethylene substituent between two adjacent molecules. The contacts were supported by the d_e mapping in the Hirshfeld surface analysis⁷⁴ based on the single-crystal structure, where d_e denotes the distance from the Hirshfeld surface to the nearest nucleus outside the surface (Fig. 4a). These intermolecular contacts led to the formation of a pitched π-stack arrangement (Fig. 3g). Notably, the Hirshfeld surface analysis also showed the dense packing via the intermolecular invasions of dimethylpropylene substituents to the molecular voids in the trans-conformer potentially locked by the presence of propylenedioxy groups and intramolecular atomic contacts, thereby separating the π-core layer and the side chain layer in contrast to the conventional alkyl chain separating strategy.^59,75,76 Contrarily, in q2P^Hex, one hexyl chain extended in the π-plane was located near the dicyanomethylene group of the molecule in the adjacent column, thereby avoiding the formation of intermolecular C⋯N contacts and leading to the formation of a π-stack arrangement. The molecule formed a windmill-like structure via intermolecular interactions involving hexyl chains; the chain in the π-plane interacted with the dicyanomethylene groups in the neighboring molecule, while the chain perpendicular to the π-plane facilitated alkyl–alkyl association with the other chains in an intercolumnar manner, possibly isolating the intracolumnar electronic interactions (Fig. 3h and 4b).

Fig. 4 Hirshfeld surface analyses (d_e mapping) of (a) q2P and (b) q2P^Hex. Here, d_e indicates the distance from the Hirshfeld surface to the nearest nucleus outside the surface. The black dotted circle represents intermolecular C⋯N contact. The details are shown in Fig. S4 (ESI†).

Based on the single-crystal structures, transfer integrals between LUMOs between the neighboring molecules were estimated using the Amsterdam density functional (ADF) program (GGA: PW91, TZP),⁷⁷ revealing the electronic structures essential for carrier transport (Fig. 3g and h). For q2P, the transfer integral t_b in the stacking direction was estimated to be 77 meV, and the transfer integrals t_p1 and t_p2 between the stacks were estimated to be 7.1 meV, indicating higher dimensionality than the intrinsic one dimensional (1D) electronic structure. Although t_p1 and t_p2 were approximately a factor of ten smaller than t_b, the intermolecular C⋯N contacts enabled the LUMO delocalized on the entire molecule including dicyanomethylene substituents at both ends (Fig. 2), contributing to the intermolecular interaction between the columns. In contrast, for q2P^Hex, the transfer integral t_a in the intracolumnar π-stacking direction was estimated to be 54 meV. This value is slightly smaller than the t_b value of q2P, although the π–π distance was shorter than that of q2P. This maybe because the displacement in the π-plane between the neighboring molecules causes differences in the overlap of the phase of the molecular orbitals. Between the adjacent stacks, intercolumnar transfer integrals were at most approximately one-hundredth (<1 meV) as large as that of t_a and contributed negligibly to intermolecular interactions. Therefore, q2P^Hex formed a 1D electronic structure. Thus, the difference in the side chains significantly affected the packing and electronic structure, along with the solubility difference.

Thin-film formation for fabrication of OFET devices

We fabricated q2P and q2P^Hex to form thin films appropriate for device applications and investigated their carrier transport properties. Although q2P exhibited some degree of solubility (as described in the synthesis section), it was not sufficient to form highly ordered thin films via solution coating. Thus, the q2P film was fabricated on Si/SiO₂ substrates pretreated with parylene C via vacuum deposition. In contrast, the q2P^Hex film, which showed good solubility in anisole and o-dichlorobenzene (ODCB), was fabricated via a blade-coating method using their solutions. Atomic force microscopy (AFM) was performed to investigate the crystallinity and homogeneity of the resulting thin films (Fig. 5a, b, and Fig. S5, ESI†). The q2P film was a polycrystalline thin film consisting of microcrystals less than a few hundred nanometers in size and possessing several grain boundaries, with a root mean-square roughness (R_rms) of 1.65 nm. In contrast, the q2P^Hex film was a relatively large and flat crystalline thin film composed of multiple crystalline domains, with an R_rms of 1.07 nm in one domain. The high flatness and crystallinity of the film were confirmed by crossed-Nicols polarized images (Fig. 5c, d and Fig. S6, ESI†). The molecular orientations in the crystalline thin films were determined using powder X-ray diffraction (PXRD) patterns (Fig. 6). Although the q2P film exhibited a low-intensity pattern (Fig. 6a), 2θ was estimated from the lowest angle peak to be 10.14°, corresponding to the d-space of 8.723 Å. This d-space is the half of the c-axis of the unit cell, reflecting the crystal structure and space group P2₁/c determined by single crystal structure analysis. This indicates that the molecules in the q2P vacuum-deposited film were preferentially deposited in the c-axis direction with respect to the substrate, and the ab-plane (molecular stacking direction) was parallel to the substrate. However, such low intensity and broad peaks suggest that the q2P film was less ordered and nearly amorphous. In contrast, the q2P^Hex film exhibited a sharp peak pattern (Fig. 6b), indicating that the film was highly ordered. Crystallinity was not affected by the additional annealing process after coating (Fig. S7, ESI†). The d-space estimated from the lowest angle peak (2θ = 5.34°) was 16.55 Å, which corresponded to the height of the unit cell. These results indicate that the ab-plane in the blade-coated q2P^Hex film was predominantly parallel to the substrate similar to q2P. These molecular orientations are preferable for effective conduction, because orientations with significant columnar interactions, along with the transverse interactions for q2P^Hex, can be located parallel to the FET channels between the source and drain electrodes (Fig. 6c). The high crystallinity and wide-sized crystalline domain of the blade-coated q2P^Hex film, despite the lower transfer integral values and dimensionality, impart high carrier transport properties.

Fig. 5 AFM images of a (a) vacuum-deposited q2P film and (b) blade-coated q2P^Hex film. Crossed Nicols polarized micrographs of the blade-coated q2P^Hex film in (c) bright and (d) dark images.

Fig. 6 PXRD patterns of (a) the vacuum-deposited q2P film and (b) blade-coated q2P^Hex film. The black lines are simulated 00l diffraction patterns based on the single-crystal structural analysis. The left and right axes represent the experimental and simulated intensity, respectively. (c) Illustration of the molecular orientation of q2P^Hex in an active later. The ab-plane in the crystal structure is parallel to the substrate.

Field-effect transistor properties

The carrier transport performance of the q2P and q2P^Hex films as n-type semiconducting layers in FET devices was examined. To this end, we deposited the thin films in the active layer of bottom-gate/top-contact type FET devices and measured the electron mobility under atmospheric conditions. The mobility of the q2P film in air was evaluated to be 3.2 × 10⁻⁵ cm² V⁻¹ s⁻¹ (maximum mobility: 3.4 × 10⁻⁵ cm² V⁻¹ s⁻¹; Fig. 7a, Fig. S9 and Table S3, ESI†), which is almost identical to that in vacuum (3.5 × 10⁻⁵ cm² V⁻¹ s⁻¹; maximum mobility: 3.6 × 10⁻⁵ cm² V⁻¹ s⁻¹). In contrast, the q2P^Hex thin film exhibited mobilities of 7.5 × 10⁻³ cm² V⁻¹ s⁻¹ (maximum mobility: 1.9 × 10⁻² cm² V⁻¹ s⁻¹) in air, 1.6 × 10⁻² cm² V⁻¹ s⁻¹ (maximum mobility: 2.6 × 10⁻² cm² V⁻¹ s⁻¹) in vacuum, and 4.6 × 10⁻² cm² V⁻¹ s⁻¹ in a nitrogen atmosphere (Fig. 7b, Fig. S8, S9 and Table S3, ESI†). The mobility of q2P^Hex was approximately 1000 times higher than that of q2P. This result indicates that the high crystallinity of the q2P^Hex film overcomes the low value of the transfer integrals and the low dimensionality of the intermolecular interactions. For comparison, OFET properties of spin-coated q2P^Hex films were investigated. The mobilities of spin-coated q2P^Hex films were 1–2 orders of magnitude lower than those of blade-coated q2P^Hex films due to the lower quality of the spin-coated films compared to the blade-coated films (Fig. S11, S12 and Table S5, ESI†).

Fig. 7 OFET properties of (a) q2P (vacuum-deposited film, maximum mobility (μ_max): 3.6 × 10⁻⁵ cm² V⁻¹ s⁻¹) and (b) q2P^Hex (blade-coated film from 0.15 wt% anisole solution, μ_max: 2.6 × 10⁻² cm² V⁻¹ s⁻¹). OFET properties of q2P^Hex film using 0.15 wt% ODCB solution (μ_max: 4.6 × 10⁻² cm² V⁻¹ s⁻¹) are shown in Fig. S8 (see ESI†). V_D = 80 V. The red and blue lines represent transfer characteristics in vacuum and air, respectively. The tangent lines for the V_G–(I_D)^1/2 curves were shown in black dashed lines. The relatively high V_th may originate from the film quality around the channel electrodes and the large energy barrier between the LUMO levels of these compounds (–4.05 eV for q2P; –3.97 eV for q2P^Hex; Fig. 2) and the work function of Au (approximately 5.1 eV relative to vacuum level).

Notably, the OFET maintained stable performances in air for more than one month for both films; approximately half and 90% of the initial mobilities were maintained for q2P and q2P^Hex, respectively (Fig. 8, Fig. S10 and Table S4, ESI†). The high air stability may be attributed to the molecular designs with deep LUMOs, which were experimentally confirmed from the CV measurements. Specifically, q2P^Hex exhibited well-balanced characteristics with high crystallinity, good solution processability, and comparatively high mobility and is applicable to air-stable n-type organic semiconductors.

Fig. 8 Air stabilities of q2P and q2P^Hex OFETs. The open red triangle and filled red triangles represent the mobilities of q2P in vacuum and air, respectively. The open blue circle and filled blue circles represent the mobilities of q2P^Hex in vacuum and air, respectively.

Conclusions

We developed quinoidal EDOT-based oligomers and demonstrated their applications as n-type organic semiconductors. DFT calculations suggest that each oligomer qnP (n = 2–5) can exhibit air-stable electron-transport properties, as their deep LUMO levels were estimated to be approximately −4.0 eV. Electrochemical, optical, crystallographic, and OFET properties were investigated for the shortest dimers. Crystal structure analysis revealed that q2P exhibited a pitched π-stack arrangement with intermolecular interactions with dimensionality higher than 1D, whereas q2P^Hex exhibited a π-stack arrangement with 1D intermolecular interactions. In both cases, intramolecular atomic contacts were observed, leading to rigid planarity due to the effect of conformational lock. Evaluation of the OFET properties showed that the q2P film (a polycrystalline thin film consisting of extremely fine crystals) fabricated via vacuum deposition exhibited an electron mobility of approximately 10⁻⁵ cm² V⁻¹ s⁻¹. Contrarily, the q2P^Hex film (a good quality crystalline thin film comprising multiple crystalline domains) fabricated via the blade-coating method exhibited an electron mobility of approximately 10⁻² cm² V⁻¹, which is 1000 times higher than that of q2P. The investigation of the change over time of the mobilities of these films in air storage over one month showed that the films maintained the same level of the initial mobilities, indicating air stability. Thus, this study demonstrated that quinoidal EDOT-based dimers exhibit good solution processability and air stability and form crystalline thin films, achieving good crystallinity and solubility, which are important for organic semiconductors.

First-principles calculations predicted that the LUMO levels were maintained suitable for n-type organic semiconductors, while the HOMO levels increased significantly with increasing oligomer length. Above the trimer, the electronic states were predicted; this can help realize air-stable ambipolar organic semiconductors. Considering the optimized side-chain structure and the high crystallinity and effective intermolecular interactions demonstrated in this study, EDOT-based oligomers show potential as next-generation high-performance electron transport materials and are expected to expand into ambipolar semiconductors. Future innovations in semiconductor materials are expected through systematic structure–transport property correlation studies using various structural parameters such as oligomer length and arrangement of EDOT-based oligomers.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the ESI.† Crystallographic data for q2P has been deposited at the CCDC under 2413270 and can be obtained from https://doi.org/10.5517/ccdc.csd.cc2m06f9. Crystallographic data for q2P^Hex has been deposited at the CCDC under 2413271 and can be obtained from https://doi.org/10.5517/ccdc.csd.cc2m06gb.

Acknowledgements

This work was partially supported by JSPS Grants-in-Aid for Scientific Research (grant numbers: JP25K18085 to H. N.; JP22H04523, JP25K01841 to T. F.; JP22H00106 to H. M.), Transformative Research Area (B) “Multiply Programmed Layers” (grant number: JP25H01403 to T. F.; JP25H01405 to T. H.), Core-to-Core Program “Emergent Quantum Electronics in Molecular Layers” (grant number: JPJSCCA20240001 to T. F. and H. M.), JST PRESTO (grant number: JPMJPR22Q8 to T. F.). We thank Assoc. Prof. Jun-ichi Yamaura, the Institute for Solid State Physics, the University of Tokyo, for allowing us to use a powder X-ray diffractometer to measure PXRD patterns.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2413270 and 2413271. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5tc01735a

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