Higher conductivity in doped ethylenedioxythiophene (EDOT) dimers with chalcogen-substituted end groups†

Kota Onozuka ^a, Tomoko Fujino *^a, Tatsuya Miyamoto ^b, Takashi Yamakawa ^b, Hiroshi Okamoto ^b, Hiroshi Akiba ^a, Osamu Yamamuro ^a, Eiichi Kayahara ^c, Shigeru Yamago ^c, Hiroshi Oike ^d and Hatsumi Mori *^a
aThe Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan. E-mail: fujino@issp.u-tokyo.ac.jp; hmori@issp.u-tokyo.ac.jp
^bDepartment of Advanced Materials Science, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan
^cInstitute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan
^dPRESTO, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan

First published on 7th August 2024

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Introduction

As next-generation materials for electronic devices, organic conductors are gaining attention owing to their light weight, flexibility, and solution processability in printing technologies.¹ Among these materials, conductive polymer, doped poly(3,4-ethylenedioxythiophene) (PEDOT; Fig. 1a) has been extensively employed in industries owing to its properties such as high conductivity and solution processability, making it suitable for various applications including solid electrolyte capacitors, antistatic coatings, organic electroluminescence devices, and organic solar cells.^2,3 However, achieving high conductivity typically necessitates post-treatment with solvents or strong acids.^4,5 Additionally, obtaining detailed structural information about polymer materials is challenging owing to their broad dispersity,³ which makes it arduous to prepare single crystals. This structural inhomogeneity also makes it difficult to understand the conduction mechanism at an atomic level and design molecules based on it.

Consequently, monodisperse oligomer conductors have garnered significant interest as promising candidates for organic conductors to address these issues.^7,8 The chain length of oligomers can be precisely controlled through organic synthesis, facilitating the formation of single crystals. These crystals are instrumental in comprehending the conduction mechanism and designing molecules based on the structure–property relationship.

Furthermore, doped oligomers offer numerous structure control parameters that enable controlling electronic structures and physical properties. These parameters include the type, number, and sequence of constituent units. Additionally, end groups present promising avenues for structural control to fine-tune electronic structures and physical properties. Since the volume fraction of end groups is larger in oligomers than in polymers, the contribution of electronic effects of end groups is also larger in oligomers than in polymers.⁹

Recently, our group reported on single-crystalline 3,4-ethylenedioxythiophene (EDOT) oligomers in radical cation salts as a discrete oligomer model of doped PEDOT.^6,10–13 Single-crystal salts 2O·X (X = BF₄, ClO₄, and PF₆; Fig. 1b)⁶ reveal one-dimensional (1D) donor stacking structures, with room-temperature resistivities (ρ_rt) ranging from 280–43000 ohm cm (Fig. 1b). These elevated ρ_rt values result from the Mott-insulating state, where carriers are localized owing to the effective Coulomb repulsion between conducting carriers (U_eff).^14,15 To deviate from the Mott-insulating state and improve conductivity, the Mott gap should be decreased by increasing intermolecular orbital overlapping (W: bandwidth) or decreasing U_eff.¹⁶ We previously reported the reduction of U_eff by expanding the conjugate system achieved by extending the oligomer chain length, which led to increased conductivity. For instance, elongating from a dimer (2O) to trimer (3O) resulted in a tenfold increase in the conductivity of 3O·PF₆(CH₂Cl₂)_n in relation to that of 2O·PF₆, despite the weakly dimerized structure of 3O·PF₆(CH₂Cl₂)_n.¹⁰ More recently, we elongated the chain length to a tetramer (4PS) using a mixed sequence and further modulated the band-filling in a 4PS·(PF₆)_1.2(solvent)_m by controlling the stacking structure. These combined effects led to a six-order of magnitude improvement in room-temperature conductivity in relation to that of 2O·PF₆ and the realization of a metallic state above room temperature.¹² Although a decrease in U_eff was achieved by increasing the chain length and band-filling modulation, the W of 4PS·(PF₆)_1.2(solvent)_m decreased to 0.41 eV because of the increased stacking distance and one unit slipping stacking for 4PS·(PF₆)_1.2(solvent)_m. Furthermore, enhancing conductivity at room temperature necessitates an increase in W and a decrease in U_eff, thereby necessitating a decrease in U_eff/W.

In this study, we focused on examining the effect of little-explored end groups on the reduction in U_eff/W among oligomer-based conductors. Although previous studies have delved on synthesizing various end groups in EDOT oligomers, such as phenyl and alkyl groups,^8,17,18 their focus has been on synthetic methods or radical cation behavior in solutions. The impact of end groups on physical properties in solids, particularly in single crystals, has been largely overlooked,⁸ thereby impeding efforts to control physical properties via electron correlation parameters (U_eff/W). In our investigation, we explored the effect of end groups on a doped EDOT dimer by replacing methylthio (SMe) groups with methylseleno (SeMe) groups (i.e., replacing S atoms with Se atoms: 2O–2SeMe·X, where X = BF₄ and ClO₄, as shown in Fig. 1c). Notably, this chalcogen substitution led to the expansion of molecular orbital and conjugate area in the doped oligomers, consequently increasing the intermolecular interactions and W, while reducing U_eff and U_eff/W. As a result, the conductivity of the salts increased significantly, similar to the increase in conductivity observed when the chain length is extended from a dimer (2O·X) to trimer (3O·X).¹⁰ This study underscores the potential of tuning the physical properties of oligomer conductors by manipulating the end groups. This approach will promote the formation of diverse crystal structures and electronic states that can improve conductivity, thus offering valuable insights into the design of conductors based on end groups.

Results and discussion

Molecular design

First, we examined the contribution of the end groups of donors to the electronic structures for the SMe-substituted 2O·X single crystals. Previous studies utilizing density functional theory (DFT) calculations have shown that the highest occupied crystal orbital (HOCO) is distributed on the S atoms in the SMe group.⁶ This crystal orbital corresponds to that of singly occupied molecular orbitals (SOMO) of the isolated donor 2O in the radical cation state (Fig. 2a and c),⁶ calculated by the Gaussian 16 software¹⁹ at the (U)B3LYP/6-31G(d) level, potentially stabilizing the molecule in the oxidized state. Therefore, we investigated the effect of substituting chalcogen atoms from S atom of the SMe end groups with heavier Se or Te atoms on conducting properties. DFT calculations of SOMO for SeMe- and TeMe-substituted EDOT dimer (i.e., 2O–2SeMe and 2O–2TeMe) in radical cation forms revealed complete delocalization of the molecular orbitals across the molecules, including on the Se and Te atoms in the end groups (Fig. 2b, d and Fig. S1, S2, Tables S3, S4, ESI†). The SOMO expanded in the order 2O, 2O–2SeMe, and 2O–2TeMe owing to the larger atomic radius from S, Se, to Te atoms. This trend was also observed for another skeleton, 3,4-ethylenedithiothiophene dimer 2S²⁰ with chalcogen-substituted end groups (Fig. S1, S2 and Tables S5, S6, ESI†). The expanded conjugated area provided by the heavier chalcogen substitution is expected to further decrease U_eff and increase W, thereby decreasing U_eff/W to improve conductivity.

Synthetic procedures

The synthesis began with an unsubstituted EDOT dimer 2O–2H (Scheme 1). This compound was lithiated with n-butyllithium and then reacted with dimethyl diselenide, yielding 2O–2SeMe in 80% yield for the two-step transformations (Scheme 1a and Fig. S18, S19, ESI†). The structural integrity of the neutral 2O–2SeMe was confirmed by X-ray diffraction (XRD) analyses of single crystals prepared via liquid–liquid diffusion from ethanol and dichloromethane (Fig. S3 and Table S1, ESI†). Single crystals of the doped dimer 2O–2SeMe·X (X = BF₄, ClO₄) were further prepared by electrocrystallization of 2O–2SeMe in tetrahydrofuran (THF) in the presence of tetra-n-butylammonium tetrafluoroborate or perchlorate as the electrolyte at a constant current of 0.25 μA. The shape of the single crystals was needle-like, and their color was dark green. The synthesis of 2O–TeMe followed a synthetic route similar to that used for 2O–SeMe. However, an insoluble white solid in typical organic solvents was obtained, hindering the subsequent electrolytic oxidation process. To enhance solubility, we substituted a highly soluble donor 2S,²⁰ for 2O, which has a twisted structure in the neutral state, providing the donor with high solubility and stability. Thus, 2S–2Br was lithiated and then TeMe groups were introduced by reaction with dimethyl ditelluride, resulting in 2S–2TeMe in 83% yield for the two-step transformations (Scheme 1b and Fig. S20, S21, ESI†). We then attempted to electrochemically oxidize the donor in the presence of various electrolytes such as tetra-n-butylammonium salts in various organic solvents. However, single crystals of radical cationic salts were not obtained, possibly owing to the instability of the dimer during the electrochemical reaction.

Single-crystal structure of 2O–2SeMe·X

Single-crystal X-ray structure analyses of 2O–2SeMe salts at 293 K revealed a 1:1 composition of the oxidized 2O–2SeMe and the counter anion BF₄⁻ or ClO₄⁻ (Fig. 3a, Fig. S4, S5 and Table 1, Table S1, ESI†),²¹ confirming the charge-transfer degree of 2O–2SeMe donor to be +1 in the salts (i.e., monovalent radical cation 2O–2SeMe^•+). As for 2O–2SeMe·BF₄, the crystal space group is I2/a, which is isostructural to 2O·BF₄.⁶ A half of the donor molecule and a half of the anion were crystallographically independent. The inversion center and the glide plane were at the center of the donor molecule and 1D donor stacking was observed. In contrast, 2O–2SeMe·ClO₄ has a space group of Ia, which differs from those of 2O–2SeMe·BF₄ and 2O·X (X = BF₄, ClO₄).⁶ A single donor molecule and an anion were crystallographically independent without an inversion center at the donor molecules, and a 1D donor stacking structure was observed (Table S1 and Fig. S4, S5, ESI†).

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The atomic-level structures of donor molecules were further analyzed through bond-length analyses. A comparison of the bond lengths of the single-crystal structures with those of the DFT-predicted structures revealed a dependence on the charge-transfer degree (n in Fig. 2h). The DFT-optimized structures of 2O–2SeMe simulated the bond lengths in the neutral state (n = 0), radical cation state (n = 1), and dication state (n = 2) at the (U)B3LYP/6-31G(d) level using the Gaussian 16 software.¹⁹ For n = 0, C–C bond lengths a and c are significantly shorter compared to b and d, indicating the double-bond character (Fig. 2e and Table S7, ESI†), for which the contribution of the benzenoid structure is dominant. For n = 1, bond lengths a–d are comparable owing to the resonance structure (Table S3, ESI†). For n = 2, bond lengths a and c are significantly longer compared to b and d, indicating the single-bond character on the basis of the quinoid structure (Table S8, ESI†). With reference to these predictions, the degree of the bond-length alterations observed in single-crystal 2O–2SeMe and 2O–2SeMe·BF₄ was comparable to that simulated for n = 0 and 1, respectively (Fig. 2f). In contrast, the bond-length alteration for single-crystal 2O–2SeMe·ClO₄ was different from that simulated for n = 1, although the n value was determined to be 1 by the ratio of donor and anion in the single crystals structure (Table 1, Table S1 and Fig. S5, ESI†). Observed lengths a–c are shorter compared to d–g, indicating stronger double-bond character (1.32–1.36 Å) similar to that in the case of n = 0, while d–g were longer (1.41–1.45 Å) similar to that in the case of n = 2 (Fig. 2g), indicating the asymmetric molecular structure. This observation implies intramolecular charge localization, as has been suggested for the elongated oligomer (tetramer) salt with SMe end groups at low temperatures.¹² The charge localization in the donor molecule, which has never been observed in short oligomers such as dimer and trimer salts, can be characteristic of heavier atom-substituted oligomers that may contribute toward expanding the conjugate area (Fig. 2c and d).

The π-stacking distance between donor molecules in 2O–2SeMe·BF₄ is 3.494 Å (Fig. 3b), which is larger than that in 2O·BF₄ (3.462 Å).⁶ Moreover, in 2O–2SeMe·ClO₄, the π-stacking distance is 3.507 Å, which is larger than that in 2O·ClO₄ (3.471 Å)⁶ because of the substitution with larger Se atoms in relation to that with S atoms. The distances between Se–Se atoms in the donor stacking direction are 3.5604(6) and 3.6315(6) Å for 2O–2SeMe·BF₄ and 3.568(1) Å and 3.647(1) Å for 2O–2SeMe·ClO₄ (Fig. 3b and Fig. S6, ESI†). These values are larger than the S–S distances observed for 2O·X (2O·BF₄: 3.521(1) and 3.572(1) Å and 2O·ClO₄: 3.5227(8) and 3.5878(9) Å).⁶ These distances are shorter than double the van der Waals radius, d_SeSe = 3.80 Å and d_SS = 3.60 Å, respectively.²³ The percentage of d_SS to double the van der Waals radius of 2O·BF₄ is 99% and that of d_SeSe for 2O–2SeMe·BF₄ is 95%; the lower percentage for 2O–2SeMe·BF₄ implies increased intermolecular interactions between donors.

First-principles calculations

The increase in intermolecular interactions of 2O–SeMe·BF₄ and 2O–SeMe·ClO₄ in relation to that of their counterparts was theoretically confirmed. We calculated intracolumnar transfer integrals for the two nearest intracolumnar donors (t in Fig. 3b and Fig. S9, Table S2, ESI†) using the Amsterdam density functional (ADF) software,²⁴ which were found to be 285 and 268 meV, respectively. These values are larger than that of 2O·BF₄ (238 meV) and 2O·ClO₄ (250 meV; Fig. S9 and Table S2, ESI†). On the other hand, the intercolumnar transfer integral was less than 1 meV, indicating that 2O–SeMe·X has 1D intermolecular interaction (Fig. S9 and Table S2, ESI†).

The band structures of 2O–2SeMe·X were calculated using the first-principles calculation OpenMX software package,²² where electron correlations near the Fermi level are not considered. The calculation results revealed a metallic 1D band dispersion, the density of states (DOS), and the Fermi surface without a gap at the Fermi level (Fig. S12 and S13 ESI†). The W value, based on the DOS of 2O–2SeMe·BF₄ was 1.153 eV (Fig. 4 and Fig. S12a, ESI†). This value exceeds that of 2O·BF₄ (0.998 eV),⁶ as predicted by the increased t values. The W value based on the DOS of 2O–2SeMe·ClO₄ was 1.133 eV (Fig. S10 and S12b, ESI†), exceeding that of 2O·ClO₄ (0.959 eV).⁶ These results suggest that chalcogen substitution causes an increase in t and the corresponding W possibly owing to the enhanced intermolecular orbital interactions through the Se–Se interactions although the π-stacking distance increased.

Notably, an inversion between the HOMO-derived and HOMO−1-derived bands was observed near −0.4 eV around the Γ point in 2O–SeMe·X (Fig. 4 and Fig. S10, ESI†). This band inversion might result from the expansion of the conjugate area, which highly disperses the HOMO-derived band. The crossing HOMO and HOMO−1 bands may form a multiorbital system,^25–28 potentially contributing to the electronic structures in contrast to the S-substituted analogs.

Additionally, the crystal orbitals of 2O–2SeMe·BF₄ derived from the HOCO band at the Γ point corresponded to the SOMO of the 2O–2SeMe radical cation (Fig. 2d and Table S3, ESI†) and extended to the Se atoms of both end groups. The orbital distribution was also observed for 2O–2SeMe·ClO₄ (Fig. S11, ESI†) in which the charge was localized in an intramolecular manner, as apparent in the bond-length analyses (Fig. 2g). The extended crystal orbitals to the entire donor molecule observed for both 2O–2SeMe·BF₄ and 2O–2SeMe·ClO₄ may expand the conjugate area of donor in relation to those for SMe-substituted 2O·BF₄ and 2O·ClO₄.

Optical characteristics

Considering the extended crystal orbitals encompassing the entire donor molecule, including end groups, substituting chalcogen atoms with heavier, large-sized Se atoms in the π-conjugate system may lead to a decrease in the effective on-site Coulomb interaction (U_eff). To explore these effects, we investigated the Se-substitution effects on U_eff by analyzing polarized infrared (IR) reflectivity spectra of the radical cation salts. We measured the reflectivity of single-crystal 2O–SeMe·BF₄ and 2O–SeMe·ClO₄ at room temperature with respect to the π-stacking direction. The peak top energies associated with the optical Mott gap were determined to be 0.67 eV for 2O–SeMe·BF₄ and 0.64 eV for 2O–SeMe·ClO₄ (Fig. 5 and Table 1). These values for 2O–SeMe·X were lower than those for 2O·X (2O·BF₄:0.74 eV and 2O·ClO₄:0.74 eV).¹¹ These results suggest that the optical Mott gap efficiently decreases, indicating a decrease in the corresponding U_eff. The combined effects of an increase in W and a decrease in U_eff lead to a favorable reduction in U_eff/W, thereby enhancing the conductive properties.

Magnetic properties

Further insights into the electronic structures were gained through magnetic property measurement. The static magnetic susceptibility (χ) of 2O–2SeMe·X (X = BF₄ and ClO₄) polycrystals was measured using a superconducting quantum interference device (SQUID) in the temperature range of 4.2–300 K. For 2O–SeMe·BF₄, χ at 300 K was found to be 9.69 × 10⁻⁵ emu mol⁻¹. These values gradually decreased with decreasing temperature, experiencing a drop of approximately 20% at around 170 K, followed by a rapid increase below 50 K (Fig. S14a, ESI†). For 2O–2SeMe·ClO₄, the χ at 300 K was 1.77 × 10⁻⁴ emu mol⁻¹, which decreased slowly with lowering temperature, reducing by approximately 20% at around 170 K, and then increasing rapidly below 50 K (Fig. S14b, ESI†). To understand the origin of the decrease in χ at around 170 K, single-crystal X-ray structure analysis was conducted below transition temperatures at 123 K for 2O–2SeMe·BF₄ and 133 K for 2O–2SeMe·ClO₄ (Table S1 and Fig. S7, S8, ESI†) The analysis of both salts revealed molecular dimerization where the glide planes disappeared. This stacking dimerization can be attributed to the instability of the 1D electronic structure, as observed for 2O·X.⁶ The remaining χ (approximately 80%) after the transition around 170 K can be attributed to the formation of the multiorbital system^25–28 originating from the inversion of HOMO and HOMO−1 bands (Fig. 4 and Fig. S10, ESI†). The increase observed below 50 K may be due to Curie impurities.

Electrical resistivity

The decrease in U_eff/W due to the heavier chalcogen substitution, as suggested by DFT calculations based on single-crystal structures and optical measurement, effectively reduced electrical resistivity (ρ). The ρ values were measured for 2O–2SeMe·X using the two-probe method. The ρ_rt value of 2O–SeMe·BF₄ was 30 ohm cm (Fig. 6), which is an order of magnitude lower than that of 2O·BF₄, which was 280 ohm cm.⁶ This compound exhibited semiconducting behavior with an activation energy (E_a) of 0.100 eV in the temperature range of 184–300 K (Table 1), a value lower than that of 2O·BF₄ (0.177 eV).⁶ Moreover, a significant increase in ρ was observed at approximately 170 K, attributed to stacking dimerization, as indicated by single-crystal X-ray structural analysis at 117 K and a rapid drop in χ around 170 K. Differential scanning calorimetry (DSC; Fig. S16 and S17, ESI†) also revealed an exothermic peak, suggesting a phase transition in this temperature range for polycrystals. Nearly no hysteresis of resistivity was observed during the cooling and heating processes, suggesting a nearly second-order phase transition accompanying the structural change.

On the other hand, the ρ_rt of 2O–2SeMe·ClO₄ was 350 ohm cm, an order of magnitude lower than that of 2O·ClO₄, which was 4300 ohm cm (Fig. 6 and Table 1).⁶ Semiconducting behavior was also observed for 2O–2SeMe·ClO₄ with E_a = 0.235 eV, exceeding that of 2O·ClO₄ (0.223 eV).⁶ Although the crystal structure analysis of SMe-substituted donor 2O in 2O·ClO₄ was symmetric with a glide plane at the center of the molecule at 293 K,⁶ the Se-substituted donor 2O–2SeMe in 2O–2SeMe·ClO₄ was asymmetric with charge localization in the molecule. This intramolecular charge localization might account for the slightly increased E_a of 2O–2SeMe·ClO₄ in relation to that of 2O·ClO₄.

Comparative discussion of 2O–2SeMe·X with 2O·X

Finally, we discuss the impact of chalcogen substitution in the end groups on syntheses, crystal and electronic structures, and the physical properties. Despite the increased stacking distances from 2O·X to Se-substituted 2O–2SeMe·X (X = BF₄, ClO₄), the introduction of heavier chalcogen atoms led to enhanced intermolecular interactions. DFT calculations also revealed an increased transfer integral and W. Notably, an inversion of the HOMO and HOMO−1 bands around the Γ point was observed in 2O–2SeMe·X owing to the expansion of the conjugate area, resulting in a characteristic multiorbital system in the band structure.^25–28 Moreover, polarized reflectivity spectra, when applying the electric field along each π-stacking direction, indicated a reduction in the optical Mott gap energy in 2O–2SeMe·X in relation to that in 2O·X, leading to a decrease in U_eff. This dual effect of increasing W and decreasing U_eff effectively reduced U_eff/W. As a result, the ρ_rt values of 2O–2SeMe·X decreased by one order of magnitude in relation to that of 2O·X. This one-order decrease is almost similar to the decrease observed from the dimer salt (2O·PF₆) to the trimer salt (3O·PF₆(CH₂Cl₂)_n), extending the conjugate chain length.¹⁰ For the three isostructural salts with I2/a (i.e., 2O·ClO₄, 2O·BF₄, and 2O–2SeMe·BF₄), the electron correlation parameter, U^exp_eff/W [= (W + 2E_a)/W],¹⁴ as suggested by W derived from the first-principles calculations and E_a from resistivity measurement, decreased in the order 2O·ClO₄ (1.47), 2O·BF₄ (1.35), and 2O–2SeMe·BF₄ (1.17) (Table 1). As evident from Fig. 7, the electrical resistivity at room temperature decreased with the lowering of the electronic correlation parameter, U^exp_eff/W, from 2O·ClO₄, 2O·BF₄, to 2O–2SeMe·BF₄, owing to the heavier chalcogen substitution of the end groups and anion effects. These results indicate that increasing the conjugated area due to heavier chalcogen substitution effectively reduces U_eff/W and increases conductivity.

Experimental section

Apparatus

Single-crystal XRD was performed using a Rigaku HyPix-6000HE X-ray diffractometer (Mo Kα, λ = 0.71073 Å). Resistivity measurement was performed using a HUSO HECS 994C instrument with a direct current/voltage generator (ADVANTEST R6142), and digital multimeter (KEYTHELY 2001). Microscopic mid-IR and ultraviolet-visible (UV-vis) spectroscopy was performed using an IRT-5000 (JASCO) equipped with FT/IR-6100 (JASCO) and MSV-5200TSO (JASCO), respectively. Static magnetic susceptibility measurement was conducted using a Quantum Design SQUID magnetometer (MPMS-XL). DSC was performed using a Diamond DSC (PerkinElmer, Inc.).

Materials

The reagents were purchased from commercial suppliers and used as received. 2O–2SeMe and 2S–2TeMe was synthesized in an argon atmosphere (see ESI† for the details). The single crystals of 2O–2SeMe·X were prepared using an electrocrystallization method.

Single-crystal XRD structural analyses

The single-crystal structures were obtained using a direct method (SHELXT²⁹ version 2018/2) and refined using the full-matrix least-squares technique (SHELXL version 2018/3) using the Olex2-1.2 (OlexSys) software.³⁰ Anisotropic thermal parameters were applied to all non-hydrogen atoms. Hydrogen atoms were geometrically generated.

Theoretical calculations

Electrical resistivity measurement

Temperature-dependent ρ measurement of the 2O–2SeMe·X single crystal was performed using a two-probe method. The sample was prepared by connecting gold wires (15 mm in diameter) to both ends of the single crystal along its long axis (the a-axis) using conductive carbon paste. To achieve suitable hardness, each conductive paste was mixed with ethylene glycol monobutyl ether acetate prior to use. Additionally, each gold wire was connected to another gold wire using silver paste as a buffer to minimize mechanical stress on the sample during measurement, and both ends of the wires were bridged to the electrode using silver paste. Prior to the temperature-dependent resistance (ρ) measurement, the ohmic behavior of the samples was confirmed by examining the current–voltage characteristics at room temperature from −1 to 1 V. At a constant DC voltage of 0.5 V within the ohmic region (Fig. S15, ESI†), the temperature-dependent resistance R of the sample was measured after cooling from room temperature to approximately 10 K, followed by heating to room temperature at a rate of approximately 1 K min⁻¹ to obtain the ρ–T plot. The plot was then fitted using the Arrhenius model, expressed as logρ = a + b*(1/T), to estimate the activation energy (E_a in Table 1).

DSC

DSC for polycrystals of 2O–2SeMe·X (X = BF₄, ClO₄) were performed under a nitrogen atmosphere. The measurement was performed within a temperature range of 100 to 298 K, utilizing a cooling and heating rate of 5 K min⁻¹ (Fig. S16 and S17, ESI†).

Optical reflection spectroscopy

The steady-state polarized reflectivity spectra in the mid-IR region (0.08–0.97 eV) and UV-vis-near IR region (0.46–6.2 eV) were analyzed using mid-IR and UV-vis spectrometers equipped with a polarizer. The spectra were obtained by applying an electric field (E) perpendicular to the π-stacking direction, which is the a-axis for 2O–2SeMe·X compounds (where X = BF₄ and ClO₄).

Static magnetic susceptibility measurement

The static magnetic susceptibility of the polycrystalline samples of 2O–2SeMe·X was measured by subjecting the samples to a static magnetic field of 50000 O_e upon cooling the sample from 300 to 50 K at a rate of 2 K min⁻¹, from 49 to 10 K at 1 K min⁻¹, and from 10 to 2 K at 0.5 K min⁻¹ (Fig. S14, ESI†).

Conclusion

In this study, we established a method for chalcogen substitution at the end groups of EDOT oligomers and demonstrated their impact on the crystal and electronic structures, as well as the enhanced conducting properties of the doped chalcogen-substituted oligomers. The introduction of end groups containing heavier atoms (such as Se and Te atoms) in the final step of the synthetic procedure may diversify the structures of these end groups, enabling the construction of rich and systematic oligomer libraries. The doped Se-substituted oligomers, denoted as 2O–2SeMe·X (X = BF₄ and ClO₄), extended the conjugate area of the donor molecules, enhanced the intracolumnar transfer integral and W, and reduced U_eff. The ρ_rt of the 2O–2SeMe·X compounds decreased by an order of magnitude in relation to that of S-substituted 2O·X, owing to the reduction in the electronic correlation parameter, U_eff/W, resulting from the extension of the conjugate system. The multiorbital system within the band structure, arising from the inversion of HOMO and HOMO−1 bands, may also significantly influence the electronic structures and physical properties of doped chalcogen-substituted oligomers. This study provides valuable insights into the design of oligomer-based conductors, with a particular focus on end groups, which can lead to the creation of a diverse array of structures that enhance conductivity. By strategically designing end groups to leverage high degrees of freedom of oligomer motifs, it is possible to expand the repertoire of oligomer-based conductor structures and achieve precise control over their conductive properties toward the development of diverse electronic devices with tailored functionalities.

Author contributions

T. F. and H. M. directed this study. K. O., E. K. and S. Y. performed the synthesis procedures. K. O. performed electrical conductivity. K. O., T. M., T. Y., and H. Ok. performed the optical measurement. K. O. performed magnetic measurement. H. A., and O. Y. performed DSC. K. O., and H. Oi. performed the theoretical calculations for the band structures. K. O. performed single-crystal XRD analyses. K. O., T. F., and, H. M. wrote the manuscript, and all authors discussed the results and commented on the manuscript.

Data availability

The crystallographic data (CIF files) for the structures reported in this article have been deposited with the Cambridge Crystallographic Data Centre (CCDC), under deposition numbers 2349871 (2O–2SeMe), 2349872 (2O–2SeMe·BF₄ at 293 K), 2352190 (2O–2SeMe·BF₄ at 123 K), 2349870 (2O–2SeMe·ClO₄ at 293 K), and 2352192 (2O–2SeMe·ClO₄ at 133 K). These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. All experimental data within the article and its ESI† are available from the corresponding authors upon requested.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was partially supported by JSPS Grants-in-Aid for Scientific Research (no. JP16H04010, JP17K18746, JP18H05225, JP21K18597, and JP22H00106 to H. M.; JP20H05206, JP21K05018, and JP22H04523 to T. F.), Research Fellowships for Young Scientists (JP23KJ0577 to K. O.), JST PRESTO (JPMJPR22Q8 to T. F.), MEXT Grants-in-Aid for Scientific Research on Innovative Areas “Hydrogenomics” (JP18H05516 to H. M.), a research grant from Technology Foundation, the Naito Foundation, the Kao Foundation for Arts and Science to T. F., Fujimori Science and Technology Foundation to K. O., Amano Institute of Technology to K. O., and the University Fellowship Founding Project for Innovation Creation in Science and Technology from The University of Tokyo to K. O. The authors thank the Supercomputer Center, The Institute for Solid State Physics, and The University of Tokyo for use of the facilities. We thank Idemitsu Kosan Co., Ltd. for their support throughout the study.

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Footnote

† Electronic supplementary information (ESI) available: Theoretical calculations, single-crystal X-ray diffraction analyses, magnetic measurements, differential scattering calorimetry, and NMR measurements. CCDC 2349870–2349872, 2352190 and 2352192. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4tc02346c

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