Boron-Stabilized Planar Neutral π-Radicals with Well-Balanced

Communication Cite This: J. Am. Chem. Soc. 2017, 139, 14336-14339

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Boron-Stabilized Planar Neutral π‑Radicals with Well-Balanced Ambipolar Charge-Transport Properties Tomokatsu Kushida,† Shusuke Shirai,† Naoki Ando,† Toshihiro Okamoto,*,‡ Hiroyuki Ishii,§ Hiroyuki Matsui,‡ Masakazu Yamagishi,‡ Takafumi Uemura,‡ Junto Tsurumi,‡ Shun Watanabe,‡ Jun Takeya,*,‡ and Shigehiro Yamaguchi*,† †

Department of Chemistry, Graduate School of Science, Integrated Research Consortium on Chemical Sciences (IRCCS), and Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Furo, Chikusa, Nagoya 464-8602, Japan ‡ Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan § Division of Applied Physics, Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan S Supporting Information *

allow an easier tuning of their electronic structures, including the on-site Coulomb energy (U) and the transfer integral (t), which ultimately leads to the desired Mott-insulator transistors.9,10 Importantly, their singly occupied and unoccupied molecular orbitals [SOMO(α) and SOMO(β), respectively] share virtually identical spatial distributions. Hence, this compound class should exhibit identical valence and conduction bandwidths in the condensed state, which are important for hole and electron transport, respectively (Figure 1a). A crucial requisite for such πradicals is to have a planar structure, which allows the formation of two-dimensional (2-D) solid-state alignments, e.g., herringbone-2 or brickwork-type5a,11 packing structures, favorable for effective charge-carrier transport. However, neutral π-radicals are in general labile and their stabilization represents a long-standing issue that hampers applications. For instance, triphenylmethyl radical, a representative organic neutral π-radical, easily forms a σ-dimer in the solid state and reacts with molecular oxygen.12 Steric protection of the radical center by, e.g., polychlorination of the phenyl rings13 represents one approach to increase the stability. Enhancing the spin delocalization through the expansion of the π-conjugation in such π-radicals is another. For that purpose, planarization of the entire π-skeleton has been proven to be particularly effective. For example, Morita and co-workers have reported highly stable planar trioxotriangulene radicals.7,14 In this Communication, we disclose a new design strategy for stable neutral π-radicals, which is based on the introduction of a tricoordinate boron atom into the triphenylmethyl skeleton, so that the spin density is effectively delocalized through the vacant p-orbital of the boron atom (Figure 1b). In the decade, the borylsubstituted neutral π-radicals have been developed, in which the spin density of the radical moiety is effectively delocalized over the tricoordinated boron atom.15 Further planarization of the entire framework should then furnish 1 with enhanced spin delocalization. Radical 1 can be regarded as a combination of a planarized triphenylmethyl radical 2 and a planarized triphenylborane 3. Hellwinkel has reported that 2 does not form σ-

ABSTRACT: Organic neutral π-monoradicals are promising semiconductors with balanced ambipolar carriertransport abilities, which arise from virtually identical spatial distribution of their singly occupied and unoccupied molecular orbitals, SOMO(α) and SOMO(β), respectively. Herein, we disclose a boron-stabilized triphenylmethyl radical that shows outstanding thermal stability and resistance toward atmospheric conditions due to the substantial spin delocalization. The radical is used to fabricate organic Mott-insulator transistors that operate at room temperature, wherein the radical exhibits wellbalanced ambipolar carrier transport properties.

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rganic semiconductors are crucial components in organic electronics, which are of paramount importance for the development of lightweight and flexible device technologies. One of the most desirable features of such semiconductors is the wellbalanced ambipolar transport of charged carriers, which enables the fabrication of self-equalized complementary circuits, highgain sensors, and low-power-consumption memory devices.1 However, most organic semiconductors exhibit unipolar, i.e., either hole- or electron-transport properties. This restriction is due to a different distribution of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Consequently, the orbital overlap relevant to hole or electron transport becomes different in the condensed state.2 Although a number of organic semiconductors with ambipolar transport characteristics have been reported,3 most of them have been synthesized without targeting this feature deliberately. The development of a design strategy that delivers semiconductors with intrinsically balanced ambipolar transport properties thus remains an important research target. Neutral organic π-monoradicals represent a promising class of materials to exhibit such properties.4 These compounds have attracted considerable attention due to their broad applications as organic metals,5 conductors,6 and electrode materials.7 In comparison with multicomponent organic charge-transfer complexes and salts,8 neutral one-component π-radicals should © 2017 American Chemical Society

Received: May 26, 2017 Published: October 4, 2017 14336

DOI: 10.1021/jacs.7b05471 J. Am. Chem. Soc. 2017, 139, 14336−14339

Communication

Journal of the American Chemical Society

atoms. However, the obtained data were of sufficient quality to determine a herringbone-like packing structure of 1a (vide inf ra), which exhibits neither π- nor σ-dimers. For the discussion of bond lengths and angles, we conducted the X-ray crystallographic analysis of 1b (Figure S3), which showed a highly planar structure with bond-angle sums of 360° around the B1 and C1 atoms. The B−C bond lengths [1.503(5)−1.511(5) Å] are slightly shorter than those in the parent planarized triphenylborane 3 [1.519(2)−1.520(2) Å].17 In contrast, the bonds between the central carbon atom and the ipso carbon atoms of the benzene rings, i.e., C1−C2, C1−C13, and C1−C14 [1.452(5)−1.465(5) Å], are slightly longer than the C1−Cipso bonds in 2 [1.443(1)− 1.447(2) Å] (Figure S4). These differences should most likely be attributed to the electronic interaction between the boron atom and the carbon radical center via the benzene moieties. DFT calculations at the UB3LYP/6-31+G(d) level of theory on the crystal structure of 1b showed that the Wiberg bond indices20 for the B−C bonds (0.97) are slightly increased relative to those in 3 (0.95). In addition, a natural bond orbital (NBO) analysis revealed that the occupancy of the vacant p-orbital on the boron atom in 1b (0.38) is slightly higher than that in 3 (0.34).21 Although these differences seem rather small, these perturbations play a crucial role for the outstanding stability of 1. Electron paramagnetic resonance (EPR) spectroscopy measurements demonstrated a significant contribution of the boron atom to the delocalization of the spin density in 1a. The EPR spectrum of 1a in THF showed a well-separated hyperfine splitting structure (Figure 2a). The spectrum was reproduced

Figure 1. (a) Relationship between the electronic structure of an isolated molecule and the band structure in the condensed state of organic neutral π-radical. (b) Molecular design for a triphenylmethyl radical stabilized by a boron atom, chemical structures for 1−4, and the comparable spatial distribution of SOMO(α) and SOMO(β) in 1a.

dimers, unlike the parent triphenylmethyl radical.16 On the other hand, we have recently demonstrated that 3 is extraordinary resistant toward air and moisture, despite the absence of steric protection of the boron atom,17 and serves as a useful scaffold for various intriguing materials.18 Herein, we report that 1 exhibit outstanding stability and well-balanced ambipolar chargetransport properties. We succeeded in preparing a singlecrystalline thin film of 1 using the solution crystallized technique,19 and in fabricating organic Mott-insulator transistors that operate at room temperature. The synthesis of 1a is described in detail in the Supporting Information. In brief, 1a was obtained in four steps from carbonyl-bridged planarized triphenylborane 4a (Scheme S1).17 A butyl-substituted derivative 1b, whose unsymmetric substitution pattern is crucial for the structural determination (vide inf ra), was synthesized in the same manner. Radicals 1 showed remarkable stability under ambient conditions. Their purification was accomplished by column chromatography on silica gel in air. To quantify the stability of 1a in the presence of oxygen, the degradation in toluene upon exposure to air was monitored by UV−vis absorption spectroscopy and compared to that of boron-free 2 (Figure S1). While 2 rapidly decomposed under these conditions (rate constant: 2.0 × 104 M−1 h−1), 1a remained intact, and its decomposition rate constant (19 M−1h−1) was over 3 orders of magnitude lower. In addition, 1a and 1b exhibit high thermal stability (Figure S2). Their decomposition temperatures for 5% weight loss (1a, Td5 = 292 °C; 1b, Td5 = 326 °C) under an N2 atmosphere are much higher than that of 2 (223 °C). These differences demonstrate the impact of the boron moiety on the stabilization of the triphenylmethyl radical. Although we attempted an X-ray crystallographic analysis of 1a, the orientation of the molecule could not be determined due to positional disorder between the central carbon and boron

Figure 2. (a) Electron paramagnetic resonance (EPR) spectrum of 1a in THF at 195 K (blue) together with its simulated spectrum (red). (b) Simulation-derived hyperfine coupling constants for 1a.

well by a simulation analysis, based on which the hyperfine coupling constants were estimated (Figure 2b). The value for the boron atom [a(11B) = 1.0 G] is indicative of substantial spin delocalization over the boron moiety. A comparison of the hyperfine coupling constants between 1a and 2 revealed crucial insight: the coupling constant of the hydrogen atoms on the para-positions relative to the central carbon atom in 2 (3.1 G)16 is higher than those of the parahydrogen atoms in 1a [a(Ha) = 2.9 G; a(Hd) = 2.3 G]. As the spin density, ρ, is correlated to the hyperfine coupling constants, a, in the EPR spectrum via the McConnell equation according to a = Qρ, wherein Q represents a proportional constant,22 these differences demonstrate that the boron atom effectively decreases the spin density on the triphenylmethyl moiety. The observed hyperfine coupling constants were corroborated by DFT calculations (Figure S6). To confirm the radical character of 1a in the solid state, we performed the magnetic susceptibility measurement, which confirmed Curie spin per molecule of almost unity (0.95) as shown in Figure S7, indicating that 1a maintains its radical character even in the solid state. 14337

DOI: 10.1021/jacs.7b05471 J. Am. Chem. Soc. 2017, 139, 14336−14339

Communication

Journal of the American Chemical Society The incorporation of the boron moiety into the triphenylmethyl radical framework also altered the redox properties. The cyclic voltammogram of 1a in THF (Figure S5) showed reversible redox waves for the oxidation and reduction processes. The half-wave potential (Eox 1/2) of the oxidation (−0.08 V vs Fc/ Fc+) was positively shifted (ΔEox 1/2 = 0.28 V) relative to that of 2 (−0.36 V). A more pronounced shift was observed for the reduction potential, where the Ered 1/2 value for 1a (−1.28 V) was positively shifted compared to that of 2 (−1.76 V). Thus, the incorporation of the boron moiety substantially enhances the electron-accepting properties of the radical, which may contribute to its greater stability toward aerial oxidation. For applications of the radicals as semiconductors, an important parameter is the on-site Coulomb repulsion energy U between the SOMO(α) and SOMO(β), which can be estimated from cyclic voltammetry. The difference between the oxidation (M0 → M+) and reduction (M0 → M−) potentials suggests a U value of 1.2 eV for 1a, which is much larger than those of previously reported charge-transfer complexes that act as Mott insulators, e.g., κ-type BEDT-TTF salts (U = 0.1−0.4 eV).23 The large U value of 1a promises potential utility for this material as a Mott insulator with a relatively large Mott− Hubbard energy gap.8a The charge transport properties of 1a were evaluated in singlecrystal-based field effect transistors (FETs). The devices with single-crystalline channels of 1a were fabricated using the solution-crystallized edge-cast method (Figure 3a,b).19 The

Figure 4. Crystal structures of 1a viewed along the (a) a axis and (b) b axis, together with transfer integrals determined by DFT calculations at the UB3LYP/6-31G(d) level of theory. (c) First Brillouin zone of 1a and (d) its band structure calculated using the unrestricted PBE functional and the 6-31G(d) basis set. Crystal structure of 1a with P21 space group was employed without including disorder for calculations and k-point mesh of 6×6×6 was used.

Notably, the carrier mobility, μ, is inversely proportional to the effective mass (m*) according to μ = eτ/m*, where τ refers to the relaxation time. As summarized in Table 1, the calculated m* Table 1. Effective Mass Calculation for 1aa electrons holes

m1/m0

m2/m0

m3/m0

5.2 (a+0.45c) 5.0 (a+0.49c)

11 (b) 8.4 (b)

14 (−0.39a + c) 43 (−0.49a + c)

a m1, m2, and m3 refer to the effective mass along the three Cartesian axes; vectors in parentheses indicate the direction of the corresponding principal axes.

Figure 3. Field-effect transistor containing 1a: (a) schematic drawing of the device structure; (b) optical microscopy image of the transistor device; (c) ID−VG characteristics of the device.

values for electrons and holes are almost comparable. Thus, the symmetric band structure in combination with the large Mott− Hubbard energy gap should be responsible for the well-balanced ambipolar transistor characteristics observed at room temperature. In summary, we designed and synthesized remarkably stable neutral π-radicals that contain a boron atom in a planarized triphenylmethyl radical framework. The incorporation of the boron atom enhances the spin delocalization, leading to substantial stabilization. In addition, the planar structure resulted in the formation of a herringbone-like packing structure. The radical exhibits ambipolar transport characteristics with wellbalanced mobilities for electrons and holes, which can be regarded as an example for an organic Mott-insulator transistor that operates at room temperature. Notably, the radicals exhibit a relatively large on-site Coulomb repulsion (∼1 eV), which cannot be achieved by conventional organic charge-transfer complexes. Further investigations into neutral radicals toward the realization of Mott transistors are currently underway in our laboratory.

surface of the SiO2/doped-Si wafers was modified with a spincoated benzocyclobutene (BCB) polymer layer (thickness: 54 nm).24 Figure 3c displays the transfer characteristics of the device in the saturation regime with drain voltage (VD) values of −80 and +100 V. Importantly, the transfer curves exhibit almost symmetric ambipolar characteristics with well-balanced mobilities of 4.5 × 10−3 and 1.1 × 10−2 cm2V−1s−1) for electrons and holes, respectively, which demonstrates that the Mott-insulator transistor operates at room temperature. To elucidate the origin of the symmetric ambipolar characteristics, the transfer integrals for the SOMOs of 1a in the crystalline state were calculated at the UB3LYP/6-31G(d) level of theory (Figure 4a,b). Although the dimethylmethylene groups in 1a hamper the formation of a densely packed alignment, 1a retains certain transfer integrals in the herringbone-like structure. The band structure for its crystal structure was calculated using the unrestricted PBE functional and the 6-31G(d) basis set (Figure 4c,d). The valence and conduction bands consist of SOMOs, and their band structures exhibit comparable energy dispersion. 14338

DOI: 10.1021/jacs.7b05471 J. Am. Chem. Soc. 2017, 139, 14336−14339

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Journal of the American Chemical Society

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b05471. Experimental details, crystal structures, and properties, as well as device fabrication protocols, including Scheme S1 and Figures S1−S26 (PDF) X-ray crystallographic data for 1a, 1b, and 2 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] *[email protected] ORCID

Shun Watanabe: 0000-0001-7377-6043 Shigehiro Yamaguchi: 0000-0003-0072-8969 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by JSPS KAKENHI grants 24109007 and 15H02163 (S.Y.). The authors would like to express their gratitude to Prof. K. Awaga, Prof. M. M. Matsushita, Dr. H. Yoshikawa, Dr. Y. Shuku, and Mr. K. Matsuura (Nagoya Univ.) for EPR spectroscopy and magnetic susceptibility measurements and valuable discussions.

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REFERENCES

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DOI: 10.1021/jacs.7b05471 J. Am. Chem. Soc. 2017, 139, 14336−14339