Bisanthra-thianthrene: synthesis, structure and oxidation properties

Abstract

A bisanthra-thianthrene has been synthesized and the oxidation property was studied to show that the bis(tetracene radical cation) linked by a dithiin ring is more stable than the π-expanded dithianonacene dication.

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Acenes, typical aromatic hydrocarbons composed of linearly fused benzene rings, are currently of great interest from a practical standpoint as functional organic materials.^1–4 Given the adjustability in properties such as stability, solubility and energy level, higher acenes including heteroatoms have attracted ongoing interest as useful semiconductors and chromophores.^5–8 In particular, thiophene-included acenes are receiving attention as superior semiconducting materials.^9–12 On the contrary, heteroacenes with sulfur atoms in a six-membered ring have been less studied because of the sulfur atoms' break in π-electron conjugation.¹³ Chi et al. recently reported stable 5,12-dithiapentacene with a 24π quinoidal structure.¹⁴ This compound is stable for the two Kekulé structures at ground state while parent pentacene has only one Kekulé structure.

Another strategy for attaining aromatic dithiaacene is the two-electron oxidation of thianthrene (THA). THA is known to have a bent form at the central dithiin-ring (Fig. 1) and the bilateral benzene rings have no electronic interaction with each other in the neutral state, while the two-electron oxidation of THA gives the closed-shell dication of THA (THA²⁺) with a planar 14π aromatic system similar to anthracene.^15–17 Bisanthra-thianthrene (BA-THA) is a larger family of THA. The investigation of oxidation behavior of BA-THA is important for understanding the fundamental properties of dithiaacenes. In this paper, we report the synthesis, crystal structure, and optical and electrochemical properties of 5,5′,12,12′-tetraphenyl-bisanthra-thianthrene (3).

Fig. 1 Chemical structures of thianthrene (THA) and bisanthrathianthrene (BA-THA).

Synthesis of compound 3 is shown in Scheme 1. Compound 1, prepared according to a previously reported procedure,¹⁸ was reacted with sodium methoxide in a mixed solvent of methanol and toluene (1 : 1) under air to give compound 2 in 80% yield. Heating a mixture of compound 2 and copper powder at 300 °C for 4 h gave 3 in 59% yield. The structures of 2 and 3 were confirmed by ¹H and ¹³C NMR, mass spectroscopy and single crystal X-ray diffraction analysis (Fig. 2 and S1–S7, ESI†). The single crystals were obtained by diffusion crystallizations with toluene/methanol for 2 and chloroform/methanol for 3, with the crystallographic data summarized in Fig. 2 and S6–S7, ESI† and Tables S1 and S2, and ESI.† The structure of 2 shows planar tetracene units connected in a zigzag shape on the central tetrathiocine unit. The unit cell of 3 includes two independent molecules: 3a has warped tetracene planes (Fig. 2c and d) and 3b has planar ones (Fig. 2e and f). The dihedral angles between tetracene units calculated from six atoms in the central dithiin-rings are 125° and 134°, respectively.

Scheme 1 Synthesis of 3.

Fig. 2 ORTEP drawing of 2 and 3: (a) top view and (b) side view of 2, (c) top view and (d) side view of 3a, and (e) top view and (f) side view of 3b. Solvent molecules are omitted for clarity. Thermal ellipsoids represent 50% probability.

To investigate the oxidized species of 3, oxidation potentials of compound 3 were measured in comparison with compounds 1 and 2 using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in dichloromethane containing 0.1 M n-Bu₄NPF₆ as an electrolyte. Tetracene 1 shows a reversible oxidation peak at 0.50 V (vs. Fc/Fc⁺) and an irreversible oxidation peak at 1.02 V (vs. Fc/Fc⁺) (Fig. 3).

Fig. 3 Cyclic voltammograms (CV) and differential pulse voltammograms (DPV) of (a) 1, (b) 2, and (c) 3 in dichloromethane. Conditions; 0.1 M n-Bu₄NPF₆/CH₂Cl₂, scan rate = 0.1 V s⁻¹, [1] = 1.5 mM, [2] = saturated, [3] = 1.1 mM, working electrode: glassy carbon; counter electrode: Pt; reference electrode; Ag/AgNO₃; electrolyte: n-Bu₄NPF₆.

Bistetracene 2 shows a broad reversible oxidation wave at 0.58 V (vs. Fc/Fc⁺) as the first two-electron oxidation, and an irreversible oxidation wave at 0.98 V (vs. Fc/Fc⁺) as the second two-oxidation. Compound 3 shows two waves at 0.43 and 0.54 V (vs. Fc/Fc⁺) as the first and second one-electron oxidation. The third and fourth oxidation potentials are observed as a two-electron oxidation peak at 0.98 V (vs. Fc/Fc⁺). THA was reported to have two reversible one-electron oxidation potentials around 0.84 and 1.4 V (vs. Fc/Fc⁺) in acetonitrile.¹⁹ The radical cation and dication delocalized over the THA molecule and the oxidation occurred step-by-step. For compound 3, the oxidation occurred on each tetracene unit separately and the first oxidation influenced the second oxidation slightly; thus the difference of the first and second oxidation potentials was only 0.11 V.

To investigate the oxidation properties of 3 in comparison with tetracene 1, the UV-Vis-NIR absorption spectra of 3 in the presence of tris(4-bromophenyl)aminium-hexachloroantimonate ((BrPh)₃NSbCl₆) as an oxidant (Fig. 4b and c) were measured. By adding (BrPh)₃NSbCl₆ into a dichloromethane solution of 1, the peaks at 419, 451, 480, and 514 nm disappeared and new absorption peaks at 439, 729, 802 and 921 nm appeared. By adding 1 eq. of oxidant, the color of the solution changed from yellow to light green. This oxidation from neutral 1 to radical cation 1˙⁺ quantitatively occurred and showed the isosbestic points at 468 nm and 526 nm. In the case of 3, the peaks at 424, 450, 480, and 513 nm decreased at 1 equiv. addition of (BrPh)₃NSbCl₆, while the weak peaks at 508, 807 and 914 nm appeared. By adding 2 equiv. of (BrPh)₃NSbCl₆, the color of the solution changed from yellow to purple and the absorption band at 508, 807 and 914 nm increased. This absorption change suggested the oxidation of 3 with 2 equiv. of the oxidant gave dication 3²⁽˙⁺⁾ as a quantitative direct conversion and the two radical cations are independent tetracene radical cations. These absorption spectra did not show additional change by adding an excess amount of (BrPh)₃NSbCl₆.

Fig. 4 (a) UV-Vis absorption and fluorescence spectra of 1, 2 and 3 in CH₂Cl₂. UV-Vis-NIR absorption spectra of 1 (b) and 3 (c) in the presence of oxidant (BrPh)₃NSbCl₆ in dichloromethane and acetonitrile at room temperature. (BrPh)₃NSbCl₆ was added to solution of 1 and 3 in CH₂Cl₂ (3 ml) by 0.1 equal amount and 0.2 equal amount of the solution (0.5 μl) in acetonitrile respectively.

The absorption peaks of the tetracene radical cation were also observed by spectroelectrochemical analysis (Fig. S8, ESI†). The absorption spectrum of 1 at 0.8 V (vs. Fc/Fc⁺) has peaks at 439, 729, 802 and 921 nm, which are the same as the absorption peaks in the presence of 1 equiv. of (BrPh)₃NSbCl₆ (Fig. 4a). For compound 3, the absorption spectra were measured at 0.5 and 0.8 V. At 0.5 V, the peaks of 727, 809, and 914 nm are observed, increasing at 0.8 V without changing the wavelength of peaks. It suggests that these NIR-absorptions are derived from the tetracene radical cation. The two-electronic oxidation of 3 occurs at each tetracene of two tetracene units independently.

To confirm the oxidized structure, the electronic structures of 1 and 3 in the presence of (BrPh)₃NSbCl₆ were investigated by electron spin resonance (ESR) measurement (Fig. 5). A dichloromethane solution of 1 (0.3 mM) in the presence of 1 equiv. (BrPh)₃NSbCl₆ was measured under an argon atmosphere at −90 °C. The solution of 1 exhibited a signal with g value = 2.0029 and the simulated hyperfine coupling constant values (hfccs) were 4.4, 1.8, and 1.3G. This spectrum was in correlation with a simulated radical cation of 1 (1˙⁺) as shown in Fig. 4a. The ESR spectrum of 3 (8.0 mM) in the presence of 2 equiv. of (BrPh)₃NSbCl₆ showed a similar g value and hfccs to 1˙⁺, which suggested two-electron oxidation of 3 gives bis(radical cation, 3²⁽˙⁺⁾), in which each radical localized to each tetracene unit. This result was supported by the spin densities of 1˙⁺ and 3²⁽˙⁺⁾ calculated at the B3LYP/6-31G(d) level using the Gaussian 09 software package (Fig. S9–12, ESI†).^20–22 The calculated spin densities of 1˙⁺ and 3²⁽˙⁺⁾ were highest at the peri-positions (1, 4, 5, 6, 7, 10, 11, 12 positions) of tetracene (Fig. S13, ESI†). The NMR spectra of 1 and 3 in the presence of (BrPh)₃NSbCl₆ also suggested the existence of radicals because no signal was observed on ¹H NMR measurement (Fig. S3, ESI†). From these experimental results, the structure of 3²⁽˙⁺⁾ as two tetracene radical cations linked by 1,4-dithiin-ring was assigned.

Fig. 5 ESR spectra of 1˙⁺ (a) and 3²⁽˙⁺⁾ (b) with (BrPh)₃NSbCl₆ at −90 °C with the simulations and the simulated hyperfine coupling constant values in dichloromethane.

Finally, we performed density functional theory (DFT) calculations at the B3LYP/6-31G(d) level as well as nuclear independent chemical shift (NICS) calculations at the GIAO-B3LYP/6-31G(d) level for THA and 3 and their dications (Fig. S14–18, ESI†).²³ The central dithiin ring of THA shows non-aromatic character at the neutral condition (NICS value: −2.42 ppm) (Fig. S19, ESI†). The dications of THA (THA²⁺) are predicted to have planar structures in both singlet and triplet states and only singlet dication gives aromaticity on the dithiin ring (singlet: −8.01 ppm; triplet: 3.20 ppm). The calculated singlet–triplet energy difference of THA²⁺, ΔE_S–T is −11.2 kcal mol⁻¹ (Table S3, ESI†), indicating that the ground state of the dication is a singlet, in agreement with the literature.¹¹ The C–S bond lengths of singlet and triplet THA²⁺ are clearly shorter than those of neutral THA, suggesting that a bond alternation exists on C–S–C, and THA²⁺ is planar like anthracene.

Compound 3 showed a bent structure the same as THA and its dication species were also bent in both singlet and triplet states. ΔE_S–T is only −0.29 kcal mol⁻¹ (Table S3, ESI†), suggesting the radicals act like two doublet states in the molecule. The C–S bond lengths of singlet and triplet dications were predicted to be similar with the neutral one, and the NICS values on central dithiin ring also suggested non-aromatic properties in both singlet and triplet states. DFT calculations showed that the two radical electrons of 3²⁽˙⁺⁾ were localized at 5, 6, 11, 12 positions of BA-THA's tetracene moieties separately. The HOMOs of singlet bis(radical cation) with the α and β spins were localized separately on one of two tetracene units of the molecule (Fig. 6 and S11, ESI†), indicating that the dications of compound 3 can be regarded as two independent mono-radical tetracene cations linked by two sulfur atoms. These phenomena were also observed for BA-THA (Fig. S15–18, ESI†), and were different from THA, in which the positive charges were delocalized on bilateral benzene rings through the sulfur atoms.

Fig. 6 HOMOs of singlet dication of THA and BA-THA (3) calculated at the B3LYP/6-31G(d) level.

In conclusion, we synthesized a bis(tetracene radical cation) by two-electron oxidation of bisanthrathianthrene 3. The two tetracene radical cations are independent, linked by a dithiin-ring. This is because bis(radical cation) structure generates four Kekulé structures in a molecule. If the two radical cations could be located on sulfur atoms to give a dithianonacene-like structure, there is only one Kekulé structure, which would be very unstable considering Clar's rule. Finally, we believe that this independent biradical may help the realization of next-generation organic electronic devices.

Acknowledgements

This work was partly supported by CREST JST and Grants-in-Aid for Scientific Research (No. 25288092, 25620061, 26288038, 26620167, 26105004, 16H02286, and 15H00876 ‘AnApple’), the Green Photonics Project in NAIST, and the program for promoting the enhancement of research universities in NAIST supported by MEXT. We thank Prof. Haruyuki Nakano, Kyushu University and Prof. Ko Furukawa, Niigata university for their valuable discussion. We thank Ms Yoshiko Nishikawa in NAIST for the measurement of mass spectra, Mr Fumio Asanoma in NAIST for the measurement of ESR spectra, and Mr Shouhei Katao in NAIST for the single-crystal structure analysis and Prof. Leigh McDowell in NAIST for his advice during manuscript preparation.

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Footnote

† Electronic supplementary information (ESI) available: Synthetic details, X-ray crystal structural analysis, spectroelectro-chemical analysis, and DFT calculations. CCDC 1447852 and 1447853. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra13036d

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