Helically Twisted Tetracene: Synthesis, Crystal Structure, and Photophysical Properties of Hexabenzo[a,c,fg,j,l,op]tetracene

Abstract

The synthesis, X-ray crystal structure, and photophysical properties of unsubstituted hexabenzo[a,c,fg,j,l,op]tetracene are described. Unlike the previously reported tert-butyl-substituted analogues, unsubstituted hexabenzo[a,c,fg,j,l,op]tetracene showed a helically twisted conformation in the solid state. Density functional theory calculations on the possible conformers were also studied.

Key words

hexabenzotetracene - twisted acenes - polycyclic aromatic hydrocarbons - nonplanar π-system

Acenes, a class of polycyclic aromatic hydrocarbons (PAHs) consisting of linearly fused benzene rings, can be twisted by bulky substituents, benzannulation, or a combination of the two.[1] Owing to their nonplanar and chiral structures, helically twisted acenes (Figure [1, a]) have been paid attention by organic chemists. Because of the growing interest and importance of nonplanar PAHs in materials science,[2] efficient synthetic methods such as direct C–H arylation reaction of PAHs have been highly demanded.[3]

Figure 1 (a) Twisted acenes. (b) Two conformations (‘waggling’ and ‘helical’) of hexabenzo[a,c,fg,j,l,op]tetracenes.

Recently, we developed a K-region-selective annulative π-extension (APEX) reaction of PAHs as an effective way to grow the π-system of PAHs in a step-economical fashion.[4] We herein report the single-step synthesis of unsubstituted hexabenzo[a,c,fg,j,l,op]tetracene (1) from pyrene and dibenzosilole using our palladium-catalyzed APEX reaction. Helically twisted structure of 1 was revealed by X-ray crystallography. These results were somewhat surprising because previously reported X-ray structures of tert-butyl-substituted hexabenzo[a,c,fg,j,l,op]tetracenes[5] were not ‘helical’ but rather ‘waggling’[6] conformations (Figure [1, b]). Adding to the synthesis and X-ray crystallography of 1, conformational analysis of 1 was also investigated by density functional theory (DFT) calculations.

Unsubstituted hexabenzo[a,c,fg,j,l,op]tetracene (1) was synthesized by a palladium-catalyzed double C–H/C–Si coupling, a so-called APEX reaction. A mixture of pyrene (1 equiv) and 5,5-dimethyldibenzo[b,d]silole (3 equiv) in 1,2-dichloroethane was heated under reflux conditions for 2.5 hours in the presence of Pd(MeCN)₄(BF₄)₂ (5 mol%) and o-chloranil (4 equiv) to afford 1 in 58% yield (Scheme [1]).[7] The product was identified by NMR spectroscopy (¹H and ¹³C) and high-resolution mass spectrometry. In spite of the unsubstituted PAH structure, 1 showed sufficient solubility for purification and analysis (8.9 mg/mL in 1,1,2,2-tetrachloroethane), which may be due to the nonplanar structure. High thermostability of 1 was also observed when the crystal of 1 was heated over 300 °C.

Scheme 1 Synthesis of hexabenzo[a,c,fg,j,l,op]tetracene (1)

The structure of 1 was confirmed by X-ray crystallography.[8] Suitable single crystal of 1 was obtained from tetrahydrofuran/pentane solution. Unlike previously reported waggling conformations of tert-butyl-substituted hexabenzo[a,c,fg,j,l,op]tetracene derivatives,[5] 1 adopts a helically twisted conformation as shown in Figure [2] (a). The end-to-end twisted angle of the tetracene moiety of 1 is 80.5°, which is higher than those of previously reported unsubstituted benzannulated acenes.[9] The highly twisted structure of 1 was realized by the combination of four [4]helicene substructures. The packing mode of 1 in the crystal is shown in Figure [2] (b). Similar to dibenzo[g,p]chrysene,[10] one-dimensional π-π stacking of the nonplanar π-systems was found.

The conformation of 1 was studied by DFT calculations with the B3LYP/6-31G(d) level of theory. Seven isomers including two pairs of enantiomers of 1 could be generated by the combination of the chirality of each [4]helicene moiety ( P or M ) as shown in Figure [3]. The twisted form 1a observed by X-ray crystallography was the most stable conformation among the five ground states 1a–e. The Gibbs free energy of the waggling conformation 1b is 0.6 kcal mol^–1, which is higher than that of 1a. The conformation of 1d, which was observed in a chlorinated hexabenzo[a,c,fg,j,l,op]tetracene derivative,[11] has 3.7 kcal mol^–1 higher energy in the case of 1. The other conformer 1e was much unstable (17.4 kcal mol^–1) relative to 1a. Then, the isomerization pathway from 1a to 1b was determined to gain insight into whether conformation 1a was a thermodynamic product or a kinetic product. From 1a to 1b, one intermediate 1c and two transition states (TSa–c and TSb–c, Figure [4]) were found and the rate-determining step was determined to be TSb–c. Two additional transition states, TSc–d and TSc–e, were also found (see the Supporting Information for detail). Considering the energy value of TSb–c (10.6 kcal mol^–1) relative to that of 1a, the isomerization between 1a to 1b can easily proceed under ambient temperature. It was also found that the isomerization barrier was almost the same (11.1 kcal mol^–1) in the case of the di-tert-butyl derivative of 1 (1′). Thus, the helical and waggling conformations of 1 interconverted into the solution state, and the conformation in the crystalline state was controlled by the packing interaction: π-π interaction (1) or C–H-π interaction (1′).[5]

The photophysical properties of 1 in the solution state were measured. Figure [5] shows absorption and fluorescence spectra of the diluted cyclohexane solution of 1. The absorption peaks were observed at 267, 310, 324, 375, and 394 nm, and the fluorescence spectrum had two peaks at 418 and 430 nm when a solution was excited at 325 nm. A bluish-green fluorescence of 1 with a moderate quantum yield (Φ _F = 0.31) was observed.

Figure 2 (a) ORTEP drawings of 1 with 50% probability; all hydrogen atoms are omitted for clarity. (b) Packing structure of 1.

Figure 3 Seven conformers of 1 with Gibbs free energies (kcal mol^–1) relative to that of 1a. Chirality of each [4]helicene moiety is represented by P or M .

In summary, we have investigated the synthesis, X-ray crystal structure, and photophysical properties of unsubstituted hexabenzo[a,c,fg,j,l,op]tetracene (1). The palladium-catalyzed oxidized double C–H/C–Si coupling, an APEX reaction, smoothly supplied 1 in one step from commercially available materials. Unlike the previously reported derivatives of 1, the structure of 1 observed by X-ray crystallography was a helical conformation. The twist angle of the tetracene moiety of 1 (80.5°) was the highest among the tetracene moieties in unsubstituted benzannulated acenes. Further structural analysis by using DFT calculations indicated that helical (1a) and waggling conformations (1b) could be interconverted in the solution state.

Figure 4 Isomerization pathway from 1a to 1b calculated with the B3LYP/6-31G(d) level of theory with Gibbs free energy values (1 atm, 298.15 K, gas phase).

Figure 5 UV–vis absorption (solid line) and fluorescence (broken line) of cyclohexane solution of 1

Acknowledgment

This work was supported by the ERATO program from JST (K.I.) and the Funding Program for KAKENHI from MEXT (26810057 to H.I.) and the Sumitomo Foundation (141495 to H.I.). We thank Ms. Hiromi Torii and Ms. Akiko Gocho (Nagoya Univ.) for the assistance of the synthesis of 1. Calculations were performed using the resources of the Research Center for Computational Science, Okazaki, Japan. ITbM is supported by the World Premier International Research Center (WPI) Initiative, Japan.

• References and Notes

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• 7 Synthesis of Hexabenzo[a,c,fg,j,l,op]tetracene (1) To a 10 mL Schlenk tube containing a magnetic stirring bar were added pyrene (81.0 mg, 400 μmol), 5,5-dimethyldibenzo[b,d]silole (253 mg, 1.20 mmol), o-chloranil (394 mg, 1.60 mmol), and Pd(MeCN)₄(BF₄)₂ (8.9 mg, 20 μmol). After dry DCE (4 mL) was added via a syringe through the septum, the septum was then replaced with argon-balloon-equipped condenser. The resultant mixture was stirred at reflux for 2.5 h. After the reaction mixture was cooled down to room temperature, this mixture was passed through a short pad of silica gel, washed with CH₂Cl₂, and solvents were removed under reduced pressure. The residue was subjected to silica gel chromatography (hexane–CH₂Cl₂, 10:1) to afford 1 (116 mg, 58%) as a white solid. ¹H NMR (600 MHz, CD₂Cl₂): δ = 7.70 (t, J = 7.1 Hz, 4 H), 7.77 (t, J = 7.1 Hz, 4 H), 8.12 (t, J = 7.5 Hz, 2 H), 8.81 (d, J = 8.2 Hz, 4 H), 8.88 (d, J = 8.2 Hz, 4 H), 8.99 (d, J = 7.5 Hz, 4 H). ¹³C NMR (150 MHz, C₂D₂Cl₄): δ = 123.6 (CH), 124.2 (C_q), 125.6 (CH), 125.8 (CH), 126.8 (CH), 127.0 (CH), 127.7 (C_q), 128.1 (C_q), 128.5 (CH), 129.3 (C_q), 130.8 (C_q). HRMS–FAB: m/z calcd for C₄₀H₂₂ [M]⁺: 502.1716; found: 502.1719; mp >300 °C. All other experimental procedures and characterization data of 1 can be found in the Supporting Information.
• 8 Crystal Data of 1 Rigaku MicroMax007HF with PILATUS diffractometer, graphite-monochromated Mo Kα radiation (λ = 0.71075 Å), formula: C₄₄H₂₂, FW = 502.58, T = 123(2) K, monoclinic, P2₁/a, a = 7.839(4) Å, b = 26.538(11) Å, c = 11.640(5) Å, β = 98.627(12)°, V = 2394.1(19) ų, Z = 4, D _calc = 1.394 g/cm³, μ = 0.079 mm^–1, F(000) = 1048, crystal size: 0.20 × 0.20 × 0.01 mm³, θ range: 3.04–25.00°, reflections collected: 29984, independent reflections: 4172, R _int = 0.0760, param.: 361, GOF on F ²: 1.121, R ₁ = 0.0832, wR ₂ = 0.2285 (I > 2σ(I)), R ₁ = 0.1036, wR ₂ = 0.2634 (all data) The structures were solved by direct methods with SIR-97 and refined by full-matrix least-squares techniques against F ² (SHELXL-97). The intensities were corrected for Lorentz and polarization effects. The nonhydrogen atoms were refined anisotropically. Hydrogen atoms were placed using AFIX instructions. CCDC 1450739 contains the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/getstructures.
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