9,10-Bis(5H-dibenzo[b,f]azepino)anthracene

Abstract

The title compound with a donor–π–donor (D–π–D) type triad structure was synthesized by Buchwald–Hartwig amination using 9,10-dibromoanthracene and 5H-dibenzo[b,f]azepine, and characterized by ¹H, ¹³C{¹H} NMR, HRMS, and X-ray diffraction analysis. The azepine–anthracene–azepine units are nearly orthogonal to each other, with a torsion angle of 88°. A broad and weak absorption band around 410–480 nm and the low emission character (Φ_F = 0.01) suggest the weak intramolecular charge transfer from the azepine to the anthracene unit due to the twisted structure.

1. Introduction

9,10-Disubstituted anthracenes have gained attention from various fields of chemistry owing to their two-photon absorption (TPA) behaviors and strong emissive properties applicable to organic light-emitting diodes (OLEDs) [1,2]. To date, donor–π–donor (D–π–D) [3,4,5], donor–π–acceptor (D–π–A) [6,7,8,9], and acceptor–π–acceptor (A–π–A) [10,11,12,13,14] type triad molecules with anthracene-9,10-diyl as a central π chromophore have been synthesized, and their intriguing photophysical properties derived from effective intramolecular charge transfer (CT) have also been unveiled.

A deep understanding of the relationship between the photophysical properties and the structure of donors and acceptors is essential to develop better optoelectronic materials. Konishi et al. demonstrated that the fluorescence properties of 9,10-bis(diarylamino)anthracenes with a D–π–D structural motif are highly dependent on the diarylamino groups (Figure 1): (1) a diphenylamino derivative I that has a strong CT character is highly emissive (Φ_F ≈ 1), (2) the dihydroacridyl derivative II, in which two Ar groups are tightly bridged by a methylene, is poorly emissive (Φ_F = 0.06) due to its twisted structure that reduces the CT character, and (3) the 10,11-dihydro-5H-dibenzo[b,f]azepino derivative III, in which the Ar groups are linked by an ethane-1,2-diyl, exhibits a moderate fluorescence (Φ_F = 0.75) [5]. Herein, we report the synthesis, structure, and photophysical properties of the unsaturated derivative of III, 9,10-bis(5H-dibenzo[b,f]azepino)anthracene 1, where the Ar groups are bridged by an ethene-1,2-diyl.

2. Results and Discussion

2.1. Synthesis and Characterization

Buchwald–Hartwig amination of 9,10-dibromoanthracene and 5H-dibenzo[b,f]azepine using Pd₂(dba)₃·CHCl₃, RuPhos, and LiHMDS (lithium hexamethyldisilazide) provided the desired 9,10-bis(5H-dibenzo[b,f]azepino)anthracene 1 in 41% yield (Scheme 1) [15]. Standing a hot N,N-dimethylacetamide (DMA) solution of 1 at room temperature deposited yellow crystals of 1·(DMA)₂ suitable for X-ray diffraction studies.

Figure 2 illustrates the solid-state structure of 1, which has a center of symmetry, and a side-view of the non-planar azepine ring. The anthracene unit is almost perpendicular to the azepine unit, with a torsion angle of C1–N1–C16–C17 of 88.1(1)°, suggesting a less efficient conjugation between these units. This highly twisted conformation is likely due to the steric repulsion between the anthracene and dibenzoazepine units. In fact, dibenzo[b,f]azepines, whose nitrogen atom has an ortho-substituted phenyl group, adopt similar twisted structures [16,17,18], whereas those without ortho-substituents favor a coplanar structure to realize an efficient conjugation between the Ph group and the azepine unit [19,20,21]. The azepine rings adopt a boat-shaped structure with θ₁ and θ₂ values of 43.1 and 24.0°, respectively, consistent with the reported ground-state structure of azepine rings [22].

The ¹H NMR spectrum of 1 shows two signals at δ 9.13 and 7.18 assignable to the anthracene, four signals ranging from δ 6.9 to 6.6 assignable to the fused benzene rings, and a signal at δ 6.48 assignable to the alkenyl unit of the azepine ring. Notably, the ¹Hj signal (see Scheme 1 for the atom label) observed at δ 9.13 are markedly downfield shifted compared with that of the related 9,10-bis(diarylamino)anthracenes (ca. 8.1–8.5) [4,5,23]. Considering that the NMR active nuclei such as ⁷Li situated above and/or below antiaromatic rings show downfield shifts due to the paramagnetic ring current [24,25], the low-field-shifted ¹H NMR signal can be rationalized by the shielding effect caused by the 8π antiaromatic azepine [26].

2.2. Photophysical Properties

UV–Vis absorption and emission spectra were recorded, as shown in Figure 3. The absorption spectrum consists of three peaks characteristic of anthracene (λ_abs = 364, 379, 396) and a weak broad band from ca. 410 to 480 nm arising from CT from the azepine to the anthracene [5]. Although the emission spectrum shows a peak top at 449 nm, compound 1 is poorly emissive with Φ_F of 0.01. As mentioned above, fluorescence quantum yields of 9,10-bis(diarylamino)anthracenes are highly dependent on the degree of CT from the amino group to the anthracene; the diphenylamino derivative I that has a strong CT character shows a bright emission with Φ_F of almost unity, whereas the 9,10-dihydroacridyl counterpart II has a weak CT character and shows a weak fluorescence (Φ_F = 0.06) [5]. It is worth noting that the fluorescence quantum yields (Φ_F) of III and 1 are dramatically different (0.76 vs. 0.01), although both have a C2 bridge between the aryl rings of the amino donors. Thus, the combination of the dibenzo[b,f]azepine donor and the anthracene-9,10-diyl is unsuitable for fluorescent materials, although the dibenzo[b,f]azepine has been utilized as a donor in some fluorescent materials [20,27].

3. Conclusions

The title compound was successfully synthesized and fully characterized. X-ray diffraction studies revealed the highly twisted D–π–D structure as well as the boat-shaped azepine ring structure. One of the ¹H NMR signals of the anthracene was strongly deshielded because of the ring current of the antiaromatic azepine. The weak absorption band, ranging from 410–480 nm, and the poorly emissive character (Φ_F = 0.01) suggest that only a weak intramolecular CT occurs in 1. A comparison of the fluorescence properties of 1 and related 9,10-bis(diarylamino)anthracenes unveiled that the 9-dibenzo[b,f]azepinoanthracene unit is not appropriate for highly emissive materials.

4. Materials and Methods

4.1. General Considerations

All manipulations were performed under an argon atmosphere and using standard Schlenk techniques. All reagents were purchased from Sigma-Aldrich Chemical Co. (Tokyo, Japan), FUJIFILM Wako Pure Chemical Corporation (Osaka, Japan), Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan), or Kanto Chemical Co., Inc. (Tokyo, Japan) and used as received, unless otherwise stated. Column chromatography was carried out using Wakogel^® (Richmond, VA, USA) silica 60N (particle size: 40–100 μm). ¹H and ¹³C{¹H} NMR spectra were recorded on a JEOL ECZL-500R spectrometer at 20 °C, unless otherwise stated. Due to the low solubility of 1 toward CDCl₃, the phase-covariance weighted (PCW) NMR technique was used to acquire the ¹³C{¹H} NMR data [28]. Chemical shifts are reported in δ and referenced to residual ¹H and ¹³C{¹H} NMR signals of the deuterated solvents as internal standards. Multiplicities are abbreviated as singlet (s), doublet (d), triplet (t), quartet (q), septet (sept), multiplet (m), and broad (br). The HRMS spectrum was obtained by a Bruker ultrafleXtreme using 9-nitroanthracene as a matrix. Diffraction data were collected on a Bruker APEX II with Mo Kα radiation (λ = 0.71075 Å) at −80 °C. The structures were solved by direct methods using SHELXS. The refinements were performed using SHELXL-2019/3 [29]. The positions of the non-hydrogen atoms were determined by SHELXT 2018/2 [30]. All non-hydrogen atoms were refined on F_o² anisotropically by full-matrix least-square techniques. All hydrogen atoms were placed at the calculated positions with fixed isotropic parameters. The UV–Vis absorption and emission spectra were recorded using JASCO V-650 and JASCO FP-6600 spectrometers. The absolute photoluminescence quantum yields were measured by a calibrated integrating sphere system C10027 (Hamamatsu Photonics Co. Ltd., Shizuoka, Japan).

4.2. Synthesis of 9,10-Bis(5H-dibenzo[b,f]azepino)anthracene 1

LiHMDS (1.4 M in cyclohexane, 1.7 mL, 2.4 mmol, 2.4 equiv.) was added to a solution of 9,10-dibromoanthracene (336.0 mg, 1.000 mmol, 1.0 equiv), 5H-dibenzo[b,f]azepine (465.5 mg, 2.41 mmol, 2.4 equiv), Pd₂(dba)₃·CHCl₃ (10.3 mg, 0.01 mmol, 1 mol%), and RuPhos (17.2 mg, 0.02 mmol, 2 mol%) in 1,4-dioxane (20 mL) at room temperature. The reaction mixture was refluxed overnight, at which time the solvent was removed in vacuo. The crude product was purified by short pad column chromatography (DCM), followed by washing with cold DCM to yield the title compound (386.5 mg, 0.408 mmol, 41%) as a yellow solid.

¹H NMR (500 MHz, CDCl₃, Figure S1): δ = 9.13 (dd, ³J_HH = 10 Hz, ⁴J_HH = 1.5 Hz, 4H, j), 7.48 (dd, ³J_HH = 10 Hz, ⁴J_HH = 1.5 Hz, 4H, k), 6.88 (dd, ³J_HH = 8 Hz, ⁴J_HH = 2 Hz, 4H, c), 6.74 (m, 8H, d and f), 6.68 (td, ³J_HH = 8 Hz, ⁴J_HH = 2 Hz, 4H, e), 6.48 (s, 4H, a); ¹³C{¹H} NMR (126 MHz, CDCl₃, Figure S2): δ = 151.2 (4°, b), 140.8 (4°, h), 134.2 (4°, i), 133.6 (3°, a), 132.5 (3°, b), 131.8 (4°, g), 128.9 (3°, e), 127.0 (3°, j), 126.9 (3°, k), 124.1 (3°, d), 123.6 (3°, f); Mp: 317 °C (decomp.); HRMS (MALDI-TOF) m/z calcd for C₄₂H₂₈N₂⁺ [M]⁺: 560.2252, found: 560.2299 (Figure S3).

Crystal data for C₅₀H₄₆N₄O₂ [1·(DMA)₂] (M = 734.91 g/mol) are as follows: triclinic, space group P-1 (no. 2), a = 8.2771(10) Å, b = 10.8044(12) Å, c = 12.0198(14) Å, α = 64.896(2)°, β = 89.075(2)°, γ = 81.773(2)°, V = 962.17(19) ų, Z = 1, T = 190(2) K, μ(MoKα) = 0.71075 mm⁻¹, D_calc = 1.268 g/cm³; 5628 reflections measured (3.7° ≤ 2θ ≤ 51.9°), with 3730 unique reflections (Rint = 0.0177), all of which were used in all calculations. The final R₁ was 0.0391 (I > 2σ(I)) and wR₂ was 0.1093 (all data), GOF = 1.013.