Letter pubs.acs.org/OrgLett
Synthesis, Structures, and Optoelectronic Properties of Pyrene-Fused Thioxanthenes Shiqian Zhang, Zhiqiang Liu,* and Qi Fang* State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China S Supporting Information *
ABSTRACT: A series of pyrene-fused thioxanthenes have been synthesized via a new concise route, and their crystal structures and photophysical properties have been fully investigated. The eight-ring fused dipyrene−thioxanthene (DPTA) can crystallize to monoclinic and triclinic X-ray structures, and their precursor has been isolated as two stable atropisomers with different photophysical properties. The EHOMO becomes higher and the Eg become narrower as more thioxanthene unit being fused with pyrene.
P
Scheme 1. Synthetic Approach to PTAs
yrene is a classical polycyclic aromatic hydrocarbon (PAH).1,2 It has been extensively studied as a readily derivatized precursor to fluorophores or organic semiconductors due to its high fluorescence quantum yield as well as π-conjugation and intermolecular π−π interaction.3 Pyrene has 10 peripheral reactive sites, which can be classified into three sets of chemically inequivalent sites. There are thus many possibilities to tune optoelectronic properties of pyrene derivatives through precise functionalization.4−6 The regioselective preparation of pyrene-based heteroatomembedded large PAHs is particularly interesting since the introduction of an electron-rich/deficient heteroatom may tune the electronic structure and also increase the noncovalent interactions in the solid state. Although pyrene can be easily derivatized at the 1-, 3-, 6-, and 8-positions,7−9 the synthesis of multisubstituted pyrene, especially heterocycle-fused pyrene, is still highly challenging. We recently synthesized three regioisomeric sulfur-bridged pyrene thienoacenes as a new class of materials for organic field-effect transistor (OFET) applications.10 Synthesis of other isomers of pyrene-fused thioxanthenes (PTAs), especially through functionalization of the K-region of pyrene and development of efficient annulation reactions, is attractive. Herein, we develop a new route to prepare PTAs starting from the K-region of pyrene in good yield. The synthesis, crystal structures, and photophysical properties are discussed. As shown in Scheme 1, our new strategy of extending pyrene from the K region is different from our previous methods for synthesizing PTAs.10 The key intermediate 4,5-bis(triflates)pyrene 3 was prepared starting from the Ru-catalyzed oxidation of pyrene followed by reduction to the diol and finally triflation.11−13 After Suzuki coupling between 3 and 2-(melthylthio)benzeneboronic acid, we fortunately harvested three fractions by silica gel chromatography. These fractions were characterized by NMR and single-crystal X-ray diffractions as monosubstituted 4 and a pair of bis-substituted atropisomers 6A/6B, respectively. The production of 4 may be attributed to proto-demetalation after oxidative addition.14 © XXXX American Chemical Society
It should be pointed out that 6A and 6B can be cleanly separated using normal silica gel chromatography (Figure S52), even though most of reported atropisomers need to be isolated via HPLC or other precision instruments. The 1H NMR spectra of 6A and 6B (Figure 1) clearly indicate that they are different compounds. The 1H NMR signals of pyrene moiety are almost the same, while those of the phenyl moiety are obviously different
Figure 1. 1H NMR and X-ray structures of 6A (top) and 6B (bottom). Received: January 26, 2017
A
DOI: 10.1021/acs.orglett.7b00276 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters (see Figure 1). These two atropisomers are very stable, and we did not observe any interconversion of these isomers even after 24 h of refluxing in toluene, as checked using 1H NMR. For the final ring-annulation step, oxidation of the sulfur to a sulfone was necessary in our previous route, but direct S−C bond formation is more desirable among the reported ways to build sulfur-bridged heteroaromatic rings.15−19 Inspired by Takimiya’s synthesis of benzothieno[3,2-b][1]benzothiophene (BTBT) derivatives via stilbene intermediates, we tried to treat o-methylthio precursors with excess I2 to build ring-fused products20,21 and successfully obtained 5 in high yield from 4. To our surprise, the only product 5 was proved to be the [3,4]-fused six-membered ring isomer, not the [4,5]-fused five-membered ring. Furthermore, treating atropisomers 6A and 6B under the same conditions led to the same product dipyrene−thioxanthene (DPTA), which is a highly π-extended heteroarenes with up to eight fused 6-membered rings, together with a trace of half-closed intermediate 7. Note that the present two-step synthesis of PTAs starting from pseudohalopyrene (3), via Suzuki coupling and a direct ring-closing, should be the shortest route to produce these π-extended heteroaromatic systems, and it could be a versatile way to obtain various novel heteroarenes. All of the new compounds were fully characterized by 1 H NMR, 13C NMR, and high-resolution mass spectrometry. All compounds exhibit good thermal and oxidative stability in air. Crystal structures of 4, 5, 6, 7, and DPTA were exclusively determined by single-crystal X-ray diffraction. As shown in Figure 2, molecule 5 is almost perfectly planar. The torsion angle between the pyrenyl and the phenyl rings is
Figure 3. X-ray structures and packing diagrams of 6A (left) and 6B (right).
6B (2242.8 Å3) is smaller than that of 6A (2319.5 Å3) and short intermolecular S···S contact of 3.310 Å can be observed in 6B. Two crystal polymorphisms, monoclinic m-DPTA (P21/n) and triclinic t-DPTA (P-1) have been grown from different solvents, i.e., a dual mixture of dichloromethane (DCM) and petroleum ether for m-DPTA and pure DCM for t-DPTA, respectively. As shown in Figures 4 and 5, the packing styles of m-
Figure 4. X-ray structure and packing diagram of monoclinic m-DPTA crystal.
Figure 5. (Left) ORTEP drawing of t-DPTA at 50% probability level showing a helical spring-washer-like molecular structure. (Right) Packing diagram of triclinic t-DPTA crystal.
Figure 2. X-ray structure and packing diagram of 5.
DPTA and t-DPTA are different, and the two terminal phenyl groups of the molecule in both m-DPTA and t-DPTA crystals are bent symmetrically to the opposite side of pyrene plane to adopt a helical structure with C2 symmetry (quasi C2 in crystal and strict C2 in free by DFT/6-311g(d) optimization). However, the length of the C−C bonds connecting the pyrene and phenyl moieties (C5−C17 in Figure 5) are 1.477 Å for m-DPTA and 1.474 Å for t-DPTA, which are the shortest bond lengths among the same bonds in PTAs in this work and mean the efficient extended π-conjugation is due to ring fusion. Although the X-ray molecular structures of m-DPTA and t-DPTA are very similar, there are distinct differences. The dihedral angle between the two terminal phenyl planes is 50.3° in m-DPTA, and that of t-DPTA is 62.4° (Figures 4 and 5). Accordingly, the C24···C22 distances in m-DPTA and t-DPTA are 2.992 and 3.133 Å, respectively. Therefore, DPTA exhibits the feature of a molecular spring washer; i.e., the thickness of the molecule is sensitive to its environment, such as solvent and pressure. As shown in Figure 6 and Table 1, the absorption spectra of all the unfused precursors are quite similar to those of pyrene and
only 1.43°. The length of the C−C bond connecting the phenyl and pyrenyl moieties is 1.491 Å, which is shorter than that of compound 4 (1.513 Å) and demonstrates the increase of the π-conjugation with the annulation. Compound 5 adopts face-to-face columnar and herringbone-like structures with a π−π distance of 3.554 Å in the crystals. Compound 6A can be defined as the anti-isomer and is essentially C2 symmetric in the crystal with the two S-methyl groups being on the opposite sides of the pyrene plane (Figure 1). The C2 molecular point group has been confirmed by the results of a DFT/6-311g(d) optimization. On the other hand, the X-ray structure of 6B is roughly mirror symmetric with its two S-methyl groups being on the same side of the pyrene plane, and the molecular geometry can be optimized to be in the Cs point group by the DFT/6-311g(d) calculation; therefore, the isomer 6B can be defined as a syn-isomer. The intramolecular S···S distance in 6A (anti-6) is 6.366 Å, which is much longer than that in 6B (syn-6) (3.893 Å). The packing of 6A and 6B is also very distinctive (Figure 3). Typically, 6A molecules are packed along the c-axis and no short S···S intermolecular interaction is detected, while 6B packs more tightly than 6A, for cell volume of B
DOI: 10.1021/acs.orglett.7b00276 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
Figure 6. Absorption (left) and fluorescence emission (right) spectra of PTAs in hexane (5 × 10−6 mol·L−1).
Figure 7. Solid-state emission spectra (left) of 6A and 6B and cyclic voltammogram (right) of PTAs in CH2Cl2 solution (1 × 10−3 mol·L−1).
reported regioisomers,10 keeping the well-resolved vibronic structures. Although the molecular structures of 6A and 6B in solids are different, their absorption and emission spectra in solution are basically the same. The absorption spectra of the ring-closed PTAs are greatly red-shifted compared to those of their unfused precursors, indicating a smaller HOMO−LUMO gap due to ring fusion. For example, the λabs of 5 is at 426.5 nm, while that of 4 at 340 nm. Compound 7 has one S-bridged fusion, and its λabs is close to that of 5. As more rings are fused with pyrene, a larger spectral red-shift was observed. DPTA has two S-bridged fusions, and its λabs is expended to 498 nm, which is red-shifted 153 nm relative to its precursors 6A/6B. The emission band of DPTA extends to near 700 nm with the λem = 553 nm, which is also red-shifted 155 nm relative to that of 6A/6B. The crystalline samples of 6A and 6B exhibited quite different solid-state emission properties (Figure 7). Although 6A presents a normal emission spectrum with one peak around 400 nm, 6B
presents a broad emission spectrum with several peaks ranging from 400 to 700 nm. At present, the correlations of this long wavelength of emission to the molecular structure and packing mode in the crystal are still unclear. In view of applications, 6A seems to be a blue emitter, and 6B may be used as a singlecomponent white-light emission material.22 The emission spectra of the single single-crystal samples of mDPTA and t-DPTA were also recorded (Figure S60). The crystal emission peak of t-DPTA is at 606 nm, while that of m-DPTA is shifted to 623 nm. This difference must be related to the different packing structures. TD-DFT-calculated singlet excitations (B3LYP/6-311g(d)) were used to rationalize the absorption spectra, and the results are listed in Table 1 and Tables S4−S9. The calculated longest absorption λabs of these compounds all correspond to the HOMO → LUMO transition and are in good agreement with that of their experimental absorption spectra (Table 1 and Figure 6).
Table 1. Structure Data and Optoelectronic Properties of PTAs T (K) space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) ρ (g/cm3) R [I > 2σ (I)] C−C precision (Å) Δρmax, Δρmin (e/Å3) λema (nm) Φb (%) λabsc (nm) λemd (nm) Φe (%) τf (ns) redox potentials E1,g E2g Eoxg peak value (eV) EHOMO (eV) Egh (eV) ELUMO (eV)
4
5
6A
6B
7
m-DPTA
t-DPTA
300 P21/c 10.2837 (2) 14.8777 (2) 10.9847 (2) 90.00 102.391 (1) 90.00 1641.49 (5) 1.313 0.0598 0.0046 0.324, −0.187 479
298 P21 8.320(3) 3.9035(14) 21.515(8) 90.00 91.080(5) 90.00 698.6(4) 1.466 0.0641 0.0055 0.634, −0.322 513 7.9 426.5 (408.7) 430 11.1 2.3 0.78 0.90 −5.15 2.85 −2.30
295 P21/n 12.2546(2) 15.9304(2) 12.3845(2) 90.00 106.387(1) 90.00 2319.50(6) 1.279 0.0403 0.0018 0.274, −0.195 406 31.7 344.0 (340.4) 398 1.0 5.8
296 P21/c 9.0815(7) 9.8004(7) 25.2369(18) 90.00 93.118(1) 90.00 2242.8(3) 1.323 0.0448 0.0023 0.421, −0.647 418 (522) 5.4 344.5 (347.4) 399 0.5 3.4 1.27 −5.53 3.48 −2.05
296 P21/n 12.0014(2) 14.5055(2) 12.1322(2) 90.00 114.591(10) 90.00 1920.49(5) 1.434 0.0499 0.0027 0.457, −0.311 623 4.2 497.5 (498.1) 553 1.5 2.3 0.66, 1.06 0.71, 1.11 −4.97 2.32 −2.65
296 P-1 8.7367(14) 10.0971(16) 11.6822(19) 106.933(2) 90.765(2) 108.096(2) 931.0(3) 1.479 0.0486 0.0039 0.248, −0.288 606
1.33, 1.57 −5.58 3.50 −2.08
300 P21/c 11.4244(8) 12.1615(8) 15.7616(11) 90.00 106.018(1) 90.00 2104.9(2) 1.359 0.0512 0.0030 0.452, −0.432 501 3.7 422.0 (415.7) 456 6.7 1.8 0.93, 1.24 0.99, 1.30 −5.25 2.82 −2.43
340.0 (336.5) 396 0.8 5.2 1.31 −5.56 3.53 −2.03
λem is the solid emission peak at the shortest wavelength, with λex = λabs. bSolid emission quantum yield was obtained using a calibrated integrating sphere. cλabs is the absorption peak in hexane at the longest wavelength, and the data in the parentheses are the calculated peak wavelengths by the TD-DFT/6-311g(d) method. dλem is the fluorescence peak in hexane at the shortest wavelength, with λex = λabs. eDetermined in hexane using coumarin 307 as reference. fDecay curves were recorded at λem with λex = λabs. gValues recorded vs Ag/AgCl. hEnergy gap was calculated from the onset of the absorption. a
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DOI: 10.1021/acs.orglett.7b00276 Org. Lett. XXXX, XXX, XXX−XXX
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The electron-donor abilities of these PTAs were assessed by cyclic voltammetry (CV) (Figure 7 and Figures S48−S51). The ferrocene/ferrocium (Fc/Fc+) couple was used as a standard (the HOMO energy level was taken to be −4.8 eV). The CV of the ring-opened precursors (4, 6A, 6B) show irreversible oxidation, while those of the ring-fused PTAs (5, 7, DPTA) show reversible oxidation. The first oxidation peak Eox of 4 was measured to be 1.31 eV and that of ferrocene is 0.55 eV; thus, the HOMO of 4 is − [4.8 + (1.31 − 0.55)] = −5.56 eV. In the same way, the HOMO of 5 and DPTA are −5.15 and −4.97 eV, respectively. These results demonstrate that with the increase in π-conjugation (from 4 to 5 and to DPTA) the HOMO energy level becomes higher, and the electron-donating ability can be ranked as 4 < 5 < DPTA. We noted that DPTA exhibits two redox couples, which corresponds to two steps of charge transfer and implies that the electronic donating may likely happen at the bridge S atom. In conclusion, we have developed a simple and efficient fivestep route to synthesize a series of π-extend sulfur-bridged pyrene derivatives, finally leading to sulfur-containing DPTA with high HOMO level and narrow energy gap. Two atropisomeric precursors 6A and 6B have quite different properties, such as chromatographic behaviors, packing styles, and solid-state emissions. These compounds may be used as potential organicelectronic materials.
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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/acs.orglett.7b00276.
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Letter
Synthesis procedure, characterization, NMR, HRMS, CV, X-ray structure, and DFT of PTAs (PDF) Crystallographic data of 4 (CIF) Crystallographic data of 5 (CIF) Crystallographic data of 6A (CIF) Crystallographic data of 6B (CIF) Crystallographic data of 7 (CIF) Crystallographic data of m-DPTA (CIF) Crystallographic data of t-DPTA (CIF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Zhiqiang Liu: 0000-0001-7863-1759 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21472116, 20972089, and 21672130) and the State Key Laboratory of Crystal Materials.
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DEDICATION Dedicated to Prof. Thomas C. W. Mak on the occasion of his 80th birthday. D
DOI: 10.1021/acs.orglett.7b00276 Org. Lett. XXXX, XXX, XXX−XXX
