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Article Cite This: ACS Omega 2018, 3, 5866−5875
Synthesis and Optical Properties of Donor−Acceptor-Type 1,3,5,9Tetraarylpyrenes: Controlling Intramolecular Charge-Transfer Pathways by the Change of π‑Conjugation Directions for Emission Color Modulations Rui Liu,†,§ Huijuan Ran,†,§ Zhen Zhao,† Xueli Yang,† Jiali Zhang,† Lijuan Chen,† Huaming Sun,‡ and Jian-Yong Hu*,† †
Key Laboratory of Applied Surface and Colloid Chemistry, National Ministry of Education, Shaanxi Key Laboratory for Advanced Energy Devices, Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xian 710119, China ‡ National Demonstration Center for Experimental Chemistry Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xian 710119, China S Supporting Information *
ABSTRACT: In dipolar organic π-conjugated molecules, variable photophysical properties can be realized through efficient excited-state intramolecular charge transfer (ICT), which essentially depends on the π-conjugation patterns. Herein, we report a controllable regioselective strategy for synthesis and optical properties of two donor−acceptor (DA)type 1,3,5,9-tetraarylpyrenes (i.e., 1,3-A/5,9-D (4b) and 1,3D/5,9-A (4c)) by covalently integrating two phenyl rings and two p-OMe/CHO-substituted phenyl units into the 2-tertbutylpyrene building block, in which the two phenyl rings substituted at the 1,3-positions act as acceptors for 4b or as donors for 4c and the two p-OMe or p-CHO-substituted phenyl moieties substituted at the K-region of 5,9-positions act as donors for 4b or as acceptors for 4c, respectively. Density functional theory calculations on their frontier molecular orbitals and UV−vis absorption of S0 → S1 transition theoretically predicted that the change of π-conjugation directions in the two DA pyrenes could be realized through a variety of substitution patterns, implying that the dissimilar ground-state and excited-state electronic structures exist in each molecule. Their single-crystal X-ray analysis reveal their highly twisted conformations that are beneficial for inhibiting the π-aggregations, which are strikingly different from the normal 1,3,5,9-tetraphenylpyrenes (4a) and related 1,3,6,8-tetraarylpyrenes. Indeed, experimental investigations on their optical properties demonstrated that the excited-state ICT pathways can be successfully controlled by the change of π-conjugation directions through the variety of substitution positions, resulting in the modulations of emission color from deep-blue to green in solution. Moreover, for the present DA pyrenes, highly fluorescent emissions with moderate-to-high quantum yields both in the thin film and in the doped poly(methyl methacrylate) film were obtained, suggesting them as promising emitting materials for the fabrication of organic light-emitting diodes.
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INTRODUCTION
molecules with individual electronic structures and ICT pathways for future organic optoelectronics and biological medicine is of great importance. As one of the organic π-conjugated systems, pyrene has attracted considerable attention due to its specific fluorescence characteristics, such as the Ham effect14 and the excimer emission.15 In fact, pyrene and its derivatives have been widely used as fluorescence probes16 for applications in biomedicine, as semiconductors17 for applications in organic optoelectronics, and as absorption materials18−22 for applications in nonlinear optics in the past two decades. It is well known that pyrene
For organic π-conjugated systems, investigating the relationships of substitution positions, π-conjugation patterns, intramolecular charge-transfer pathways, and variable photophysical properties is of interest. Dipolar organic molecules with donor−acceptor (DA)-type structures have been considered as ideal materials for optoelectronic devices,1,2 nonlinear optics,3−8 and biological applications9 in recent years. For such materials, a suitable choice of the electron donor and acceptor groups selectively substituted in the periphery of the organic π-conjugated system can provide highly variable photophysical properties through efficient excited-state intramolecular charge transfer (ICT),10 which essentially depends on the static π-conjugation pattern (such as degree, pathway, etc.).11−13 Therefore, developing new DA-type π-conjugated © 2018 American Chemical Society
Received: March 29, 2018 Accepted: May 7, 2018 Published: May 31, 2018 5866
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Figure 1. Schematic representations of possible charge-transfer conjugation pathways in donor−acceptor substituted pyrenes. D = donor and A = acceptor.
influenced by the ICT processes through the change of the substitution positions (see Figure 1), thereby making these molecules potential candidates as optoelectronic materials,24,26,29 solvent polarity sensors,28 photostable fluorophores,32 etc. As previously mentioned, since the dynamic ICT processes essentially depend on the corresponding static π-conjugation patterns,11−13 assuming that the change of π-conjugation directions in the dipolar organic π-conjugated molecules could be realized by the variety of the substitution patterns, in which case variable photophysical properties are achievable through the corresponding ICT process. As per this concept, for the above-pointed three substitution patterns at the 1-, 3-, 6-, 8- and/or 2-, 7- and/or 4-, 5-, 9-, 10-positions of the DA pyrenes, this hypothesis is nearly impossible because of the equivalent activity of the site on the pyrene ring. Meanwhile, the realization of the change of π-conjugation directions in the fusion site of the 2,7-/4,5- and/or 1,8-/4,5-position-substituted DA pyrenes is also inadvisable due to the lack of straightforward strategy for modifying the synthetic routes.28 Interestingly enough, Yamato et al. recently reported an effective approach to brominate 2-tert-butylpyrene both at the active site of 1,3-positions and the K-region of 5,9positions.35,36 The proposed bromination mechanism is that
exhibits strong positional dependence along the long axis (active site of 1-, 3-, 6-, and 8-positions and plane node of 2-, 7positions) and the short axis (K-region of 4-, 5-, 9- and 10positions). Generally, pyrene possesses the equivalent activity of the site at the 1-, 3-, 6-, and 8-positions that makes asymmetric functionalization of the pyrene ring to remain a big challenge. Meanwhile, the substitution of D/A groups at different positions of the pyrene would bring interesting optoelectronic properties, which make the fine tuning of photophysical properties of the pyrenes to be possible through the ICT processes.23 For example, the photophysical properties of 2- and 2,7-substituted pyrenes have significant differences in their optical behaviors when compared to the more typical 1substituted analogues.23 Thanks to the chemists’ efforts and the progress in modern synthetic methodology, pyrene has recently been used as a building block for the construction of dipolar organic-conjugated molecules24−34 (Figure 1). These DA pyrenes developed up to date can be classified into three substitution patterns: (i) at the active site of 1-, 3-, 6-, and 8positions24−26,30,32−34 (Figure 1A), (ii) at the plane node of 2-, 7-positions29,31,34 and the K-region of 4-, 5-, 9-, 10positions27,28 (Figure 1B, left), and (iii) at the fusion site of 2,7-/4,5- and/or 1,8-/4,5-positions28 (Figure 1B, right). The photophysical properties for the DA pyrenes are strikingly 5867
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ACS Omega Scheme 1. Synthesis of DA-Type 1,3,5,9-Tetraarylpyrenes
Figure 2. (A) Optimized geometries and (B) calculated FMOs for the compounds 4a (left), 4b (middle), and 4c (right).
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the bromo atoms first substituted at the 1- and 3-positions of pyrene37 could sterically hinder the 4- and 10-positions, thereby enabling regioselective substitutions at the 5- and 9-positions because the tert-butyl group at the 7-position plays a role to protect the ring against electrophilic attack at the 6- and 8positions.24 Accordingly, this approach theoretically provides a reasonable and useful synthetic strategy for the construction of DA-type pyrenes substituted at the 1-, 3-, 5-, and 9-positions. Herein, we present an effective approach for the synthesis and optical properties of two dipolar organic molecules from 2tert-butylpyrene, in which two phenyl rings substituted at the 1,3-positions act as acceptors for 4b or as donors for 4c and two p-OMe or p-CHO-substituted phenyl moieties substituted at the K-region of 5,9-positions act as donors for 4b or as acceptors for 4c, respectively (Scheme 1). We demonstrated that the fluorescence emission colors of the investigated DA pyrenes in solution are well modulated by solely varying the attachment of p-donor (OMe)- and p-acceptor (CHO)substituted phenyl groups to the K-region (5- and 9-positions, deep-blue for 4b and green for 4c), indicating that the ICT pathways in the present DA pyrenes can be controlled by the change of π-conjugation directions through the variety of substitution patterns (Figure 1C).
RESULTS AND DISCUSSION Quantum Chemistry Computation. Density functional theory (DFT) calculations on the optimized geometries and the electron density distribution of the frontier molecular orbitals (FMOs) were first performed by the Gaussian 16 program, using the B3LYP functional and the polarized 6-31G* basis set.38 The calculated dipole moments were 0.36 for 4a (derived from its weak polar symmetrical structure) and 4.37 for 4c, higher compared to the value of 2.30 for 4b (derived from the stronger polar symmetrical structure of 4c). As shown in Figure 2A, the dihedral angles between the pyrene core and the phenyl planes in the optimized geometries of 4a−c are ca. 54−58°, which should be beneficial for inhibiting the π−π stacking in the solid state. Figure 2B depicts the separated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) orbitals of 4 in their optimized geometries. Apparently, for 4a and 4b, the HOMO is predominantly located over the entire pyrene framework in each molecule, while the aryl substituted groups have limited contributions to this system. However, the HOMO of 4c is mainly located on the pyrene core and the 1,3-substituted phenyl rings have notable contributions to this molecule. In contrast, when compared to that of 4a, the LUMO of 4b is 5868
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ACS Omega Table 1. Photophysical Properties of the Compounds 4 λabs, max (nm)a compd
CH2Cl2 (ε × 105)
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373 (0.43) 377 (0.90) 375 (0.63)
4a 4b 4c
λem, max (nm) film 380 375 376
Φf
CH2Cl2
film/doped filmb
CH2Cl2/film/doped filmsb
Eg (eV)c
Eox1/2 (V)d
412 419 493
410/nd 432/408 461/430
0.92/0.75/nd 0.74/0.36/0.65 0.61/0.29/0.52
3.43 3.05 2.96
1.48 1.08 1.26
HOMO/LUMO (eV)e −5.88/−2.45 −5.48/−2.43 −5.66/−2.70
Tm/Td (°C) 335/350 312/365 297/352
Mole absorption coefficient (ε): M−1 cm−1. bThe compounds 4b and 4c 0.5 wt %-doped in poly(methyl methacrylate) (PMMA) thin films. Optical band gap estimated from onset point (onset) of absorption band: Eopt = 1240/λonset. dEox1/2 is half-wave potentials of the oxidation waves, potentials vs calomel electrode, working electrode glassy carbon, 0.1 M Bu4NPF6-CH2Cl2. eHOMO and LUMO energy levels were calculated according to the equations: HOMO = −(4.4 eV + Eox1/2) and LUMO = HOMO + Eg. a c
Figure 3. Single-crystal structures of 4b (left) and 4c (right): (A) top view (up) and (B) side view (bottom).
expected 1,3,5,9-tetraarylpyrenes in isolated yields of 65−74% (Scheme 1). The structure of 4 was fully characterized by 1H/13C NMR spectra, Fourier transform infrared (FT-IR) spectroscopy, mass spectroscopy, as well as elemental analysis (the spectroscopic analysis of 4a is same as the literature,35 see Supporting Information). The two DA compounds 4b and 4c were highly soluble in common organic solvents, such as cyclohexane, toluene, CH2Cl2, CHCl3, THF, acetonitrile, and N,Ndimethylformamide. The thermal properties (Figure S21) of 4b and 4c were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The thermal data for the pyrenes 4 are listed in Table 1. Single-Crystal X-ray Structures. Single-crystal X-ray analysis41 also confirmed the structures of 4b and 4c (Tables S2−S4). A noticeable feature in the molecular structure of 4b and 4c is highly twisted conformation between the pyrene core and the phenyl groups, which is slightly different from 4a.35 The dihedral angles between the pyrene core and the phenyl planes are ca. 58.4, 62.7, 61.8, and 67.1° for 4b and 62.8, 50.2, 74.2, and −77.9° for 4c (Figure 3). Comparing their computed optimized geometries, it has found that for 4b, it is almost identical; however, a different geometry was observed in the crystal structure of 4c, which might be due to the existence of the hydrogen-bonding interactions originating from the substituents of p-CHO groups (Figures 2A and S9). This conformation is more effective for inhibiting π−π stacking and releasing the steric interactions in the thin film compared to that of 4a.35 In the two crystals, C−H···π interactions (only for
mainly located both on the center pyrene core and the 1,3substituted phenyl rings, implying the two phenyl rings play the electron-accepting nature; similarly, the LUMO of 4c is predominantly located on both p-CHO-substituted phenyl groups at the 5,9-positions of K-region and the pyrene core. The different HOMO/LUMO distributions of 4a/b and 4c imply that the dissimilar ground-state and/or excited-state electronic structures exist in each molecule, which theoretically originate from the change of π-conjugation directions through the variety of substitution patterns (Figure 1C), thereby variable photophysical properties are highly expected. The computed UV−vis absorption (Figure S1) of S0 → S1 transition of 4 also gave similar theoretical evidences for implying their dissimilar electronic structures. This can be further illustrated by the calculated values in Figure 2B and Table S1 in the Supporting Information. Synthesis and Characterization. There could be several possible bromination conditions to prepare the novel bromide precursor of 1,3-diphenyl-5,9-dibromopyrene used for synthesizing the proposed DA architectures (4). After many synthetic attempts, we finally found that the bromination condition39 in CH2Cl2 at a low temperature of −78 °C is very effective. According to the reported procedures, the 2-tert-butylpyrene (1),40 7-tert-butyl-1,3-dibromopyrene,39 and 7-tert-butyl-1,3diphenylpyrene (2)37 were synthesized in high yields and the treatment of which with bromine (2 equiv) in CH2Cl2 at −78 °C surprisingly gave the desired 5,9-dibromopyrene (3) in high isolated yield of 71%. Then, Suzuki cross-coupling reaction of (3) with the corresponding arylboronic acids yielded the 5869
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Figure 4. (a) UV−vis absorption and (b) emission spectra of the compounds 4 recorded in CH2Cl2 at ca. ∼10−5−10−6 M. (c) Emission spectra of 4b and 4c in thin film and in 0.5 wt %-doped PMMA film. (d) The 1931 CIE chromaticity coordinates of 4b and 4c in CH2Cl2 and in a thin film.
Upon excitation, the three compounds clearly exhibit broad and featureless emission spectra indicating that their S1 states are identified as 1CT states, which is also confirmed by the spatial separation of their FMOs. In addition, the emission maxima at 412 nm for 4a and 419 nm for 4b (deep-blue) reveal a smaller blue shift than the reported 1,3,5,9-tetrakis(4methoxyphenyl)pyrene 35 (λmax = 421 nm). The three compounds have a slight red-shift order of 1,3,5,9-tetrakis(4methoxyphenyl) pyrene (4OMe) > 4b (2OMe) > 4a (H), implying the excited-state electronic contribution is mainly attributed to the π-conjugation degree from the donor units of OMe. However, very impressively, the 4c with two CHO moieties caused a remarkable red shift (∼24 nm) and a broadened emission maximum (λmax) at 493 nm (green) compared to those of the 1,3,5,9-tetrakis(4-formylphenyl)pyrene35 with four CHO moieties (λmax = 469 nm), indicating that the excited-state electronic contribution is predominantly attributed to the ICT processes.23 The results further demonstrated that the 4c with 1,3-D/5,9-A structure has more efficient excited-state ICTs than those of 4b with 1,3-A/ 5,9-D structure, indicating that the variety of substitution positions can realize the change of π-conjugation directions. The 1CT energies of 4 calculated from the onset of their prompt emission spectra are 3.20, 3.14, and 2.88 eV. All photophysical data are fully summarized in Table 1. Thin-Film Characterization. The thin-film emission spectrum of the two DA compounds displayed a prominent maximum emission in the blue region at 432 nm for 4b and 461 nm for 4c (Figure 4c and Table 1). Interestingly, the emission of 4b is drastically red shifted by 13 nm related to the spectrum
4b, Figure S4) and hydrogen-bonding interactions (Figure S9) were observed and they can largely influence their molecular geometries and effectively inhibit the excimer emission in the solid film.42,43 Packing structures of 4b and 4c can be best described as an irregular packing form without J-aggregations44−47 or H-aggregations48 for 4b (Figures S5 and S6) and a one-dimensional columnar structure along the crystallographic c axis with H-aggregations48 (π-stacking (ca. 4.57 Å)) for 4c (Figure S10), which is strikingly different from 4a35 and related 1,3,6,8-tetraarylpyrenes,49 with a two-dimensional selfassembled planar solid-state structure for the former and a sandwichlike three-dimensional structure for the latter overlapping the porous two-dimensional networks.35 Optical Properties. Figure 4 shows the optical properties of the compounds 4 investigated in CH2Cl2 and in a thin film. As shown in Figure 4a, three compounds reveal broad S0 → 1 CT absorption bands at 350−400 nm, which can be attributed to the ICT processes.23 Interestingly, for 4a and 4b, characteristic of their spectra is almost the same with a broad and featured fine structure due to their weak polar structure resulting in weak ICT processes; however, 4c exhibits a more broader and less well-resolved spectra, which can be attributed to its stronger ICT processes originated from its stronger polar structure. It should be noted that this difference of S0 → 1CT absorption bands between 4b and 4c was mainly contributed from their individual electronic structures and ICT pathways. The results are also in well agreement with the DFT calculations on both FMOs and UV−vis absorption of S0 → S1 transition (Figure S1). 5870
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Figure 5. (a) Emission spectra of 4c in different organic solvents. (b) Emission colors of 4c in different solvents under 365 nm UV illumination.
quantum yields recorded in dilute CH2Cl2 and a thin film are 74 and 36% for 4b and 61 and 29% for 4c, respectively. In contrast, the 0.5 wt %-doped PMMA films of 4b and 4c exhibited higher quantum yields of 65 and 52%, respectively. The higher quantum yields of the doped PMMA film stem from the weaker aggregation-caused quenching compared to that of thin film. Electrochemical Properties. To investigate the electrochemical properties of 4b and 4c, the oxidation potentials were measured in CH2Cl2 solution. As shown in Figure S27, the oxidation potentials are around +1.08 ± 0.05 V for 4b and +1.26 ± 0.05 V for 4c. The oxidation potential of 4b is appreciably lower than that of 4c (+0.18 V). Thus, the two OMe substitutions in the para position of phenyl ring could slightly reduce its ionization potential. The HOMO and LUMO energy levels of 4b and 4c were estimated as −5.48, −5.66 and −2.43, −2.70 eV, respectively (Table 1), which is in agreement with the theoretically calculated data (Figure 2b).
in CH2Cl2 solution, which is ascribed to the formation of intermolecular C−H···π interactions and the different dielectric constants,50 whereas the emission spectrum of 4c shows a large hypsochromic shift (∼32 nm) with respect to the spectrum in CH2Cl2 solution, which probably attributed to its packing structures with H-aggregations.48 The results were also consistent with their single-crystal analysis. Solvatochromism Effect. As shown in Figures 5 and S22− S24, the spectral properties of the solvatochromism for 4 were investigated in different solvents with varying polarity (cyclohexane; 1,4-dioxane; ethyl ether; tetrahydrofuran; dichloromethane; dimethylformamide; and acetonitrile). The 4b shows slightly solvatochromic absorption (∼3 nm) and emission (∼7 nm) due to its weak ICT progress (Figure S23). In contrast, the emission spectra of 4c are highly sensitive toward the solvent polarity, whereas the absorption spectra exhibit slight shift (Figure S24). So, it could be interpreted that the excited state is more sensitive to the solvent polarity than that of the ground state. With increasing the solvent polarity from cyclohexane to acetonitrile, the 4c exhibits a remarkable red shift (∼76 nm, Figure 5a). This phenomenon was further evaluated by the relationship between the Stokes-shifts in various solvents and the Lippert equation,51,52 which shows a linear correlation between these two factors (Figure S25). The trend in the slope of the Lippert−Mataga plots follows the order of 4c > 4b > 4a, and the value of the slope of the fitting line for 4c (10 007) is far higher than that for 4b (645), indicating the intramolecular excited state has a larger dipolar moment for the former than for the latter. The results also indicated that the different solvatochromism behaviors between 4b and 4c were mainly attributed to their dissimilar ICT pathways and effectiveness, which originate from the change of π-conjugation directions through the variety of substitution positions. Doped PMMA Film Characterization. We also examined the emission spectra of 4b and 4c in a 0.5 wt %-doped poly(methyl methacrylate) (PMMA) film.53 We observed a deep-blue emission with a maximum peak at 408 nm for 4b and 430 nm for 4c (Figure 4c), which is almost identical to the corresponding emission in nonpolar cyclohexane solution, and might be due to the formation of π−π stacking interactions, C− H···π interactions, and/or hydrogen-bonding interactions that can be further suppressed in the doped films. The results suggest that these newly developed DA pyrenes might be promising candidates for the fabrication of deep-blue host− guest-based organic light-emitting diodes (OLEDs).54,55 The
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CONCLUSIONS
In summary, we have established a reliable and straightforward access to the 1,3-A/5,9-D- and 1,3-D/5,9-A-tetraaryl-substituted pyrenes from the 2-tert-butylpyrene. DFT calculations on their FMOs and UV−vis absorption of S0 → S1 transition theoretically predicted that the change of π-conjugation directions in the two DA pyrenes could be realized through the variety of substitution pattern, implying that the dissimilar ground-state and excited-state electronic structures existed in each molecule. Their single-crystal X-ray analysis reveals their highly twisted conformations that are beneficial for inhibiting the π-aggregations and are strikingly different from the normal 1,3,5,9-tetraarylpyrenes and related 1,3,6,8-tetraarylpyrenes. Exactly, experimental investigations on their photophysical properties demonstrated that the excited-state ICT pathways can be successfully controlled by the change of π-conjugation directions through the variety of substitution positions, resulting in the modulations of emission color from deepblue to green in solution. Moreover, for the present DA pyrenes, highly fluorescent emissions with moderate-to-high quantum yields both in the neat film and in the doped PMMA film were obtained, suggesting them as promising emitting materials for the fabrication of OLEDs. We believe that this effective synthetic strategy might be applied to the other πconjugated systems for the development of new dipolar organic molecules. We thus hope that the present DA pyrenes will 5871
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solution was dried (MgSO4) and condensed under reduced pressure. Recrystallization from pure hexane gave 1,3-dibromo7-tert-butylpyrene (0.61 g) in 76% yield as light green silver flakes. (lit 38.) 1H NMR (300 MHz, CDCl3): d = 8.48 (s, 1H), 8.36 (d, J = 9 Hz, 2H), 8.35 (s, 2H), 8.21 (d, J = 9 Hz, 2H), 1.59 (s, 9H). Synthesis of 7-tert-Butyl-1,3-diphenylpyrene (2). A mixture of 1,3-dibromo- 7-tert-butylpyrene (200 mg, 0.5 mmol), phenylboronic acid (300 mg, 2.0 mmol) in toluene (20 mL), and ethanol (10 mL) at room temperature was stirred under argon, and a 2 M aqueous solution of K2CO3 (10 mL) and [Pd(PPh3)4] (70 mg, 0.06 mmol) were added. The mixture was heated to 110 °C overnight under stirring. After cooling to room temperature, the mixture was quenched with water, extracted with CH2Cl2, and washed with water and brine. The organic extracts were dried with MgSO4, and the solvent was evaporated. The residue was purified by column chromatography eluting with CH2Cl2/n-hexane (1:2) and then recrystallized from hexane to give 7-tert-butyl-1,3-diphenylpyrene (2) (124 mg) in 63% yield as white silver flakes. Mp 186 °C (lit 36.). 1H NMR (300 MHz, CDCl3): δ = 1.59 (s, 9H, tBu), 7.44−7.69 (m, 10H, Ar-H), 7.94 (s, 1H, pyrene-H2), 8.01 (d, J = 9.2 Hz, 2H, pyrene-H4,10), 8.18 (d, J = 9.2 Hz, 2H, pyreneH5,9), 8.20 (s, 2H, pyrene-H6,8) ppm. Synthesis of 7-tert-Butyl-1,3-diphenyl-5,9-dibromopyrene (3). A solution of Br2 (0.03 mL, 0.5 mmol) and CH2Cl2 (2 mL) was added dropwise to a mixture of 7-tertbutyl-1,3-diphenylpyrene (100 mg, 0.5 mmol) in CH2Cl2 (4 mL) at −78 °C for 6 min and stirred for 20 min under N2 atmosphere. The resulting mixture was allowed to stir overnight. The reaction mixture was poured into ice water and neutralized with a 10% aqueous solution of Na2S2O3. The mixture solution was extracted with dichloromethane. The organic layer was washed with water and brine and then the solution was dried (MgSO4) and condensed under reduced pressure. The residue was purified by column chromatography eluting with CH2Cl2/n-hexane (1:7). Recrystallization from CH2Cl2/n-hexane (2:1) gave 3 (166 mg, 71%) as a light-yellow powder. 1H NMR (300 MHz, CDCl3): δ = 8.73 (s, 2H), 8.54 (s, 2H), 7.96 (s, 1H), 7.46−7.66 (m, 10H), 1.64 (s, 9H) ppm. 13 C NMR (100 MHz, CDCl3) δ = 150.72, 140.21, 137.57, 130.62, 130.09, 129.76, 128.90, 128.73, 127.82, 124.94, 123.86, 123.73, 123.26, 35.90, 32.01. Anal. calcd for C32H24Br2: C, 67.63; H, 4.26; found: C, 67.96; H, 4.64. Synthesis of 7-tert-Butyl-1,3,5,9-tetrakisphenylpyrene (4a). A mixture of 7-tert-butyl-1,3-diphenyl-5,9-dibromopyrene (248 mg, 0.44 mmol), phenylboronic acid (134 mg, 1.10 mmol) in toluene (20 mL), and ethanol (12 mL) at room temperature was stirred under N2 atmosphere, and a 2 M aqueous solution of K2CO3 (8 mL) and [Pd(PPh3)4] (100 mg) were added. The mixture was heated to 110 °C under stirring for 16 h. After cooling to room temperature, the mixture was quenched with water, extracted with toluene, and washed with water and brine. The organic extracts were dried with MgSO4, and the solvent was evaporated. The residue was purified by column chromatography eluting with CH2Cl2/n-hexane (1:7) and recrystallized from CH2Cl2/n-hexane (2:1) to give 4a (160 mg, 65%) as a white powder. (lit 34. mp 335 °C) νmax (KBr)/ cm−1 3055.1, 3026.2, 2961.5, 2901.6, 2866.4, 2360.2, 2341.56, 1593.4, 1490.1, 1367.0, 1256.8, 1177.0, 1072.1, 1003.3, 896.8, 762.7, 701.5, 624.1, 609.1, 597.3. 1H NMR (300 MHz, CDCl3): δ = 1.36 (s, 9H, tBu), 7.40−7.54 (m, 12H, Ar-H), 7.63−7.70 (m, 8H, Ar-H), 7.98 (s, 1H, pyrene-H), 8.16 (s, 2H, pyrene-H)
contribute to the development of organic optoelectronic materials in near future.
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EXPERIMENTAL SECTION Materials and Instruments. All materials were obtained from commercial suppliers and used without purification unless otherwise noted. The 1H/13C NMR spectra (300 MHz) were recorded on a Bruker Avance 300 MHz FT-300 NMR spectrometer referenced to 7.26 and 77.0 ppm for chloroform-D solvent with SiMe4 as an internal reference: J-values are given in Hz. FT-IR spectra were measured for samples as KBr pellets in a Thermo Fisher Nicolet Is10 spectrophotometer. Mass spectra were obtained with a Bruke microflex mass spectrometer. Elemental analyses were performed by Vario EL III Elementar Analysensysteme GmbH. UV−vis spectra were obtained with a PerkinElmer Lambda 950 UV−vis spectrometer in various organic solvents. Fluorescence spectroscopic studies were performed in various organic solvents in a semimicro fluorescence cell (Hellma, 104F-QS, 10 × 4 mm2, 1400 μL). Photoluminescence spectra were obtained using a Shimadzu F-7000 spectrophotometer. Fluorescence quantum yields were measured using absolute methods with a Japan Hamamatsu C9920-06G. Thermogravimetric analysis (TGA) was performed using a USA Waters Q600 under nitrogen atmosphere at a heating rate of 10 °C min−1. Differential scanning calorimetry (DSC) was performed using a METTLER TOLEDU DSC instrument under nitrogen atmosphere at a heating rate of 10 °C min−1. Electrochemical properties of HOMO and LUMO energy levels were determined by an electrochemical analyzer. The thin films were prepared by a solution process as follows: 6 mg of the sample was dissolved in 1 mL of CH2Cl2 solution, and the solution was placed on the substrate, which was then rotated at high speed to spread the fluid by centrifugal force. The quantum chemistry calculation was performed with the Gaussian 16W (B3LYP/6-31G* basis set) software package. Synthesis of 2-tert-Butylpyrene (1). To a stirred solution of 25.0 g (0.123 mol) of pyrene and 13.8 g (0.149 mol) of 2chloro-2-methylpropane in 100 mL of CH2Cl2 at 0 °C was added 17.6 g of anhydrous AlCl3 in one portion. After the mixture was stirred for 3 h at room temperature, it was poured into a large excess of ice water. The reaction mixture was extracted with dichloromethane. The combined organic extracts were washed with water and brine, the CH2Cl2 layer was separated, dried with anhydrous MgSO4 and evaporated, and the residue was crystallized from hexane. Further recrystallization from hexane gave pure 2-tert-butylpyrene (1) in 63% yield (20.0 g, 0.0775 mol) as yellowish silver plates. Mp 110−112 °C (lit 39.). 1H NMR spectrum completely agreed with the reported values. 1H NMR (300 MHz, CDCl3): δ = 1.59 (s, 9H, tBu), 8.18 (d, J = 9.2 Hz, 2H, pyrene-H), 8.30 (s, 2H, pyreneH), 8.37 (d, J = 9.2 Hz, 2H, pyrene-H), 8.47 (s, 1H, pyrene-H) ppm. Synthesis of 1,3-Dibromo-7-tert-butylpyrene. A solution of Br2 (0.20 mL, 3.87 mmol) in anhydrous CH2Cl2 (20 mL) was slowly added to a degassed solution of 2-tertbutylpyrene (0.5 g, 1.94 mmol) in anhydrous CH2Cl2 (10 mL) at −78 °C under argon atmosphere. The resulting mixture was allowed to slowly warm up to room temperature and was stirred overnight. The reaction mixture was poured into ice water and neutralized with a 10% aqueous solution of Na2S2O3. The mixture solution was extracted with dichloromethane. The organic layer was washed with water and brine and then the 5872
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ACS Omega and 8.29 (s, 2H, pyrene-H) ppm. 13C NMR (100 MHz, CDCl3) δ = 148.73, 141.42, 141.19, 139.87, 137.31, 130.72, 130.57, 130.24, 129.79, 128.54, 128.48, 127.51, 127.37, 127.33, 125.67, 124.56, 124.22, 121.48, 35.56, 31.79. MS: m/z calcd for C44H34 562.74; found 562.10 [M+]. Anal. calcd for C44H34: C, 93.91; H, 6.09; found: C, 93.61; H, 6.45. Synthesis of 7-tert-Butyl-1,3-diphenyl-5,9-di(4methoxyphenyl)pyrene (4b). A mixture of 7-tert-butyl-1,3diphenyl-5,9-dibromopyrene (200 mg, 0.35 mmol), 4-methoxyphenylboronic acid (133 mg, 0.87 mmol) in toluene (20 mL), and ethanol (12 mL) at room temperature was stirred under N2 atmosphere, and a 2 M aqueous solution of K2CO3 (8 mL) and [Pd(PPh3)4] (100 mg) were added. The mixture was heated to 110 °C under stirring for 16 h. After cooling to room temperature, the mixture was quenched with water, extracted with toluene, and washed with water and brine. The organic extracts were dried with MgSO4, and the solvent was evaporated. The residue was purified by column chromatography eluting with CH2Cl2/n-hexane (1:7) and recrystallized from CH2Cl2/n-hexane (2:1) to give 4b (150 mg, 68%) as a light-yellow powder. Mp 302 °C. νmax (KBr)/cm−1 3029.18, 2959.52, 2361.44, 1608.44, 1515.42, 1501.89, 1245.33, 1174.73, 1035.70, 833.10, 702.96, 611.46, 590.62. 1H NMR (300 MHz, CDCl3): δ = 1.38 (s, 9H, tBu), 3.92 (s, 6H, OMe), 7.08 (d, J = 9 Hz, 4H), 7.69, (d, J = 3 Hz, 4H), 7.43−7.59 (m, 10H), 7.96 (s, 1H, pyrene-H), 8.13 (s, 2H, pyrene-H), 8.32 (s, 2H, pyreneH) ppm. 13C NMR (100 MHz, CDCl3) δ = 157.86, 147.43, 140.07, 138.26, 135.80, 132.63, 130.10, 129.60, 129.52, 128.52, 127.31, 126.24, 126.07, 124.32, 123.17, 123.08, 120.26, 112.69, 54.29, 34.37, 30.66. MS: m/z calcd for C46H38O2 622.79; found 621.89 [M+]. Anal. calcd for C46H38O2: C, 88.71; H, 6.15; found: C, 88.61; H, 6.00. Synthesis of 7-tert-Butyl-1,3-diphenyl-5,9-di(4formylphenyl)pyrene (4c). A mixture of 7-tert-butyl-1,3diphenyl-5,9-dibromopyrene (200 mg, 0.35 mmol), 4-formylphenylboronic acid (131 mg, 0.87 mmol) in toluene (20 mL), and ethanol (12 mL) at room temperature was stirred under N2 atmosphere, and a 2 M aqueous solution of K2CO3 (8 mL) and [Pd(PPh3)4] (100 mg) were added. The mixture was heated to 110 °C under stirring for 16 h. After cooling to room temperature, the mixture was quenched with water, extracted with toluene, and washed with water and brine. The organic extracts were dried with MgSO4, and the solvent was evaporated. The residue was purified by column chromatography eluting with CH2Cl2/n-hexane (1:7) and recrystallized from CH2Cl2/n-hexane (2:1) to give 4c (160 mg, 74%) as a white solid. Mp 289 °C. νmax (KBr)/cm−1 2963.63, 2360.34, 1700.56, 1603.40, 1603.40, 1207.23, 835.92, 700.35, 537.66. 1H NMR (300 MHz, CDCl3): δ = 1.36 (s, 9H, tBu), 7.44−7.58 (m, 6H), 7.70 (d, J = 9 Hz, 4H), 7.83 (d, J = 9 Hz, 4H), 8.03 (s, 1H, pyrene-H), 8.08 (d, J = 9 Hz, 4H), 8.19 (s, 2H, pyrene-H), 8.23 (s, 2H, pyrene-H), 10.15 (s, 2H, CHO) ppm. 13C NMR (100 MHz, CDCl3) δ = 192.13, 149.28, 147.75, 140.73, 138.56, 138.20, 135.58, 130.89, 130.64, 130.05, 128.65, 127.62, 127. 06, 126.20, 124.96, 124.25, 121.32, 35.61, 31.76. MS: m/z calcd for C46H34O2 618.10; found 618.76 [M+]. Anal. calcd for C46H34O2: C, 89.29; H, 5.54; found: C, 89.15; H, 5.36.
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Theoretical calculations, crystallographic data, and spectroscopic data (PDF) Crystallographic data of 4b (CCDC 1567741) (CIF) Crystallographic data of 4c (CCDC 1577231) (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jian-Yong Hu: 0000-0002-6197-7531 Author Contributions §
R.L. and H.R. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National Key Research & Development Program (2016YFB0401000), the Natural Science Foundation of Shaanxi Province (2016JZ014), and the Fundamental Research Funds for the Central Universities (GK01010).
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