Dialkoxybenzo[j]fluoranthenes: synthesis, structures, photophysical properties, and optical waveguide application

Abstract

A series of dialkoxybenzo[j]fluoranthene derivatives were readily and efficiently synthesized in gram scale starting from the commercial 6- or 7-methoxy-1-tetralone. The crystal structures of the BjF derivatives were described, and their structure–optical properties in solution and in the solid state were investigated. Moreover, this kind of organic material also exhibited excellent optical waveguide behavior owing to their large Stokes shifts and high crystallinity.

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Introduction

Polycyclic aromatic hydrocarbons (PAHs) have become one of the largest class of organic molecules owing to their various molecular structural forms.¹ The specific structural characteristics and unique physical and electronic properties of PAHs^2–6 allow them to be widely used in organic semiconducting devices,^7–9 such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), organic thin-film transistors (OTFTs), liquid crystals and organic solar cells. The isomeric benzofluoranthenes (benzo[a]fluoranthene, BaF; benzo[b]fluoranthene, BbF; benzo[j]fluoranthene, BjF; and benzo[k]fluoranthene, BkF)¹⁰ are a class of important PAHs, and they have attracted much attention because of their cytotoxic and tumorigenic activity,¹¹ and unique photoelectric properties.^12,13

Compared with BkF derivatives¹⁴ with convenient, available, easily modification, and wide potential applications in organic semiconductors or luminescent materials,¹⁵ few examples¹⁶ on the synthesis and applications of substituted benzo[j]fluoranthenes have been reported. Especially, most of the synthetic methods not only involved harsh reaction conditions and multi-step routes, but also gave the products in poor yields.¹⁷ Moreover, studies on their electro-optical properties and potential applications in organic optoelectronic materials are also limited.¹⁸ So, it is very necessary to develop a convenient and efficient route to the synthesis of BjF derivatives, and explore their photoelectric properties.

Herein, we report the convenient and efficient synthesis of substituted benzo[j]fluoranthene derivatives starting from the commercial materials. The structures of the BjF derivatives, and their photophysical properties in both solution and solid states are described in details. Moreover, a practical application of the BjF derivatives in optical waveguide is also reported.

Results and discussion

Synthesis of BjF derivatives

The synthetic route of dimethoxy substituted BjF derivatives 4a–b is depicted in Scheme 1. Starting from the commercial available 7-methoxy-1-tetralone 1a, diene 2a was first prepared in 72% yield according to the literature method,¹⁹ By treatment of 2a with 1,4-benzoquinone (1,4-BQ) in CH₃CO₂H : HCl (v/v, 15 : 1) at reflux temperature for 8 h, BjF derivative 4a was obtained in 60% yield. Similarly, BjF derivative 4b could be synthesized starting from 6-methoxy-1-tetralone 1b. As an alternative way, compounds 4a and 4b could also be synthesized by the treatment of 1-tetralone derivatives 1a–b with zinc dust in the presence of TMSCl and HCl, and then followed by the oxidative dehydrogenation of 1,2,3,6b,7,8-hexahydrobenzo[j]fluoranthene derivatives 3a–b with 1,4-BQ in refluxed toluene. According to route 2, BjF derivative 4a could also be conveniently prepared in gram scale. As the picture shown in Scheme 1, 6.3 g of 4a could be obtained from 11.8 g of 7-methoxy-1-tetralone, which provided a basis for its further functionalization and practical applications.

Scheme 1 Synthesis of BjF derivatives 4a and 4b.

Demethylation of compound 4a with boron tribromide in dry dichloromethane produced dihydroxyl-substituted BjF derivative 5 in 98% yield, which was then treated by brominated alkanes in acetone in the presence of anhydrous K₂CO₃ to give BjF derivatives 4c–e in high yields (Scheme 2).

Scheme 2 Synthesis of BjF derivatives 4c–e.

X-ray crystallographic structures

The single crystals of BjF derivatives 4a, 4b and 4d suitable for X-ray diffraction analysis were obtained by their slow evaporation in CH₂Cl₂ at ambient temperature. As shown in Fig. 1a–c, 4a, 4b and 4d all have planar BjF cores. But it was found that depending on the position and size of the substituents, the BjF molecules could take different packing patterns by the intermolecular π–π interactions. For molecule 4a, wavy packing pattern (Fig. 1d) was found, while molecule 4b showed cross-herringbone-like packing pattern (Fig. 1e) in which the molecule can take close packing structure probably owing to the two methoxyl groups in far away from each other (11.37 Å) than those of 4a (9.22 Å). In the case of 4d, it could form a layered packing pattern (Fig. 1f) by the CH–π interactions between the adjacent molecules with the distances of 2.82–2.88 Å, and the π–π interactions between the adjacent molecules with the distance of 3.35 Å. Because of the two long alkyl chains in 4d, effective intermolecular π–π stacking between the adjacent molecules could be avoided, which might subsequently prevent its fluorescence quenching in the solid state.

Fig. 1 Crystal structures of 4a (a), 4b (b) and 4d (c), and packing patterns of 4a (d), 4b (e) and 4d (f).

Photophysical properties

The photophysical properties of BjF derivatives 4a–e in CH₂Cl₂ were first investigated. The photophysical data were summarized in Table 1, and their absorption and emission spectra were shown in Fig. 2a and b. Correspondingly, the fluorescent photos of 4a–e in CH₂Cl₂ under 365 nm irradiation were shown in Fig. 2c. It was found that the maximum absorption and emission of 4a were at 407 and 518 nm, respectively, which indicated that it has a large Stokes shifts (111 nm). Compared with 4a, compound 4b with different positions of the methoxyl groups has obvious blue-shift of its maximum absorption from 407 to 389 nm, and slightly red-shift maximum emission (from 518 to 531 nm). Especially, we found that the molar absorptivity (ε) and quantum yield (φ_f) of 4b were much less than 4a. To investigate the effect of the size of the substituents on the photophysical properties of the BjF skeleton, we further prepared a series of different alkoxy substituted derivatives 4c–e starting from 4a. Besides slightly high quantum yields of 4c–e were found, their absorption and emission spectra were very similar to those of 4a, which suggested that the size of the alkoxyl group has no significant effect on the photophysical property of the BjF skeleton in solution.

Table 1 Photophysical properties of 4a–e in CH₂Cl₂^a

Compd	λmax abs (nm)	log ε (M^−1 cm^−1)	λem (nm)	Δλ^b (nm)	φf^c (%)
a All spectra were recorded in CH2Cl2 (c = 1.0 × 10^−5 M) at room temperature.b Stokes shift = λem,max − λabs,max.c Absolute fluorescence quantum yield, measured using a Hamamatsu Photonics Quantaurus QY.
4a	407	4.37	518	111	7.8
4b	389	3.79	531	142	3.5
4c	408	4.24	518	110	8.2
4d	409	4.13	519	110	8.2
4e	409	4.15	518	109	8.3

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Fig. 2 (a) UV-vis, and (b) fluorescence spectra of 4a–e in CH₂Cl₂ (c = 1.0 × 10⁻⁵ M) at room temperature. (c) Photographs of 4a–e in CH₂Cl₂ (c = 1.0 × 10⁻⁵ M) under irradiation at 365 nm.

It was also found that BjF derivatives 4a–e exhibited strong fluorescent emission in the solid state, and the photophysical data were summarized in Table 2. As shown in Fig. 3, the maximum emission band of 4a in the spin-coated film is similar to that one in solution. Although its maximum absorption band is slightly red-shift, the Stokes shift of 104 nm is still large. Moreover, the fluorescence quantum yield of 4a (14.0%) was found to be bigger than that one in solution, which might be due to the steric hindrance of the methoxyl groups to impede the intermolecular π–π stacking to some extent. Similarly, it was found that 4b showed bigger fluorescence quantum yield in the solid state (10.5%) than that one in solution. Compared with 4a, BjF derivatives 4c–e with bigger alkoxyl groups could impede the intermolecular π–π stacking interactions to a greater extent, which would result in the increase of the fluorescence quantum yields in the solid state. In addition, it was also found that the thickness of the films which could be determined by AFM topography images showed little influence on their fluorescence spectra (Fig. S18†). Thus, it can be concluded that just by changing the size and position of the substituents, the increased of fluorescence quantum yields for the BjF derivatives could be achieved, which might make these molecules have wide potential applications in optical materials.

Table 2 Photophysical properties of 4a–e in the spin-coated films^a

Compd	λmax abs (nm)	λem (nm)	Δλ^b (nm)	φf^c (%)
a Spin-coated films prepared from CH2Cl2 solution (5 mg mL^−1).b Stokes shift = λem,max − λabs,max.c Absolute fluorescence quantum yield, measured by a Hamamatsu Photonics Quantaurus QY.
4a	416	520	104	14.0
4b	404	520	116	10.5
4c	415	510	95	17.3
4d	415	510	95	17.7
4e	417	513	96	17.1

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Fig. 3 (a) Fluorescence spectra of 4a–e in the spin-coated films at room temperature. (b) Photographs of the spin-coated films of 4a–e under irradiation at 365 nm.

Waveguide behavior

The strong fluorescence in solid state and easily crystallized properties of the BjF derivatives encouraged us to further explore their applications in optical waveguides. As a result, the microrod of 4d (43 nm) was prepared by an anti-solvent diffusion method. As shown in Fig. 4a, when the different local positions of the microrod were excited by a focused laser (351 nm), blue emissions were detected from the ends of the microrod. These phenomena indicated that the molecule has the typical optical waveguide behavior. Furthermore, in order to investigate the optical waveguide of compound 4d, the propagation loss of the materials was evaluated by detecting the spatially resolved spectra of the emitted light with respect to the distance travelled. And Fig. 4b illustrates the corresponding signals which were detected from the microrod terminus by changing the position of the excitation laser beam. The inset of Fig. 4b exhibits the change of the emission intensity detected at the ends of the microrod decreased with the propagation distance, optical loss coefficient for microrods of 4d at 512 nm was estimated to be 18.84 dB mm⁻¹, which is lower than usual organic materials.²⁰ There are two factors which may contribute to this property. First, the large Stokes shifts effectively reduced the emission spectral overlap with its absorption spectra, which can minify the self-absorption of light during propagation. Second, the high crystallinity and smooth surface can avoid scattering during this process, which diminishes the optical loss. The above reasons contributed to the excellent optical waveguide behavior of this kind of organic materials.

Fig. 4 (a) Bright-field and PL images obtained from a single 4d 1D nanowire by exciting the wire at different positions. (b) Spatially resolved PL spectra from the tip of the nanowire for different separation distances between the excitation spot and tip of the wire shown in (a). The inset shows the plot of the peak intensity at 512 nm vs. the propagation distance.

Conclusions

We have provided two convenient and efficient routes for the synthesis of dialkoxyl substituted BjF derivatives in even gram scale by two-step reactions starting from commercial materials. The photophysical properties of the BjF derivatives were studied in details, and it was found that compounds 4a–e exhibited strong fluorescence and large Stokes shifts in solid states. Moreover, by changing the size and position of the substituents, the BjF derivatives displays different packing patterns, which could influence the intermolecular π–π stacking, and thus increase the fluorescence quantum yields in solid state. Furthermore, the optical waveguide property of a BjF derivative was also detected, and it exhibited excellent optical waveguide behavior, which might be attributed to its large Stokes shifts and high crystallinity.

Experimental section

6,11-Dimethoxy-1,2,3,6b,7,8-hexahydrobenzo[j]fluoranthene (3a)

To a mixture of 7-methoxy-1-tetralone (8.8 g, 0.05 mol) and zinc dust (6.5 g, 0.10 mol) in THF (170 mL) at −78 °C was added chlorotrimethylsilane (12.95 mL, 0.15 mol) and 37% HCl (20 mL), respectively. After 30 minutes, the mixture was stirred at room temperature for 9 h, and then heated at 60 °C for 20 minutes. The reaction mixture was dissolved in CH₂Cl₂, washed with water, dried over anhydrous MgSO₄, and then concentrated under reduced pressure. The residue was recrystallized from ethanol to give product 3a (5.45 g, 68%) as white powder. Mp: 167–168 °C. ¹H NMR (300 MHz, CDCl₃): δ 7.12 (d, J = 8.1 Hz, 2H), 7.01 (d, J = 8.1 Hz, 1H), 6.74 (dd, J = 8.4, 2.1 Hz, 1H), 6.65 (d, J = 8.1 Hz, 1H), 3.86 (d, J = 1.4 Hz, 3H), 3.82 (d, J = 1.5 Hz, 3H), 3.58 (d, J = 10.8 Hz, 1H), 3.00–2.98 (m, 2H), 2.95–2.82 (m, 2H), 2.79–2.73 (m, 2H), 2.17–2.06 (m, 1H), 1.88–1.72 (m, 1H), 1.36–1.30 (m, 1H). ¹³C NMR (75 MHz, CDCl₃): δ 157.7, 154.5, 145.0, 138.3, 134.6, 132.9, 130.9, 129.9, 129.7, 126.4, 126.0, 112.4, 111.5, 107.7, 55.5, 55.4, 48.8, 30.0, 27.8, 26.7, 25.5, 24.2. HR MS (ESI): m/z calcd for [M + H]⁺ C₂₂H₂₃O₂: 319.1619, found 319.1690.

5,10-Dimethoxy-1,2,3,6b,7,8-hexahydrobenzo[j]fluoranthene (3b)

According to the same method as described above, 3b (5.7 g, 70% yield) as pale yellow solid was obtained starting from 6-methoxy-1-tetralone (8.8 g, 0.05 mol). Mp: 154–155 °C. ¹H NMR (400 MHz, CDCl₃): δ 7.53 (d, J = 8.5 Hz, 1H), 6.89 (s, 1H), 6.83–6.78 (m, 1H), 6.77 (s, 1H), 6.64 (s, 1H), 3.82 (d, J = 5.6 Hz, 6H), 3.45 (d, J = 13.2 Hz, 1H), 3.14–2.80 (m, 5H), 2.60–2.51 (m, 1H), 2.14–2.10 (m, 1H), 1.91–1.78 (m, 1H), 1.49–1.43 (m, 1H). ¹³C NMR (101 MHz, CDCl₃): δ 158.2, 157.8, 145.9, 138.1, 137.3, 135.1, 133.0, 130.2, 127.2, 127.1, 114.0, 112.2, 110.7, 107.3, 55.7, 55.3, 49.6, 31.2, 28.5, 27.5, 25.3, 23.9. HRMS (ESI) m/z calcd for [M + H]⁺ C₂₂H₂₂O₂ 319.1619, found 319.1690.

General procedures for the synthesis of BjF derivatives 4a and 4b

Method A. A mixture of diene 2a or 2b (2 g, 6.29 mmol) and 1,4-benzoquinone (6.79 g, 62.90 mmol) in acetic acid (150 mL) and HCl (10 mL) was stirred at reflux temperature for 8 h, and then concentrated under reduced pressure. To the residue was added CH₂Cl₂, and the solution was washed with water, dried over anhydrous MgSO₄. After the solvent was removed, the residue was chromatographed on silica gel to afford 4a (1.28 g, 64%) or 4b (1.2 g, 60%) as luminous yellow solid.

Method B. The mixture of compound 3a or 3b (0.75 g, 2.35 mmol) and 1,4-benzoquinone (1.27 g, 11.75 mmol) in toluene (100 mL). was refluxed for 12 h. The reaction mixture was cooled to room temperature, and concentrated under reduced pressure. The residue was then subjected to flash column chromatography with CH₂Cl₂ : petroleum ether (1 : 1, v/v) as eluent to afford 4a (0.66 g, 89%) or 4b (0.65 g, 87%) as luminous yellow solid.

6,11-Dimethoxybenzo[j]fluoranthene (4a)

Mp: 178–180 °C. ¹H NMR (300 MHz, CDCl₃): δ 8.40 (d, J = 7.1 Hz, 1H), 8.22 (d, J = 8.3 Hz, 1H), 7.99 (d, J = 1.9 Hz, 1H), 7.89 (d, J = 8.9 Hz, 1H), 7.84 (d, J = 4.7 Hz, 1H), 7.83–7.77 (m, 2H), 7.61 (t, J = 7.6 Hz, 1H), 7.36 (d, J = 8.9 Hz, 1H), 7.14 (dd, J = 9.0, 2.3 Hz, 1H), 4.18 (s, 3H), 4.07 (s, 3H). ¹³C NMR (75 MHz, CDCl₃): δ 158.4, 155.7, 137.4, 136.7, 133.3, 131.5, 131.1, 130.7, 130.1, 128.7, 127.9, 125.9, 125.7, 125.0, 124.1, 120.8, 120.5, 117.2, 114.9, 102.9, 56.0, 55.4. HRMS (ESI): m/z calcd for [M + H]⁺ C₂₂H₁₆O₂ 313.1150, found 313.1219.

6,11-Dimethoxybenzo[j]fluoranthene (4b)

Mp: 172–173 °C. ¹H NMR (400 MHz, CDCl₃): δ 8.57 (d, J = 9.1 Hz, 1H), 8.21 (d, J = 6.9 Hz, 1H), 7.92 (d, J = 8.3 Hz, 1H), 7.74–7.68 (m, 2H), 7.62 (d, J = 7.2 Hz, 1H), 7.59 (s, 1H), 7.27 (d, J = 2.3 Hz, 1H), 7.24 (s, 1H), 7.20 (s, 1H), 7.09 (s, 1H), 3.97 (d, J = 15.1 Hz, 6H). ¹³C NMR (101 MHz, CDCl₃): δ 160.4, 157.2, 138.8, 137.4, 135.8, 135.7, 135.0, 130.1, 128.5, 127.8, 126.8, 126.0, 125.9, 125.8, 121.7, 120.4, 119.7, 113.5, 107.2, 104.6, 55.9, 55.3. HRMS (ESI): m/z calcd for C₂₂H₁₇O₂ [M + H]⁺ 313.1150, found 313.1218.

6, 11-Dihydroxybenzo[j]fluoranthene (5)

Compound 4a (3.1 g, 0.01 mol) was treated with boron bromide (9.4 mL, 0.1 mol) in dry dichloroethane (50 mL) at 0 °C for 10 h. The reaction mixture was quenched with NaHCO₃ aqueous solution. The precipitated yellow solid was filtered, washed with water and dried under vacuum to give 5 (2.9 g, 98%) as yellow-green solid. Mp: >300 °C. ¹H NMR (300 MHz, acetone-d₆): δ 8.49 (d, J = 7.1 Hz, 1H), 8.23–8.18 (m, 2H), 7.92–7.83 (m, 4H), 7.63 (t, J = 7.6 Hz, 1H), 7.35 (d, J = 8.7 Hz, 1H), 7.15 (dd, J = 8.9, 2.0 Hz, 1H). ¹³C NMR (101 MHz, CDCl₃): δ 156.3, 153.9, 137.3, 136.3, 133.5, 131.8, 130.1, 128.1, 127.9, 125.8, 125.2, 124.9, 123.8, 120.6, 119.9, 118.0, 117.1. HRMS (ESI): m/z calcd for [M + H]⁺ C₂₀H₁₃O₂ 285.0837, found 285.0905.

General procedure for the synthesis of BjF derivatives 4c–e

To a mixture of compound 5 (1.0 g, 3.4 mmol) and potassium carbonate (3.0 equiv.) in acetone (100 mL) was added brominated alkane (3.0 equiv. per hydroxy group). The reaction mixture was refluxed for 24 h, and then concentrated under reduced pressure. To the residue was added CH₂Cl₂, the solution was then washed with water, and dried over anhydrous MgSO₄. After the solvent was removed, the residue was chromatographed on silica gel with CH₂Cl₂ : petroleum ether (1 : 1, v/v) as eluent to yield the product as yellow solid.

6, 11-Diethoxybenzo[j]fluoranthene (4c)

Yield: 90%. Mp: 188–189 °C. ¹H NMR (300 MHz, CDCl₃): δ 8.38 (d, J = 7.0 Hz, 1H), 8.22 (d, J = 8.2 Hz, 1H), 7.99 (s, 1H), 7.93–7.73 (m, 4H), 7.60 (t, J = 7.6 Hz, 1H), 7.32 (d, J = 8.9 Hz, 1H), 7.14 (d, J = 8.9 Hz, 1H), 4.41 (d, J = 6.9 Hz, 2H), 4.36–4.23 (m, 2H), 1.64 (t, J = 6.9 Hz, 3H), 1.56 (dd, J = 12.6, 5.7 Hz, 3H). ¹³C NMR (75 MHz, CDCl₃): δ 157.8, 155.2, 137.4, 136.7, 133.3, 131.6, 131.1, 130.7, 130.0, 128.7, 127.9, 125.9, 125.6, 124.9, 124.0, 120.7, 120.6, 117.4, 115.8, 103.7, 64.4, 63.6, 15.2, 14.9. HRMS (ESI): m/z calcd for [M + H]⁺ C₂₄H₂₁O₂ 341.1536, found 341.1532.

6, 11-Dibutoxybenzo[j]fluoranthene (4d)

Yield: 92%. Mp: 150–151 °C. ¹H NMR (300 MHz, CDCl₃): δ 8.39 (d, J = 7.1 Hz, 1H), 8.21 (d, J = 8.3 Hz, 1H), 7.99 (d, J = 2.1 Hz, 1H), 7.89–7.77 (m, 4H), 7.66–7.55 (m, 1H), 7.34 (d, J = 8.9 Hz, 1H), 7.14 (dd, J = 8.9, 2.3 Hz, 1H), 4.35 (t, J = 6.4 Hz, 2H), 4.24 (t, J = 6.4 Hz, 2H), 2.07–1.87 (m, 4H), 1.73–1.58 (m, 4H), 1.10–1.06 (m, 3H), 1.06–1.02 (m, 3H). ¹³C NMR (75 MHz, CDCl₃): δ 158.0, 155.3, 137.5, 136.7, 133.3, 131.6, 131.0, 130.7, 130.0, 128.6, 127.9, 125.8, 125.7, 124.9, 124.0, 120.61, 120.57, 117.5, 115.8, 103.7, 68.5, 67.8, 31.7, 31.6, 19.5, 19.4, 13.99, 13.96. HRMS (ESI): m/z calcd for [M + H]⁺ C₂₆H₂₉O₂ 397.2162, found 397.2155.

6, 11-Didodecoxybenzo[j]fluoranthene (4e)

Yield: 90%. Mp: 103–104 °C. ¹H NMR (300 MHz, CDCl₃): δ 8.39 (d, J = 7.1 Hz, 1H), 8.22 (d, J = 8.3 Hz, 1H), 7.99 (d, J = 1.9 Hz, 1H), 7.87–7.78 (m, 4H), 7.65–7.57 (m, 1H), 7.34 (d, J = 8.9 Hz, 1H), 7.14 (dd, J = 8.9, 2.2 Hz, 1H), 4.35 (t, J = 6.4 Hz, 2H), 4.24 (t, J = 6.5 Hz, 2H), 2.09–1.86 (m, 4H), 1.61 (dd, J = 18.7, 11.2 Hz, 4H), 1.27 (s, 32H), 0.88 (t, J = 6.1 Hz, 6H). ¹³C NMR (75 MHz, CDCl₃): δ 158.0, 155.3, 137.5, 136.7, 133.3, 131.6, 131.0, 130.6, 130.0, 128.6, 127.9, 125.8, 125.6, 124.9, 124.0, 120.6, 120.6, 117.5, 115.8, 103.7, 68.8, 68.1, 32.0, 29.7, 29.68, 29.63, 29.58, 29.54, 29.4, 26.3, 26.2, 22.7, 14.1. HRMS (ESI): m/z calcd for [M + H]⁺ C₄₄H₆₁O₂ 621.4661, found 621.4650.

Acknowledgements

We thank the National Natural Science Foundation of China (21272264, 21332008), the National Basic Research Program (2011CB932501, 2015CB856502), and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12010400) for financial support.

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Footnote

† Electronic supplementary information (ESI) available: Copies of ¹H and ¹³C NMR spectra of 3a, 3b and 4a–e. UV-vis spectra of 4a–e in the spin-coated films at room temperature. Crystal data of 3a, 4a, 4b and 4d. CCDC 1037207, 1037219, 1038631 and 1037208. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra17112h

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