4-(10-Phenyl-9-Anthracenyl)-1,2-Benzenediol

Abstract

The title compound, 4-(10-phenyl-9-anthracenyl)-1,2-benzenediol, was synthesized using a two-step protocol. In the first step, 9-phenyl,10-bromoanthracene was coupled to 3,4-dimethoyphenylboronic acid by employing Pd(PPh₃)₄ as the catalyst and potassium carbonate as the base. Methoxy group removal was effected using HBr in the presence of acetic acid in the second step. Overall, two novel 9,10-diphenylanthracence-based compounds were synthesized in this work; standard spectroscopic techniques and high-resolution mass spectrometry were employed in their characterization. The photophysical properties of these compounds are also reported. These compounds are potentially useful as sensors, catalysts and building blocks for larger architectures.

1. Introduction

The compound, 9,10-diphenylanthracene (DPA), is extensively utilized in industrial arenas due to its excellent luminescence properties [1,2]. For example, it is used as a sensitizer in chemiluminescent reactions. DPA is also well-known as the compound that gives blue glowsticks and organic light-emitting diodes (OLEDs) their signature colors and luminescence [3]. There is keen interest in functionalizing DPA, as it is a means of tuning its luminescent properties, as well as developing synthetic routes for polymers, molecular wires and generally, larger architectures featuring this compound [4,5]. Functionalization via installation of a catechol moiety is important as the metal-binding properties of the resulting compounds can be harnessed for use as siderophore mimics, sensors or catalysts. The synthesis, characterization and photophysical properties of two novel DPA-based compounds are reported herein. This includes the title compound 2, which features a catechol moiety.

Compounds 1 and 2 (this work) are shown in Figure 1 alongside the only closely related compounds that have been reported. A DPA-based compound bearing catechol moieties (3) was synthesized and used as a ligand for conjugated coordinated polymers [6]. An anthracene compound appended with a catechol moiety, 9-(3,4-dihydroxyphenyl)anthracene (4), was synthesized and used as a ligand; the magnetic properties of its cobalt complexes were also studied [7]. In an expansion of that work, (boron-dipyrromethene) BODIPY was attached to this ligand (5); the magnetic and fluorescent properties of its cobalt complexes were studied [8]. Other DPA-based compounds bearing methoxy groups (6 and 7) have been studied computationally as candidates for solar cells and are used as dyes in test kits for the detection of occult blood [9,10].

2. Results and Discussion

2.1. Synthesis

The synthetic route to title compound 2 is shown in Scheme 1. The Suzuki cross-coupling reaction of 10-phenyl-9-bromoanthracene with 3,4-dimethoxyphenylboronic acid employing Pd(PPh₃)₄ as the catalyst in the presence of K₂CO₃ as the base gave 1 in 84% yield. Deprotection of 1 was effected using HBr/CH₃COOH, which afforded 2 in 40% yield. The procedures that were used for both coupling and deprotection steps were modified from those reported by King and co-workers in the synthesis of dendrimers containing the alkoxyalkylbiphenyl motifs [11]. We note that the second step of this protocol employs the milder HBr/CH₃COOH cleavage conditions for the methoxy group compared to the use of BBr₃ or Me₃SiI [12]. Compound 1 was obtained as a cream-colored semi-crystalline solid, whereas title compound 2 was obtained as a light yellow crystalline solid. Compounds 1 and 2 are soluble in a variety of solvents including acetonitrile, acetone, ethyl acetate, THF, dichloromethane, DMSO and methanol. These compounds have brilliant blue fluorescence in the solid state and in solutions of various solvents, visible when irradiated with UV light.

2.2. Characterization

The formation of compounds 1 and 2 was supported by spectroscopic data (¹H NMR, ¹³C NMR and IR), along with high-resolution mass spectrometry (HRMS).

2.2.1. Analysis of Compound 1

The ¹H NMR data of 1 (Figure S1) show the characteristic resonances due to the hydrogens of the 10-phenylanthracene moiety between 7.45 and 7.78 ppm. Resonances that are due to the hydrogens of the aryl group, originating from the boronic acid, are found between 7.36 and 7.86 ppm. Singlets due to the methoxy group hydrogens (OCH₃) are found at 3.82 and 3.98 ppm. The aromatic carbons have resonances between 110 and 150 ppm in the ¹³C NMR spectrum (Figure S2). The aromatic carbons bearing the methoxy groups are found at 148 and 149 ppm. This spectrum also features the overlapping resonances due to the carbons of the methoxy groups at 56.4 ppm. Distinguishing features in the IR spectrum of compound 1 (Figure S7) are the series of weak bands from 2800–3000 cm⁻¹ due to the C-H stretching vibrations from the methoxy group and the aromatic moieties. Another series of bands, weak to medium in their intensity, are present between 1500 and 1600 cm⁻¹. These bands are due to the C=C stretching vibrations of the aromatic moieties. HRMS (ESI-positive mode) shows a peak corresponding to the (M+H)⁺ ion; the observed mass was 391.1670, which is in excellent agreement with the theoretical mass of 391.1673 (Figure S5).

2.2.2. Analysis of Compound 2

The ¹H NMR data of 1 (Figure 2) shows the characteristic resonances due to the hydrogens of the 10-phenylanthracene moiety within a range of 7.36–7.86 ppm. Resonances which were due to the aryl group originating from the boronic acid were found between 6.80 and 7.15 ppm. Singlets due to catechol hydroxyl hydrogens (OHs) were found at 8.17 and 8.22 ppm. The full ¹H NMR spectrum is shown in Figure S1.

Key resonances seen in the ¹³C NMR spectrum (Figure S4) are due to the carbons of the catechol moiety at ca. 145 ppm and the aromatic carbons that are found between 115 and 145 ppm. The aromatic carbons bearing the hydroxyl groups are found at ca. 145 ppm. The IR spectrum (Figure S8) of compound 2 showed a sharp band due to the O-H stretching frequency at 3500 cm⁻¹, followed by two broader bands centered at 3300 cm⁻¹ due to hydrogen bonding of the O-H group. The bands due to the C-H stretching vibrations originating from the aromatic moieties are found between 2900 and 3100 cm⁻¹. Another series of bands from 1500 to 1700 cm⁻¹ are present due to the C=C stretching vibrations of the aromatic moieties. HRMS (ESI-negative mode) showed a peak corresponding to the (M-H)⁻ ion in the spectrum of 2; the observed mass was 361.1238, which is in excellent agreement with the theoretical mass of 361.1234 (Figure S8).

2.3. Photophysical Studies

The photophysical properties of 1 and 2 were determined in acetonitrile and are compared to DPA (

Table 1. The photophysical properties of compounds 1 and 2 in acetonitrile at room temperature.

Compound	Absorption	Emission
	λmax (nm) (log ε a)	λmax (nm)	Stokes’ Shift	Φ (%)
DPA b	259 (5.0), 338 (3.6), 354 (3.80), 374 (4.04), 394 (4.00)	408, 438	15	95 c
1	259 (5.35), 338 (3.92), 355 (4.24), 374 (4.45), 394 (4.43)	434	40	70
2	259 (5.47), 338 (4.00), 355 (4.32), 374 (4.53), 394 (4.51)	437	43	63

^a = ε- extinction coefficient (M⁻¹ cm⁻¹); ^b = ε, λ_max for absorption and λ_max for emission were measured in this work; ^c = quantum yield of DPA in ethanol [13].

). The absorption (UV-Vis) spectra (Figure 3) of 1 and 2, as expected, show identical features to those of DPA. The peak with λ_max at ca. 260 nm is due to π to π* transitions from the anthryl moiety and the aryl group bearing the methoxy or catechol moieties. The peaks, with λ_max values between 342 and 395 nm, are vibronic bands due the π to π* transitions of the anthryl moiety. This fine vibrational structure is characteristic of this moiety and is attributed to its rigidity. Molar extinction coefficient values for these peaks are high, with log values ranging from 4.0 to 5.45, indicating the great efficiency of the absorption process in these compounds.

The fluorescence spectra (Figure 3) that were produced by excitation of the complexes at ca. 374 nm show an emission peak, with λ_max at ca. 435 nm, and is attributed to the π* to π transitions; this is consistent with the strong blue, fluorescent nature of these compounds. We note that the emission peaks of compounds 1 and 2 are broader and slightly red-shifted relative to DPA. Quantum yields for compounds 1 (70%) and 2 (63%) are very respectable but lower than DPA [13].

3. Materials and Methods

10-Phenyl-9-bromoanthracene was synthesized according to the method outlined in the literature [14]. All other starting materials for the synthesis of compounds 1 and 2 and compounds that were used in the quantum yield measurements were purchased from Sigma Aldrich and used as received. Toluene was obtained from a JC Meyer solvent dispensing system and used without further purification. All other solvents were of reagent or spectroscopic grade and were degassed if necessary.

NMR spectra were recorded on Bruker DPX-300, DPX-400 400 and AVIII HD 500 MHz spectrometers and calibrated using residual solvent peaks. Chemical shifts were reported in δ (parts per million) and coupling constants were reported as J values (hertz). High-resolution ESI mass spectrometry was performed on a Thermo LTQ Orbitrap Discovery using a flow injection method in the range 100–2000 Da. Absorption spectra were obtained on a Perkin Elmer Lambda 35 UV-Vis spectrophotometer. Fluorescence excitation and emission spectra were acquired on a Horiba FluoroMax-4. IR Spectra were acquired on a PerkinElmer Frontier Spectrometer.

Protocol for Absorption and Fluorescence Measurements: All solutions were prepared in dry degassed CH₃CN. A solution of concentration 0.386 mM was prepared for each compound. A 1 in 10 dilution was then performed on this solution, followed by another 1 in 10 dilution on the subsequent solution. This gave a solution with a concentration of 3.86 μM. The spectrum was then taken.

Protocol for IR Measurements: A small sample of each compound was placed on the diamond crystal platform of the ATR module. The swivel pressure tower was then used to press the sample onto the ATR crystal to ensure good contact. A background measurement was taken without any sample, and then sample measurements were taken; 16 scans were used for each measurement.

Determination of Quantum Yields [15]: All solutions were prepared in dry degassed solvent. The solution concentration (7.0 μM) was chosen based on the molar extinction coefficient of the compounds, such that the stock solution had an absorbance of ca. 0.1 at the excitation wavelength. To minimize the potential for inner filter effects, dilutions were performed yielding solutions of concentrations of 5.6 μM, 4.2 μM, 2.8 μM and 1.4 μM. The UV-vis spectrum was recorded (220–600 nm), noting the absorbance at the excitation wavelengths.

Subsequently, the full visible-range fluorescence spectra were recorded for all solutions. The resultant fluorescence spectra were integrated across the emission range. The integration values were plotted against the absorbance values for all solutions at the excitation wavelengths. The plots were then fit to linear trendlines. The gradients of these trendlines were used to calculate the quantum yield according to the following:

$${\varphi }_x={\varphi }_{st}\left({\displaystyle \frac{{grad}_x}{{grad}_{st}}}\right)\left({\displaystyle \frac{{\eta }_x^2}{{\eta }_{st}^2}}\right)$$

where ϕ_x, η_x and grad_x represent the fluorescence quantum yield, refractive index and gradient of the aforementioned linear trendline. respectively. Subscripts denote the identity of the solution, whether analyte (“x”) or standard (“st”). DPA in ethanol (Φ = 95%) was used as the standard. The gradients of compounds 1 and 2 were compared to the standard compound.

Compound 1. 9-Bromo-10-phenyl-anthracene (1.03 g, 3.1 mmol), Pd(PPh₃)₄ (0.4 g, 0.35 mmol), K₂CO₃ (0.737 g, 5.3 mmol) and 3,4-dimethoxyphenylboronic acid (1.03 g, 5.7 mmol) were added to a Schlenk flask. Three evacuation-fill with N₂ cycles were then performed. A toluene/ethanol/water solvent system (3:3:1, 63 mL) was deoxygenated via 3 freeze–pump–thaw cycles and added to the Schlenk flask using a cannula. The reaction mixture was heated to reflux for 40 h. The reaction mixture was then diluted with equal volumes of water and CH₂Cl₂ and the layers separated. The aqueous layer was extracted three times with CH₂Cl₂. The combined CH₂Cl₂ extracts were washed with water and brine, dried with MgSO₄ and filtered. The solvent was removed in vacuo. The resulting solid was then washed with copious amounts of ethanol to yield the product as a cream-coloured semi-crystalline solid (1.04 g, 84%).

¹H NMR (500 MHz, DMSO-d₆, 300 K) δ 7.78–7.67 (m, 4H, 10-phenylanthracene), 7.71–7.60 (m, 3H. 10-phenylanthracene), 7.55–7.45 (m, 6H, 10-phenylanthracene), 7.29 (d, J = 8.1 Hz, 1H, Ar(OMe)₂), 7.10 (d, J = 1.9 Hz, 1H, Ar(OMe)₂), 7.05 (dd, J = 8.0, 1.9 Hz, 1H, Ar(OMe)₂), 3.98 (s, 3H, Ar(OMe)₂), and 3.82 (s, 3H, Ar(OMe)₂).

¹³C NMR (125 MHz, CDCl₃) δ 148.87, 148.38, 139.08, 137.06, 136.99, 131.48, 131.34, 131.31, 130.15, 129.89, 128.44, 127.49, 127.05, 126.98, 125.03, 123.56, 114.43, 111.12, 76.80, 56.04, and 55.99.

IR (ATR): ν = 3065, 2954, 2912, 2837, 1580, 1578, 1508, 1463, 1440, 1386, and 1384 cm⁻¹.

HR-ESI-MS m/z: [M + H]⁺ calcd. for C₂₈H₂₂O₂ 391.1693; found 391. 1670.

UV-vis (acetonitrile) λ_max (log ε) [λ_max (nm) and ε (cm⁻¹ M⁻¹)]: 259 (5.35), 338 (3.92), 355 (4.24), 374 (4.45), and 394 (4.43). Fluorescence (acetonitrile) nm: 434.

Compound 2. Compound 1 (180 mg, 0.46 mmol) was added to a round bottom flask along with HBr (48% in acetic acid) (2 mL). The reaction mixture was heated at 115 °C for 48 h. The reaction was cooled and neutralized with a NaHCO₃ solution. The aqueous layer was then extracted twice with CH₂Cl₂, and the combined CH₂Cl₂ extracts were washed with water and dried with MgSO₄. Following evaporation of the solvent, the product was isolated via preparative TLC (1:4 acetone: hexanes) and recrystallized using the same solvent system to yield 2 as a light yellow crystalline solid (67 mg, 40%).

¹H NMR (500 MHz, Acetone-d₆) δ 8.21 (s, 1H, ArH(OH)₂), 8.17 (s, 1H, ArH(OH)_2, (d, J = 22.1 Hz, 2H), 7.87–7.75 (m, 2H, 10-phenylanthracene), 7.72–7.59 (m, 5H, 10-phenylanthracene), 7.52–7.37 (m, 6H, 10-phenylanthracene), 7.13 (d, J = 7.9 Hz, 1H, ArH(OH)₂), 6.97 (d, J = 2.0 Hz, 1H, ArH(OH)₂), 6.82 (dd, J = 7.9, 2.0 Hz, 1H, ArH(OH)₂).

¹³C NMR (214 MHz, Acetone-d₆) δ 145.17, 144.82, 139.00, 137.42, 136.88,131.12, 130.16, 129.85, 128.55, 127.59, 127.06, 126.52, 125.13, 124.92, 122.68, 118.18, and 115.42.

IR (ATR): ν = 3505, 3396, 3300, 3199, 3052, 2923, 2854, 1725, 1608, 1528, 1431, 1363, and 1302 cm⁻¹.

HR-ESI-MS m/z: [M-H]⁻ calcd. for C₂₆H₁₈O₂ for 361.1238; found 361.1234.

UV-vis (acetonitrile) λ_max (log ε) [λ_max (nm) and ε (cm⁻¹ M⁻¹)]: 259 (5.47), 338 (4.00), 355 (4.32), 374 (4.53), and 394 (4.51). Fluorescence (acetonitrile) nm: 437.

4. Conclusions

In summary, we report the synthesis, characterization and photophysical studies of novel DPA-based compounds, including the title compound, 4-(10-phenyl-9-anthracenyl)-1,2-benzenediol. Both compounds are potentially useful building blocks for other DPA-based architectures including polymers, metal complexes and sensors.