Synthesis of highly emissive 1,8-diaryl anthracene derivatives and fabrication of their micro/nanostructures

Abstract

We report a new route for the synthesis of 1,8-diaryl anthracene derivatives (1–6) starting from anthraquinone. The reduction of 1,8-dichoroanthraquinone, followed by aryl–aryl coupling using modified Suzuki–Miyaura reaction conditions, furnished 1,8-diarylanthracene derivatives in a good yield. Detailed photophysical and electrochemical studies show that these anthracene derivatives emit in the blue region with a narrow FWHM and provide high quantum yields (upto 75%). The HOMO and LUMO energy levels of these compounds are in the ranges of −5.62 to 5.71 and 2.68–2.79 eV, respectively. Furthermore, anthracene derivative 5 was used for a surfactant assisted self assembling process to obtain micro/nanostructures. Compound 5 formed microplates in polyvinylpyrrolidone (PVP) and nanowires with an average diameter of ∼290 nm in cetyltrimethylammonium bromide (CTAB). The absorption and emission properties of the micro/nanoassemblies of compound 5 showed a red shift of about 12 nm. It was also noticed that the emission intensity of 5 was retained even in the nanoassemblies, and was found to be comparable with that of the solution of 5 in organic solvents. DFT calculations suggest that multiple hydrogen bonding interactions are possible between two interacting monomers of 5. We believe that the possibility of H-bonding interactions in 5 is one of the factors that may aid the formation of micro/nanoassemblies. The blue emitting properties, compatible HOMO and LUMO energy levels, and highly blue emitting micro/nanoassemblies of these compounds make them suitable materials for organic light emitting devices (OLEDs).

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Introduction

Anthracene derivatives have attracted a great deal of attention in recent times owing to their various chemical, photophysical and medical applications.^1–3 For example, they have been frequently used as fluorescence probes and singlet oxygen acceptors, as well as in optoelectronic devices, including organic thin-film transistors and solar cells; they have also been used for the upconversion of energy using the triplet–triplet annihilation process.^4–9 Moreover, anthracene derivatives possessing high fluorescence quantum yields and good chemical photostability have found applications in organic light emitting devices (OLEDs).¹⁰ Recently, it has been shown that triplet–triplet annihilation (or triplet fusion) and thermally-activated delayed fluorescence in anthracene based OLEDs leads to very high external quantum efficiency (16.5%).^11,12 However, unsubstituted anthracene has a very low fluorescence quantum yield and readily crystallizes in thin film, which limits its application in solid state devices. Consequently, several attempts have been made to improve the emission and thin film properties of anthracene derivatives. In particular, phenyl substitution at the 9 and 10 positions of anthracene has been found to be a successful protocol towards improving the photophysical properties of anthracene.^13,14 The phenyl rings of diphenylanthracene (DPA) are found to be out of the plane on the anthracene core, which causes a marginal bathochromic shift and also prevents the crystallization of DPA in a thin film.¹⁴ It was demonstrated that the film forming properties of DPA can be improved by introducing bulky aryl-based substituents at the 9 and 10 positions or other positions (2 and 6) of anthracene.¹⁵ While substitution at the 9 and 10 positions of anthracene is well reported and studied in the literature, substitution at other positions of anthracene has rarely been explored. Recently, the synthesis of 2,6-diphenylanthracene was reported using 2,6-dihydroxyanthracene as a starting material, which was obtained by the reduction of 2,6-dihydroxyanthraquinone (Scheme 1a).¹⁶ Similarly, the synthesis of 1,8-diphenylanthracene has been reported by the reaction of phenyl magnesium bromide with 1,8-dichloroanthracene, which was obtained by the reduction of 1,8-dichloroanthraquinone.¹⁷ However, this method employs Grignard reagents, which are not easy to prepare, and requires perfectly anhydrous reaction conditions (Scheme 1b); therefore, this method is less practical. It is worth mentioning that the preparation of Grignard reagents has limitations for several organic functional groups. Alternatively, to synthesize aryl substituted anthracene, a convenient approach is aryl–aryl bond formation using Suzuki–Miyaura, Stille or Negishi coupling reactions.¹⁸ Such coupling reactions require 1,8-haloanthracene derivatives, which are difficult to synthesize, because the 9 and 10 positions of anthracene are more active towards halogenation. In this study, we have synthesized 1,8-dichloroanthracene from a readily available starting material, 1,8-dichloroanthraquinone. 1,8-Dichloroanthracene is further subjected to Suzuki–Miyaura coupling using [1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene](3-chloropyridyl)palladium(II) dichloride (PEPPSI-iPr)^18h to obtain six different 1,8-aryl substituted anthracene derivatives (1–6). Substituents on the phenyl group, such as methoxy (3), acetyl (4) fluoro (5) and trifluoromethyl (6) groups, are expected to facilitate intermolecular hydrogen bonding interactions. Moreover, substitution at various positions of polycyclic aromatic systems aids aggregate or micro/nanoassembly formation, which in turn improves the optical properties of the material.¹⁹ Lee et al. reported self-assembled nanoribbons and nanorods by varying the solubility of DPA molecules in solution.^19b More recently, surfactant assisted nanowires and microrods of DPA molecules were synthesized and used in OLEDs; it was also noted that the nanowire is the best shape for heterojunction light emitting devices.²⁰ Therefore, surfactant assisted micro/nano-sized self-assembly formations of new anthracene derivatives (1–6) were also attempted, and successful attempts for 5 are reported in this article.

Scheme 1 Literature methods used to synthesize aryl substituted anthracenes.

Results and discussion

Typically, the Suzuki–Miyaura coupling reaction employs the reaction of a bromo or iodo-substituted aryl substrate with an aryl boronic acid in the presence of Pd⁰ and base to produce diaryl derivatives.^18a However, the Suzuki–Miyaura coupling reaction is not efficient with chloro-substituted aryl substrates in the presence of the conventional catalyst Pd(PPh₃)₄. To overcome this problem, we have taken advantage of a recently reported Suzuki–Miyaura coupling reaction with a chloro aryl substrate in the presence of a new catalyst, Pd–PEPPSI-iPr. Using this strategy, we have synthesized 1,8-diaryl anthracene derivatives (1–6) from compound 7, as shown in Scheme 2. Compound 7 is obtained by the reduction of commercially available 1,8-dichloroanthraquinone.^17d Compound 7 is then reacted with substituted aryl boronic acid, Na₂CO₃, toluene, THF and water in the presence of a catalytic amount of Pd–PEPPSI-iPr (5–7 mol%) at 85 °C for 16–18 h. The usual reaction workup and purification by column chromatography afforded 1,8-diarylanthracene derivatives 1–6 as white or off white solids (except 3 and 4, which are slightly yellow) in 52% to 77% yield.

Scheme 2 Synthesis of anthracene derivatives 1–6.

All compounds were identified using several spectroscopic techniques such as ¹H-NMR, ¹³C-NMR, and mass spectrometry, and their characterization data have been summarized in the Experimental section. A representative ¹H-NMR spectrum of 1,8-di(4-fluorophenyl)anthracene, 5, is shown in Fig. 1. Singlets at 8.57 and 8.50 ppm were assigned to the protons at the 9- and 10-positions. The proton at the 9-position of substituted anthracene is shifted downfield compared to that of unsubstituted anthracene; this may be due to the presence of an electron withdrawing aryl group in the neighboring position. A doublet for two protons at 8.07 ppm and a triplet for two protons at 7.54 ppm were assigned to the protons at the 4- and 3-positions, respectively. The proton at the 2-position appeared as a doublet at 7.40 ppm. The 4-fluorophenyl protons showed two different doublets for four protons each at 7.46 and 7.15 ppm. It can be noticed in Fig. 1 that the signals for the protons are slightly broader, presumably because of H-bonding (vide infra). Signals related to the molecular mass ion were observed in MALDI for compounds 1–6 (see ESI†), which further confirms the formation of 1–6. All the compounds are fairly soluble in common organic solvents. The melting points of these compounds are high.

Fig. 1 ¹H NMR of 1,8-(4-fluorophenyl) anthracene, 5, in CDCl₃.

UV-visible absorption spectroscopic studies of 1–6 were carried out in toluene (Fig. 2) and in drop casted film (see ESI†). The toluene solution of 1 shows an absorption maximum (λ_max) at 395 nm and other high energy absorption peaks at 354 and 375 nm (π–π* transition). Compounds 2–6 also showed similar patterns in their absorption spectra, with a marginal blue or red shift of 4–5 nm depending on the substituents. Drop casted thin film absorption spectra showed slight broad peaks and a bathochromic shift of about 2–3 nm, except for 5. Compounds 3 and 5 showed large bathochromic shifts of 17 nm and 11 nm, respectively, which shows intermolecular interaction in the thin films. An similar trend was observed in the emission spectra, with fluorescence maxima at 432 nm for 1 and 432 to 440 nm for compounds 2–6 (Fig. 2). The fluorescence quantum yields of these anthracene derivatives were calculated in toluene using 9,10-diphenylanthracene (0.95 in ethanol)¹³ as a standard; the results have been summarized in Table 1. Fluorescence lifetime data for these compounds (1–6), recorded using the TCSPC technique, are provided in Table 1. Single exponential fit was observed, and the spectra are provided in the ESI.†

Fig. 2 Absorption (above) and emission (below) spectra of compounds 1–6 obtained in toluene.

Table 1 Photophysical data for compounds 1–6

Comp.	λabs^a (nm)	λabs^b (nm)	λem^a (nm)	Φ^c	τ^a (ns)
a In toluene.b In drop casted thin film.c Relative quantum yields were calculated using 9,10-DPA as reference (ethanol, Φ = 0.95).
DPA	371, 392	—	404, 425	0.95	0.93
1	375, 396	381, 401	416, 432	0.75	3.54
2	371, 390	373, 393	410, 432	0.71	3.60
3	379, 399	391, 416	419, 440	0.65
4	369, 387	378, 400	431	0.45	2.58
5	374, 394	382, 405	409, 431	0.72	3.79
6	374, 394	376, 397	410, 432	0.67	3.75

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Compounds 1–6 were studied by cyclic voltammetry to obtain insight into their oxidation potentials, which together with photophysical studies, give information on their HOMO and LUMO energy levels. Cyclic voltammetry and differential pulse voltammetry were performed in CH₃CN at a scan rate of 100 mV s⁻¹ using tetrabutylammonium hexafluorophosphate (TBAHP) as the supporting electrolyte. In these experimental conditions, only oxidation waves were observed for 1–6 (with the exception of 3); reduction waves were not observed. A comparison of the first oxidation waves of compounds 1–6, with the exception of 3, is shown in Fig. 3; the data are presented in Table 2. The 1,8-diaryl substituted anthracenes are relatively difficult to oxidize compared to 9,10-diphenylanthracene (the E_ox of DPA is lower by 0.1 eV), which is clearly evident from the data presented in Table 2.²¹ The E_HOMO values of the new anthracene derivatives were calculated using their oxidation peak potentials and using the internal standard Fc/Fc⁺ value of 4.8 eV versus vacuum. The E_LUMO values were calculated using the E_HOMO values and optical band gaps for the respective compounds obtained from the onset of their absorption spectra in drop casted film. The E_HOMO and E_LUMO values of 1–6 were found to be ∼−5.6 eV and −2.7 eV, respectively. The HOMO and LUMO energy levels of 1–6 suggest that these compounds can be used in OLEDs as blue emitters.

Fig. 3 Cyclic voltammograms of 1–6 (except for 3) with ferrocene in CH₃CN.

Table 2 Electrochemical data for compounds 1–6

Comp.	Eox (V)	Eg^a (eV)	EHOMO^b (eV)	ELUMO^c (eV)
a Optical band gap, calculated from absorption spectrum obtained in drop-casted films.b EHOMO calculated from oxidation potentials (vs. ferrocene), i.e. EHOMO = −(Δox w.r.t. Fc/Fc^+ + 4.8 eV).c ELUMO = EHOMO − Eg; nd = not detected.
DPA	1.22	—	—	—
1	1.32	−2.92	−5.63	−2.71
2	1.26	−2.94	−5.62	−2.68
3	nd	−2.83	—	—
4	1.34	−2.86	−5.71	−2.85
5	1.30	−2.92	−5.66	−2.74
6	1.32	−2.93	−5.68	−2.75

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Furthermore, the surfactant assisted micro/nano self-assembly formation of 1–6 were studied. Out of these anthracene derivatives, only 5 provided well-organized nano-sized assemblies. Fig. 4 shows the SEM images of the micro/nanostructures of 5 with different morphologies, which were obtained by the surfactant-assisted precipitation method. Different surfactants, such as PVP and CTAB, were used in this procedure (details are given in the Experimental section). As shown in Fig. 4(A), discontinuous rectangular plates were observed when 5 (in THF) was added to an aqueous solution of PVP with vigorous stirring. The inset in Fig. 4(A) shows a magnified image of the rectangular plates with smooth surfaces and different sizes. Different surfactants led to changes in the morphology of 5. The CTAB assisted formation of 5 in THF gave nanowires, as shown in Fig. 4(B). It can be observed that the nanowires of 5 (inset Fig. 4(B)) have virtually uniform diameters and show continuous structures. The average length and diameter of the nanowires were found to be around 100 μm and 290 nm, respectively. We believe that the inter-molecular H-bonding interactions in 5, facilitated by the presence of the terminal fluorine group on the phenyl substituent, may be the reason for the formation of nano-sized assemblies of 5.

Fig. 4 SEM images of different morphologies of 5 dissolved in THF/surfactant/water. (A) Rectangular plate-like shapes for 5 in PVP as surfactant; the inset shows a magnified area. (B) Nanowires for 5 in CTAB as surfactant; the inset shows a magnified area.

The micro/nanostructures of 5 were studied with UV-visible and emission spectroscopy. The UV-vis absorption spectra showed a red shift with a broad tail absorption in a longer wavelength region, as shown in Fig. 5. The absorption spectra of 5 in PVP and CTAB showed an 18 nm red shift compared to the spectrum of compound 5 in THF. The long tail absorption is attributed to the scattering effect caused by the micro/nanoparticles. The emission spectra of 5 and its assemblies in PVP and CTAB showed a similar trend to that observed in the UV-vis absorption spectra. A bathochromic shift of 10 nm and a broad tail was observed in the emission spectra as well. These findings in the absorption and emission spectra further prove the formation of surfactant assisted micro/nano-sized assemblies.

Fig. 5 Absorption and emission spectra of micro/nanoassemblies of 5 in PVP, CTAB and THF surfactants.

To identify the nature of the non-bonding interactions in the substituted anthracenes, we performed computational investigations on 1 and 5 using density functional theory (DFT) methods. All calculations were performed using the Gaussian 09 suite of programmes.²² Several chemically intuitive guess structures of dimers of 1 and 5 were considered for geometry optimization at the B3LYP/6-311G** level of theory,²³ and optimized structures were further identified as minima using harmonic force constant analysis at the same level of theory. Only the lowest-energy structures are reported in this study. The calculated interaction energies between two interacting monomers were corrected by the counterpoise (CP) method.²⁴

The substitution of a fluoro group on the phenyl substituted anthracene allows the possibility of H-bonding interactions between two monomers. Consequently, the two monomers of 5 were found to be more closely placed in the optimized structures than the two monomers of 1. The shortest inter-atomic distance between two monomers of 5 is 2.45 Å (average), whereas in the case of 1, the two monomers are separated by 3.63 Å (Fig. 6). These H-bonding interactions also lead to the strong interactions between two monomers in 5. The calculated interaction energies between two interacting monomers in 5 and 1 are −1.86 and −0.12 kcal mol⁻¹, respectively. Furthermore, to assign H-bonding interactions in more than two units of 5, the tetramer structure of 5 was optimized at the B3LYP/6-311G** level of theory. The optimized structure of the tetramer of 5, delineating multiple hydrogen bonding interactions between the interacting monomers, is shown in Fig. 6. Our calculations evidently suggest that multiple hydrogen bonding interactions are possible in 5, which may aid the formation of nano-sized self assemblies via the surfactant assisted precipitation method.²⁵

Fig. 6 Optimized geometries of dimers of 1 (A) and 5 (B) and the tetramer of 5 (C).

Conclusions

We have synthesized six anthracene derivatives having different aryl groups at the 1 and 8 positions of anthracene by applying Suzuki–Miyaura coupling reaction conditions to 1,8-dichloroanthracene. Substitutions, such as methoxy, acetyl, fluoro, and trifluoromethyl groups, on the phenyl groups were expected to facilitate intermolecular hydrogen bonding interactions. Photophysical characterizations of these derivatives show that the absorption and emission maxima of these derivatives are slightly bathochromically shifted with reference to DPA. Overall, these anthracene derivatives emit in the blue region and show high extinction coefficients and fluorescence quantum yields. Furthermore, surfactant assisted micro/nano-sized assembly formations of these anthracene derivatives were attempted. The nano-sized assemblies were successfully obtained only in the case of anthracene derivative 5, which has fluoro substituents on the phenyl groups. We have noticed that anthracene derivative 5 provides nanostructures with different shapes in different surfactants. In PVP/THF solution, rectangular microplates of 5 were obtained, whereas in CTAB/THF solution, nanowires with uniform diameter were observed. Theoretical investigations of anthracene derivative 5 have established different possibilities for the intermolecular H-bonding interactions in 5. Such H-bonding interactions may favor the formation of micro/nano-sized orderly structures, as observed in the case of 5. In all, we believe that the photophysical and electrochemical parameters of anthracene derivatives 1–6 can be exploited in organic light emitting devices. Currently, we are in the process of establishing a means to explore the possible applications of these compounds in OLEDs.

Experimental

Chemicals

All general chemicals and solvents were procured from S D. Fine Chemicals, India and Sigma-Aldrich. Column chromatography was performed using silica gel obtained from Sisco Research Laboratories, India. Tetrabutylammonium hexafluorophosphate was purchased from Aldrich and used without further purification.

Instrumentation

¹H and ¹³C-NMR (δ in parts per million) spectra were obtained using a Bruker 500 MHz spectrometer. Tetramethylsilane (TMS) was used as an internal reference for recording the ¹H-NMR spectra in CDCl₃ (residual proton; δ = 7.26 ppm). MS spectra were obtained on a Bruker MALDI-TOF instrument. UV-vis spectra were obtained at room temperature on a Shimadzu 1800 instrument. Fluorescence emission measurements were carried out at room temperature using a Horiba Fluoromax 4 instrument. For emission, compounds 1–6 were excited at 380 nm. Cyclic voltammetry measurements were carried out using an electrochemical analyzer (620D, CH Instruments Co.) at room temperature utilizing a three-electrode configuration consisting of a Pt disc (working electrode), platinum wire (auxiliary electrode), and standard calomel (reference electrode) electrodes. The experiments were performed in dry acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphate as the supporting electrolyte. The sizes and shapes of the micro/nanostructures were observed on a SEM (Quanta-200 ESEM, FEI) at an accelerating voltage of 5 kV.

Characterization

1,8-Dichloro anthracene, 7. M.P.: 155 °C (lit.²⁰ 156–157 °C) ¹H-NMR (δ in ppm): 9.29 (s, 1H), 8.50 (s, 1H), 7.98 (d, 2H, J = 8.5 Hz), 7.66 (d, 2H, J = 7.1 Hz), 7.44 (t, 2H, J = 8.0 Hz).

General procedure for the synthesis of anthracene derivatives 1–6

A two necked round bottom flask was charged with compound 7 (1 equiv.), Na₂CO₃ (4–6 equiv.) and the corresponding aryl boronic acid (2.4 equiv.) under Ar atmosphere. A mixture of toluene, THF and water in equal volume ratio was added, and the temperature was raised to 60 °C. Pd–PEPPSI-iPr (5–7 mol%) was added and the reaction was refluxed at 85 °C. After 12 h, a fresh 2 mol% of catalyst was added, if required based on TLC analysis. The reaction mixture was further stirred at 85 °C for 16–18 h. The reaction mixture was cooled and extracted with dichloromethane, and the organic layers were washed with brine and water. The solvents were removed by rotary evaporator under vacuum; the crude product obtained was purified by column chromatography using hexane and ethyl acetate to afford compounds 1–6 (∼50–70%) as white or off white solids.

1,8-Diphenyl anthracene, 1. A mixture of 7 (0.12 g, 0.5 mmol), phenylboronic acid (0.15 g, 1.2 mmol) and Na₂CO₃ (0.40 g, 4 mmol) in water/THF/toluene (1 : 1 : 1, 15 mL) was stirred under argon for 5 min, and IPr-PEPPSI (0.05 g, 0.07 mmol) was added. The reaction mixture was refluxed at 85 °C for 18 h. Silica gel column chromatography of the crude compound afforded compound 1 as a white solid. Yield: (0.11 g, 69%). M.P.: 190 °C. FTIR (KBr, cm⁻¹): ν 3029, 2922, 1545, 1490, 1444, 1323, 1167, 1072, 879, 745, 695, 603, 549. ¹H NMR (500 MHz, CDCl₃): δ 8.65 (s, 1H), 8.57 (s, 1H), 8.07 (d, 2H, J = 8.5 Hz), 7.55 (t, 2H, J = 6.9 Hz), 7.52 (d, 4H, J = 7.3 Hz), 7.43 (m, 6H), 7.39 (d, 2H, J = 7.3 Hz) ppm. ¹³C NMR (500 MHz, CDCl₃) δ 140.5, 140.4, 131.8, 129.9, 128.0, 127.6, 127.1, 126.7, 126.1, 125.2, 123.9 ppm. MALDI-TOF mass calcd. for C₂₆H₁₈ (M)⁺: 330.14; found: 330.55 (M+).

1,8-Dinaphthyl anthracene, 2. A mixture of 7 (0.12 g, 0.5 mmol), naphthylboronic acid (0.21 g, 1.2 mmol) and Na₂CO₃ (0.40 g, 4 mmol) in water/THF/toluene (1 : 1 : 1, 15 mL) was stirred under argon for 5 min, and IPr-PEPPSI (0.05 g, 0.07 mmol) was added. The reaction mixture was refluxed at 85 °C for 18 h. Silica gel column chromatography of the crude compound afforded compound 2 as a white solid. Yield: (0.11 g, 52%). M.P.: 208 °C. FTIR (KBr, cm⁻¹): ν 3031, 2660, 1550, 1494, 1442, 1326, 1160. ¹H NMR (500 MHz, CDCl₃): δ 8.63 (d, 1H, J = 8.7 Hz), 8.14 (t, 2H, J = 8.5 Hz), 7.85 (d, 1H, J = 8.1 Hz), 7.65 (t, 4H, J = 7.6 Hz), 7.56–7.57 (m, 2H), 7.41–7.46 (m, 4H), 7.30 (t, 1H, J = 8.0 Hz), 7.23–7.29 (m, 3H), 6.87 (t, 1H, J = 7.5 Hz), 7.04–7.09 (m, 2H), 7.20 (t, 1H, J = 7.5 Hz) ppm. ¹³C NMR (500 MHz, CDCl₃): δ = 140.5, 131.8, 129.9, 128.1, 127.6, 127.1, 126.7, 126.1, 125.2, 123.9 ppm. MALDI-TOF mass calcd. for C₃₄H₂₂ (M)⁺: 430.17; found: 429.95 (M+).

1,8-Di(4-methoxyphenyl) anthracene, 3. A mixture of 7 (0.12 g, 0.5 mmol), 4-methoxyphenylboronic acid (0.18 g, 1.2 mmol) and Na₂CO₃ (0.40 g, 4 mmol) in water/THF/toluene (1 : 1 : 1, 15 mL) was stirred under argon for 5 min, and IPr-PEPPSI (0.05 g, 0.07 mmol) was added. The reaction mixture was refluxed at 85 °C for 18 h. Silica gel column chromatography of the crude compound afforded compound 3 as a slightly yellow solid (0.11 g, 58%). M.P.: above 220 °C. FTIR (KBr, cm⁻¹): ν 3036, 2923, 1609, 1534, 1508, 1439, 1317, 1287, 1243, 1173, 1098, 1030, 885, 872, 828, 735, 555. ¹H NMR (500 MHz, CDCl₃): δ 8.67 (s, 1H), 8.54 (s, 1H), 8.03 (d, 2H, J = 8.5 Hz), 7.54 (t, 2H, J = 6.9 Hz), 7.46 (d, 4H, J = 8.5 Hz), 7.41 (d, 2H, J = 6.6 Hz), 7.00 (d, 4H, J = 8.6 Hz), 3.90 (s, 6H, OCH₃) ppm. MALDI-TOF mass calcd. for C₂₈H₂₂O₂ (M)⁺: 390.16; found: 389.84 (M+).

1,8-Di(4-actylphenyl) anthracene, 4. A mixture of 7 (0.12 g, 0.5 mmol), 4-acetylphenylboronic acid (0.196 g, 1.2 mmol) and Na₂CO₃ (0.40 g, 4 mmol) in water/THF/toluene (1 : 1 : 1, 15 mL) was stirred under argon for 5 min, and IPr-PEPPSI (0.05 g, 0.07 mmol) was added. The reaction mixture was refluxed at 85 °C for 18 h. Silica gel column chromatography of the crude compound afforded compound 4 as an off-white (yellowish) solid (77%, 0.16 g). M.P.: 175 °C. FTIR (KBr, cm⁻¹): ν 3038, 2916, 1670 (CO), 1552, 1455, 1424, 1400, 1312, 1356, 1262, 1167, 1091, 1016, 957, 828, 794, 732, 671, 593, 472. ¹H NMR (500 MHz, CDCl₃): δ 8.60 (s, 1H), 8.42 (s, 1H), 8.09 (d, 2H, J = 8.55 Hz), 8.03 (d, 2H, J = 8.1 Hz), 7.56–7.61 (m, 6H), 7.52 (m, 2H), 7.47 (d, 2H, J = 6.7 Hz), 2.67 (s, 6H, COCH₃) ppm. HRMS mass calcd. for C₃₀H₂₂O₂ (M + H)⁺: 414.16; found: 384.42 (M − 2CH₃), 414.90 (M+).

1,8-Di(4-fluorophenyl) anthracene, 5. A mixture of 7 (0.12 g, 0.5 mmol), 4-fluorophenylboronic acid (0.17 g, 1.2 mmol) and Na₂CO₃ (0.40 g, 4 mmol) in water/THF/toluene (1 : 1 : 1, 15 mL) was stirred under argon for 5 min, and IPr-PEPPSI (0.05 g, 0.07 mmol) was added. The reaction mixture was refluxed at 85 °C for 18 h. Silica gel column chromatography of the crude compound afforded compound 5 as an off-white solid (0.12 g, 67%). M.P.: 204 °C. FTIR (KBr, cm⁻¹): ν 3037, 2917, 1598, 1503, 1443, 1462, 1326, 1218, 1157, 1093, 1015, 940, 906, 834, 791, 564, 525. ¹H NMR (500 MHz, CDCl₃): δ 8.57 (s, 1H), 8.50 (s, 1H), 8.07 (d, 2H, J = 8.2 Hz), 7.54 (t, 2H, J = 7.2 Hz), 7.39–7.45 (m, 6H), 7.13 (t, 4H, J = 8.3 Hz) ppm. ¹³C NMR (500 MHz, CDCl₃): δ = 163.18, 161.22, 139.32, 136.29, 131.75, 131.40, 130.09, 127.75, 126.88, 126.26, 125.22, 123.35, 116.15, 115.97, 115.07, 107.46 ppm. MALDI-TOF mass calcd. for C₂₆H₁₆F₂ (M)⁺: 366.12; found: 365.71 (M+).

1,8-Di(4-trifluoromethoxyphenyl) anthracene, 6. A mixture of 7 (0.12 g, 0.5 mmol), 4-(trifluoromethoxy)phenylboronic acid (0.23 g, 1.2 mmol) and Na₂CO₃ (0.40 g, 4 mmol) in water/THF/toluene (1 : 1 : 1, 15 mL) was stirred under argon for 5 min, and IPr-PEPPSI (0.05 g, 0.07 mmol) was added. The reaction mixture was refluxed at 85 °C for 18 h. Silica gel column chromatography of the crude compound afforded compound 6 as a white solid (0.17 g, 75%). M. P. 218 °C. FTIR (KBr, cm⁻¹): ν 2925, 1612, 1549, 1405, 1322, 1184, 1102, 1064, 978, 948, 839, 792, 743, 705, 631, 614, 603, 542. ¹H NMR (500 MHz, CDCl₃): δ 8.60 (s, 1H), 8.30 (s, 1H), 8.12 (d, 2H, J = 8.2 Hz), 7.67–7.75 (m, 5H), 7.56–7.60 (m, 5H), 7.45 (d, 2H, J = 6.3 Hz) ppm. ¹³C NMR (500 MHz, CDCl₃): δ 143.82, 138.92, 131.69, 130.17, 129.84, 128.32, 127.56, 126.97, 126.16, 125.88, 124.96, 123.02 ppm. MALDI-TOF mass calcd. for C₂₈H₁₆F₆ (M + H)⁺: 466.11; found: 465.97 (M+).

Methods for micro/nanoassembly formation. The plate shaped microcrystals of 5 were prepared by adding 0.5 mL of 0.5 mM 5/THF solution to 4 mL water containing PVP (0.5 mg mL⁻¹) with vigorous stirring. After stirring for 10 min, the solution was left undisturbed overnight at room temperature before characterization. When PVP was replaced by CTAB and the other conditions remained the same, nanowires rather than microplates were produced. Preparation of SEM samples: an aliquot (20 μL) of the solution was placed on a silica wafer and dried over 24 h.

Acknowledgements

We thank the Centre for Excellence in Basic Sciences, Mumbai for providing research facilities. Partial funding was provided by the Department of Science and Technology, India (SR/FT/CS-87/2010). We thank the Tata Institute of Fundamental Research, Mumbai for providing the NMR and mass spectroscopy facilities. We also thank the National Centre for Nanoscience and Nanotechnology, University of Mumbai and the Bhabha Atomic Research Center, Mumbai for SEM and TCSPC facilities.

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Footnote

† Electronic supplementary information (ESI) available: NMR, mass spectra, absorption and emission spectra and differential pulse voltammograms of compounds 1–6. See DOI: 10.1039/c5ra19564k

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