2,7-Bis(pyridin-3-ylethynyl)fluoren-9-one

Abstract

2,7-bis(pyridin-3-ylethynyl)fluoren-9-one [(3-PyE)₂FO] was synthesized in one step by the Sonogashira coupling reaction between 3-ethynylpyridine and 2,7-dibromofluoren-9-one. The title compound was fully characterized, and its crystal structure was determined through single-crystal XRD analysis.

1. Introduction

The construction of novel supramolecular architectures and functional materials starts with the design of appropriate building blocks with determined functional and geometrical properties [1]. Organic linkers, including fluorophores in their skeletons, can be designed to impart peculiar luminescence properties to the final supramolecular net-works. Fluorene and their oxidized fluorenone derivatives have been employed to develop new materials that are very promising for photonic and optoelectronic applications, such as optical second harmonic (SGH) and terahertz (THz) generation, light emitting diodes, biomarkers, solar cells, and sensors [2,3,4]. During recent decades, discrete fluorene-based building blocks have been extensively studied, featuring either a single fluorene [5] or fluorenone [6] moiety or multiple fluorene cores, such as spirobifluorene [7] and cofacial bifluorene derivatives [8].

Most fluorenone derivatives recently reported in the literature, such as 2,7-diphenylfluoren-9-one [(Ph)₂FO] [9], 2,7-di([1,1′-biphenyl]-4-yl)fluoren-9-one [(4-BPy)₂FO] [10], and 2,7-bis(phenylethynyl)fluoren-9-one [(PhE)₂FO] [11] (Figure 1), do not feature donor sites suitable for metal coordination and find applications only for the preparation of purely organic materials. To overcome this limitation and extend their use to the fabrication of hybrid metal-organic frameworks, fluorenone carboxylate derivatives can be prepared by including charged donor sites in the structure, such as fluoren-9-one-2,7-dicarboxylate [12]. Neutral supramolecular spacers can also be prepared by exploiting neutral donor sites, such as pyridyl groups; see, for example, 2,7-di(pyridin-3-yl)fluoren-9-one [(3-Py)₂FO] [6], 2,7-di(pyridin-4-yl)fluoren-9-one [(4-Py)₂FO] [13,14,15] and (2,7-bis(2-(pyridin-3-yl)vinyl)fluoren-9-one [(4-PyV)₂FO] [16] (Figure 1). Apart from these few examples, the design of pyridyl-functionalized fluorenone derivatives is still largely unexplored. Here, we report the synthesis and full characterization of the novel pyridyl-functionalized fluorenone 2,7-bis(pyridin-3-ylethynyl)fluoren-9-one [(3-PyE)₂FO] (Scheme 1).

2. Results and Discussion

Compound (3-PyE)₂FO was synthesized by adapting a procedure previously reported for the preparation of 2,5-bis(pyridin-3-ylethynyl)thiophene [17]. Two equivalents of 3-ethynylpyridine were reacted with 2,7-dibromofluoren-9-one in dry diethylamine under inert atmosphere by following a classical Pd/Cu-catalyzed tandem Sonogashira coupling reaction (Scheme 1) [18].

The crude product was recrystallized to give pure (3-PyE)₂FO as light-orange crystals with a 26% yield. FTIR analysis showed the stretching modes of alkynyl and carbonyl moieties at 2206 and 1716 cm⁻¹, respectively, with ¹H and ¹³C{¹H} NMR spectra in agreement with the chemical structure of (3-PyE)₂FO. The absorption and emission properties of (3-PyE)₂FO were investigated in solution and in the solid state (Figure 2). The UV-Vis spectrum of (3-PyE)₂FO in CHCl₃ exhibited four maxima centered at 304, 339, 349, and 436 nm, and a photoluminescence emission was observed at 554 nm ($\mathsf{\Phi }$ = 0.04, λ_ex = 350 nm). Solid-state measurements showed a broad absorption in the diffuse reflectance spectrum in the range of 200–500 nm and a strong emission band of 553 nm, similar to what was observed in solution.

A crystal suitable for X-ray diffraction analysis was selected and structurally characterized. The compound (3-PyE)₂FO crystallized in the monoclinic space group P2₁/c with a single molecule in the asymmetric unit, as shown in Figure 3. The compound adopted an almost planar geometry with one pyridyl ring coplanar with the fluorenone ring (torsion angle ≃ 1°), and the other twisted with respect to the fluorenone plane by about 33°.

Crystal data for (3-PyE)₂FO: C₂₇H₁₄N₂O (M_r = 382.40 g mol⁻¹), monoclinic, P2₁/c (No. 14), a = 6.6190(1) Å, b = 13.7115(1) Å, c = 22.1683(3) Å, β = 110.080(1)°, α = γ = 90°, V = 1889.62(4) ų, T = 100(2) K, Z = 4, Z′ = 1, µ(Cu K_a) = 0.653. A total of 57,856 reflections were measured, and 3441 unique reflections (R_int = 0.0247) were used in all calculations. The final wR₂ was 0.1204 (all data), and R₁ was 0.0455 (I ≥ 2 σ(I)).

A full interaction map [19] was calculated for this structure (based on pre-extracted IsoStar interaction data from the CSD), which implies that crystal packing was driven by a combination π–π interactions and CH∙∙∙O and CH∙∙∙N hydrogen bonds (Figure S1, Supplementary Materials). Adjacent molecules of (3-PyE)₂FO are pillared along the a-axis through intermolecular π–π interactions between fluorenone and pyridyl moieties (3.66–3.81 Å, Figure 4a and Figure S1). Molecules located on vicinal pillars are rotated by about 61°, generating a zig-zag packing motif (Figure 4b), strongly resembling that found in the α-phase of (PhE)₂FO [11].

The presence of the oxygen and nitrogen atoms in (3-PyE)₂FO promotes, respectively, the formation of two weak CH∙∙∙O hydrogen bonds with para-positioned hydrogen atoms of pyridyl rings (interactions a and b in Figure 4a) belonging to two molecule units located on the adjacent pillars and two CH∙∙∙N hydrogen bonds involving hydrogen atoms located at the fluorenone core (interactions c and d in Figure 4a; see Table S1 for details).

3. Materials and Methods

3.1. General

Solvents and reagents were purchased from TCI, FluoroChem, and Aldrich. Diethylamine was distilled over LiAlH₄ and degassed prior to use. Syntheses were carried out under dry dinitrogen atmosphere using standard Schlenk equipment. FT-IR measurements were recorded at room temperature on a Thermo-Nicolet 5700 spectrometer using KBr pellets with a KBr beam splitter and KBr windows (4000−400 cm⁻¹, resolution 4 cm⁻¹). NMR spectra were carried out in CDCl₃ at room temperature using a Bruker Avance III HD 600 spectrometer. Chemical shifts are reported in ppm (δ) and were calibrated to the solvent residue. Coupling constants J are reported in hertz (Hz). Positive ESI-MS spectra were recorded with a high-resolution LTQ Orbitrap Elite™ mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). The solutions were infused at a flow rate of 5.00 μL/min into the ESI source. Spectra were recorded in the range of m/z 300–600 with a resolution of 240,000 (FWHM). The instrumental conditions were as follows: a spray voltage of 3500 V, a capillary temperature of 275 °C, sheath gas 12 (arbitrary units), auxiliary gas 3 (arbitrary units), sweep gas 0 (arbitrary units), and a probe heater temperature of 50 °C. Elemental analysis was carried out with a CHNS/O PE 2400 series II elemental analyzer (T = 925 °C). The melting point was determined on a FALC mod. C (up to 290 °C) apparatus. UV-Vis measurements in solution were recorded in the range of 190–1100 nm using quartz cuvettes with a path length of 1.0 cm by means of an Agilent Cary 5000 UV−vis−NIR dual-beam spectrophotometer. Diffuse reflectance measurements were carried out on an Agilent Cary 5000 UV−vis−NIR dual-beam spectrophotometer equipped with a diffuse reflectance accessory. Photoluminescence emission spectra in solution and solid state were collected either on a Varian Cary Eclipse spectrofluorometer or a Perkin Elmer LS55 fluorescence spectrometer. The photoluminescence quantum yield Φ was determined according to Equation (1) and using rhodamine 6G as a standard (${\mathsf{\Phi }}_{\mathrm{ST}}$ = 0.95 in ethanol) [20].

$${\mathsf{\Phi }}_{\mathrm{X}}={\mathsf{\Phi }}_{\mathrm{ST}}\left(\frac{{\mathrm{Grad}}_{\mathrm{X}}}{{\mathrm{Grad}}_{\mathrm{ST}}}\right)\left(\frac{{\mathsf{\eta }}_{\mathrm{X}}^2}{{\mathsf{\eta }}_{\mathrm{ST}}^2}\right)$$

(1)

where the subscripts ST and X denote standard and sample, respectively; Grad is the gradient from the plot of integrated fluorescence intensity vs. absorbance; and η is the refractive index of the solvent.

X-ray diffraction data for (3-PyE)₂FO were collected at 100 (2) K on a Rigaku 007HF diffractometer equipped with Varimax confocal mirrors, an AFC11 goniometer, and a HyPix 6000 detector. The structure was solved with the ShelXT [21] solution program using dual methods, and the model was refined with ShelXL 2018/3 [22] using full-matrix least-squares minimization on F². Olex2 1.5 [23] was used as the graphical interface.

3.2. Synthesis of 2,7-Bis(pyridin-3-ylethynyl)fluoren-9-one [(3-PyE)₂FO]

3-Ethynylpyridine (517 mg; 5.0 mmol), 2,7-dibromofluoren-9-one (845 mg; 2.5 mmol), copper iodide (24 mg; 0.13 mmol), and Pd(PPh₃)₂Cl₂ (90 mg; 0.13 mmol) were dried under vacuum for 20 min, and freshly distilled and degassed diethylamine (25 mL) was added under dry N₂ atmosphere. The yellow mixture was heated to 55 °C and stirred under inert atmosphere for 24 h. The mixture slowly turned red in 30 min, and the day after, an intense yellow color was observed. After cooling to RT, a NH₄Cl saturated aqueous solution (100 mL) was added, and the precipitate was separated by filtration under vacuum. The crude yellow product was recrystallized from the DCM/EtOAc mixture to yield the title compound as light-orange crystals (230 mg; 0.60 mmol; Y = 24%). M. p. = 249 °C. ESI (+)-MS (MeCN solution) m/z 383.1195 for [C₂₇H₁₅N₂O]⁺ [M–H]⁺. Elemental analysis calculated (%) for C₂₇H₁₄N₂O: C 84.80, H 3.69, N 7.33. Found: C 84.20, H 3.45, N 7.21. FTIR (KBr, 4000–400 cm⁻¹): 3053 w, 3035 w, 3003 w, 2206 w νC≡C, 1959 w, 1928 w, 1896 w, 1863 w, 1807 w, 1782 w, 1716 s, 1614 ms, 1604 ms, 1581 mw, 1556 m, 1479 vs, 1464 ms, 1427 m, 1400 ms, 1385 w, 1315 m, 1245 m, 1209 w, 1194 m, 1186 m, 1115 m, 1020 m, 1014 m, 982 mw, 949 w, 922 w, 902 mw, 831 m, 833 ms, 802 ms, 783 s, 735 w, 702 s, 650 m, 623 m, 578 w, 540 m, 515 mw, 473 w, 428 w, 405 mw cm⁻¹. ¹H NMR (600 MHz, CDCl₃) δ: 8.78 (d, J = 1.4 Hz, 2H), 8.58 (dd, J = 4.9, 1.6 Hz, 2H), 7.86–7.82 (m, 4H), 7.70 (dd, J = 7.7, 1.4 Hz, 2H), 7.57 (d, J = 7.7 Hz, 2H), 7.33 (dd, J = 7.7, 4.9 Hz, 2H) ppm. ¹³C{¹H} NMR (151 MHz, CDCl₃) δ: 192.0, 152.2, 148.9, 143.7, 138.9, 138.1, 134.7, 127.7, 124.0, 123.3, 120.9, 120.2, 91.9, 88.1 ppm.

4. Conclusions

The novel visible-emitting fluorophore 2,7-bis(pyridin-3-ylethynyl)fluoren-9-one [(3-PyE)₂FO] was successfully prepared by the Sonogashira coupling reaction, and its full characterization was presented. Further studies are ongoing in our laboratories to evaluate the potential use of (3-PyE)₂FO as a building block for the formation of luminescent supramolecular assemblies.