Synthesis, structure and photophysical properties of near-infrared 3,5-diarylbenzoBODIPY fluorophores

Abstract

A set of 3,5-diarylated benzoBODIPYs were prepared within three steps from commercial isoindolin-1-one via Vilsmeier Haack, Suzuki coupling and a one-pot POCl₃-promoted self-condensation in concert with a BF₃ complexation. These benzoBODIPYs showed strong absorption and intense fluorescence emission in far-red to NIR region (λ^max_abs = 634–706 nm, λ^max_em = 663–747 nm, Φ = 0.35–0.99) tunable via the variation of the 3,5-aryl substituents.

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Introduction

Near-infrared (NIR) organic chromophores,¹ especially those with high molar absorptivities, bright fluorescence and good stabilities have attracted wide research interest over the past decades and have found applications in materials and biomedicine areas, for example as efficient photosensitizers in organic photovoltaics² and as labeling/imaging agents in biological related systems.³ Boron dipyrromethenes (BODIPYs, Fig. 1) have excellent photophysical properties, like bright fluorescence and excellent photostability,⁴ and have been applied in a highly diverse research fields, including photovoltaics,⁵ bioimaging,⁶ sensing,⁷ and photodynamic therapy.⁸ Conventional BODIPYs generally absorb/emit between 470 and 530 nm.⁴ Long-wavelength NIR BODIPYs can be achieved via structural modifications,^9,10 for example, through extending the conjugation at 3,5-positions,^10–13 and the annulations of aromatic compounds onto the pyrrolic positions of the chromophore,^14–19 for example the annulations of benzene-moieties to form the so-called “benzoBODIPYs”.^20–24 Theoretically, there are two isomeric structures of benzoBODIPYs ([a] and [b]-fusion, Fig. 1a) with the annulations occurred at a and b-bond positions. Most benzoBODIPYs appeared in the literature are [a]-benzoBODIPYs^21–24 with intense fluorescence emission that is in great contrast to the dark fluorescence observed in [b]-benzene-annulated isomers.²⁰

Fig. 1 (a) Chemical structure of [a]- and [b]-benzoBODIPYs; (b) current available (routes A–C) and potential (route D) synthetic routes for [a]-benzoBODIPYs; (c) synthesis of unsymmetrical 3-chlorobenzoBODIPYs and related pyrrolylbenzoBODIPYs recently reported by our group.

Their synthesis were generally achieved via: (i) a thermal retro Diels–Alder reaction (above 200 °C, route A, Fig. 1b) developed by Ono and coworker,²¹ in which the preparation of bicyclo-octadiene-fused pyrrole precursor was required, (ii) the condensation of di- and tetrahydroisoindoles reported by Vicente and coworkers,²² in which the oxidative dehydrogenation with DDQ was required (route B, Fig. 1b), and (iii) the total synthesis from 2-acetylphenols²³ (route C, Fig. 1b). On the other hand, the direct condensation of isoindoles (route D, Fig. 1b) although straightforward is still inaccessible due to the stability issue of the dipyrromethene intermediate.

Recently, we have applied 3-chloro-1-formyl(acetyl)isoindoles for the condensation with pyrrole in the synthesis of unsymmetrical [a]-benzoBODIPYs and pyrrolylbenzoBODIPYs (Fig. 1c).²⁴ Herein we report a three-step synthesis and photophysical properties of brightly NIR fluorescent 3,5-arylated benzoBODIPYs from commercial reagents based on an POCl₃-promoted self-condensation of 3-aryl-1-formylisoindoles (Scheme 1).

Scheme 1 Synthesis of 3,5-diarylbenzoBODIPYs 1a–e from commercial isoindolin-1-one via Vilsmeier–Haack, Suzuki coupling and POCl₃-promoted self-condensation reactions.

Results and discussion

Synthesis

Our initial attempts to symmetrical 3,5-dihalogenobenzoBODIPY 1X from an acid promoted self-condensation²⁵ of 3-halogeno-1-formylisoindole in POCl₃/dichloromethane or HCl/methanol (Scheme S1†) failed. Instead, only a complicated polar mixture was obtained. We attributed it to the poor stability of the 3,5-dihalogenobenzodipyrromethene intermediate. We rationalized that the converting of 3-halogeno substituent, like bromo to aryl group may be helpful for the stabilization of the dipyrromethene intermediate and may provide an opportunity to get the direct access to symmetrical [a]-benzoBODIPYs.

The key synthetic precursor 3 ^24,26 was generated in 92% yield from commercial isoindolin-1-one via Vilsmeier–Haack reaction (Scheme 1). The Suzuki coupling on 3 with 4-methoxyphenylboronic acid smoothly generated the desired 3-(4-methoxyphenyl)-1-formylisoindole 2a as a stable light yellow in 72% yield. Subsequent POCl₃-promoted self-condensation of 2a in dichloromethane smoothly proceeded at room temperature and generated the target 3,5-di(4-methoxyphenyl) benzoBODIPYs 1a in 64% isolated yield. Therefore, the installation of aryl group at 3-position of 1-formylisoindole framework indeed stabilized the thus formed dipyrromethene intermediate, which can even be isolated from the reaction mixture.

To test the versatility of this reaction, we further prepared 3-aryl-1-formylisoindole 2b–d in 53–83% isolated yields from the Suzuki coupling of 3 with substituted phenylboronic acid and 5-thiophene-2-boronic acid. Subsequent self-condensation of 2b–d smoothly proceeded. After subsequent BF₃ complexation with N,N-diisopropylamine (DIPA) and BF₃·Et₂O, the desired 3,5-diarylbenzoBODIPYs 1b–d were generated in 22–53% overall yields started from commercial isoindolin-1-one. Compounds 2a–e and 3,5-diarylbenzoBODIPYs 1a–e were characterized via NMR and HRMS. BenzoBODIPYs 1a, 1c and 1d were further characterized by X-ray analysis.

X-ray structure

Crystals of benzoBODIPYs 1a, 1c and 1d suitable for X-ray analysis were obtained by slow diffusion of methanol into their dichloromethane solutions at room temperature. As shown in Fig. 2, all these benzoBODIPYs showed nearly planar dipyrrin structures with dihedral angles between the two pyrrole units less than 6.4° (Table S2, ESI†). This nearly planar benzoBODIPY core results in the well extended π-conjugation and the observed bathochromic shift of the spectra. The dihedral angles between 3,5-phenyls and the dipyrrin core is in the range of 40.8° to 56.9° (Table S2, ESI†), indicates the intramolecular hydrogen bondings between F atoms and the hydrogen atoms on 3,5-phenyl moieties. Multiple intermolecular C–H⋯F hydrogen bonds between F atoms and various hydrogen atoms are formed due to the strong electron negativity of the F atom. These intermolecular hydrogen bondings help the establishment of the crystal packing structure and make these BODIPYs nearly parallel to each other in a head-to-tail orientation (Fig. S2, S4, and S6, ESI†).

Fig. 2 Top and front views of the X-ray structures of 1a, 1c and 1d. C, light gray; H, gray; N, blue; B, dark yellow; F, bright green; O, red.

Photophysical properties

These 3,5-diarylated benzoBODIPYs are soluble in common organic solvents, like toluene, dichloromethane, THF, methanol and acetonitrile. Their absorption and emission spectra in these solvents were studied as summarized in the Table 1 and S3 (ESI).† As shown in Fig. 3, benzoBODIPYs 1a–e each showed a characteristic absorption and emission band similar to most BODIPYs,⁴ with the maximal absorption ascribed to the S₀ → S₁ (π–π*) transition. In dichloromethane, benzoBODIPYs 1a–e each showed strong absorption between 634 and 706 nm, and bright NIR fluorescence emission in the range of 663 and 747 nm. In comparison with their non-fused BODIPY analogs,^27,28 these benzoBODIPYs each showed more than 80 nm bathochromic shift in both their absorption and emission spectra. The longest absorption and emission (706 and 747 nm, respectively) were observed in 3,5-(5-methylthiophene)benzoBODIPY 1e in toluene. Electronic properties of the phenyl substituents also affected their absorption and emission properties. BenzoBODIPY 1a possessing strong electron-donating methoxy group at the para-position of 3,5-phenyl groups showed the longer absorption and emission maximum (658 nm and 690, respectively) relative to other 3,5-diphenylbenzoBODIPYs 1b–d in toluene.

Table 1 Photophysical properties of benzoBODIPYs 1a–e in toluene at room temperature

BenzoBODIPYs	λ^maxabs (nm)	λ^maxem (nm)	log εmax	Φ^a	Stokes shift (cm^−1)
a The fluorescence quantum yields were calculated using ZnPc in DMF solution (Φ = 0.28) as reference for 1a–d and 1,3,5,7-(4-methoxy)phenylazaBODIPY (Φ = 0.36 in chloroform) for 1e, respectively. The standard errors are less than 5%.
1a	658	690	4.84	0.70	705
1b	653	681	5.04	0.76	630
1c	646	674	4.94	0.78	643
1d	648	679	5.01	0.75	704
1e	706	747	4.86	0.35	777

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Fig. 3 Overlaid and normalized (a) absorption spectrum and (b) fluorescence emission spectra of benzoBODIPYs 1a–e in dichloromethane (excited at 620 nm for 1a, excited at 610 nm for 1b–d and excited at 660 nm for 1e).

Increase of the polarity of solvents from toluene to dichloromethane, THF, methanol and acetonitrile led to 12–16 and 10–11 nm blue-shift of the absorption and emission maximum, respectively (Table S3, ESI†). All these benzoBODIPYs except 1e showed intense fluorescence with the fluorescence quantum yields between 0.70 and 0.99 in all the solvents investigated. These values were higher than that of their non-fused BODIPY analogs.²⁸ The relatively low fluorescence quantum yields (0.30–0.36) observed in 1e may associated with the rotation of thiophene moieties and the heavy atom effect of sulfur atoms.²⁹ Only a slight variation of the fluorescence quantum yields were observed for these benzoBODIPYs with the variation of the solvent polarity. This may closely associated with their rigid structures. Our result is in good consistence with literature reported aromatic-ring-fused π-extended chromophores.²²

These benzoBODIPYs showed excellent photostabilities as demonstrated by 1a, 1d and 1e in toluene in Fig. 4. During the period (70 min) of strong irradiation with a 500 W Xe lamp, more than 90% of 1a, 1d and 1e remained, while only 26% of the well-known commercialized Indocyanine Green (ICG) and 18% of methylene blue were left under this condition.

Fig. 4 Comparison of the photostabilities of 1a, 1d and 1e with methylene blue and Indocyanine Green (1 × 10⁻⁵ M) in DMF under continuous irradiation with a 500 W Xe lamp over 70 min; 28 mW cm⁻²; 25 °C.

DFT calculation

For a better understanding of the optical properties of these benzoBODIPYs, their transitional energies were calculated using time-dependent density functional theory (TD-DFT) at the B3LYP/6-31G(d,p) level (Table S4, ESI†). Non-fused 3,5-diarylBODIPY A as an analogue of 1a was also investigated and was used as a reference compound for comparison. The calculated absorption maximum was in good agreement with the absorption spectra of these benzoBODIPYs in dichloromethane. The longest absorption maximum for all these benzoBODIPYs were mainly contributed by the HOMO–LUMO transition (Table S4, ESI†). In comparison with reference compound A, benzene-annulation resulted in the increased HOMO (0.37 eV) and LUMO (0.14 eV) energies and the overall decrease of the bandgap (0.23 eV) for 1a (Fig. 5). Electron-donating substituents on 3,5-phenyls destabilize both HOMO and LUMO. For example, the HOMO/LUMO energies decreased from −4.95/−2.64 eV (1c) to −4.76/−2.45 (1b) and −4.64/−2.37 eV (1a). Interestingly, in comparison with 1a, 1e showed a decreased LUMO level from −2.37 to −2.56 eV with the HOMO energy level remained unchanged (Fig. 5). This led to a decreased HOMO–LUMO energy gap (0.19 eV), and is in good agreement with a 48 nm further red-shift in the absorption spectra in toluene relative to 1a.

Fig. 5 The frontier molecular orbitals (MOs) of 1a–e and reference compound A. Calculations are based on ground state geometry by DFT at the B3LYP/6-31G* level using Gaussian 09. The reported HOMO LUMO energy and bandgap are in eV.

As shown in Fig. 5, a stronger electron-density distribution on the annulated benzene moieties was found in the HOMO orbitals in comparison to that in the LUMO orbitals. This is in agreement with that benzene annulation (1a) largely increased the HOMO energy level with respect to non-fused A. Both HOMO and LUMO are also localized on the 3,5-aryl groups in these benzoBODIPYs. Interestingly, a stronger HOMO and LUMO delocalization on the 5-methylthiophene units was observed in 1e relative to those on phenyls in 1a–d. This may indicate thiophene moieties in 1e provide a better conjugation with the benzoBODIPY chromophore than that of phenyl moieties in 1a–d.

Conclusion

In conclusion, a set of photostable 3,5-diarylated benzoBODIPYs with strong absorption and emission in the far-red to NIR region (λ^max_abs ≤ 706 nm, λ^max_em ≤ 745 nm) were synthesized within three steps from commercial isoindolin-1-one. In comparison with the non-fused BODIPY analogue, these benzoBODIPYs showed a more than 80 nm red-shift in both absorption and emission maximum and an enhanced fluorescence. Their optical properties can be easily modulated via the variation of the electronic properties of 3,5-diaryl moieties. DFT calculations indicated the localization of HOMO and LUMO at the two annulated benzene moieties and the 3,5-aryl groups. The electron-donating groups on the 3,5-aryl groups caused a destabilized HOMO and LUMO, while the 3,5-thiophenes induced an increased HOMO and a decreased LUMO, and a better conjugation with benzoBODIPY chromophore than that of phenyl moieties.

Experimental section

General

Reagents and solvents were used as received from commercial suppliers unless noted otherwise. All reactions were performed in oven-dried or flame-dried glassware unless otherwise stated, and were monitored by TLC using 0.25 mm silica gel plates with UV indicator (60F-254). ¹H and ¹³C NMR were recorded on 300 MHz or 500 MHz NMR spectrometers at room temperature. Chemical shifts (δ) are given in ppm relative to CDCl₃ (7.26 ppm for ¹H and 77 ppm for ¹³C) or to internal TMS.³⁰ High-resolution mass spectra (HRMS) were obtained using APCI-TOF or MALDI-TOF in positive mode. Photostabilities were studied by continuous irradiation with a Xe lamp (500 W) in DMF.

UV-visible absorption spectra and fluorescence emission spectra were recorded on a commercial spectrophotometer (Shimadzu UV-2450 and Hitachi F-4500) at room temperature. Relative fluorescence quantum efficiencies of dyes were obtained by comparing the areas under the corrected emission spectrum of the test sample in various solvents with reference compound. Fluorescence quantum yields were calculated using ZnPc (ϕ = 0.28, in DMF)^31a and 1,7-diphenyl-3,5-di(4-methoxyphenyl)-azadipyrromethene (ϕ = 0.36 in chloroform).^31b Non-degassed, spectroscopic grade solvents and a 10 mm quartz cuvette were used. Dilute solutions (0.01 < A < 0.05) were used to minimize the reabsorption effects. Quantum yields were determined using the following equation:³²

The subscripts x and r refer respectively to our sample x and reference (standard) fluorophore r with known quantum yield Φ_r in a specific solvent, F stands for the spectrally corrected, integrated fluorescence spectra, A(λ_ex) denotes the absorbance at the used excitation wavelength λ_ex, and n represents the refractive index of the solvent (in principle at the average emission wavelength).

Crystals of dyes 1a (CCDC 1445553), 1c (CCDC 1445552) and 1d (CCDC 1445551)† suitable for X-ray analysis were obtained by slow diffusion of methanol into their dichloromethane solutions at room temperature. A suitable crystal was chosen and mounted on a glass fiber using grease. Data were collected using a diffractometer equipped with a graphite crystal monochromator situated in the incident beam for data collection at room temperature. Cell parameters were retrieved using SMART³³ software and refined using SAINT³⁴ on all observed reflections. The determination of unit cell parameters and data collections were performed with Mo Kα radiation (λ) at 0.71073 Å. Data reduction was performed using the SAINT software, which corrects for Lp and decay. The structure was solved by the direct method using the SHELXS-974 program and refined by least squares method on F², SHELXL-97,³⁵ incorporated in SHELXTL V5.10.³⁶

Computational details

The ground state geometry was optimized by using DFT method at B3LYP/6-31G (d) level. The same method was used for vibrational analysis to verify that the optimized structures correspond to local minima on the energy surface. TD-DFT computations were used to obtain the vertical excitation energies and oscillator strengths at the optimized ground state equilibrium geometries under the B3LYP/6-31+G(d,p) theoretical level. TD-DFT calculated molecules in dichloromethane were done using the Self-Consistent Reaction Field (SCRF) method and Polarizable Continuum Model (PCM). All of the calculations were carried out by the methods implemented in Gaussian 09 package.³⁷

Synthesis

Compound 3 was synthesized according to literatures.^24,26

General procedure for the preparation of compound 2. To compound 3 (400 mg, 1.6 mmol), arylboronic acids (3.5 mmol), and Pd(PPh₃)₄ (60 mg, 0.05 mmol) in a Schlenk flask under argon was added aqueous Na₂CO₃ (1 M, 5 mL), and freshly distilled dry toluene (20 mL). This reaction mixture was then degassed via three freeze–pump–thaw cycles before purging with argon again. The Schlenk flask was sealed and heated to 85 °C for 12 h. After cooling down to room temperature, the reaction mixture was washed with water (30 mL × 3). Organic layers were combined, dried over anhydrous Na₂SO₄, and dried under vacuum. The crude product was purified through chromatograph on silica using CH₂Cl₂, and was refluxed for 3 h in ethanol (120 mL) containing aqueous sodium hydroxide (4 M, 20 mL). Solvent was removed in vacuo. The resultant solid was dissolved in ethyl acetate (100 mL), neutralized with HCl (3 M), and washed with brine. Organic layers were combined, dried under anhydrous Na₂SO₄, filtrated and concentrated under vacuum. The residue was purified through column chromatography on silica using CH₂Cl₂ as eluent to afford 2 as yellow solids.
Compound 2a. Compound 2a was prepared in 72% yield (289 mg) from 3 (400 mg, 1.6 mmol) and 4-methoxyphenylboronic acid (532 mg, 3.5 mmol). ¹H NMR (300 MHz, CDCl₃) δ 9.87 (s, 1H), 7.98 (d, J = 6.0 Hz, 2H), 7.77 (d, J = 6.0 Hz, 2H), 7.41 (br, 1H), 7.26 (br, 1H), 7.09 (d, J = 9.0 Hz, 2H), 3.90 (s, 3H). ¹³C NMR (75 MHz, CDCl₃) δ 173.0, 160.4, 135.7, 133.2, 129.5, 127.4, 123.9, 123.5, 123.0, 122.1, 121.8, 117.4, 114.6, 55.4. HRMS (ESI) calcd for C₁₆H₁₃NO₂ [M + H]⁺: 252.1025, found 252.1019.
Compound 2b. Compound 2b was prepared in 73% yield (323 mg) from 3 (400 mg, 1.6 mmol) and 4-tert-butylphenylboronic acid (623 mg, 3.5 mmol). ¹H NMR (300 MHz, CDCl₃) δ 9.90 (s, 1H), 8.04–7.98 (m, 2H), 7.78 (d, J = 6.0 Hz, 2H), 7.58 (d, J = 9.0 Hz, 2H), 7.41 (t, J = 7.5 Hz, 1H), 7.24 (t, J = 6.0 Hz, 1H), 1.39 (s, 9H). ¹³C NMR (75 MHz, CDCl₃) δ 173.6, 152.5, 134.4, 132.4, 127.7, 127.4, 127.3, 126.4, 123.9, 123.7, 122.1, 121.9, 117.5, 34.9, 31.2. HRMS (ESI) calcd for C₁₉H₁₉NO [M + H]⁺: 278.1545, found 278.1539.
Compound 2c. Compound 2c was prepared in 53% yield (202 mg) from 3 (400 mg, 1.6 mmol) and 4-fluorophenylboronic acid (490 mg, 3.5 mmol). ¹H NMR (300 MHz, CDCl₃) δ 9.91 (s, 1H), 8.02–7.93 (m, 2H), 7.83 (brs, 2H), 7.43 (t, J = 7.5 Hz, 1H), 7.29–7, 24 (m, 3H). ¹³C NMR (75 MHz, CDCl₃) δ 174.3, 164.6, 162.6, 133.3, 132.5, 129.9, 127.9, 127.2, 124.4, 124.2, 122.5, 121.9, 118.0, 116.9. F–C coupling was not observed due to the limited solubility. HRMS (ESI) calcd for C₁₅H₁₀FNO [M + H]⁺: 240.0825, found 240.0819.
Compound 2d. Compound 2d was prepared in 70% yield (281 mg) from 3 (400 mg, 1.6 mmol) and 3-methoxyphenylboronic acid (532 mg, 3.5 mmol). ¹H NMR (300 MHz, CDCl₃) δ 12.08 (s, 1H), 9.89 (s, 1H), 8.02–7.97 (m, 4H), 7.43 (brs, 1H), 7.25 (t, J = 7.5 Hz, 1H), 7.00 (brs, 1H), 3.90 (s, 3H). ¹³C NMR (75 MHz, CDCl₃) δ 173.7, 160.2, 134.8, 132.7, 131.7, 130.3, 127.5, 124.1, 123.9, 122.0, 120.4, 117.5, 115.2, 113.0, 55.5. HRMS (ESI) calcd for C₁₆H₁₃NO₂ [M + H]⁺: 252.1025, found 252.1019.
Compound 2e. Compound 2e was prepared in 65% yield (236 mg) from 3 (400 mg, 1.6 mmol) and 5-methylthiophene-2-boronic acid (497 mg, 3.5 mmol). ¹H NMR (300 MHz, CDCl₃) δ 9.86 (s, 1H), 8.07 (d, J = 9.0 Hz, 1H), 7.97 (d, J = 9.0 Hz, 1H), 7.55 (d, J = 3.0 Hz, 1H), 7.41 (t, J = 7.5 Hz, 1H), 7.29–7.24 (m, 1H), 6.88 (d, J = 3.0 Hz, 1H), 2.59 (s, 3H). ¹³C NMR (75 MHz, CDCl₃) δ 171.9, 141.1, 131.2, 128.9, 127.9, 126.6, 125.6, 125.5, 122.9, 122.6, 121.0, 120.5, 116.5, 14.4. HRMS (APCI) calcd for C₁₄H₁₂NOS [M + H]⁺: 242.0640, found 242.0634.

General procedure for the preparation of compound 1. To compound 2 (1.0 mmol) in CH₂Cl₂ (20 mL) were added POCl₃ (94 μL, 1.0 mmol) in CH₂Cl₂ (1 mL) at ice-cold condition under argon. The reaction mixture was stirred at room temperature for 4 h. To the resultant solution was added N,N-diisopropylamine (1 mL). After stirring for 10 min, BF₃·OEt₂ (1.2 mL) was added to the solution. The mixture was stirred at room temperature for 2 h. After cooling down to room temperature, the reaction mixture was washed with brine. Organic layers were combined, dried over anhydrous Na₂SO₄, filtrated and concentrated under vacuum. The residue was purified through column chromatography on silica using petroleum ether and CH₂Cl₂ (v/v = 10/1) as eluent to afford 1 as green powders.
Compound 1a. Compound 1a was prepared in 64% yield (161 mg) from 2a (254 mg, 1.0 mmol). ¹H NMR (500 MHz, CDCl₃) δ 7.79 (brs, 6H), 7.63 (brs, 2H), 7.44 (t, J = 7.5 Hz, 3H), 7.26–7.23 (m, 3H), 7.04 (d, J = 5.0 Hz, 4H), 3.88 (s, 6H). ¹³C NMR (75 MHz, CDCl₃): δ 160.6, 151.3, 134.2, 131.8, 130.4, 128.8, 127.5, 125.1, 123.7, 123.6, 118.9, 113.9, 113.5, 55.3. HRMS (APCI) calcd for C₃₁H₂₄O₂N₂BF₂ [M + H]⁺: 505.1899, found 505.1898.
Compound 1b. Compound 1b was prepared in 73% yield (202 mg) from 2b (278 mg, 1.0 mmol). ¹H NMR (300 MHz, CDCl₃) δ 7.91 (d, J = 6.0 Hz, 4H), 7.80 (d, J = 9.0 Hz, 4H), 7.68 (d, J = 9.0 Hz, 2H), 7.53–7.44 (m, 7H), 1.38 (s, 18H). ¹³C NMR (75 MHz, CDCl₃): δ 152.5, 151.8, 134.2, 130.7, 130.0, 128.9, 128.2, 127.6, 125.3, 125.1, 123.9, 118.9, 114.3, 34.9, 31.3. HRMS (APCI) calcd for C₃₇H₃₆N₂BF₂ [M + H]⁺: 557.2940, found 557.2944.
Compound 1c. Compound 1c was prepared in 41% yield (98 mg) from 2c (120 mg, 0.5 mmol). ¹H NMR (300 MHz, CDCl₃) δ 7.93 (d, J = 9.0 Hz, 6H), 7.86–7.79 (m, 4H), 7.61 (d, J = 9.0 Hz, 2H), 7.49 (t, J = 7.5 Hz, 2H), 7.30–7.17 (m, 7H). ¹³C NMR (125 MHz, CDCl₃) δ 165.0, 163.0, 151.1, 134.6, 132.7, 131.0, 129.6, 128.0, 127.4, 125.9, 123.8, 119.4, 116.0, 115.9. F–C couping was not observed due to its limited solubility. HRMS (APCI) calcd for C₂₉H₁₈N₂BF₄ [M + H]⁺: 481.1499, found 481.1494.
Compound 1d. Compound 1d was prepared in 69% yield (174 mg) from 2d (254 mg, 1.0 mmol). ¹H NMR (300 MHz, CDCl₃) δ 7.91 (brs, 4H), 7.69 (brs, 2H), 7.44–7.40 (m, 9H), 7.02 (brs, 2H), 3.85 (s, 6H). ¹³C NMR (75 MHz, CDCl₃) δ 159.2, 151.6, 134.3, 132.2, 130.5, 129.2, 129.1, 127.7, 125.3, 123.7, 122.7, 118.9, 115.8, 115.4, 110.0, 55.3. HRMS (APCI) calcd for C₃₁H₂₄O₂N₂BF₂ [M + H]⁺: 505.1899, found 505.1895.
Compound 1e. Compound 1e was prepared in 46% yield (111 mg) from 2f (120 mg, 0.5 mmol). ¹H NMR (300 MHz, CDCl₃) δ 8.06 (d, J = 7.5 Hz, 2H), 7.89–7.86 (m, 4H), 7.69 (brs, 1H), 7.45 (t, J = 7.5 Hz, 2H), 7.32–7.30 (m, 2H), 6.95 (d, J = 3.0 Hz, 2H), 2.61 (s, 6H). ¹³C NMR was not available due to poor solubility. HRMS (MALDI-TOF) calcd for C₂₇H₂₀N₂S₂BF₂ [M + H]⁺: 485.1129, found 485.1198.

Acknowledgements

This work is supported by the National Nature Science Foundation of China (Grants No. 21372011, 21402001 and 21472002) and Nature Science Foundation of Anhui Province (Grants No. 1508085J07). The numerical calculations in this paper have been done on the supercomputing system in the Supercomputing Center of University of Science and Technology of China.

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Footnote

† Electronic supplementary information (ESI) available: Crystal structure determination and CIF files, additional photophysical data and spectra, copies of NMR spectra and high resolution mass spectra for all new compounds, calculated frontier orbitals and absorption spectra. CCDC 1445551–1445553. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra04412c

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