A Family of Highly Fluorescent and Unsymmetric Bis(BF2

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Letter Cite This: Org. Lett. 2018, 20, 4462−4466

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A Family of Highly Fluorescent and Unsymmetric Bis(BF2) Chromophore Containing Both Pyrrole and N‑Heteroarene Derivatives: BOPPY Changjiang Yu,† Zhenlong Huang,‡ Xinru Wang,† Wei Miao,† Qinghua Wu,† Wai-Yeung Wong,§ Erhong Hao,*,† Yi Xiao,*,‡ and Lijuan Jiao*,†

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†

The Key Laboratory of Functional Molecular Solids, Ministry of Education; School of Chemistry and Materials Science, Anhui Normal University, Wuhu, Anhui 241000, China ‡ State Key Laboratory of Fine Chemicals, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China § Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, China S Supporting Information *

ABSTRACT: A fundamental, highly fluorescent, and easily accessible scaffold named BOPPY is reported. The use of hydrazine as a bridging linkage between pyrrole and N-heteroarenes enables the binding of two BF2 units to provide sufficient rigidity of the unsymmetric core skeleton. These resultant unsymmetrical BOPPYs are thus highly fluorescent in their solutions and solid powder states and exhibit high molar absorption coefficients (42200−47000 M−1 cm−1), large Stokes shifts, excellent photostability, and insensitivity to pH. More importantly, these BOPPYs showed efficient two-photon absorption in the wide spectral range of 700−900 nm, making them well suited for two-photon fluorescence microscopy imaging in living cells.

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simple two-step reaction from commercially available, stable formylpyrrole, is a symmetric, highly fluorescent compound rigidified by two BF2 units. It has quickly attracted wide applications in energy-transfer cascades, probes, fluorescence imaging reagents, sensitizers for solar cells, and photodynamic therapy.6−8 Herein, we present a new family of unsymmetrical bis(BF2) fluorophore containing both pyrrole and N-heteroarene derivatives (BOPPY, Figure 1), which was motivated by several considerations. First, this facile synthetic method would provide an excellent diversity of target dyes from corresponding easily accessible N-heteroarene derivatives. Second, the unsymmetric core skeleton may provide larger Stokes shifts for the resulting dyes,5 and the new ring system might have interesting fluorescence properties. These new BOPPYs were prepared for the first time via a simple one-pot procedure from commercially available substances, featuring high fluorescence in solution and solid states, large Stokes shifts, and excellent photo- and chemostability. Moreover, these dyes also show good two-photon absorption cross sections of more than 900 GM in the near-infrared region and have been further successfully used in one-photon microscopy (OPM) and two-photon microscopy (TPM) imaging of living cells. The reaction of pyrrole-2-carboxaldehyde 3a with 2hydrazinylpyridine 4a (Scheme 1) in the presence of ptoluenesulfonic acid (PTSA) smoothly gave the corresponding

ontinuous efforts have been devoted to the development of novel small-molecule heterocyclic chromophores, which have become essential to many applications ranging from bioimaging, sensing, and therapy to photovoltaic, and optoelectronics.1 Recently, conjugated organic boron complexes have been explored as interesting and versatile fluorescent dyes.2,3 Among them, BODIPY (Figure 1) dyes,

Figure 1. Chemical structures of BODIPY core, BOPHY core, and the rationally designed BOPPY core in this contribution.

as one of the most successful fluorophores, have attracted diverse research efforts with remarkable achievements due to their easy synthesis, rich chemistry, and tunable photophysical properties.3,4 The success of the BODIPY dyes has spurred investigations into similar systems containing heteroarene or pyrrole derivatives,5 such as the recently developed BOPHY dyes6,7 (Figure 1). In particular, BOPHY, synthesized via a © 2018 American Chemical Society

Received: June 5, 2018 Published: July 23, 2018 4462

DOI: 10.1021/acs.orglett.8b01752 Org. Lett. 2018, 20, 4462−4466

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Organic Letters

constants of them further support their unsymmetrical structures.

Scheme 1. Syntheses of 1 from Formylpyrroles 3a−c and 4a−c

Figure 2. X-ray structures of 1ba (a), 1ca (b), 1bb (c), 1cb (d), 1bc (e), 1cc (f), 2a (g), and 2b (h) with thermal ellipsoids at 50% probability level. C, gray; N, blue; B, bright yellow; F, bright green; Cl, green; S, yellow. Hydrogen atoms were omitted for clarity.

condensation product which readily reacted with BF3·OEt2 in the presence of N,N-diisopropylethylamine. This one-pot reaction gave a new product with strong fluorescence as BOPPY 1aa in 50% yield. BOPPY analogs 1ba and 1ca were also synthesized in 42% and 46% yields, respectively, starting from the corresponding formylpyrroles 3b and 3c. To test the versatility of this reaction, commercially available 2-chloro-6hydrazinylpyridine 4b and 2-hydrazinylbenzo[d]thiazole 4c were then applied for condensation with corresponding formylpyrroles 3a−c, with subsequent BF3·OEt2 complexation via the same one-pot procedure, from which the desired BOPPY dyes 1ab, 1bb, 1cb, 1ac, 1bc, and 1cc were isolated in 23−54% yields (Scheme 1). These halogen groups on BOPPYs 1ab, 1bb, and 1cb provide valuable reactive sites for the facile postfunctionalization of these dyes with various functionalities, as demonstrated in Scheme 2. For example, 1bb readily underwent nucleophilic

The crystals unambiguously reveal the structural distinctions for these dyes, as shown in Figure 2, which show the planar core of pyrrolic, pyridyl, or benzo[d]thiazole rings at the periphery, and two newly formed BF2-containing units including a five- and six-membered ring in the center. Selected bond lengths, dihedral angles, and data collection parameters are summarized in Tables S1−S3 in the Supporting Information (SI). In the solid state, most dyes are rigidly planar, and the dihedral angles between the pyrrolic and pyridyl/benzo[d]thiazole rings are less than 6.4°. However, larger dihedral angles are observed for 2,4-methyl-3-ethylpyrrole substituted BOPPYs 1ca and 1cb (15.3° and 16.1°, respectively), indicating slightly distorted conformations of these compounds. The thiophene ring is coplanar with the BOPPY core in 2b, where the dihedral angle is less than 0.5° between the BOPPY core and the thiophene ring. Each boron atom is coordinated in a tetrahedral geometry by two nitrogen and two fluorine atoms. Among the four B−N bonds, the bond length between boron and the pyrrolic nitrogen is shortest (around 1.52 Å), while others are around 1.56 Å (Table S1, SI). The different B−N bond lengths support the unsymmetrical nature of BOPPY dyes. These BOPPY dyes generally exhibit excellent optical properties with a strong broad absorption and intense emission in visible regions in several solvents studied, as summarized in Table 1 as well as Table S4 and Figures S9−S19 (SI). The parent BOPPY 1aa exhibits two well-split absorption maxima at 393 and 413 nm in dichloromethane, with extinction coefficients of 4.48 × 104 and 4.43 × 104 M−1 cm−1, respectively. BOPPY 1aa also exhibits dual fluorescence emission maxima at 432 and 462 nm (Figures 3 and S9), respectively, with a quantum yield of 0.79 in dichloromethane. Similar dual absorption and emission maxima are observed for most of the other BOPPY dyes (Figures S10−S19). These dyes show large Stokes shifts in the range of 2427−4463 cm−1 (Table S4). Gradual red shifts of the absorption and emission bands for these dyes were observed with the installation of alkyl substituents on the pyrrolic position or chloride on the pyridyl ring of the chromophore (Figures S20−S22). For example, in comparison with 1aa, the absorption bands for 2,4dimethyl-3-ethylpyrrole substituted 1ca were red shifted to 411 and 430 nm (Figure S20), while the absorption bands for 2-

Scheme 2. Syntheses 2a and 2b from 1bb

substitution reactions with n-butylamine in the presence of base in refluxing 1,2-dichloroethane, from which BOPPY 2a was generated in 82% yield. A similar reaction has found many applications on BODIPYs with a halogen atom on the 3-, 5-, or 8-position.9 BOPPY 1bb also showed good reactivity toward Stille cross-coupling reaction, giving thiophene substituted 2b in 91% yield (Scheme 2). All these BOPPYs are stable toward light, humidity, and air. They were readily purified via silica column chromatography to give crystalline solids after evaporation of solvent and were characterized by 1H NMR, 13 C NMR, 19F NMR, HRMS, and X-ray crystal analysis (Figures 2 and S1−S8). The four sets of F−F coupling 4463

DOI: 10.1021/acs.orglett.8b01752 Org. Lett. 2018, 20, 4462−4466

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Table 1. Photophyscial Properties of BOPPYs 1 and 2 in Dichloromethane and Solid Powder States at Room Temperature dichloromethane dyes 1aa 1ba 1ca 1ab 1bb 1cb 1ac 1bc 1cc 2a 2b

solid powder

λabsmax/nm (log ε) 393 408 411 402 417 422 378 386 391 421 430

(4.47), (4.47), (4.48), (4.58), (4.51), (4.43), (4.41), (4.50), (4.73), (4.53), (4.45)

413 428 430 422 436 442 394 404 410 443

(4.45) (4.43) (4.45) (4.51) (4.47) (4.38) (4.36) (4.51) (4.77) (4.64)

λemmax/nm (τ/ns, ϕ) 432, 462 (2.73, 0.79) 447, 478 (2.89, 0.57) 469(sh), 491 (2.45, 0.50) 444, 471 (2.96, 0.65) 489 (2.22, 0.41) 499 (1.78, 0.38) 409, 432, 463 (1.84, 0.62) 420, 441, 461 (2.28, 0.74) 423, 445, 456 (2.13, 0.77) 464, 483 (2.05, 0.44) 520 (0.65, 0.08)

λemmax/nm (ϕ) 484, 557 (0.21) 490, 508 (0.18) 498, 575(sh) (0.22) 500(sh), 528 (0.48) 506, 537 (0.19) 503, 543, 579 (0.24) 480, 507(sh) (0.27) 479(sh), 500 (0.15) 466, 499, 537 (0.31) 530 (0.16) 579 (0.09)

red-shifted and high solid-state fluorescence of these BOPPYs may be related to the presence of the strengthened intermolecular interactions in the solid state due to the closeness of the molecules (packing effect). Recently, similar results have been observed for some boron-containing fluorophores.5,10 In their crystal-packing structures, these dyes all give well-ordered head-to-tail packing structures with negligible π−π interactions. More importantly, BOPPYs 1ba, 1bb, 1bc, 1ca, 1cb, and 1cc all adopt coplanar inclined arrangements of their transition dipoles with slip angles of 25.0°−35.8° (Figure 4a, Figures S1−S8). These arrangements are characteristic of J-type solid state packing as previously reported.10

chloropyridine substituted 1cb were red shifted to 422 and 442 nm (Figure S21). In general, slight blue shifts of the absorption were observed for most of these BOPPY dyes with the increase of the polarity of the system from hexane, toluene, dichloromethane, tetrahydrofuran, to methanol (Table S4). While this variation of the solvent polarity caused little variation of the fluorescence emission maxima; however, reduced fluorescence quantum yields were obtained in more polar solvents for almost all these dyes, such as in acetonitrile. Apolar environments improve their fluorescence quantum yields while polar ones reduce their fluorescence quantum yields (Table S4). The similar appearance was observed for their corresponding fluorescence lifetimes (Table S4, Figures S23−S72). This solvent-dependent fluorescence may be attributed to the solvation−relaxation process (a rapid nonradiative transition) in the excited states of these dyes, as indicated by the fluorescence lifetimes of these dyes (Table S4).

Figure 4. (a) Crystal-packing pattern of BOPPY 1ba between the adjacent interlayered crystals. Interlayer distance is 3.73 Å. Slip angle of 35.8° for coplanar inclined arrangements of its transition dipole. (b) Photo of solid state fluorescence of 1aa, 1ba, and 1ca taken under a hand-held UV (365 nm) lamp. Figure 3. Normalized UV−vis (solid lines) and fluorescence spectra (dot lines) of 1aa in organic solvents of hexane (black), toluene (cyan), dichloromethane (blue), tetrahydrofuran (orange), methanol (green), and the solid powder state (red).

The BOPPY dyes displayed good photostability (Figures S84−S87). A commercial 1,3,5,7-tetramethylBODIPY (BDP) dye was chosen as the reference compound. Solutions of 1aa, 1ca, and BDP in acetonitrile were irradiated under a 50 W LED lamp. After 3 h, the absorbance dropped by ∼50% for BDP where the absorbance of 1aa and 1ca did not show noticeable changes, suggesting that BOPPY dyes are much more photostable than BDP. The BOPPY dyes also exhibited excellent chemostability. The stability toward pH was tested with 1aa and 1ca as examples. A solution of 1aa or 1ca in aqueous PBS buffer solution was added to a HCl or NaOH solution while monitoring UV−vis absorption (Figures S88 and S89). The absorption spectra remained almost unchanged in the pH ranges from 1 to 13 tested.

Most of these BOPPYs show strong fluorescence emission in the visible range of light in their solid powder states (Table 1, Figures 3, 4b, and S73−S83) with high fluorescence quantum yields of 0.09−0.48. Their broad fluorescence emission bands with maxima ranging from 466 to 625 in the solid powder states are red-shifted with respect to their corresponding emission bands in solutions. For example, 1ab shows emission maxima at 511 and 536 nm with a fluorescent quantum yield of 0.48 in the solid powder state (Table 1), where in dichloromethane 1ab gives emission maxima at 444 and 471 nm with a fluorescent quantum yield of 0.65. The observed 4464

DOI: 10.1021/acs.orglett.8b01752 Org. Lett. 2018, 20, 4462−4466

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Organic Letters The cyclic voltammetry of BOPPYs (Figures S90−93 and Table S5) show irreversible reductions with Epc ranging from −1.39 to −1.69 V. Irreversible oxidations were obtained for 1aa, 1ba, 1ab, and 1bc with Epa at 1.59, 1.50, 1.67, and 1.63 V, respectively, while reversible oxidations were found for 1ca, 1bb, 1cb, 1cc, and 2b with half-wave potentials at 1.31, 1.42, 1.36, 1.48, and 1.33 V, respectively. As expected, the installation of electron-donating alkyl substituents on the pyrrolic position of BOPPY shifts the oxidation and reduction to low potentials. The chlorine group on the pyridyl ring of the chromophore mainly shifts the reduction to more positive potentials. To further understand the electronic properties of these dyes, the ground state geometry was optimized by using the density functional theory (DFT) method at the TPSSh/6311g* level on BOPPYs 1ca, 1cb, and 1cc. Their HOMO and LUMO are all well distributed (Figure S94), and the HOMO and LUMO energy levels are −5.38 and −2.48 eV for 1ca, −5.48 and −2.61 eV for 1cb, and −5.57 and −2.43 eV for 1cc, respectively, which are in good agreement with electrochemistry data. The time-dependent DFT (TDDFT) predicted that the strong bands observed in the visible region of these dyes are the S1 → S0 transition mainly contributed by HOMO → LUMO (Figures S95−S97 and Table S6). Additional weak absorption bands in the UV region are the S2 → S0 transition. We then calculated the excited state geometry of 1ca, 1cb, and 1cc using the B3LYP in 6-31G* level, and the three BOPPYs all show planar conformations in their excited S1 state. However, some bond lengths in the S1 state changed significantly. For example, the N−N bond length of 1ca in the S1 geometry is 0.05 Å lesser than that of the S0 state. These results indicate that, similar to previously reported BOPHY dyes, the double absorption bands observed in the visible range belong to vibronic progressions of the same excited state and the splitted emission bands are possible 0−0 and 0−1 transitions of the same vibronic progression.1e,6c,7a,b,8d However, the precise lifetime for each split emission was not possible to obtain due to instrumentation limits and the lifetimes reported above might be ascribed to the formation of S1 in the slowest process.7b In order to evaluate the potential of our BOPPY in TPM imaging, we determined the two-photon absorption (TPA) cross sections of 1cb by the femtosecond two-photon excited fluorescence (TPEF) technique. In methanol, the two-photon excitation spectra of 1cb in the relatively wide spectral range of 700−900 nm was recorded in Figure S98, which exhibited two excitation bands, with one peak TPEF cross section δ of 989 GM (Goeppert−Mayer, 10−50 cm4 s/photon) at 770 nm. This TPEF activity of 1cb is thus a considerable value, because the active cross sections of more than 50 GM are sufficient for TPM imaging, according to previous reports in the development of two-photon probes.11 It should be noted that the maximum one-photon absorption of 1cb at 450 nm reveals that the optimal two-photon excitation activity may be located over 900 nm. 1cb exhibited good solubility in phosphate buffer saline (PBS) containing 0.1% DMSO (Figure S99), which was then employed for imaging in living cells. MCF-7 cells were stained with the dye 1cb for 10 min and imaged with both TPM and OPM. As shown in Figure 5, 1cb passed through cell membranes fast and gave strong fluorescence signals in cells. Compared with OPM, TPM imaging of 1cb exhibited higher spatial resolution and weaker background interference due to two-photon excitation using the near-infrared laser.

Figure 5. OPM, TPM, and phase contrast images of MCF-7 cells stained with 1cb (5 μM) for 10 min and washed three times with PBS. (A) OPM imaging of 1cb (λex = 405 nm, λem = 450−550 nm); (B) TPM imaging of 1cb (λex = 770 nm, λem = 450−550 nm); (C) different interference contrast; scale bar: 20 μm.

In summary, we have developed the diversity-oriented efficient one-pot synthesis of a series of BOPPY dyes, which represent a new structural motif for highly fluorescent pyrrolebased BF2 dyes. The facile one-pot synthesis starting from commercially available precursors provides excellent diversity of these BOPPY dyes, while the preparation is modular and tolerates various pyrrole and N-heteroarene moieties. These BOPPYs showed excellent photophysical properties, including high fluorescence quantum yields in both the solid and solution state, high two-photon absorption cross sections in the biological window, large Stokes shifts, excellent stability, good molar absorptivity, and tunable absorption/emission profiles via simple variation of the starting pyrrole derivatives and heteroarene derivatives or further postfunctionalizations such as the nucleophilic substitution and Stille coupling reactions demonstrated in this work. Preliminary results for TPM cell imaging provided higher resolution than OPM confocal imaging, supporting further investigation of these dyes for bioimaging applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b01752. Experimental details, NMR, and additional photophysical data (PDF) Accession Codes

CCDC 1542853−1542855, 1542857−1542858, 1542860− 1542861, and 1542864 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Changjiang Yu: 0000-0002-9509-7778 Wai-Yeung Wong: 0000-0002-9949-7525 Erhong Hao: 0000-0001-7234-4994 Lijuan Jiao: 0000-0002-3895-9642 4465

DOI: 10.1021/acs.orglett.8b01752 Org. Lett. 2018, 20, 4462−4466

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Organic Letters Notes

Lévêque, P.; Heiser, T.; Retailleau, P.; Ziessel, R. Chem. Commun. 2015, 51, 14742. (c) Zhang, C.; Zhao, J. J. Mater. Chem. C 2016, 4, 1623. (d) Li, X.; Ji, G.; Son, Y. Dyes Pigm. 2016, 124, 232. (9) (a) Leen, V.; Yuan, P.; Wang, L.; Boens, N.; Dehaen, W. Org. Lett. 2012, 14, 6150. (b) Zhao, N.; Xuan, S.; Fronczek, F. R.; Smith, K. M.; Vicente, M. G. H. J. Org. Chem. 2015, 80, 8377. (10) (a) Choi, S.; Bouffard, J.; Kim, Y. Chem. Sci. 2014, 5, 751. (b) Kim, S.; Bouffard, J.; Kim, Y. Chem. - Eur. J. 2015, 21, 17459. (11) (a) Lei, Z.; Yue, P.; Wang, X.; Li, X.; Li, Y.; He, H.; Luo, X.; Meng, X.; Chen, J.; Qian, X.; Yang, Y. Chem. Commun. 2017, 53, 10938. (b) Sadowski, B.; Kita, H.; Grzybowski, M.; Kamada, K.; Gryko, D. T. J. Org. Chem. 2017, 82, 7254. (c) Yu, J.; Bian, H.; Man, H.; Li, N.; Xie, L.; Xiao, Y. Dyes Pigm. 2018, 149, 851.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the National Nature Science Foundation of China (21672006, 21672007, 21402001, and 21472002) and Anhui Province (1508085J07), Hong Kong Polytechnic University (1-YW2T), and Anhui Normal University Doctoral up Starting Foundation (2017XJJ28) for financial support.

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DOI: 10.1021/acs.orglett.8b01752 Org. Lett. 2018, 20, 4462−4466