Letter Cite This: Org. Lett. 2018, 20, 4103−4106
pubs.acs.org/OrgLett
Synthesis of 1,3-Azaphospholes with Pyrrolo[1,2‑a]quinoline Skeleton and Their Optical Applications Di Wu,† Lingfeng Chen,‡ Shuangliang Ma,† Huiying Luo,§,∥ Jing Cao,*,§ Runfeng Chen,*,‡ Zheng Duan,*,† and Francois Mathey*,†
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†
College of Chemistry and Molecular Engineering, International Phosphorus Laboratory, International Joint Research Laboratory for Functional Organophosphorus Materials of Henan Province, Zhengzhou University, Zhengzhou 450001, P. R. China ‡ Key Laboratory for Organic Electronics and Information Displays & Jiangsu Key Laboratory for Biosensors, Institute of Advanced Materials, Nanjing University of Posts & Telecommunications, Nanjing 210023, P. R. China § Department of Anatomy, College of Basic Medicine, Zhengzhou University, Zhengzhou 450001, P. R. China ∥ Guangzhou University of Chinese Medicine, Guangzhou 510000, P. R. China S Supporting Information *
ABSTRACT: A facile synthesis of 1,3-azaphospholes with a pyrrolo[1,2-a]quinoline skeleton has been described. These new annulated 1,3-azaphospholes exhibit good photoelectric performance and can be used as the emitting dopant in organic lightemitting diodes (OLEDs) and dye for bioimaging.
ecently, incorporating a phosphorus atom into πconjugated systems has attracted much attention for the production of novel molecules and materials with unusual properties and applications.1 The design, synthesis, and characterization of new phosphorus-bridged π-conjugated molecules is a challenging and essential step toward new phosphorus-based organic light-emitting diodes (OLEDs),2 bioimaging reagents,3 and probes.4 Pyrrolo[1,2-a]quinoline 1, one of the basic fused-ring moieties in heterocyclic organic chemistry, has a wide range of applications in organic functional materials,5 pharmaceuticals,6 and biological studies.7 Intrigued by the performance of the pyrrolo[1,2-a]quinoline moiety in material science, we targeted the incorporation of the P atom into a pyrrolo[1,2-a]quinoline skeleton to form the unknown annulated 1,3-azaphospholes 2 (Figure 1). It was thus expected that the embedded phosphorus atom not only induces perturbations of the electronic structure of the πbackbone but also facilitates tuning the physical properties of the resulting molecules by simple chemical modifications of the P center, as well as allows for the synthesis of new fluorophores
R
with some structural diversity. In the present study, we set out to synthesize and study the annulated 1,3-azaphospholes 2. N-Aryl pyrroles 3 were synthesized from commercially available anilines and 2,5-dimethoxytetrahydrofuran8 (Scheme 1a). Following Lautens’s procedure,9 3b was converted into Scheme 1. Synthesis of Pyrrolo[1,2-a]quinolines 1a and 1b
pyrroloquinoline 1a through palladium catalyzed annulation with norbornadiene in toluene at high temperature (Scheme 1b). Pyrrolo[1,2-f ]phenanthridine 1b was prepared by the Pdcatalyzed annulation of arynes with 3c, which was developed by Larock et al.10 (Scheme 1c). Product 3b was transformed into 5 through the reaction between aryllithium 4 and 9,10dibromophenanthrene. The following palladium-catalyzed Received: May 26, 2018 Published: June 22, 2018
Figure 1. Incorporation of phosphorus into pyrrolo[1,2-a]quinolone. © 2018 American Chemical Society
4103
DOI: 10.1021/acs.orglett.8b01663 Org. Lett. 2018, 20, 4103−4106
Letter
Organic Letters intramolecular arylation reactions then afforded dibenzo[i,k]pyrrolo[1,2-f ]phenanthridine 1c (Scheme 2). Scheme 2. Synthesis of Pyrrolo[1,2-a]quinolone 1c
Figure 3. X-ray crystal structure of 2cO with top (left) and packing (right) views. The level set for thermal ellipsoids of all atoms is 30%.
H4 and H16 (Figure 3) in the A bay side (Scheme 3) results in a slight twisting of this part. This intramolecular repulsion does not exist in the opposite bay region (B). In this case, the B bay region is coplanar. The distortion from planarity is seen most clearly by comparing the torsional angles along the two bay region: ∠C4−C5−C14−C15 = 18.0(3)°, ∠C5−C14−C15− C16 = 20.2(4)° (A) vs ∠C9−C8−C13−C24 = 2.3(3)°, ∠C8− C13−C24−C23 = 0.3(5)° (B). The twisting of the A bay area out of planarity can explain the absence of π−π stacking between the adjacent molecules. This packing mode might be beneficial for preventing excimer formation and Dexter-type energy transfer leading to the loss of the excited energy. Each compound shows a strong absorption band at 295− 318 nm, which is assignable to a π−π* transition, and a weaker band at 378−425 nm, which can be ascribed to an intramolecular charge transfer (CT) transition (Figure 4).
The synthesis of annulated 1,3-azaphospholes relies primarily on the reaction of dilithiobiaryls and dichlorophosphins.11 To our delight, in the presence of TMEDA, the bay regions of pyrrolo[1,2-a]quinoline derivatives 1 were dilithiated selectively and easily with n-butyllithium to form 6, and subsequent cyclization with dichlorophosphine provided the tervalent 1,3-azaphospholes 2 (Scheme 3), which were Scheme 3. Synthesis of 1,3-Azaphospholes
converted into the more stable azaphosphole oxides (X = O) or sulfides (X = S). Benzo[d]pyrrolo[1,2-a][1,3]azaphosphole oxide 2dO was prepared for the sake of comparison.12 The structures of these compounds were assigned unambiguously based on the results from the NMR spectroscopy, X-ray crystallography, elemental analysis, and mass spectrometry measurements. The single crystals of 2aO and 2cO suitable for X-ray analysis were obtained by recrystallization from solutions of dichloromethane and hexane. The crystal structure of 2aO revealed a highly planar conformation (Figure 2). In the packing structure, 2aO forms a slipped π−π stacked structure, in which the intermolecular distance is approximately 3.487 Å. Compound 2cO crystallizes in the monoclinic space group P21/c as enantiomers (Figure 3). The steric repulsion between
Figure 4. UV−vis absorption and fluorescence spectra of the azaphospholes in CH2Cl2 at rt.
Fluorescence maxima appeared in the yellow to green region with large Stokes shifts (Table 1). There was little overlap in the absorption and fluorescence spectra, thus suggesting that the designed structures of 2a−c were advantageous for suppressing luminescence quenching through Förster resonance energy transfer. Compared with 2dO (λmax = 290 nm), the absoption bands of azaphospholes 2a−c are bathochromic shifted, due to the increase in conjugation. In addition, the fluorescence quantum yields (Φf) of 2acO(S) (8−97%) are higher than 2dO (4%) (Table 1). Compared with sulfides (2aS, 2cS), more efficient emissions were observed with the corresponding oxides (2aO, 2cO), respectively. In CH2Cl2, 2cO showed the highest absolute fluorescence quantum yield at 97%. In light of the high thermal stability (Table 1) and photoluminescent quantum efficiency of 2cO with a rigid and fused molecular structure, electroluminescent devices were fabricated using 2cO as the emitting dopant in the following device structure: ITO/MoO3 (3 nm)/mCP (25 nm)/DMACDPS: x % 2cO (15 nm)/PO-T2T (45 nm)/LiF (1 nm)/Al (100 nm). In the devices, the widely used blue thermally
Figure 2. X-ray crystal structure of 2aO with top (left) view and packing (right) view. The level set for thermal ellipsoids of all atoms is 30%. Hydrogen atoms of the packing view were omitted for clarity. 4104
DOI: 10.1021/acs.orglett.8b01663 Org. Lett. 2018, 20, 4103−4106
Letter
Organic Letters
that was very similar to its fluorescent spectrum at all the different doping concentrations (Figure 5c, Figure S4). Therefore, these TADF-sensitized OLEDs exhibit good device performance at very low doping concentrations, and at the optimal concentration of 1.5 wt %, the best device performance was observed, showing low turn-on voltage (2.72 V) and high maximum current efficiency, power efficiency, and external quantum efficiency (EQE) up to 19.21 cd/A, 14.03 lm/W, and 6.41%, respectively (Table 2). Even at a luminescence of
Table 1. Photophysical Data, Melting Point, and Decomposition Temperature compd
λmax (nm)a
log ε
2aO 2aS 2bO 2cO
412 407 378 425
3.44 3.50 3.23 3.89
2cS 2dO
422 290
4.02 3.56
λem (nm)a
Stokes shift (cm−1)
Φ fb
496 490 462 490 (512)f 493 378
4110 4161 4810 3121
0.36 0.08 0.25 0.97g
−e 205 224 273
97 299 362 399
3412 8028
0.24 0.04
240 156
438 294
tm td (°C)c (°C)d
Table 2. EL Performance of Devices
−5
Measured in CH2Cl2 (1 × 10 M). Emission maxima upon excitation at the absorption maximum wavelengths. bMeasured relative to quinine sulfate (H2SO4, 0.1 M), ±15%. cMelting point measurements were performed by DSC. dDecomposition temperatures were defined as 5% of weight loss under argon, 10 °C/min. e Not observed. fValues in parentheses were measured as powder. g Absolute fluorescence quantum yields determined by using a calibrated integrating sphere system. a
doping rate (wt %)
Von (V)
luminancemax (cd/m2)
CEmax (cd/A)a
PEmax (lm/W)b
EQEmax (%)c
1.0 1.5 2.0
2.72 2.72 2.72
9870 10 407 11 101
15.70 19.21 15.12
11.21 14.03 10.60
5.39 6.41 5.06
a
Maximum current efficiency. bMaximum power efficiency. cMaximum external quantum efficiency.
10 000 cd/m2, the efficiencies are also as high as 19.05 cd/A and 12.12 lm/W, suggesting a very low efficiency roll-off of only 0.8%. The high EQE, which is larger than the theoretical up-limit of 5% of fluorescent OLEDs, represents direct evidence for the TADF-sensitized feature of these devices.14 The highly efficient and stable OLEDs should be closely related to the highly luminescent and stable emitter of the newly constructed 2cO with the highly fused molecular structure. Interestingly, 2cO is membrane permeable and no significant cytotoxicity (Supporting Information (SI), Figure S5) was observed in cultured PC12 cells when using 2 μM of 2cO for live-cell imaging. Strong green or blue fluorescence was detected inside the cells when stimulated by laser λex = 480/30 nm (Figure 6A, a) and λex = 360/40 nm (Figure 6B,
activated delayed fluorescence (TADF) molecule of bis[4-(9,9dimethyl-9,10-dihydroacridine)phenyl]sulfone (DMAC-DPS) is used as the host to sensitize the emission of the doped 2cO at different weight concentrations (x) in the range 1.0%, 1.5%, and 2.0% in the emitting layer.13 Whereas, ITO and Al are the anode and the cathode, MoO3 and LiF are the hole- and electron-injection materials, and 1,3-bis(9-carbazolyl)benzene (mCP) and (1,3,5-triazine-2,4,6-triyl)tris(benzene-3,1-diyl))tris(diphenylphosphine oxide) (PO-T2T) serve as the holeand electron-transport molecules, respectively. From the energy-level schematic diagram (Figure 5a), balanced hole
Figure 6. Fluorescence and bright field images of PC12 cells treated with 2cO at 2 μM for 2 h (A, B) or 24 h (a, b). Images were acquired with λex = 360/40 nm (A, a), λex = 480/30 nm (B, b), bright field image (C, c).
Figure 5. (a) Energy level schematic diagram of the TADF-sensitized OLEDs. (b) Luminance−Voltage−Current density characteristics of the devices based on 2cO. (c) Electroluminescent (EL) spectra and images (inset) of TADF-sensitized OLEDs with 2cO doping concentrations of 1.5 wt % at different driving voltages. Photograph of the electroluminescence image was taken at 6 V. (d) Efficiencies− luminance curves of the OLEDs.
b), respectively. The signal of 2cO was still detected even after 24 h (Figure 6a, b). To examine the potential for in vivo imaging, 2cO was intravenously injected into rats. Both visceral tissue and nerve tissue could be fluorescence stained with 2cO in 2 h after injection (SI, Figure S6A−G, a−g). In conclusion, we have synthesized a series of phosphorus embedded π-systems from a facile method. These new annulated 1,3-azaphospholes exhibit electronic absorptions in the visible and UV ranges. The physical properties, such as absorption and emission band, and quantum yields can be tuned by chemical modification of the phosphorus atom. Good
and electron injection and transport can be established with the harmonious frontier orbital energy levels of the different layers. Also, complete energy transfers from the TADF host of DMAC-DPS to the guest of 2cO for the TADF-sensitized fluorescence were evidenced by the overlapped emission spectra of DMAC-DPS and absorption bands of 2cO, leading to only the characteristic green electroluminescence of 2cO 4105
DOI: 10.1021/acs.orglett.8b01663 Org. Lett. 2018, 20, 4103−4106
Letter
Organic Letters
Szű cs, R.; Lescop, C.; Réau, R.; Nyulászi, L.; Bouit, P.-A.; Hissler, M. Organometallics 2017, 36, 2502. (2) (a) Chen, H.; Delaunay, W.; Li, J.; Wang, Z. Y.; Bouit, P. A.; Tondelier, D.; Geffroy, B.; Mathey, F.; Duan, Z.; Réau, R.; Hissler, M. Org. Lett. 2013, 15, 330. (b) Chen, H.; Pascal, S.; Wang, Z.; Bouit, P.A.; Wang, Z.; Zhang, Y.; Tondelier, D.; Geffroy, B.; Reau, R.; Mathey, F.; Duan, Z.; Hissler, M. Chem. - Eur. J. 2014, 20, 9784. (c) Riobé, F.; Szű cs, R.; Bouit, P. A.; Tondelier, D.; Geffroy, B.; Aparicio, F.; Buendía, J.; Sánchez, L.; Réau, R.; Nyulászi, L.; Hissler, M. Chem. Eur. J. 2015, 21, 6547. (d) Joly, D.; Bouit, P.-A.; Hissler, M. J. Mater. Chem. C 2016, 4, 3686. (e) Zhou, Y.; Yang, S.; Li, J.; He, G.; Duan, Z.; Mathey, F. Dalton Trans. 2016, 45, 18308. (3) (a) Liang, X.; Mack, J.; Zheng, L.-M.; Shen, Z.; Kobayashi, N. Inorg. Chem. 2014, 53, 2797. (b) Chai, X.; Cui, X.; Wang, B.; Yang, F.; Cai, Y.; Wu, Q.; Wang, T. Chem. - Eur. J. 2015, 21, 16754. (c) Wang, C.; Fukazawa, A.; Taki, M.; Sato, Y.; Higashiyama, T.; Yamaguchi, S. Angew. Chem., Int. Ed. 2015, 54, 15213. (d) Huang, H.; Luo, H.; Tao, G.; Cai, W.; Cao, J.; Duan, Z.; Mathey, F. Org. Lett. 2018, 20, 1027. (4) (a) Yamaguchi, E.; Wang, C.; Fukazawa, A.; Taki, M.; Sato, Y.; Sasaki, T.; Ueda, M.; Sasaki, N.; Higashiyama, T.; Yamaguchi, S. Angew. Chem., Int. Ed. 2015, 54, 4539. (b) Gong, P.; Ye, K.; Sun, J.; Chen, P.; Xue, P.; Yang, H.; Lu, R. RSC Adv. 2015, 5, 94990. (c) Taki, M.; Ogasawara, H.; Osaki, H.; Fukazawa, A.; Sato, Y.; Ogasawara, K.; Higashiyama, T.; Yamaguchi, S. Chem. Commun. 2015, 51, 11880. (5) (a) Ahmed, E.; Briseno, A. L.; Xia, Y.; Jenekhe, S. A. J. Am. Chem. Soc. 2008, 130, 1118. (b) Yan, L.; Zhao, D.; Lan, J.; Cheng, Y.; Guo, Q.; Li, X.; Wu, N.; You, J. Org. Biomol. Chem. 2013, 11, 7966. (c) Mathew, S.; Astani, N. A.; Curchod, B. F. E.; Delcamp, J. H.; Marszalek, M.; Frey, J.; Rothlisberger, U.; Nazeeruddin, M. K.; Gratzel, M. J. Mater. Chem. A 2016, 4, 2332. (6) (a) Prunier, H.; Rault, S.; Lancelot, J.-C.; Robba, M.; Renard, P.; Delagrange, P.; Pfeiffer, B.; Caignard, D.-H.; Misslin, R.; Hamon, M. J. Med. Chem. 1997, 40, 1808. (b) Guillon, J.; Le Borgne, M.; Rimbault, C.; Moreau, S.; Savrimoutou, S.; Pinaud, N.; Baratin, S.; Marchivie, M.; Roche, S.; Bollacke, A.; Pecci, A.; Alvarez, L.; Desplat, V.; Jose, J. Eur. J. Med. Chem. 2013, 65, 205. (c) Lv, W.; Budke, B.; Pawlowski, M.; Connell, P. P.; Kozikowski, A. P. J. Med. Chem. 2016, 59, 4511. (7) (a) Hazra, A.; Mondal, S.; Maity, A.; Naskar, S.; Saha, P.; Paira, R.; Sahu, K. B.; Paira, P.; Ghosh, S.; Sinha, C.; Samanta, A.; Banerjee, S.; Mondal, N. B. Eur. J. Med. Chem. 2011, 46, 2132. (b) Ganihigama, D. U.; Sureram, S.; Sangher, S.; Hongmanee, P.; Aree, T.; Mahidol, C.; Ruchirawat, S.; Kittakoop, P. Eur. J. Med. Chem. 2015, 89, 1. (8) Deng, H. J.; Fang, Y. J.; Chen, G. W.; Liu, M. C.; Wu, H. Y.; Chen, J. X. Appl. Organomet. Chem. 2012, 26, 164. (9) Hulcoop, D. G.; Lautens, M. Org. Lett. 2007, 9, 1761. (10) Liu, Z.; Larock, R. C. J. Org. Chem. 2007, 72, 223. (11) Hibner-Kulicka, P.; Joule, J. A.; Skalik, J.; Balczewski, P. RSC Adv. 2017, 7, 9194. (12) Cheeseman, G. W. H.; Greenberg, S. G. J. Organomet. Chem. 1979, 166, 139. (13) Tao, Y.; Yuan, K.; Chen, T.; Xu, P.; Li, H.; Chen, R.; Zheng, C.; Zhang, L.; Huang, W. Adv. Mater. 2014, 26, 7931. (14) Zhang, D.; Song, X.; Cai, M.; Duan, L. Adv. Mater. 2018, 30, 1705250.
EL performance and bioimaging applications of 2cO were observed, owing to its high luminescent quantum efficiency and stable emission in various environments. These results demonstrate the significant advantages of incorporating a phosphorus atom into π-conjugated systems, providing important clues for the design and invention of organic fluorophores.
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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.8b01663. Experimental section; fluorescence spectra of 2cO in solution and as powder; cyclic voltammograms; electrochemical data; TGA and DSC graphs; electroluminescent spectra of TADF-sensitized OLEDs; cell proliferation of PC12 cells; immunofluorescence of the tissues; X-ray crystallographic studies of 2aO and 2cO; NMR spectra (PDF) Accession Codes
CCDC 1558400 and 917810 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: *E-mail: *E-mail: *E-mail:
[email protected] (Z.D.).
[email protected] (F.M.).
[email protected] (R.C.).
[email protected] (J.C.).
ORCID
Runfeng Chen: 0000-0003-0222-0296 Zheng Duan: 0000-0002-0173-663X Francois Mathey: 0000-0002-1637-5259 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (21672193, 21272218, 21072179, 20702050, 21674049, 81671091) and Zhengzhou University, for their financial support.
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REFERENCES
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DOI: 10.1021/acs.orglett.8b01663 Org. Lett. 2018, 20, 4103−4106
