Synthesis of 1,3-Azaphospholes with Pyrrolo[1,2- a]quinoline Skeleton and Their Optical Applications

Article abstract of DOI:10.1021/acs.orglett.8b01663

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 light-emitting diodes (OLEDs) and dye for bioimaging.

Full text of DOI:10.1021/acs.orglett.8b01663

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
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*,†
†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 perform-
ance and can be used as the emitting dopant in organic light-
emitting diodes (OLEDs) and dye for bioimaging.
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
ecently, incorporating a phosphorus atom into π-
Rconjugated 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
Scheme 1. Synthesis of Pyrrolo[1,2-a]quinolines 1a and 1b
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 pharmaceut-
icals,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 π-
backbone 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
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 Pd-
catalyzed 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 9,10-
dibromophenanthrene. The following palladium-catalyzed
Received: May 26, 2018
Figure 1. Incorporation of phosphorus into pyrrolo[1,2-a]quinolone.
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01663
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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 dichlorophos-
phins.11 To our delight, in the presence of TMEDA, the bay
regions of pyrrolo[1,2-a]quinoline derivatives 1 were dilithi-
ated 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 Forster reso-
nance 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 2a-
cO(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)/DMAC-
DPS: 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.
B
DOI: 10.1021/acs.orglett.8b01663
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Letter
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
λ
λ
Stokes shift
tm
td
d
maxa
compd (nm)
em a
(nm)
b
c
log ε
(cm−1
)
Φf
(°C) (°C)
e
2aO
2aS
2bO
2cO
412
407
378
425
3.44 496
3.50 490
3.23 462
3.89 490
4110
4161
4810
3121
0.36
0.08
0.25
0.97
−
97
205
224
273
299
362
399
g
f
(512)
2cS
2dO
422
290
4.02 493
3.56 378
3412
8028
0.24
0.04
240
156
438
294
Table 2. EL Performance of Devices
a
Measured in CH2Cl2 (1 × 10−5 M). Emission maxima upon
doping rate
(wt %)
Von
(V)
luminancemax
(cd/m2)
CE
(cd/A)
PE
EQEmax
maxa
max b
c
b
excitation at the absorption maximum wavelengths. Measured
relative to quinine sulfate (H2SO4, 0.1 M), 15%. Melting point
measurements were performed by DSC. Decomposition temper-
(lm/W)
(%)
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
d
atures were defined as 5% of weight loss under argon, 10 °C/min.
e
f
Not observed. Values in parentheses were measured as powder.
Absolute fluorescence quantum yields determined by using a
a
b
c
g
Maximum current efficiency. Maximum power efficiency. Max-
imum external quantum efficiency.
calibrated integrating sphere system.
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
activated delayed fluorescence (TADF) molecule of bis[4-(9,9-
dimethyl-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 hole-
and electron-transport molecules, respectively. From the
energy-level schematic diagram (Figure 5a), balanced hole
was detected inside the cells when stimulated by laser λex
=
480/30 nm (Figure 6A, a) and λex = 360/40 nm (Figure 6B,
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
C
DOI: 10.1021/acs.orglett.8b01663
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
̋
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Szucs, R.; Lescop, C.; Reau, R.; Nyulaszi, L.; Bouit, P.-A.; Hissler, M.
Organometallics 2017, 36, 2502.
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
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
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Experimental section; fluorescence spectra of 2cO in
solution and as powder; cyclic voltammograms; electro-
chemical data; TGA and DSC graphs; electrolumines-
cent spectra of TADF-sensitized OLEDs; cell prolifer-
ation 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 data_request@ccdc.cam.ac.uk, or
by contacting The Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223
336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: duanzheng@zzu.edu.cn (Z.D.).
*E-mail: frmathey@yahoo.fr (F.M.).
*E-mail: iamrfchen@njupt.edu.cn (R.C.).
*E-mail: caojing73@126.com (J.C.).
ORCID
Runfeng Chen: 0000-0003-0222-0296
Zheng Duan: 0000-0002-0173-663X
Francois Mathey: 0000-0002-1637-5259
Notes
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Adv. 2017, 7, 9194.
(12) Cheeseman, G. W. H.; Greenberg, S. G. J. Organomet. Chem.
1979, 166, 139.
The authors declare no competing financial interest.
(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.
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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