Thieno[3,2-b]thiophene fused BODIPYs: synthesis, near-infrared luminescence and photosensitive properties

Article abstract of DOI:10.1039/C9OB00030E

The fusion of π-sufficient heteroaryl moieties has proven to be an effective strategy for achieving the red shift of the main spectral bands of BODIPY. In this paper, thieno[3,2-b]thiophene-fused BODIPY derivatives 1 and 2 have been designed and characterized by various spectroscopic methods, and their photosensitive properties have also been explored. Both dyes absorb in the near-infrared region with extremely high molar extinction coefficients, due to the extension of π-conjugation by fusion of the thieno[3,2-b]thiophene moiety. Their fluorescence quantum yields and singlet oxygen generation properties are significantly affected by iodine substitutions; dye 2 displays a moderate singlet oxygen generation value of 0.32, which makes it a potential NIR photosensitizer for photodynamic therapy of cancer in future research.

Full text of DOI:10.1039/C9OB00030E

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Received 00th January 20xx,
Thieno[3,2-b]thiophene fused BODIPYs: synthesis, near-infrared
luminescence and photosensitive properties
Yijuan Sun, Zhirong Qu, Zhikuan Zhou, Lizhi Gai* and Hua Lu*
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The fusion of π-sufficient heteroaryl moieties has proven to be an effective strategy for achieving red shift of the main
spectral bands of BODIPY. In this paper, thieno[3,2-b]thiophene-fused BODIPY derivatives 1 and 2 have been designed and
characterized with various spectroscopic methods, and their photosensitive properties have also been explored. Both
dyes absorb in the near-infrared region with extremely high molar extinction coefficients, due to extention of π-
conjugation by fusion of thieno[3,2-b]thiophene moiety. Their fluorescent quantum yields and singlet oxygen generation
properties are significantly affected by iodine substitutions, dye 2 displays moderate singlet oxygen generation value of
0.32, providing it as a potential NIR photosensitizer for photodynamic therapy of cancer in future research.
moieties result in significant red-shift of 80 and 63 nm
compared to the model BODIPY 1a, respectively, while the
emission band maxima lie at 583 and 571 nm. These dyes
have very large molar absorption coefficients rivaling those of
cyanine dyes. Although meso-position substituent often have
only a minor influence on the absorption and emission maxima
of BODIPY dyes, it is noteworthy that the introduction of a
strong electron-withdrawing CF3 group results in a red-shift of
ca. 50 nm without obvious decreasing of molar absorption
coefficients.
Introduction
Among many important types of chromophores, BODIPY (4,4-
difluoro-4-bora-3a,4a-diaza-s-indacene) derivatives have
attracted tremendous attention as fluorescence dye due to
their unique and famous spectroscopic properties1 such as: (i)
excellent photochemical stabilities, (ii) narrow absorption and
emission bands, (iii) large molar absorption coefficients and
high fluorescence quantum yields, and (iv) their chemical
versatility. BODIPY dyes have significant utility in a variety of
applications, such as labeling reagents,2 organic photovoltaics
(OPVs),3 organic light-emitting diodes (OLEDs),4 photodynamic
therapy,5 etc. However, since their absorption/emission
wavelengths typically range between 470 and 530 nm,1 it
restricts some applications such as biological imaging and
photosensitizing which generally require dyes that absorb and
emit at longer wavelengths in the far-red or NIR region. To
solve this research problem, various structural modifications
on the skeleton have been reported for BODIPYs1,6: 1)
Replacement of the meso carbon atom with a nitrogen atom
to form aza-BODIPY dyes; 2) Rigidification of rotatable
moieties; 3) Extension of π conjugation by introduction of aryl,
vinyl, styryl and arylethynyl substituents; 4) The extension of
the π conjugation by the fusion of aryl building blocks to the
pyrrole unit. The fusion of π-sufficient heteroaryl moieties on
the BODIPY skeleton can be regarded as one of the most
effective strategies for the rational design of red/NIR region
BODIPYs.7 For example, the fusion of furan and thiophene
Scheme 1 Chemical structures and photophysical data of BODIPYs, heterocycle
ring fused BODIPYs and the rationally designed BODIPY in this paper.
With this in mind, thieno[3,2-b]thiophene, which have strong
electron-donating ability and efficient extension of the π
system, is employed to fuse the BODIPY core.8 Herein we
report the synthesis, photophysical properties, and the singlet
oxygen generation properties of thieno[3,2-b]thiophene-fused
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BODIPY derivatives with strong absorption in the NIR region. Single crystals of
Journal Name
1
were obtained for X-ray structure analysis
View Article Online
DOI: 10.1039/C9OB00030E
The electron-withdrawing CF3 group was introduced at meso by the slow diffusion of hexane into CH2Cl2 solutions. X-ray
position for increment of deep red shift.7g,h BODIPY
2 bearing diffraction analysis of 1 confirmed incorporation of thieno[3,2-
two iodines were prepared to evaluate the heavy-atom effect b]thiophene groups into the [b]-fused positions of pyrrole rings
on the improvement of singlet oxygen generation capability of BODIPY skeleton(Fig. 1). The dihedral angle between the
through the enhanced intersystem crossing (ISC) efficiency.
thieno[2',3':4,5]thieno[3,2-b]pyrrole moieties is 17.89° (Fig. 1).
In the packing structure, the average intermolecular distance
between neighbouring thieno[3,2-b]thiophene planes is
approximately 3.56 Å, molecular packing adopt coplanar
inclined arrangements of their transition dipoles moments
with slip angles of 35.0° (Fig. 1d), which are characteristic of J-
type solid state packing as previously reported.7h
Results and discussion
Spectroscopic properties
Scheme 2. Synthetic route of thieno[3,2-b]thiophene-fused BODIPY dyes 1 and 2. (i)
NaOEt, Ethyl azidoacetate, EtOH, room temperature, 6 h; (ii) toluene, reflux, overnight;
(iii) NaOH, ethanol, 1 h; (iv) trifluoroacetic acid, trifluoroacetic anhydride, reflux, 1 h,
BF3OEt2, trimethylamine; DCM, room temperature, 1 h; (v) NIS, DCM.
Synthesis and characterization
The synthetic protocols of thieno[3,2-b]thiophene-fused
BODIPY was depicted in Scheme 2. Thieno[3,2-b]thiophene-2-
carbaldehyde
published
6 was synthesized in 86% yield according to a
Fig. 2 The absorption and emission spectra of
λex=640nm for , λex=650 nm for
1 and 2 in DCM solution.
procedure.9
7H-thieno[2',3':4,5]thieno[3,2-
5, was obtained following the
1
2.
b]pyrrole-6-carboxylic acid
Table 1. Spectroscopic and photophysical properties of 1-2 in various solvents at
298K
established Hemetsberger–Knittel10 synthesis between
6
and
ethylazidoacetate, and then upon cyclization gave ethyl 7H-
λabs
[nm]
εmax
λem
△
vabs-em
τ
Kr
Knr
Solvent
ΦF
[nm] [cm-1]
[ns] [108s-1] [108s-1]
thieno[2',3':4,5]thieno[3,2-b]pyrrole-6-carboxylate
subjected to hydrolysis reaction to give corresponding
BODIPY
4
.
4
3
was
. The
Hexane 690 224600 693
Toluene 699 216300 704
63
0.21
0.19
0.21
0.13
0.09
0.09
0.03
0.07
0.07
0.04
0.03
0.07
4.49
4.55
4.33
3.08
2.08
1.85
3.89
2.40
1.90
1.05
0.54
0.58
0.47
0.42
0.48
0.42
0.43
0.49
0.08
0.29
0.37
0.38
1.76
1.78
1.82
2.82
4.38
4.92
2.49
3.88
4.89
9.14
1
was then synthesized through boron difluoride
101
123
103
187
208
39
chelation of the meso CF3 substituted dipyrromethene unit,
which was obtained through a TFA-mediated condensation of
DCM
THF
695 204500 701
694 194700 699
1
2
3
with TFA anhydride, under basic conditions in CH2Cl2 at room
temperature. was obtained in 60% yield via an iodination
reaction of with N-Iodosuccinimide (NIS).
MeOH 689 175800 698
MeCN 688 174100 698
Hexane 714 233300 716
Toluene 726 250400 732
2
1
113
153
57
DCM
THF
720 234900 728
721 222400 724
MeOH 714 175800 721
MeCN 713 213700 721
136
156
0.56 17.96
1.21 16.03
The spectroscopic properties of thieno[3,2-b]thiophene fused
BODIPYs investigated in solvents of different polarity are
outlined in Figure 2, ESI. S1, S2 and Table 1.
In
dichloromethane (DCM), and have intensive absorption
1
2
Fig.1 (a) Top views of the molecular structure of
at 50% probability. (b) Packing diagram of . H atoms are omitted for clarity. (c)
Side views of the molecular structure. (d) Crystal-packing pattern of BODIPY
between the adjacent interlayered crystals. Slip angle of 35.0° for coplanar
inclined arrangements of its transition dipole.
1, with the thermal ellipsoids set
with large absorption coefficients, the characteristic cyanine-
type band of BODIPY dyes is observed at 695 and 720 nm,
which can be readily assigned to the S0–S1 transition.11 Few
broader, much weaker band around 300-500 nm can be
attributed to the second or third transition (Fig.S2 in ESI). The
1
1
X-Ray crystal structure analysis
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absorption maxima of thieno[3,2-b]thiophene fused BODIPYs
are red-shifted by 142 and 82 nm, respectively, relative to
those observed for the BDP-1b and furan-fused BDP-2b, which
suggests that the fusion of thieno[3,2-b]thiophene is a
promising way to achieve NIR dyes. Compared to thiophene-
fused BDP-3b, one more thiophene-fused leads to red-shift of
72 nm and almost three-fold increasing of molar extinction
coefficient (εmax: 72200 M-1 cm-1 for BDP-3b; 194700 M-1 cm-1
ARTICLE
1
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DOI: 10.1039/C9OB00030E
for
1 in THF), which is suitable for imaging cell and
photosensitizer. The introduction of iodine results in a red-
shift of 25 nm and 27 nm in the absorption and emission band
with lower quantum yield, due to the heavy atom effect and
the increased internal conversion. The fluorescent spectra are
typical mirror-image relationship of the lowest-energy
absorption spectra. The absorptive maxima slightly depend on
the polarity of the solvent, with a blue shift of ca. 11 nm when
the solvent polarity changes on moving from the relatively
non-polar toluene to more highly polar acetonitrile. A similar
trend is observed in the emissive maxima. These features
suggest that emission occurs from the weakly polar, relaxed
Franck–Condon excited state. The very small Stokes shift is
observed and generally increases on going from a hexane to an
acetonitrile, due to little different of permanent dipole
moments between the ground state and the excited state. The
fluorescence decay profiles of dyes could be described by a
single-exponential fit with fluorescence lifetime in the range of
nanosecond in all of the solvents investigated, similar to the
lifetime data of reported BODIPY systems.7h The fluorescence
Fig.3 Selected transition energies and wave functions of BDP-1b
calculated using the DFT method with CAM-B3LYP functional and 6-31G(d, p)
basis set (LanL2DZ basis for I atom)
, BDP-3b, 1 and
2
Table 2 Selected transition energies and wave functions of BDP-1b
and calculated using the TDDFT method with CAM-B3LYP functional and 6-
31G(d, p) basis set (LanL2DZ basis for I atom)
, BDP-3b, 1
2
Energy
[eV]
λ
Statea
fb
Orbitals (coefficient)c
HOMO LUMO (98%)
LUMO (100%)
[nm]
BDP-
1b
BDP-
3b
S1
2.81
441
0.52
→
S1
S1
S1
2.55
2.35
2.28
487
528
545
0.85
0.81
0.91
HOMO
HOMO
HOMO
→
1
2
→
LUMO (96%)
LUMO (96%)
→
a Excited state. b Oscillator strength. c MOs involved in the transitions.
quantum yields of
1 had a positive solvatokinetic behavior
from 0.21 in hexane to 0.09 in MeCN, this trends is reflected
by the rates of nonradiative decay. The fluorescence quantum
In order to gain a deeper understanding of the effect of ring
fusion on electronic properties, Nuclear Independent Chemical
Shift (NICS (0))14,15 values were calculated using the DFT-GIAO
method with the 6-31g(d, p) for the optimized structures of
yields of
2 are considerably lower than those of 1 in different
solvent, due to the heavy-atom effect of iodine atoms
enhancing the rate of intersystem crossing of the dyes.
classic BODIPY and
pyrrole rings are -6.50 and -6.10 for meso-CF3-BODIPY and
1 (Fig. S5 in ESI). The NICS(0) values of
Singlet Oxygen generation
1,
In order to estimate the ability of generation of singlet oxygen
respectively, suggesting the fusion of dithieno[3,2-b]thiophene
unit have less influence on the aromatic characteristics of
pyrrole rings. The NICS value of thiophene of thieno[3,2-
b]thiophene rings (0) is -5.63 and -10.20, suggesting the
aromatic characteristics of the electronic delocalization in both
the thiophene rings significantly differs.16 The ground-state
geometry optimizations were directly taken from the single
crystal data and performed by DFT with CAM-B3LYP functional
and 6-31G(d, p) basis set (LanL2DZ basis for I atom) ( Fig. 3 and
Table 2, Table S1-S4 in ESI ).15 UV/Vis spectra were simulated
by utilizing the first 20 excited states by TD-DFT with the same
approach. For 1-2, the calculated absorption spectra in the gas
phase are blue-shifted compared with experimental values,
similar to reported BODIPY systems, and their sequence agrees
rather well with the measured absorption spectra in solution.17
It is clear that the narrowing of the HOMO–LUMO gap is due
primarily to a destabilization of the HOMO caused by fusion of
thieno[3,2-b]thiophene moiety, resulting in a red-shift of
absorption and emission bands.
BODIPY
1 and 2, 1,3-diphenylisobenzofuran (DPBF), a well-
known singlet oxygen indicator was chosen as a standard.12
The mixture of BODIPY sensitizer
1 or 2 and DPBF was
irradiated with a 690 nm laser beam (100 mW cm−2, Fig S3).
The slope of the graph obtained by plotting the changes in
optical density against time is used to calculate the quantum
yield of singlet oxygen. Compared to the value that of
di-iodio substituted gave a better production of singlet
oxygen with the value of 0.32. These results were validated by
1 (0.10),
2
1
monitoring the phosphorescence spectrum of O2 at ca.1270
nm (Fig. S3 and S4 in ESI).13
DFT Calculation
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The lowest energy is mostly consisting of the HOMO−LUMO yield). 1H NMR (400 MHz, CDCl3) δ 7.40 (d, J = 8 Hz, 1H), 7.28(s, 1H),
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DOI: 10.1039/C9OB00030E
transition, HOMO and LUMO are all well distributed over the 7.20(s, 1H), 4.50 (dd, J = 8 Hz, 2 H), 1.45 (t, 3H).
entire conjugated π-system. The TDDFT calculations predict
Synthesis of 7H-thieno[2', 3':4, 5]thieno[3,2-b]pyrrole-6-carboxylic
acid (3)
that the lowest energy band of
1 and 2 are red-shift of 87 and
104 nm compared with BDP-1b, which is in good agreement
Compound 4 (1.38 g, 5.5 mmol) was dissolved in EtOH (50 mL),
NaOH (3.1 g, 77.5 mmol) in water (25 mL) was added, and the
mixture was refluxed for 1 h. The reaction was then cooled to room
temperature and chilled in an ice bath to acidify the mixture with
concentrated HCl. The precipitate was filtered, washed with water,
and dried under vacuum to afford 3 as grey solid (1.2 g, 98% yield).
1H NMR (400 MHz, DMSO-d6) δ 12.61 (s, 1H), 12.05 (s, 1H), 7.17 (d, J
= 4 Hz, 1H), 6.99 (s, 1H).
with the experimental data.
Conclusions
In conclusion, thieno[3,2-b]thiophene-fused BODIPY featuring
an intensive absorption in the NIR region have been
synthesized and fully characterized. Thieno[3,2-b]thiophene
fused on the BODIPY core greatly redshifted the absorption
Synthesis of 1
and fluorescence band. Diiodio-substituted BODIPY
2 was
found to exhibit efficient singlet oxygen generation with a
quantum yield of 0.32 upon irradiation with 690 nm laser.
Efforts to improve the water solubility and targeting of this
type BODIPY dyes for cancer cells are currently underway.
Compound 3 (0.8 g, 3.6 mmol) was dissolved in trifluoroacetic acid
(TFA) (50 mL) and heated to 40 oC for 10 min. Subsequently,
trifluoroacetic anhydride (17 mL) was added and the temperature
was then raised to 80oC with continued stirring for 4 h. The
resulting deep blue reaction solution was then allowed to cool to
room temperature and poured into an aqueous NaHCO3 solution
containing crushed ice. The resultant precipitate was then filtered
and dried in vacuo. The dry solid was then dissolved in CH2Cl2 (250
mL) and stirred for 5 min at room temperature under a nitrogen
atmosphere. Next, boron trifluoride dietherate (7 mL) and
triethylamine (5 mL) were added and the reaction mixture was
stirred at room temperature for 1 h. Following evaporation of the
solvent, the crude product was purified by flash chromatography on
Experimental
Synthesis
Synthesis of thieno[3,2-b]thiophene-2-carbaldehyde (6)
N, N-dimethylformide (16 mL, 210 mmol) was added phosphorus
o
chloride oxide (2.2 mL, 23.6 mmol) under argon at 0 C. And then it
was added to a solution of thieno[3, 2-b]thiophene (2.24 g, 16
o
1
mmol) in 1,2-dichloroethane (100 mL) and heated for 4 h at 70 C.
silica gel (CH2Cl2) to afford 1 as blue solid (300 mg, 17% yield). H
Subsequently, it was neutralized with Na2CO3, the crude product
was extracted with dichloromethane, washed with brine and water,
and dried over anhydrous sodium sulfate. After removing solvent
under reduced pressure, the residue was purified by column
chromatography to yield yellowish powders 6 (1.86 g, 86% yield).
1H NMR (400 MHz, CDCl3) δ 9.98 (s, 1H), 7.95(s, 1H) 7.70 (d, J = 4 Hz,
1H), 7.33 (d, J = 4 Hz, 1H).
NMR (400 MHz, THF-d8) δ 8.06 (d, J = 4 Hz, 2H), 7.49 (s, 2H), 7.38 (d,
J = 4 Hz, 2H). 11B NMR (128 MHz, CDCl3) δ 0.56 (t, J = 24.3 Hz, BF2)
ppm. 19F NMR (376 MHz, CDCl3) δ -51.46 (t, CF3), -160.61 (q, BF2).
UV/vis (CH2Cl2). λ max (ε) = 695 nm (204500 dm3 mol-1 cm-1); HRMS-
ESI: m/z: calcd [C18H6BF5N2S4]+ m/z =483.9427, found m/z=
483.9422.
Synthesis of 2
Synthesis
of
ethyl
(Z)-2-azido-3-(thieno[3,2-b]thiophen-2-
NIS (225 mg, 1 mmol) was added to the dichloromethane solution
of 1（60 mg, 0.124 mmol）and the mixture was then stirred at
room temperature overnight, the residue was chromatographed on
yl)acrylate (5)
Thieno[3,2-b]thiophene-2-carbaldehyde 6 (1.1 g, 6.63 mmol) and
ethyl 2-azidoacetate (1.72 g, 13.3 mmol) were dissolved in 60 mL of
ethanol, and the mixture was dropwise added to a solution of
sodium ethoxide in ethanol at 0oC and stirred for 2h, then room
temperature for 9 h. After the reaction was completed, added 150
mL of saturated NH4Cl, extracted with dichloromethane, dried over
anhydrous sodium sulphate. After removing solvent under reduced
pressure, the residue was purified by column chromatography to
1
silica gel to give 2 as green solid (45 mg, 50% yield). H NMR (400
MHz, DMSO-d6) δ 7.88 (s, 2H), 7.67(s, 2H). 11B NMR (128 MHz, CDCl3)
δ 0.44 (t, J = 23.8 Hz, BF2) ppm. 19F NMR (376 MHz, CDCl3) δ -51.46
(t, CF3), -160.62 (q, BF2). UV/vis (CH2Cl2), λ max (ε) = 720 nm
(234900 dm3 mol-1 cm-1); HRMS-ESI: m/z: calcd [C18H4BF5I2N2S4]+
m/z =735.7358, found m/z= 735.7387.
Spectroscopic Measurements
1
yield colorless solid 5 (370 mg, 20%). H NMR (400 MHz, CDCl3) δ
UV-Vis spectra were recorded on
a
Shimadzu UV-3000
7.50 (t, 2 H), 7.27(s, 1 H), 7.17 (s, 1 H), 4.40 (dd, J = 8 Hz, 2 H), 1.42
spectrophotometer. Fluorescence spectra were measured on a
Hitachi F-2700 FL spectrophotometer with a 150 W xenon arc lamp
as light source. Samples for absorption and emission measurements
(m, 3 H).
Synthesis of ethyl 7H-thieno[2',3':4,5]thieno[3,2-b]pyrrole-6-
carboxylate (4)
were contained in
1 cm × 1 cm quartz cuvettes. For all
Compound 5 (480 mg, 1.72 mmol) was dissolved in 10 mL of p-
xylene, heated to reflux for 1 h. The solvent was removed under
reduced pressure, the residue was purified by column
chromatography using silica gel and dichloromethane/petroleum
ether (1/1; v/v) as the eluent to obtain a white solid 4 (320 mg, 74%
measurement, the temperature was kept constant at (298 ± 2) K.
Dilute solution with an absorbance of less than 0.05 at the excited
wavelength was used for the measurement of fluorescent quantum
yields, The luminescence quantum yields in solution were measured
by using fluorescein with excited wavelength ZnPc (Φ=0.28, in DMF)
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as a reference.18 The quantum yield Ф as a function solvent polarity
Acknowledgements
View Article Online
DOI: 10.1039/C9OB00030E
is calculated using the following equation.19
Financial support was provided by the National Natural Science
Foundation of China (Nos. 21801057, 21871072, 21501073,
21471042); Collaborative Innovation Center for the
Manufacture of Fluorine and Silicone Fine Chemicals and
Materials (FSi2018A027); Organosilicon chemistry innovation
team and research funding project of Hangzhou Normal
University (2018QDL002). Theoretical calculations were carried
out at the Computational Center for Molecular Design of
Organosilicon Compounds, Hangzhou Normal University.
2
Фsample=Фstd
[
][ sample][
]
(1)
Where subscript sample and std denote the sample and standard,
respectively, Φ is quantum yield, I is the integrated emission
intensity, A stands for the absorbance, n is refractive index.
The fluorescence lifetimes of the samples were determined with a
Horiba JobinYvon Fluorolog- 3 spectrofluorimeter. Absorption and
emission measurements were carried out in 1×1 cm quartz cuvettes.
The goodness of the fit of the single decays as judged by reduced
chi-squared (χ2 ) and autocorrelation function C(j) of the residuals
R
was below χ2 <1.2.
R
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When the fluorescence decays were monoexponential, the rate
constants of radiative (kr) and nonradiative (knr) deactivation were
calculated from the measured fluorescence quantum yield (ΦF) and
fluorescence lifetime (τ) according to eqs 2 and 3:
1
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(2)
(3)
knr = (1-ΦF)/τ
2
3
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Computational details
The ground state structures of compounds 1 and 2 are optimized
using the density functional theory (DFT) method with CAM-B3LYP
functional and 6-31G(d, p) basis set (LanL2DZ basis for I atom).
LanL2DZ basis set was assigned to the elements of I atom, which
guarantees a reasonable balance of the computational cost and the
reliability of the results. The absorption properties were predicted
by time-dependent (TD-DFT) method with the same basis set. All of
the calculations were performed with the Gaussian09 program
package.15
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X-ray structure determination
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,
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Compound 1: C18H6B1F5N2S4; A brown block-like crystal of the
approximate dimensions was measured. Monoclinic, space group P
-1, a = 8.209(2) Å, b = 9.719(3) Å, c = 12.587(5) Å, α = 111.424(6), β
= 104.496(7), γ = 96.556(4), V = 881.2(5) Å3, Z = 2, F(000) = 484, ρ =
1.825 Mg m–3, R1 = 0.0443, wR2 = 0.1154, GOF = 1.095. CCDC No.
1875790 for 1 containing the supplementary crystallographic data
for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ,
UK; fax: (+44)1223–336–033; E-mail: deposit@ccdc.cam.ac.uk).
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Conflicts of interest
There are no conflicts to declare.
Choi, J. Bouffard and Y. Kim, Chem. Sci., 2014, 5, 751; (i) J.
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Jiao, New J. Chem., 2016, 40, 5966.
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