Benzofuran-fused phosphole: Synthesis, electronic, and electroluminescence properties

Article abstract of DOI:10.1021/ol303260d

A synthetic route to novel benzofuran-fused phosphole derivatives 3-5 is described. These compounds showed optical and electrochemical properties that differ from their benzothiophene analog. Preliminary results show that 4 can be used as an emitter in OLEDs, illustrating the potential of these new compounds for opto-electronic applications.

Full text of DOI:10.1021/ol303260d

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Benzofuran-fused Phosphole: Synthesis,
Electronic, and Electroluminescence
Properties
Hui Chen,† Wylliam Delaunay,‡ Jing Li,† Zuoyong Wang,† Pierre-Antoine Bouit,‡
†
‡
Denis Tondelier,§ Bernard Geffroy, Franc-ois Mathey,†,^ Zheng Duan,* Regis Reau,*
ꢀ
ꢀ
and Muriel Hissler*‡
International Phosphorus Laboratory, College of Chemistry and Molecular Engineering,
Zhengzhou University, Zhengzhou 450001, P. R. China, Institut des Sciences Chimiques
ꢀ
de Rennes, CNRS - Universite de Rennes 1, Campus de Beaulieu, 35042 Rennes, France,
Laboratoire de Physique des Interfaces et Couches Minces, CNRS UMR 7647,
Ecole Polytechnique, 91128 Palaiseau, France, Laboratoire de Chimie des Surfaces et
Interfaces, CEA Saclay, IRAMIS, SPCSI, 91191 Gif-sur-Yvette, France, and Nanyang
Technological University, CBC-SPMS, 21 Nanyang Link, Singapore 637371, Singapore
*regis.reau@univ-rennes1.fr, muriel.hissler@univ-rennes1.fr, duanzheng@zzu.edu.cn
Received November 27, 2012
ABSTRACT
A synthetic route to novel benzofuran-fused phosphole derivatives 3ꢀ5 is described. These compounds showed optical and electrochemical
properties that differ from their benzothiophene analog. Preliminary results show that 4 can be used as an emitter in OLEDs, illustrating the
potential of these new compounds for opto-electronic applications.
π-Conjugated organic systems are highly versatile ma-
terials for the development of efficient electronic devices.1
In particular, a great variety of S-containing π-conjugated
derivatives based on the thiophene scaffold have proven to
be very efficient semiconducting materials for opto-
electronic devices (Organic Light Emitting Diodes (OLED),
Organic Solar Cells (OSC), and Field Effect Transistors
(OFET)).2 In the last years, other heteroles appeared as
† Zhengzhou University.
‡
ꢀ
CNRS - Universite de Rennes 1.
§ Ecole Polytechnique.
CEA Saclay.
(2) (a) Roncali, J. Chem. Rev. 1997, 97, 173. (b) Mishra, A.; Ma, C.-Q.;
^ Nanyang Technological University.
€
Bauerle, P. Chem. Rev. 2009, 109, 1141. For specific reviews on OSC,
€
ꢀ
(1) (a) Mullen, K., Scherf, U., Eds. Organic Light Emitting Devices:
OFET and OLEDs, see: (c) Thomson, B. C.; Frechet, J. M. J. Angew.
Synthesis Properties and Applications; Wiley-VCH: Weinheim, Germany,
2006. (b) Cheng, Y.-J.; Yang, S.-H.; Hsu, C.-S. Chem. Rev. 2009, 109, 5868.
(c) Usta, H.; Facchetti, A.; Marks, T. J. Acc. Chem. Res. 2011, 44, 501.
Chem., Int. Ed. 2008, 47, 58. (d) Allard, S.; Forster, M.; Souharce, B.;
Thiem, H.; Scherf, U. Angew. Chem., Int. Ed. 2008, 47, 4070. (e) Gather,
M. C.; Kohnen, A.; Meerholz, K. Adv. Mater. 2011, 23, 233.
€
r
10.1021/ol303260d
XXXX American Chemical Society

appealing building blocks for the tailoring of conjugated
materials since they possess electronic properties that
markedly differ from those of thiophene. For example,
the aromatic character of furan and phosphole is less
pronounced than in thiophene, resulting in an increased
exocyclic conjugation ability of their π-systems.3 However,
this less pronounced aromatic character also confers to
these heteroles a lower stability which can be circumvented
by performing appropriate structural modifications on
these heteroles. Stable furan-containing π-systems A and
R-oligofurans4 B showing promising results as an active
layer in organic/hybrid solar cells and OFETs have been
described very recently.5 Likewise, the use of phosphole
scaffolds for constructing organic materials AꢀC (Figure 1)
has started in the past decade.6 In these materials, exploiting
the reactivity and coordination chemistry of the reactive
P-centers allowed controlling both their HOMOꢀLUMO
gap and solid-state organization.7 Indeed, this unique mo-
lecular engineering afforded conjugated P-derivatives that
can be used as advanced materials in OLEDs, including
white-emitting devices or for the development of fibers, gels
and liquid crystals.8 Derivatives of type A or C associating
phosphole and thiophene moieties have been widely inves-
tigated.6 In contrast, mixed phospholeꢀfuran compounds
are quite rare. Only linear systems A are known to date, and
these compounds display a moderate emission property,
precluding their use in optoelectronic devices.9 Therefore, we
have been interested in P,O-based acenes associating phosp-
hole and furan rings since structure rigidification generally
improves the emission properties of π-conjugated organic
systems.
Figure 1. Phosphole-based π-conjugated systems A, B and C.
In this paper, we describe the synthesis, structure, and
electronic properties of the first mixed phosphole-furan
acene 2 (Scheme 1). Proper derivatization of this scaffold
using the reactivity of the σ3,λ3-P center afforded com-
pounds exhibiting good thermal stability and high emis-
sion quantum yields. These appealing properties have been
exploited for the fabrication of OLEDs.
To design stable mixed furanꢀphosphole π-systems, the
fused heteroacenic structure 2 (Scheme 1) was targeted since
the benzofuran moiety exhibits a good stability due to the
annelation of the two aromatic rings. The target compound
2 was synthesized by using a modified synthetic procedure
reported by Baumgartner et al. for the preparation of mixed
thiopheneꢀphosphole heteroacenes C (Figure 1).6b,7f The
iodo-substituted benzofuran 1 was subsequently treated
with 2 equivalents of n-BuLi and one equivalent of PhPCl2
at ꢀ78 ꢀC affording compound 2 (Scheme 1) (31P NMR:
ꢀ44.0 ppm). Since σ3,λ3-phospholes are generally too sensi-
tive toward oxygen to be used as material in OLEDs, this
compound was derivatized in good yields by in situ oxidation
into thiooxophosphole 3 (31P NMR: þ17.0 ppm) and
oxophosphole 4 (31P NMR: þ11.9 ppm) (Scheme 1). The
corresponding gold complex 5 (31P NMR: ꢀ8.0 ppm) was
also readily prepared by reacting 2 with AuCl(tetrahydro-
thiophene (tht)) (see Scheme 1). These novel air-stable
derivatives were purified by column chromatography and
fully characterized by multinuclear NMR spectroscopy and
high-resolution mass spectrometry (HR-MS).
(3) (a) Salzner, U.; Lagowski, J. B P.; Pickup, G.; Poirier, R. A.
Synth. Met. 1998, 96, 177. (b) Cyranski, M. K.; Krygowski, T. M.;
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Katritzky, A. R.; Von Rague Schleyer, P. J. Org. Chem. 2002, 67, 1333.
(4) (a) Gidron, O.; Diskin-Posner, Y.; Bendikov, M. J. Am. Chem.
Soc. 2010, 132, 2148. (b) Bunz, U. H. F. Angew. Chem., Int. Ed. 2010, 49,
5037.
(5) (a) Mitsui, C.; Tsuji, H.; Sato, Y.; Nakamura, E. Chem.ꢀAsian J.
2012, 7, 1443. (b) Mitsui, C.; Soeda, J.; Miwa, K.; Tsuji, H.; Takeya, J.;
Nakamura, E. J. Am. Chem. Soc. 2012, 134, 5448. (c) Walker, B.;
Tamayo, A. B.; Dang, X.-D.; Zalar, P.; Seo, J. H.; Garcia, A.; Tantiwiwat,
M.; Nguyen, T.-Q. Adv. Funct. Mater. 2009, 19, 3063. (d) Wu, C.-C.; Hung,
W.-Y.; Liu, T.-L.; Zhang, L.-Z.; Luh, T.-Y. J. Appl. Phys. 2003, 93, 5465. (e)
Aleveque, O.; Frere, P.; Leriche, P.; Breton, T.; Cravino, A.; Roncali, J.
J. Mater. Chem. 2009, 19, 3648. (f) Woo, C. H.; Beaujuge, P. M.; Holcombe,
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T. W.; Lee, O. P.; Frechet, J. M. J. J. Am. Chem. Soc. 2010, 132, 15547. (g)
Lin, J. T.; Chen, P.-C.; Yen, Y.-S.; Hsu, Y.-C.; Chou, H.-H.; Yeh, M.-C. P.
Org. Lett. 2008, 11, 97.
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(6) (a) Hissler, M.; Dyer, P. W.; Reau, R. Coord. Chem. Rev. 2003,
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244, 1. (b) Baumgartner, T.; Reau, R. Chem. Rev. 2006, 106, 4681.
(7) (a) Bouit, P.-A.; Escande, A.; Szucs, R.; Szieberth, D.; Lescop, C.;
ꢀ
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Nyulaszi, L.; Hissler, M.; Reau, R. J. Am. Chem. Soc. 2012, 134, 6524.
(b) Ren, Y.; Kan, W. H.; Henderson, M. A.; Bomben, P. G.; Berlinguette,
C. P.; Thangadurai, V.; Baumgartner, T. J. Am. Chem. Soc. 2011, 133,
17014. (c) Deschamps, E.; Ricard, L.; Mathey, F. Angew. Chem., Int. Ed.
Engl. 1994, 33, 1158. (d) Matano, Y.; Nakashima, M.; Imahori, H. Angew.
Chem., Int. Ed. 2009, 48, 4002. (e) Yavari, K.; Moussa, S.; Ben Hassine, B.;
Retailleau, P.; Voituriez, A.; Marinetti, A. Angew. Chem., Int. Ed. 2012, 51,
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6748. (f) Dienes, Y.; Eggenstein, M.; Karpati, T.; Sutherland, T. C.;
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Nyulaszi, L.; Baumgartner, T. Chem.;Eur. J. 2008, 14, 9878. (g) Fukaza-
wa, A.; Hara, M.; Okamoto, T.; Son, E.-C.; Xu, C.; Tamao, K.; Yama-
guchi, S. Org. Lett. 2008, 10, 913. (h) Fukazawa, A.; Yamada, H.;
Yamaguchi, S. Angew. Chem., Int. Ed. 2008, 47, 5582.
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K. Inorg. Chem. 2011, 50, 8516.
Scheme 1. Synthetic Routes to FuranꢀPhosphole
Derivatives 2ꢀ5
(8) (a) Chen, H.; Delaunay, W.; Yu, L.; Joly, D.; Wang, Z.; Li, J.;
Wang, Z.; Lescop, C.; Tondelier, D.; Geffroy, B.; Duan, Z.; Hissler, M.;
ꢀ
Mathey, F.; Reau, R. Angew. Chem., Int. Ed. 2012, 51, 214. (b) Fave, C.;
ꢀ
Cho, T.-Y.; Hissler, M.; Chen, C.-W.; Luh, T.-Y.; Wu, C.-C.; Reau, R.
J. Am. Chem. Soc. 2003, 125, 9254. (c) Joly, D.; Tondelier, D.; Deborde,
V.; Delaunay, W.; Thomas, A.; Bhanuprakash, K.; Geffroy, B.; Hissler,
ꢀ
M.; Reau, R. Adv. Funct. Mater. 2012, 22, 567. (d) Ren, Y.; Baumgartner,
T. Dalton Trans. 2012, 41, 7792.
(9) (a) Matano, Y.; Saito, A.; Fujita, M.; Imahori, H. Heteroatom
Chem. 2011, 22, 457. (b) Deschamps, E.; Ricard, L.; Mathey, F.
Heteroatom Chem. 1991, 2, 377. (c) Tran Huy, N. H.; Lu, Y.; Nien Ah
Qune, L. F.; Mathey, F. J. Organomet. Chem. 201210.1016/j.
jorganchem.2012.08.032.
B
Org. Lett., Vol. XX, No. XX, XXXX

These results show that the P-center of the mixed furan-
phosphole 2 retains the versatile reactivity of phosphole
derivatives. It is interesting to note that the 13C NMR
chemical shifts of the phosphole rings of furan derivatives
3ꢀ5 are markedly different from those of their thiophene
analogs of type C (Figure 1 and SI), suggesting that the
different electronic properties (π-donor ability and
electronegativity) of the O and S atoms strongly influence
the electronic distribution within the fused frameworks.7f
The structures of 3ꢀ5 were determined by X-ray diffrac-
tion. Since no solid-state structure of fused heteropentacene
gold-complex is known to date, the discussion will focus on
this compound. This study confirmed the proposed struc-
ture and shows that the conjugated system including the five
conjugated rings is fully planar (Figure 2a). As already
observed for other AuI-phosphole complexes, the PꢀAuꢀ
Cl fragment is not linear (PꢀAuꢀCl = 173ꢀ).8b The
phosphorus atom exhibits a distorted tetrahedral geometry
In contrast to what is observed with P,S-monomers C
(Figure 1), the thiooxo and -oxo-phospholes 3,4 do not
organize in the solid state via πꢀπ interactions (see SI).
This information is of prime importance for further in-
corporation of the compounds in OLEDs devices, since
intermolecular πꢀπ interactions induce strongly modified
solid state emission behavior.11
Optical properties of 3ꢀ5were studied in CH2Cl2 by means
of UVꢀvis absorption and fluorescence (Figures 3, S7 (SI)
and Table 1). The three compounds show broad absorption in
the visible range (λmax = 380 nm, see Figure S7, SI) attributed
to a πꢀπ* transition of the conjugated backbone confirmed
by density functional theory calculations (DFT) at the
B3LYP/6-31þG (d, p) level (Figure 3). The substitution
of the P atom only weakly modifies the optical transitions.
Compared to their thiophene analogs C, 3ꢀ5 display
slightly blue-shifted transition.7f This observation is also in
accordance with the measurements showing a reduced
HOMOꢀLUMO gap in the oligo-thiophene series
compared to its oligofuran analog.4 The effect of the P-
environment is more pronounced on the emission properties
of 3ꢀ5 (Figure 3). A gradual decrease of the maximal
emission wavelength is observed in the series 4-3-5, as
already observed on phosphole oligomers.12,8b P-substitu-
tion also affects the fluorescence quantum yield (Table 1).
Notably, 4 and 5 display high quantum yield (65ꢀ73%),
making these derivatives good candidates for use as emitters
in light-emitting device. Furthermore, the solid-state emis-
sion matches the emission in diluted solution as shown
Figure 4 in the case of compound 4.
˚
with usual PꢀC bond lengths (1.79 A). It is interesting to
note that the furan rings display a dissymmetric character.
The single CꢀO and double CdC bonds within the PCCO
˚
path are quasi-similar (1.36 ( 1 A) and notably shorter than
˚
their benzo-counterpart (1.40 A). This dissymmetry suggests
that the O-atom is effectively conjugated with the PꢀCdC
double bound substituted by an electron-withdrawing σ3,
λ4-P moiety (see Table S1, SI). Two features are worth noting.
First, this structural molecular feature is observed for all
compounds 3-5 (see Figures S4, S5 and Table S1, SI), having
similar backbone and withdrawing σ3,λ4-P moieties. Second,
this effect is not observed for the corresponding thiophene-
thiooxophosphole derivatives C (Figure 1) characterized by
X-ray diffraction studies,7f showing that the impact of repla-
cing S by O within this planar conjugated platform. Remark-
ably, complex 5 crystallizes as dimers that are closely
Table 1. Photophysical and Electrochemical Data
c
λabsa [nm] log ε λemb [nm] ΦF
Eox d [V] Ered d [V]
1
1
packed due to a combination of both πꢀπ interactions
10
˚
3
4
5
391
390
380
4.4
4.3
4.5
480
491
470
0.03
0.65
0.73
þ1.09
þ1.20
þ1.17e
ꢀ2.06e
ꢀ2.08e
ꢀ2.03e
(πꢀπ distances, 3.40 A) and aurophilic interaction (dAuꢀAu
=
2.99 A) (Figure 2b). Furthermore, these dimers are engaged
˚
˚
in intermolecular πꢀπ interactions (d = 3.55 A) resulting in
the formation of infinite π-stacked columns (Figure 2c).
a In CH2Cl2 (10ꢀ5 M). b In CH2Cl2 (10ꢀ5 M) with (λex = 370 nm).
c Measured relative to quinine sulfate (H2SO4, 0.1 M), φref = 0.546. d In
CH2Cl2 with Bu4NþPF6ꢀ (0.2 M) at a scan rate of 100 mVsꢀ1. Eox (Ered) =
1/2(Epc þ Epa) for reversible process, otherwise Eox = Epa. Potentials vs
ferrocene/ferrocenium. e Reversible process.
Redox properties of 3ꢀ5 were studied by means of cyclic
voltammetry (CH2Cl2, 0.2 M Bu4NPF6, v = 100 mVsꢀ1).
All compounds display amphoteric redox character with
oxidation and reduction waves at relatively low potentials
(Table 1). Typically, 3 displays an irreversible oxidation
wave (Eox = þ1.09 V vs Fe) and a reversible reduction
wave (Ered = ꢀ2.06 V vs Fe). P-substitution has a weak
impact on the redox properties, as illustrated by the data
presented in Table 1. Compared to its thiophene analog,
3ꢀ5 display lower oxidation and reduction potential, with
(10) No proof of formation of these AuꢀAu interactions in solution
was observed
(11) Note that the absence of intramolecular interaction might be
detrimental to the charge transport in the device.
(12) Hay, C.; Hissler, M.; Fischmeister, C.; Rault-Berthelot, J.;
Figure 2. (a) View of crystallographic structure of 5, (b) view of
the dimers and (c) view of the packing along the x axis.
ꢀ
Toupet, L.; Nyulaszi, L.; Reau, R. Chem.;Eur. J. 2001, 7, 4222.
Org. Lett., Vol. XX, No. XX, XXXX
C

Figure 4. Photoluminescence of 4 in solution (CH2Cl2, 10ꢀ5 M)
(blue) and in the solid state (red); electroluminescence of device
B (green) and device A (purple).
Figure 3. (a) HOMO of 4, (b) LUMO of 4, and (c) normalized
emission spectra in CH2Cl2 of 3 (red), 4 (blue) and 5 (green).
Table 2. Electroluminescent Performance of Devices A and B
a global increase of the electrochemical gap.7f These data
fit with the observed general trend that exchanging sulfur
for oxygen in conjugated five-membered oligomers results
in changes such as increasing the HOMOꢀLUMO gap
and lowering the oxidation potential.4
power
a
b
b
λEL Von
B30
EQE30
[%]
efficiency
[lm.W1ꢀ b
device [nm] [V] [cd.cmꢀ2
]
]
xb
yb
A
B
532 5.66
500 5.70
175
0.45
2.29
0.33
1.55
0.32 0.47
0.22 0.43
1248
Since compounds 3ꢀ5 are developed as emitters for
OLEDs, it was of interest to investigate their thermal
stability. The compounds 3 and 4 display decomposition
temperature above 250 ꢀC and a melting point at 220 ꢀC
indicating good thermal stabilities which is a critical issue
for device stability and lifetime. Note that compound 5 is
less thermally stable than its analogs 3 and 4 with decom-
position temperature of 240 ꢀC.
a Measured for B = 0.1 cd mꢀ2 b Measured at 30 mA cmꢀ2
.
not observed, indicating a quantitative energy transfer from
the DPVBi to the emitter 4. The external quantum efficiency
of the device is close to the one of the DPVBi reference. In
this case, the efficacity of the device is not limited by the
charge transport of 4, which explains the better properties of
device B compared to device A.
Taking in account the thermal stabilities and the physi-
cal properties of compounds 3ꢀ5, only compound 4 was
used as emitting material (EM, pure or doped in a DPVBi
matrix) in multilayered OLEDs having a classical structure
(indium tin oxide (ITO)/copper phthalocyanine (CuPc)
(10 nm)/N,N0-diphenyl-N,N0-bis(1-naphthylphenyl)-1,10-
biphenyl-4,40-diamine (R-NPB) (50 nm)/EM (15 nm)/
4,40-bis (2,20-diphenylvinyl) biphenyl (DPVBi) (35 nm)/
bathocuproine (BCP) (10 nm)/tris(8-hydroxyquinolinato-
)aluminum (Alq3) (10 nm)/LiF (1.2 nm) /Al (100 nm)
configuration, see Figure S9, SI). In a first device A, mixed
phosphole-furan 4 was used as pure emitter. In this case,
the emission wavelength (λEL = 532 nm) is red-shifted by
ca. 40 nm compared to the solid-state photoluminescence
of4 (Figure4). However, the increaseofthecurrent density
does not impact the emission wavelength of device A (see
Figures S10, S12, SI), proving that this emission is not due
to degradation of the material. In this device, luminance and
external quantum efficiency are moderate (Table 2). It is
believed that charge transport in 4 is low, causing this low
efficiency. Derivative 4 was then used as dopant in a DPVBi
matrix (doping rate 3.6% w, device B). In this case, the
electroluminescence wavelength which is not affected by the
increase of the current density, matches the emission of 4 in
the solid state (Figures 4, S13, SI). The emission of DPVBi is
In conclusion, three novel benzofuran-fused phosphole
heteroacenes 3ꢀ5 were synthesized and fully characterized.
These P,O-containing acenes display distinct properties com-
pared to their S-analogs. The high emission quantum yields
of 4 associated with good thermal stability allowed the use of
this novel scaffold in OLED emitting in the blue-green region.
Acknowledgment. This research is supported by the
ꢁ
ꢀ
Ministere de la Recherche et de l’Enseignement Superieur,
the University of Rennes 1, the CNRS (AIL FOM), IUF.
NSFC (No. 21072179) and Henan Science and Technology
Department (114300510007) of China. COST CM0802
(Phoscinet) is also acknowledged. The authors are grateful
ꢀ
to C. Lescop (CNRS - Universite de Rennes 1) for X-ray
diffraction studies and to J. Troles (CNRS - Universite de
ꢀ
Rennes 1) for the DSC measurements.
Supporting Information Available. Synthetic proce-
dure, complete characterizations, X-ray crystallographic
data and CIF files for 3ꢀ5, computational details, device
preparation. This material is available free of charge via
the Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX