Facile synthesis of biphenyl-fused BODIPY and its property

Article abstract of DOI:10.1021/ol2033916

A biphenyl-fused BODIPY was synthesized through a facile oxidative cyclization of peripheral aryl-substituents at the β-position of the BODIPY unit. The extended π-system of the fused BODIPY induces near-infrared (NIR) absorption and strong π - π interact

Full text of DOI:10.1021/ol2033916

ORGANIC
LETTERS
2012
Vol. 14, No. 3
866–869
Facile Synthesis of Biphenyl-Fused
BODIPY and Its Property
Yosuke Hayashi,† Naoki Obata,‡,§ Masatomo Tamaru,† Shigeru Yamaguchi,‡
Yutaka Matsuo,‡ Akinori Saeki,^ Shu Seki,^ Yuka Kureishi,z Shohei Saito,z
Shigehiro Yamaguchi,z and Hiroshi Shinokubo*,†
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University,
Nagoya 464-8603, Japan, Department of Chemistry, The University of Tokyo, Hongo,
Bunkyo-ku, Tokyo 113-0033, Japan, Department of Applied Chemistry,
Graduate School of Engineering, Osaka University, Yamada-oka, Suita, Osaka 565-0871,
Japan, Mitsubishi Chemical Group Science and Technology Research Center, Inc.,
Yokohama 227-8502, Japan, and Department of Chemistry, Graduate School of Science,
Nagoya University, Nagoya 464-8602, Japan
hshino@apchem.nagoya-u.ac.jp
Received December 19, 2011
ABSTRACT
A biphenyl-fused BODIPY was synthesized through a facile oxidative cyclization of peripheral aryl-substituents at the β-position of the BODIPY
unit. The extended π-system of the fused BODIPY induces near-infrared (NIR) absorption and strong πꢀπ interactions in the solid state. These
features are beneficial for the application of the dye as a functional material. The biphenyl-fused BODIPY dye was demonstrated to exhibit
photocurrent conversion ability on the basis of its n-type semiconducting property.
Boron dipyrrins (BODIPYs) are currently attracting
much interest in a wide variety of research areas such as
labeling reagents, fluorescent switches, chemosensors, non-
linear optical materials, and photovoltaics owing to their
advantageous photophysical properties such as photo-
stability, large extinction coefficients, and high lumines-
cence efficiency.1 The parent BODIPY dye (Scheme 1)
shows absorption and emission around 500 nm. The addi-
tion of near-infrared (NIR) absorbing/emitting property
to the BODIPY dye makes BODIPYs more useful materi-
als in applications to solar cells as well as biolabeling.
Scheme 1. General Structure and Numbering of meso-Substi-
tuted BODIPYs
† Graduate School of Engineering, Nagoya University
‡ The University of Tokyo
§ Mitsubishi Chemical Group Science and Technology Research Center,
Inc.
Hence, various methods to access NIR BODIPY dyes
have been developed, including extension of π-conjugation,
introduction of intramolecular charge transfer (ICT) char-
acter,2,3 and incorporation of a nitrogen atom in the
skeleton.4 To extend π-conjugation, introduction of
π-conjugated fragments such as ethynyl and vinyl groups
or fusion of BODIPYs with aryl groups are often employed.
^ Osaka University
z Graduate School of Science, Nagoya University
(1) (a) Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891. (b)
Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed. 2008, 47,
1184. (c) Loudet, A.; Burgess, K. In Handbook of Porphyrin Science;
Kadish, K. M., Smith, K. M., Guilard, K. R., Eds.; World Scientific:
Hackensack, NJ, 2010; Vol. 8, p 1. (d) Ziessel, R.; Ulrich, G.; Harriman
New J. Chem. 2007, 31, 496. (e) Buyukcakir, O.; Bozdemir, O. A.;
Kolemen, S.; Erbas, S.; Akkaya, E. U. Org. Lett. 2009, 11, 4644.
r
10.1021/ol2033916
Published on Web 01/24/2012
2012 American Chemical Society

In particular, aryl-fused BODIPY dyes possess high rigid-
ity in their structures. High rigidity of π-systems leads to a
high fluorescence quantum yield in solution as well as
strong intermolecular πꢀπ interactions in the solid state,
which are beneficial for applications as π-functional ma-
terials. The conventional method for synthesis of aryl-
fused BODIPY dyes is a condensationꢀoxidation seque-
nce starting with π-extended fused pyrroles such as furo-
pyrrole.3aꢀc In contrast, the oxidative fusion strategy of
BODIPYs with peripheral aryl moieties is considered to
be a straightforward alternative route. However, such
examples are limited to the recent work by Wu et al.,
who reported synthesis of perylene- and porphyrin-fused
BODIPYs through an oxidative fusion reaction withmeso-
substituents.3d,e Along this line, we decided to develop
a novel strategy for synthesis of aryl-fused BODIPYs
through an oxidative cyclization of β-aryl groups. Here,
we report the synthetic procedure as well as application of a
biphenyl-fused BODIPY dye as an n-type semiconducting
material.
Scheme 2 shows the synthetic route and condition for
a biphenyl-fused BODIPY. SuzukiꢀMiyaura coupling
of 2,6-dibromo BODIPY 25 with 2-biphenylboronic
acid afforded biphenyl BODIPY 3 in 82% yield. Ox-
idative cyclization of biphenyl BODIPY 3 by PIFAꢀ
Figure 1. (a) UVꢀvis absorption spectra of 1 (R = 2,4,6-
trimethylphenyl; black), 3 (red), and 4 (blue). (b) Fluorescence
spectra of 1 (black), 3 (red), and 4 (blue) measured in CH2Cl2. (c)
UVꢀvis absorption spectrum of thin film state of 4.
6
BF3 OEt2 at ꢀ78 ꢀC proceeded smoothly to furnish
3
biphenyl-fused BODIPY 4 as a sole product in 65%
yield.
Single crystal X-ray diffraction analysis revealed the
molecular structure of 4 (Figure 2). BODIPY 4 exhibited
π-interactions through peripheral fused-biphenyl moieties,
in which the interplanar distance is 3.48 A. Interestingly,
the BODIPY units adopt a 1-D infinite stack (Figure 2c),
which is known to be favorable for organic semiconductors.7
The intermolecular πꢀπ interactions in the solid state were
further confirmed by the UVꢀvis absorption spectrum in
Figure 1a,b shows the absorption and emission spectra
of fused BODIPY 4 in CH2Cl2, which exhibit substantial
red-shifts in comparison to biphenyl BODIPY 3 due
to a decrease in the HOMOꢀLUMO gap (vide infra).
BODIPY 4 showed an intense absorption band at 673 nm
with rather high absorption coefficients (ε(λmax) = 1.4 ꢁ
105 Mꢀ1 cmꢀ1) (Table 1). The fluorescence quantum yield
of 4 is high enough, despite the rather small HOMOꢀLU-
MO gap (Φ = 0.51). The lower fluorescence quantum
yield of 3 could be due to partial charge transfer character
of 3 (Supporting Information).
(2) (a) Deniz, E.; Lsbasar, G. C.; Bozdemir, O. A.; Yildirim, L. T.;
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Scheme 2. Synthesis of a Biphenyl-Fused BODIPYa
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R.; Carril, M.; Urtiaga, M. K.; Solans, X.; Tellitu, I.; Domınguez, E.
´
J. Org. Chem. 2005, 70, 3178.
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Chem. Soc. 2001, 123, 9482. (b) Lloyd, M. T.; Mayer, A. C.; Subramanian,
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a Ar = 2,4,6-trimethylphenyl, PIFA = [Bis(trifluoroacetoxy)iodo]-
benzene.
Org. Lett., Vol. 14, No. 3, 2012
867

the solid state. As shown in Figure 1c, the UVꢀvis absorp-
tion spectrum of compound 3 in thin film exhibited a
substantial red shift in comparison to that in solution
(Δλ = 34 nm). No emission was observed in the solid state.
time-resolved microwave conductivity (FP-TRMC) of the
polycrystalline film of 4.11 The TRMC measurement of
transient photoconductivity confirms that 4 has actually
good carrier mobility (a minimum mobility; μmin = 9 ꢁ
10ꢀ3 cmꢀ1 Vꢀ1 sꢀ1) determined by the maximum yield of
photogenerated charge carriers upon excitation at 355 nm
derived from transient photocurrent traces.
Table 1. Summary of Optical Properties of BODIPYs in
Solution
compound
λmax (nm)
ε (Mꢀ1 cmꢀ1
)
λ
em (nm)
Φf
1
2
3
4
501
538
573
673
6.3 ꢁ 104
5.8 ꢁ 104
4.8 ꢁ 104
1.4 ꢁ 105
514
577
616
692
0.92a
0.14a
0.58
0.51
a Values taken from ref 5.
Electrochemical properties of BODIPY 3 and 4 were
examined by cyclic voltammometry (Table 2). In the case
of 3, a quasi-reversible oxidation wave and reversible reduc-
tion wave were observed at 0.89 and ꢀ1.34 V, respectively.
In the case of 4, reversible oxidation and reduction waves
were observed at 0.80 and ꢀ1.05 V, respectively (vs Fc/Fcþ).
Remarkably, reduction of 4 occurred at rather positive
potential. Consequently, expansion of π-conjugation and
the electron-withdrawing boron atom significantly low-
ered the LUMO level (ꢀ3.75 eV) of 4,8 which is almost
equal to representative n-type semiconducting materials
such as perylene bisimides and fullerenes.
Figure 2. X-ray crystal structure of 4. (a) Top view of 4, (b) side
view of 4, and (c) packing structure of 4. The thermal ellipsoids
were scaled to the 50% probability level. meso-Aryl substituents
were omitted for clarity except (c).
Table 2. Summary of Electrochemical Properties of BODIPYsa
compound
E1/2red1/V
E1/2ox1/V
LUMO level/eVb
3
4
ꢀ1.34
ꢀ1.05
0.89
0.80
ꢀ3.46
ꢀ3.75
To clarify whether BODIPY 4 can be used as an n-type
semiconducting material for a device application, a pꢀn
heterojunction solar cell was fabricated.12,13 Tetrabenzo-
porphyrin (BP) was employed as an electron donor.14 The
current densityꢀvoltage (JꢀV) characteristics under
AM 1.5G simulated solar illumination at an intensity of
100 mW/cm2 and corresponding external quantum effi-
ciency (EQE) spectrum are shown in Figure 3. The device
actually showed a performance with an open-circuit vol-
tage (Voc) of 0.51 V, a shot-circuit current density (Jsc) of
a Measurements were performed in CH2Cl2 solution containing
TBAPF6 (0.1 M) as a supporting electrolyte with a scan rate of
100 mV/s. Platinum, platinum wire, and Ag/AgClO4 electrodes were
used as working, counter, and reference electrodes, respectively. b Values
from the vacuum level were estimated by the following equation: LUMO
level = ꢀ(4.8 þ E1/2red) eV
Because an application of a BODIPY skeleton as n-type
semiconducting materials has not been reported so far,
except for BODIPY-polymers,9,10the n-type semiconduct-
ing property was examined. To evaluate the intrinsic
charge-carrier mobility, we measured the flash-photolysis
(12) Applications of BODIPY monomers as a p-type OPV material
or a photosensitizer in an OPV device, see: (a) Rousseau, T.; Cravino, A.;
Ripaud, E.; Leriche, P.; Rihn, S.; De Nicola, A.; Ziessel, R.; Roncali, J.
Chem. Commun. 2010, 49, 5082. (b) Rousseau, T.; Cravino, A.; Bura, T.;
Ulrich, G.; Ziessel, R.; Roncali, J. Chem. Commun. 2009, 1673. (c) Kubo,
Y.; Watanabe, K.; Nishiyabu, R.; Hata, R.; Murakami, A.; Shoda, T.;
Ota, H. Org. Lett. 2011, 13, 4574.
(8) The LUMO level was estimated by using the following equation:
LUMO level = ꢀ(4.8 þ E1/2red) eV. Wong, W.-Y.; Wang, X.-Z.; He, Z.;
Djurii, A. B.; Yip, C.-T.; Cheung, K.-Y.; Wang, H.; Mak, C. S. K.;
Chan, W.-K. Nat. Mater. 2007, 6, 521.
(9) Popere, C. B.; Della Pella, A. M.; Thayumanavan, S. Macro-
molecules 2011, 44, 4767.
(10) Related compounds that show n-type semiconducting property,
see: (a) Ono, K.; Yamaguchi, H.; Taga, K.; Saito, K.; Nishida, J.-I.;
Yamashita, Y. Org. Lett. 2009, 11, 149. (b) Ono, K.; Hashizume, J.;
Yamaguchi, H.; Tomura, M.; Nishida, J.-I.; Yamashita, Y. Org. Lett.
2009, 11, 4326.
(11) (a) Acharya, A.; Seki, S.; Koizumi, Y.; Saeki, A.; Tagawa, S.
J. Phys. Chem. B 2005, 109, 20174. (b) Acharya, A.; Seki, S.; Saeki, A.;
Koizumi, Y.; Tagawa, S. Chem. Phys. Lett. 2005, 404, 356.
(13) For recent examples of nonfullerene n-type semiconduncting
materials for OPVs, see: (a) Sonar, P.; Ng, G. M.; Lin, T. T.; Dodaba-
lapur, A.; Chen., Z. K. J. Mater. Chem. 2010, 20, 3626. (b) Brunetti, F.;
Gong, X.; Tong, M.; Heeger, A.; Wudl, F. Angew. Chem., Int. Ed. 2010,
49, 532. (c) Woo, C. H.; Holcombe, T. W.; Unruh, D. A.; Sellinger, A.;
ꢀ
Frechet, J. M. J. Chem. Mater. 2010, 19, 1892.
(14) (a) Ito, S.; Murashima, T.; Uno, H.; Ono, N. Chem. Commun.
1998, 1661. (b) Aramaki, S.; Sakai, Y.; Ono, N. Appl. Phys. Lett. 2004,
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Nakamura, E. J. Am. Chem. Soc. 2009, 131, 16048.
868
Org. Lett., Vol. 14, No. 3, 2012

2.9 mA/cm2, and a fill factor (FF) of 0.35. The power
conversion efficiency (PCE) of 0.52% was obtained. This
means that biphenyl-fused BODIPY 4 can act as an n-type
semiconducting material as expected from the packing
structure as well as low LUMO level.
Figure 4. (a) HOMO and (b) LUMO of 4 calculated at the
B3LYP/6-31G(d) level.
The calculated tHOMO (54.7 meV) and tLUMO (68.8
meV) are substantially large in the 1-D infinite stack to
support high carrier mobility of BODIPY 4. On the basis of
these analyses, fused BODIPY 4 would also act as a hole-
transporting material.
In summary, we have developed a novel synthetic
procedure for a biphenyl-fused BODIPY through Su-
zukiꢀMiyaura couplingꢀoxidative cyclization se-
quence. The biphenyl-fused BODIPY has a low
LUMO level and exhibits strong πꢀπ interactions in
the solid state. These features allow the use of the
biphenyl-fused BODIPY as an n-type organic photo-
voltaic (OPV) material. Synthesis of aryl-fused BODI-
PYs toward further applications in materials science is
currently underway.
Figure 3. JꢀV curves for the device; glass/ITO/PEDOT:PSS/
BP/BODIPY 4/NBphen/Al. Photocurrent is measured under
AM1.5G illumination (100 mW/cm2, 1 sun). Inset: External
quantum efficiency (EQE) as function of wavelength of the OPV
device.
To understand the carrier mobility, frontier orbitals of
4 were calculated at the B3LYP/6-31G(d) level by the DFT
method (Figure 4). Both HOMO and LUMO of 4 are well
delocalized on the fused biphenyl moiety, offering effective
orbital interactions between the stacked π-systems. We
also evaluated transfer integrals of HOMO and LUMO
(tHOMO and tLUMO) on the basis of the crystal struc-
ture of 4 by is the Amsterdam Density Functional
(ADF) program package (PW91/DZP level of theory).15,16
Acknowledgment. This work at Nagoya University was
supported by Grant-in-Aids for Scientific Research
(No. 23108705 “pi-Space”) from MEXT, Japan and the
Global COE program in Chemistry of Nagoya University.
H.S. acknowledges Asahi Glass Foundation for financial
support.
Supporting Information Available. General procedures,
spectral data for compounds, and CIF file for the X-ray
analysis of 4. This material is available free of charge via
the Internet at http://pubs.acs.org.
(15) ADF2010.01; SCM, Theoretical Chemistry; Vrije Universiteit:
Amsterdam; http://www.scm.com.
(16) (a) Senthilkumar, K.; Grozema, F. C.; Bickelhaupt, F. M.;
Siebbeles, L. D. A. J. Chem. Phys. 2003, 119, 9809. (b) Prins, P.;
Senthilkumar, K.; Grozema, F. C.; Jonkheijm, P.; Schenning, A. P. H.
J.; Meijer, E. W.; Siebbeles, L. D. A. J. Phys. Chem. B 2005, 109, 18267.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 3, 2012
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