Impacts of dibenzo- and dithieno-fused structures at the b, g bonds in the bodipy skeleton

Article abstract of DOI:10.1246/cl.130360

Dibenzo[b,g]-fused and dithieno[b,g]-fused BODIPY derivatives were synthesized, and the electronic effects of the introducing dibenzo- and dithieno-fused structures at both of the b and g bonds in the BODIPY skeleton were investigated. Electrochemical and

Full text of DOI:10.1246/cl.130360

doi:10.1246/cl.130360
986
Published on the web June 1, 2013
Impacts of Dibenzo- and Dithieno-fused Structures at the b, g Bonds in the BODIPY Skeleton
Hiroyuki Shimogawa,1 Haruki Mori,1 Atsushi Wakamiya,*1,2 and Yasujiro Murata*1
1Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011
2PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012
(Received April 18, 2013; CL-130360; E-mail: wakamiya@scl.kyoto-u.ac.jp, yasujiro@scl.kyoto-u.ac.jp)
CF3
Dibenzo[b,g]-fused and dithieno[b,g]-fused BODIPY deriv-
Hexyl
F3C
CF3
F3C
CF3
atives were synthesized, and the electronic effects of the
introducing dibenzo- and dithieno-fused structures at both of
the b and g bonds in the BODIPY skeleton were investigated.
Electrochemical and photophysical measurements demonstrated
that these structural modifications effectively enhance the
electron-accepting ability of the BODIPY skeleton leading to
the red-shifted absorption.
a
b
S
S
N
N
N
N
N
N
g
B
B
B
Mes
F
F
F F
F
F
1
2
3
Figure 1. Ring-fused BODIPY derivatives 1, 2, and 3.
BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) skel-
eton has unique electrochemical and optical properties, such
as intense absorption profiles, high photostability, and high
electron-accepting ability.1 Various structural modifications on
this skeleton have been extensively studied to demonstrate their
significant utilities in a variety of applications, such as dye-
sensitized solar cells (DSSCs),2 organic photovoltaics (OPVs),3
organic light-emitting diodes (OLEDs),4 photodynamic therapy,5
etc. A simple structural modification to expand the ³-conjuga-
tion would be the introduction of a ring-fused structure to the
BODIPY skeleton. Although several derivatives having the ring-
fused structures at the a bond have been reported,3b,6 the b bond-
fused derivatives are still limited to a few examples,3c,7,8 such as
the difurano[b,g]-fused7b,7c and bisbiphenyl[b,g]-fused deriva-
tives.3c We recently reported monobenzo[b]-fused BODIPY 1
and demonstrated that the benzene-fusing at the a bond in the
BODIPY skeleton increases the HOMO level, whereas the
benzene-fusing at the b bond effectively decreases the LUMO
level (Figure 1).8 The latter is important for molecular design
toward NIR dyes or electron-transporting materials with air
stability.9 In the course of our study to develop fully ³-expanded
systems by aromatic ring-fusing at both of the b and g bonds of
BODIPY skeleton, we have been interested in dibenzo[b,g]-
and dithieno[b,g]-fused BODIPYs. In this paper, we report the
synthesis and properties of dibenzo[b,g]- and dithieno[b,g]-fused
BODIPY derivatives 2 and 3, in which the 3,5-bis(trifluoro-
methyl)phenyl group was introduced at 8-position for increment
of solubility and the electron-accepting ability (Figure 1). We
discuss the effects of aromatic ring-fusing at both the b and g
bonds in the BODIPY skeleton.
(a)
1) RCHO, TFA
2) DDQ
3) i-Pr2EtN
BF3•OEt2
R
S
S
S
N
N
B
(R = 3,5-(CF3)2C6H3)
N
H
CH2Cl2
rt.
F
F
3
6%
(b)
1) MeMgI
reflux, 1 h
2) RCOCl
rt., 12 h
Na
liq. NH3
R
O
N
N
N
H
MeOH
–78 °C, 30 min.
THF
H
H
4
5
79%
66%
R
R
1) BF3•OEt2
2) i-Pr2EtN
DDQ
4
+ 5
N
N
N
N
CH2Cl2
rt., 1 h
CH2Cl2
rt., 5 h
B
B
F
F
F F
2
58%
6
42%
(R = 3,5-(CF3)2C6H3)
Scheme 1. Synthetic route for BODIPYs 2 and 3.
Subsequently, addition of BF3¢OEt2 as a Lewis acid into the
mixture of 4 and 5, followed by treatment with i-Pr2EtN directly
produced tetrahydrodibenzo[b,g]-fused BODIPY 6 in 42% yield
as dark purple solids. Finally, oxidation of 6 with DDQ afforded
dibenzo[b,g]-fused BODIPY 2 in 58% yield as dark blue solids
with gold luster. All compounds of 2, 3, and 6 are stable toward
air and water and could be purified by silica gel column
chromatography. BODIPY 6 was used as a reference compound
in the following discussion.
To evaluate the electronic effects of introducing ring-fused
structures at both of the b and g bonds in the BODIPY skeleton,
cyclic voltammetry of 2, 3, and 6 in CH2Cl2 was conducted
(Figure 2). While tetrahydrodibenzo[b,g]-fused BODIPY 6
exhibited a reversible wave at E1/2 = ¹1.33 V (vs. Fc/Fc+)
and an irreversible wave at Epc = ¹2.09 V, dibenzo[b,g]-fused
BODIPY 2 showed two reversible reduction waves at the
significantly less negative potentials of E1/2 = ¹0.67 and
¹1.62 V, respectively. The first reduction potential of 2 is much
less negative even compared to that of monobenzo[b]-fused
derivative 1 (E1/2 = ¹1.05 V),8 demonstrating the significant
effect of introduction of benzo-fused structures at both of the b
Scheme 1 shows our synthetic route for BODIPYs 2 and 3.
Dithieno[b,g]-fused BODIPY 3 was synthesized as dark purple
solids with a gold luster according to the conventional method.10
In the case of dibenzo[b,g]-fused BODIPY 2, the similar
synthetic route using 1H-indole as a starting material was not
suitable due to the lower reactivity for an electrophilic
substitution reaction at the 2-position of indole compared to
that of pyrrole.8 Therefore, we synthesized dibenzo[b,g]-fused
BODIPY 2 by another method using 4,7-dihydro-1H-indole (4)
as a key intermediate (Scheme 1b). Thus, 4 was prepared by the
Birch reduction of 1H-indole,11 followed by acylation to give 5.
Chem. Lett. 2013, 42, 986-988
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

987
E
pc = –2.09 V
–7.6
–9.8
R
R
B
C5
C6
C3
–7.8
C4
C7
C8
6
2
C13
C12
C2
NN1 NN2
B
N
N
C1
C9
E1/2 = –1.33 V
–13.2
–11.3
C10
C11
F
F
F
F
1' A
B
(R = 3,5-(CF3)2C6H3)
N
H
–9.0
–6.7
–9.0
–6.7
R
E
1/2 = –1.62 V
R
C5
C6
Current
C3
E1/2 = –0.67 V
C7
C4
indole
C8
C2
C17
C16
C13
C12
Epc = –1.85 V
NN1 NN2
B
N
N
C1
C9
B
F F
C14
C10
C15
C11
F
F
3
E
1/2 = –0.97 V
2
(R = 3,5-(CF3)2C6H3)
–8.8
–8.3
–8.8
–8.3
R
R
B
–11.9
C5
C6
–14.2
0
–0.5
–1.0
–1.5
–2.0
–2.5
C3
C7
C4
C8
Potential/V vs. Fc/Fc+
C2
S
NN1 NN2
B
S
S
S
S
N
N
C1
C9
C11
N
H
C13
Figure 2. Cyclic voltammograms of 2, 3, and 6 in CH2Cl2 (1 mM),
C12
C10
F
F
F
F
¹
measured with (n-Bu)4N+PF6 (0.1 M) as the supporting electrolyte at a
thienopyrrole
3
¹1
(R = 3,5-(CF3)2C6H3)
scan rate of 100 mV s
.
Figure 4. The NICS(0) values (ppm) calculated at the HF/6-
31+G(d,p)// B3LYP/6-31G(d) level.
CH2Cl2,12 6 and 3 showed moderate emission band at -em
=
579 nm (ΦF = 0.30) and a weak emission band at -em = 606 nm
(ΦF = 0.06), respectively, whereas no emission band was
observed for dibenzo[b,g]-fused derivative 2.
To gain deeper insights into the effects of aromatic ring-
fusing in the BODIPY skeleton, DFT calculations (B3LYP/6-
31G(d)) were conducted on 2 and 3, together with 1¤, in which
one of the benzo-fused structures is removed from 2, and 6 for
comparison. The compounds 1¤, 2, and 3 could be represented
by the possible resonance structures as shown in Figure 4. In the
optimized structures of unsymmetrical monobenzo-fused deriv-
ative 1¤, the distances of the N1-C1 (1.335 ¡) and C4-C5
(1.398 ¡) bonds are shorter than those of the N2-C9 (1.364 ¡)
and C5-C6 (1.415 ¡) bonds, respectively, indicating the more
significant contribution of the canonical structure of the form
B between two possible resonance structures. Dibenzo-fused
derivative 2 and dithieno-fused derivative 3 were optimized
with C1 symmetry, but the ³-conjugated skeletons are almost
symmetric corresponding to the resonance structures in Figure 4.
In the optimized structure of 2, the C2-C3 and C7-C8 (1.399 ¡)
bonds are shortened and the C1-C2 and C8-C9 (1.442 ¡) bonds
are longer than the C7-C8 (1.404 ¡) and the C8-C9 (1.438 ¡)
bonds in 1¤, respectively. This structural feature indicates that
introducing of benzo-fused structures at both sides of the
BODIPY skeleton coincidentally enhances the nonaromatic
quinoidal character of the benzene rings and the electron-
accepting azafulvene character. These features are also supported
by the results of NICS calculations at the HF/6-31+G(d,p) as
shown in Figure 4; the NICS(0) values (ppm) for the benzene
(¹6.7) and pyrrole (¹9.0) rings of the indole substructure in 2
are less negative compared to those of the benzene (¹7.8) and
pyrrole (¹9.8) in 1¤ as well as the benzene (¹11.3) and pyrrole
(¹13.2) in the parent indole. A similar structural feature of
enhancement of azafulvene character was also observed in the
optimized structure for dithieno-fused derivative 3. The N1-C1
and N2-C9 bonds (1.349 ¡) and the C2-C3 and C7-C8 bonds
(1.384 ¡) are even shorter than those of 1¤ and 2. The NICS (0)
values for the thiophene (¹8.3) and pyrrole (¹8.8) are also less
Figure 3. UV-vis absorption spectra of 2, 3, and 6 in CH2Cl2 with their
absorption data. The pictures of their CH2Cl2 solutions are included.
and g bonds on increasing the electron-accepting ability. In the
case of dithieno[b,g]-fused derivative, 3 exhibited a reversible
wave at E1/2 = ¹0.97 V and an irreversible wave at Epc
=
¹1.85 V. The first reduction potential of 3 is also less negative
than that of 6 but is negatively shifted by 0.30 V compared to
that of 2. This difference is likely due to the ³-electron-donating
ability of thiophene rings. In the oxidation process, dithie-
no[b,g]-fused 3 as well as 6 showed an irreversible oxidation
wave at less positive potentials of Epa = +1.03 V for 3 and
+0.92 V for 6 compared to that of 2 (Epa = +1.20 V),
respectively.12
In the absorption spectra in CH2Cl2 (Figure 3), tetrahydro-
dibenzo[b,g]-fused derivative 6 showed an absorption band
(full width at half-maximum (FWHM) = 1000 cm¹1) at -abs
=
556 nm (log ¾ = 4.59), whereas dibenzo[b,g]-fused derivative
2 showed a relatively broad band (FWHM = 2500 cm¹1) at
-
abs = 581 nm (log ¾ = 4.65) with a tail up to 660 nm. The
absorption band for 2 is red-shifted even compared to that of
monobenzo[b]-fused derivative 1 (-abs = 539 nm),8 indicating
the significant effect of the dibenzo[b,g]-fused structure on the
absorption profile. In the case of dithieno[b,g]-fused derivative
3, an intense band (FWHM = 1100 cm¹1) was observed at
-
abs = 582 nm (log ¾ = 4.99). In the fluorescence spectra in
Chem. Lett. 2013, 42, 986-988
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

988
group at the 8-position is probably responsible for no emission
in 2.
In summary, we demonstrated that introducing aromatic
ring-fused structures at both b and g bonds in the BODIPY
skeleton perturbs the electronic structure to enhance the quinoid
and azafulvene character in the indole or thienopyrrole subunits,
which effectively increases the electron-accepting ability. The
comparison of electrochemical and photophysical properties of
2, 3, and 6 as well as the results of DFT calculations revealed
that the effects of dibenzo[b,g]-fusing are more significant than
that of monobenzo[b]-fusing. Further synthetic study of more
extended BODIPY oligomers with the benzo- or thieno-fused
structure is now in progress in our laboratory.
This study was supported financially by PRESTO project
from Japan Science and Technology Agency (JST). This study
was carried out with the NMR spectrometer in the JURC at ICR,
Kyoto Univ.
References and Notes
1
a) A. Loudet, K. Burgess, Chem. Rev. 2007, 107, 4891. b) R. Ziessel,
G. Ulrich, A. Harriman, New J. Chem. 2007, 31, 496. c) G. Ulrich,
R. Ziessel, A. Harriman, Angew. Chem., Int. Ed. 2008, 47, 1184.
a) S. Hattori, K. Ohkubo, Y. Urano, H. Sunahara, T. Nagano, Y.
Wada, N. V. Tkachenko, H. Lemmetyinen, S. Fukuzumi, J. Phys.
Chem. B 2005, 109, 15368. b) S. Erten-Ela, M. D. Yilmaz, B. Icli, Y.
Dede, S. Icli, E. U. Akkaya, Org. Lett. 2008, 10, 3299. c) D.
Kumaresan, R. P. Thummel, T. Bura, G. Ulrich, R. Ziessel, Chem.®
Eur. J. 2009, 15, 6335.
2
3
a) T. Rousseau, A. Cravino, T. Bura, G. Ulrich, R. Ziessel, J.
Roncali, Chem. Commun. 2009, 1673. b) Y. Kubo, K. Watanabe, R.
Nishiyabu, R. Hata, A. Murakami, T. Shoda, H. Ota, Org. Lett. 2011,
13, 4574. c) Y. Hayashi, N. Obata, M. Tamaru, S. Yamaguchi, Y.
Matsuo, A. Saeki, S. Seki, Y. Kureishi, S. Saito, S. Yamaguchi, H.
Shinokubo, Org. Lett. 2012, 14, 866.
Figure 5. Pictorial presentation of the frontier orbitals, a plot of the
Kohn-Sham HOMOs and LUMO energy levels (B3LYP/6-31G(d)), and
optical transitions with oscillator strength (CAM-B3LYP/6-31G(d)//
B3LYP/6-31G(d)).
4
5
6
L. Bonardi, H. Kanaan, F. Camerel, P. Jolinat, P. Retailleau, R.
Ziessel, Adv. Funct. Mater. 2008, 18, 401.
T. Yogo, Y. Urano, Y. Ishitsuka, F. Maniwa, T. Nagano, J. Am. Chem.
Soc. 2005, 127, 12162.
a) A. Haefele, G. Ulrich, P. Retailleau, R. Ziessel, Tetrahedron Lett.
2008, 49, 3716. b) G. Ulrich, S. Goeb, A. De Nicola, P. Retailleau, R.
Ziessel, J. Org. Chem. 2011, 76, 4489. c) C. Yu, Y. Xu, L. Jiao,
J. Zhou, Z. Wang, E. Hao, Chem.®Eur. J. 2012, 18, 6437. d) M.
Nakamura, H. Tahara, K. Takahashi, T. Nagata, H. Uoyama, D.
Kuzuhara, S. Mori, T. Okujima, H. Yamada, H. Uno, Org. Biomol.
Chem. 2012, 10, 6840.
a) J. Chen, A. Burghart, A. Derecskei-Kovacs, K. Burgess, J. Org.
Chem. 2000, 65, 2900. b) K. Umezawa, Y. Nakamura, H. Makino, D.
Citterio, K. Suzuki, J. Am. Chem. Soc. 2008, 130, 1550. c) K.
Umezawa, A. Matsui, Y. Nakamura, D. Citterio, K. Suzuki, Chem.®
Eur. J. 2009, 15, 1096. d) S. G. Awuah, J. Polreis, V. Biradar, Y. You,
Org. Lett. 2011, 13, 3884. e) Y. Ni, W. Zeng, K.-W. Huang, J. Wu,
Chem. Commun. 2013, 49, 1217. f) Y. Yang, Q. Guo, H. Chen, Z.
Zhou, Z. Guo, Z. Shen, Chem. Commun. 2013, 49, 3940.
A. Wakamiya, T. Murakami, S. Yamaguchi, Chem. Sci. 2013, 4,
1002.
negative than those of the thiophene (¹11.9) and pyrrole
(¹14.2) rings in the parent thienopyrrole.
These structural perturbations affect the frontier orbitals of 2
and 3. Dibenzo[b,g]-fused derivative 2 and dithieno[b,g]-fused
derivative 3 have low-lying LUMO levels compared to that
of 6. The HOMO and LUMO levels in 3 are slightly elevated
compared to those of 2. These results are in quantitative
agreement with the redox potentials in CV. It should be noted
that the LUMO level of 2 is lower than that of monobenzo-fused
1¤ (¹3.38 eV),12 indicating that benzene-fusing at both side of
the b and g bonds in the BODIPY skeleton is significantly
effective in enhancing the electron-accepting ability. TD DFT
calculations (CAM-B3LYP/6-31G(d)) revealed that the longest
absorption bands in 3 and 6 correspond to the transition from the
HOMO to the LUMO, whereas the broad absorption band in 2
is assignable to the multitransitions mainly containing the
contributions from the HOMO¹1 to the LUMO as well as the
HOMO to the LUMO (Figure 5). Surprisingly, the lowest
energy transition of 2 has a larger contribution from the
HOMO¹1 to the LUMO than that from the HOMO to the
LUMO, and has smaller oscillator strength ( f = 0.055) than
those for 3 ( f = 0.809) and 6 ( f = 0.547). As the radiative rate
constant is known to be proportional to the oscillator strength,13
this electronic structural feature as well as the rotation of the aryl
7
8
9
a) W. Zhao, E. M. Carreira, Angew. Chem., Int. Ed. 2005, 44, 1677.
b) W. Zhao, E. M. Carreira, Chem.®Eur. J. 2006, 12, 7254. c) B. A.
Jones, A. Facchetti, M. R. Wasielewski, T. J. Marks, J. Am. Chem.
Soc. 2007, 129, 15259.
10 A. B. Nepomnyashchii, M. Bröring, J. Ahrens, A. J. Bard, J. Am.
Chem. Soc. 2011, 133, 8633.
11 H. Çavdar, N. Saraçoğlu, Tetrahedron 2005, 61, 2401.
12 Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.html.
13 N. J. Turro, V. Ramamurthy, J. C. Scaiano, Principles of Molecular
Photochemistry: An Introduction, University Science Books, Sausa-
lito, 2009.
Chem. Lett. 2013, 42, 986-988
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/