Benzene-fused BODIPY and fully-fused BODIPY dimer: Impacts of the ring-fusing at the b bond in the BODIPY skeleton

Article abstract of DOI:10.1039/c2sc21768f

4,4-Difluoro-4-bora-3a,4a-diaza-s-indacenes (BODIPYs) are fascinating dyes with great potentials for various applications. To establish the design principle for the modification of the BODIPY skeleton, we now investigate the electronic impacts of the introduction of ring-fused structures. The DFT calculations revealed that while the benzene-fusing at the a bond in the BODIPY skeleton increases the HOMO level, the benzene-fusing at the b bond leads to a decreased LUMO level. Based on these results, a benzo[b]-fused BODIPY 1 and a fully-fused BODIPY dimer 2 were synthesized. X-ray crystal structure analysis demonstrated that the benzo[b]-fused structure significantly perturbs the π-conjugation with enhancement of an electron-accepting azafulvene character. In the ring-fused BODIPY dimer 2, the central benzene ring is largely deviated from the aromatic benzene geometry. As a consequence, 2 has a significantly low-lying LUMO delocalized along the periphery. In cyclic voltammograms, the benzene-fused BODIPY 1 and fully-fused dimer 2 indeed showed reversible reduction waves at much less negative potentials (E_1/2 = -1.05 V vs. Fc/Fc⁺ for 1, -0.85 V for 2), relative to a non-fused BODIPY 3 (E_1/2 = -1.40 V). In addition, the benzene-fused BODIPYs showed broad and intense absorption bands in the Vis-NIR region. In particular, the BODIPY dimer 2 showed an intense absorption at 629 nm with a broad shoulder band reaching 900 nm. Corresponding to the red-shifted absorption, compounds 1 and 2 showed weak fluorescence in the deep red region (λ_em = 690 nm, Φ_F < 0.01) and in the near-infrared region (940 nm, Φ_F < 0.01), respectively.

Full text of DOI:10.1039/c2sc21768f

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EDGE ARTICLE
Benzene-Fused BODIPY and Fully-Fused BODIPY Dimer: Impacts of
the Ring-Fusing at the b Bond in the BODIPY Skeleton
Atsushi Wakamiya,*a Takanori Murakamib and Shigehiro Yamaguchi*b
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
4,4-Difluoro-4-bora-3a,4a-diaza-s-indacenes (BODIPYs) are fascinating dyes with great potentials for
various applications. To establish the design principle for the modification of the BODIPY skeleton, we
now investigate the electronic impacts of the introduction of ring-fused structures. The DFT calculations
revealed that while the benzene-fusing at the a bond in the BODIPY skeleton increases the HOMO level,
10 the benzene-fusing at the b bond leads to a decreased LUMO level. Based on these results, a benzo[b]-
fused BODIPY 1 and a fully-fused BODIPY dimer 2 were synthesized. X-ray crystal structure analysis
demonstrated that the benzo[b]-fused structure significantly perturbs the π-conjugation with enhancement
of an electron-accepting azafluvene character. In the ring-fused BODIPY dimer 2, the central benzene
ring is largely deviated from the aromatic benzene geometry. As a consequence, 2 has a significantly low-
15 lying LUMO delocalized along the periphery. In cyclic voltammograms, the benzene-fused BODIPY 1
and fully-fused dimer 2 indeed showed reversible reduction waves at much less negative potentials (E1/2
=
–1.05 V vs Fc/Fc+ for 1, –0.85 V for 2), relative to a non-fused BODDIPY 3 (E1/2 = –1.40 V). In addition,
the benzene-fused BODIPYs showed broad and intense absorption bands in the Vis-NIR region. In
particular, the BODIPY dimer 2 showed an intense absorption at 629 nm with a broad shoulder band
20 reaching 900 nm. Corresponding to the red-shifted absorption, compounds 1 and 2 showed week
fluorescence in deep red region (λem = 690 nm, ΦF <0.01) and in near-infrared region (940 nm, ΦF <0.01),
respectively.
HOMO-LUMO gap dyes, decreasing the LUMO level while
Introduction
BODIPYs, 4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes, are
25 fascinating dyes with some notable features, such as an excellent
stability, intense absorption profiles, and insensitivity to the
polarity of the environment.1 Various structural modifications of
the BODIPY skeleton at the periphery2 or at the boron atom3 have
been extensively studied in order to alter their photophysical
30 properties, and, thereby, their significant utilities in a variety of
applications, including fluorescent probes for biomolecules,4
photodynamic therapy,5 laser dyes,6 emissive materials in
OLEDs,7 and organic photovoltaics,8 have been demonstrated.
Among them, a particularly urgent challenge is the application to
35 organic photovoltaics as the dyes absorbing light in the near
infrared (NIR) region.9 For the molecular design of narrow
a Institute for Chemical Research, Kyoto University and PRESTO, JST,
Gokasho, Uji, Kyoto, 611-0011, Japan. Fax: 81 774 38 3178; Tel: 81 38
3173; E-mail:wakamiya@scl.kyoto-u.ac.jp
b Department of Chemistry, Graduate School of Science, and Research
Center for Materials Science, Nagoya University and CREST, JST,
Chikusa, Nagoya, 464-8602, Japan. E-mail: yamaguchi@chem.nagoya-
u.ac.jp
†Electronic Supplementary Information (ESI) available: Experimental
procedures, analytical data for compounds prepared in this work. CCDC
reference numbers 899844–899846. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/b000000x/
Fig. 1 Benzene-fused BODIPYs: (a) Effect of the benzene-fused
structures on the electronic structures (B3LYP/6-31G(d)) and (b)
structures of the BODIPY derivatives synthesized in this study.
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Chemical Science
Page 2 of 6
maintaining a moderate increment of the HOMO level is 55 the pyrrole (ESI). Therefore, we examined alternative routes and
important for the stability toward air oxidation. To this end, how
to efficiently expand the π conjugation is a key issue. Whereas
the direct dimerization or oligomerization at the 2- or 3-position
of the BODIPY skeleton might be a straightforward way, this
finally obtained 1 by the direct nucleophilic reaction of a 2-
lithioindole derivative with the ketopyrrole, as shown in Scheme
1. Thus, starting from the N-Boc-indole derivative 4, the ortho-
View Article Online
5
metalation using EtZn(µ-Et)(µ-TMP)Li as a base15 followed by
often suffers from steric congestion caused by neighboring 60 treatment with I2 produced a 2-iodoindole derivative 5. Lithiation
substituents, resulting in a twisted non-coplanar conformation.10
As part of our chemistry of the fully ring-fused planar π-
conjugated materials,11 we are now interested in introducing the
of 5 with t-BuLi generated a 2-lithioindole derivative, which was
reacted with the ketopyrrole 6 to give a hydroxy derivative 7.
Without isolation, this compound was successively treated with
CF3COOH in CH2Cl2 and BF3·OEt2 in the presence of Et3N.
10 fused structure to the BODIPY skeleton.12–14 While several
derivatives having the fused structures at the a bond have been 65 Purification by silica gel column chromatography gave us the
reported,12 the b bond-fused derivatives are still limited to only a
few examples,13 such as the difurano[b,g]-fused derivatives. We
now disclose the synthesis of benzo[b]-fused BODIPY 1 and its
15 dimer 2 consisting of 7 fully-fused rings (Fig. 1). Based on the
benzene-fused BODIPY 1 in 34% yield (overall for the 3 steps)
as a dark purple solid. On the basis of this methodology, using
benzodipyrrole 8 as the starting material, the benzene-fused
BODIPY dimer 2 was also successfully synthesized in 20% yield
study of their structure properties relationships, we will discuss 70 (overall for the 3 steps) as a dark green solid. Both compounds 1
the impacts of the b bond-fused structure.
and 2 are stable toward air and water and soluble in the common
organic solvents, such as CHCl3 and THF. Compound 3 was also
We first elucidated how the benzene-fused structure alters the
electronic structure of the BODIPY skeleton. The DFT
20 calculations at the B3LYP/6-31G(d) level of theory revealed that
the electronic structure is highly dependent on the position where
the benzene ring is fused. While the benzene-fusing at the a bond
in the BODIPY skeleton increases the HOMO energy level, the
benzene-fusing at the b bond leads to a decreased LUMO level
25 along with a slight increase in the HOMO level, as shown in Fig.
1. Consequently, the benzo[b]-fused derivative has a narrower
HOMO–LUMO gap. The BODIPY skeleton can be regarded as a
combination of the electron-donating pyrrole and electron-
accepting azafulvene moieties. The aforementioned effects may
30 be interpreted such that, while the benzo[a]-fused structure
enhances the character of the benzo[c]pyrrole substructure,
thereby resulting in the destabilization of the HOMO energy level,
the benzo[b]-fused structure enhances the azafulvene character to
decrease the LUMO level and make the HOMO–LUMO gap
35 narrower while maintaining a moderate increment of the HOMO
level. These electronic effects of the benzo[b]-fused structure
would be beneficial to the molecular design of air-stable NIR
dyes. To demonstrate the validity of the design, we
experimentally investigated the benzo[b]-fused derivatives 1 and
40 2, in which the p-CF3- and p-hexyl-substituted phenyl groups
were introduced on the peripheral positions in order to increase
the electron-accepting ability and solubility, respectively. As a
reference compound, we also examined a non-benzene-fused
BODIPY 3 (Fig. 1).
R2 OH
R1
Boc
R1
N
O
b
R3
R3
R
+
N
N
R2
N
Boc
Boc
Boc
4 (R = H)
5 ( R = I)
6
7
a, 99%
R2
R2
R1
R1
c
d
R1 = 4-hexyl-C6H4
R2 = 4-CF3-C6H4
R3 R3 = mesityl
N
N
NH
N
B
R3
F
F
1
34% (in 3 steps)
R1
R1
R1
R1
R2
R2
O
b, c, d
R3
N
N
+ 2
R
R
N
R2
N
N
N
N
B
F
B
Boc
F
F
Boc
Boc
F
R3
R3
8 (R = H)
9 ( R = I)
2
a, 95%
20% (in 3 steps)
Scheme 1 Reagents and conditions: (a) EtZn(µ-Et)(µ-TMP)Li, THF, –
78 °C, then I2; (b) t-BuLi, THF, –78 °C; (c) CF3COOH, CH2Cl2, rt.;
(d) BF3·OEt2, Et3N, toluene, reflux.
prepared according to the conventional method (see ESI).
Structural features of benzo[b]-fused BODIPY derivatives
75
The effect of the benzene-fused structure in compound 1 was
first studied based on the X-ray structure analysis. As shown in
Fig. 2a, the BODIPY skeleton has a planar structure with the
torsion angles among the fused four ring skeletons of less than
6.1°. The terminal mesityl group is arranged nearly perpendicular
45 Results and discussion
80 to the BODIPY plane, while the other aryl groups at the C5 and
C7 positions are arranged with the tilt angles of 57.2° and 54.7°,
respectively. Notably, the bond distances (Å) of C1–N1
(1.349(3)), C2–C3 (1.360(4)), and C4–C5 (1.388(4)) are
substantially shorter than those of C9–N2 (1.374(3)), C7–C8
85 (1.425(4)), and C5–C6 (1.423(4)), respectively. These differences
indicate the more significant contribution of form A between two
possible resonance structures, as shown in Fig. 2b. The form A is
a canonical structure that consists of the aromatic benzene and
azafluvene substructures.
Synthesis of a series of benzo[b]-fused BODIPY derivatives
The synthesis of unsymmetrically substituted BODIPYs is
generally achieved based on the condensation of a ketopyrrole
with another pyrrole in the presence of acid.1 According to this
50 methodology, we first attempted the synthesis of the benzene-
fused BODIPY 1. However, the reaction of the indole 4 with
ketopyrrole 6 in the presence of trifluoroacetic acid only
produced 1 in poor yield (~3%), probably due to the lower
electrophilicity of the indole at the 2-position compared to that of
2 | Journal Name, [year], [vol], 00–00
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This consideration is supported by the quantum chemical
calculations. Thus, the structural optimization for a model
(a)
F
F
F
F
compound 1ʹ (R1 = tolyl) calculated at the B3LYP/6-31G(d) level
of theory well reproduced the crystal structure (ESI). The
NICS(0) values for the optimized structure calculated at the
HF/6-31+G(d,p) level are summarized in Fig. 2c. The NICS(0)
value for the terminal pyrrole ring is –6.6, which is less negative
than that (–9.6) of the pyrrole ring in the indole substructure and,
moreover, even compared to that of the pyrrole ring (–8.0) in the
F
F
C11
C10
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C7
C8
C9
5
C6
N1
C5
C4
N2
B1
F
N
C3
F
B
N
C1
C2
F
F
10 non-benzene-fused derivative 3. This result implies that the
nonaromatic azafulvene character is enhanced by introducing the
benzo[b]-fused structure in 1. However, the NICS(0) values for
the benzene (–7.8) and pyrrole (–9.6) rings of the indole
substructure in 1ʹ are also less negative compared to those of the
15 benzene (–11.3) and pyrrole (–13.2) rings in the parent indole.
This fact suggests that the form B, which has a quinoid structure
for the indole moiety, should also contribute to some extent to the
resonance equilibrium.
(b)
–7.6
R1
–7.9
R1
–13.3
–12.5
R2
–6.5
R2
N
B
N
N
H
N
H
N
N
B
F
F
F
F
1,7-dihydrobenzo[1,2-b:5,4-b']dipyrrole
R3
R3
2'
(R1 = p-tolyl, R2 = 4-CF3-C6H5, R3 = mesityl)
(a)
(b)
F
F
R'
R
F
R1
R1
(c)
–0.160
–0.091 –0.091
R2
R2
–
+
N
F
N
B
R"
N+0.244 +0.244 N
–0.287
F
N
N
B
B
F
F
A
F
F
R3
R3
C
C3
C5
B1
C4
C7
C8
C6
Fig. 3 Structural features and aromaticity in 2: (a) ORTEP drawing
(50% probability for thermal ellipsoids), where the pentyl group is
omitted for clarity; (b) NICS(0) values (ppm) for 2' and 1,7-
dihydrobenzo[1,2-b:5,4-b’]dipyrrole calculated at the HF/6-
C2
C13
C1
N1
N2
R'
C12
C9
C10
R
F
F
C11
31+G(d,p)//B3LYP/6-31G(d) level; (c)
a representation of the
N
F
N
B
resonance structure with a quinoid character. The atomic charges on
the benzene rings for 2' obtained by NPA (B3LYP/6-31G(d) are
included.
R"
F
B
(c)
–9.6
R2
R2
–8.0
R1
–7.8
instead, the quinoid character is enhanced in the benzodipyrrole
moiety.
–11.3
–13.2
–6.6
N
F
N
N
N
B
B
R3
R3
N
H
R3
F
F
F
35 Electronic Structures of benzo[b]-fused BODIPY derivatives
These structural features of 2 were reproduced in the optimized
1'
3
indole
(R1 = p-tolyl, R2 = 4-CF3-C6H5, R3 = mesityl)
Fig. 2 Structural features and aromaticity in 1: (a) ORTEP drawing
(50% probability for thermal ellipsoids), where the pentyl group is
omitted for clarity; (b) resonance structures; (c) NICS(0) values (ppm)
for 1', 3, and indole, calculated at the HF/6-31+G(d,p)//B3LYP/6-
31G(d) level.
structure (B3LYP/6-31G(d)) for a model compound 2' (R1
=
tolyl), which also showed a substantial bond alternation in the
central benzene ring (ESI). The NICS(0) values calculated for the
40 terminal pyrrole ring and the central benzene ring (HF/6-
31+G(d,p)) are –6.5 and –7.9, respectively. These values are
much less negative compared to those for the pyrrole ring (–8.0)
in the parent BODIPY 3 and the benzene ring (–13.3) in the
parent benzodipyrroles, implying the lower aromaticity in these
45 ring skeletons. The natural population analysis (NPA) (B3LYP/6-
31G(d)) showed a deflection in the atomic charges on the
benzene ring in 2' as shown in Fig. 3c. Taking these results into
account, the structure of 2 might be best depicted as form C in
Fig. 3c, which has the characteristics of the terminal azafulvene
50 substructures and the zwitterionic character of the central
benzodipyrrole substructure. In conjunction with this
consideration, the calculations also indicated the interesting
electronic structure of compound 2' as shown in Fig. 4. The
LUMO of 2' is delocalized along the periphery and consequently
55 lies in a substantially lower energy level compared to that of 1'.
A similar structural feature was also observed in the crystal
20 structure of the 7-ring fused dimer 2 (Fig. 3a). This compound
has a symmetrical plane at the center of the molecule. The bond
distances (Å) of C1–N1 (1.340(4)), C2–C3 (1.367(5)), C4–C5
(1.383(5)) are shorter than those of C9–N2 (1.368(4)), C7–C8
(1.434(4)), and C5–C6 (1.417(5)), respectively (Fig. 3). These
25 results again demonstrate that the azafulvene character is
enhanced in both of the terminal pyrrole rings by the introduction
of the benzene-fused structure. In addition, it is worth noting that
the bond distance of C8–C9 (1.443(4)) is substantially longer
than those of C8–C11 (1.395(4)) and C9–C10 (1.387(4)). This
30 implies that the central benzene ring in the BODIPY dimer
significantly deviates from the aromatic benzene geometry and,
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LUMO
LUMO
LUMO
–2.78 eV
LUMO
–3.07 eV
–3.31 eV
LUMO
–4.97 eV
–5.63 eV
HOMO
–5.58 eV
–5.96 eV
1'
–5.79 eV
HOMO–1
HOMO
3
2'
Fig. 5 Cyclic voltammograms of 1–3 in CH2Cl2 (1 mM), measured with
–
(n-Bu)4N+PF6 (0.1 M) as a supporting electrolyte at a scan rate of 100
mVs–1.
upon oxidation at the unprotected terminal benzene ring. On the
other hand, the benzene-fused dimer 2 showed both reversible
oxidation and reduction processes at the significantly less positive
and less negative potentials of E1/2 = +0.56 V and –0.85 V,
30 respectively. The differences between these values and those for
3 amount to –0.51 and +0.55 V, respectively, demonstrating the
significant impacts of the dimerization through the benzene-fused
structure. Notably, the first reduction potential of the dimer 2 is
less negative even compared to that of C60 (E1/2 = –0.98 V vs
35 Fc/Fc+),16 indicative of its potential use as an electron-accepting
material. In addition, the dimer 2 also showed a reversible second
reduction process at E1/2 = –1.31 V, suggesting the formation of
stable reduced species to the dianion under the stated
measurement conditions.
HOMO
HOMO
HOMO
HOMO–1
HOMO–1
Fig. 4 Pictorial presentation of the frontier orbitals and a plot of the
Kohn-Sham HOMOs and LUMO energy levels for 1'–3 (B3LYP/6-
31G(d)).
In the occupied molecular orbitals, beside the inherent π-orbital
of the BODIPY skeleton that appeared as the HOMO–1, 2' has a
higher-lying HOMO that originated from the benzodipyrrole
skeleton.
Based on the structural features, the benzene-fusing at the b
bond in the BODIPY skeleton significantly alters the electronic
structure. In particular, the dimerization of the BODIPY skeletons
through the benzene-fused structure is effective to attain the
narrow HOMO–LUMO gap. These electronic effects indeed
40
Impacts of the Benzo[b]-fusing on Photophysical Properties
5
Fig. 6 shows the Vis-NIR absorption spectra of compounds 1–
3 in THF. While the non-fused BODIPY 3 showed an intense
absorption band with the maximum (λmax) at 519 nm (log ε =
45 4.80), the benzo[b]-fused derivative 1 showed a relatively broad
band at λmax = 539 nm (log ε = 4.73), which is red-shifted by 20
10 result in several characteristic electrochemical and photophysical
properties.
15
Impacts of the Benzo[b]-fusing on Electrochemical Properties
" = 629 nm
(log ! = 5.01)
Fig. 5 shows the cyclic voltammograms of 1 and 2, together
15 with that of 3 for comparison, which were measured in CH2Cl2
with (n-Bu)4NPF6 as the supporting electrolyte. In comparison
with the non-fused BODIPY 3, while the benzo[b]-fused 1
showed an irreversible oxidation wave at only a slightly less
positive potential, it exhibited a reversible reduction process at a
20 much less negative potential by 0.35 V. This comparison
demonstrates that the benzene-fused structure at the b bond is
indeed effective to decrease the LUMO level. In the oxidation
process for 1, the new weak oxidation peaks at around 0.80 V and
+0.20 V appeared after the oxidation process at +1.05 V (ESI).
25 This result implies that 1 might undergo electropolymerization
10
1
" = 539 nm
(log ! = 4.71)
" = 519 nm
( log ! = 4.80)
–
m
c
1
3
1
–
2
M
4
0
1
/
!
5
0
" = 753 nm
(log ! = 4.21)
400
500
600
700
Wavelength / nm
800
900
1000
Fig. 6 UV-vis-NIR absorption spectra of 1–3 in THF with their
absorption data. The pictures of their THF solutions are included.
4 | Journal Name, [year], [vol], 00–00
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Page 5 of 6
Chemical Science
Shinokubo, Angew. Chem. Int. Ed. 2011, 50, 2280; (d) J.-s. Lu, H. Fu,
Y. Zhang, Z. J. Jakubek, Y. Tao and S. Wang, Angew. Chem. Int. Ed.
2011, 50, 11658.
nm relative to 3. The band of 1 is broadened to 620 nm, which
makes the color of the solution of 1 reddish-purple as shown in
Fig 6. A more striking difference was observed for the benzene-
fused dimer 2. Thus, this compound showed a very intense
absorption band at the more red-shifted λmax of 629 nm with a
significantly high molar extinction coefficient (ε) of 1.0 × 105 M–
1cm–1, and, in addition, a broad shoulder band with a tail up to
900 nm with the ε of 1.5 × 104 M–1cm–1. These features are
beneficial to absorbing light in the long wavelength region when
10 used as a dye for photovoltaics. According to the TD-DFT
calculations of the model compound 2' (B3LYP/6-31G(d)), while
the intense absorption at λmax = 629 nm is assignable to the
transition from the HOMO–1 to the LUMO, the broad absorption
band in the near IR region is attributed to the transition from the
15 HOMO to the LUMO (Fig. 4, ESI). The absorption spectra of 1
and 2 were insensitive to the solvents (1: 543 nm in cyclohexane,
538 nm in DMF, 2: 637 and 789 nm in cyclohexane, 622 and 771
nm in DMF, ESI), indicative of small difference in their dipole
moments between the ground state and the excited state. In the
20 fluorescence spectra in THF, compounds 1 and 2 showed week
emissions in deep red region (λem = 690 nm, ΦF <0.01) and in
near-infrared region (940 nm, ΦF <0.01), respectively (ESI). The
reference compound 3 also showed a week emission at λem = 550
nm (ΦF = 0.02). One reason for the low fluorescence quantum
25 yields in these compounds is probably due to the rotation of aryl
group on the peripheral positions in the BODIPY skeleton.2b
3
4
(a) G. Ulrich, C. Goze, M. Guardigli, A. Roda and R. Ziessel, Angew.
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In summary, the impacts of the ring-fusing in the BODIPY
skeleton is highly dependent on the position at which the ring is
30 introduced. The benzene-fusing at the b bond is an effective way
to decrease the LUMO level. In particular, the dimerization of the
BODIPY skeleton through the benzene-fused structure
significantly alters the electronic structure by attaining a low-
lying LUMO as well as the narrow HOMO–LUMO gap. The
35 produced compound showed intriguing electrochemical and
photophysical properties, such as the NIR absorption. These
results would provide an important guideline for further structural
modification of the BODIPY frameworks. Further synthetic study
of more extended BODIPY oligomers with the benzene-fused
40 structure is now in progress in our laboratory.
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Acknowledgements
We thank Prof. M. Minoura at Kitasato University and Prof. T.
Sasamori at Kyoto University for valuable suggestions on X-ray
crystal structure analysis of compound 3. This work was
45 supported by Grants-in-Aid (19685004 and 19675001) from the
Ministry of Education, Culture, Sports, Science, and Technology,
Japan.
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