Red-emissive polyphenylated BODIPY derivatives: Effect of peripheral phenyl groups on the photophysical and electrochemical properties

Article abstract of DOI:10.1246/cl.2008.1094

A series of red-emissive BODIPY derivatives 2-4 having five to seven phenyl groups on their periphery were synthesized, and their photophysical and electrochemical properties were investigated. While the 8-phenyl-substituted derivatives only have moderate fluorescence quantum yields in solution, they can show intense red fluorescence in the PMMA films. The peripheral phenyl groups also affect the redox reversibility in cyclic voltammetry. Copyright

Full text of DOI:10.1246/cl.2008.1094

1094
Chemistry Letters Vol.37, No.10 (2008)
Red-emissive Polyphenylated BODIPY Derivatives:
Effect of Peripheral Phenyl Groups on the Photophysical
and Electrochemical Properties
Atsushi Wakamiya,ꢀ Naoya Sugita, and Shigehiro Yamaguchiꢀ
Department of Chemistry, Graduate School of Science, Nagoya University,
Furo, Chikusa-ku, Nagoya 464-8602
(Received July 10, 2008; CL-080683;
E-mail: wakamiya@mbox.chem.nagoya-u.ac.jp, yamaguchi@mbox.chem.nagoya-u.ac.jp)
H
A series of red-emissive BODIPY derivatives 2–4 having
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
OHC
Ph
Ph
b
five to seven phenyl groups on their periphery were synthesized,
and their photophysical and electrochemical properties were
investigated. While the 8-phenyl-substituted derivatives only
have moderate fluorescence quantum yields in solution, they
can show intense red fluorescence in the PMMA films. The
peripheral phenyl groups also affect the redox reversibility in
cyclic voltammetry.
a
Ph
Ph
N
N
F
N
H
N
H
B
F
5
6
3
17%
Ph
Ph
Ph
Ph
Ph
R
c
R
R
N
N
F
Ph
N
H
B
Ph
F
7 (R = H)
5 (R = Ph)
2 (R = H) 54%
4 (R = Ph) 53%
Scheme 1. Reagents and conditions: a, POCl3, DMF; bꢁ, i) 5,
4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) de-
rivatives have attracted much attention as fluorescence materials
because of their excellent stability, intense absorption profiles,
and high fluorescence quantum yields.1 Significant efforts have
been devoted to their structural modification in order to tune
their fluorescence properties, so that some derivatives indeed
show great potential not only as biological probes1a but also
for various applications, such as organic light emitting diodes,2
laser dyes,3 and sensitizers for solar cells.4 As a new family of
BODIPY derivatives, we have now focused our attention on
an all-phenyl-substituted BODIPY. The incorporation of phenyl
groups on the periphery of the fluorophore can lead to some
characteristic properties, such as increased morphological stabi-
lity5a as well as aggregation-induced emission as seen in the
tetraphenylsiloles.5b As for the phenyl-substituted BODIPY,
the 1,3,5,7-tetraphenyl derivative 1 was reported in the
literature.6 In this study, we synthesized a series of BODIPY de-
rivatives having five to seven phenyl groups on the periphery,
i.e., the 1,3,5,7,8-pentaphenyl 2, 1,2,3,5,6,7-hexaphenyl 3,
and 1,2,3,5,6,7,8-heptaphenyl derivatives 4 (Figure 1) and
investigated the effects of the peripheral phenyl groups on their
properties.
ꢁ
.
POCl3, CH2Cl2, 0 C, ii) Et3N, BF3 OEt2, toluene, 100 C; c,
i) benzaldehyde, TFA, CH2Cl2, then DDQ, ii) Et3N, BF3 OEt2,
.
toluene, 100 ꢁC or CH2Cl2.
b)
a)
B
N
B
F
N
N
N
F
Figure 2. ORTEP drawings of a) 3 and b) 4. Thermal ellipsoids
are drawn at the 50% probability.
In a similar manner, the pentaphenyl derivative 2 was synthe-
sized from 2,4-diphenylpyrrole (7)8 as a deep red solid in 54%
yield.9
The X-ray crystallographic analysis of 3 and 4 revealed that
the peripheral phenyl groups are arranged in a propeller-like
fashion having dihedral angles with the central BODIPY plane
of 34.4–76.5ꢁ (Figure 2).10 In 4, the distance between the centers
of the 8-phenyl ring and the 1- or 7-phenyl ring are 3.71 and
As shown in Scheme 1, the hexa- and heptaphenyl BODIPY
derivatives 3 and 4 were synthesized using 2,3,4-triphenyl-
pyrrole (5) as a key precursor, which was prepared by the
threefold Suzuki–Miyaura coupling of 2,3,4-tribromopyrrole7
with PhB(OH)2. The reaction of 5 with formylpyrrole 6 or benz-
˚
3.78 A, respectively, which are shorter than those for the phenyl
˚
groups at the 1, 2, and 3 positions (4.59–4.85 A in 3 and 4). This
suggests a substantial steric congestion in the consecutive three
phenyl-substituted moieties at the 1, 7, and 8 positions. A similar
steric congestion was also observed for the pentaphenyl deriva-
.
aldehyde followed by treatment with BF3 Et2O and Et3N afford-
ed 3 and 4 as deep red solids in 17 and 53% yields, respectively.
˚
tive 2 with the distances of 3.75 and 3.78 A between the 8-phenyl
group and the 1- or 7-phenyl group, respectively.10
Table 1 summarizes the photophysical properties of the
BODIPY derivatives. In the absorption spectra in THF, all the
derivatives 1–4 showed strong absorption bands with maximum
wavelengths (ꢀmax) around 561–578 nm (log " ¼ 4:7{4:9),
which are red-shifted by about 60–70 nm compared to the
1,3,5,7-tetramethyl BODIPY derivative (8: 509 nm)11a and
1,3,5,7-tetramethyl-8-phenyl BODIPY derivative (9: 500
nm).11b According to the DFT and TDDFT calculations (see
R
R
B
8
1
7
H
H
2
6
N
F
N
F
N
F
N
F
3
5
B
1: R = H
2: R = Ph
3: R = H
4: R = Ph
Figure 1. Phenyl-substituted BODIPY derivatives 1–4.
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.10 (2008)
1095
Table 1. Photophysical properties of the BODIPY derivatives
the generated radical cation and radical anion have sufficient sta-
bilities under these conditions. This is in contrast to the fact that
the 1,3,5,7-tetramethyl derivative 8 shows irreversible oxidation
and reduction waves.12a In the case of the tetraphenyl BODIPY
Compd
ꢀ
max/nm
ꢀ
em/nm
(ꢂF)a
571 (4.87) 601 (0.92)
570 607 (0.83)
561 (4.78) 597 (0.31)
560 601 (0.87)
576 (4.79) 608 (0.96)
572 608 (0.75)
573 (4.87) 613 (0.20)
Lifetime
ꢃs/ns
Decay rate constants
kr/108 sꢃ1 knr/108 sꢃ1
(log ")
1 (THF)
(film)b
5.2
5.6
2.1
5.8
5.3
5.4
1.4
6.7
6.1
3.1
1.8
1.5
1.4
1.5
1.8
1.4
1.4
1.1
1.5
1.8
0.15
0.30
3.2
1, while a reversible oxidation wave was observed at E1=2
þ0:83 V, the reversibility of the reduction wave (Epc
¼
2 (THF)
(film)b
¼
0.22
0.075
0.46
5.7
ꢃ1:34 V) is substantially decreased. For stabilization of
their radical anion, the protection of the reactive 8-position
may be important.12b
3 (THF)
(film)b
4 (THF)
(film)b
8 (toluene)11a 509
9 (THF)11b
500
In summary, we synthesized a series of phenyl-substituted
BODIPY derivatives. The study of their structure–property
relationships revealed several findings, such as the effect of
the 8-phenyl group on the fluorescence, their high fluorescence
quantum yields in the polymer films, and the high reversibility
in the redox process. These findings would be important for
further structural modification of the BODIPY skeleton. These
results also suggest the potential use of these compounds as
red-emitting dopant materials.
570
609 (0.73)
516 (0.92)
510 (0.56)
0.40
0.13
1.4
aAbsolute quantum yield determined by a calibrated integrating sphere system.
bSpin-coated film prepared from a CH2Cl2 solution of PMMA (50 mg/mL)
containing 1 wt % of the sample.
Supporting Information),9 these substantial red shifts are mainly
due to the ꢁ conjugation expanded with the 3,5-phenyl groups as
well as the electron-withdrawing effect of the peripheral phenyl
groups. The former effect is responsible for the elevation of
the HOMO levels, while the latter contributes to the decrease
in the LUMO levels.
In the fluorescence spectra in THF, compounds 1–4 showed
red emissions with the maxima (ꢀem) at 597–613 nm, which
are also red-shifted by about 100 nm compared to 811a and
9.11b Although the ꢀem of 1–4 are comparable to each other,
significant difference was observed in their quantum yields
(ꢂF). While the tetraphenyl and hexaphenyl BODIPYs 1 and 3
have extremely high ꢂF values of 0.92 and 0.96, respectively,
those of the pentaphenyl and heptaphenyl BODIPYs 2 (0.31)
and 4 (0.20) are only moderate. This difference should be ascrib-
able to the effect of the phenyl group at the 8 position.
To gain an insight into the origin of the difference in ꢂF
values, the excited state dynamics was investigated. On the basis
of the ꢂF and fluorescence lifetime (ꢃs), the radiative (kr) and
nonradiative (knr) decay rate constants from the singlet excited
state were determined. While the kr values of 1–4 are almost
constant in the range of 1:4{1:8 ꢂ 108 sꢃ1, the knr values of 2
(3:2 ꢂ 108 sꢃ1) and 4 (5:7 ꢂ 108 sꢃ1) are much greater than
those of 1 (1:5 ꢂ 107 sꢃ1) and 3 (7:5 ꢂ 106 sꢃ1). As observed
in the X-ray structures of 2 and 4, the steric congestion around
the 8-phenyl group may accelerate internal conversion.
This research was supported by Grants-in-Aid for Scientific
Research (Nos. 19675001 and 19685004) from the Ministry of
Education, Culture, Sports, Science and Technology, Japan.
This paper is dedicated to Professor Ryoji Noyori on the
occasion of his 70th birthday.
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To confirm this consideration, we also measured the
fluorescence spectra in a rigid medium with a low-concentration
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Published on the web (Advance View) October 5, 2008; doi:10.1246/cl.2008.1094