Efficient light absorbers based on thiophene-fused boron dipyrromethene (BODIPY) dyes

Article abstract of DOI:10.1016/j.bmc.2013.03.050

We present the development of the thiophene-fused boron dipyrromethene derivatives as efficient light absorbers. The two strategies for the evolution of the optical properties such as the peak positions of absorption wavelengths and molar extinct coeffici

Full text of DOI:10.1016/j.bmc.2013.03.050

Bioorganic & Medicinal Chemistry 21 (2013) 2715–2719
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry
journal homepage: www.elsevier.com/locate/bmc
Efficient light absorbers based on thiophene-fused boron
dipyrromethene (BODIPY) dyes
⇑
Kazuo Tanaka, Honami Yamane, Ryousuke Yoshii, Yoshiki Chujo
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
We present the development of the thiophene-fused boron dipyrromethene derivatives as efficient light
absorbers. The two strategies for the evolution of the optical properties such as the peak positions of
absorption wavelengths and molar extinct coefficients were established by the substituent effects: by
introducing iodine groups, the bathochromic shifts of the peak positions (+15 nm) and the enhancement
of molar extinct coefficients were simultaneously received owing to the heavy atom effect. Next, it was
found that the modification with the trifluoromethyl group contributed to the large bathochromic shift
Received 30 January 2013
Revised 8 March 2013
Accepted 10 March 2013
Available online 1 April 2013
Keywords:
BODIPY
Absorption
Heavy atom effect
(
+60 nm) because of the lowering effect on the lowest unoccupied molecular orbital of the dye by the
À1
À1
substituent. Finally, we obtained the dyes with large molar extinct coefficients (184,140 M cm at
5
À1
À1
92 nm, 72,180 M cm at 623 nm), sharp absorption bands, and low emissions.
Ó 2013 Elsevier Ltd. All rights reserved.
6
1
. Introduction
Efficient light-absorbers are useful for the applications as a wide
bioprobes based on the photochemistry of BODIPY. By regulating
energy transfer involving the BODIPYs, they can monitor the bior-
eactions with the changes of emission intensity from the probe
with high sensitivity and specificity. Moreover, hetero ring-fused
In particular, thiophene-fused
BODIPYs (Fig. 1) were reported as a photosensitizer to generate
variety of optical materials in biochemistry and biology. For exam-
ple, for the construction of artificial photosynthesis systems, the
light-absorbing molecules which have a large molar extinct coeffi-
cient are a key material as an energy receiver to improve conver-
sion efficiency from light energy to the driving force of chemical
7
–9
BODIPYs have been synthesized.
1
0
singlet oxygen. By the modification with sulfur or iodine ele-
ments for receiving the heavy atom effect, the intersystem crossing
after photo-excitation to these BODIPYs can be readily induced.
Accordingly, the triplet-excited states of the BODIPYs efficiently
generate the singlet oxygen via a sensitizing reaction. The superior
ability of light absorbing contributes to enhancing the sensitizing
efficiency. It should be mentioned that these BODIPYs have sharp
spectra of light-absorption and emission from the red-light to the
near-infrared region. Based on these optical properties of thio-
phene-fused BODIPYs, we were inspired to develop an efficient
light absorber without emission. In particular, we aimed to develop
new series of thiophene-fused BODIPYs to improve the light-
absorption ability in the red-light region.
Herein, we report the synthesis and the efficient light-absorbing
ability of the new series of thiophene-fused BODIPYs. The substitu-
ent effects of two distinct groups on the optical properties were
examined to improve molar extinct coefficients and the peak posi-
tion of absorption wavelengths. By employing the heavy atom ef-
fect of iodine, the evolution of the optical properties such as the
position of absorption bands, the enhancement of absorption abil-
ity, and the suppression of the emission can be simultaneously
achieved. Next, according to the data of the molecular orbital of
the dye calculated by computer simulation, we were inspired the
introduction of the trifluoromethyl group into the thiophene-fused
1
reactions. As another instance, the light-absorbing ability with
the specific wavelength region is feasible for the regulation of pho-
tosynthesis. It is known as the Emerson enhancement effect that
the photosynthesis in several types of algae can be enhanced by
2
,3
the light irradiation in the wavelength range around 650 nm.
In contrast, the light over 690 nm can suppress the efficiency.
These phenomena were observed in other plants.4 This fact sug-
gests the scenario that by loading the efficient light absorbers to
the light around 650 nm on the wall of green houses, the sunlight
can be transformed to the light which has the suppression effect on
weed growths. Thus, the lights with the specific wavelength region
around 650 nm or over 690 nm are a valid tool for precisely con-
trolling the plant growth.
Boron dipyrromethene (BODIPY) is well known as a fluorophore
for biochemical and biotechnological usages.5 Because of strong
emission intensity, low environmental dependency of optical prop-
erties and high stability to photo-degradation, BODIPY can be used
as a conventional fluorophore for a marker or a labeling reagent
under various situations. Nagano and co-workers reported the
⇑
Corresponding author. Tel.: +81 75 383 2604; fax: +81 75 383 2605.
E-mail address: chujo@chujo.synchem.kyoto-u.ac.jp (Y. Chujo).
0
968-0896/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmc.2013.03.050

2
716
K. Tanaka et al. / Bioorg. Med. Chem. 21 (2013) 2715–2719
R3
B
orthoformate (36.3 g, 245 mmol) was added. After stirring at
0 °C for 30 min, excess amounts of cyclopentyl methyl ether
CPME) and satd NaHCO aq were poured into the reaction solu-
tion. The organic layer was washed with brine and water, dried
over MgSO , filtrated and condensed by evaporation. The silica
H C
CH3
3
5
(
3
S
S
N
F
N
F
R1
R2
4
gel column chromatography with the mixture eluent (hex-
ane:ethyl acetate = 2:1) afforded 4 as a brown solid (7.11 g,
43.0 mmol, 70%). Because of low stability, the compound 4 was
used for the next step immediately after checking the spectrum
TB: R =R =R =H
1
2
3
TB-I: R =I, R =R =H
1
2
3
TB-I2: R =R =I, R =H
1
2
3
TB-F: R =R =H, R =CF
1
2
3
3
1
1
3
of H NMR. H NMR (CDCl ) d: 9.75 (1H, s), 9.10 (1H, s), 7.44 (1H,
Figure 1. Chemical structures of the thiophene-fused BODIPYs used in this study.
d, J = 12.43 Hz), 6.94 (1H, d, J = 11.70 Hz), 2.54 (3H, s). HRMS
+
(
EI+) m/z calcd [M] : 165.0248, found: 165.0244.
BODIPY. Finally, the selective light absorption in the red-light re-
gion was accomplished.
2
.4. TB (5)
2
. Experimental section
To the solution of 4 (7.11 g, 43.0 mmol) in dichloromethane
(
3
215 mL), POCl (7.92 g, 51.7 mmol) was added dropwise at 0 °C.
1H (400 MHz), 13C (100 MHz), and 11B (128 MHz) NMR spectra
After stirring at room temperature for 3 days in the dark, triethyl-
amine (30.0 mL, 215 mmol) was added dropwise at 0 °C. After stir-
ring at 0 °C for 15 min, BF
dropwise, and then the mixture was stirred at room temperature
for 2 days. The reaction was quenched by adding 100 mL of water,
and the products were extracted with dichloromethane. The organ-
1
were recorded on JEOL JNM-EX400 spectrometers. In the H NMR
1
3
and C NMR spectra was used tetramethylsilane (TMS) as an inter-
nal standard in CDCl
, and 11B NMR spectra were referenced exter-
nally to BF (sealed capillary). UV–vis absorption spectra were
ÁOEt
3 2
ÁEt O (38 mL, 308 mmol) was added
3
3
2
recorded on a SHIMADZU UV-3600 spectrophotometer. Fluores-
cence emission spectra and absolute quantum yields were
recorded on a HORIBA JOBIN YVON Fluoromax-4P spectrofluorom-
eter equipped with the integrating sphere. 2-Acetyl-3-bromothio-
phene (1) was purchased from Aldrich and used without further
purification. Computations were performed using the GAUSSIAN 03
4
ic layer was washed with water twice and brine, dried over MgSO ,
filtrated, and condensed by evaporation. The residue was passed
through the silica gel column with the mixture eluent (hex-
ane:ethyl acetate = 3:1). The resulting product was dissolved in
THF, and 5 was obtained as a red purple solid (0.684 g, 2.07 mmol,
10%) from the reprecipitation in methanol. H NMR (CDCl
1
1
1
suite of programs.
3
) d: 7.64
(
2H, d, J = 10.26 Hz), 7.40 (1H, s), 7.11 (2H, d, J = 9.53 Hz), 2.47 (6H,
1
3
2
.1. Ethyl 6-methyl-4H-thieno[3,2-b]pyrrole-5-carboxylate (2)
3
s). C NMR (CDCl ) d: 158.5, 140.9, 139.9, 132.1, 131.8, 123.6,
1
1
1
+
3
14.0, 11.0. B NMR (CDCl ) d: 0.39. HRMS (EI+) m/z calcd [M] :
The synthesis was performed according to the previous re-
332.0425, found: 332.0425.
1
2
port:
to the mixture containing 1 (8.62 g, 42.0 mmol), CuI
(34.2 g, 105 mmol) in DMSO
42 mL), ethyl isocyanoacetate (5.23 g, 46.2 mmol) was added
(
(
0.80 g, 4.20 mmol), and CsCO
3
2.5. TB-I (6) and TB-I2 (7)
dropwise at room temperature. After stirring at 50 °C for 4 h, the
products were extracted with dichloromethane. The organic layer
was washed with brine twice, dried over MgSO and filtrated.
4
The filtrate was condensed with evaporation, and the silica gel col-
umn chromatography with hexane as a mixture eluent (hex-
The mixture containing 5 (0.312 g, 0.934 mmol) and N-iodosuc-
cinimide (0.210 g, 0.934 mmol) in chloroform (140 mL) and acetic
acid (14 mL) was stirred at room temperature for 24 h in the dark
under Ar atomosphere. Then, the reaction solution was washed
with NaOH aq (2.5 M, 200 mL) twice, water, and brine, dried over
ane:ethyl acetate = 7:1) gave 2 as a pale yellow solid (5.35 g,
4
MgSO , and filtrated. After evaporation, the products were sus-
1
2
3
5.6 mmol, 61%). H NMR (CDCl ) d: 8.88 (1H, s), 7.30 (1H, d,
pended onto silica gel. TB-I (0.137 g, 32%) and TB-I2 (0.129 g,
23%) were purified with the silica gel colum chromatography with
toluene as an eluent as a green silver and a black powder, respec-
tively. Because of poor solubility of TB-I in conventional organic
J = 10.51 Hz), 6.91 (1H, d, J = 10.02 Hz), 4.37 (2H, q, J = 28.10 Hz),
2
1
.53 (3H, s), 1.40 (3H, t, J = 24.19 Hz). 13C NMR (CDCl
39.6, 129.1, 126.6, 123.2, 120.3, 111.42, 60.5, 14.7, 12.3. HRMS
3
) d: 162.3,
+
13
(
ESI) m/z calcd [M+H] : 210.0583, found: 210.0582.
solvents, the clear spectrum of C NMR was not obtained. TB-I:
1
3
H NMR (CDCl ) d: 7.68 (1H, d, J = 12.70 Hz), 7.42 (1H, s), 7.40
11
2
.2. 6-Methyl-4H-thieno[3,2-b]pyrrole-5-carboxylic acid (3)
The mixture containing 2 (6.00 g, 28.7 mmol) and NaOH aq
(1H, s), 7.09 (1H, d, J = 14.16 Hz), 2.43 (3H, s), 2.41 (3H, s).
NMR (CDCl
457.9404. TB-I2: H NMR (THF-d
(6H, s). C NMR (CDCl
95.4, 9.7. B NMR (CDCl
582.8285, found: 582.8292.
B
+
3
) d: 0.293. HRMS (EI+) m/z calcd [M] : 457.9391, found:
1
8
) d: 7.85 (1H, s), 7.39 (2H, s), 2.42
1
3
(
17.2 g, 430 mmol in 120 mL) in 225 mL of ethanol was refluxed
3
) d: 157.3, 139.7, 135.6, 130.7, 125.6, 123.6,
1
1
À
for 1 h. After cooling to room temperature, concd HCl (10%) was
added dropwise to acidify. The products were extracted with
dichloromethane, and the organic layer was washed with brine,
3
) d: 0.195. HRMS (ESI) m/z calcd [MÀH] :
dried over MgSO
as a dark purple solid (4.96 g, 27.3 mmol, 95%). H NMR
DMSO-d d: 12.44 (1H, s), 11.51 (1H, s), 7.46 (1H, d,
J = 10.72 Hz), 6.92 (1H, d, J = 11.21 Hz), 2.40 (3H, s). C NMR
4
and filtrated. Evaporation of the filtrate yielded
2.6. TB-F (8)
1
3
(
6
)
The mixture of 3 (1.0 g, 5.52 mmol) in trifluoroacetic acid
(50 mL) was stirred at 40 °C for 40 min under Ar atomosphere,
and then trifluoroacetic anhydride (2.29 mL, 16.6 mmol) was
added dropwise to the reaction solution. After stirring at 80 °C
for 1 h and cooling to room temperature, the deep blue solution
was poured into the mixture with water (100 mL) and toluene
13
(
DMSO-d
3
) d: 162.8, 139.8, 128.5, 124.6, 123.1, 118.0, 112.0, 11.8.
À
HRMS (ESI) m/z calcd [MÀH] : 180.0125, found: 180.0118.
2
.3. 6-Methyl-4H-thieno[3,2-b]pyrrole-5-carbaldehyde (4)
(
100 mL). After neutralization by adding NaHCO
were extracted with toluene. The organic layer was washed with
satd NaHCO aq, water, and brine, dried over MgSO , and filtrated.
3
, the products
The solution of 3 (11.1 g, 61.2 mmol) dissolved in trifluoroacetic
acid (200 mL) was stirred at 50 °C for 20 min, and then triethyl
3
4

K. Tanaka et al. / Bioorg. Med. Chem. 21 (2013) 2715–2719
2717
After evaporartion, the crude products containing the aldehyde
were used for the next step without further purification because
of low stability. The residue was dissolved in 50 mL of toluene,
and triethylamine (1.54 mL, 22.07 mmol) was added dropwise to
the solution. After stirring at room temperature for 15 min,
BF
3
ÁEt
2
O (2.72 mL, 22.1 mmol) was added dropwise. After stirring
at room temperature for 10 min, the mixture was heated at 80 °C
and stirred for 2 h. After quenching the reaction by adding water,
the product was extracted with toluene. The organic layer was
4
washed with brine twice, dried over MgSO , and filtrated. After
evaporation, the silica gel column chromatography with the mix-
ture eluent (toluene:ethyl acetate = 9:1) was performed. The
resulting solid was dissolved in THF, and TB-F was obtained from
the precipitation in methanol as a glossy green solid (7.2 mg,
1
0
.7%). H NMR (CDCl
3
) d: 7.67 (2H, d, J = 16.57 Hz), 7.05 (2H, d,
1
3
J = 16.32 Hz).
C NMR (CDCl ) d: 159.1, 143.8, 138.1, 136.5,
3
11
1
3
33.4, 128.6, 122.3 (q, J = 275 Hz), 114.5, 15.4. B NMR (CDCl )
d: 0.195. HRMS (EI+) m/z calcd [M] : 400.0299, found: 400.0282.
Figure 2. UV–vis absorption (10
lM) and photoluminescence spectra (1
l
M) of TB
+
in THF. The wavelength of the excitation light was at 530 nm.
3
. Results and discussion
Table 1
Optical properties of thiophene-fused BODIPYs
We designed the thiophene-fused BODIPYs as shown in Figure
À1
À1
a
1. The synthesis of TB derivatives is outlined in Scheme 1. Until
k
max,abs (nm)
e
(M cm
)
U
F
the preparation of the precursors before the ligand formation, all
reactions proceeded in good yields. Because of the instability of
the aldehyde 4 and the ligand moiety before the introduction of
TB
562
577
592
623
127,000
170,000
184,000
72,200
0.04
TB-I
TB-I2
TB-F
<0.01
<0.01
<0.01
boron, the reaction yields in the formation of boron complexes
were relatively low. From the characterization with 1H and
11
B
a
Determined as an absolute value.
NMR spectroscopies and mass measurements, the products have
the expected structures. The dyes were obtained as colored solids
with the solubility in conventional organic solvents such as chloro-
form and THF. During the measurements, the decomposition and
photo-degradation were subtly observed.
F
suppressed (U = 0.04). The low emission intensity of TB should be
originated from the fact that the decay of the excited state of TB
should proceed via the triplet-excited state. Indeed, new emission
bands with long lifetimes at the similar peak position and in the
longer wavelength region than that of the emission at room tem-
perature at 77 K assigned as a delayed fluorescence and phospho-
rescence, respectively. These data clearly indicate that TB is a
promising seed compound for realizing expected optical properties
such as sharp spectra, large molar extinct coefficients and low
emissions.
The optical properties of TB were investigated. The absorption
band was observed with the peak at 562 nm from UV–vis absorp-
tion measurement (Fig. 2). The molar extinct coefficient exceeded
5
À1
À1
1
0 M cm (Table 1). These data mean that TB possesses the
sharp and large absorption in the visible region. From the photolu-
minescence spectrum, although the emission band was obtained
with the peak at 571 nm, the quantum yield of TB can be efficiently
O
CH3
CH3
CH3
S
a
S
b
S
c
S
CH3
OEt
OH
N
N
N
CHO
Br
O
e
O
1
2
3
4
H C
CH3
H C
CH3
H C
CH3
3
3
3
d
and
S
S
S
S
S
S
N
F
N
F
N
F
N
F
N
F
N
B
B
B
F
I
I
I
5
, TB
6, TB-I
7, TB-I2
CF3
H C
CH3
3
f,g
3
S
S
N
F
N
F
B
8, TB-F
Scheme 1. Synthetic outlines for thiophene-fused BODIPYs. Reagents and conditions: (a) ethyl cyanoacetate, CuI, Cs
reflux, 1 h, 95%; (c) (i) trifluoroacetic acid, 50 °C, 20 min, (ii) CH(OEt) 50 °C, 30 min, 70%; (d) (i) POCl , dichloromethane, rt, 3 days, (ii) triethylamine, BF
e) N-iodosuccinimide, acetic acid, chloroform, rt, 24 h, 32% for TB-I, 23% for TB-I2; (f) (i) trifluoroacetic acid, 40 °C, 40 min, (ii) trifluoroacetic anhydride, 80 °C, 1 h; (g)
BF O, triethylamine, toluene, 80 °C, 2 h, 0.7% (in two steps).
ÁEt
2
CO
3
2
, DMSO, 50 °C, 4 h, 61%; (b) NaOH, H O, ethanol,
3
3
3
2
ÁEt O, rt, 2 days, 10%;
(
3
2

2
718
K. Tanaka et al. / Bioorg. Med. Chem. 21 (2013) 2715–2719
dine into BODIPYs is valid for regulating the peak positions of
absorption.
To explore another method for the modulation of the peak posi-
tion of absorption wavelength, initially we carried out the density
functional theory (DFT) calculation of TB with a B3LYP/SVP meth-
od. According to the computer modeling on the shape of the lowest
unoccupied molecular orbital (LUMO) of TB, the localization of the
orbital was found at the meso position (Fig. 4). Based on this result,
we designed the new dye, TB-F, having a trifluoromethyl group.
The strong ability of electron withdrawing can lower the energy le-
vel of LUMO and decrease the gap width of the energy level be-
tween the frontier orbitals. Thus, it can be expected that drastic
bathochromic shift of the absorption band should be received.
The synthesis of TB-F was executed with similar procedures as
other TB derivatives. Because of low stability of the intermediate
before boron complexation, the reaction yield in the formation of
the BODIPY ring was low. The desired compound was obtained
as a solid, and it was confirmed that TB-F has enough solubility
in conventional organic solvents such as chloroform and THF for
the optical measurements. During the measurements, less decom-
position and photo-degradation were also observed with TB-F.
Figure 5 presents the absorption spectrum of TB-F. As we ex-
pected, TB-F showed the large absorption band at longer wave-
length region by +60 nm than that of TB (kmax = 623 nm).
Similarly as TB, the emission band was mainly obtained between
Figure 3. Absorption changes of TB by introducing iodine groups. The spectra were
obtained from the solutions containing the dyes (10 M) in THF.
l
Next, for tuning the optical properties of TB, an iodide group
was introduced via the heavy atom effect. To maximize the expres-
sion of the heavy atom effect, iodide groups were directly attached
to the thiophene rings.13 Figure 3 clearly represents that the
absorption bands of TB-Is showed the bathochromic shifts than
that of TB. By introducing a single iodide group, the peak positions
were moved by 15 nm to the red-light region. These data can be
supported by the results from computer calculations. The gap
widths of the energy levels between the frontier orbitals were nar-
rowed by increasing the number of the iodine substituents (Fig. 4).
Moreover, the molar extinct coefficients were enhanced by
increasing the number of iodide groups. According to the previous
report on the photophysical properties of iodinated BODIPY deriv-
5
50 nm and 650 nm. This absorption wavelength is adequate for
3
generating the light which can inhibit the photosynthesis. Fur-
thermore, the emission was subtly obtained from TB-F (
0.01). These data suggest that the efficient light absorbers for
U
F
<
the red light can be prepared based on the thiophene-fused BODI-
PYs. In addition, these optical properties suggest another possibil-
ity for using the thiophene-fused BODIPYs as
a quencher.
Comparing to the conventional efficient absorbers called as QSY
1
4
atives, these changes could be owing to the heavy atom effect. In-
deed, the iodinated TB derivatives hardly showed emissions (
0.01). The heavy atom effect of iodine could contribute to inhibit-
ing the fluorescence emission. In summary, the introduction of io-
À1
À1 15
À1
À1 16
(
e
max = 90,000 cm
the thiophene-fused BODIPYs have several advantages as
quencher besides of the intrinsic merits of BODIPY dyes. It should
M
)
and BHQ (
e
max = 35,600 cm
M ),
U
F
a
<
Figure 4. The changes in energy levels and shapes of frontier orbitals by introducing a trifluoromethyl group into the meso position in TB. The DFT calculations were
performed with a B3LYP/SVP method.

K. Tanaka et al. / Bioorg. Med. Chem. 21 (2013) 2715–2719
2719
Scientific Research on Innovative Areas ‘New Polymeric Materials
Based on Element-Blocks (No. 2401)’ (24102013) of The Ministry
of Education, Culture, Sports, Science, and Technology, Japan.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmc.2013.03.050.
References and notes
1
.
.
Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Chem. Rev. 2010, 110,
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2
4
3.
4.
5.
Dring, M. J.; Lüning, K. Mar. Biol. 1985, 87, 109.
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Leen, V.; Dehaen, W. Chem. Soc. Rev. 2012, 41, 1130.
Figure 5. Absorption changes of TB by introducing a trifluoromethyl group. The
spectra were obtained from the solutions containing the dyes (10 M) in THF.
l
6.
(a) Kobayashi, T.; Komatsu, T.; Kamiya, M.; Campos, C.; González-Gaitán, M.;
Terai, T.; Hanaoka, K.; Nagano, T.; Urano, Y. J. Am. Chem. Soc. 2012, 134, 11153;
(
b) Komatsu, T.; Oushiki, D.; Takeda, A.; Miyamura, M.; Ueno, T.; Terai, T.;
be emphasized that the molar extinct coefficients of the thiophene-
fused BODIPYs are two or three times larger than those of the con-
Hanaoka, K.; Urano, Y.; Mineno, T.; Nagano, T. Chem. Commun. 2011, 47, 10055;
(c) Komatsu, T.; Urano, Y.; Fujikawa, Y.; Kobayashi, T.; Kojima, H.; Terai, T.;
Hanaoka, K.; Nagano, T. Chem. Commun. 2009, 7015.
1
5,16
ventional absorbers.
Thus, we can say that the thiophene-fused
7
8
9
.
.
.
Umezawa, K.; Nakamura, Y.; Makino, H.; Citterio, D.; Suzuki, J. J. Am. Chem. Soc.
BODIPYs are promised to be an efficient quencher on the biotech-
2
008, 130, 1550.
Umezawa, K.; Matsui, A.; Nakamura, Y.; Citterio, D.; Suzuki, K. Chem. Eur. J.
009, 15, 1096.
1
7
nological assays.
. Conclusions
We demonstrate the validity of the TB skeleton for the design of
2
(a) Banfi, S.; Caruso, E.; Zaza, S.; Mancini, M.; Gariboldi, M. B.; Monti, E. J.
Photochem. Photobiol., B 2012, 114, 52; (b) Chen, Y.; Zhao, J.; Xie, L.; Guo, H.; Li,
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4
an efficient light absorber. We established two manners for evolv-
ing the optical properties of TB: by employing the heavy atom ef-
fect, the peak positions can be shifted to the red-light region. The
enhancement of molar extinct coefficients was also obtained. It
was found that the introduction of the strong electron-withdraw-
ing group at the meso position in the BODIPY skeleton was respon-
sible for the drastic bathochromic shift in the absorption spectrum.
Finally, we obtained the series of efficient light absorbers for the
red light. These compounds have suitable optical properties for
generating the light to control photosynthesis and plant growth.
Furthermore, thiophene-fused BODIPYs with the efficient
light-absorbing ability are promised to be applicable for efficient
sensitizers. Our materials and chemical modification methods for
modulating the optical properties presented here could be versatile
for developing efficient photo-responsive bio-related materials to
control the biological activities and efficient quenchers on the bio-
technological assays with labelled biomolecules.
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Acknowledgements
This work was partially supported by ‘The Adaptable and Seam-
less Technology Transfer Program’ through target-driven R&D, Ja-
pan Science and Technology Agency (JST) and a Grant-in-Aid for
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