an important advance in BODIPY studies. The replace-
ments of fluoride in F-BODIPY with alcohol or phenol
about the effect of an electron-withdrawing carboxylate on
the boron atom in BODIPY. Herein we found that mono-
and di-AcO substituted BODIPYs exhibited excellent
fluorescence quantum yields and photostability. During
preparation of this manuscript, a stability study of BODIPY
5
affording alkoxy or aroxy BODIPYs, as well as B,
6
O-chelated BODIPYs, were also reported to be useful
strategies for adjusting fluorescence properties. F-BODIPY
dyes are generally quite hydrophobic. For biological and
medical applications, functionalized BODIPYs providing
good water solubility are preferred. We have been inter-
0
dyes indicated that a 4,4 -dichloroacetoxy analog of
0
BODIPY was formed when 4,4 -dimethoxy BODIPY was
mixed with an excess of dichloroacetic acid in methylene
7
10
chloride. However, no fluorescence study of the 4,
ested in novel BODIPY and aza-BODIPY dyes. Apart
0
from modifications on the pyrrole moiety with water
solubilizing functionalities, we are also interested in the
novel replacement on the boron atom. Although many
4 -dichloroacetoxy BODIPY was explored.
0
0
4-AcO-4 -F-BODIPY 1 and 4,4 -(AcO) -BODIPY 2
2
were successfully obtained by utilizing TMSOAc gener-
11
ated in situ from acetic acid and TMSCl (Scheme 1).
3ꢀ6
studies on boron substitution were documented,
the
new type of boron modification is highly desired. Herein
we wish to report novel BODIPY dyes with acetoxy (AcO)
substituent(s) on the boron center of BODIPY dyes.
It is well-known that the electronic nature of the sub-
stituents on the BODIPY core and boron atom affects
With 5 equiv of TMSOAc, 1 (12%) and 2 (13%) were
isolated. With 20 equiv of TMSOAc, 18% of 1 and 37% of
2 were obtained.
8
,9
the fluorescent properties. The reported replacement of
F-BODIPYs on the boron atom with an alkoxy or aroxy
group indicated that flexible alkoxy and electron-donating
aroxy groups significantly diminished the fluorescent
Scheme 1. Synthesis of BODIPY Analogs 1 and 2
5
c
quantum yields of BODIPYs. It is noteworthy that a
BODIPY dye with an electron-withdrawing aroxy group
4
g,5c
on the boron atom exhibited high fluorescent yield.
Inspired by the aforementioned reports, we are curious
Both compounds 1 and 2 are highly fluorescent. The
properties of 1 and 2 were evaluated in comparison with
(
4) (a) Kee, H. L.; Kirmaier, C.; Yu, L.; Thamyongkit, P.; Young-
blood, W. J.; Calder, M. E.; Ramos, L.; Noll, B. C.; Bocian, D. F.;
ꢀ
1
Scheidt, W. R.; Birge, R. R.; Lindsey, J. S.; Holten, D. J. Phys. Chem. B
005, 109, 20433. (b) Goze, C.; Ulrich, G.; Mallon, L. J.; Allen, B. D.;
Harriman, A.; Ziessel, R. J. Am. Chem. Soc. 2006, 128, 10231. (c) Ulrich,
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82. (d) Choi, S. H.; Pang, K.; Kim, K.; Churchill, D. G. Inorg. Chem.
TM-BDP (λ = 509 nm; λem = 516 nm; ε = 80 000 M
abs
2
ꢀ1
4a
cm ; Φ = 0.92 in CHCl ). The spectra of absorption
f
3
and fluorescence of 1 and 2 are shown in Figure 1. Both 1
and 2 absorb at 510 nm and emit at 517 nm, which are very
similar to those of the parent TM-BDP. Both 1 and 2
possess an identical Stokes shift to TM-BDP (7 nm).
Monosubstituted species 1 possesses an almost identical
9
2
007, 46, 10564. (e) Ulrich, G.; Goeb, S.; De Nicola, A.; Retailleau, P.;
Ziessel, R. Synlett 2007, 1517. (f) Li, L.; Nguyen, B.; Burgess, K. Bioorg.
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(
ꢀ
1
ꢀ1
extinction coefficient (ε = 73600 M cm ) to the parent
1
TM-BDP and a slightly higher quantum yield (Φ = 0.96),
f
ꢀ
1
ꢀ1
5
compared to disubstituted 2 (ε = 72 500 M cm
;
J. Org. Chem. 2011, 76, 4489. (l) Landrum, M.; Smertenko, A.; Edwards,
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f
tionality provides a narrower full width at half-maximum
(fwhm = 14 nm for 1; fwhm = 16 nm for 2) compared with
the parent TM-BDP (fwhm = 19 nm).
(5) (a) Kim, H.; Burghart, A.; Welch, M. B.; Reibenspies, J.; Burgess,
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moisture. When pH-dependence experiments were ex-
plored, we found that 1 and 2 were less degraded in basic
conditions than the parent TM-BDP; however, they were
more susceptible to lose boron and to generate dipyrrin in
acidic conditions (Figure 2).
The dyes 1 and 2 are more polar than the parent
TM-BDP and, thus, have improved water solubility. While
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2
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(
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(
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(
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1
(
(
(
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2
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Org. Lett., Vol. 14, No. 1, 2012
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