Highly water-soluble neutral BODIPY dyes with controllable fluorescence quantum yields

Article abstract of DOI:10.1021/ol102758z

A series of novel highly water-soluble neutral BODIPY dyes have been obtained by functionalization of BODIPY dyes with branched oligo (ethylene glycol)methyl ether groups at positions 8, 2 and 6 or 4 and 4'. Use of an ortho-substituent group of branched o

Full text of DOI:10.1021/ol102758z

ORGANIC
LETTERS
2011
Vol. 13, No. 3
438-441
Highly Water-Soluble Neutral BODIPY
Dyes with Controllable Fluorescence
Quantum Yields
Shilei Zhu,† Jingtuo Zhang,† Giri Vegesna,† Fen-Tair Luo,‡ Sarah A. Green,† and
Haiying Liu*,†
Department of Chemistry, Michigan Technological UniVersity,
Houghton, Michigan 49931, United States, and Institute of Chemistry,
Academia Sinica, Taipei, Taiwan 11529, Republic of China
hyliu@mtu.edu
Received November 13, 2010
ABSTRACT
A series of novel highly water-soluble neutral BODIPY dyes have been obtained by functionalization of BODIPY dyes with branched oligo(ethylene
glycol)methyl ether groups at positions 8, 2 and 6 or 4 and 4′. Use of an ortho-substituent group of branched oligo(ethylene glycol)methyl
ether on the meso-phenyl ring of BODIPY dyes and replacement of the fluorine atoms of BODIPY dyes at positions 4 and 4′ with methyloxy
or ethynyl subunits significantly enhance fluorescence quantum yields of BODIPY dyes.
BODIPY (4,4′-difluoro-4-bora-3a,4a-diaza-s-indacene) dyes
have gained a great deal of attention recently because of their
many distinctive and desirable properties such as high
extinction coefficients, narrow absorption and emission
bands, high quantum efficiencies of fluorescence, relative
insensitivity to environmental perturbations, and resistance
to photobleaching.1 Biological and medical applications of
the BODIPY dyes require good water solubility and resis-
tance to the formation of nonfluorescent dimer and higher
aggregates. Reported strategies to make these dyes water-
soluble typically involve introduction of oligo(ethylene
glycol), N,N-bis(2-hydroxyethyl) amine, carbohydrates, nu-
cleotides, or ionic hydrophilic groups such as carboxylic acid,
sulfonic acid, or ammonium groups to BODIPY dyes.2
However, neutral water-soluble BODIPY dyes have advan-
tages over ionic ones because they avoid potential nonspecific
interactions through electrostatic interactions between
BODIPY dyes and proteins or other biomolecules in biologi-
† Michigan Technological University.
‡ Academia Sinica.
(1) (a) Loudet, A.; Burgess, K. Chem. ReV. 2007, 107, 4891. (b) Ulrich,
G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed. 2008, 47, 1184.
10.1021/ol102758z  2011 American Chemical Society
Published on Web 12/22/2010

cal and medical applications. It is very important to develop
general approaches to significantly enhance the water
solubility of neutral BODIPY dyes with controlled fluores-
cence quantum yields.
derivative (7) with 2,4-dimethylpyrrole in the presence of a
catalytic amount of trifluoroacetic acid (TFA) and followed
by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ) and chelation with BF3-etherate in the presence of
N,N-diisopropylethylamine (DIPEA).4 BODIPY dye (A) is
highly water-soluble. Highly fluorescent E-BODIPY dye (B)
was also prepared by replacing the usual fluorine atoms of
BODIPY dye (A) with ethynyl tri(ethylene glycol)methyl
ether (8) via a Grignard reaction. In order to evaluate the
effect of functionalization of BODIPY dye (A) on water
solubility, we iodized BODIPY dye A at positions 2 and 6,
affording 2,6-diiodo BODIPY dye (C), and palladium-
catalyzed Sonogashira coupled 2,6-diiodo BODIPY dye (C)
with trimethylsilylacetylene, followed by deprotection of
trimethylsilyl groups in the presence of NaF, affording 2,6-
diethynyl BODIPY dye (D).4b,c Replacement of the usual
fluorine atoms of BODIPY dye A with methyloxy subunits
introduced some steric hindrance at the BODIPY core and
resulted in BODIPY dye with methyloxy subunits at positions
4,4′ (E). BODIPY dyes (B, C, D, and E) are highly water-
soluble, and further functionalization of BODIPY dye (A)
does not affect water solubility of new BODIPY dyes,
indicating that the strong hydrophilic character of branched
oligo(ethylene glycol)methyl ether residues considerably
enhances interactions of BODIPY dyes with water, thus
significantly increasing their water solubility.
We hypothesized that incorporation of branched oligo(et-
hylene glycol)methyl ether into BODIPY dyes could ef-
fectively enhance enthalpic interactions of BODIPY dyes
with water and significantly increase the water solubility of
BODIPY dyes, and that introduction of steric hindrance at
the meso, 4,4’-positions of BODIPY dyes could significantly
reduce their aggregation through π-π stacking interactions
between BOIDIPY cores in aqueous solution and consider-
ably enhance their fluorescence quantum yields. In this letter,
we have introduced branched oligo(ethylene glycol)methyl
ether to the meso, 2,6-, 4,4’-positions BODIPY dyes and
demonstrated significantly enhanced fluorescence quantum
yields of the new dyes (the abstract scheme). These neutral
BODIPY dyes are highly water-soluble because of the strong
hydrophilic nature of oligo(ethylene glycol)methyl ether
residues. These approaches offer very efficient ways to
prepare different BODIPY dyes with emission ranging from
green to deep red regions.
In order to demonstrate the feasibility of using branched
oligo(ethylene glycol)methyl ether to enhance the water
solubility of BODIPY dyes, we first introduced branched
oligo(ethylene glycol)methyl ether to BODIPY dyes at the
meso position (Scheme 1). Compound 4 was prepared
BODIPY dye A displays weak fluorescence in 0.5 M
phosphate buffer solution (PBS) (pH 7.4) with a fluorescence
quantum yield of only 4.2% while it becomes highly fluorescent
with fluorescence quantum yields of 68% and 61% in methylene
chloride and ethanol, respectively. The low fluorescence
quantum yield of BODIPY dye A may be attributed to self-
quenching because of the likely aggregation of the dye in
aqueous solution through π-π stacking or hydrophobic interac-
tions between BODIPY cores. In order to prove this hypothesis,
we replaced the usual fluorine atoms of BODIPY dye A with
ethynyl subunits to introduce steric hindrance to the BODIPY
core, resulting in BODIPY dye (B) with a significantly enhanced
fluorescence quantum yield of 35.7% in PBS solution. The side
chains of ethynyl tri(ethylene glycol)methyl ether in BODIPY
dye (B) provide steric hindrance and reduce the aggregation of
BODIPY dye in aqueous solution. Iodization of BODIPY dye
(A) at positions 2 and 6 results in 2,6-diiodo BODIPY dye (C),
Scheme 1
.
Synthetic Route to Highly Water-Soluble BODIPY
Dyes at the Meso Position
(2) (a) Atilgan, S.; Ozdemir, T.; Akkaya, E. U. Org. Lett. 2008, 10,
4065. (b) Jiao, L. J.; Li, J. L.; Zhang, S. Z.; Wei, C.; Hao, E. H. New
J. Chem. 2009, 33, 1888. (c) Yen, V. H.; Micouin, L.; Ronet, C.; Gachelin,
G.; Bonin, M. ChemBioChem 2003, 4, 27. (d) Giessler, K.; Griesser, H.;
Gohringer, D.; Sabirov, T.; Richert, C. Eur. J. Org. Chem. 2010, 3611. (e)
¨
Dilek, O.; Bane, S. L. Bioorg. Med. Chem. Lett. 2009, 19, 6911. (f) Dodani,
S. C.; He, Q.; Chang, C. J. J. Am. Chem. Soc. 2009, 131, 18020. (g) Niu,
S. L.; Ulrich, G.; Retailleau, P.; Harrowfield, J.; Ziessel, R. Tetrahedron
Lett. 2009, 50, 3840. (h) Niu, S. L.; Ulrich, G.; Ziessel, R.; Kiss, A.; Renard,
P. Y.; Romieu, A. Org. Lett. 2009, 11, 2049. (i) Li, L. L.; Han, J. Y.;
Nguyen, B.; Burgess, K. J. Org. Chem. 2008, 73, 1963.
according to a reported procedure3 and further brominated
with PBr3 in methylene chloride at 40 °C, affording bromi-
nated branched oligo(ethylene glycol)methyl ether (5). The
benzaldehyde derivative bearing branched oligo(ethylene
glycol)methyl ether residues (7) was prepared by reacting
3,4-dihydroxybenzaldehyde with compound 5 under basic
conditions. BODIPY dye substituted with branched oligo-
(ethylene glycol)methyl ether at the meso position (A) was
prepared through the condensation of the benzaldehyde
(3) Lee, M.; Jeong, Y. S.; Cho, B. K.; Oh, N. K.; Zin, W. C. Chem.sEur.
J. 2002, 8, 876.
(4) (a) Meng, G.; Velayudham, S.; Smith, A.; Luck, R.; Liu, H. Y.
Macromolecules 2009, 42, 1995. (b) Donuru, V. R.; Vegesna, G. K.;
Velayudham, S.; Meng, G.; Liu, H. Y. J. Polym. Sci., Part A: Polym. Chem.
2009, 47, 5354. (c) Donuru, V. R.; Vegesna, G. K.; Velayudham, S.; Green,
S.; Liu, H. Y. Chem. Mater. 2009, 21, 2130. (d) Donuru, V. R.; Zhu, S.;
Green, S.; Liu, H. Y. Polymer 2010, 51, 5359.
Org. Lett., Vol. 13, No. 3, 2011
439

which has an extremely low fluorescence quantum yield of 1.2%
in PBS solution because of efficient intersystem crossing
induced by the heavy atom effect of iodine.4a,b Replacement
of iodo groups of BODIPY dye (C) at positions 2 and 6
with ethynyl groups yielded 2,6-diethynyl BODIPY dye (D),
which shows a low fluorescence quantum yield of 1.9%,
attributed to potential dye aggregation with enhanced π-con-
jugation in aqueous solution. Replacement of the fluorine
atoms of BODIPY dye (D) with methyloxy subunits gave
BODIPY dye (E), which displays an enhanced fluorescence
quantum yield of 8.9% due to the slightly increased steric
hindrance of the methyloxy subunits on the BODIPY core.
We hypothesized that further introduction of a bulky group
to an ortho position on the meso-phenyl ring of BODIPY dyes
would additionally enhance fluorescence quantum yields of
BODIPY dyes through a greater disruption of π-π stacking
interactions between the BODIPY cores. In order to prove our
hypothesis, we prepared BODIPY dye F with one ortho-
substituent group of branched oligo(ethylene glycol)methyl ether
from the aldehyde 10 via a similar approach to that used to
prepare BODIPY dye A (Scheme 2). As we expected, BODIPY
methyl ether at the meso position with those at positions 3 and
5, suggested by a strong absorption peak at 383 nm, attributed
to phenyl units at 3,5-positions, the relatively weak absorption
of the BODIPY core at 507 nm, and a shoulder peak at 605
nm (please see Figure 40 in the Supporting Information).
In order to further show the versatility of these approaches,
we introduced branched oligo(ethylene glycol)methyl ether to
BODIPY dyes at 4,4’-, 2,6-positions. We introduced a tethered
spacer of tetra(ethylene glycol) to the branched oligo(ethylene
glycol)methyl ether in order to reduce spatial hindrance around
the BODIPY dye by reacting compound 4 with ethynyl-
functionalized tetra(ethylene glycol) tosylate (13) in dry THF
in the presence of sodium hydride. BODIPY dye (I) bearing
branched oligo(ethylene glycol)methyl ether residues at posi-
tions 4 and 4′ was prepared by replacing the usual fluorine atoms
of F-BODIPY dye (15) with ethynyl-functionalized oligo(eth-
ylene glycol)methyl ether (14) via a Grignard reaction. BODIPY
dye J bearing branched oligo(ethylene glycol)methyl ether
residues at the positions 2 and 6 was prepared by palladium-
catalyzed Sonogashira coupling of 2,6-diiodo BODIPY dye (16)
with ethynyl-functionalized branched oligo(ethylene glycol)m-
ethyl ether (14) (Scheme 3). Both BODIPY dyes I and J are
Scheme 2. Synthetic Route to BODIPY Dyes Bearing
Monostyryl and Distyryl Groups at Positions 3 and 5
Scheme 3. Synthetic Route to BODIPY Dyes Bearing Branched
Oligo(ethylene glycol)methyl Ether at 4,4’-, and 2,6-Positions
extremely soluble in water, and both are highly fluorescent in
aqueous solution. Thus, introduction of side chain serves the
dual purposes of aqueous solubility and inscreasing fluorescence
quantum yields by reducing aggregation. BODIPY dye I with
the side chains at the 4,4′-positions is considerably more
fluorescent in PBS solution (fluorescence quantum yield 68%)
than BODIPY dye J substituted at the 2,6-positions (fluores-
cence quantum yield 34%). So, substitution at the 4,4′-positions
is more effective at preventing aggregation than 2,6-substitution
(Table 1).
All BODIPY dyes (A-J) are easily soluble not only in water
but also in common solvents. The absorption properties of
BODIPY dye A in PBS solution are characterized by a strong
S0fS1 (π-π*) transition at 499 nm and a weaker broad band
around 350 nm attributed to the S0fS2 (π-π*) transition (Table
dye F is highly fluorescent with a fluorescence quantum yield
of 31.2% in PBS solution which we attribute to enhanced steric
hindrance of the bulky ortho-substituent group on the meso-
phenyl ring of BODIPY dye F. BODIPY dyes with extended
π-conjugation systems have strong hydrophobic features and
are typically insoluble in water. In order to further demonstrate
the feasibility of using branched oligo(ethylene glycol)methyl
ether to promote the water solubility of BODIPY dyes with an
extended π-conjugation system, we prepared BODIPY dyes
bearing monostyryl and distyryl groups at positions 3, 3 and 5
(G and H) by condensation of methyl substituents of BODIPY
dye F at positions 3, 3 and 5 with aldehyde derivative 11.4d
Both BODIPY dyes G and H are readily soluble in aqueous
solution and have fluorescence quantum yields of 21.2% and
1.3%, respectively. The unexpectedly low fluorescence quantum
yield of BODIPY dye H may arise from rotation of vinyl bonds
caused by interactions between flexible oligo(ethylene glycol)-
(5) Gabe, Y.; Urano, Y.; Kikuchi, K.; Kojima, H.; Nagano, T. J. Am.
Chem. Soc. 2004, 126, 3357.
440
Org. Lett., Vol. 13, No. 3, 2011

Table 1. Absorption and Emission Maxima, and Fluorescence Quantum Yields of BODIPY Dyes in Organic Solvents and 0.5 M
Phosphate Buffer Solution (PBS) (pH 7.4)a
BODIPY dye
solvent
absorption maximum (nm)
emission maximum (nm)
fluorescence quantum yield (%)
A
CH2Cl2
ethanol
PBS
CH2Cl2
PBS
CH2Cl2
PBS
CH2Cl2
ethanol
PBS
CH2Cl2
PBS
CH2Cl2
PBS
CH2Cl2
PBS
CH2Cl2
PBS
501
500
499
499
496
534
533
539
539
536
538
534
504
501
580
577
659
660
495
491
547
543
511
509
510
508
507
548
550
552
554
555
552
550
514
511
596
598
676
676
507
501
563
563
68.0
61.0
4.2
81.0
35.7
1.9
B
C
D
1.2
47.9
43.0
1.9
51.4
8.9
91.5
31.2
30.7
21.2
6.1
E
F
G
H
I
1.3
CH2Cl2
PBS
CH2Cl2
PBS
79.0
68.0
39
J
33.6
a Quantum yields of BODIPY dyes were determined by use of fluorescein (fluorescence quantum yield of 0.85 in 0.1 N NaOH) as a standard.5
1).1a Replacement of the usual fluorine atoms of BODIPY dye
A with ethynyl subunits at 4,4′-positions causes slight blue shifts
of BODIPY dye B in both absorption and emission compared
with its precursor BODIPY dye A.1b These shifts may arise
from decreased aggregation of BODIPY dye B in PBS solution
due to the bulky ethynyl substituents, which also results in the
observed increase in fluorescence quantum yield. Introduction
of 2,6-diiodo substituents to the BODIPY dye A to give
BODIPY C leads to large red shifts (33 and 38 nm) in both
the absorption and fluorescence maxima, respectively, as 2,6-
diiodo substituents function as auxochromes (Table 1). 2,6-
Diethynylation of 2,6-diiodo-tetramethyl BODIPY dye (C)
results in a slight red shift for BODIPY dye D due to the
enhanced conjugation compared with BODIPY dye A. Re-
placement of the usual fluorine atoms of BODIPY dye D with
methyloxy subunits at positions 4,4′ causes minor blue shifts
of BODIPY dye E in both absorption and emission compared
with its precursor BODIPY dye D (Table 1), which may arise
from less aggregation of BODIPY dye E due to slightly
enhanced steric hindrance from the methyloxy subunits. BO-
DIPY dye F in PBS solution displays absorption and emission
peaks at 501 and 511 nm, respectively. BODIPY dyes with
monostyryl and distyryl substituents at 3, 3 and 5 positions (G
and H) in PBS solution show significant red shifts in both
absorption and emission due to their extended π-conjugation
as compared with their precursor BODIPY dye (F). BODIPY
dyes G and H in aqueous solution show absorption maxima at
577 and 660 nm and fluorescence maxima at 598 and 676 nm,
respectively (Table 1 and Supporting Information). BODIPY
dye I in aqueous solution shows absorption and fluorescence
maxima at 491 and 501 nm, respectively. Absorption and
fluorescence spectra of BODIPY dye J in PBS solution are red-
shifted with absorption and fluorescence maxima at 543 and
563 nm, respectively, because of its extended π-conjugation
compared with those of its precursor BODIPY dye (15) in
methylene chloride.
In conclusion, we have developed effective ways to
prepare highly water-soluble neutral BODIPY dyes with
emissions from green to deep red regions through func-
tionalization of BODIPY dyes at positions 8, 2 and 6 or
4 and 4′ with branched oligo(ethylene glycol)methyl ether
residues. Fluorescence quantum yields of BODIPY dyes
can be manipulated by introducing branched oligo(ethyl-
ene glycol)methyl ether residues to an ortho position on
the meso-phenyl ring of BODIPY dyes, replacing the
fluorine atoms of BODIPY dyes at 4,4′-positions with
methyloxy or ethynyl subunits or attaching branched
oligo(ethylene glycol)methyl ether residues to positions
2 and 6 of BODIPY dyes.
Acknowledgment. H.Y. Liu acknowledges an NIH SBIR
grant (contract number: 1R43CA134039-01), the 21st Cen-
tury Jobs Fund of Michigan (Contract Number: 06-1-P1-
0283), and an NSF grant for financial support.
Supporting Information Available: Detailed synthesis,
characterization, and optical properties of BODIPY dyes.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL102758Z
Org. Lett., Vol. 13, No. 3, 2011
441