A BODIPY - Borane dyad for the selective complexation of cyanide ion

Article abstract of DOI:10.1021/om701237t

The coupling of borane as a donor and BODIPY as an acceptor leads to a boron-based receptor (3) that shows a 3-fold enhancement in fluorescence response for the selective sensing of cyanide ion by virtue of intramolecular energytransfer transitions.

Full text of DOI:10.1021/om701237t

1022
Organometallics 2008, 27, 1022–1025
A BODIPY-Borane Dyad for the Selective Complexation of
Cyanide Ion
Jung Oh Huh, Youngkyu Do,* and Min Hyung Lee*
Department of Chemistry, School of Molecular Science-BK21 and Center for Molecular Design and
Synthesis, KAIST, Daejeon 305-701, Republic of Korea
ReceiVed December 11, 2007
overcome by use of ammonium borane.9 Because of the
Summary: The coupling of borane as a donor and BODIPY as
an acceptor leads to a boron-based receptor (3) that shows a
3-fold enhancement in fluorescence response for the selectiVe
sensing of cyanide ion by Virtue of intramolecular energy-
transfer transitions.
requirement of high sensitivity for cyanide detection, the
enhanced signal transduction from the binding event should
further be considered for the borane-based receptors that usually
utilize a direct change in the absorption and fluorescence
intensity at the boron center. An effective method to achieve
this would exploit energy-transfer transitions to signal a binding
event: for example, using a highly fluorescent donor–acceptor
system. Although recent examples have described such energy-
transfer transitions for fluoride sensing,10 little is known for
cyanide sensing and the three-coordinate boron center has been
regarded as an electron/energy acceptor. The reverse donor–ac-
ceptor systems in which the excitation energy from borane
activates a well-defined, highly fluorescent acceptor transition
are thus quite intriguing and could provide versatile methods
to design a novel sensor scheme.
Cyanide ion sensing is of great interest because of the high
toxicity of cyanide ion in physiological systems,1 as well as
environmental concerns arising from the widespread industrial
uses of cyanide.2 Among various analytical methods, optical
sensors have recently attracted considerable attention, owing
to their simple, inexpensive, and fast detection of cyanide by
monitoring a change in color and/or fluorescence intensity as
the result of a binding event. Although recently reported
chemosensors based on organic3 and organometallic4 com-
pounds achieve high selectivity and sensitivity for the detection
of cyanide, it is still worthwhile to search for a highly selective
system that can operate at very low concentrations of cyanide,
such as occur in physiologically relevant systems.
Cyanide sensors based on organoboron receptors have rarely
been investigated, in spite of the strong B-CN bonding nature.
Several organoboron compounds5–7 showing high affinity for
cyanide are known, but they are often incompatible with the
presence of other anions, fluoride in particular, which readily
binds to the boron center.5,6,8 Very recently, an elegant study
demonstrated that cyanide selectivity over fluoride can be
In this report, we combined three-coordinate borane as a
donor and highly fluorescent BODIPY as an acceptor to
construct a discrete and highly fluorescent borane-based receptor
that utilizes intramolecular excitation energy-transfer transitions
for the selective detection of cyanide ion.
A Suzuki-type coupling reaction between a BODIPY deriva-
tive (1)11 and a boronic acid bearing a borane moiety (2)12
afforded the desired Lewis acid 3 as an orange-red solid in 51%
1
yield (Scheme 1). While the H and 13C NMR spectra show
the expected resonances corresponding to the BODIPY and
triarylborane moieties, two 11B NMR signals detected at δ 76.8
and 0.68 ppm confirm the presence of both base-free trigonal-
planar and four-coordinate boron centers, respectively, in 3. The
latter 11B nucleus is coupled to the fluorine atoms, giving rise
to a sharp triplet (1JB-F ) 33.0 Hz) comparable to those found
in the known BODIPY derivatives.13,14 These features are
* To whom correspondence should be addressed. E-mail: ykdo@kaist.ac.kr
(Y.D.); lmh74@kaist.ac.kr (M.H.L.).
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10.1021/om701237t CCC: $40.75
2008 American Chemical Society
Publication on Web 02/20/2008

Communications
Organometallics, Vol. 27, No. 6, 2008 1023
Figure 1. Crystal structures of 3 (left) and [3-CN]- in [NEt4][3-CN] (right, molecule A only) (50% probability ellipsoids). The [NEt4]+,
solvent molecules, and H atoms are omitted for clarity.
Scheme 1
clearly established from the crystal structure of 3, which exhibits
a trigonal-planar geometry (∑(C-B-C) ) 359.9°) around the
boron atom of the borane moiety and four-coordination of the
boron atom by the fluorine and nitrogen atoms in the BODIPY
group (Figure 1, left). The dihedral angle of 78.2° between the
BODIPY ring fragment and the adjacent phenylene ring reveals
a highly twisted conformation between the two rings. In contrast,
the biphenylene bridging unit forms a phenylene-phenylene
dihedral angle of 4.1(1)°, showing its considerable planarity,
which further extends to the trigonal boron plane.
for molecule A and 80.3° for molecule B), as similarly observed
in 3, the biphenylene bridging unit shows an apparent distortion
(
dihedral ) 32.1(3)° for molecule A and 33.1(3)° for molecule
B), possibly due to the disruption of the extended conjugation
after cyanide complexation.
To examine the cyanide binding properties of 3, UV–vis and
PL experiments were carried out. Compound 3 features two
major absorption bands at 330 nm (log ꢀ ) 4.49) and 501 nm
(log ꢀ ) 4.92) in THF assignable to the dominant π-pπ(B)
transition in the borane6,9,18 and the π-π* transition in the
BODIPY19 moiety, respectively (Figure 2).
Compound 3 is readily converted into the cyanide adduct
[3-CN]- when treated with Et4NCN in CH2Cl2 (Scheme 1). In
addition to the triplet 11B NMR signal detected at δ 0.43 ppm
(1JB-F ) 33.5 Hz) which is almost identical with that observed
in the neutral 3, the signal at δ -13.6 ppm confirms the
existence of an additional four-coordinate boron center,9,15,16
indicating the binding of cyanide to the borane moiety. To
elucidate this binding nature, the crystal structure of [3-CN]-
was determined from an X-ray diffraction study. The asymmetric
unit contains two independent molecules, denoted as A and B,
that have very similar structures. As shown in Figure 1 (right),
the cyanide group is coordinated to the boron center of the
borane moiety through the B-CN linkage. The B(1)-CN bond
lengths of 1.622(9) Å (molecule A) and 1.620(9) Å (molecule
B), which are comparable to those observed in triarylcyanobo-
rates9,16,17 and slightly shorter than those of B(1)-Caryl bonds
(1.64–1.68 Å), indicate the presence of the usual polar covalent
B-CN linkage. While the BODIPY ring fragment is almost
From a comparison with the absorption spectra of the closely
related mononuclear compounds 1 and Mes2B(p-biphenyl) (4),
one can note that the wavelengths of the absorption maxima
(λmax) of the two fluorophores in 3 show almost no change, thus
indicating the presence of little electronic communication
between the two moieties (Figure S3 in the Supporting Informa-
tion). The orthogonal arrangement of the BODIPY and the
adjacent phenylene ring fragments is probably responsible for
this feature.13,20,21 Addition of cyanide to a THF solution of 3
leads to a decrease in the intensity of the absorption band of
the borane moiety at 330 nm, while the band of the BODIPY
fragment at 501 nm remains unaffected throughout the titrations
(Figure 2). This result indicates not only the formation of the
1:1 complex [3-CN]- with a stability constant that exceeds 107
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16565. (b) Melaïmi, M.; Solé, S.; Chiu, C.-W.; Wang, H.; Gabbaï, F. P.
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Commun. 2004, 1284–1285. (d) Yamaguchi, S.; Akiyama, S.; Tamao, K.
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perpendicular to the adjacent phenylene ring (
) 89.9°
dihedral
(14) Ulrich, G.; Ziessel, R. J. Org. Chem. 2004, 69, 2070–2083.
(15) Hannant, M. H.; Wright, J. A.; Lancaster, S. J.; Hughes, D. L.;
Horton, P. N.; Bochmann, M. Dalton Trans. 2006, 2415–2426.
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Green, J. C.; Schilling, R. E.; Anderson, G. D. W.; Rees, L. H. Dalton
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1024 Organometallics, Vol. 27, No. 6, 2008
Communications
moiety (Φ ) 0.32, λex ) 330 nm) and by direct excitation in
the BODIPY (Φ ) 0.56, λex ) 480 nm), respectively, indicates
that intramolecular energy transfer occurs with 57% efficiency
for 3. This result is also in good agreement with the overlapping
feature between the excitation spectrum recorded for the
BODIPY emission and the absorption band ascribable to the
borane moiety (Figure S9 in the Supporting Information).13
Upon addition of cyanide, the emission intensity of 3 at 510
nm gradually decreases and finally reaches the residual emission
derived from [3-CN]- (Figure 3, right). This result can be
correlated to the decrease in energy transfer from the borane
moiety to the BODIPY group as cyanide binds to the boron
center. Moreover, the quantum yields of 3 (Φ ) 0.32) and
[3-CN]- (Φ ) 0.30) measured in THF with 330 nm excitation
are very similar and, thus, it can be suggested that the additional
quenching effect accompanying the formation of [3-CN]- is
negligible in the observed emission. A comparison with the same
emission quenching experiments using 4 reveals a 3-fold
enhancement effect in 3 (Figure 3, left).
Figure 2. Spectral changes in the UV–vis absorption of a solution
of 3 in THF (1.0 × 10-5 M) upon addition of Bu4NCN (0–15 µM).
The inset shows the absorbance at 330 nm as a function of [CN-]
(9). The line corresponds to the binding isotherm calculated with
K ) 5 × 107 M-1
.
The presence of 10 equiv of tetrabutylammonium salts of
-
other anions such as Cl-, Br-, I-, OAc-, NO3-, H2PO4
,
SCN-, HSO4-, and ClO4 does not significantly affect the
UV–vis spectrum and the emission band of 3 in THF (Figure
S10 in the Supporting Information), indicating high selectivity
for cyanide. These results also reflect the high stability of the
BODIPY moiety toward such anions. In sharp contrast, the
addition of fluoride gives rise not only to a spectral change of
absorption with a slow disappearance of the band at 501 nm but
also to a color change from greenish to colorless, indicating
decomposition of the BODIPY moiety. The change of a triplet
11B signal into a singlet signal in the presence of excess fluoride
is in good agreement with this observation (Figure S12 in the
Supporting Information).23 Furthermore, the emission color of a
solution of 3 in the presence of fluoride is more vividly differenti-
ated from that of the cyanide-containing solution (Figure S11 in
the Supporting Information). To obtain a more detailed insight into
this feature, UV–vis titrations with incremental amounts of fluoride
ion were carried out. The results show that 3 also has a strong
affinity for fluoride which, however, induces decomposition of the
BODIPY moiety over time (Figure S6 in the Supporting Informa-
tion). Although 3 responds to a small amount of fluoride ion, these
results could further provide a distinction for the detection of
cyanide over fluoride. Finally, by taking advantage of the high
fluorescence and strong binding affinity of 3, the emission response
at a very low concentration of cyanide was investigated using a
0.1 µM solution of 3. The fluorescence intensity change indicates
that 3 remains highly sensitive even at a submicromolar level of
cyanide (Figure S13 in the Supporting Information).
-
Figure 3. Comparison of the fluorescence response of solutions of
3 (right) and 4 (left) in THF (λex 330 nm; 1.0 × 10-6 M) upon
addition of Bu4NCN (0–2 µM).
M-1 but also the invariant absorption feature of the BODIPY
in 3 and [3-CN]- originating from the similar, nonplanar
arrangement of the BODIPY moieties in both compounds.
The independent absorption characteristics of the two fluo-
rophores in 3 thus provide a basis for the utilization of 3 as a
donor–acceptor pair for intramolecular energy-transfer transi-
tions. The emission spectrum of 3 irradiated at 330 nm
exclusively exhibits a very intense green fluorescence (λem 510
nm; Figure 3, right) while that of 4 at the same excitation gives
a blue fluorescence (λem 394 nm; Figure 3, left).
This result implies an energy transfer from the borane moiety
induced by 330 nm irradiation to the BODIPY fragment, where
subsequent emission occurs. The observation of the apparent
emission bands ascribable to 1 and 4, respectively, from the
solution of an equimolar mixture (λex ) 330 nm; Figure S7 in
the Supporting Information) clarifies such an intramolecular
energy-transfer transition phenomenon in 3.13,20,22 A comparison
of the quantum yields obtained by irradiation in the borane
In conclusion, the coupling of borane as a donor and BODIPY
as an acceptor leads to an unprecedented boron-based cyanide ion
receptor (3) that exploits discrete intramolecular energy-transfer
transitions. In addition to high selectivity for cyanide over physi-
ologically and environmentally relevant anions, the highly sensitive
fluorescence response allows the detection of an extremely low
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R. W.; Lindsey, J. S. J. Am. Chem. Soc. 1994, 116, 9759–9760.
(23) A similar decomposition of a BODIPY derivative by fluoride has
been reported. The authors suggested the possibilities of fluoride attack on
the B-N bond to form a B-F bond or degradation of the BODIPY ring
initiated by abstraction of methyl protons. See: Coskun, A.; Akkaya, E. U.
Tetrahedron Lett. 2004, 45, 4947–4949.
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R. J. Phys. Chem. A 2006, 110, 7994–8002. (b) Wan, C.-W.; Burghart, A.;
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Communications
Organometallics, Vol. 27, No. 6, 2008 1025
Supporting Information Available: Text, tables, figures, and
CIF files giving experimental details, X-ray crystallographic data,
and UV–vis and PL spectra. This material is available free of charge
via the Internet at http://pubs.acs.org.
level of cyanide. Studies on modifying the given system to make
it viable in an aqueous medium are in progress.
Acknowledgment. We gratefully acknowledge the Korea
Research Foundation (No. KRF-20070458000), the CMDS,
and the BK 21 project.
OM701237T