Charge-transfer emission in nonplanar three-coordinate organoboron compounds for fluorescent sensing of fluoride

Article abstract of DOI:10.1002/anie.200601286

A real turn-on: Two nonplanar three-coordinate boron compounds were synthesized as fluorescent sensors for fluoride that utilize charge-transfer emission. The fluorescence of one of the sensors turns on (see picture) and the other turns off in the presenc

Full text of DOI:10.1002/anie.200601286

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Sensors
produces intense solvent-dependent fluorescent emission
(e.g. lem = 492 nm, Fp = 0.67 in CH2Cl2) originating from the
charge transfer between the amino and the boron centers.[2k]
We have observed that the addition of fluoride to a solution of
BNPB causes fluorescent quenching due to the occupation of
the boron pp orbital by electrons from Fꢀ, which effectively
blocks intramolecular charge transfer, thus causing decrease
in emission intensity. Direct evidence for Fꢀ binding to the
boron center in BNPB comes from our 19F NMR study of a
(nBu4N)F solution (TBAF) titrated with BNPB: the spectra
revealed distinct 19F chemical shifts for bound and unbound
fluoride. We have also established that BNPB binds to
fluoride exclusively even in the presence of other halides such
as Clꢀ or Brꢀ, which do not cause a significant change in
fluorescence when added to the BNPB solution. This high
selectivity for Fꢀ by BNPB is consistent with previously
reported fluoride sensors based on three-coordinate organo-
boron compounds in which the boron center is protected by
substituent groups at the ortho positions.[3] Although BNPB is
potentially useful for selective detection of Fꢀ, it is a “turn-
off” sensor; that is, fluorescence is quenched in the presence
of fluoride. For practical applications, signal detection is more
effective in “turn-on” sensors. Even better are “turn-on”
sensors that display distinct color changes for fast and
efficient sensing.[3f] In our search for “turn-on” organoboron
sensors based on intramolecular charge-transfer emission, we
synthesized organoboron compound 1 using the procedure
shown in Scheme 1. The donor N(Ph)(1-naphthyl) and the
acceptor B(mesityl)2 in 1 are linked to two separate biphenyl
groups, which are further connected by a naphthalene unit.
Thus, the donor and the acceptor groups in 1 have a nonplanar
arrangement. Molecule 1 displays solvent-dependent fluores-
cence (lem = 504 nm, Fp = 0.10 in CH2Cl2), which is character-
istic of charge-transfer emission. Molecular orbital calcula-
tions (Gaussian03)[6] confirmed that the HOMO consists
indeed of contributions of the aminobiphenyl portion, and the
LUMOof the (mesityl) 2B(biphenyl) portion. Molecular
modeling studies show that the two biphenyl units in 1 are
approximately orthogonal to the naphthalene ring, and, as a
result, charge transfer from the amine to the boron unit most
likely occurs through space rather than through the aromatic
linker. In contrast to the behavior of BNPB, whose emission is
quenched upon addition of Fꢀ, the emission spectrum of a
solution of 1 shifts to a shorter wavelength (lmax = 453 nm, in
CH2Cl2) upon addition of TBAF and the emission intensity is
drastically enhanced (Figure 1). The color of the emission of 1
changes vividly from green to blue (Figure 1) after addition of
Fꢀ. Hence, compound 1 can be described as a “turn-on”
sensor for fluoride. A possible explanation for the color
change is the presence of dual fluorescent pathways in 1, that
is, charge-transfer emission between the N donor and the B
acceptor as well as p*!p emission localized on the N donor.
The binding of Fꢀ to the B center blocks the charge-transfer
transition, thus inhibiting the green emission, but the blue
emission of the N(Ph)(1-naphthyl)(biphenyl) portion is
activated simultaneously (Scheme 2). The closely related
molecules 4,4’-bis[(1-naphthyl)(phenyl)amino]biphenyl and
1,8-bis{4-[(1-naphthyl)(phenyl)amino]biphenyl-4’-yl}naph-
thalene[7] exhibit similar blue emission (see the Supporting
DOI: 10.1002/anie.200601286
Charge-Transfer Emission in Nonplanar Three-
Coordinate Organoboron Compounds for
Fluorescent Sensing of Fluoride**
Xiang Yang Liu, Dong Ren Bai, and Suning Wang*
Three-coordinate organoboron compounds possess an empty
pp orbital on the boron center and hence have a tendency to
display intense intramolecular charge-transfer transitions
when an appropriate electron donor is present. Such donor–
acceptor charge-transfer properties have enabled a number of
important applications of three-coordinate boron compounds
in materials chemistry such as nonlinear optical materials,[1]
charge-transport materials, and emitters in organic light
emitting devices (OLEDs).[2] Recently it has been demon-
strated by several research groups that three-coordinate
boron compounds can also be used as effective colorimetric,
fluorescent, or ratiometric sensors for the detection of
fluoride by utilizing the empty pp orbital on the boron
center.[3] Selective detection of fluoride is of current interest
because of their importance to human health, their impact on
the environment, and their association with nerve agents.[4,5]
Previously reported fluorescent sensors based on three-
coordinate boron compounds operate on the principle that
the binding of fluoride to the boron center disrupts or
perturbs the pp–p conjugation between the boron center and
the aromatic chromophore, thus inducing a change in
fluorescent signal.[3d] Although the donor–acceptor charge-
transfer properties of three-coordinate boron compounds
have been exploited extensively for applications in nonlinear
optics and OLEDs, surprisingly little investigation has been
done on their utility in fluorescent sensor applications. Our
preliminary investigation indicates that charge-transfer fluo-
rescence of three-coordinate boron compounds can be very
sensitive and selective for fluoride, and that “turn-on”
fluorescent sensors for fluoride can be made by manipulating
the geometry of the donor and acceptor in the molecule.
All previously reported three-coordinate boron com-
pounds that produce intense charge-transfer fluorescence
are planar conjugated systems,[1–3] an example of which is
BNPB, reported by us recently.[2k] BNPB has an N(Ph)(1-
naphthyl) donor and a B(mesityl)2 acceptor group that are
linked by a 4,4’-biphenyl group (see Scheme 2). BNPB
[*] Dr. X. Y. Liu, D. R. Bai, Prof. Dr. S. Wang
Department of Chemistry
Queen’s University
Kingston, Ontario, K7L3N6 (Canada)
Fax: (+1)613-533-6669
E-mail: wangs@chem.queensu.ca
[**] This work was supported by the Natural Sciences and Engineering
Research Council of Canada.
Supportinginformation for this article is available on the WWW
under http://www.angewandte.org or from the author.
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emission quantum yields of BNPB and 1
are most likely the consequence of the
linker between the donor and the
acceptor, namely, a linear aromatic linker
versus a nonlinear, nonconjugated linker.
To appreciate the importance of the
N(Ph)(1-naphthyl)(biphenyl) group in the
“turn-on” sensing of 1, diboron molecule 2
in which the N(Ph)(1-naphthyl) unit is
replaced by B(mesityl)2 was synthesized
(Scheme 1). Molecule 2 displays solvent-
dependent fluorescence (see the Support-
ing Information), which is consistent with
charge-transfer emission. 2 fluoresces at a
much shorter wavelength than 1 and has a
much higher quantum efficiency (e.g.
lem = 414 nm, Fp = 0.98 in CH2Cl2).
Molecular-orbital calculations (Gaus-
sian03) revealed that the HOMO of 2
consists predominantly of the naphthyl
ring whereas the LUMOis dominated by
the pp orbitals of the two boron centers
and the biphenyl units. The lowest elec-
tronic transition in 2 can be therefore
attributed to charge transfer between the
naphthyl ring and the two boron centers.
The response of 2 to fluoride is in sharp
contrast to that of 1: instead of fluorescent
enhancement and a color change, the
Scheme 1. Synthesis of fluorescent fluoride sensors 1 and 2.
addition of Fꢀ to a solution of 2 quenches
the emission from 2 (Figure 2). 2 is there-
fore a “turn-off” sensor for fluoride. The behavior of 2 in the
presence of Fꢀ resembles that of BNPB, which is not
surprising since in both molecules the donor and the acceptor
are connected through a linear aromatic linker (biphenyl).
The fluorescent quenching of 2 by Fꢀ is due to the occupation
of the empty pp orbital of boron by the Fꢀ ligand—the same
operating mechanism as in BNPB. The contrasting fluores-
cent properties of 2 and its response to fluoride establish
unequivocally that the aminobiphenyl unit in 1 is indeed
Figure 1. The emission spectra of 1 (7.4”10ꢀ6 m in CH2Cl2) upon
addition of TBAF.
Information), which supports the hypothesis that the
observed blue emission of Fꢀ-bound 1 is indeed from the
(phenyl)(1-naphthyl)aminobiphenyl unit. One possible factor
that is responsible for the distinct behavior of 1 and BNPB is
the difference in their fluorescent quantum efficiencies (0.10
versus 0.67). The emission quantum efficiency of the amino-
biphenyl unit in 1 and BNPB is likely between 0.10 and 0.67,
and, as a consequence, the quenching of the charge-transfer
emission by fluoride leads to an increase in emission intensity
for 1, but a decrease for BNPB. The drastically different
Scheme 2. Operatingprinciple of “turn-on” and “turn-off” sensors for
fluoride.
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or DMFas the solvent is the difficulty of recovering the sensor
molecules by using water, as these solvents are water-miscible.
When mixed solvents such as THFand H2Oor DMF and H 2O
are used for the fluoride titration experiments, no apparent
fluorescence change was observed; this observation can be
attributed to the formation of hydrogen bonds between
fluoride ions and water molecules, which compete with the
binding to the boron center.
In summary, we have demonstrated that a nonplanar
linker, such as 1,8-bis(4,4’-biphenyl)naphthalene, can be used
effectively to bring either two electron acceptor groups (e.g.
diaryl boron groups), or an electron donor and an acceptor
(e.g. a diaryl amino group and a diaryl boron group) together
to form molecules that are stable in water and display distinct
fluorescent responses to fluoride. By linking a nitrogen donor
and a boron acceptor group in a nonplanar arrangement (as in
1), it is possible to produce through-space donor–acceptor
charge-transfer emission that can be switched off by the
addition of fluoride, which in turn activates the fluorescence
from the donor chromophore, thus producing a sensitive and
selective “turn-on” fluorescent sensor for fluoride. Energy-
transfer systems involving three-coordinate boron com-
pounds that exploit dual signaling pathways for the detection
of fluoride ions have been reported previously.[3f] Compound
1 demonstrates that dual signal pathways in nonplanar three-
coordinate boron compounds can also be modulated by
intramolecular charge transfer and exploited effectively for
the detection of anions such as fluoride.
Figure 2. The emission spectra of 2 (9.1”10ꢀ6 m in CH2Cl2) upon the
addition of TBAF.
responsible for the fluorescent color change and intensity
enhancement of 1 induced by Fꢀ.
The binding of fluoride to 1 and 2 was verified by titration
experiments with 19F NMR spectroscopy. The addition of 1 or
2 to a solution containing excess TBAF results in two distinct
19F chemical shifts, which correspond to bound and unbound
Fꢀ ions, respectively. Furthermore, the NMR data indicated
that 1 forms a 1:1 complex with Fꢀ whereas 2 forms a 1:2
complex. SolØ and Gabbai demonstrated that two unsaturated
boron groups attached directly to the 1 and 8 positions of
naphthalene can capture an Fꢀ ion, which acts as a bridging
ligand and results in a complex with an exceptionally large Experimental Section
binding constant.[3g] We did not observe any evidence of
bridging Fꢀ ligands in the NMR experiments with 2.
Molecular-modeling studies and geometry optimization with
the Gaussian03 program suite showed that the two boron
centers in 2 are approximately 10 Š apart, which is too far for
an Fꢀ bridge. The binding constants of 1 and 2 with fluoride
were determined from the fluorescence titration data (see the
Supporting Information for details) to be approximately 4.0 ”
104 mꢀ1 and 9.0 ” 108 mꢀ2, respectively, which are comparable
to previously reported three-coordinate monoboron com-
pounds.[3] (The binding constant of 2 is seemingly large
because it is the overall binding constant for two fluoride
ions.) The fluorescence spectra of 1 and 2 (Figures 1 and 2)
change significantly upon addition of around two equivalents
of Fꢀ ions. This corresponds to a detection limit for Fꢀof
between 0.015 and 0.020 mm under the experimental con-
ditions used; thus, 1 and 2 are very sensitive to Fꢀ. Moreover,
they do not respond to chloride, bromide, or iodide ions, they
can be recovered fully from the titration solution, and the
original color and intensity of their fluorescence can be
restored fully by adding water to extract the fluoride ions (for
organic solvents such as CH2Cl2 that are not miscible with
water; see the Supporting Information). The use of water to
reverse fluoride binding to three-coordinate boron molecules
Excitation and emission spectra were recorded on
a Photon
Technologies International QuantaMaster Model C-60 spectrometer.
Fluorescence titrations were carried out by adding stock solutions of
TBAF in CH2Cl2 to the solutions of 1 or 2 in CH2Cl2. Binding
constants were obtained by using the fitting methods described by
Connors.[8] Elemental analyses were performed by Canadian Micro-
analytical Service Ltd., Delta, British Columbia, Canada. The
Gaussian03 program suite[6] was used for all molecular-geometry
optimization and molecular-orbital calculations. The calculations
were carried out at the B3LYP level of theory with 6-311G** as the
basis set. 1,8-Diiodonaphthalene[9] and 4-iodo-4’-(1-naphthylphenyl-
amino)biphenyl[10] were synthesized by modified literature methods.
The experimental and synthetic details are provided in the Supporting
Information.
Received: April 1, 2006
Published online: July 19, 2006
Keywords: boron · charge transfer · fluorescence · fluorides ·
.
sensors
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