Highly sensitive sensory materials for fluoride ions based on the dithieno[3,2-b:2′,3′-d]phosphole system

Article abstract of DOI:10.1021/ol052911p

The newly developed functionalization of an unsubstituted dithieno[3,2-b:2′,3′-d]phosphole at the 5,5′-positions gives access to bis(pinacoleboryl) species that can be utilized as sensory materials for fluoride ions. The fluoride-triggered response of the air- and moisture-stable boryl-functionalized dithienophosphole oxide manifests itself in the generation of a new fluorescence emission that can be detected at very low analyte concentrations (ppm) or even with the naked eye upon irradiation with UV light (366 nm).

Full text of DOI:10.1021/ol052911p

ORGANIC
LETTERS
2006
Vol. 8, No. 3
495-497
Highly Sensitive Sensory Materials for
Fluoride Ions Based on the
Dithieno[3,2-b:2′,3′-d]phosphole System
Toni Neumann, Yvonne Dienes, and Thomas Baumgartner*
Institute of Inorganic Chemistry, RWTH-Aachen UniVersity, Landoltweg 1,
52074 Aachen, Germany
thomas.baumgartner@ac.rwth-aachen.de
Received December 1, 2005
ABSTRACT
The newly developed functionalization of an unsubstituted dithieno[3,2-b:2
′
,3′
-d]phosphole at the 5,5
′
-positions gives access to bis(pinacoleboryl)
species that can be utilized as sensory materials for fluoride ions. The fluoride-triggered response of the air- and moisture-stable boryl-
functionalized dithienophosphole oxide manifests itself in the generation of a new fluorescence emission that can be detected at very low
analyte concentrations (ppm) or even with the naked eye upon irradiation with UV light (366 nm).
A great deal of attention has recently been focused on the
selective sensing of anions by means of synthetic molecular
or polymeric sensors.1 In this context, the detection of
fluoride (F-) is of particular interest as it plays an essential
role in a broad range of biological, medical, and chemical
processes and applications such as dental care,2 treatment
of osteoporosis,3 fluorinaton of water supplies,4 or even in
chemical and nuclear warfare agents.5 As a result of this
diversity, it seems important to develop very sensitive and
selective methods to detect the fluoride ion under environ-
mental conditions. Popular approaches to access fluoride-
triggered responses involve hydrogen bonding with the
fluoride ion or strong Lewis acid-base interactions, such
as the strong affinity of electron-deficient boron compounds
toward fluoride.6-9 Over the past years, a variety of detection
methods have been utilized in these sensor materials includ-
ing electrochemical6 or colorimetric responses7,8 as well as
(1) (a) Beer, P. D.; Gale, P. A. Angew. Chem., Int. Ed. 2001, 40, 486.
(b) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. ReV. 2000, 100,
2537. (c) Bianchi, E.; Bowman-James, K.; Garcia-Espana, E. Supramo-
lecular Chemistry of Anions; Wiley-VCH: New York, 1997.
(2) Kirk, K. L. Biochemistry of the Halogens and Inorganic Halides;
Plenum Press: New York, 1991.
(3) (a) Briancon, D. ReV. Rheum. 1997, 64, 78. (b) Kleerekoper, M.
Endocrinol. Metab. Clin. North Am. 1998, 27, 441. (c) Kissa, E. Clin. Chem.
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(4) (a) ASTM International; Standard Test Methods for Fluoride Ions
in Water; D1179-04; DSTM: West Conshohocken, PA, 2005. (b)
Dreisbuch, R. H.; Handbook of Poisoning; Lange Medical Publishers: Los
Altos, CA, 1980.
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Soc. 2000, 122, 5399. (b) Zhang, S.-W.; Swager, T. M. J. Am. Chem. Soc.
2003, 125, 3420.
(6) (a) Nicolas, M.; Fabre, B.; Simonet, J. Chem. Commun. 1999, 1881.
(b) Aldridge, S.; Bresner, C.; Falls, I. A.; Coles, S. J.; Hursthouse, M. B.
Chem. Commun. 2002, 740.
(7) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2000,
122, 6793.
(8) (a) Yamaguchi, S.; Akiyama S.; Tamao, K. J. Am. Chem. Soc. 2001,
123, 11372. (b) Sole´, S.; Gabba¨ı, F. Chem. Commun. 2004, 1284. (c) Badr,
I. H. A.; Meyerhoff, M. E. J. Am. Chem. Soc. 2005, 127, 5318. (d) Cho, E.
J.; Ryu, J.; Lee, Y. J.; Nam, K. C. Org. Lett. 2005, 7, 2607.
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1365. (b) Anzenbacher, P., Jr.; Jursikova´, K.; Sessler, J. L. J. Am. Chem.
Soc. 2000, 122, 9350. (c) Arimori, S.; Davidson, M. G.; Fyles, T. M.;
Hibbert, T. G.; James, T. D.; Kociok-Ko¨hn, G. I. Chem. Commun. 2004,
1640, (d) Melaimi, M.; Gabba¨ı, F. J. Am. Chem. Soc. 2005, 127, 9680. (e)
Xu, S.; Chen, K.; Tian, H. J. Mater. Chem. 2005, 15, 2676.
10.1021/ol052911p CCC: $33.50
© 2006 American Chemical Society
Published on Web 01/04/2006

fluorescence changes.9 The latter, very convenient method,
is intriguingly attractive for sensing as it exhibits great
sensitivity at very low analyte concentrations down to the
micromolar scale,9b which is required for detection of
physiological fluoride3c as well as fluoride in drinking
water.4,8c However, most of the fluorescence-based sensory
materials employed rely on changes in the emission intensi-
ties that are less reliable than the detection of a new emission
at a different wavelength.9,10
δ ) 29.0 ppm. The blue luminescent compound 2 exhibits
pronounced optoelectronic properties with a maximum
wavelength of absorption at 376 nm, an emission at 419 nm,
and a very high photoluminescence (PL) quantum yield
efficiency of φPL ) 0.6113 that is typical for dithienophos-
pholes.12 Compared to the nonfunctionalized dithienophos-
phole 1 (λex ) 346 nm; λem ) 424 nm),12b the value for
absorption experiences a red shift of about 30 nm, supporting
the electron-accepting properties of the boryl groups, whereas
the emission is almost not affected.
An ideal sensory material for the fluoride ion at the parts
per million scale could therefore combine the strong affinity
of boron centers to fluoride with the generation of a new
fluorescence emission upon exposure. With respect to
stability under environmental conditions, the use of boronic
esters would be particularly favorable. Known fluorescence-
based sensor systems that utilize boron-fluoride interactions
are sensitive toward oxygen and moisture,8a,9d or they are
based on boronic acids, which involve complicated equilibria
through the formation of various fluoroborate species.10c
Boronic esters such as pinacole borane, on the other hand,
provide the necessary stability as well as selectivity toward
one fluoride ion only.11 The use of a π-conjugated material
with very favorable optoelectronic properties as central
fluorophore would also be very helpful for the successful
design of an appropriate sensor.7 In the context of our work
on materials for molecular electronics and optoelectronics,
we have established the novel dithieno[3,2-b:2′,3′-d]phos-
phole moiety that exhibits very promising photophysical
properties in terms of wavelength, intensity, and tunability.12
This system was expected to be an excellent fluorescent relay
for a corresponding boron-based sensory material. The
desired 5,5′-bis(pinacoleboryl) functionalized system can be
accessed by a newly developed general procedure via
lithiation of the unsubstituted dithienophosphole 1 with LDA
followed by the addition of 2 equiv of isopropoxy(pinacole)-
borane (iPrOB(pin)) at -78 °C in THF (Scheme 1). The
However, to gain the necessary stability at environmental
conditions, we targeted an oxidized phosphole species that
has been found to fulfill these requirements before.12 The
oxidized bis(boryl)dithienophosphole 3 is accessible in a
manner similar to that described for 2 followed by in situ
oxidation of the functionalized phosphole intermediate with
aqueous tert-butylhydroperoxide to give the air- and moisture-
stable product 3. Analysis by 31P NMR spectroscopy clearly
supports the oxidation of the dithienophosphole moiety with
a resonance at δ ) 16.5 ppm (δ 11B ) 28.5 ppm).
The fluorescence spectrum of the strongly blue luminesc-
ing 3 shows a maximum wavelength for absorption at λex
398 nm that is red shifted about 20 nm from the one for 2,
whereas the maximum wavelength for emission at λem
)
)
452 nm shows a red shift of about 30 nm supporting the
successful oxidation.12 The oxidized boryl-substituted species
3 also exhibits a very good PL quantum yield efficiency of
φPL ) 0.53.13
Treatment of a 1 × 10-4 M or 1 × 10-5 M undegassed
CH2Cl2 solution of 3 with a stoichiometric amount of
Bu4NF results in a significant red shift of the maximum
wavelengths for absorption and emission with λex ) 415 nm
and λem ) 485 nm, now displaying a blue-green emission
with a photoluminescence quantum yield of φPL ) 0.5513
(Figure 1, top). Owing to the formation of the ionic species
4 (δ 31P ) 17.9; δ 11B ) 5.8; δ 19F ) -132.2 ppm), the
intensities for absorption and emission, as observed by
fluorescence spectroscopy, drop to some extent but still
remain significantly strong. It is important to note that the
observed change in the fluorescence emission in a solution
of 3 toward the fluoro-substituted borate 4 can even be
detected, for the first time for B-F interactions,10c at the
micromolar (ppm) scale (see Supporting Information), which
is attributed to the extraordinary optoelectronic properties
of the dithienophosphole moiety. By contrast, addition of
Bu4NCl, Bu4NBr, or Bu4NI does not affect the fluorescence
properties of 3 at all, resulting in fluorescence spectra similar
to the one observed for genuine 3, supporting the high
selectivity of the boryl-functionalized dithienophosphole
toward fluoride ions. Slight differences from the spectrum
Scheme 1. Synthesis of Boryl-Functionalized
Dithieno[3,2-b:2′,3′-d]phospholes and Reaction with Fluoride
(10) (a) Kim, T.-H.; Swager, T. M. Angew. Chem., Int. Ed. 2003, 42,
4803. (b) Liu, B.; Tian, H. J. Mater. Chem. 2005, 15, 2681. (c) Badugu,
R.; Lakowicz, J. R.; Geddes, C. D. Sens. Actuators, B 2005, 104, 103.
(11) Aldridge, S.; Bresner, C.; Fallis, I. A.; Coles, S. J.; Hursthouse, M.
B. Chem Commun. 2002, 740.
(12) (a) Baumgartner, T.; Bergmans, W.; Ka´rpa´ti, T.; Neumann, T.;
Nieger, M.; Nyula´szi, L. Chem. Eur. J. 2005, 11, 4687. (b) Baumgartner,
T.; Neumann T.; Wirges, B. Angew. Chem., Int. Ed. 2004, 43, 6197.
(13) Relative to quinine sulfate (0.1 M H2SO4 solution). See: Demas,
N. J.; Crosby, G. A. J. Chem. Phys. 1971, 75, 991.
multinuclear NMR data support the successful functional-
ization of the dithienophosphole unit showing a 31P NMR
resonance at δ ) -24.3 ppm and a 11B NMR resonance at
496
Org. Lett., Vol. 8, No. 3, 2006

Figure 2. Absorption (left) and emission (right) spectra of 4 in
CH2Cl2 and water (c ≈ 1 × 10-4 M).
dependent response in water, although at neutral pH, such
as in drinking water or physiological fluids, the interaction
can be assumed to be minimal. In general, environmental
and physiological sensing applications do not experience
significant pH changes.10c
It should be mentioned in this context that the same
selectivity toward the fluoride ion is also observed with the
phosphole species 2. However, the fluorescence emission of
the corresponding bis(fluoroborate) still occurs in the blue
region of the optical spectrum (λex ) 377 nm; λem ) 454
nm) which is inferior to the blue-green emission observed
for 4 in terms of signal transduction; the latter even allows
for a naked-eye detection of fluoride ions when irridiated
with UV light.
Figure 1. Absorption (left) and emission (right) spectra of 3 (c ≈
1 × 10-4 M) in the presence of various halides (top) and emission
spectra with different fluoride concentrations added (bottom).
In conclusion, we have synthesized the first boryl-
substituted dithieno[3,2-b:2′,3′-d]phosphole derivatives. The
corresponding phosphole oxide 3 displays a highly sensitive,
selective sensory material for the fluoride ion. The novel,
phosphaorganic sensor combines the strong affinity of boron
centers toward fluoride with the generation of a new emission
that can be detected unambiguously in solution at room
temperature, even at very low concentrations (ppm) under
environmental conditions. Further investigations will include
the potential of dithienophosphole-based sensors, particularly
toward water soluble systems, and the utility of 2 and 3 as
building blocks for extended π-conjugated materials in
molecular electronics.
of 3 can be noticed in some decreased intensities for the
mixtures, particularly for Bu4NI, that can be explained by
the heaVy-atom effect of the iodide.
Furthermore, the addition of fluoride to the CH2Cl2
solution of 3 can be monitored ratiometrically by a gradual
red shift of the emission from λem ) 452 nm (no F-) to λem
) 485 nm (excess F-), potentially also allowing for a
quantitative determination of fluoride in dilute solutions by
using the dithienophosphole 3 (Figure 1, bottom). The
derivation of two binding constants (lgK1 ) 4.02(8); lgK2
) 3.79(8)) obtained from a fluorescence titration indicates
a stepwise addition of fluoride. This is further supported by
the 11B NMR data of 3×F with two resonances at δ ) 5.8
and 28.5 ppm indicating one borane and one borate moiety.
Although 3 is not water soluble itself, fluoride ions can
nevertheless also be detected in aqueous solutions, as the
fluoroborate 4 is soluble in water. Addition of solid 3 to
fluoride containing undegassed water results in a blue-green
emission of the resulting solution with features (λex ) 391
nm; λem ) 478 nm) similar to those observed in CH2Cl2
(Figure 2). The formation of 4 in water is further supported
by multinuclear NMR studies showing values comparable
to those in CH2Cl2 (see Supporting Information). The di-
thienophosphole sensor 3 is likely to also have a pH-
Acknowledgment. We thank Prof. Dr. J. Okuda for his
generous support of this work. Financial support from the
Fonds der Chemischen Industrie (FCI), the Deutsche For-
schungsgemeinschaft (DFG), and the Bundesministerium fu¨r
Bildung und Forschung (BMBF) is gratefully acknowledged.
Supporting Information Available: Experimental pro-
cedures for the preparation of 2 and 3; NMR data of 2, 3,
3×F, and 4; and fluorescence data of 4 under varying
conditions. This material is available free of charge via the
Internet at http://pubs.acs.org.
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