Fluorescent carbazolylurea anion receptors

Article abstract of DOI:10.1039/b900178f

A series of fluorescent carbazolylurea base anion receptors have been synthesised that show a high affinity for oxo-anions (particularly bicarbonate and acetate). The fluorescence of dicarbazolylurea (1) is quenched upon addition of benzoate anions in DMS

Full text of DOI:10.1039/b900178f

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Volume 7 | Number 9 | 7 May 2009 | Pages 1737–1988
ISSN 1477-0520
COMMUNICATION
Jennifer R. Hiscock et al.
Fluorescent carbazolylurea anion
receptors
1477-0520(2009)7:9;1-6

COMMUNICATION
www.rsc.org/obc | Organic & Biomolecular Chemistry
Fluorescent carbazolylurea anion receptors†‡
Jennifer R. Hiscock,a Claudia Caltagirone,b Mark E. Light,a Michael B. Hursthousea and Philip A. Gale*a
Received 6th January 2009, Accepted 11th February 2009
First published as an Advance Article on the web 20th February 2009
DOI: 10.1039/b900178f
A series of fluorescent carbazolylurea base anion receptors
have been synthesised that show a high affinity for oxo-anions
(particularly bicarbonate and acetate). The fluorescence of
dicarbazolylurea (1) is quenched upon addition of benzoate
anions in DMSO–0.5% water.
Our interest in anion complexation1 has led us to investigate
compounds that are easy to make and yet have high affinities and
selectivities for anionic guests.2 Work from the groups of Jeong,3
Jurczak,4 Beer,5 Sessler,6 Pfeffer7 and ourselves8 has shown that
indoles, carbazoles, indolocarbazoles and biindoles are effective
components of hydrogen bond donating anion receptors.9 In
a recent collaboration with Albrecht, we found that 2,7-disub-
stituted indoles were effective receptors for carboxylates.10 Build-
ing on this work, we discovered that diindolylurea compounds
1
Anion complexation studies were performed using H NMR
titration techniques in DMSO-d6–0.5% water with stability con-
stants obtained from the titration curves using the EQNMR
computer program.14 The results (see Table 1) show that all the
compounds bind acetate strongly. NMR titrations with dihydro-
gen phosphate show strong binding but the titration curves for
compounds 1 and 2 show features at 1 and 2 equivalents of anion
that cannot be fitted adequately to a binding model (see ESI).
Titrations with fluoride caused broadening of the NH resonances
in all cases, and thus it was not possible to determine an accurate
stability constant. Shifts of the NH resonances suggest weak
complex formation (i.e. a continuous downfield shift – see ESI for
compounds 2 and 3). Binding studies were also conducted with
tetraethylammonium bicarbonate. Strong binding was observed
in all cases with stability constants >104 M-1.
have very high affinities for oxoanions and selectively bind
dihydrogen phosphate in DMSO–water mixtures.11 In this com-
munication we report the synthesis and anion binding prop-
erties of related carbazolylureas (1–3). In addition to forming
strong complexes with oxoanionic guests, the fluorescence of
these easy-to-make receptors is perturbed in the presence of
anions, allowing them to be employed as fluorescent anion
sensors.12
1-Nitrocarbazole was synthesised via a modification to a
literature procedure (see ESI‡).13 This was then reduced with
hydrogen over 10% Pd/C to afford the 1-aminocarbazole. This
compound was then dissolved in a mixture of dichloromethane
and saturated aqueous NaHCO3, and one equivalent of triphos-
gene added to afford the isocyanate in quantitative yield. This
compound proved to be very reactive and hence was used
immediately. Compounds 1 and 2 were synthesised in 41% and
26% respective yields by addition of a solution of the isocyanate in
dichloromethane to a solution of 1-aminocarbazole or 7-amino-
indole and triethylamine in chloroform. Compound 3 was pre-
pared in 69% yield by addition of a solution of phenyliso-
cyanate in dichloromethane to a solution of 1-aminocarbazole in
dichloromethane.
Crystals of the acetate complex of compound 1 were grown
by slow evaporation of a DMSO solution of the receptor in the
presence of excess tetrabutylammonium acetate.§ The structure
(shown in Fig. 1) reveals that the receptor binds acetate via
˚
four NH ◊ ◊ ◊ O hydrogen bonds (N1 ◊ ◊ ◊ O2 2.781(2) A; N2 ◊ ◊ ◊ O2
Table 1 The stability constants (M-1) of compounds 1, 2 and 3 with
a variety of anionic guests (added as tetrabutylammonium salts except
bicarbonate which was added as a tetraethylammonium salt) at 298 K
in DMSO-d6–0.5% water as determined by following urea NH resonance
adjacent to the carbazole. In all cases 1 : 1 receptor: anion stoichiometry
was observed. Errors estimated to be no more than 15%
aSchool of Chemistry, University of Southampton, Southampton, SO17 1BJ,
UK. E-mail: philip.gale@soton.ac.uk; Fax: +44 (0)23 8059 6805; Tel: +44
(0)23 8059 3332
Anion
Compound 1
>104
Compound 2 Compound 3
bDipartimento di Chimica Inorganica ed Analitica, Universita` degli Studi di
Cagliari, S.S. 554 Bivio per Sestu, 09042, Monserrato, CA, Italy. E-mail:
ccaltagirone@unica.it; Fax: +39 0706754456; Tel: +39 0706754452
Acetate
Benzoate
Dihydrogen phosphate
Chloride
Bicarbonate
>104
>104
3420
6140
85
5670
5880
† Dedicated to Professor Roeland Nolte on the occasion of his 65th
birthday.
‡ Electronic supplementary information (ESI) available: Experimental
details, 1H and 13C NMR spectra, NMR titration curves and fluores-
cence spectra. CCDC reference numbers 715669 and 715670. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b900178f
a
a
102
139
>104
>104
>104
a NMR titration curve could not be fitted to a 1 : 1 or 1 : 2 receptor:anion
binding stoichimetry but indicates strong binding.
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©

In order to investigate the ability of receptors 1–3 to act as
luminescent sensors, fluorescence studies were performed in a
DMSO–0.5% water mixture. Under these conditions receptor 1
shows an intense fluorescence emission (U = 0.549) with maxima
at 363 nm and 376 nm when excited at 270 nm. As shown in Fig. 3
upon addition of increasing amounts of tetrabutylammonium
benzoate, a selective quenching of the fluorescent emission was
observed (Ires = 10%).
Fig. 1 Two views of the X-ray crystal structure of the acetate complex
of receptor 1. The tetrabutylammonium counter-cation and selected
hydrogen atoms have been omitted for clarity.
˚
˚
˚
2.809(2) A; N3 ◊ ◊ ◊ O3 2.926(2) A; N4 ◊ ◊ ◊ O3 2.820(2) A). One of
the carbazoles is in the plane of the urea group whilst the other
twists out of plane by 39.9◦.
Crystals of the benzoate complex of compound 1 were grown
by slow evaporation of a DMSO solution of the receptor in
the presence of excess tetrabutylammonium benzoate.¶ The unit
cell contains two crystallographically distinct benzoate complexes
which adopt similar conformations. One example is shown in
Fig. 2. As with the acetate complex, in both benzoate complexes
one of the carbazoles is in the plane of the urea whilst the other
is twisted out of plane by 37.1◦ or 43.0◦. Each benzoate anion
is bound by four hydrogen bonds in the range N ◊ ◊ ◊ O 2.762(5)–
Fig. 3 Fluorescence quenching of receptor 1 in DMSO–0.5% water upon
addition of increasing amounts of tetrabutylammonium benzoate.
Upon addition of the tetrabutylammonium salts of acetate,
fluoride chloride and dihydrogen phosphate and the tetraethylam-
monium salt of bicarbonate, less dramatic effects were observed
(Fig. 4). In fact, only acetate caused a partial quenching of the
emission (Ires = 50%) while a stronger quenching upon addition
of fluoride was observed but only when an excess of anion was
added (more than three equivalents), which may be indicative of
deprotonation.15 A slight perturbation of the fluorescence emis-
sion of 1 was observed in the presence of dihydrogen phosphate
(Ires = 68%) and bicarbonate (Ires = 73%). Chloride did not effect
the emission of the system. The trend in fluorescence quenching
˚
2.996(5) A.
1
is similar to that found for the stability constants by H-NMR
titrations with the exception for benzoate. This may be due to a
p–p interaction in the excited state between the aromatic electrons
of the carbazole moieties and the substrate.
Fig. 2 Two views of the X-ray crystal structure of one of the two
crystallographically distinct benzoate complexes present in the unit cell of
the tetrabutylammonium benzoate complex of receptor 1. Tetrabutylam-
monium counter-cations and selected hydrogen atoms have been omitted
for clarity.
Fig. 4 Effect of increasing anion concentration upon the relative fluores-
cence emission of receptor 1 in DMSO–0.5% water.
1782 | Org. Biomol. Chem., 2009, 7, 1781–1783
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©

Similar but less pronounced quenching effects caused by
benzoate were observed with receptor 2 (U = 0.562 and 3 (U =
0.501) (see ESI). Thus, under these conditions receptor 1 acts as a
selective ON–OFF fluorescent sensor for benzoate.
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Compounds 1–3 are easy-to-make analogues of the diindoly-
lureas, binding acetate and bicarbonate strongly. The fluorescence
of compound 1 is selectively quenched by benzoate. We are con-
tinuing to explore the anion complexation and sensing properties
of indole and carbazole ureas, and the results of these studies will
be reported in due course.
5 M. J. Chmielewski, L. Zhao, A. Brown, D. Curiel, M. R. Sambrook, A.
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Acknowledgements
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P.A.G. thanks the EPSRC for a DTA studentship (JRH).
C.C. would like to thank the Italian Ministero dell’Istruzione,
dell’Universita` e della Ricerca Scientifica (MIUR) for financial
support (Project PRIN-2007C8RW53).
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Notes and references
§ Crystal data were collected on a Bruker Nonius KappaCCD with a Mo
rotating anode generator; standard procedures were followed. Crystal data
for compound 1·tetrabutylammonium acetate: C43H57N5O3, M = 691.94,
T = 120(2) K, monoclinic, space group P21/n, a = 11.0933(2), b =
◦
3
˚
˚
26.7833(6), c = 13.2013(3) A, b = 98.0560(10) , V = 3883.60(14) A ,
rc = 1.183 g cm -3, m = 0.075 mm-1, Z = 4, reflections collected: 40088,
independent reflections: 8914 (Rint = 0.0815), final R indices [I > 2sI]:
R1 = 0.0674, wR2 = 0.1354, R indices (all data): R1 = 0.1274. wR2
=
0.1613.
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◦
◦
˚
10.980(5), b = 13.774(5), c = 15.173(5) A, a = 68.760(5) , b = 85.232(5) ,
◦
3
-3
-1
˚
g = 81.541(5) , V = 2114.5(14) A , rc = 1.184 g cm , m = 0.074 mm ,
Z = 2, reflections collected: 44837, independent reflections: 9728 (Rint
=
0.1052), final R indices [I > 2sI]: R1 = 0.0661, wR2 = 0.1368, R indices
(all data): R1 = 0.1190. wR2 = 0.1589.
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The Royal Society of Chemistry 2009
Org. Biomol. Chem., 2009, 7, 1781–1783 | 1783
©