Fluorescent carbazolylurea- and carbazolylthiourea-based anion receptors and sensors

Article abstract of DOI:10.1080/10610271003637087

A series of carbazolylurea- and carbazolylthiourea-based receptors have been synthesised and their anion complexation and fluorescence properties were studied. The urea compounds show selectivity for oxo-anion complexation over chloride and the fluorescence of compound 1 is selectively quenched by benzoate anions in DMSO/0.5% water. However, the thiourea compounds show reduced anion affinities compared to the urea analogues.

Full text of DOI:10.1080/10610271003637087

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Fluorescent carbazolylurea- and carbazolylthiourea-
based anion receptors and sensors
Jennifer R. Hiscock a , Philip A. Gale a , Claudia Caltagirone b , Michael B. Hursthouse a &
Mark E. Light a
a School of Chemistry , University of Southampton , Southampton, SO17 1BJ, UK
b Dipartimento di Chimica Inorganica ed Analitica , Universita' degli Studi di Cagliari , S.S.
554 Bivio per Sestu, Monserrato (CA), Italy
Published online: 07 Jun 2010.
To cite this article: Jennifer R. Hiscock , Philip A. Gale , Claudia Caltagirone , Michael B. Hursthouse & Mark E. Light (2010)
Fluorescent carbazolylurea- and carbazolylthiourea-based anion receptors and sensors, Supramolecular Chemistry, 22:11-12,
647-652, DOI: 10.1080/10610271003637087
To link to this article: http://dx.doi.org/10.1080/10610271003637087
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Supramolecular Chemistry
Vol. 22, Nos. 11–12, November–December 2010, 647–652
Fluorescent carbazolylurea- and carbazolylthiourea-based anion receptors and sensors
Jennifer R. Hiscocka, Philip A. Galea*, Claudia Caltagironeb, Michael B. Hursthousea and Mark E. Lighta
b
aSchool of Chemistry, University of Southampton, Southampton SO17 1BJ, UK; Dipartimento di Chimica Inorganica ed Analitica,
Universita’ degli Studi di Cagliari, S.S. 554 Bivio per Sestu, Monserrato (CA), Italy
(Received 6 November 2009; final version received 20 December 2009)
A series of carbazolylurea- and carbazolylthiourea-based receptors have been synthesised and their anion complexation and
fluorescence properties were studied. The urea compounds show selectivity for oxo-anion complexation over chloride and
the fluorescence of compound 1 is selectively quenched by benzoate anions in DMSO/0.5% water. However, the thiourea
compounds show reduced anion affinities compared to the urea analogues.
Keywords: anion binding; indole; hydrogen bonding; crystallography
Introduction
pendant additional hydrogen bond donor groups, the pKa
of the anion is reduced to such an extent that it can be
deprotonated by dihydrogen phosphate free in solution (6).
We wished to study diindolylurea analogues and
synthesised a series of carbazole urea and thiourea
compounds and studied their anion complexation and
fluorescence properties. Aspects of this work have been
communicated previously (7).
Interest in new hydrogen bond donor receptors for anionic
species has led our group and others to begin to explore
indole, carbazole, indolocarbazoles and biindoles as
components of new anion complexation agents (1). We
have recently reported that indole-appended isophthala-
mides and pyridine-2,6-dicarboxamides function as
selective fluoride binding agents (2). Our interest in urea
O
S
N
H
N
H
N
H
N
H
NH
HN
HN
NH
HN
HN
1
4
O
O
S
S
N
H
N
H
N
H
N
H
2
5
NH
NH
NH
NH
3
6
N
H
N
H
N
H
N
H
(3) as a receptor for oxo-anions led us to synthesise oxo-
anion selective urea-functionalised 2,7-disubstituted
indoles (in collaboration with Albrecht and Triyanti) (4)
and, most recently, both functionalised and unfunctiona-
lised diindolylureas that have particularly high affinities for
dihydrogen phosphate anions (5). In fact, when dihydro-
gen phosphate is bound to diindolylureas containing
Experimental
General remarks
All reactions were performed using oven-dried glassware
under a slight positive pressure of nitrogen/argon (as
specified). 1H NMR (300 MHz) and 13C{1H} NMR
(75 MHz) spectra were determined on a Bruker AV300
*Corresponding author. Email: philip.gale@soton.ac.uk
ISSN 1061-0278 print/ISSN 1029-0478 online
q 2010 Taylor & Francis
DOI: 10.1080/10610271003637087
http://www.informaworld.com

648
J.R. Hiscock et al.
spectrometer. 1H NMR (400 MHz) and 13C NMR
(100 MHz) spectra were determined on a Bruker AV400
spectrometer. Chemical shifts for 1H NMR are reported in
parts per million (ppm) and calibrated to the solvent peak
set, with coupling constants reported in Hz. The following
abbreviations are used for spin multiplicity: s ¼ singlet,
d ¼ doublet, t ¼ triplet, m ¼ multiplet. Chemical shifts
for 13C{1H} NMR are reported in ppm, relative to the
central line of a septet at d ¼ 39.52 ppm for deuterio-
dimethylsulphoxide. Infrared (IR) spectra were recorded
on a Matterson Satellite (ATR). FT-IR are reported in
wavenumbers (cm21). All solvents and starting materials
were purchased from chemical sources where available.
NMR titrations were performed by adding aliquots of
the putative anionic guest (as the tetrabutylammonium
(TBA) or tetraethylammonium (TEA)) salt (0.15 M) in a
solution of the receptor (0.01 M) in DMSO-d6 to a solution
of the receptor (0.01 M). Fluorescence and UV–vis
titrations were performed by adding aliquots of the
putative anionic guest (as the TBA or TEA salt in the case
of bicarbonate) (1 £ 1023 M) in a solution of the receptor
(1 £ 1025 M) in DMSO/0.5% water. All fluorescence and
UV–vis spectra were recorded on a Varian Cary Eclipse
fluorescence spectrophotometer and Thermo Nicolet
Evolution 300 spectrophotometer, respectively. Lumines-
cence quantum yields were determined using quinine
sulphate in a 1 M H2SO4 aqueous solution (F ¼ 0.546) as
a reference.
dryness and the product was isolated by filtration and
recrystallized from ether/methanol by slow evaporation to
give a light green solid. Yield 36%, 0.15 g; mp 2218C; 1H
NMR (300 MHz, DMSO-d6): d 7.13–7.20 (m, 4H), 7.33
(d, J ¼ 7.7 Hz, 2H), 7.40–7.45 (m, 2H), 7.61 (d,
J ¼ 8.0 Hz, 2H), 8.03 (d, J ¼ 7.7 Hz, 2H), 8.12 (d,
J ¼ 7.7 Hz, 2H), 9.71 (s, thiourea NH, 2H), 11.27 (s,
indole NH, 2H). 13C NMR (75 MHz, DMSO-d6): d 111.2
(CH), 118.5 (CH), 118.6 (CH), 118.7 (CH), 120.2 (CH),
122.7 (C), 123.1 (C), 124.0 (C), 124.2 (CH), 125.7 (CH),
136.3 (C), 139.8 (C), 180.8 (CS); IR (film): n ¼ 3389,
3281, 1148 cm21. LR-MS (ES2): m/z: 405 [M 2 H]2;
HR-MS (ESþ): m/z: act: 407.1332 [M þ H]þ cal:
407.1325 [M þ H]þ.
1-(9H-Carbazol-1-yl)-3-(1H-indol-7-yl)thiourea (5)
A solution of 7-aminoindole (0.31 g, 2.35 mM) and 1-
isothiocyanatocarbazole (0.53 g, 2.36 mM) was stirred in
dry pyridine (20 ml) at room temperature for 48 h. The
solution was washed with hexane and evaporated to
dryness and the oil was dissolved in hot methanol (2 ml)
and left to cool to room temperature. White crystals were
isolated by filtration after 24 h and washed with hexane
(25 ml) and dried under vacuum. Yield 48%, 0.41 g; mp
1
1448C; H NMR (300 MHz, DMSO-d6): d 6.48 (dd, 1
J1 ¼ 1.83 Hz, J2 ¼ 2.55 Hz, ArH), 6.99 (t, J ¼ 7.7 Hz, 1
ArH), 7.06–7.20 (m, 3 ArH), 7.29 (d, J ¼ 7.7 Hz, 1 ArH),
7.38–7.47 (m, 3 ArH), 7.57 (d, J ¼ 8.1 Hz, 1 ArH), 8.00
(d, J ¼ 7.7 Hz, 1 ArH), 8.12 (d, J ¼ 7.7 Hz, 1 ArH), 9.59
(s, thiourea NH, 1H), 9.61 (s, NH, 1H), 11.11 (s, NH, 1H),
11.19 (s, NH, 1H); 13C NMR (75 MHz, DMSO-d6): d
101.6 (ArCH), 111.2 (ArCH), 118.4 (ArCH), 118.6
(ArCH), 118.6 (ArCH), 118.6 (ArCH), 118.8 (ArCH),
119.3 (ArCH), 120.2 (ArCH), 122.7 (ArC), 123.2 (ArC),
123.6 (ArC), 123.9 (ArC), 124.0 (ArCH), 125.4 (ArCH),
125.7 (ArCH), 129.3 (ArC), 132.0 (ArC), 136.2 (ArC),
139.7 (ArC), 180.7 (CS); IR (film): n ¼ 3380, 3280,
1210 cm21; LR-MS (ES2): m/z: 355 [M 2 H]2; HR-MS
(ESþ): m/z: act: 357.1168 [M þ H]þ cal: 357.1166
[M þ H]þ.
Compounds 1–3 were synthesised by our previously
published procedures (7).
1-Aminocarbazole
1-Nitrocarbazole was synthesised according to a literature
procedure (8) modified by separation of 1- and 3-
nitrocarbazoles via chloroform flash chromatography (see
the Supplementary Information for details, available
online). Characterisation data agreed with previously
published characterisation data (9).
1-Isothiocyanato-9H-carbazole
1-Isothiocyanato-9H-carbazole was prepared by the
reaction of 1-aminocarbazole with thiophosgene in a
mixture of dichloromethane (DCM) and a saturated
aqueous solution of NaHCO3 (see the Supplementary
Information for details).
1-(9H-Carbazol-1-yl)-3-phenylthiourea (6)
A solution of phenylisothiocyanate (0.41 g, 3.00 mM) in
chloroform (20 ml) was added dropwise to a stirring
solution of 1-aminocarbazole (0.21 g, 1.18 mM) in chloro-
form (20 ml). The solution was then heated to reflux under
argon for 48 h. The solution was then reduced in volume to
2 ml, and DCM (10 ml) and hexane (20 ml) were added.
The solution was then sonicated for 5 min and a dark green
semi-solid was removed and a white precipitate was
collected, washed with hexane (10 ml) and dried under
vacuum. Yield 41%, 0.16 g; mp 1698C; 1H NMR
1,3-di(9H-Carbazol-1-yl)thiourea (4)
A solution of isothiocyanate (0.23 g, 1.04 mM) in chloro-
form (20 ml) was added dropwise to a solution of 1-
aminocarbazole (0.19 g, 1.04 mM) in chloroform (25 ml).
The solution was heated at reflux overnight with
triethylamine (2 ml). The solution was evaporated to

Supramolecular Chemistry
649
Table 1. The stability constants (Ka, M21) of compounds 1–3
with a variety of anionic guests (added as TBA salts except
bicarbonate which was added as a TEA salt) at 298 K in DMSO-
d6/0.5% water as determined by the following urea NH resonance
adjacent to the carbazole.
(300 MHz, DMSO-d6): d 7.11–7.19 (m, 3 ArH), 7.28–
7.41 (m, 4 ArH), 7.53–7.60 (m, 3 ArH), 8.01 (d,
J ¼ 7.7 Hz, 1 ArH), 8.11 (d, J ¼ 7.7 Hz, 1 ArH), 9.65 (s,
thiourea NH, 1H), 9.83 (s, thiourea NH, 1H), 11.14 (s,
indole NH, 1H); 13C NMR (75 MHz, DMSO-d6): d 111.3
(ArCH), 118.3 (ArCH), 118.5 (ArCH), 118.7 (ArCH),
120.2 (ArCH), 122.6 (ArC), 123.2 (ArCH), 123.9 (ArC),
124.5 (ArCH), 125.7 (ArCH), 128.4 (ArCH), 136.0 (ArC),
139.6 (ArC), 139.8 (ArC), 180.4 (CS) (there are
overlapping signals in the 13C NMR resulting in fewer
than expected carbon resonances); IR (film): n ¼ 3300,
3320, 1220 cm21; LR-MS (ES-): m/z: 316 [M 2 H]2; HR-
MS (ESþ): m/z: act: 318.1059 [M þ H]þ cal: 318.1031
[M þ H]þ.
Anion
Compound 1
.104
5670
Compound 2
.104
5880
Compound 3
Acetate
.104
3420
6140
Benzoate
Dihydrogen
phosphate
Chloride
a
a
102
.104
139
.104
85
.104
Bicarbonate
Notes: In all cases, a 1:1 receptor:anion stoichiometry was observed. Errors were
estimated to be no more than ^15%.
a NMR titration curve could not be fitted to a 1:1 or 1:2 receptor:anion
binding stoichiometry but indicates strong binding.
Crystal data for compound 1, tetrabutylammonium
benzoate, C48H59N5O3, Mr ¼ 754.00, T ¼ 120(2) K, tri-
clinic, space group P1, a ¼ 10.980(5), b ¼ 13.774(5),
This is unusual as thioureas are more acidic than ureas, and
hence would be expected to form stronger hydrogen
bonding interactions. Interestingly though, the reduction in
affinity in thiourea analogues of urea-based receptors was
also observed with diindolylurea-based receptors, which
was attributed to either size or conformational inter-
conversion effects (5). The work presented here further
highlights that thiourea-based receptors do not always
possess higher affinities for anions than urea-based
systems.
˚
c ¼ 15.173(5) A, a ¼ 68.760(5)8, b ¼ 85.232(5)8,
3
g ¼ 81.541(5)8, V ¼ 2114.5(14) A , rcalc ¼ 1.184 g cm23
,
˚
m ¼ 0.074 mm21, Z ¼ 2, reflections collected: 44,837,
independent reflections: 9728 (Rint ¼ 0.1052), final R
indices [I . 2sI ]: R1 ¼ 0.0661, wR2 ¼ 0.1368, R indices
(all data): R1 ¼ 0.1190, wR2 ¼ 0.1589 (7).
Results and discussion
Figure 1 shows a comparison of the 1H NMR titration
curves for compounds 1 and 4 with tetrabutylammonium
benzoate in DMSO-d6/0.5% water solution. Upon addition
of excess benzoate salt to compound 1 (dicarbazolylurea),
the urea NH shifts downfield by 2.3 ppm while the indole
NH shifts downfield by approximately 1 ppm. Compound
4 is the thiourea analogue of compound 1. In this case,
while the thiourea NH groups shift downfield by 2.8 ppm,
the indole NH shifts downfield by only 0.4 ppm. The lower
affinity of the thiourea analogues of our original
diindolylurea compounds was attributed to a conformation
effect that caused the thiourea receptors to adopt a twisted
The synthesis of compounds 1–3 has been communicated
previously (7). 1-Aminocarbazole was prepared by
literature methods (8). This was converted to isothio-
cyanate by the reaction with thiophosgene in CH2Cl2/
sat. NaHCO3(aq). Compounds 4 and 5 were prepared by
the reaction of isothiocyanate with 1-aminocarbazole or
7-aminoindole, respectively. Compound 6 was prepared by
the reaction of phenyl isothiocyanate with 1-aminocarba-
zole in chloroform.
Stability constants for the compounds with a range of
1
anionic guests were determined using H NMR titration
techniques with stability constants calculated using the
EQNMR computer program (Table 1) (10). The results
show that all the compounds bind acetate and bicarbonate
strongly. NMR titrations with H2PO24 show titration
profiles usually indicative of 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 (7). It is possible that an anion
deprotonation process is occurring in these cases (6).
Titrations with F2 resulted in the broadening of the NH
resonances in all cases, 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 the Supplementary
Information for compounds 2 and 3).
Table 2. The stability constants (Ka, M21) of compounds 4–6
with a variety of anionic guests (added as TBA salts except
bicarbonate which was added as a TEA salt) at 298 K in DMSO-
d6/0.5% water as determined by the following urea NH resonance
adjacent to the carbazole.
Anion
Compound 4
Compound 5
Compound 6
Acetate
2230
658
687
1800
675
1340
1780
870
Benzoate
Dihydrogen
phosphate
Chloride
a
15
17
23
a
a
a
Bicarbonate
Notes: In all cases, a 1:1 receptor:anion stoichiometry was observed. Errors were
estimated to be no more than ^15%.
a
Compounds 4–6 displayed reduced affinities for
anions as compared to the urea analogues 1–3 (Table 2).
Peak broadening prevented a stability constant from being determined in these
cases.

650
J.R. Hiscock et al.
Figure 1. Proton NMR titration curves for compounds (a) 1 and (b) 4 with tetrabutylammonium benzoate in DMSO-d6/0.5% water
showing shifts of indole and urea NH groups.
conformation due to the large size of the sulphur atom
present in these compounds (5). The NMR evidence
presented here is consistent with this theory in that if the
compound is adopting a twisted conformation, we would
expect a less linear relationship between the carbazole NH
hydrogen bond donors and the benzoate oxygen atoms
resulting in a weaker interaction.
In order to investigate the ability of compounds 1–3 to
act as sensors, fluorescence studies were performed in a
DMSO/0.5% water solution. Under these conditions,
receptor 1 displays an intense fluorescence emission
(F ¼ 0.549) with maxima at 363 and 376 nm when excited
at 270 nm. As shown in Figure 3, upon addition of
increasing amounts of tetrabutylammonium benzoate, a
selective quenching of the fluorescence emission was
observed (Ires ¼ 10%).
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 confor-
mations. In both benzoate complexes, one of the
carbazoles is in the plane of the urea while the other is
twisted out of plane by 37.18 or 43.08 (Figure 2). Each
benzaote anion is bound by four hydrogen bonds in the
Addition of the TBA salts of fluoride, chloride,
acetate, dihydrogen phosphate and tetraethyl-
ammonium bicarbonate resulted in significantly less
dramatic fluorescence quenching, as shown in Figure 4.
Addition of acetate resulted in only a partial quenching of
the emission (Ires ¼ 50%) while a stronger quenching of
the fluorescence emission was observed upon addition of
.3.0 equivalents of fluoride, a finding which may be
indicative of receptor deprotonation (11). Smaller
perturbations of the fluorescent emission of 1 were
observed in the presence of H2PO24 (Ires ¼ 68%) and
˚
range N· · ·O 2.762(5)–2.996(5) A (7).
Figure 2. 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. TBA counter cations and selected hydrogen atoms
have been omitted for clarity.
Figure 3. Fluorescence quenching of receptor 1 in DMSO/0.5%
water upon addition of increasing amounts of
tetrabutylammonium benzoate (for benzoate concentrations for
each fluorescence spectrum, see Figure 4).

Supramolecular Chemistry
Acknowledgements
651
We thank the EPSRC for studentship funding (J.R.H) and for
access to the crystallographic facilities at the University of
Southampton. C.C. would like to thank the Italian Ministero
`
dell’Istruzione, dell’Universita e della Ricerca Scientifica
(MIUR) (Project PRIN-2007C8RW53) and Universita degli
`
Studi di Cagliari (fondo 5%) for financial support.
References
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relative fluorescence emission of receptor 1 in DMSO/0.5%
water.
HCO23 (Ires ¼ 73%). Chloride did not affect the emission
of the system. The trend in fluorescence quenching is
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aromatic groups of the receptor and the guest.
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Compounds 4–6 did not reveal any selectivity for the
anionic guests considered in this study and had
significantly lower quantum yields (F ¼ 0.01, 0.0025
and 0.0019 for 4, 5 and 6, respectively) than those
observed for compounds 1–3. When excited at 332 nm
(compounds 4 and 5) and 344 nm (compound 6), they
showed two maxima in their fluorescence emission, at 379
and 395 nm in DMSO/0.5% water, in the case of 4, a less
structured spectrum with a maximum at 390 nm in the case
of compound 5 and a maximum at 385 nm with a shoulder
at 404 nm in the case of compound 6. Only compound 4 in
the presence of tetrabutylammonium benzoate showed a
partial quenching of the fluorescence (Ires ¼ 54%), which
is much less pronounced than that observed with the
same anion and urea analogue 1 (Ires ¼ 10%). With the
other anions, only negligible changes in fluorescence
were observed upon addition to the three thiourea
receptors.
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Conclusions
Carbazole ureas are effective hosts for oxo-anions
binding acetate selectively in DMSO-d6/water solutions.
Compound 1, a dicarbazolylurea, functions as a selective
fluorescent sensor for benzoate anions. In contrast to these
results, however, thiourea-based analogues show reduced
affinities and selectivity for anionic guests and show
reduced perturbations to their fluorescence properties
in the presence of anions. These findings further high-
light that thiourea-based anion receptors do not
always have higher affinities for guests than urea-based
analogues.
(4) Bates, G.W.; Triyanti; Light, M.E.; Albrecht, M.; Gale,
P.A. J. Org. Chem. 2007, 72, 8921–8927, see also: Makuc,
ˇ ˇ
D., Lenarcic, M., Bates, G.W., Gale, P.A., Plavec, J. Org.
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