Aryl substituted adamantane-dipyrromethanes: Chromogenic and fluorescent anion sensors

Article abstract of DOI:10.1016/j.tet.2012.12.026

New adamantane-dipyrromethane anion receptors were synthesized and their photophysical properties and anion binding investigated. In addition, a nitro derivative has been characterized by single crystal X-ray analysis. In the crystal structure, the nitro derivative is complexed with CH₃CN molecules, indicating that dipyrromethanes can in principle be employed as neutral molecule hosts. Anion binding ability in CH₃CN solutions with TBA salts (F^-, Cl^-, Br^-, AcO^-, HSO4-, NO3- and H₂PO4-) was investigated by UV-vis, fluorescence and/or ¹H NMR spectroscopy, and the association constants of the corresponding complexes were determined. The receptors form 1:1 complexes with the highest association constants with F^- and AcO^-. Change of the substituent at the phenyldipyrromethanes results in different photophysical properties, leading to different spectroscopic responses in the presence of anions. Therefore, differently substituted phenyldipyrromethanes can be used as colorimetric or fluorescent anion sensors.

Full text of DOI:10.1016/j.tet.2012.12.026

Tetrahedron 69 (2013) 1725e1734
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Aryl substituted adamantaneedipyrromethanes: chromogenic and
fluorescent anion sensors
b
c
c
a,
*
ꢀ
ꢁ a
ꢁ a
ꢁ
Marija Aleskovic , Nikola Basaric , Ivan Halasz , Xing Liang , Wenwu Qin , Kata Mlinaric-Majerski
a
ꢀ
ꢁ
ꢀ
Department of Organic Chemistry and Biochemistry, RuCer Boskovic Institute, Bijenicka cesta 54, 10000 Zagreb, Croatia
b
ꢀ
ꢁ
ꢀ
Division of Material Chemistry, RuCer Boskovic Institute, Bijenicka cesta 54, 10000 Zagreb, Croatia
c Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province and State Key Laboratory of Applied Organic Chemistry, College of Chemistry and
Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
New adamantaneedipyrromethane anion receptors were synthesized and their photophysical properties
and anion binding investigated. In addition, a nitro derivative has been characterized by single crystal
X-ray analysis. In the crystal structure, the nitro derivative is complexed with CH3CN molecules,
indicating that dipyrromethanes can in principle be employed as neutral molecule hosts. Anion binding
ability in CH3CN solutions with TBA salts (Fꢀ, Clꢀ, Brꢀ, AcOꢀ, HSOꢀ4 , NOꢀ3 and H2POꢀ4 ) was investigated by
UVevis, fluorescence and/or 1H NMR spectroscopy, and the association constants of the corresponding
complexes were determined. The receptors form 1:1 complexes with the highest association constants
with Fꢀ and AcOꢀ. Change of the substituent at the phenyldipyrromethanes results in different
photophysical properties, leading to different spectroscopic responses in the presence of anions.
Therefore, differently substituted phenyldipyrromethanes can be used as colorimetric or fluorescent
anion sensors.
Received 19 September 2012
Received in revised form 22 November 2012
Accepted 11 December 2012
Available online 19 December 2012
Keywords:
Adamantanes
Anion sensors
UVevis spectrometry
Fluorescence spectrometry
Dipyrromethanes
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
adamantylurea7 derivatives. We have recently investigated a series
of simple adamantaneedipyrromethanes wherein adamantane is
Within the field of supramolecular chemistry there is a contin-
uous flow of conducted research related to anion receptors due to
highlighted importance and presence of anions in many scientific
areas, as well as in biological and technological processes.1 Although
supramolecular chemists have designed various complex supra-
molecular systems based on acyclic and cyclic pyrrole receptors,2
simple pyrrole anion receptors potent in complexing anions with
high selectivity and affinity are still a topic of the investigation.3 It is
well known that even simple receptors bearing only two pyrrole
units (i.e., DPQ systems) can bind anions with appreciable selec-
tivity and high binding constants.4 In addition, substitution of the
pyrrole moiety with the additional H-bond donating groups, and/or
introduction of a variety of chromophores leads to the development
of very effective hosts and sensors.3 Such systems are able to create
structural recognition motifs for anionic species that can be very
diverse in their nature, and can bind from simple inorganic anions1
to macromolecules such as peptides or DNA.5
a rigid spacer and pyrroles are the H-bond donors of the receptor
system.6a,c Incorporation of the rigid and bulky adamantane moiety
into receptors diminishes the rotational mobility of the pyrroles,
which can lead to the preorganization of the receptors for the anion
capture. Therefore, the stability constants of the complexes were
higher than those of the phenyl substituted dipyrromethane
receptor. Furthermore, 1H NMR titration experiments performed in
CDCl3 and X-ray analysis revealed the unexpected stoichiometries
of the complexes with Fꢀ and AcOꢀ, wherein the anion is bound by
four H-bonds in the cavity created between two dipyrromethane
units from two host molecules.6a,c
1H NMR spectroscopy is a very informative technique for the
binding studies since it also gives structural information. However,
it is not applicable in sensor technology. On the other hand, sensors
with optical response (UVevis or fluorescence) are of particular
interest because detection of a substrate can be achieved fast and
with low detection limits, the methods are inexpensive, easy to use,
and the instrumentation is highly accessible. In addition, receptors
with ‘naked eye detection’ are getting even more popular because
semiquantitative analysis can be performed outside the typical
laboratory environment. It is known that chromophores, which are
directly attached to the binding region of the receptor usually
exhibit significant spectral changes (chromogenic or fluorogenic).8
A current line of research in our group is oriented at the study of
anionic receptors with polycyclic cage units, i.e., synthesis and
investigation of anion binding properties of adamantylpyrrole6 and
ꢁ
* Corresponding author. E-mail address: majerski@irb.hr (K. Mlinaric-Majerski).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.12.026

ꢀ ꢁ
M. Aleskovic et al. / Tetrahedron 69 (2013) 1725e1734
1726
Hence, even simple chromophore-based anion hosts can be very
effective in signalling process of molecular recognition, as it is
shown in many examples from the literature.4,9 Therefore, our next
step in the design of anion sensor molecules was introduction of
a chromophore into the adamantaneedipyrromethane receptors so
that the observation of binding can be monitored by spectropho-
tometric methods.
1 R = H, 69 %
CHO
3
R = NO2, 62 %
H2,
Pd/C
5 R = NH2, 65 %
10
7
R = CN, 73 %
R
H
N
TFA, CH2Cl2
Herein we report the synthesis and anion binding studies
of new receptors 1e8. The receptor molecules bear the ada-
mantaneedipyrromethane moiety and aryl groups directly
attached to the pyrroles. The aryl groups were functionalized with
electron-withdrawing groups (CN or NO2), which should
electronically enhance the binding ability of the nearby pyrrole
H-bond donors, or electron-donating group that should introduce
additional hydrogen bonding sites (NH2 groups). The rigid
adamantane moiety of the receptors composed of adamant-1-yl or
adamant-2,2-diyl group should have different conformational
freedom and effect the anion binding ability and selectivity.
Depending on the H-bonding ability, and the availability of
H-bonds for the complexation (determined by the electronic and
structural effects) the study should highlight the optimal sub-
stitution of the receptor molecules and favourable structural motifs
for strong and selective anion binding. The investigated anions in
this report are characterized by different size, geometry and
basicity (Fꢀ, Clꢀ, Brꢀ, AcOꢀ, NOꢀ3 , HSOꢀ4 and H2POꢀ4 ).
TFA, CH2Cl2
O
9a
R = H
2 R = H, 66 %
9b R = NO2
9c
4
R = NO2, 68 %
H2,
Pd/C
R = CN
6 R = NH2, 72 %
8
R = CN, 62 %
11
Scheme 1.
2.2. Photophysical properties of 1e8
Decay of the fluorescence measured by single photon timing
(SPT) can be fitted to single exponential function (except for 7 and 8
in cyclohexane), giving fluorescence lifetimes in the range 2e9 ns.
Comparison of the UVevis andfluorescence spectra of1, 2, 7 and8
taken in CH3CN and cyclohexane reveal that increase of solvent po-
larity induces small solvatochromic shifts in the absorption and
relatively large shift in the fluorescence spectra. For example, Fig. S7
(see Supplementary data) demonstrates the influence of the solvent
polarity on the fluorescence spectra of 8 (taken in cyclohexane, ethyl
acetate, and CH3CN). Furthermore, fluorescence lifetimes and fluo-
rescence quantum yields increase with the increase of the solvent
polarity, corresponding to a decrease of both deactivation rate con-
stants, radiative and non-radiative. For example, for 1 in cyclohexane
kF and kNR are 2.9ꢁ108 and 6.9ꢁ108 sꢀ1, and in CH3CN the constants
are 2.1ꢁ108 and 1.7ꢁ108 sꢀ1. These findings suggest that excitation to
S1 leads to an increase of the dipole moment. That is, excitation to S1
has a partial CT character that is more stabilized in more polar sol-
vents. Therefore, it is anticipated that binding of anions should in-
duce changes in both absorption and fluorescence spectra (vide
infra). It is interesting to note that fluorescence spectra of 2 and 8 are
z10e20 nm bathochromically shifted compared to 1, and 7,
respectively. This finding demonstrates that change of the aliphatic
adamantane spacer also changes the photophysical properties. Pre-
sumably, there is some vibronic coupling between two arylpyrrole
moieties in 2,2-bissubstituted adamantyl derivatives 2 and 8.
HN
R
NH
R
NH
R
HN
R
1 R = H
2 R = H
3
4
R = NO2
R = NO2
5 R = NH2
6 R = NH2
7
8
R = CN
R = CN
2. Results and discussion
2.1. Synthesis
2.3. Anion binding studies
New dipyrromethane derivatives 1e4, 7 and 8 were obtained in
very good yields from the corresponding adamantane carbonyl
derivatives 10 or 11 and 2-phenylpyrrole precursors 9aec,10 which
were obtained in an optimized Suzuki coupling reaction between
N-Boc-2-pyrrole boronic acid and the corresponding aryl bromide.
Amino derivatives 5 and 6 were obtained by catalytic hydrogena-
tion from the nitro analogues 3 and 4, respectively (Scheme 1).
For the use of new receptors as optical sensors it is important to
investigate their photophysical properties. Therefore, we measured
UVevis and fluorescence spectra of 1e8 in CH3CN. In addition, we
measured quantum yields of fluorescence and lifetimes of the
singlet excited states in CH3CN and cyclohexane (Table 1). In CH3CN
solution all receptors exhibit a strong absorption band at z300 nm
The binding studies of new receptors 1e8 with the TBA salts of
the following anions: Fꢀ, Clꢀ, Brꢀ, AcOꢀ, NOꢀ3 , HSOꢀ4 and H2POꢀ4 ,
were performed by UVevis and fluorescence titrations in CH3CN
solution. Dependences of the UVevis and fluorescence spectra on
anion concentrations were processed by multivariate non-linear
regression analysis by use of Specfit program11 to determine the
complex stoichiometries and the stability constants. The stability
constants are compiled in Table 2. In addition, some of the fluo-
rescence titrations were performed by a ratiometric method to
additionally verify the obtained values of the stability constants
and demonstrate the applicability of the receptor molecules as
fluorescence ratiometric sensors (vide infra).
(log
additional low-energy absorption bands can be seen in the spectra,
for 3 and 4 at z410 nm (log
/Mꢀ1 cmꢀ1z3.5), for 5 and 6 at
3
/Mꢀ1 cmꢀ1z4.2e4.6). On substitution of the phenylpyrrole,
Addition of basic anions (Fꢀ, AcOꢀ and H2PO4ꢀ) to the CH3CN
solution of 1 or 2 induced pronounced changes in their UVevis
spectra. For example, addition of Fꢀ to the CH3CN solution of 2
resulted in a small bathochromic shift and an increase of the
absorbance at z300 nm (Fig. 1). Formation of three clear isosbestic
points (293, 309 and 325 nm) indicated a presence of two coloured
species in an equilibrium. Addition of less basic anions made
3
320 nm, and for 7 and 8 at 350 nm. All receptors except nitro
derivatives 3 and 4, which are nonfluorescent, display structureless
fluorescence spectra in
at 360e450 nm, and relatively strong emission with
a
CH3CN solution with the maxima
¼0.4e0.7.
F

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M. Aleskovic et al. / Tetrahedron 69 (2013) 1725e1734
1727
Table 1
Photophysical properties of 1e8
Compd
Solvent
labs/nm
log
3
/Mꢀ1 cmꢀ1
lem/nm
Fa
sb/ns
1
CH3CN
Cyclohexane
CH3CN
Cyclohexane
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
Cyclohexane
304
4.59
dc
360
338
380
358
d
0.55ꢂ0.01
0.30ꢂ0.01
0.66ꢂ0.02
0.53ꢂ0.02
d
2.63ꢂ0.02
1.02ꢂ0.01
3.01ꢂ0.02
1.59ꢂ0.01
d
dc
2
306
4.52
dc
dc
3
4
5
6
7
299 and w405
302 and w415
280 and 320
285 and 320
304 and 348
dc
4.37 and 3.65
4.31 and 3.48
4.28 and 4.29
4.26 and 4.33
4.30 and 4.26
dc
d
d
d
381
378
430
411
0.41ꢂ0.01
0.43ꢂ0.05
0.70ꢂ0.02
0.44ꢂ0.01
2.85ꢂ0.01
3.07ꢂ0.01
7.97ꢂ0.01
3.5ꢂ0.4 (32)
6.3ꢂ0.3 (68)
9.33ꢂ0.09
8
CH3CN
Cyclohexane
304 and 355
4.27 and 4.26
450
421
0.52ꢂ0.02
0.43ꢂ0.03
dc
dc
3.93ꢂ0.01 (75)
7.1ꢂ0.3 (25)
a
Measured by use of quinine sulfate in 0.05 M aqueous H2SO4 as reference.
Measured by single photon timing method.
Compound not soluble in cyclohexane.
b
c
Table 2
The association constants (log
bꢂs
(log
Fꢀ
b
)) of the complexes of receptors 1e8 with anions determined by UVevis and fluorescence titrationsa
Compd
Method
Clꢀ
Brꢀ
AcOꢀ
NOꢀ3
H2POꢀ4
HSOꢀ4
1
UVevis
Fluo
4.50ꢂ0.02
4.31ꢂ0.02
<2
1.99ꢂ0.07
db
3.80ꢂ0.10
3.97ꢂ0.02
db
db
3.48ꢂ0.01
3.25ꢂ0.01
3.29ꢂ0.02c
3.11ꢂ0.02
3.02ꢂ0.02
3.25ꢂ0.09c
3.40ꢂ0.01
3.44ꢂ0.01
2.75ꢂ0.04
2.78ꢂ0.06
2.61ꢂ0.13c
2.55ꢂ0.06
2.83ꢂ0.03
2.58ꢂ0.14c
4.07ꢂ0.01
3.97ꢂ0.01
4.14ꢂ0.04c
3.55ꢂ0.01
3.43ꢂ0.01
<2
<2
z1
2
UVevis
Fluo
4.51ꢂ0.03
4.78ꢂ0.02
3.02ꢂ0.04
3.13ꢂ0.03
1.99ꢂ0.03
<2
4.03ꢂ0.03
4.06ꢂ0.02
3.78ꢂ0.04c
3.95ꢂ0.03
3.88ꢂ0.05
4.56ꢂ0.12
4.81ꢂ0.20
<2
db
db
1.67ꢂ0.20
3
4
5
UVevis
UVevis
UVevis
Fluo
4.99ꢂ0.02
5.31ꢂ0.03
4.04ꢂ0.07
3.71ꢂ0.04
2.21ꢂ0.04
2.98ꢂ0.03
db
db
db
<2
db
db
1.68ꢂ0.02
1.77ꢂ0.05
5.96ꢂ0.02d
8.04ꢂ0.02d
2.13ꢂ0.02
db
<2
db
6
7
8
UVevis
Fluo
3.84ꢂ0.04
4.09ꢂ0.01
2.94ꢂ0.07
2.18ꢂ0.08
<2
<2
4.27ꢂ0.20
3.95ꢂ0.06
<2
2.57ꢂ0.20
5.97ꢂ0.02d
8.10ꢂ0.02d
UVevis
Fluo
5.31ꢂ0.04
4.96ꢂ0.02
2.35ꢂ0.02
2.37ꢂ0.03
1.63ꢂ0.04
<2
4.04ꢂ0.02
4.05ꢂ0.01
4.08ꢂ0.03c
3.78ꢂ0.06
4.09ꢂ0.01
4.07ꢂ0.02c
db
db
1.92ꢂ0.01
2.42ꢂ0.04
UVevis
Fluo
5.33ꢂ0.03
5.18ꢂ0.03
3.18ꢂ0.02
3.16ꢂ0.04
2.08ꢂ0.02
2.19ꢂ0.09
<2
2.43ꢂ0.10
1.88ꢂ0.05
<2
The titrations were performed at 25 ꢃC in a CH3CN solution. The anions were added as TBA salts. The complexes are formed in 1:1 stoichiometry giving /molꢀ1 dm3,
b
a
1:1 stoichiometry is confirmed by mole ratio method.
b
No binding interaction observed.
c
The average association constants were determined from direct emission fluorimetric titration by processing data taken from one fluorescence wavelength or ratiometric
fluorimetric titration.
d
The titration data gave the best fit to 1:2 model (receptor/anion) giving b
/molꢀ2 dm6.
smaller (Clꢀ, Brꢀ and HSO4ꢀ) or no (NOꢀ3 ) changes in the UVevis
spectra, so in some cases fitting was rather difficult, and therefore,
some log values were estimated to be <2. The observed changes
of the UVevis spectra of receptors 1 and 2 enable their application
as sensors for Fꢀ, Clꢀ or H2POꢀ4 . Particularly noticeable is the change
on addition of Fꢀ, which may enable use of 1 and 2 as ‘naked eye’
colorimetric sensors (graphical abstract).
In contrast to the hyperchromic changes observed in the UVevis
spectra, addition of basic anions (Fꢀ, AcOꢀ, H2PO4ꢀ, and Clꢀ) to the
CH3CN solution of 1 and 2 resulted in fluorescence quenching
(Fig. 2 and Supplementary data, Figs. S64eS77). The quenching was
not observed on addition of a large excess of less basic anions (Brꢀ,
HSOꢀ4 , and NOꢀ3 ). Non-linear curve-fitting after the fluorescence
titration by the Specfit program enabled estimation of the stability
constants compiled in Table 2. The constant determined by UVevis
and fluorescence titration generally agree very well. However,
some smaller differences between the values obtained by these two
methods can be observed for the most basic anion Fꢀ, which may be
attributed to a partial proton transfer taking place in the excited
state. For example, in the fluorescence spectra of 2, on addition of
Fꢀ, a new shoulder appears in the fluorescence spectra at z500 nm,
0,8
0,6
0,4
0,2
0,0
b
250
275
300
325
λ / nm
350
375
Fig. 1. UVevis titration of the CH3CN solution of 2 (c¼2ꢁ10ꢀ5 M) with addition of Fꢀ.
The spectra were corrected for dilution. Inset: absorbance of the CH3CN solution of 2 at
340 nm versus mole ratio of Fꢀ/2 (- exp. values, calcd values).

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M. Aleskovic et al. / Tetrahedron 69 (2013) 1725e1734
1728
1
and 2 binding site is somewhat shielded by the phenyl
substituents, so it would be sterically very demanding to surround
the anions with two receptors.
700
600
500
400
300
200
100
0
Compared to nonsubstituted receptors
1 and 2, amino
derivatives 5 and 6 were anticipated to form more stable complexes
with anions due to an introduction of the additional hydrogen bond
donors (NH2 groups). The amino groups along with the pyrrole NH
protons could in principle create an optimal surrounding for the
stabilization of the negative charge of anion through a net of
hydrogen bonds and thus increase the stability of the complex.
Although the anticipated trend of increased association constants
was not observed (Table 2), amino derivatives 5 and 6 showed
selective spectral response in the presence of Fꢀ and HSO4ꢀ in the
UVevis and fluorescence spectra. On the other hand, little evidence
of anion binding was seen for other tested anions (Clꢀ, Brꢀ and
NOꢀ3 ), so in some cases calculated association constants are rather
uncertain or the recognition can not be observed at all by UVevis or
fluorimetric titration experiments.
350
400
λ / nm
450
500
550
An addition of Fꢀ to the CH3CN solution of 5 or 6 induced
quenching of the fluorescence with concomitant bathochromic
shift of the spectral maximum. The fluorescence emission intensity
at 380 nm decreases, with an increase at z400 nm (Fig. 4). Addition
of a large excess of other basic anions (i.e., H2POꢀ4 ) to the solutions
of hosts 5 or 6 quenched florescence without shifting the maximum
in the spectra. The shifting of the maximum in the fluorescence
spectra was not seen with other receptors with either of the tested
anions. Therefore, receptors 5 and 6 can be applied as ratiometric
fluorescence sensors for Fꢀ.
Fig. 2. Fluorescence titration of the CH3CN solution of 2 (c¼2ꢁ10ꢀ6 M) with addition
of Fꢀ, lex¼310 nm. The spectra were corrected for dilution. Inset: fluorescence emission
of the CH3CN solution of 2 at 380 nm versus concentration of Fꢀ (- exp. values,
calcd values).
which may be attributed to the fluorescence of the deprotonated
receptor 2. Fꢀ is a strong base that can deprotonate pyrrole NH
while simultaneously forming the HFꢀ2 species.12 Furthermore, it is
known that molecules exhibit different acidity and basicity in the S0
and S1 states.13 Therefore, smaller differences between the associ-
ation constants obtained from UVevis and fluorescence may be due
to the change of the acidity/basicity of the molecules on excitation.
Nevertheless, the pronounced quenching of fluorescence for hosts
1 and 2 with Fꢀ, not seen with other anions, renders them as flu-
orogenic sensors for Fꢀ (Fig. 3).
700
600
500
400
300
200
100
0
350
400
450
500
550
λ / nm
Fig. 4. Fluorescence titration of the CH3CN solution of 6 (c¼4ꢁ10ꢀ6 M) with the
addition of Fꢀ, lex¼320 nm. The spectra were corrected for dilution. Inset: emission of
the CH3CN solution of 6 at 370 and 430 nm versus mole ratio of Fꢀ/6.
Basic Fꢀ can in principle induce deprotonation of the pyrrole NH,
which in turn, may be responsible for the observed bathochromic
shift.11 However, the binding of Fꢀ in CH3CN by 5 or 6 showed
Fig. 3. Application of 2 as fluorogenic anion sensor (comparison of the fluorescence
emission of 2 in the CH3CN after addition of 100 equiv of different TBA salts).
a small hyperchromic effect (DAz0.03) and a small bathochromic
shift (Dlz0.5) in the UVevis spectra (see Supplementary data, Figs.
S36 and S43). If Fꢀ had deprotonated the pyrrole NH, the generated
negative charge would induce an enhancement in the intensity of
the electrical dipole with the direct consequence of a significant red
shift of the absorption band. Since it was not observed, we can
conclude that the observed spectral change in the fluorescence
spectra is result of the hydrogen bonded complex with Fꢀ, and not of
deprotonation. Probably, the introduction of the electron-donating
group decreased acidity of the pyrrole NH. Additionally, we per-
formed 1H NMR titrations of 6 with Fꢀ in CD3CN to probe for the NH
deprotonation and get structural information about the complex
The findings obtained by UVevis and fluorescence titrations for
receptors 1 and 2 strongly indicate formation of the ground state
complexes of the receptors with anions. All the complexes are
formed in stoichiometry 1:1. Overall, the most stable complexes are
observed with the most basic anions Fꢀ, H2PO4ꢀ and AcOꢀ. The
binding of the anions is accomplished through hydrogen bonds
with two pyrrole NH protons, in contrast to the previous series of
adamantaneedipyrromethanes, wherein formation of 1:2 (host/
anion) stoichiometries was observed.6a,c Probably, in the receptors

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M. Aleskovic et al. / Tetrahedron 69 (2013) 1725e1734
1729
geometries. The addition of even a small quantity of Fꢀ (0.125 equiv)
700
600
500
400
300
200
100
0
resulted in a significant change in the 1H NMR spectra of 6, and the
excess of Fꢀ shifted the NH signal downfield for z5 ppm (see
Supplementary data, Fig. S113). Since we have not observed dis-
appearance of the NH signal, deprotonation does not take place in
the concentration range for 1H NMR experiments. Consequently, we
can say that receptors 5 and 6 have selective ratiometric fluores-
cence response only in the presence of Fꢀ.
UVevis titrations of receptors 5 and 6 with HSOꢀ4 showed some
unexpected spectral changes. Upon addition of TBAHSO4 to the
solution of 6 in CH3CN, absorption bands at z285 and z320 nm
disappeared and the new band at z300 nm arose (Fig. 5). Such
a change in the UVevis spectra of 5 and 6 was not seen in the ti-
trations with other tested anions, even in the presence of a large
excess. More interestingly, the analysis of the experimental data
indicated complexes of 1:2 (host/anion) stoichiometry with much
higher association constants. Unique spectral response in the
presence of HSOꢀ4 and specific stoichiometry of the complex call in
question if the changes observed in the UVevis spectra are due to
different process then H-bonding complexation. Bowman-James,14
and Gale3g have reported that HSOꢀ4 (an acidic anion) induces
protonation of the receptors, which have basic (amino) sub-
stituents. Therefore, we tentatively assign the observed spectral
changes to the proton transfer between the HSOꢀ4 and the amino
groups. Furthermore, in the fluorescence titrations of 5 or 6 in
CH3CN solution with TBAHSO4, quenching of the fluorescence was
observed with only a small shift of the spectral maximum (Fig. 6).
Analysis of the experimental data gave approximately two orders of
magnitude higher values of the binding constants than those that
were determined by UVevis. Consequently, the UVevis and fluo-
rescence titration experiments with HSOꢀ4 indicate that binding of
HSOꢀ4 is not a simple process, since it is probably accompanied by
proton transfer.
350
400
450
500
550
λ / nm
Fig. 6. Fluorescence titration of the CH3CN solution of 6 (c¼3ꢁ10ꢀ6 M) with the
addition of HSOꢀ4
,
lex¼320 nm. The spectra were corrected for dilution. Inset: emission
of the CH3CN solution of 6 at 385 nm versus concentration of HSOꢀ4 (- exp. values,
calcd values).
and 2, addition of Clꢀ, Brꢀ, HSO4ꢀ and NO3ꢀ to the CH3CN solution of
3 and 4 had a negligible influence on the UVevis spectra. The
estimated stability constants of the corresponding 1:1 complexes
are similar to those obtained for unsubstituted analogues 1 and 2
(Table 2), and in some instances (Brꢀ, NO3ꢀ) no anion coordination
was observed. On the contrary, more basic anions such as Fꢀ, AcOꢀ
and H2POꢀ4 induced more pronounced changes in the UVevis
spectra (Fig. 7). Generally, an increase of the absorbance at
z300 nm is observed, and only in the titration with Fꢀ a small
bathochromic shift of the absorption band occurred (z5 nm).
Addition of anions induced larger spectral shifts on the low-energy
band (350e550 nm). However, due to a low absorptivity of the
band, these changes are not applicable for the sensing. Although it
was expected that nitro compounds 3 and 4 (compared to dipyr-
romethanes 1 and 2) would form complexes with much higher
association constants, only a small increase of the complex stability
was observed for Fꢀ (Table 2).
1,0
0,8
0,6
0,4
0,2
0,0
1,00
0,75
0,50
0,25
0,00
275
300
325
350
375
λ / nm
Fig. 5. UVevis titration of the CH3CN solution of 6 (c¼3ꢁ10ꢀ5 M) with addition of
HSOꢀ4 . The spectra were corrected for dilution. Inset: absorbance of the CH3CN solution
of 6 at 300 nm versus mole ratio of HSOꢀ4 /6 (- exp. values,
calcd values).
Therefore, the estimated binding constants do not represent real
association constants corresponding to the complex of 1:2
stoichiometry, but to
a cumulative constant comprising the
stability constant of the H-bonded complex, proton transfer (PT),
and the stability constant of the ion pair formed by PT. Different
values of the association constants obtained from UVevis and
fluorescence titrations are probably due to the change of basicity of
the amino group of the receptors on excitation to S1.
300
350
400
λ /nm
450
500
550
Fig. 7. UVevis titration of the CH3CN solution of 4 (c¼4ꢁ10ꢀ5 M) with addition of
H2POꢀ4 . The spectra were corrected for dilution. Inset: absorbance of the CH3CN
solution of 4 at 330 nm versus mole ratio of H2POꢀ4 /4 (- exp. values,
calcd
Nitro analogues 3 and 4 are nonfluorescent, so we could only
perform UVevis titrations. Similar to the titrations of receptors 1
values).

ꢀ ꢁ
M. Aleskovic et al. / Tetrahedron 69 (2013) 1725e1734
1730
From the numerous attempts to grow single crystals of new
1,0
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
receptors and/or their complexes, only crystals of receptor 4 were
suitable for X-ray analysis. Interestingly, in the crystal structure of
4, the molecule adopts a conformation with two pyrrolyl moieties
pointing to the same directions. The pyrroles NH form two hydro-
gen bonds with neutral CH3CN molecule (Fig. 8). Similar confor-
mations were observed earlier in the crystal structures of the anion
complexes of unsubstituted dipyrromethanes.6c On the other hand,
in the crystal structure of the unsubstituted dipyrromethane
receptors, the pyrroles adopt a conformation wherein NH are
pointing to the opposite directions.6c Thus, as demonstrated by the
crystal structure of the CH3CN complex, most probably the struc-
ture of receptor 4 is preorganized for binding of anions or neutral
molecules.
280
300
320
340
360
380
400
420
λ / nm
Fig. 9. UVevis titration of the CH3CN solution of 8 (c¼4ꢁ10ꢀ5 M) with addition of Fꢀ.
The spectra were corrected for dilution. Inset: absorbance of the CH3CN solution of 8 at
350 nm versus concentration of Fꢀ (- exp. values,
calcd values).
900
600
300
0
Fig. 8. ORTEP view of the asymmetric unit of compound 4. Thermal ellipsoids are
drawn at 50% probability. Acetonitrile molecule forms two hydrogen bonds towards
ꢂ
ꢂ
pyrrole nitrogen atoms: N2eH2n/N5 (d(N2eH2n)¼0.871 A, d(H2neN5)¼2.432 A,
:(N2eH2neN5)¼161.8ꢃ) and N1eH1n/N5 (d(N1eH1n)¼0.827 A, d(H1neN5)¼
ꢂ
400
450
500
λ /nm
550
600
ꢃ
ꢂ
2.324 A, :(N1eH1neN5)¼154.1 ).
Fig. 10. Fluorescence titration of the CH3CN solution of 8 (c¼4ꢁ10ꢀ6 M) with the
addition of Fꢀ, lex¼355 nm. The spectra were corrected for dilution. Inset: emission of
the CH3CN solution of 8 at 455 nm versus concentration of Fꢀ (- exp. values, calcd
values).
In line with other dipyrromethane receptors, the cyano-
substituted derivatives 7 and 8 form the most stable complexes
with basic anions Fꢀ, AcOꢀ and H2POꢀ4 (Table 2) and have a low
affinity towards Clꢀ, Brꢀ and HSO4ꢀ, but no binding of NOꢀ3 was
observed. An addition of TBAF to the CH3CN solution of 8 caused red
shift of the absorption band (z15 nm) and a small hyperchromic
shifting of the other signals. Since addition of basic Fꢀ did not result
in the disappearance of the pyrrole NH signals, we can discard
deprotonation of the pyrrole NH, at least in the concentration range
for the NMR experiments. Stoichiometry of the 1:1 complex was
confirmed by mole ratio method, which was also performed by 1H
NMR spectroscopy (Fig. 11).
effect (DAz0.15, Fig. 9). Furthermore, formation of three clear
isosbestic points (288, 325 and 360 nm) indicated existence of two
coloured species in equilibrium, i.e., molecules of the receptor and
its complex. Therefore, from the UVevis titration of 7 and 8 with
F
ꢀ there was no indication that the deprotonation of the pyrrole NH
takes place, even though we increased its acidity by CN group.
In the fluorescence titration of the CH3CN solutions of 7 and 8,
quenching of the emission was observed by addition of Fꢀ (Fig. 10),
H2POꢀ4 and AcOꢀ whereas a large excess of other anions did not
induce any significant change (see Supplementary data, Figs.
S92eS105). The binding of Fꢀ by cyano receptors, as well as con-
formation changes of the receptors upon binding were investigated
by 1H NMR titration of 8 with TBAF in CD3CN. The addition of
2.5 equiv of Fꢀ to the CD3CN solution of 8 shifted the pyrrole NH
Since we were not able to grow single crystals of complexes of 7
or 8, we studied conformation of the receptor 8 and its complex in
the solution by NMR, using NOESY experiments. The interactions
seen in the NOESY spectrum of receptor 8 showed that there is
a free rotation around the single bond between the pyrrole and the
phenyl ring, as well as between the pyrrole and the adamantane
moiety (Scheme 2). Thus, the NOESY experiment did not indicate
the existence of a preferable conformer of 8.
In the presence of Fꢀ, on the other hand, the interactions
between the pyrrole NH protons and the ortho-phenyl protons can
be seen, in accordance with the observed shift of the phenyl signal
in the titration experiments. The finding suggests that the com-
plexation of Fꢀ leads to the conformation in which pyrrole groups
signal downfield from the initial value of
d 9.2e14.8 ppm (see
Supplementary data, Fig. S114). In the aromatic region of the 1H
spectrum, a prominent downfield shift was also observed to the
signal of the ortho-phenyl proton (Ddz0.8 ppm), and only negligible

ꢀ
ꢁ
M. Aleskovic et al. / Tetrahedron 69 (2013) 1725e1734
1731
Fꢀ, AcOꢀ and H2POꢀ4 . Since these anions are very different in their
geometry it is obvious that the basicity of the anion played the most
important role in the molecular recognitionprocess regardless of the
change in the functionality of the receptors binding site. However,
there is a slight difference in the stability constants of the complexes
between the adamant-1-yl and adamant-2,2-diyl substituted
receptors and other halide anions tested. The receptors 2, 4, 6 and 8,
which have a 2-adamantyl backbone, form the complexes with
Clꢀ and Brꢀ with about an order of magnitude higher association
constants than the receptors 1, 3, 5 and 7 (Table 2). This valuable
finding is important for designing the next generation of the
receptor molecules with the polycyclic rigid spacers.
15
14
13
12
11
10
9
δ
4. Experimental section
4.1. General
0,0
0,5
1,0
1,5
2,0
2,5
1H and 13C NMR spectra were recorded on a on a Bruker
Spectrometer at 300 or 600 MHz. All NMR spectra were measured
in deuterated solvents using tetramethylsilane as a reference. High-
resolution mass spectra (HRMS) were measured on an Applied
Biosystems 4800 Plus MALDI TOF/TOF instrument. IR spectra were
recorded on FT-IR ABB Bonem MB 102 spectrophotometer. UVevis
spectra were recorded on Varian Cary 100 spectrometer equipped
with a thermostating device. For fluorescence measurements, Cary
Eclipse Varian and Hitachi F-4500 spectrofluorimeters were used.
Fluorescence decay histograms were obtained on an Edinburgh
instrument OB920. Single crystal was measured on an Oxford
Diffraction Xcalibur 3. Melting points were obtained using an
Original Kofler Mikroheitztisch apparatus (Reichert, Wien) and are
uncorrected.
n (F-) / n (8)
Fig. 11. Mole ratio method for stoichiometry determination of complex 8$Fꢀ in CD3CN.
NC
NC
δ+
A-
δ−
A-
δ+
HN
N H
CN
H
H
N
N
δ−
CN
Adamantane-1-carbaldehyde (10),16 adamantane-2-one (11),17
and 2-phenylpyrrole precursors (9aec)10 were prepared accord-
ing to the procedure described in the literature. The new receptors
1e4, 7 and 8 were additionally dried in Memmert vacuum drying
oven to remove residual solvent.
8
8 + A-
Scheme 2.
are oriented in the same direction forming the H-bonds with the
anion, while the cyanide groups are turned out from the cleft with
the anion (Scheme 2).
4.2. General procedure for preparation of receptors 1e4, 7
It is known that modification of binding sites by introduction of
electron-withdrawing groups increases the acidity of nearby
hydrogen bond donors. In turn, this increases the strength of the
interactions within the complex and results in the formation of
complexes with higher association constants.15 Accordingly, it was
anticipatedthatintroductionoftheNO2 and CN groups to the receptor
molecules would improve the binding ability for anions, compared to
the unsubstituted analogues 1 and 2, increasing the corresponding
association constants. Whereas nitro group only increased stability of
the Fꢀ complexes, cyano derivatives displayed stronger binding with
all anions except AcOꢀ. However, the effect of the electron-with-
drawing groups was more demonstrated through the spectral
changes observed on titration with anions. Absorption spectra of the
receptors with NO2 (3 and 4) and CN substituents (7 and 8) have
partial charge transfer character. The hydrogen bond interaction
between anion and the pyrroles increases the electron density on the
pyrroles, which further increases the charge transfer character in both
ground and excited states. Therefore, we observed large spectral
changes in the titration experiments with 7 and 8, which render them
as useful UVevis and fluorescence sensors for anions.
and 8
In a two-neck flask under a stream of nitrogen carbaldehyde 10
or ketone 11 (1 mmol) and the corresponding phenylpyrroles 9aec
(2 mmol) were dissolved in a small volume of dry CH2Cl2 (ca.
5e10 mL). By use of a syringe, to the reaction mixture was added
TFA (0.1 mmol) and the mixture was stirred at rt, while the progress
of the reaction was followed by TLC using CH2Cl2 as an eluent. Upon
the disappearance of the starting 2-phenylpyrrole, the reaction was
quenched by the addition of an aqueous solution of NaOH (0.1 M,
w20 mL) and the layers were separated. The aqueous layer was
extracted two more times using CH2Cl2 (2ꢁ20 mL), the organic
extracts were washed with water (20 mL), collected and dried over
anhydrous MgSO4. After filtration, the solvent was evaporated on
a rotary evaporator and the obtained dark residue was purified by
chromatography, crystallization or washing with solvent.
4.2.1. 1-[Di(5-phenyl-1H-pyrrol-2-yl)methyl]adamantane
Obtained by reacting adamantane-1-carbaldehyde (10, 103 mg,
0.63 mmol) with 2-phenylpyrrole (9a, 180 mg, 1.26 mmol) in the
(1).
presence of TFA (10
mL, 0.13 mmol) in dry CH2Cl2 (5 mL). After
3. Conclusion
stirring over 7 days at rt the reaction was quenched and worked-up
following the general procedure. After two chromatographies on
silica gel using 0/50% CH2Cl2 in hexane as eluent reaction fur-
nished 187 mg of product 1 in the form of colourless crystals. The
obtained compound was further dried on 60 ꢃC and 1000 Pa for 3
days. Colourless crystals, 187 mg, 69%; mp 92e94 ꢃC; UV (CH3CN)
New adamantaneedipyrromethane anion receptors 1e8 were
synthesized and their binding ability in the CH3CN solutions with
various TBA salts (Fꢀ, Clꢀ, Brꢀ, AcOꢀ, HSO4ꢀ, NOꢀ3 and H2POꢀ4 ) was
investigated by UVevis, fluorescence and/or 1H NMR spectroscopy.
Overall, the most stable complexes were formed with basic anions
lmax (log
): 304 nm (4.59 Mꢀ1 cmꢀ1); IR (KBr) nmax 3430 (m), 2901
3

ꢀ ꢁ
M. Aleskovic et al. / Tetrahedron 69 (2013) 1725e1734
1732
(m), 2845 (w), 1604 (w), 1506 (m), 753 (s), 692 (m) cmꢀ1; 1H NMR
(CDCl3, 300 MHz) 1.58e1.72 (m, 12H), 1.97 (br s, 3H), 3.62 (s, 1H),
2.7 Hz), 6.38 (dd, 2H, J¼3.5, 2.7 Hz), 7.20e7.24 (m, 2H), 7.43e7.48 (m,
2H), 7.54 (dd, 2H, J¼8.1,1.2 Hz), 7.66 (dd, 2H, J¼8.1,1.2 Hz), 8.79 (br s,
d
6.17e6.24 (m, 2H), 6.44e6.52 (m, 2H), 7.12e7.19 (m, 2H), 7.29e7.34
2H) ppm; 13C NMR (CDCl3, 150 MHz)
d
27.1 (2ꢁCH), 33.4 (4ꢁCH2),
(m, 4H), 7.37e7.44 (m, 4H), 8.27 (br s, 2H) ppm; 13C NMR (CDCl3,
33.5 (2ꢁCH), 37.8 (CH2), 45.3 (C), 106.3 (2ꢁCH), 111.5 (2ꢁCH), 124.5
(2ꢁCH),124.7 (2ꢁC),126.0 (2ꢁCH),126.9 (2ꢁC),130.2 (2ꢁCH),132.1
(2ꢁCH), 140.5 (2ꢁC), 147.3 (2ꢁC) ppm; HRMS calculated for
C30H29N4O4 ([MþH]þ): 509.2183; observed: 509.2187.
75 MHz)
d
28.6 (3ꢁCH), 36.7 (3ꢁCH2), 37.0 (C), 40.4 (3ꢁCH2), 51.9
(CH), 106.2 (2ꢁCH), 108.5 (2ꢁCH), 123.3 (4ꢁCH), 125.7 (2ꢁCH),
128.7 (4ꢁCH), 130.2 (2ꢁC), 131.2 (2ꢁC), 132.6 (2ꢁC) ppm; HRMS
calculated for C31H33N2 ([MþH]þ): 433.2644; observed: 433.2650.
4.2.5. 1-{Di[5-(2-cyanophenyl)-1H-pyrrol-2-yl]methyl}adamantane
4.2.2. 2,2-Di(5-phenyl-1H-pyrrol-2-yl)adamantane
(2). Obtained
(7). Obtained by reacting carbaldehyde 10 (120 mg, 0.7 mmol) with
by reacting adamantane-2-one (11, 95 mg, 0.63 mmol) with 2-
phenylpyrrole (9a, 180 mg, 1.26 mmol) in the presence of TFA
(10 mL, 0.13 mmol) in dry CH2Cl2 (5 mL). After stirring over 5 days at
rt the reaction was quenched and worked-up following the general
procedure. After two chromatographies on silica gel using 0/50%
CH2Cl2 in hexane as eluent reaction furnished 173 mg of product 2
in the form of colourless crystals. The obtained compound was
further dried on 60 ꢃC and 1000 Pa for 3 days. Colourless crystals,
phenylpyrrole 9c (245 mg, 1.4 mmol) in the presence of TFA (11 mL,
0.1 mmol) in dry CH2Cl2 (10 mL). After stirring over 6 days at rt the
reaction was quenched and worked-up following the general pro-
cedure. To remove unreacted phenylpyrrole precursor 9c the crude
mixture was washed with the CH2Cl2 and hexane mixture. Ana-
lytically pure compound (256 mg) in the form of white powder was
obtained by crystallization from methanol, which was further dried
on 60 ꢃC and 1000 Pa for 24 h. White powder, 256 mg, 73%; mp
173 mg, 66%; mp 75e77 ꢃC; UV (CH3CN) lmax (log
(4.52 Mꢀ1 cmꢀ1); IR (KBr) nmax 3448 (m), 2901 (m), 2853 (w), 1603
(m), 1503 (m), 1207 (w), 751 (s) cmꢀ1 1H NMR (CDCl3, 300 MHz)
1.73e1.90 (m, 8H), 2.23e2.32 (m, 4H), 2.72 (br s, 2H), 6.11e6.19
(m, 2H), 6.32e6.39 (m, 2H), 7.08e7.16 (m, 2H), 7.26e7.38 (m, 8H),
7.97 (br s, 2H) ppm; 13C NMR (CDCl3, 75 MHz)
(4ꢁCH2), 33.9 (2ꢁCH), 38.0 (1ꢁCH2), 45.4 (1ꢁC), 105.7 (2ꢁCH),
106.2 (2ꢁCH), 123.3 (4ꢁCH), 125.6 (2ꢁCH), 128.6 (4ꢁCH), 130.5
(2ꢁC), 132.8 (2ꢁC), 139.0 (2ꢁC) ppm; HRMS calculated for
C30H29N2 ([MꢀH]þ): 417.2331; observed: 417.2323.
3
): 306 nm
216e218 ꢃC; UV (CH3CN) lmax (log
4.26 Mꢀ1 cmꢀ1); IR (KBr) nmax 3349 (m), 3325 (m), 2906 (s), 2850
(m), 2226 (s), 1594 (m), 1502 (s), 1268 (w), 1196 (w), 761 (s) cmꢀ1
1H NMR (CD3CN, 600 MHz)
1.57e1.62 (m, 3H), 1.65e1.72 (m, 9H),
2.07e2.10 (br s, 2H), 3.79 (s, 2H), 6.29 (dd, 2H, J¼3.5, 2.6 Hz), 6.78
(dd, 2H, J¼3.5, 2.6 Hz), 7.26e7.32 (m, 2H), 7.59e7.63 (m, 4H),
7.68e7.72 (m, 2H), 9.47 (br s, 2H) ppm; 1H NMR (CDCl3, 300 MHz)
3
): 304 and 348 nm: (4.30 and
;
;
d
d
d
27.3 (2ꢁCH), 33.6
d
1.56e1.75 (m, 12H), 1.99 (br s, 3H), 3.67 (s, 1H), 6.31 (dd, 2H, J¼3.5,
2.7 Hz), 6.74e6.82 (m, 2H), 7.14e7.23 (m, 2H), 7.51 (dt, 2H, J¼7.4,
1.3 Hz), 7.56e7.66 (m, 4H), 9.12 (br s, 2H) ppm; 13C NMR (CDCl3,
75 MHz)
d
28.5 (3ꢁCH), 36.6 (3ꢁCH2), 37.5 (C), 40.3 (3ꢁCH2), 52.0
4.2.3. 1-{Di[5-(2-nitrophenyl)-1H-pyrrol-2-yl]methyl}adamantane
(3). Obtained by reacting carbaldehyde 10 (362 mg, 2.2 mmol) with
phenylpyrrole 9b (835 mg, 4.4 mmol) in the presence of TFA (33 mL,
(CH), 105.0 (2ꢁC), 109.5 (2ꢁCH), 110.8 (2ꢁCH), 120.5 (2ꢁC), 125.2
(2ꢁCH), 126.1 (2ꢁCH), 126.7 (2ꢁC), 132.9 (2ꢁC), 133.2 (2ꢁC), 133.8
(2ꢁCH), 135.4 (2ꢁC) ppm; HRMS: calculated for C33H30N4 ([M]þ):
482.2465; observed: 482.2462.
0.44 mmol) in dry CH2Cl2 (10 mL). After stirring over 4 days at rt the
reaction was quenched and worked-up following the general pro-
cedure. After chromatography on silica gel using CH2Cl2 as eluent
and the subsequent rechromatography on silica gel using 0/5%
EtOAc in CH2Cl2 as eluent reaction furnished 712 mg of product 3 in
the form of red crystals. The obtained compound was further dried
on 35 ꢃC and 1000 Pa for 20 h. Red crystals, 712 mg, 62%; mp
4.2.6. 2,2-Di[5-(2-cyanophenyl)-1H-pyrrol-2-yl]adamantane
(8). Obtained by reacting ketone 11 (90 mg, 0.6 mmol) with
phenylpyrrole 9c (200 mg, 1.2 mmol) in the presence of TFA (8 mL,
0.1 mmol) in dry CH2Cl2 (10 mL). After stirring over 5 days at rt the
reaction was quenched and worked-up following the general
procedure. To remove unreacted phenylpyrrole precursor 9c the
crude mixture was washed with the CH2Cl2 hexane mixture.
Analytically pure compound (172 mg) in the form of white powder
was obtained by chromatography on preparative TLC with 10%
diethyl ether in CH2Cl2 as eluent. The compound was further dried
on 60 ꢃC and 1000 Pa for 48 h. White powder, 172 mg, 62%; mp
88e90 ꢃC; UV (CH3CN) lmax (log
(KBr) nmax 3424 (m), 2901 (m), 2846 (m), 1605 (w), 1521 (s), 1349
(w), 778 (m), 743 (m) cmꢀ1; 1H NMR (CDCl3, 600 MHz)
1.57e1.63
3
): 299 nm (4.37 Mꢀ1 cmꢀ1); IR
d
(m, 9H), 1.64e1.68 (m, 3H), 1.98 (br s, 3H), 3.59 (s, 1H), 6.22 (dd, 2H,
J¼3.5, 2.7 Hz), 6.47 (dd, 2H, J¼3.5, 2.7 Hz), 7.28 (dt, 1H, J¼7.8, 1.1 Hz),
7.50 (dt, 1H, J¼7.8, 1.2 Hz), 7.58 (dd, 1H, J¼8.1, 1.2 Hz), 7.69 (dd, 1H,
J¼8.1, 1.1 Hz), 8.83 (br s, 2H) ppm; 13C NMR (CDCl3, 150 MHz)
d
28.6
230e231 ꢃC; UV (CH3CN) lmax (log
): 304 and 355 nm (4.27 and
3
(3ꢁCH), 36.6 (3ꢁCH2), 37.3 (C), 40.2 (3ꢁCH2), 51.8 (CH), 109.1
(2ꢁCH), 111.7 (2ꢁCH), 124.5 (2ꢁCH), 124.7 (2ꢁC), 126.3 (2ꢁCH),
126.8 (2ꢁC), 130.3 (2ꢁCH), 132.2 (2ꢁCH), 132.9 (2ꢁC), 147.5
(2ꢁC) ppm; HRMS calculated for C31H30N4NaO4 ([MþNa]þ):
545.2159; observed: 545.2136.
4.26 Mꢀ1 cmꢀ1); IR (KBr) nmax 3446 (m), 3142 (m), 3377 (m), 3317
(m), 2902 (s), 2854 (m), 2226 (s), 1602 (s), 1498 (s), 1216 (m), 1055
(w), 757 (s) cmꢀ1; 1H NMR (CD3CN, 600 MHz)
d 1.77e1.86 (m, 8H),
2.14e2.20 (m, 4H), 2.96 (br s, 2H), 6.18 (dd, 2H, J¼3.6, 2.7 Hz), 6.66
(dd, 2H, J¼3.6, 2.7 Hz), 7.25e7.29 (m, 2H), 7.57e7.60 (m, 4H),
7.66e7.70 (m, 2H), 9.25 (br s, 2H) ppm; 1H NMR (CDCl3, 300 MHz)
4.2.4. 2,2-Di[5-(2-nitrophenyl)-1H-pyrrol-2-yl]adamantane
d 1.72e1.99 (m, 8H), 2.15e2.99 (m, 4H), 2.82 (br s, 2H), 6.22 (dd, 2H,
(4). Obtained by reacting ketone 11 (223 mg, 1.5 mmol) with phe-
J¼3.6, 2.7 Hz), 6.67 (dd, 2H, J¼3.6, 2.7 Hz), 7.08e7.19 (m, 2H),
nylpyrrole 9b (558 mg, 3 mmol) in the presence of TFA (33
mL,
4.41e7.51 (m, 2H), 7.53e7.61 (m, 4H), 9.12 (br s, 2H) ppm; 13C NMR
0.44 mmol) in dry CH2Cl2 (10 mL). After stirring over 4 days at rt the
reaction was quenched and worked-up following the general pro-
cedure. After chromatography on silica gel using 0/5% EtOAc in
CH2Cl2 as eluent reaction furnished 518 mg of product 4 in the form
of red crystals. The obtained compound was further dried on 35 ꢃC
and 1000 Pa for 20 h. Red crystals, 518 mg, 68%; mp 198e200 ꢃC; UV
(CDCl3,75 MHz)
d
27.1 (2ꢁCH), 33.3 (2ꢁCH), 33.5 (4ꢁCH2), 37.7
(CH2), 45.5 (C), 104.5 (2ꢁC), 106.6 (2ꢁCH), 110.5 (2ꢁCH), 120.8
(2ꢁC), 124.9 (2ꢁCH), 125.9 (2ꢁCH), 126.6 (2ꢁC), 132.8 (2ꢁCH),
133.7 (2ꢁCH), 135.6 (2ꢁC), 140.9 (2ꢁC) ppm; HRMS calculated for
C32H28N4 ([M]þ): 468.2387; observed: 468.2380.
(CH3CN) lmax (log
3
): 302 nm (4.31 Mꢀ1 cmꢀ1); IR (KBr) nmax 3449
4.3. General procedure for preparation of receptors 5 and 6
(s), 2926 (w), 2897 (w), 2878 (w), 1605 (w), 1521 (s), 1487 (m), 1333
(s),1292 (m),1206 (m),1038 (m), 850 (w), 781 (m), 743 (m) cmꢀ1; 1H
Nitro compounds 3 and 4 were submitted to hydrogenation on
10% Pd/C in abs methanol in a Paar apparatus at 60 psi of hydrogen
on rt. After the disappearance of the characteristic red colour of the
NMR (CDCl3, 600 MHz)
d
1.76 (br s, 2H), 1.82 (d, 4H, J¼12.2 Hz), 1.86
(br s, 2H), 2.19 (d, 4H, J¼12.2 Hz), 2.73 (br s, 2H), 6.15 (dd, 2H, J¼3.5,

ꢀ
ꢁ
M. Aleskovic et al. / Tetrahedron 69 (2013) 1725e1734
1733
compounds the catalyst was filtered off, and the solvent was
evaporated under reduced pressure to afford crude products of
amino derivatives 5 and 6, which were further chromatographed
on a silica gel column.
z0.08. The solution of the receptor was placed in a quartz cuvette
(3 mL) and small volumes (10 L) of the solutions of anion are added:
m
TBAF (1 M in THF, containing <5 wt % H2O, diluted with CH3CN to
1ꢁ10ꢀ3 M), or TBACl, TBABr, TBAOAc, TBAHSO4 or TBAH2PO4 (from
1ꢁ10ꢀ3 to 1ꢁ10ꢀ1 M in CH3CN). After each addition, fluorescence
spectra were recorded. The titrations were performed at 25 ꢃC. The
obtained data were processed using SPECFIT program.
In other set of titrations experiments freshly prepared samples
in 1-cm quartz cells were used to perform all UVevis absorption
and emission measurements. All spectra were recorded at 20 ꢃC
using undegassed samples. The titration experiments were carried
out by adding small quantities of a stock solution of tetrabuty-
lammonium salt in acetonitrile to a much larger volume (25 mL) of
solutions of compounds. The data were processed as specified in
Supplementary data.
4.3.1. 1-{Di[5-(2-aminophenyl)-1H-pyrrol-2-yl]methyl}adamantane
(5). Following the general procedure amino compound 5 was
obtained by hydrogenation of nitro derivative 3 (100 mg, 0.2 mmol)
dissolved in abs methanol (40 mL) with 10% Pd/C catalyst (40 mg).
After 5 h under pressure of 4070 hPa of hydrogen the reaction was
completed. Purification by chromatography on a silica gel column
with 0/1% MeOH in CH2Cl2 as eluent afforded 58 mg of the product
5 in the form of colourless crystals. Colourless crystals, 58 mg, 65%;
mp 113e115 ꢃC; UV (CH3CN) lmax (log
4.29 Mꢀ1 cmꢀ1); IR (KBr) nmax 3420 (s), 2902 (s), 2846 (w),1615 (m),
749 (m) cmꢀ1 1H NMR (CDCl3, 300 MHz)
1.51e1.71 (m, 12H),
3 ): 280 and 320 nm (4.28 and
;
d
1.90e2.0 (br s, 3H), 3.59 (s, 1H), 3.93 (br s, 4H), 6.12e6.23 (m, 2H),
6.29e6.42 (m, 2H), 6.69e6.87 (m, 4H), 6.99e7.11 (m, 2H), 7.19e7.24
4.6. 1H NMR titrations
(m, 2H), 8.48 (br s, 2H) ppm; 13C NMR (CDCl3, 75 MHz)
d
28.6
To a 0.5 mL of the CD3CN solution of anion receptor (cz0.01 M)
was added a solution of TBAF (1 M in THF, containing <5 wt % H2O)
or TBAHSO4 (z0.1 M in CD3CN). After each addition, 1H NMR
spectra were recorded.
(3ꢁCH), 36.7 (3ꢁCH2), 37.1 (C), 40.3 (3ꢁCH2), 51.7 (CH), 107.5
(2ꢁCH),107.8 (2ꢁCH),116.5 (2ꢁCH),119.1 (2ꢁCH),119.8 (2ꢁC),127.3
(2ꢁCH), 127.6 (2ꢁC), 127.9 (2ꢁCH), 130.5 (2ꢁC), 142.8 (2ꢁC) ppm;
HRMS calculated for C31H34N4: 462.2778; observed: 462.2794.
4.7. Steady state and time-resolved fluorescence
4.3.2. 2,2-Di[5-(2-aminophenyl)-1H-pyrrol-2-yl]adamantane
(6). Following the general procedure amino compound 6 was
obtained by hydrogenation of nitro derivative 4 (220 mg, 0.4 mmol)
dissolved in abs methanol (75 mL) with 10% Pd/C catalyst (80 mg).
After 6 h under pressure of 3999 hPa of hydrogen the reaction was
completed. Purification by chromatography on a silica gel column
with 2/10% diethyl ether in CH2Cl2 as eluent afforded 139 mg of
the product 6 in the form of colourless crystals. White powder,
measurements
The compounds were dissolved in CH3CN (HPLC grade), or
cyclohexane (UV-spectroscopy grade) and the concentrations were
adjusted to have absorbance at the excitation wavelength <0.1.
Solutions were purged with argon for 30 min prior to analysis. The
measurements were performed at 20 ꢃC. Fluorescence quantum
yields were determined by comparison of the integral of the
139 mg, 72%; mp 168e169 ꢃC; UV (CH3CN) lmax (log
3 ): 285 and
emission bands with the quinine sulfate in 0.5 M H2SO4 (
F
¼0.54).18
320 nm (4.26 and 4.33 Mꢀ1 cmꢀ1); IR (KBr) nmax 3430 (m), 3333
Typically, three absorption traces were recorded (and averaged)
and three fluorescence emission traces, excited at three different
wavelengths. Three quantum yields were calculated and the mean
value reported.
(m), 2902 (s), 2850 (s), 1610 (s), 1502 (s), 1474 (m), 1445 (w), 1200
(w), 773 (s), 745 (s) cmꢀ1 1H NMR (CD3CN, 600 MHz)
; d 1.77e1.83
(m, 8H), 2.17e2.21 (m, 4H), 2.9 (br s, 2H), 4.23 (s, 4H), 6.06 (dd, 2H,
J¼3.4, 2.8 Hz), 6.19 (dd, 2H, J¼3.4, 2.8 Hz), 6.69 (dt, 1H, J¼7.4,
0.9 Hz), 6.73 (dd, 1H, J¼7.9, 0.9 Hz), 6.97e7.00 (m, 1H), 7.14 (dd, 2H,
J¼7.6, 1.4 Hz), 8.89 (br s, 2H) ppm; 1H NMR (CDCl3, 300 MHz)
Fluorescence decay histograms were obtained on an Edinburgh
instrument OB920, equipped with light emitting diode (excitation
wavelength 310 nm), using time-correlated single photon counting
technique in 1023 channels. Histograms of the instrument response
functions (using LUDOX scatterer), and sample decays were recor-
ded until they typically reached 3ꢁ103 counts in the peak channel.
The half width of the instrument response function was typically
w1.5 ns. The time increment per channel was 0.02, 0.049 or
0.098 ns. Obtained histograms were fitted as sums of exponential
using Gaussian-weighted non-linear least-squares fitting based on
MarquardteLevenberg minimization implemented in the software
package of the instrument. The fitting parameters (decay times and
pre-exponential factors) were determined by minimizing the
d
1.71e1.91 (m, 8H), 2.19e2.33 (m, 4H), 2.70 (br s, 2H), 3.82 (s, 4H),
6.07e6.13 (m, 2H), 6.20e6.27 (m, 2H), 6.68e6.82 (m, 4H), 7.02 (dt,
1H, J¼7.8, 1.3 Hz), 7.16 (dd,1H, J¼7.6, 1.1 Hz), 8.30 (br s, 2H) ppm; 13C
NMR (CDCl3, 75 MHz)
d
27.4 (2ꢁCH), 33.6 (4ꢁCH2), 33.7 (2ꢁCH),
38.0 (CH2), 45.4 (C), 105.2 (2ꢁCH), 106.8 (2ꢁCH), 116.5 (2ꢁCH),
119.1 (2ꢁCH), 120.1 (2ꢁC), 127.2 (2ꢁCH), 127.8 (2ꢁC), 127.9 (2ꢁCH),
138.5 (2ꢁC), 142.8 (2ꢁC) ppm; HRMS calculated for C30H32N4:
448.2621; observed: 448.2625.
4.4. UVevis titrations
reduced chi-square
c
2. Additional graphical method was used to
The receptor was dissolved in spectroscopic grade CH3CN in the
concentration range 10ꢀ5 M, corresponding to the maximum of
absorbance in the range 0.6e0.9. The solution of the receptor was
judge the quality of the fit that included plots of surfaces (‘carpets’)
of the weighted residuals versus channel number.
placed in a quartz cuvette (1 mL) and small volumes (5e20
mL) of
4.8. Crystallography
the following anion solutions were added: TBAF (1 M in THF,
containing <5 wt % H2O, diluted with CH3CN to 1ꢁ10ꢀ3 M), TBACl,
Single crystal was obtained by slow evaporation of CH3CN/
hexane (w1:1) solution of the receptor at the 4 ꢃC. Single crystal
diffraction data were collected from the crystal glued on a glass
TBABr, TBAOAc, TBAHSO4 or TBAH2PO4 (from 1ꢁ10ꢀ3 to 1ꢁ10ꢀ1
M
in CH3CN). After each addition, UVevis spectra were recorded. The
titrations were performed at 25 ꢃC. The spectrometric data were
processed using SPECFIT program.
fibre tip. Diffraction intensity data were collected by
u-scans on an
Oxford Diffraction Xcalibur 3 using graphite-monochromated Mo
19
ꢂ
K
a
radiation (
l
¼0.71073 A) and reduced using the CrysAlis
4.5. Fluorescence titrations
program package.
A
summary of general and crystal data,
intensity data collection and final refinement parameters are
presented in Table S1 (Supplementary data). The structures were
solved by direct methods using SHELXS.20 The refinement
The receptor was dissolved in CH3CN in the concentration range
z10ꢀ5 M, corresponding to the absorbance maximum in the range

ꢀ ꢁ
M. Aleskovic et al. / Tetrahedron 69 (2013) 1725e1734
1734
procedure by full-matrix least-squares methods based on F2 values
against all reflections included anisotropic displacement parame-
ters for all non-H atoms. The positions of H-atoms each riding on its
parent carbon atom were determined on stereochemical grounds.
Hydrogen atoms bonded to pyrrole nitrogen atoms were located
form difference Fourier map and isotropically refined. Refinements
were performed using SHELXL-97.20 The SHELX programs operated
within the WinGX21 suite. Molecular graphics were done with
Mercury22 and ORTEP.23 Supplementary crystallographic data sets
for the structures are available through the Cambridge Structural
Database with deposition number CCDC900725. Copy of this in-
formation may be obtained free of charge from the director, CCDC,
12 Union Road, Cambridge CB2 1EZ, UK (fax: þ44 1223 336 033;
e-mail: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).
Hursthouse, M. B.; Light, M. E. Org. Biomol. Chem. 2004, 2, 2935e2941; (f)
Camiolo, S.; Gale, P. A.; Hursthouse, M. B.; Light, M. E. Tetrahedron Lett. 2002, 43,
6995e6996; (g) Navakhun, K.; Gale, P. A.; Camiolo, S.; Light, M. E.; Hursthouse,
M. B. Chem. Commun. 2002, 2084e2085; (h) Schmuck, C.; Bickert, V. Org. Lett.
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Chem. 2007, 31, 646e654; (m) Sun, Y.; Zhong, C.; Gong, R.; Mu, H.; Fu, E. J. Org.
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Acknowledgements
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ꢁ
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Tetrahedron 2009, 65, 2051e2058; (c) Aleskovic, M.; Basaric, N.; Mlinaric-Ma-
jerski, K.; Molcanov, K.; Kojic-Prodic, B.; Kesharwani, M. K.; Ganguly, B.
ꢀ
ꢁ
ꢁ
The support of The Croatian Ministry of Science, Education and
Sports (Grant no. 098-0982933-2911), the ChineseeCroatian
Bilateral Project and the ‘International Cooperation Program of
Gansu Province’ (1104WCGA182) is gratefully acknowledged. The
authors thank Prof. C. Bohne and Prof. P. Wan and the University of
Victoria for the use of single photon timing set-up. We thank
Tetrahedron 2010, 66, 1689e1698.
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67, 3846e3857; (b) Blazek, V.; Mlinaric-Majerski, K.; Qin, W.; Basaric, N. J.
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