Calix[2]-m-benzo[4]phyrin with aggregation-induced enhanced-emission characteristics: Application as a HgII chemosensor

Article abstract of DOI:10.1002/chem.201100046

Quenched by mercury: A hybrid, core-modified, expanded calixphyrin was synthesized, confirmed by single-crystal X-ray analysis, and found to exhibit aggregation-induced enhanced-emission (AIEE) characteristics. The aggregate formation was confirmed by HR-

Full text of DOI:10.1002/chem.201100046

DOI: 10.1002/chem.201100046
Calix[2]-m-benzo[4]phyrin with Aggregation-Induced Enhanced-Emission
Characteristics: Application as a HgII Chemosensor
P. S. Salini,[a] Ajesh P. Thomas,[c] R. Sabarinathan,[a] S. Ramakrishnan,[a]
K. C. Gowri Sreedevi,[a] M. L. P. Reddy,[b] and A. Srinivasan*[c]
In recent years, solid-state fluorescent materials have re-
ceived much attention in diverse fields, such as fluorescent
biological labels, sensors, and light-emitting diodes.[1] Many
conjugated organic luminophores are highly emissive in
dilute solutions, however, when fabricated into thin films,
they suffer an aggregation-caused quenching (ACQ) effect,
which limits their practical applications.[1] To solve this prob-
lem, recently, some organic molecules, which show an in-
tense emission in the aggregated state, whereas they exhibit
almost no or weak emission in dilute solutions, were report-
ed. This unique phenomenon is widely researched as aggre-
gation-induced emission (AIE)[2] and aggregation-induced
enhanced emission (AIEE).[3] However, the potential utility
of this phenomenon has not been explored in macrocycles.
On the other hand, molecular sensors for HgII ions have re-
ceived considerable attention due to the extremely toxic
impact exerted by HgII on our environment.[4] Many efforts
have been made to design various chemosensors specific to
HgII ions.[5] The major challenges involved in the creation of
HgII sensors are lack of selectivity and specificity.
The syntheses of calixbenzophyrins are outlined in
Scheme 1. The synthetic methodology followed here is basi-
cally an acid-catalyzed condensation reaction. Stirring a so-
Scheme 1. Syntheses of calix[n]-m-benzo[m]phyrins.
lution of 1[7] with pentafluorobenzaldehyde in dichlorome-
thane in the presence of trifluoroacetic acid (TFA) followed
by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ) resulted in calix[n]-m-benzo[m]phyrins (n=2,3,4;
m=4,6,8) in 20% (M-1), 10% (M-2), and 5% (M-3) yield,
respectively. All the macrocycles were fully characterized by
electronic, FAB-MS, and NMR spectral studies, and the
structure of M-1 was finally confirmed by single-crystal X-
ray-diffraction analysis.[8] All the derivatives are soluble in
common organic solvents, but insoluble in water.
Calixphyrins, polypyrrolic macrocycles, are known for the
coordination chemistry,[6] but reports on metal-ion sensing
are very rare. Herein, we wish to report the AIEE charac-
teristic of a calixbenzophyrin-derivative, M-1, and utilize
this novel property for probing HgII ions both in aqueous so-
lution and in the solid state.
[a] P. S. Salini, R. Sabarinathan, S. Ramakrishnan, K. C. G. Sreedevi
Photosciences and Photonics Section
A solution of M-1 in dilute acetonitrile shows an absorp-
tion band at 434 nm, which arises from the p–p* transition
of the dipyrrin moiety.[8] The emission spectrum shows a
band at 539 nm. We observed an anomalous behavior in the
emission spectrum of M-1 upon addition of various percen-
tages of water (0–90%), upon which the band at 539 nm is
intensified with a bathochromic shift to 550 nm (Figure 1,
left). Because water is not a good solvent for M-1, addition
of water promotes efficient calixphyrin self-aggregation,
which enhances the emission; in other words, M-1 is AIEE
active. The trajectory of the intensity change suggests that
the molecularly dissolved M-1 starts to congregate at a
water fraction of 50% and the population of the aggregate
continues to increase as the water fraction increases from 50
to 90% (Figure 1, left, inset). To have a quantitative picture,
we estimated the photoluminescence (PL) quantum yields
(FF) of M-1 in pure acetonitrile and in a 1:9 acetonitrile/
Chemical Sciences and Technology Division
National Institute for Interdisciplinary Science and Technology
(NIIST-CSIR)
Thiruvananthapuram - 695019, Kerala (India)
[b] Dr. M. L. P. Reddy
Inorganic Chemistry Section
Chemical Sciences and Technology Division
National Institute for Interdisciplinary Science and Technology
(NIIST-CSIR)
Thiruvananthapuram - 695019, Kerala (India)
[c] A. P. Thomas, Dr. A. Srinivasan
School of Chemical Sciences
National Institute of Science Education and Research (NISER)
Bhubaneswar - 751005, Orissa (India)
Fax : (+91)674-2302436
E-mail: srini@niser.ac.in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100046.
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Figure 1. Left: Emission spectra of M-1 (8.7 mm) in acetonitrile/water
mixtures with different fractions of water (fw =0–90 vol%). The inset
shows the changes in emission intensities (I) of M-1 with increased water
contents. I0 =intensity in pure acetonitrile. Right: Temperature effect on
the emission peak intensity of M-1 in acetonitrile/water (1:9 v/v, lex
=
440 nm).
water mixture. The FF values were found to be 0.8ꢁ10À3
and 3.36ꢁ10À3, respectively, which is 4.2 fold higher than
that in a pure acetonitrile. A similar trend was observed in
the previously studied AIEE systems.[9] Basically, when the
water content is increased to 50%, it reaches a critical
point, at which addition of a small amount of water signifi-
cantly weakens the solvating power of the solvent mixture.
This greatly populates the aggregates and leads to rapid in-
crease in the emission intensity. The enhancement in emis-
sion also eliminates the possible formation of delocalized
excitons or excimers, which usually quenches the emission.[9]
Further, the temperature effect on the fluorescence spec-
tra of M-1 in an acetonitrile/water mixture (1:9 v/v) was also
investigated (Figure 1, right). Upon increasing the tempera-
ture, the aggregate state of the compound at low tempera-
tures changed to a monomer-like state at high temperatures.
This proves that aggregation restricts intramolecular rota-
tions (IMR) of the pentafluorophenyl group; this rigidifies
the molecule and hence makes it more emissive. The single-
crystal X-ray studies further confirm that the IMRs in M-1
have indeed been restricted during aggregation.
Figure 2. Single-crystal X-ray structure of M-1. a) Top view, b) side view,
and c) 2-D array.
À
volved: F7 in the first molecule interacts both with C14
H14 in one of the m-phenyl units present in the second mol-
ecule, to form the second self-assembled dimer, and further
À
with C36 H36 in the m-phenyl group of the third molecule
to form the 1-D array with the distance and angles of
2.62 ꢂ, 1468 and 2.66 ꢂ, 1558, respectively.[8] By combining
these two self-assembled dimers and the 1-D array, M-1 gen-
erates the 2-D supramolecular assembly shown in Figure 2c.
Overall, the meso-pentafluorophenyl groups are effectively
packed by the intermolecular hydrogen-bonding interac-
tions; this restricts the intramolecular rotation of these units
in the solid state and leads to further emission enhancement,
as observed in the aggregated state.
The single-crystal X-ray structure of M-1 is shown in
Figure 2. The dipyrrin units contain both the amine and
imine pyrrole nitrogen atoms that form intramolecular hy-
drogen-bonding interactions with the distance and angles of
The efficient emission of M-1 in the aggregated state
prompted us to explore the potential application as chemo-
sensors. All the spectroscopic measurements were carried
out in 90% aqueous acetonitrile solutions at pH 7.4 (10 mm
phosphate buffer containing 2 mm NaCl). Compound M-1
was treated with ten equivalents of various metal ions, such
as CuII, CdII, CoII, CrIII, FeIII, KI, LiI, NiII, PbII, MnII, ZnII,
and HgII, to test the sensing ability. Among the tested metal
ions, only HgII exhibited effective fluorescence quenching
(Figure 3a). To have a quantitative picture, various equiva-
lents of HgII ions were gradually added to the M-1 solution.
As the concentration of HgII ions was increased, the emis-
sion intensity gradually quenches (Figure 3b). Also, the
competitive-recognition studies reveal that HgII ions could
À
À
N1 H1A···N2 and N3···H4A N4 are 2.08 ꢂ, 1308 and
2.02 ꢂ, 1358, respectively (Figure 2b). The m-phenyl units
and the meso-pentafluorophenyl groups are deviated from
the mean dipyrrin plane by 89.92 and 71.98, respectively.[8]
The fluorine atoms in the meso-pentafluorophenyl groups
generate intermolecular hydrogen-bonding interactions, in
which F2 interacts with the pyrrolic b-CH to form a self-as-
À
sembled dimer (C3 H3···F2: 2.57 ꢂ and 1548). In addition
to the self-assembled dimer, F7 generates a 1-D array. To
generate such interactions, three molecules of M-1 are in-
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A. Srinivasan et al.
Figure 4. HR-TEM images: a) M-1 (scale bar=1 mm) and b) M-1 in the
presence of ten equivalents of HgII ions (scale bar=50 nm).
The microrod-shaped aggregates of M-1 with a length of
2 mm (Figure 4a) dissociate into nanospheres, with a uniform
diameter of 12 nm, when ten equivalents of HgII ions were
added to M-1 (Figure 4b).
In summary, we have synthesized three hybrid, core-modi-
fied, expanded calixphyrins and exploited the AIEE charac-
teristics of M-1. To the best of our knowledge, the emission
features described herein are hitherto unknown in calixphyr-
in chemistry. We could utilize this novel property as a probe
for the detection of HgII ions both in solution as well as in
the solid state with excellent selectivity over 100 equivalents
of other physiologically important metal ions. The emission
characteristics and sensing studies are now extended to
other expanded calixbenzophyrins and the research is cur-
rently underway in our laboratory.
Figure 3. a) Fluorescence response of M-1 (8.7 mm) in acetonitrile/water
(1:9 v/v) at pH 7.4 (10 mm phosphate buffer containing 2 mm NaCl) in
the presence of ten equivalents of various metal ions: 1) CuII, 2) CdII,
3) CoII, 4) CrIII, 5) FeIII, 6) KI, 7) LiI, 8) NiII, 9) PbII, 10) MnII, 11) ZnII,
12) HgII, 13) blank. b) The emission changes of M-1 (8.7 mm) upon titra-
tion with HgII ions in acetonitrile/water (1:9 v/v) at pH 7.4 (lex =480 nm).
c) Metal ion selectivity of M-1 (8.7 mm) in acetonitrile/water (1:9 v/v) at
pH 7.4. The small, colored bars represent the changes in the emission in-
tensity of a solution of M-1 with 100 equivalents of the cation of interest:
1) CuII, 2) CdII, 3) CoII, 4) CrIII 5) FeIII 6) KI, 7) LiI, 8) NiII, 9) PbII,
, ,
Experimental Section
10) MnII, 11) ZnII, 12) HgII. The turquoise bars show the fluorescence
change that occurs upon addition of ten equivalents of HgII to the solu-
tion containing M-1 and the respective cation. d) The solid-state color
change of M-1 upon addition of HgII ions under visible light (top) and il-
lumination under UV light (bottom, lex =365 nm).
Synthetic procedure: Pentafluorobenzaldehyde (0.211 mL, 1.71 mmol)
was added to a 500 mL flask containing 1 (0.5 g, 1.71 mmol) in dry di-
chloromethane (400 mL). The reaction mixture was stirred for 10 min at
room temperature under a nitrogen atmosphere with light protection.
TFA (0.791 mL, 10.27 mmol) was added and the reaction mixture was al-
lowed to stir for 2 h. DDQ (1.164 g, 5.13 mmol) was added. The solution
was opened to air and stirred for further 2 h and the solvent was removed
under reduced pressure. Column chromatographic purification (basic alu-
mina; 0, 5, and 10% dichloromethane in hexane as eluent) of the residue
afforded M-1, M-2, and M-3 as yellow solids in 20, 10 and 5% yield, re-
spectively. Crystals suitable for X-ray analysis were obtained in chloro-
form/n-hexane.
Spectral data for M-1: m.p. 2108C; 1H NMR (500 MHz, CDCl3, 258C,
TMS): d=12.39 (brs, 2H; NH), 7.17 (m, 2H; Ar), 7.10–7.09 (m, 4H;
Ar), 7.02–7.01 (d, J=1.5 Hz, 2H; Ar), 6.32–6.31 (d, J=4 Hz, 4H; pyrrol-
ic-b-CH), 6.23–6.22 (d, J=4 Hz, 4H; pyrrolic-b-CH), 1.26 ppm (s, 24H;
CH3). 13C NMR (125 MHz, CDCl3, 258C, TMS): d=166.04, 147.9, 145.84,
139.19, 128.14, 126.13, 125.28, 123.52, 111.76, 41.27, 29.71, 28.94 ppm. MS
(FAB): m/z (%): 936.56 [M]+ (100); elemental analysis calcd (%) for
C54H42F10N4: C 69.22, H 4.52, N 5.98; found: C 69.15, H 4.49, N 5.78.
Spectral data for M-2: m.p. 1928C; 1H NMR (500 MHz, CDCl3, 258C,
TMS): d=12.52 (brs, 3H; NH), 7.19–7.06 (m, 12H; Ar), 6.25–6.24 (d, J=
4 Hz, 6H; pyrrolic-b-CH), 6.08–6.07 (d, J=4.5 Hz, 6H; pyrrolic-b-CH),
1.25 ppm (s, 36H; CH3). 13C NMR (125 MHz, CDCl3, 258C, TMS): d=
166.44, 147.31, 139.13, 128.29, 126.10, 124.53, 124, 120.82, 117.03, 41.32,
32.77, 30.06, 29.72, 29.39, 29.19, 28.87, 22.72, 14.14 ppm. MS (FAB): m/z
(%): 1404.55 [M]+ (100); elemental analysis calcd (%) for C81H63F15N6: C
69.22, H 4.52, N 5.98; found: C 69.11, H 4.41, N 5.87.
be detected even in the presence of 100 equivalents of other
metal ions (Figure 3c). The changes in the emission-spectral
data indicate a 1:1 binding mode[8] with an association con-
stant of 4.2ꢁ104 mÀ1 and the calculated detection limit is
1.4 ppm. The results were further supported by MALDI-
TOF mass-spectral analysis, in which a peak corresponding
to the 1:1 complex was observed.[8] The reversibility of the
binding mode was examined by the addition of IÀ ions.[8]
These interesting results encouraged us to investigate the
sensing ability of M-1 in the solid state. Figure 3d shows the
color change of M-1 upon addition of HgII ions in the solid
state. The color of the ligand changes from yellow in the un-
coordinated state to brick red in the mercury-bound form.
Upon irradiation of UV light, an intense pale green fluores-
cence, which quenches effectively in the presence of the
HgII ions, was observed. This result proves that M-1 is an ex-
cellent candidate for solid-state detection of HgII ions.
Finally, the aggregate formation was confirmed by high-
resolution (HR)-TEM analysis. The images of M-1 in the
absence and presence of HgII ions are shown in Figure 4.
Spectral data for M-3: m.p. 1768C; 1H NMR (500 MHz, CDCl3, 258C,
TMS): d=12.52 (brs, 4H; NH), 7.20 (s, 4H; Ar), 7.14–7.06 (m, 12H; Ar)
6.24–6.23 (d, J=4 Hz, 8H; pyrrolic-b-CH), 6.06–6.05 (d, J=4.2 Hz, 8H;
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A HgII Chemosensor
COMMUNICATION
pyrrolic-b-CH) 1.29 ppm (s, 48H; CH3). 13C NMR (125 MHz, CDCl3,
258C, TMS): d=166.21, 147.15, 138.81, 127.99, 125.87, 124.09, 116.86,
41.10, 29.46, 28.62, 22.45, 13.88 ppm. MS (FAB): m/z (%): 1874.19 [M+
2]+ (80); elemental analysis calcd (%) for C108H84F20N8: C 69.22, H 4.52,
N 5.98; found: C 68.99, H 4.47, N 5.76.
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Acknowledgements
A. S. thanks DST - New Delhi, NWP-23 and the Directors, NIIST-CSIR
and NISER for financial support. We thank Dr. Babu Varghese, SAIF,
IIT-Chennai for solving the crystal structure of M-1. S. P. S. thanks R. A.
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Keywords: calixphyrins
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Received: January 5, 2011
Published online: April 28, 2011
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