A fluorescent ammonia sensor based on a porphyrin cobalt(II)-dansyl complex

Article abstract of DOI:10.1016/j.tetlet.2011.03.048

Detecting and measuring the concentration of ammonia is of interest in many scientific and technological areas. A porphyrin based cobalt(II) complex with a dansyl fluorophore has been synthesized and investigated as a 'turn-on' fluorescent ammonia sensor. Over sixfold increase in fluorescence emission occurs upon the treatment of NH₃ to [Co(TPP)(Ds-pip)] sensor solution, resulting from NH₃-induced displacement of the axially coordinated fluorophore.

Full text of DOI:10.1016/j.tetlet.2011.03.048

Tetrahedron Letters 52 (2011) 2645–2648
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Tetrahedron Letters
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A fluorescent ammonia sensor based on a porphyrin cobalt(II)–dansyl complex
Jiyeon Kim a, Si-Hyung Lim b, Yeoil Yoon c, T. Daniel Thangadurai a, , Sungho Yoon a,
⇑
⇑
a Department of Chemistry, Kookmin University, 861-1 Jeongneung-dong, Seongbuk-gu, Seoul 136-702, Republic of Korea
b Department of Mechanical Engineering, Kookmin University, 861-1 Jeongneung-dong, Seongbuk-gu, Seoul 136-702, Republic of Korea
c Greenhouse Gas Research Center, Korea Institute of Energy Research, 71-2, Jangdong, Yuseonggu 305-343, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Detecting and measuring the concentration of ammonia is of interest in many scientific and technological
areas. A porphyrin based cobalt(II) complex with a dansyl fluorophore has been synthesized and
investigated as a ‘turn-on’ fluorescent ammonia sensor. Over sixfold increase in fluorescence emission
occurs upon the treatment of NH3 to [Co(TPP)(Ds-pip)] sensor solution, resulting from NH3-induced
displacement of the axially coordinated fluorophore.
Received 17 January 2011
Revised 7 March 2011
Accepted 10 March 2011
Available online 16 March 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Cobalt(II)porphyrin
Dansylpiperazine
Axial ligand
Ammonia
Fluorescence enhancement
1. Introduction
concomitant fluorescence turn-on. Fluorescence enhancement is
generally preferred over fluorescence quenching when monitoring
Ammonia has been widely used in the production of explosives,
fertilizers, and as an industrial coolant,1 the excess presence of
ammonia in the atmosphere, however, creates potential hazards
to human being and ecosystems. Ammonia is toxic to many aquatic
organisms even in very low concentrations and inhalation of only a
small dose of ammonia vapor causes acute poisoning to people.
Detecting and measuring the concentration of ammonia is of inter-
est in many scientific and technological areas. Although electro-
chemical ammonia sensors are widely used in environmental
monitoring, automotive and chemical industry,2 in recent years,
several new approaches have been reported, including optical gas-
eous ammonia sensors.1,3–9 They utilize the reaction of ammonia
vapor with either a pH-dependent dye material or a pH-sensitive
film,7,10 which undergoes a suitable color change.
an analyte. Herein, we report that NH3 can displace a axial
fluorophore ligand bound to cobalt(II) complex with concomitant
fluorescence increase (Scheme 1). The observed fluorescence
response of these cobalt–fluorophore conjugates may ultimately
allow their use for measuring NH3 vapor in atmosphere.
2. Results and discussion
Metalloporphyrins have been used as colorimetric gas sensors
based on characteristic color changes, resulting from coordination
of analytes at the metal center.11–14 The metal center of M(II)-
porphyrins and M(II)-phthalocyanines possesses coordinatively
unsaturated axial sites, which are potentially used in catalysis or
development of sensor. In the adsorbed state, the underlying metal
surface can occupy one of the axial sites and, as an additional
ligand, influence the electronic structure of the metal center. The
nature of this interaction, which is the main focus of this paper,
has been studied previously, in particular on the example of the
coordinated cobalt ion.15 Moreover, the axial ligation properties
of the metalloporphyrins can be utilized for the self-organization
of the molecules.
Our method in developing sensors that utilize the formation of
transition-metal ammonia complexes to trigger
a change in
fluorescence is based on a strategy that takes advantage of the
well-known fluorescence-quenching properties of transition metal
complexes with partially filled d-shells. In particular, we have been
exploring systems in which reaction of NH3 with a transition-metal
complex containing a coordinated fluorophore conjugate results in
removal of the fluorophore from the coordination sphere with
Fluorescence quenching upon recognition of the analyte is not
only disadvantageous for a high signal output but also hampers
temporal separation of spectrally similar complexes with time-
resolved fluorometry.16 Thus, it is of interest that the recognition
of NH3 by the sensor does increase the fluorescence. To enhance
⇑
Corresponding authors. Tel.: +82 2 910 4763/5440; fax: +82 2 910 4415 (T.D.T.).
E-mail addresses: danielt@kookmin.ac.kr (T.D. Thangadurai), yoona@kookmin.
ac.kr (S. Yoon).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.03.048

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J. Kim et al. / Tetrahedron Letters 52 (2011) 2645–2648
Scheme 1. Mechanism of fluorescence enhancement of [Co(TPP)(Ds-pip)] complex upon the addition of ammonia.
the fluorescence intensity of the sensor system upon treatment of
NH3, one needs to carefully design the sensor molecule containing
a fluorophore so that the photoinduced intramolecular electron
transfer (PET) responsible for fluorescence quenching is maximized
in the sensor itself, whereas the PET is minimized in the NH3-
bound state of the sensor. To find such molecule, we chose few me-
pentane over 3 days at room temperature. The structure of 1 is
depicted in Figure 2 and interesting inter-atomic angles and
distances of 1 are represented in Table S2 (Supplementary data).
The cobalt atom has a distorted square pyramidal geometry.
Co–N(1), Co–N(2), Co–N(3), and Co–N(4) distances are in the range
of 1.981 and 1.992 Å, which are very common distances for the
coordinate bond between Co(II) ion and the N donor ligands. Inter-
estingly, Co–N(5) distance (2.254 Å) is longer than those of the
other Co–N distances in 1. The N(2)–Co–N(5) angle (87.41°) is
smaller than those in other N–Co–N(5) angles (96.94°, 94.28°,
tal complexes and tested with 10 lM concentration of fluorophore
solution. In the absence of metal complex, the emission spectrum
of the Ds-pip has one strong peak at 500 nm. When the fluorophore
reacts to metal complex solutions, the emission peak decreases
(Fig. 1). Among all the complexes tested, cobalt(II)tetraphenylpor-
phyrin [Co(TPP)] complex showed a largest quenching effect than
other metal complexes. Using this large quenching behavior of
cobalt(II)porphyrin complex, we decided to prepare a fluorescence
ensemble system based on a Ds-pip ligand, which can function as a
new class of synthetic sensor for NH3.
Reaction of 1:1 molar ratio of [Co(TPP)] and Ds-pip in CH2Cl2 at
room temperature afforded [Co(TPP)(Ds-pip)] (1) complex as dark
purple color powder. Compound 1 was characterized by FT-IR, UV–
visible and single crystal X-ray diffraction studies (Supplementary
data). Especially, cubic shape single crystals of 1 were grown by
vapor diffusion of CH2Cl2 solution of [Co(TPP)(Ds-pip)] into
Figure 1. Fluorescence response of 10 lM solution of dansylpiperazine with
different coordinated metal porphyrin complexes in CH2Cl2 (excitation: 340 nm;
Figure 2. Thermal ellipsoid plot of [Co(TPP)(Ds-pip)] at 200 K in the P2(1)/c space
emission: 500 nm; slit width: 3 nm).
group (50% probability ellipsoids). Hydrogen atoms are omitted for clarity.

J. Kim et al. / Tetrahedron Letters 52 (2011) 2645–2648
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Figure 3. Fluorescence emission changes of 1 (single crystal) (10 lM) upon the addition of NH3 (0–102 equiv) in CH2Cl2 (excitation: 340 nm; emission: 500 nm; slit width:
3 nm); (b) photograph under the irradiation at 365 nm, ((i) single crystal of 1, (ii) single crystal of 1 + NH3)); (c) fluorescence intensity changes upon addition of equivalents of
ammonia in CH2Cl2.
Scheme 2. Proposed PET mechanism of fluorescence ‘on-off-on’ behavior of [Co(TPP)(Ds-pip)] complex upon the addition of NH3.
and 95.08°), indicating that the cobalt atom locates above the por-
phyrin plane with N(1), N(2), N(3), and N(4) atoms.
fluorescence quenching process might be vanished upon formation
of Cobalt(II)–porphyrin–NH3 coordination complex due to the
disassociation of axial fluorophore ligand, as a result, the fluores-
cence ‘switched on’. Hence, only the intensity of fluorescence is
modulated upon NH3 sensing, from these results it is clear that
compound 1 is behaving like an ideal PET sensor.17
The fluorescence emission changes of sensor 1 upon the addi-
tion of NH3 are illustrated in Figure 3a. Addition of 0.2 equiv of
NH3 to 1 in CH2Cl2 induces an immediate strong fluorescent
enhancement at ca. 500 nm. There is an increase of about 6-fold
of emission intensity when 26 equiv of NH3 was added to sensor
1 solution. Further addition causes no significant changes in the
spectrum as a whole. The fluorescence emission intensity changes
of 1 upon the addition of NH3 can be viewable not only by instru-
ment but also by naked eye (red ? green) (Fig. 3b). Based on our
titration experimental result, the detection limit (LOD) of the sen-
sor 1 is calculated to be ca. 40 nM (Fig. 3c).
Proposed photophysical mechanism to explain fluorescent ‘on-
off-on’ switching of the sensor system has shown in Scheme 2.
The demonstration of HOMO and LUMO levels clearly illustrate
the direction of possible PET process in the compound 1. The
electron transfer from a metal complex (PET donor) to the excited
fluorophore (PET acceptor) diminishes the fluorescence of
original fluorophore resulting in ‘switched off’. However, the PET
3. Conclusions
In conclusion, using quenching effect of a cobalt(II) ion, we
developed simple fluorescence ‘on-off-on’ signal ensemble system
for detection of NH3, based on a dansylpiperazine(porphyrin–
Co(II)) complex. The principle lying behind the so-called fluoro-
ionophore work is based on an enhancement or decrease of
fluorescence intensity of the system when the ionophore units
bind guests. This has led to the design and synthesis of fluorescent
reagents that are suitable for the detection of ions particularly
important for environmental and biological applications. Among
the known design principles for sensing, the displacement

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J. Kim et al. / Tetrahedron Letters 52 (2011) 2645–2648
(ensemble) approach is simple and convenient because a signaling
unit (indicator) is bound to a binding site (receptor) by noncova-
lent interactions.18 There are several advantages of this approach
because of the noncovalent binding between receptor and indica-
tor: (1) No synthetic effort is required to chemically link an indica-
tor and a receptor; (2) there is considerable freedom in the choice
of an appropriate indicator; and (3) signal transduction is induced
by the simple addition of an analyte to the receptor–indicator mix-
ture.19 The fluorophore, dansylpiperazine, act as an axial ligand
which coordinates with metal porphyrin complex through the
piperazine nitrogen atom. The fluorescence emission intensity of
compound 1 increases with gradual addition of NH3 in dichloro-
methane in which NH3 act as a nitrogen donor by replacing fluoro-
phore. To the best of our knowledge, [Co(TPP)(Ds-pip)] is the first
example of a metal complex capable of turn-on fluorescent detec-
tion of NH3 under nonpolar conditions. From the results we con-
cluded that compound 1 provides a very important contribution
to the fast growing field of supramolecular gas sensing. Further
applications of 1 and related derivatives for in vivo imaging/photo-
physical studies are currently in progress and will be reported in
due course.
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Acknowledgments
This work was supported by the Converging Research Center
Program through the National Research of Korea (NRF) funded
by the Ministry of Education, Science and Technology (Nos.
20090313 and 20090094261) and by a grant from the Fundamental
R&D program for Core Technology of Materials funded by the
Ministry of Knowledge Economy, Republic of Korea.
Supplementary data
20. Cambridge Cystallographic Data Center deposition number: CCDC 798455.
Copies of the data can be obtained free of charge via www.ccdc.cam.ac.uk/
conts/retrieving.html (or from the Cambridge Crystallographic Data centre, 12
Union Road, Cambridge CB2 1EZ, UK. Fax: +44 1223 36033; e-mail:
deposit@ccdc.cam.ac.uk).
Supplementary data (experimental details for synthesis, fluo-
rescence studies, X-ray studies and crystallographic data20 of 1
(CIF) are provided) associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.03.048.