Design of ICT-PET fluorescent probes for zinc(II) based on 5-aryl-2,20-bipyridines

Article abstract of DOI:10.1016/j.tetlet.2012.09.027

Rational design of selective and sensitive 'off-on' fluorescent probes for Zn(II) cations exploiting both PET and ICT mechanisms of sensing is illustrated by the synthesis and application of 5-aryl-2,20-bipyridines modified with a dipicolylaminomethyl fragment. The aryl substituent provides tuning of the properties.

Full text of DOI:10.1016/j.tetlet.2012.09.027

Tetrahedron Letters 53 (2012) 6265–6268
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Tetrahedron Letters
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Design of ICT-PET fluorescent probes for zinc(II) based on 5-aryl-2,20-bipyridines
Dmitry S. Kopchuk a, Anton M. Prokhorov a, Pavel A. Slepukhin b, Dmitry N. Kozhevnikov a,b,
⇑
a Ural Federal University, Mira 19, Ekaterinburg 620002, Russia
b I. Postovsky Institute of Organic Synthesis, Kovalevskoy 22, Ekaterinburg 620090, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 July 2012
Revised 30 August 2012
Accepted 6 September 2012
Available online 14 September 2012
Rational design of selective and sensitive ‘off-on’ fluorescent probes for Zn(II) cations exploiting both PET
and ICT mechanisms of sensing is illustrated by the synthesis and application of 5-aryl-2,20-bipyridines
modified with a dipicolylaminomethyl fragment. The aryl substituent provides tuning of the properties.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Zinc(II) sensor
Bipyridine
Photoinduced electron transfer
Intramolecular charge transfer
DFT
Zinc(II) is the second most abundant d-block metal ion in the
human brain.1 Sensitive methods for detection of zinc ions attract
attention since zinc plays a crucial role in many biological pro-
cesses.2 Fluorescence spectroscopy is a powerful method for sens-
ing and imaging metal cations at submicromolar concentrations.3,4
Photoinduced electron transfer (PET) and intermolecular charge
transfer (ICT) are the most widely used mechanisms of fluores-
cence signal modification used in the design of Zn(II) chemosen-
sors.5,6 PET provides significant enhancement of a fluorescence
signal with increased concentration of the metal ions and gives
‘off-on’ sensors.7 However, in PET probes isolation of the signaling
and sensing parts through aliphatic spacers cannot give a shift of
the fluorescence maxima. On the contrary, in ICT sensors the sig-
naling and sensing parts are combined in one conjugated system,
and binding of an analyte leads to blue or red shift of the fluores-
cence usually without significant changes of intensity, thus a ratio-
metric analysis can be applied.8 A combination of these two
techniques promises better results in ‘turn on’ ratiometric sensor
design since the effect of color alteration due to ICT would be
amplified by intensity increase due to PET.6 This idea has been
illustrated by V. Kozhevnikov when 6,600-bis(aminomethyl)-5,
500-bis-(4-bromophenyl)-2,20:60,200-terpyridine showed both a red-
shift of the maximum and an enhancement of intensity of lumines-
cence upon increasing the Zn(II) concentration in buffered aqueous
solution.9 In this case, the terpyridine moiety played the role of the
metal binding site and the chromophore simultaneously. A similar
effect was reported for 6-dipicolylaminomethyl-2-pyridylbenzimi-
dazole.10 Both PET and ICT mechanisms can be assumed for
sensors, where an amino or oxy group conjugated with a chromo-
phore takes part in binding the Zn(II) cation.11
Zinc(II) complexes of bipyridine, phenanthroline, or terpyridine
exhibit bright luminescence. Extended conjugated systems provide
significant red-shift of luminescence of the ligands and especially
Zn(II) complexes, thus a number of ICT Zn(II) sensors based on oli-
gopyridines have been described.12 In particular, 5-aryl-2,20-
bipyridines can be considered as an appropriate platform for the
design of new Zn(II) sensors.13 In order to increase the selectivity
of a sensor, the bipyridine chelating site should be extended with
specific Zn(II) ion receptors, that is, dipicolylamine (DPA).6 To
exploit the PET mechanisms the amine should be attached at the
a-position of the bipyridine through an appropriate spacer, for
example a methylene group.
The targeted ligands 1a,b were synthesized according to the
strategy shown on Scheme 1. Key intermediates, 5-aryl-60-bromo-
methyl-2,20-bipyridines 2, were obtained by the ‘1,2,4-triazine0
methodology starting from synthetically available triazines 3.14
Reaction of 2 with dipicolylamine resulted in the desired ligands
1a,b.15 The aryl substituent in 1 plays a dual role. Firstly, it in-
creases the amphiphilic properties of the ligand, which is impor-
tant for biological imaging applications.2 Secondly, the aryl
modulates absorption and emission properties of the ligand and
the Zn-complex that shifts the emission maxima to the visible area,
and increases the Stokes shift.
Zinc(II) complexes of the new ligands were isolated from the
reaction of 1 with Zn(ClO4)2. Single crystals of [Zn(1a)](ClO4)2 were
obtained by recrystallization from methanol. X-ray crystallogra-
phy16 revealed the 1:1 composition of the complex ion [Zn(1a)]2+
(Fig. 1). Pentacoordinated zinc(II) atoms have distorted trigonal
⇑
Corresponding author. Tel.: +7 343 362 3056; fax: +7 343 374 0458.
E-mail address: dnk@ios.uran.ru (D.N. Kozhevnikov).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.09.027

6266
D. S. Kopchuk et al. / Tetrahedron Letters 53 (2012) 6265–6268
lows the suggestion that 1a is a typical ‘off-on’ PET but not ICT
R
sensor. An electron-donating methoxy group in the aryl amplifies
intraligand charge-transfer in 1b resulting in a significant red-shift
(60 nm) of fluorescence upon complex formation. At the same
time, the coordination results in an eightfold increase of the fluo-
rescence intensity, thus 1b can be discussed as PET-ICT sensor.
The absorption and emission spectra of 1a on titrating Zn(ClO4)2
into the aqueous solution (HEPES buffer) are presented in Figure 2.
The absorption and emission maxima are slightly (5–10 nm)
shifted toward longer wavelengths with the addition of Zn(II).
The fluorescence intensity in an aqueous medium was enhanced
but not as dramatically (by the factor of 5.5) as in acetonitrile be-
cause of the relatively intense ambient emission of the ligand 1a in
water. However 1a can be considered as a PET fluorescent probe for
Zn(II) in aqueous solutions. The lowest-energy absorption band of
the Zn(II) complex of 1b in the HEPES buffer was 10 nm blue-
N
N
N
O
R
N
OMe
3
i, ii, iii
N
N
30-35%
R
N
N
iv
73-80%
N
N
N
Br
2a,b
1a,b
R = H (a), MeO (b)
Scheme 1. Synthesis of ligands. Reagents and conditions: (i) 2,5-norbornadiene, o-
xylene, reflux, 48 h; (ii) NaBH4, EtOH, reflux, 2 h; (iii) PBr3, CH2Cl2, 8 h; (iv) 2,20-
dipicolylamine, K2CO3, MeCN, reflux, 8 h.
Figure 1. Crystal structure of [Zn(1a)]2+. Hydrogen atoms are omitted for clarity.
Figure 2. Fluorescence enhancement upon titrating Zn(ClO4)2 (0 – 0.8 equiv, step
0.05 equiv) to the solution of 1a (10ꢀ5 M in HEPES buffer), excitation at isosbestic
point 313 nm. Inset: changes of the absorption.
bipyramidal geometry. Distances between zinc and the nitrogens
of the pyridine rings at the base of bipyramid are practically equal
(2.01–2.03 Å), while the Zn-N distances with the fourth pyridine
ring and the amino group at the vertexes of the bipyramid are sig-
nificantly longer, 2.09 and 2.25 Å correspondingly.
Photophysical properties of the new ligands 1 and their com-
plexes measured in acetonitrile are collected in Table 1. The dipic-
olylaminomethyl group does not affect the energy of electronic
transitions, so absorption and emission spectra of both ligands
and complexes are close to those of the parent arylbipyridines.
13a
At the same time, the intensities of fluorescence of the ligands
are decreased significantly due to PET. In the Zn-complexes the
lone pair of the amino group is involved in the coordination of
the metal atom that excludes PET and turns on bright fluorescence.
The enormous (35 fold) increase of quantum efficiency of lumines-
cence but with a very small red-shift upon Zn(II) coordination al-
Table 1
Photophysical data of 1 and [Zn(1)](ClO4)2 in acetonitrile
Compound
1a
Absorption, kmax, nm (
e
, 10ꢀ3 Mꢀ1cvꢀ1
)
kem, nm Ua
262 (20.5), 303 (31.5)
367
377
395
455
0.006
[Zn(1a)](ClO4)2 262 (20.0), 327 (27.1)
1b 263 (11.3), 268 (11.2), 313 (25.6)
[Zn(1b)](ClO4)2 262 (15.2), 345 (17.8)
0.21
0.12
0.94
Figure 3. Fluorescence enhancement upon titrating Zn(ClO4)2 (0 – 0.8 equiv, step
0.1 equiv) to the solution of 1b (10ꢀ5 M in HEPES buffer), excitation at isosbestic
point 327 nm. Inset: changes of the absorption.
a
Quantum yields of fluorescence were measured in acetonitrile relatively to the
anthracene (U = 0.27 in ethanol17).

D. S. Kopchuk et al. / Tetrahedron Letters 53 (2012) 6265–6268
6267
shifted in comparison with the data measured in acetonitrile, while
the absorption maximum of the ligand 1b was not sensitive to the
solvent (Fig. 3). On the contrary, the emission maximum of 1b
shifted dramatically (83 nm) toward longer wavelengths in aque-
ous solution in comparison with acetonitrile. Titrating Zn(II) into
the solution of 1b in the HEPES buffer resulted in a very small
(7 nm) red-shift of the emission maxima with a 4.5-fold enhance-
ment of the fluorescence intensity (Fig. 3). Thus in an aqueous
medium 1b worked like a PET but not like a ICT fluorescent probe
for Zn(II).
sponse was not affected by physiologically abundant metal ions
such as Na+, Ca2+, and Mg2+, although Cd2+ changed the fluores-
cence spectra similarly to Zn(II). Transition metals, such as Cu2+
Fe2+, and Mn2+ quenched the fluorescence. However Co2+, Hg2+
,
,
and particularly Ni2+ caused no significant decrease of the fluores-
cence signal. The ligand 1b showed similar selectivity.
We used DFT calculations (B3LYP/TZVP, COSMO solvation mod-
el) to understand the large differences between the photophysical
properties of the ligands and the complexes in acetonitrile and
water solutions. Geometries of the ligands 1a,b and their Zn(II)
complex cations were calculated, and geometry parameters of
[Zn(1a)]2+ were close to the crystal data described above. Addition-
ally H-bond complexes of the ligands 1a,b with a molecule of
water linked to the amino group were calculated in order to con-
sider the solvent effect. The calculated energies and localizations
of frontier molecular orbitals of the ligands 1a,b and their com-
plexes with water and Zn(II) are shown in Figure 5.
Metal-sensing selectivity was studied for 1a. The fluorescence
ratio values of 1a in response to various divalent metal ions in
the HEPES buffer are depicted in Figure 4. The fluorescence re-
The HOMO of 1a is mainly the lone pair of the amino group,
while the LUMO is p⁄ located on the bipyridine fragment. Since
the n,p⁄-excited state decays through nonradiative pathways, it ex-
plains the low intensity of the fluorescence of 1a in acetonitrile. In
the Zn complex of 1a, the HOMO has
sion from
p⁄-excited state was observed. The interaction of 1a
with a molecule of water decreases the energy of the lone pair,
thus in the 1a+HOH complex, the HOMO is but not n, that is in
water solution 1a+HOH should emit from the
p⁄-excited state,
p character, so bright emis-
p,
p
p
,
which explains the fluorescence enhancement of 1a in aqueous
media.
An electron-donating methoxy group raises energy of the
p
orbital, which becomes the HOMO of 1b in acetonitrile solution,
while the lone pair of the amino group is slightly lower in energy.
It increases the p,
p⁄ character of the lowest excited state of 1b, that
enhances the intensity of the fluorescence if compared with 1a. As
a result, the PET effect is significantly reduced for 1b both in aceto-
nitrile and particularly in water. The lowest excited state of 1b is
an ICT state, since the HOMO is mainly localized on the methoxy-
phenyl fragment, while the LUMO is on the bipyridine moiety, thus
excitation increases the polarity of 1b (Fig. 6). In contrast, the high
Figure 4. Fluorescence response of 1a (10 M) to 10 M of various metal ions (blank
bars) and followed by addition of 10 M of Zn(II) (black bars) monitored at 391 nm in
aqueous solution (25 mM HEPES, 0.1 M NaCl), excitation at 313 nm (isosbestic point
of the absorption spectra).
Figure 5. Calculated (B3LYP/TZVP) energies and localizations of frontier molecular orbitals of the ligands 1a,b and the complexes with water and Zn(II).

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D. S. Kopchuk et al. / Tetrahedron Letters 53 (2012) 6265–6268
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(A) The occupied molecular orbitals of the chromophore should
be sufficiently lower in energy than the lone pair of the
metal binding part to provide PET in aqueous media; DFT
calculations are an effective tool for this purpose.
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(B) The substituents should not decrease the polarity of the
excited state of the Zn-complex providing a large red-shift
due to ICT in polar aqueous media, that is the intraligand
charge transfer should be directed from the chelating part
toward the conjugated electron-withdrawing substituent.
15. General procedure for the preparation of ligands 1 Mixture of corresponding 60-
bromomethylbipyridine 2 (0.6 mmol), dipicolylamine (0.12 mL, 0.66 mmol),
anhydrous potassium carbonate (830 mg, 6 mmol), and dry acetonitrile
(90 mL) was stirred under reflux for 12 h. The solvent was removed under
reduced pressure. Water (50 mL) was added to the residue, and the resulting
mixture was stirred at room temperature for 5 min. The resulting precipitate
was filtered off, washed with water and recrystallized from acetonitrile.
16. CCDC 894258 contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgement
The authors thank the Russian Foundation for Basic Researches
for financial support.
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Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.
2012.09.027.