Zinc(II)-driven fluorescence quenching of a pyrene-labelled bis-2,2'- bipyridine ligand

Article abstract of DOI:10.1039/a805336g

The title compound has been synthesized and its fluorescence emission is quenched upon complexation of the Zn²⁺ in methanol solution.

Full text of DOI:10.1039/a805336g

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134 J. CHEM. RESEARCH (S), 1999
J. Chem. Research (S),
1999, 134±135$
Zinc(II)-driven Fluorescence Quenching of a
Pyrene-labelled Bis-2,2'-bipyridine Ligand$
Jean-Ernest Sohna Sohna, Pascale Jaumier and Frederic Fages*
Laboratoire de Chimie Organique et Organometallique, UMR 5802 CNRS, Universite Bordeaux I,
33405 Talence Cedex, France
The title compound has been synthesized and its fluorescence emission is quenched upon complexation of the Zn2‡ in
methanol solution.
The development of selective ¯uorescent chemosensors for
metal ions remains an active search of research.1 Two-
component supramolecular systems2 in which a ¯uorescent
chromophore and
a metal cation complexing site are
covalently linked have thus emerged as a very promising
family of photosensitive compounds for the detection and
quanti®cation of trace metal analytes in solution. Recent
e€orts have been devoted to the production of such systems
designed to detect ZnII, an extremely important biological
cation.3 In that connection, a variety of zinc ¯uorosensors
have been reported that allowed the signalling of ZnII-
ligand interaction through ¯uorescence intensity enhance-
ment3±6 or ratiometric methods.7,8 Although the d10 Zn2‡
cation is a non-quenching metal ion, several systems that
function according to
a
ZnII-driven chelation-enhanced
quenching (CHEQ) e€ect were also envisaged.9,10 They were
designed to undergo profound conformational changes upon
Zn2‡ chelation, the e€ect of which is to decrease the dis-
tance between a donor and an acceptor and, thereby, to
permit intramolecular photoinduced electron transfer (PET)
¯uorescence quenching. In line with our investigations on
receptors for transition-metal ions that incorporate the
pyrene chromophore,11 we wish to report here the synthesis
of the bis-2,2'-bipyridine ligand 1 and its non¯uorescent
analogue 2 (Scheme 1). Although 2,2'-bipyridine (bpy) is
among the most studied bidentate ligands in coordination
chemistry, its utilization as chelating site in chemosensors
Fig. 1 Electronic absorption spectra, at room temperature, in
5
methanol, of ligand 1 and its Zn2‡ complex (10 M)
structures has not been frequently reported so far.9,12
Complexation of divalent zinc was found to quench the
¯uorescence emission of 1, which was quite unexpected as
Scheme 1 Reagents and conditions: i, NaH, DMF, 90 8C, 10 h; ii, NaOH, ethanol, reflux, 6 h; iii, HOSu, DCC, DMF, room temp., 96 h
*To receive any correspondence.
this compound was not designed to contain
programmed quenching process. The mechanism of quench-
ing is discussed.
a pre-
$This is a Short Paper as de®ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1999, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).

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J. CHEM. RESEARCH (S), 1999 135
pyrene being the donor, which is consistent with our pre-
vious work.18b In contrast to the case of other ¯uorosensors
pre-programmed to display ZnII-induced ¯uorescence
quenching,9,10 the deactivation of the pyrene singlet state
within 1 would result from a direct photophysical inter-
action between the excited aromatic part and the metallic
centre. Finally, the Zn2‡ cation, well known for its photo-
physical inertness towards ¯uorescent receptors based on
nitrogen-containing ligand units other than bpy moieties,
would a€ect the photophysical properties of 1, as do open-
shell transition metal ions.2 Compound 1, incorporating a
¯uorescent chromophore tethered to bpy ligand units, thus
belongs to a new family of photosensitive ligands that could
®nd potential applications in the ®eld of metal sensoring
and molecular electronics.
Fig. 2 Corrected fluorescence spectra, at room temperature, in
6
methanol, of 1(10 M), in the absence and in the presence of
increasing amounts of ZnCl2
The synthesis of the title compound,
1 is outlined
Experimental
General methods as published elsewhere.11
Ligand 1 had dH (ppm) (CDCl3) 4.60 (4 H, d), 5.90 (2 H, s), 7.4±
8.7 (26 H, m), 9.50 (2 H, t) (Found: C, 76.1; H, 4.75; N, 11.0
in Scheme 1. The non¯uorescent model 2 was obtained
similarly by using 1-bromopropane in the ®rst alkylation
step. The electronic absorption spectrum of 1 in methanol
(Fig. 1) consists of the additive superimposition of the
absorption bands of the 2,2'-bipyridine and pyrene chromo-
phores as recorded from the corresponding reference com-
pounds 2 and 3, respectively. The addition of one molar
equivalent of ZnCl2 to a methanolic solution of 1 caused a
strong modi®cation of the position and shape of the pp*
transition bands of the bpy units (Fig. 1)13 This e€ect, also
detected for ligand 2, enabled us to determine spectrophoto-
metrically the stoichiometry of the ZnII complex of 1 by the
method of continuous variations (Job plot).14 A character-
istic triangular plot with linear segments indicated the occur-
rence of a single, strong complex, the apex being located at
a mole fraction value of 0.5. The formation in methanol of
a 1:1 Zn2‡ ligand 1 complex was also con®rmed by a
titration experiment. Moreover, treatment of 1 (30 mmol)
with one molar equivalent of ZnCl2 in DMF (5 ml) followed
by precipitation with diethyl ether a€orded the mononuclear
complex Zn2‡Á1 as a beige amorphous solid. The FAB mass
spectrum showed the molecular ion with high intensity and
C
47H35N6O3 Á0.5H2O requires C, 76.2; H, 4.9; N, 11.3%).
Ligand 2 had dH ([2H6]acetone) 0.91 (3 H, t), 1.80 (2 H, m), 3.92
(2 H, t), 4.68 (4 H, s), 7.3±8.7 (17 H, m) (Found: C, 68.4; H, 5.5;
N, 15.0. C33H30N6OÁH2O requires C, 68.7; H, 5.6; N, 14.6%).
The authors thank the CNRS, the Universite Bordeaux I,
and La Region Aquitaine for ®nancial support. Ms Eve-
Anne Lerendu is acknowledged for assistance.
Received, 9th July 1998; Accepted, 22nd October 1998
Paper E/8/05336G
References
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1
the absence of any stoichiometries other than 1:1. H NMR
spectroscopy showed the participation of both bpy units of
1 in the coordination of the metal.
The ¯uorescence spectrum of 1 in non-degassed methanol
(Fig. 2) was found to be typical of 1-alkyl pyrene deriva-
tives11 and of similar intensity (¯uorescence quantum yield
Ff ˆ 0.02). Amazingly, titration with ZnII of a methanolic
solution of 1 produced an ecient quenching of the ¯uor-
escence emission to reach a plateau value at 1 equivalent
(Fig. 2). At room temperature, in the methanol±ethanol (1:4
v:v) solvent system, the isolated mononuclear complex (vide
supra) displayed a very low ¯uorescence quantum yield
(F<0.001). Freezing of the solution at 77 K led to the total
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revival of the pyrene ¯uorescence, which suggested15
a
signi®cant contribution of a PET process to the quenching
of the photoexcited pyrene nucleus by the zinc(II)-containing
moiety. Moreover, addition of an excess of tri¯uoroacetic
acid to a methanol solution of 1 was also followed by the
extensive quenching of ¯uorescence.
The latter observation can be related to the protonation-
induced ¯uorescence quenching of the anthracene chromo-
phore bound to pyridyl-containing ligands.2,4,10,16 The
ability of the 2,2'- or 4,4'-bipyridinium cation to promote an
electron-transfer reaction from the excited state of organic
or inorganic chromophores is also well documented.17
Similarly, complexation of the hard Lewis acid zinc(II)
cation is expected to increase the reduction potential of the
bpy fragments, as reported recently,18a so that the latter
could then play the role of electron acceptor, the excited