Torsionally restricted tetradentate fluorophore: A swivelling ligand platform for ratiometric sensing of metal ions

Article abstract of DOI:10.1039/b812261j

A conformationally pre-organized ligand platform allows for restricted swivelling motions of two aryleneethynylene-appended 2,2′-bipyridyl units that function as a tight bischelator as well as a ratiometric fluorescence sensor for selected metal ions. The Royal Society of Chemistry.

Full text of DOI:10.1039/b812261j

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Torsionally restricted tetradentate fluorophore: a swivelling ligand
platform for ratiometric sensing of metal ionsw
Xuan Jiang, Byung Gyu Park, Justin A. Riddle, Bong June Zhang, Maren Pink
and Dongwhan Lee*
Received (in Berkeley, CA, USA) 17th July 2008, Accepted 13th August 2008
First published as an Advance Article on the web 14th October 2008
DOI: 10.1039/b812261j
A conformationally pre-organized ligand platform allows for
restricted swivelling motions of two aryleneethynylene-appended
2,20-bipyridyl units that function as a tight bischelator as well as
a ratiometric fluorescence sensor for selected metal ions.
Cross-conjugated electronic structures continue to attract sig-
nificant research interests in organic materials chemistry.1
Molecular cruciforms represent one such architectural motif.2,3
Scheme 1 Chemical structure of 4 and schematic representation of
metal binding through torsionally restricted structural folding.
Spatial separation of the frontier molecular orbitals (FMOs)
along each linear segment of such two-dimensional (2-D) con-
structs provides unique opportunities to control optoelectronic
properties either through covalent modification of p-conjugated
backbones4 or by interaction with chemical input signal.5
Torsional control over the conjugation paths should directly
impact the electronic properties associated with 2-D electronic
conjugation.6 This idea prompted us to explore a new
p-conjugated platform that supports two converging
2,20-bipyridyl (= bipy)7 motifs. We report that this swivelling
tetradentate ligand (i) displays fluorescence associated with
cross-linked [n,p]/[p,p]-conjugation, (ii) tightly binds metal
ions with Ka up to 107 Mꢀ1 to elicit red-shift in emission,
and (iii) thus enables ratiometric fluorescence sensing.
Bipy-containing linearly conjugated structures have been
extensively studied as fluorescence sensors for metal ions.8,9
Efforts to incorporate bipy units into 2-D p-conjugated molecular
scaffolds, however, focused predominantly on shape-persistent
macrocycles10 such as 1,11 2,12 or 3.13
We envisioned that a non-cyclic ligand 4 (Scheme 1) derived
from 2 should have a limited number of freely rotating C–C
bonds and thus provide finite trajectories for the two bipy
fragments converging at the metal center. Such conforma-
tional restriction was anticipated to enhance the binding
affinity by lowering the entropic cost of complexation. In
addition, unique electronic properties associated with the
[n,p]/[p,p]-conjugated molecular backbone could be exploited
for optical signaling of such binding events.14
The molecular C2-symmetry of 4 simplified the synthetic
operation (Scheme 2), which commenced with Sonogashira–
Hagihara coupling between 5 and 5-ethynyl-2,20-bipyridine (6),
proceeding exclusively on the iodo-substituted position of 5 to
furnish the mono-coupled product 7. A second round of
Department of Chemistry, Indiana University, 800 East Kirkwood
Avenue, Bloomington, IN 47405, USA.
E-mail: dongwhan@indiana.edu; Fax: +1-812-855-8300;
Tel: +1-812-855-9364
w Electronic supplementary information (ESI) available: Experimental
details for the synthesis, additional spectroscopic and crystallographic
data. CCDC 695503 and 695504. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/b812261j
Scheme 2 Synthetic route to 4.
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Fig. 3 FMO isosurface plots (isodensity value = 0.05 au) of DFT
models of 4 (a and b) and 6 (c and d).
Fig. 1 ORTEP diagrams of (a) 4 and (b) its Zn(II) complex
[Zn(4)(OTf)2] with thermal ellipsoids at 50% probability, where N is
blue, O is red, S is orange, and Zn is green. The tert-butyl groups of 4,
and one tert-butyl group and one OTfꢀ ligand of [Zn(4)(OTf)2] are
disordered over two positions, for which only one model is shown.
the Job plot (Fig. S3, ESIw), which pointed toward the
formation of multiple competing species in solution.
A high binding constant (Ka) of 1.7 (ꢂ0.5) ꢃ 107 Mꢀ1 (in
MeCN) was obtained by fitting DA310 vs. [Zn2+] data against a
typical 1 : 1 binding isotherm. Under similar conditions,
systematically weaker binding was observed for Cd2+ and
Hg2+, with Ka = 1.5 (ꢂ0.5) ꢃ 106 Mꢀ1 and 9.4 (ꢂ0.9) ꢃ
104 Mꢀ1, respectively (Fig. S4 and S5, ESIw). The decrease in
the binding affinity along the series Zn2+ 4 Cd2+ 4 Hg2+
might be rationalized by the inability of the torsionally
restricted ligand 4 to fully relax to accommodate metals of
larger ionic radii. Alternatively, increasing softness of the d10
metals down the row might weaken the M–Nimine bonds.
Structural preorganization of 4 apparently plays a critical
role in discrete 1 : 1 complexation with high binding constants.
This was subsequently confirmed by X-ray crystallography on
its Zn2+ complex.z As shown in Fig. 1(b), 4 supports a six-
coordinate metal center with bis(bipy)-derived four nitrogen
atoms adopting a cis-divacant octahedral geometry and two
triflate-derived oxygen atoms completing the remaining co-
ordination sites. Metal binding proceeds via rigid body swivel-
ling motions of the ortho-phenyleneethynylenebipyridyl arms
around the diethynylene hinge of 4 with no significant
structural distortion of the molecular backbone.
cross-coupling with acetone-protected acetylene produced 8,
which was subsequently converted to 9 under basic conditions.
Oxidative homocoupling of 9 completed the synthesis of 4
(Fig. 1(a)).15
In MeCN at 298 K, 4 displayed a strong (e = 78 000 Mꢀ1 cmꢀ1
)
absorption at lmax = 310 nm and broad shoulders extending to ca.
400 nm (Fig. 2). In comparison, the UV-Vis spectrum of the ‘‘half-
ligand’’ model 9 has no features above 360 nm, indicating that the
optical transitions of 4 at the longer wavelength end originate from
the p-conjugation along the diethynylene-linked biphenylene back-
bone. Consistent with this interpretation, the FMOs of 4 deter-
mined by density functional theory (DFT) calculations revealed
significant electron delocalization along the extended ligand back-
bone (Fig. 3). On the other hand, the similarity between the strong
absorption of 4 at lmax = 310 nm and that of 6 at lmax
=
297 nm points toward p–p* electronic transitions localized at the
ethynylbipyridyl ‘‘arm’’ fragment (Fig. 3). Accordingly, electronic
perturbation induced by metal coordination should have profound
effects on this region of the UV-Vis spectrum.
Addition of Zn2+ (delivered as triflate salt) to 4 in MeCN
indeed elicited a gradual decrease in intensity of the absorption
peak at lmax = 310 nm with concomitant development of a
longer wavelength transition at lmax = 327 nm (Fig. S1, ESIw).
The well-resolved isosbestic point at l = 319 nm suggested the
formation of a discrete metal complex of 4, the 1 : 1 binding
stoichiometry of which was subsequently confirmed by the
Job’s plot analysis (Fig. S2, ESIw). In contrast, the half-ligand
model 9 displayed complicated binding patterns as reflected on
In addition to providing a torsionally restricted structural
platform for high-affinity binding, the bent [n,p]-system of 4
fundamentally switched the intrinsic de-excitation mechanism
of the bipy unit. The free ligand 4 in MeCN at 293 K showed
fluorescence peaks at lmax = 390 and 415 nm with a large
Stokes shift (Fig. 2). Addition of Zn2+, however, elicited a
significant decrease in the free-ligand emission with a con-
comitant development of the intense ligand–metal complex
emission at 495 nm with an isoemissive point at 433 nm
Fig. 4 Fluorescence response of (a) 4 (0.5 mM) and (b) 9 (1.0 mM)
upon addition of Zn2+ (0–2.5 mM) in MeCN with lexc = 320 nm;
T = 293 K. Each trace corresponds to a 0.5 mM increment of [Zn2+].
Fig. 2 Normalized UV-Vis (solid lines) and fluorescence (dotted
lines) spectra of 4 (red), 6 (green) and 9 (blue) in MeCN, T = 298 K.
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(Rint = 0.0745). Final GooF = 0.946, R1 = 0.0690, wR2 = 0.1800
(with I 4 2s(I)).
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Fig. 5 Fluorescence response of 4 (0.5 mM) toward various transi-
tion-metal ions (2.5 mM) in MeCN. The response was quantified by the
ratio (I ꢀ I0)/I0, in which I0 and I denote the emission intensity at
l = 500 nm prior to and after addition of the metal ion, respectively;
lexc = 320 nm; T = 293 K.
(Fig. 4(a)). The ratio of I500/I390 with excitation at 320 nm
varied from 0.1 in the absence of Zn2+ to 25 upon treatment of
5 equiv. Zn2+ (2.5 mM), a 250-fold emission ratio increase.
Under similar conditions, the half-ligand model 9 showed only
slight decrease in the intensity of the l = 370 nm peak with no
development of longer wavelength features (Fig. 4(b)). On the
other hand, the bipy ligand fragment model 6 showed a simple
fluorescence turn-on behavior toward Zn2+ (Fig. S6, ESIw).
The significant enhancement and red-shift in the emission of 4
now enables ratiometric detection, which is advantageous over
conventional measurement at a single wavelength.8,16–20
The high affinity of 4 toward group 12 metal ions prompted
the investigation of its response profile across a wider range of
transition-metal ions. As summarized in Fig. 5, a MeCN
solution of 4 (0.5 mM) displayed strong turn-on fluorescence
response toward mM-level concentrations of Zn2+, Cd2+ and
Hg2+ as monitored by changes in the emission intensity. On
the other hand, other closed-shell ions such as Ag+ or Cu+, or
paramagnetic Cu2+, Fe2+, Mn2+, Co2+ or Ni2+ elicited
either complete quenching or negligible changes.21
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14 Coordination-induced spectral changes have previously been
reported for 2 but no information is available for its binding
stoichiometry, affinity, or the structure of putative metal
In summary, a new dynamic fluorophore was prepared, in
which swivelling motions around a rigid p-conjugated axle was
exploited to enforce two metal binding units to converge at the
metal center as a tight bischelator. A high affinity binding with
Ka up to B107 Mꢀ1 and ratiometric fluorescence response
toward selected metal ions promise potential application of
this and related ligand platforms for biological and environ-
mental sensing.20,22–24 Efforts are currently underway in order
to improve the selectivity, sensitivity, and solubility of this
first-generation prototype in aqueous environments.
complexes12
.
15 A pyrene-appended all-carbon analogue of
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This work was supported by the National Science Founda-
tion (CAREER CHE 0547251) and the US Army Research
Office (W911NF-07-1-0533). This paper is dedicated to Pro-
fessor Myunghyun Paik Suh on the occasion of her 60th
birthday.
21 Fluorescence quenching by paramagnetic or heavy metal ions has
typically been ascribed to the effect of strong spin–orbit coupling.
Closed-shell metal ions suffer less from such deleterious pathways
and (with appropriate metal–fluorophore combinations) can give
rise to enhanced emission through the inversion of the np*–pp*
ligand excited states22
.
Notes and references
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107, 1339.
24 J. S. Kim and D. T. Quang, Chem. Rev., 2007, 107, 3780.
z Crystal data for [Zn(4)(OTf)2]: C50H38F6N4O6S2Zn, M = 1034.33,
monoclinic, space group C2/c, a = 59.348(3), b = 9.9017(6),
c
= 19.2658(12) A, b = 102.291(2)1, V =
11062.0(11) A3,
T = 150(2) K, Z = 8, 35 934 reflections measured, 9700 unique
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6030 | Chem. Commun., 2008, 6028–6030