Reactivity-based fluoride detection: Evolving design principles for spring-loaded turn-on fluorescent probes

Article abstract of DOI:10.1021/ol7014187

A covalently triggered fluorescence turn-on detection scheme has been implemented for a tris(N-salicylideneamine)-derived dynamic fluorophore. Selective cleavage of strategically placed Si-O bonds by fluoride ion induces spring-loaded conformational trans

Full text of DOI:10.1021/ol7014187

ORGANIC
LETTERS
2007
Vol. 9, No. 18
3579-3582
Reactivity-Based Fluoride Detection:
Evolving Design Principles for
Spring-Loaded Turn-On Fluorescent
Probes
Xuan Jiang, Mario C. Vieweger, John C. Bollinger, Bogdan Dragnea, and
Dongwhan Lee*
Department of Chemistry, Indiana UniVersity, 800 East Kirkwood AVenue,
Bloomington, Indiana 47405
dongwhan@indiana.edu
Received June 18, 2007
ABSTRACT
A covalently triggered fluorescence turn-on detection scheme has been implemented for a tris(N-salicylideneamine)-derived dynamic fluorophore.
Selective cleavage of strategically placed Si
to fluorescence enhancement.
−O bonds by fluoride ion induces spring-loaded conformational transitions that are tightly coupled
Conformational switching drives and regulates important
signal transduction pathways in biology.1 Dynamic inter-
conversion between the on- and off-state of signaling proteins
is often mediated by reversible covalent modification of key
amino acid side residues. Prominent examples include
phosphorylation and dephosphorylation of Ser/Tyr residues
in many transmembrane proteins, in which covalently
introduced local structural distortions are amplified to global
conformational changes that initiate a cascade of events.2
This ingenious mode of operation displayed by dynamic
multi-subunit assemblies continues to inspire the design,
synthesis, and application of molecular devices operating in
a conceptually parallel manner.3
In this contribution, we describe rational molecular designs
to turn on fluorescence signals via covalently triggered
conformational switching. Installation of cleavable silylether
groups to a dynamic fluorophore system enforced structural
unfolding and fluorescence quenching. Efficient desilylation
by externally added fluoride ion, however, triggered spon-
taneous structural folding and the recovery of an intense blue
emission originating from a planar conjugated tris(N-sali-
cylideneaniline) motif.4 A general design strategy to exploit
the high selectivity inherent to reactivity-based detection
schemes constitutes the main topic of this paper.
We have recently shown that structural folding and
unfolding motions of the C3-symmetric compound 1 (Figure
1) can reversibly modify its emission properties.4c Specifi-
cally, loss of structural rigidity upon disruption of the cyclic
(1) Transduction Channels in Sensory Cells; Frings, S., Bradley, J., Eds.;
Wiley-VCH: Weinheim, 2004.
(2) Krauss, G. Biochemistry of Signal Transduction and Regulation, 3rd
ed.; Wiley-VCH: Weinheim, 2003.
(3) (a) Balzani, V.; Credi, A.; Raymo, F. M.; Stoddart, J. F. Angew.
Chem., Int. Ed. 2000, 39, 3348-3391. (b) Molecular Switches; Feringa, B.
L., Ed.; Wiley-VCH: Weinheim, 2001. (c) Balzani, V.; Venturi, M.; Credi,
A. Molecular DeVices and Machines: A Journey into the Nanoworld;
Wiley-VCH: Weinheim, 2003. (d) Bonnet, S.; Collin, J.-P.; Koizumi, M.;
Mobian, P.; Sauvage, J.-P. AdV. Mater. 2006, 18, 1239-1250.
(4) (a) Riddle, J. A.; Bollinger, J. C.; Lee, D. Angew. Chem., Int. Ed.
2005, 44, 6689-6693. (b) Riddle, J. A.; Lathrop, S. P.; Bollinger, J. C.;
Lee, D. J. Am. Chem. Soc. 2006, 128, 10986-10987. (c) Jiang, X.;
Bollinger, J. C.; Lee, D. J. Am. Chem. Soc. 2006, 128, 11732-11733. (d)
Lim, Y.-K.; Jiang, X.; Bollinger, J. C.; Lee, D. J. Mater. Chem. 2007, 17,
1969-1980. (e) Riddle, J. A.; Jiang, X.; Huffman, J.; Lee, D. Angew. Chem.,
Int. Ed. 2007, 46, in press.
10.1021/ol7014187 CCC: $37.00
© 2007 American Chemical Society
Published on Web 08/01/2007

bonding networks drives spontaneous structural folding to
B, the torsionally rigid nature of which should result in
enhanced fluorescence efficiency.
The feasibility of this “spring-loaded” mechanism was
tested against new C3-symmetric fluorophores 3a-b (Figure
1). A high-yielding Sonogashira-Hagihara cross-coupling
between 2-iodo-4-tert-butylaniline and propargyl alcohol
afforded 4, which was reacted with 1,3,5-triformylphloro-
glucinol in refluxing EtOH to furnish the desired triple Schiff
base condensation product 2 in 81% yield (Scheme 2).
Scheme 2. Synthetic Routes to 2, 3a, and 3b
Figure 1. Chemical structures of dynamic fluorophores.
hydrogen-bonding network in 1 facilitated nonradiative decay
of its excited states. This was evidenced by fluorescence
quenching of 1 by F-, which engages in competitive F-‚‚‚
H-Ohydroxy hydrogen bonds and blocks the key anchoring
points for its mobile aniline units. Although this process
represented an intriguing signal transduction via controllable
conformational switching, the turn-off mode of operation
significantly compromised its practicality as a detection
scheme for F-. In the quest for a structural platform enabling
turn-on signaling, it was immediately brought to our attention
that the molecular mechanism of fluorescence quenching
itself could be exploited to our advantage. A conceptual
reversal of this structure-property relationship has indeed
guided our mechanism-based molecular reengineering as
described below.
Simple treatment of a THF solution of 2 with TMSCl/Et3N
cleanly afforded 3a as a yellow solid in 73% yield.5,6 On
the other hand, an analogue of 3a having TBDMS groups
could be prepared by direct condensation reaction between
6 and 1,3,5-triformylphloroglucinol, which furnished 3b in
essentially quantitative (>99%) yield.
The C3-symmetric tris(N-salicylideneamine) core structure
of 2 was initially suggested by its characteristic Nenamine-H
proton doublet resonance at 13.77 ppm, which is coupled (J
) 13.2 Hz) to the Cvinyl-H doublet at 8.70 ppm.4 The proton
signal associated with the O-H groups on the peripheral
aniline fragments appeared as a triplet (J ) 5.6 Hz) at 5.36
ppm, which was significantly downfield-shifted compared
with that (1.68 ppm) of its model compound 3-phenyl-prop-
2-yn-1-ol. This observation strongly implicated their involve-
ment in the hydrogen-bonding interactions with the carbonyl
oxygen atoms at the core (Figure 1), which was subsequently
confirmed by X-ray crystallography.
Scheme 1. Mechanism of Turn-On Detection in which
Reaction between C and Non-Emissive A Furnishes Emissive B
The solid-state structure of 2, shown in Figure 2, revealed
an essentially coplanar arrangement of the entire molecule
reinforced by three CdO‚‚‚H-N hydrogen bonds surround-
ing the six-membered ring at the core. Notably, the wing-
As shown in Scheme 1, our molecular prototype A has
bulky substituents installed on the “wing-tip” O-H groups
of its three mobile components. Steric crowding and the lack
of attractive interactions prevent structural folding and furnish
torsionally flexible and therefore weakly emissive A. A
chemical reaction between A and C, however, can remove
these capping groups and expose the O-H groups. Subse-
quent formation of the key Ohydroxy-H‚‚‚Ocarbonyl hydrogen-
(5) Compound 3a was obtained as mixture of two stereoisomers of local
C3h and Cs symmetry associated with its tris(N-salicylideneamine) core.4
On the other hand, 3b was obtained exclusively as the C3h-symmetric isomer.
Despite this stereochemical inhomogeneity, similar absorption and emission
spectra were obtained for 3a and 3b.
(6) See Supporting Information.
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Org. Lett., Vol. 9, No. 18, 2007

silylation of the OH groups in 2 did not fundamentally affect
the emissive decay pathways. On the other hand, 2 and 3a
display markedly different nonradiative decay rate constants
(knr) of 2.70 and 8.00 ns-1, respectively. The increased
torsional freedom of 3a (Scheme 1) and the presence of
dangling trimethylsilyl groups apparently facilitate nonra-
diative relaxation of the excited states, thereby effectively
decreasing ΦF.7-9
Upon addition of F- (0.1 mM, nBu4NF), a CH2Cl2 solution
sample of 3a (1.0 µM) displayed >3-fold enhancement in
the fluorescence signal (Figure 3a). The time-dependent
Figure 2. X-ray structure of 2: (a) ORTEP diagram with thermal
ellipsoids at 50% probability; (b) capped-stick representation of a
dimeric unit constituting hydrogen-bonded stacks of 2 in the solid
state. All the hydrogen atoms, except for those associated with the
N-H and O-H groups, have been omitted for clarity. Two of the
three tert-butyl groups in 2 are disordered over two positions,6 for
which only one model is shown. Selected interatomic distances
(Å): O11‚‚‚O1, 2.771; O49‚‚‚O20A, 2.699; O30‚‚‚O11A, 2.689;
O1‚‚‚N5, 2.615; O39‚‚‚N43, 2.594; O20‚‚‚N24, 2.634.
tip O-H groups of 2 engage in hydrogen-bonding not only
in the expected intramolecular fashion (for O11) but also in
the rather unanticipated intermolecular fashion (for O30 and
O49), which presumably arises from solid-state packing
interactions. Specifically, the dangling O-H groups in 2
(Figure 2a) participate in either O-H‚‚‚O-H (for O30) or
O-H‚‚‚OdC (for O49) hydrogen-bonds with an adjacent
molecule of 2 to define interdigitated stacks in the lattice.
Under dilute (∼1.0 µM) conditions used for fluorescence
measurements (vide infra), however, the solution population
of 2 should be dominated by the intramolecularly hydrogen-
bonded structures shown in Figure 1. This was supported
by the linear relationship between the concentration and the
absorption/emission intensities determined for [2] ) 0-1.4
µM.
Figure 3. (a) Emission spectra of 3a (1.0 µM) (i) prior to and (ii)
after addition of nBu4NF (0.1 mM) in CH2Cl2 with λexc ) 340 nm.
(b) Fluorescence signal response of 3a (1.0 µM) to different anionic
species (50 µM) added as their tetrabutylammonium salts in CH2-
Cl2. The response was quantified as (I - I0)/I0, in which I0 and I
represent fluorescence intensity of 3a prior to and after addition of
the analyte, respectively. Measurements were made after a 20 min
period after addition at T ) 25 °C.
increase in the emission intensity displayed pseudo first-
order-type kinetics with an increasing overall rate with
increasing [F-] (Figure S3, Supporting Information), which
points toward the participation of both 3a and F- in the rate-
determining step leading to fluorescence turn-on.10
In CH2Cl2 at 298 K, 2 displays intense (ꢀ ) 93000 M-1
cm-1) visible absorptions at λmax,abs ) 420 and 440 nm
(Figure S1, Supporting Information). Upon excitation at 340
nm, 2 emits at λmax,em ) 458 nm with a small Stokes shift
(945 cm-1), which reflects the rigid molecular structure.
Notably, the fluorescence quantum yield () ΦF) of 2 is
9.7(4)%, which is significantly higher than that of 1
(4.2(2)%).4c
Silylation of the key OH groups in 2 no longer supports
the rigidifying Ohydroxy-H‚‚‚Ocarbonyl hydrogen-bonding net-
works. The complete disappearance of the longest wave-
length absorption at 440 nm and blue-shift of the transition
at 345 nm (Figure S1) presumably reflect structural distortion
and decrease in the 2-D conjugation area spanned by 3a.
This covalent modification also results in significant decrease
in the emission quantum yield, with ΦF values of 3.4(1) and
4.3(1)% determined for 3a and 3b, respectively.
Mass spectrometric analysis of the reaction mixture of 3a
+ F- revealed peaks corresponding to a [2 + Na]+ ion (m/z
) 788.3670). In addition, the UV-vis spectrum obtained
for this reaction mixture was essentially identical to that
obtained for 2 (Figures S1 and S4, Supporting Information).
These findings fully corroborate the formation of 2 as the
desilylation product of 3a. Under similar conditions, reactions
between 3b and F- did not result in any noticeable change
in the emission signal, which is presumably due to the more
t
robust nature of the O-SiMe2 Bu bond relative to the
O-SiMe3 bond against nucleophilic attack by F-.11
(7) Pyun, C.-H.; Lyle, T. A.; Daub, G. H.; Park, S.-M. Chem. Phys. Lett.
1986, 124, 48-52.
(8) Turro, N. J. Modern Molecular Photochemistry; University Science
Books: Sausalito, CA, 1991.
(9) Valeur, B. Molecular Fluorescence: Principles and Applications;
Wiley-VCH: Weinheim, Germany, 2002.
(10) Detailed kinetic analysis of process, however, was hampered by
the multiple O-SiMe3 groups in 3a which can react with F- as well as by
the formation of partially desilylated products at the early stage of the
reaction that can potentially contribute to the experimentally observed
fluorescence enhancement.
The solution dynamics of 2 and 3a were further probed
by time-correlated single photon fluorescence spectroscopy
(Figure S2, Supporting Information).6 In CH2Cl2 at 293 K,
average radiative decay rate constants (kr) of 0.290 and 0.281
ns-1 were obtained for 2 and 3a, respectively, indicating that
Org. Lett., Vol. 9, No. 18, 2007
3581

The mechanism of fluorescence turn-on in this system is
based exclusively on the Si-O bond cleavage reactions of
3a which can be effected by F- under mild conditions to
generate 2. The high-level of selectivity attained by this
reactivity-based detection scheme was convincingly dem-
onstrated by the screening of 3a against 11 different anions.
As shown in Figure 3b, only F- elicited measurable increase
in the fluorescence signal, whereas Cl-, Br-, I-, CN-, SCN-,
bond cleavage leading to cyclization of a coumarin precur-
sor,18 some of which display highly selective turn-on signal
response similar to 3a.15a-c,17,18
In summary, preprogrammed conformational switching19
of a covalently modifiable fluorophore was exploited for turn-
on detection of fluoride ion. The conceptual framework of
this spring-loaded mechanism should be generally applicable
to other chemical transformations that can restore the
structural rigidity and thereby enhance the emission proper-
ties of a dynamic fluorogenic platform. Efforts are currently
underway in our laboratory to expand the scope of this
chemistry.
-
-
-
-
NO3 , HSO4 , PF6 , ClO4 , and H2PO4- had no effect under
similar conditions.
The turn-on detection scheme shown in Figure 3 relies
critically on the inertness of 2 toward F-. Unlike the situation
with 1,4c treatment of an excess (>100 equiv) amount of F-
did not elicit any noticeable changes in the emission spectra
of 2. Use of the silylether derivative of 1 analogous to 3a
would suffer from unreacted F- that can interact with the
initially generated product 1 to cause eventual diminution
of the initial turn-on response.
Acknowledgment. This work was supported by Indiana
University, the National Science Foundation (CAREER CHE
0547251), the American Chemical Society Petroleum Re-
search Fund (42791-G3), and the Indiana METACyt Initia-
tive funded through a major grant from the Lilly Endowment,
Inc.
Increasing concerns regarding environmental protection
and national security has fueled research efforts to detect
fluoride ion released as the hydrolysis product of toxic
organophosphonate agents. Fluorescent probes developed for
fluoride ion typically integrate multidentate hydrogen-
bonding arrays as receptor units.12 The compact (ionic radius
) 1.15 Å) and basic (pKa ) 3.2) nature of fluoride ion makes
it an ideal target of such state-of-the-art “coordination”-based
detection scheme.13,14 An alternative approach that exploits
the intrinsic chemical reactivity of fluoride ion toward Lewis
acidic boron or silicon atom is a promising but much less
explored avenue. Previous examples include triarylborane-
based fluorophores that respond by perturbation of the
electronic conjugation as a result of B-F bond formation,15,16
luminescent triaryldifluorosilicates obtained by Si-F bond
formation of triarylfluorosilanes,17 or fluoride-triggered Si-O
Supporting Information Available: Experimental details
of the preparation and characterization of synthetic interme-
diates and X-ray crystallographic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
OL7014187
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