Exploiting the versatile assembly of arylpyridine fluorophores for wavelength tuning and SAR

Article abstract of DOI:10.1021/ol0272287

(Matrix presented) The facile modular assembly of polyarylpyridine fluorophores provides two important advantages in the development of fluorescent chemosensors: it allows rapid dissection of the structural requirements for fluorescent chemosensing and it allows dramatic tuning of emission wavelength by changes in a substituent remote from the binding site.

Full text of DOI:10.1021/ol0272287

ORGANIC
LETTERS
2003
Vol. 5, No. 7
967-970
Exploiting the Versatile Assembly of
Arylpyridine Fluorophores for
Wavelength Tuning and SAR
Albert G. Fang, Jesse V. Mello, and Nathaniel S. Finney*
Department of Chemistry and Biochemistry, UniVersity of California,
San Diego, La Jolla, California 92093-0358
nfinney@chem.ucsd.edu
Received November 4, 2002 (Revised Manuscript Received February 28, 2003)
ABSTRACT
The facile modular assembly of polyarylpyridine fluorophores provides two important advantages in the development of fluorescent
chemosensors: it allows rapid dissection of the structural requirements for fluorescent chemosensing and it allows dramatic tuning of emission
wavelength by changes in a substituent remote from the binding site.
The majority of fluorescent chemosensors are based on
polycyclic aromatic fluorophores, many of which are not
amenable to extensive synthetic variation.1 This in turn limits
the tuning of emission wavelengths and variation of binding
domains. We recently described a new class of ion-responsive
Biarylpyridine 1 is representative of our previous work.
Upon titration with perchlorate salts of Li+, Mg2+, or Ca2+,
significantly enhanced fluorescence emission is observed
(Figures 1 and 2).2 Notably, 1 is a dual-emission fluorophore,
fluorescent chemosensors based on biarylpyridines.2
A
defining characteristic of fluorescent chemosensors based on
bi- and triarylpyridines is their modular assembly from the
corresponding di- and trihalopyridines.2,3 We have taken
advantage of this feature in preparing a series of new
fluorescent pyridines that (1) delineate the minimal structural
requirements for metal binding and fluorescence response
in our previously reported chemosensor system and (2)
demonstrate that the modular synthetic assembly of triaryl-
pyridines facilitates straightforward tuning of the emission
wavelength.
(1) For representative reviews of the chemosensor field, see: (a) Rurack,
K.; Resch-Genger, U. Chem. Soc. ReV. 2002, 31, 116-127. (b) Valeur, B.;
Leray, I. Coord. Chem. ReV. 2000, 205, 3-40. (c) deSilva, A. P.; Gunaratne,
H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher,
J. T.; Rice, T. E. Chem. ReV. 1997, 97, 1515-1566.
Figure 1. Titration of 1 (10-5 M in CH3CN) with metal ions.
(2) Mello, J. V.; Finney, N. S. Angew. Chem., Int. Ed. 2001, 40, 1536-
1438.
a property that allows discrimination between different metals
bound by the same molecule: titration with Li+ leads to an
(3) Mello, J. V.; Finney, N. S. Org. Lett. 2001, 3, 4263-4265.
10.1021/ol0272287 CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/14/2003

Evaluation of 1-4 led to several clarifying observations.
First, in all but one instance the substructures of 1 exhibit
metal ion affinities 1-2 orders of magnitude lower than the
parent compound (Table 1).7 This indicates that cooperative
Table 1. Ka Values (M-1) for Metal-Ion Binding by 1-4
ion
1
2
3
4
Li+
6700
-
830
9
39
20
64
6
Figure 2. Fluorescent chemosensor substructures.
Na+
Mg2+
Ca2+
29
63
9
3
32
9
670
240
increase in shorter-wavelength (locally excited, LE) emission,
Mg2+ to an increase in longer-wavelength (charge transfer,
CT) emission, and Ca2+ to an increase of both emission
bands.4
Based on the symmetry of 1, it is not immediately obvious
that both ethereal side chains are required for ion recognition
or that both flanking aryl groups are required for fluorescence
response. To ascertain the minimal requirements for fluo-
rescence emission and metal ion response, we have compared
the properties of 1 and substructures 2-4 (Figure 2, Scheme
1). Fluorophore 1 was prepared as previously described.2 The
binding by both ether side chains is required for affinity,
even in cases where coordination to the pyridine nitrogen is
also involved (such cases providing CT enhancement).
The single exception in terms of diminished binding
affinity is 3, which retains most of the Ca2+-affinity of 1
but only shows an increase in CT (long wavelength)
emission. This supports our previous inference that Ca2+-
induced enhancement of both LE and CT emission in 1
reflects the presence of two distinct Ca2+ binding modes.
LE enhancement requires cooperative coordination by both
ether arms, and is thus not observed in 3, while CT
enhancement requires cooperative coordination by the
pyridine nitrogen and only one of the ether arms and is still
seen. That 2 does not bind Ca2+ is somewhat surprising, and
must reflect subtle conformational differences between 1 and
2.
Scheme 1. Syntheses of 1-4a
Second, we have identified 4 as the minimal fluorophore
(Figure 3). This is significant since it shows that a second
a Reagents and conditions: (a) NaH, BnOH, then 2,6-dibromo-
pyridine (55%); (b) n-BuLi, ArBr; ZnCl2; then 2-benzyloxy-6-
bromopyridine and cat. Pd(PPh3)4 (70%); (c) H2, cat. Pd/C; (d) Tf2O,
Py (79%, two steps); (e) ArB(OH)2, cat. Pd(OAc)2, cat. BINAP,
Cs2CO3 (64%); (f) n-BuLi, ArBr; ZnCl2; then 2-bromopyridine and
cat. Pd(PPh3)4 (46%); (g) as (f) (57%).
analogue lacking one polyether side chain, 2, was prepared
from 2,6-dibromopyridine by sequential SNAr with sodium
benzyloxide, Negishi coupling with the appropriate arylzinc,5
debenzylation, triflation, and Suzuki cross coupling with the
requisite boronic acid.6 Monoarylpyridines 3 and 4 were
prepared from 2-bromopyridine by similar Negishi couplings.
Figure 3. Response of 4 (10-5 M in CH3CN) to metal ion.
(4) Enhanced emission from the LE state results from binding-induced
conformational restriction. The emissive CT state becomes accessible upon
metal coordination by the pyridine nitrogen lone pair. The CT state can
also be induced by protonation.
(5) For a recent review, see: Negishi, E.-i. In Metal-Catalyzed Cross
Coupling Reactions; Diederich, F., Stang, P. J., Eds.; Wiley-VCH: New
York, 1998; Chapter 1.
(6) For reviews, see: (a) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95,
2457-2483. (b) Suzuki, A. J. Organomet. Chem. 1999, 576, 147-168.
Synthesis of the arylboronates is described in ref 2.
aryl substituent is not required to for the emissive properties
of these fluorophores. This relaxed structural requirement
allows much greater latitude in the design of future aryl-
pyridine fluorescent chemosensors.
Finally, the minimal fluorophore still exhibits increased
fluorescence emission, albeit in the presence of a large (ca.
968
Org. Lett., Vol. 5, No. 7, 2003

Scheme 2. Synthesis of Fluorophores 7-11a
Figure 4. Fluorophores spanning a ∼ 100 nm emission range.
1000-fold) excess of metal ions (Figure 3). While these
increases are small and nonselective in comparison to those
of 1, they are provocative. We had previously ascribed
enhanced LE emission to binding-induced conformational
restriction and enhanced CT emission to a charge-transfer
state induced by metal ion coordination to the pyridine.
Neither of these can explain the LE enhancements observed
with 4 and indicate that there is a third mechanism for
fluorescence enhancement at work, details of which will
require further study.8
In addition to facilitating mechanistic inquiry, the modular
structure of fluorescent chemosensors such as 1 provides an
opportunity to dramatically tune the emission wavelength
of bi- and triarylpyridine fluorophores. Increasing conjugation
or polarization of the core biarylpyridine by substitution at
the 4-position of the pyridine can lead to emission red-shifts
of almost 100 nm, as illustrated with 7-11 (Figures 4 and
5, Scheme 2, Table 2).
a Reagents and conditions: (a) n-BuLi, ArBr; ZnCl2; then 2,6-
dibromopyridine and cat. Pd(PPh3)4 (95%); (b) LiOH, THF/H2O
(100%); (c) (COCl)2; NaN3; TFA; (d) K2CO3, CH3OH (78%); (e)
HCl, CH3CN, NaNO2; KI (52%); (f) n-BuLi, ArBr; ZnCl2; then
13 and cat. Pd(PPh3)4 (98%); (g) ArB(OH)2, cat. Pd2(dba)3, cat.
P(t-Bu)3, Cs2CO3, (82%); (h) as (f) (71%); as (g) (31%); (j) n-BuLi,
ArBr; ZnCl2; then 12 (80%); (k) 2-aminophenol, EDCl (97%); (l)
230 °C (87%); (m) ArB(OH)2, cat. Pd2(dba)3, cat. [HP(t-Bu)3]BF4,
Cs2CO3 (78%).
converted to ester 12 and then trihalopyridine 13 as previ-
ously described.3,9 Intermediate 13 was easily elaborated to
8 and 9 by chemoselective Negishi coupling of the 4-iodo
substituent with the appropriate arylzinc reagent, followed
by Suzuki coupling with 4-methoxy-2-methylphenylboronic
acid.10 Similarly, Negishi coupling of 12 provided ready
access to pyridine 10. The remaining fluorophore 11 was
prepared by saponification of 12, coupling of the free acid
with 2-aminophenol, thermal dehydrative cyclization, and
finally Suzuki coupling with 4-methoxy-2-methylphenyl-
boronic acid.
Figure 5. Emission of fluorophores 7-11 in CH3CN.
Fluorophore 7 exhibits absorption and emission spectra
identical to those of 1 (Table 2, Figure 5). Replacing the
The unsubstituted control fluorophore 7 can be prepared
from 2,6-dibromopyridine as described above. The remaining
fluorophores ultimately derive from citrazinic acid (2,6-
dihydroxy-4-pyridinecarboxylic acid). Citrazinic acid can be
Table 2. Select Spectral Parameters for 7-11 in CH3CN11
7
8
9
10
11
e (×10-3
)
14.8
290
350
60
9.3
305
380
75
33.8
310
400
90
4.4
315
420
105
0.23
14.6
300
440
140
0.14
(7) Titrations were carried out by adding aliquots of metal perchlorates
in spectroscopic grade CH3CN to 10-5 M solutions of fluorophore in the
same solvent. See the Supporting Information for details. Binding constants
were determined by nonlinear least-squares fitting of plots of I/I0 vs log-
[metal ion] using the Prism3cx software package (GraphPad, Inc., San
Diego, CA). All data are consistent with formation of 1:1 M/L complexes.
λ
exc (nm)
λem (nm)
∆λ (nm)
φ
0.03
0.20
0.21
Org. Lett., Vol. 5, No. 7, 2003
969

hydrogen at the 4 position of 7 with a phenyl group (8) leads
to an immediate red-shift of ∼25 nm in the LE emission
band. Extending the conjugation further by installation of a
2-benzofuranyl group (9) provides a further red-shift of ∼20
nm. Equally pronounced effects arise from increasing the
polarization of the fluorophore: placing an electron-
withdrawing group at the 4 position of the pyridine (10)
provides a full 70 nm red-shift, such that LE emission from
10 overlaps with the CT emission of 1 and 5. Finally,
installation of a 2-benzoxazolyl provides a total red-shift in
LE emission of 90 nmsremarkable for simply varying one
substituent of a triarylpyridine.
Modular construction is a defining characteristic of
fluorescent chemosensors based on bi- and triarylpyridines.
We have shown here that this feature facilitates the study of
structure-activity relationships in novel chemosensor sys-
tems and also allows dramatic tuning of emission wavelength
by variation of a substituent remote from the potential
binding site of these chemosensors. We anticipate that the
synthesis and observations described here will be of use in
the many other applications of substituted pyridines.
(8) One plausible hypothesis is that metal coordination to the OCH3 group
leads to Stark effect-induced alteration of the T1 energy and thus less
efficient nonradiative decay. For a leading reference on such effects in
hydrocarbon crown ethers, see: (a) Ghosh, S.; Petrin, M.; Maki, A. H.;
Sousa, L. R. J. Chem. Phys. 1988, 88, 2913-2918. (b) Ghosh, S.; Petrin,
M.; Maki, A. H.; Sousa, L. R. J. Chem. Phys. 1987, 87, 4315-4323.
(9) Since our original report (ref 3), we have found that the addition of
acetonitrile as a cosolvent during diazotization/displacement improves the
reliability of the conversion of 4-amino-2,6-dichloro-pyridine to 13. See
the Supporting Information for details.
Acknowledgment. We thank the National Science Foun-
dation for support of this research (CHE-9876333) and the
Departmental NMR facilities (CHE-9709183).
(10) While not always required, we find the use of P(t-Bu)3 typically
provides the best yields for Suzuki coupling to 2,6-dichloropyridine
derivatives. For early reports of the utility of this phosphine, see: (a) Old,
D. W.; Wolfe, J. P.; Buchwald, S. L. J. Am. Chem. Soc. 1998, 120, 9722-
9723. (b) Littke, A. F.; Fu, G. C. Angew. Chem., Int. Ed. Engl. 1997, 37,
33387-3388. For the more recent use of this phosphine as its HBF4 salt,
see: Netherton, M. R.; Fu, G. C. Org. Lett. 2001, 3, 4295-4298.
(11) Excitation and emission maxima are rounded to the nearest 5 nm.
Quantum yields were determined by standard methods (Lakowicz, J. R.
Principles of Fluorescence Spectroscopy, 2nd ed.; Kluwer Academic: New
York, 1999). See the Supporting Information for details.
Supporting Information Available: Complete experi-
mental details and tabulated spectroscopic data for 1-15;
1H NMR spectra for 7-11. This material is available free
of charge via the Internet at http://pubs.acs.org.
OL0272287
970
Org. Lett., Vol. 5, No. 7, 2003