5-Aryl-2,2′-bipyridines as tunable fluorophores

Article abstract of DOI:10.1016/j.tetlet.2006.07.111

Readily available 5-aryl-2,2′-bipyridines and their derivatives are a unique family of fine-tunable chromophores, where changes in the substitution patterns, solvation and coordination are translated into dramatic spectral changes. They exhibit sensitive and selective response to zinc ions with dramatic increases in emission intensity or significant red-shifts of emission maxima.

Full text of DOI:10.1016/j.tetlet.2006.07.111

Tetrahedron Letters 47 (2006) 7025–7029
5-Aryl-2,20-bipyridines as tunable fluorophores
b
b
Dmitry N. Kozhevnikov,a,b, Olga V. Shabunina, Dmitry S. Kopchuk,
*
Pavel A. Slepukhina and Valery N. Kozhevnikova,b
aInstitute of Organic Synthesis, Ural Branch of Russian Academy of Sciences, Kovalevskoy 22, Ekaterinburg 620219, Russia
bUral State Technical University, Mira 19, Ekaterinburg 620002, Russia
Received 22 June 2006; revised 11 July 2006; accepted 21 July 2006
Available online 10 August 2006
Abstract—Readily available 5-aryl-2,20-bipyridines and their derivatives are a unique family of fine-tunable chromophores, where
changes in the substitution patterns, solvation and coordination are translated into dramatic spectral changes. They exhibit sensitive
and selective response to zinc ions with dramatic increases in emission intensity or significant red-shifts of emission maxima.
Ó 2006 Elsevier Ltd. All rights reserved.
Luminescent organic/organometallic compounds have
attracted much attention because of their potential
applications in electroluminescent displays.1–3 Lumines-
cent chelate complexes have been shown to be particu-
larly useful in electroluminescent (EL) displays because
of their relatively high stability.4,5 A critical element in
designing and fabricating such materials is the control
of their emission wavelength.6 One of the approaches
for controlling the emitted color of organic materials
is to append fluorescent chromophores to a polymeric
backbone or to blend such dyes into inert polymeric
matrices.7–9 Ideally, one would like to utilize one family
of modular chromophores and tune their photophysical
characteristics at will.10
Here, we describe a novel series of highly fluorescent
2,20-bipyridines. Tuning of the emission wavelength of
any of the derivatives is possible by changing the nature
of its substituents and it can be further modulated by
protonation or metal ion coordination. Since the most
intense electronic transition of the 2,20-bipyridine skele-
ton is polarized along the 5,50-positions,16 it was
shown10 that increasing the conjugation along this axis
provides highly fluorescent derivatives of 1,10-phenan-
throline. The 5-position of bipyridines is considered
the best for introduction of aromatic substituents, for
example, 5-manisyl-2,20-bipyridines have been previ-
ously shown to exhibit higher emission quantum yields
compared with the 2- and 4-manisyl analogues.15 More-
over, increasing polarization along the 5,50-axis may
lead to a significant red-shift of the emission maximum.
Unsymmetrical arylbipyridines are attractive candidates
for this purpose. In addition, aryl moieties at the b-posi-
tion do not affect the coordination behavior of the
ligand.
Metal complexes of polypyridines are widely used
luminophores. However, the parent 2,20-bipyridine,
2,20:60,200-terpyridine and 1,10-phenanthroline possess
extremely low fluorescence quantum yields and undesir-
able short emission wavelengths, which can be explained
by emission from the n–p* excited state.11,12 Introduc-
tion of conjugated electron-donor moieties, for example,
pyrrolylethenyl,13 phenylethynyl,10 aminophenyl14 or
manisyl15 leads to an increase in quantum yields and a
shift of emission wavelength in the visible area. A distin-
guishing feature of such ligands is a sensitive response
toward Zn2+ ions with a significant red-shift in emission.
R
R
N
N
N
N
N
N
2a-c
1a-c
R = H (a), Me (b), OMe (c)
5-Aryl-2,20-bipyridines 1a–c were obtained according to
*
Corresponding author. Tel.: +7 343 3623341; fax: +7 343 3741189;
e-mail: dnk@ios.uran.ru
a recently reported method that involves initial synthesis
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.07.111

7026
D. N. Kozhevnikov et al. / Tetrahedron Letters 47 (2006) 7025–7029
of 6-aryl-3-pyridyl-1,2,4-triazines 2a–c and their conver-
sion to substituted bipyridines 1 through an inverse
electron demand Diels–Alder reaction with 2,5-norbor-
nadiene (Scheme 1).17
emission quantum yield of 1a, since the n–p* excited
state often decays through non-radiative pathways.
We observed that acidification of an ethanolic solution
of 1a (addition of excess CF3COOH) led to a bathochro-
mic shift in the longest wavelength absorption band.
The fluorescent emission band underwent a 74 nm red-
shift of emission maxima and some associated increase
in integrated emission intensity. This data can be ex-
plained by stabilization of the intramolecular charge
transfer (ICT) excited state which is expected to occur
due to an increase in the electron deficient nature of
the bipyridine upon protonation.
Table 1 summarizes the photophysical data of 5-aryl-
2,20-bipyridines 1a-c in ethanol at room temperature.
The lowest energy absorbance maxima of the bipyri-
dines 1a–c in solutions is not affected by the nature of
the aryl substituent or the solvent polarity. On the other
hand, the aryl substituent has a significant influence on
the emission characteristics (Table 1 and Fig. 1). Phenyl-
bipyridine 1a (R = H) possesses a very weak emission,
which is slightly blue-shifted upon increasing solvent
polarity. This indicates a greater contribution of the
n–p* excited states in fluorescence and explains the low
It is known that coordination to zinc(II) shifts the
absorption bands of polypyridine units to the red.
COOH
COOMe
HOOC
N
HOH2C
N
SeO2/dioxane
reflux, 2 h
COOMe
Ar
N
Ar
Ar
N
N
NH2
OHC
AcOH
N
N
4
N
N
N
N
m
-xylene
reflux, 16 h
OH
5a-c
r.t. 14 h, then reflux 0.5 h
COOMe
COOMe
3a-c
55-72%
6a-c
Ar
Ar
N
N
OHC
N
N
N
N
N
m
-xylene
AcOH
reflux, 10-12 h
64-76%
r.t. 1 h, then 90 ºC 1 h
7a-c
8a-c
77-86%
MeO
Me
Ar =
(c)
(a)
(b)
Scheme 1.
Table 1. Photophysical data for arylbipyridines 1, 3 and 7
Compound
kmax [nm]a
kem [nm]b
(UF)c
k
max(Zn2+) [nm]d
k
em(Zn2+) [nm]e
k
max(H+) [nm]f
k
em(H+) [nm]g
1a
1b
1c
3a
3b
3c
7a
7b
7c
300
303
312
314
319
329
359
363
0.05
0.38
0.80
0.05
0.39
0.36
0.019
0.12
0.62
322
326
339
337
339
355
357
362
365
375
320
337
350
315
322
331
362
366
378
433
438
525
385
404, 428
443, 510sh.
455, 517sh.
440, 520
517
397, 430sh.
405sh., 442
405, 426
436
517
398, 430sh.
435
399, 430sh.
382
400, 428sh.
440, 510sh.
369
315sh., 325, 340sh.
317sh., 328, 341sh.
318sh., 330, 343sh.
382
432
517
a Absorption maxima in EtOH.
b Emission maxima in EtOH.
c Fluorescence quantum yields were measured using anthracene as the standard (U = 0.27 in EtOH8).
d Absorption maxima in EtOH after addition of excess Zn(ClO4)2.
e Emission maxima in EtOH after addition of excess Zn(ClO4)2.
f Absorption maxima in EtOH after addition of excess CF3COOH.
g Emission maxima in EtOH after addition of excess CF3COOH.

D. N. Kozhevnikov et al. / Tetrahedron Letters 47 (2006) 7025–7029
7027
packing shows that there are two types of stacks involv-
ing the same array of intermolecular forces but possess-
ing p–p stacking interactions aligning along different
directions, hence a herringbone-type pattern is created
(see Supplementary data).
Introduction of a weak electron-donor group such as
methyl results in a significant change of the photophys-
ical properties of 1b (R = Me). While the absorption
spectra and the emission maxima of 1b are similar to
those of 1a, the emission quantum yield of 1b is seven
times as large as that of 1a. The low dependence of
the emission of 1b on solvent polarity and the high emis-
sion intensity suggest that the emitting excited state of
1b is a p–p* state. Protonation of 1b led to changes in
the absorption and emission spectra similar to those of
1a. However, addition of zinc ions to a solution of 1b re-
sulted in bright purple emission with a 34 nm red-shift,
which is greater than that of 1a. Moreover, the emission
spectrum of the Zn(II) complex of 1b, in polar solvents,
has a shoulder on the long-wavelength side which can be
attributed to the ICT state contribution, which is not
very substantial.
Figure 1. Normalized UV–vis absorbance and fluorescence spectra of
1a (solid), 1b (dash) and 1c (dot) recorded in EtOH.
Titrating Zn2+ (as perchlorate or chloride) into a solu-
tion of 1a in ethanol led to a bathochromic shift similar
to that of the protonated ligand. However, in contrast to
protonation, coordination to zinc resulted only in a
small red-shift of the emission maxima of 1a, but an
extremely enhanced emission intensity (by a factor of
14, when excited at the isosbestic point (308 nm)). The
emission maxima of 1a was blue-shifted upon increasing
the solvent polarity. This indicates that the emitting
excited state is a p–p* state, not an ICT excited state.
Substituting phenylbipyridine with a methoxy group (1c,
R = MeO) resulted in a 40 nm red-shift in the emission
and an extremely enhanced fluorescence intensity
(UF = 0.80) in ethanolic solution. The emission maxima
of 1c is more susceptible to solvent polarity than those
of 1a,b. The red-shift observed for 1c upon increasing
solvent polarity is indicative of stabilization of the ICT
excited state. Protonation and Zn(II) coordination re-
sulted in a large red-shift of the emission (Fig. 3). Addi-
tion of excess CF3COOH to a solution of 1c in EtOH
caused yellow–green fluorescence with decreasing fluo-
rescence intensity. A shift of approximately 45 nm was
We isolated complex [Zn(1a)Cl2] from the reaction of 1a
with ZnCl2. Single crystals of [Zn(1a)Cl2] suitable for
X-ray diffraction were grown from acetonitrile.18 The
molecular structure of the complex [Zn(1a)Cl2] is shown
in Figure 2, and selected bond distances are given in the
caption. The bipyridine fragment is essentially planar
(torsion angle is 1.13°), while the torsion angle between
the pyridine ring and the aromatic substituent is 29.26°.
The Zn atom adopts a tetrahedral coordination geo-
metry and the measured Zn–N(1)/(2) distances are
observed upon addition of Zn2+ ions to 1c leading to
a bright blue emission.
To overcome the shortcomings peculiar to arylbipyri-
dines 1, such as short-wavelength excitation and insuffi-
cient red-shift of emission, we decided to introduce an
electron-withdrawing group (methoxycarbonyl) oppo-
˚
˚
2.059 and 2.060 A, and the Zn–Cl(1)/(2) distances are
2.194 and 2.218 A. Complex face-to-face p–p stacking
interactions between the phenylbipyridine ligands are
also evident, the interplanar separations are in the range
˚
3.4–3.7 A, and the glide-related complexes are linked in
a head-to-tail fashion to generate a supramolecular
architecture of infinite chains. Inspection of the crystal
˚
Figure 2. ORTEP view of [Zn(1a)Cl2]. Selected bonds length, (A):
Zn(1)–N(1) 2.0593, Zn(1)–N(2) 2.0602, Zn(1)–Cl(1) 2.1944, Zn(1)–
Cl(2) 2.2175.
Figure 3. Normalized UV–vis absorbance and fluorescence spectra of
1c (solid), 1c + Zn2+ (dash) and 1c + H+ (dot) recorded in EtOH.

7028
D. N. Kozhevnikov et al. / Tetrahedron Letters 47 (2006) 7025–7029
site to the aryl substituent. 5-Aryl-50-methoxycarbonyl-
2,20-bipyridines 3a–c were synthesized using the method
described for bipyridines 1a–c (Scheme 1). The key
reagent in this case is 5-methoxycarbonylpyridine-2-
carboxaldehyde 4 which was obtained from the 2,5-
dicarboxylic acid through esterification, mono-reduction
of the ester group at C-2 of the pyridine19 and oxidation
of the resulting 2-hydroxymethyl-5-methoxycarbonyl-
pyridine with SeO2. The reaction of hydrazones 5a–c
with aldehyde 4 resulted in 6-aryl-3-(50-methoxycar-
bonyl-2-pyridyl)-1,2,4-triazines 6a–c. Refluxing triazines
6a–c with 2,5-norbornadiene in m-xylene gave target
bipyridines 3a–c. It should be noted that derivatives
of 2,20-bipyridine-5-carboxylic acid are important
building-blocks for supramolecular chemistry.20–23 The
method described herein makes compounds of this type
readily available with tunable structures.
changes as observed for neutral ethanolic solutions.
Such behavior can be explained by the decreasing
basicity of the pyridine ring upon introduction of the
electron-withdrawing methoxycarbonyl group. Thus,
esters 3a–c can be utilized as fluorescent probes for
Zn(II) in acidic solutions.
Extension of the conjugation in arylbipyridines by fus-
ing an additional aromatic ring to one of the pyridines
resulted in changes in the photophysical properties very
similar to those caused by introduction of the ester
group. 2-(5-Arylpyridin-2-yl)quinolines 7a–c were ob-
tained from 2-quinolinecarboxaldehyde and hydrazones
5a–c (Scheme 1) through intermediate 3-quinolyl-1,2,4-
triazines 8a–c. As was expected, the absorption maxima
of pyridylquinolines 7a–c were red-shifted (20–25 nm)
compared to those of 1a–c (Table 1 and Fig. 4). Similar
moderate red-shifts (10–35 nm) of the fluorescence
maxima of 7a–c occurred. A decrease in the emission
quantum yields of pyridylquinolines 7a–c compared to
those of bipyridines 1a–c was observed. However, the
fluorescence intensity of 7a–c was still strongly depen-
dent on the aromatic substituents. Phenyl- and tolyl-
substituted pyridylquinolines 7a,b possess lower
emission quantum yields (UF = 0.019 and 0.12, respec-
tively), while 7c (R = MeO) was intensely fluorescent
chromophore (UF = 0.62) (Table 1). The weak emission
of 7a,b was slightly red-shifted upon increasing the sol-
vent polarity: 8–12 nm when compared with the emis-
sion maxima in toluene and methanol. Obviously, 7a,b
emits from locally excited states: p–p* and n–p* excited
states with considerable contribution of the latter, which
was additionally confirmed by the low fluorescence
intensity of 7a,b. In contrast, a significant red-shift of
the intense emission of 7c in polar solvents (emission
maximum is 375 nm in toluene, 396 nm in CH2Cl2 and
434 nm in methanol) indicated, that the ICT excited state
is the main contributing factor in the emission of 7c.
Introduction of the ester group resulted in red-shifts of
the absorption (13–17 nm) and emission (23–41 nm)
maxima of bipyridinecarboxylates 3a–c compared to
those of 1a–c (see Table 1 and Fig. 4). The absorption
and emission spectra of 3a–c exhibited the same depen-
dence on the aryl substituents as arylbipyridines 1a–c.
Phenyl derivative 3a possessed weak fluorescence
(UF = 0.05), while tolyl and methoxyphenyl derivatives
3b,c exhibited bright purple and blue fluorescence
(UF = 0.39 and 0.36). Addition of Zn2+ to ethanolic
solutions of esters 3a–c caused changes in absorption
and emission similar to those of 1a–c, that is, a dramatic
increase in the emission intensity of phenyl derivative 3a
(by a factor of 10) and a slight decrease in the fluores-
cence intensity of methoxyphenylbipyridine 3c. How-
ever, coordination of esters 3a–c to Zn(II) resulted in
significantly higher red-shifts of the fluorescence max-
ima (23–77 nm) compared to those of bipyridines 1a–c
(15–43 nm), due to the electron-withdrawing ester group
stabilizing the ICT state.
Unexpectedly, the photophysical properties of esters
3a–c were insensitive to protonation. Addition of a large
excess of CF3COOH to solutions of 3a–c had no influ-
ence on the absorption and emission maxima. At the
same time, addition of Zn(II) to solutions of 3a–c in
EtOH, containing excess CF3COOH, caused the same
Addition of excess Zn2+ to ethanolic solutions of quin-
olines 7a–c resulted in the expected bathochromic shift
of the absorption maxima (30–35 nm). Zinc complexes
of quinolines 7a–c exhibit intense blue (7a,b) or yel-
low–green (7c) fluorescence (see Table 1). The longer
red-shifts of the emissions of quinolines 7a–c in compar-
ison with pyridines 1a–c can be explained by a larger
contribution of the ICT excited state in the emission.
Coordination of phenyl- and tolyl-quinolines 7a,b to
zinc dramatically increases the emission intensity, like
pyridines 1a,b by a factor of 25 for 1a and 5 for 1b.
The emission intensity of methoxyphenylquinoline
7c was slightly decreased upon addition of Zn2+
.
In conclusion, 5-aryl-2,20-bipyridines and their deriva-
tives are a unique family of fine-tunable chromophores,
where changes in the substitution patterns, solvation,
and coordination are translated into dramatic spectral
changes. Unique selective responses allow new ligands
to be considered as sensitive probes for zinc(II). The rel-
atively simple synthesis and availability of numerous
precursors make the preparation of other arylbipyri-
dine-based chromophores with diverse photophysical
properties facile.
Figure 4. Normalized UV–vis absorbance and fluorescence spectra of
1c (solid), 3c (dash) and 7c (dot) recorded in EtOH.

D. N. Kozhevnikov et al. / Tetrahedron Letters 47 (2006) 7025–7029
7029
11. Kotlicka, J.; Grabowski, Z. R. J. Photochem. 1979, 413–
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Acknowledgements
12. Albano, G.; Balzani, V.; Constable, E. C.; Maestri, M.;
Smith, D. R. Inorg. Chim. Acta 1998, 277, 225–231.
13. Ajayaghosh, A.; Carol, P.; Sreejith, S. J. Am. Chem. Soc.
2005, 127, 14962–14963.
The authors thank the Russian Foundation for Basic
Research (Grants 05-03-32134 and 04-03-96106) for
financial support.
14. Goodall, W.; Williams, J. A. G. Chem. Commun. 2001,
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15. Loren, J. C.; Siegel, J. S. Angew. Chem., Int. Ed. 2001, 40,
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Supplementary data
16. Bosnich, B. Acc. Chem. Res. 1969, 2, 266–273.
17. Kozhevnikov, V. N.; Kozhevnikov, D. N.; Shabunina, O. V.;
Rusinov, V. L.; Chupakhin, O. N. Tetrahedron Lett. 2005,
46, 1791–1793.
Supplementary data containing experimental details
can be found, in the online version, at doi:10.1016/
j.tetlet.2006.07.111.
18. Complex [Zn(1a)Cl2]. A solution of bipyridine 1a (50 mg,
0.15 mmol) in acetonitrile (30 mL) was added to a solution
of ZnCl2 (21 mg, 0.15 mmol) in acetonitrile (30 mL). The
resulting colorless solution was kept for 23 days at rt. The
resulting colorless crystals were filtered off. Crystal data
for [Zn(1a)Cl2] were measured with an Xcalibur 3 CCD
(graphite monochromator, MoKna): C16H12Cl2N2Zn,
FW = 368.55, monoclinic, a = 33.670(3), b = 5.9039(2),
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