V-shaped 4,6-bis(arylvinyl)pyrimidine oligomers: Synthesis and optical properties

Article abstract of DOI:10.1021/jo900107u

(Chemical Equation Presented) A series of V-shaped 4,6-bis(arylvinyl) pyrimidines have been efficiently prepared by aldol condensation between 4,6-dimethylpyrimidine and the appropriate aromatic aldehyde. The methodology also proved successful when dendri

Full text of DOI:10.1021/jo900107u

V-Shaped 4,6-Bis(arylvinyl)pyrimidine Oligomers: Synthesis and
Optical Properties
Sylvain Achelle,†,‡ Ines Nouira,† Bernd Pfaffinger,† Yvan Ramondenc,† Nelly Ple´,*,† and
Julia´n Rodr´ıguez-Lo´pez*,‡
Institut de Chimie Organique Fine (IRCOF) UMR-CNRS 6014, INSA et UniVersite´ de Rouen,
BP08 76131 Mont-Saint-Aignan, France, and Facultad de Qu´ımica, UniVersidad de Castilla-La Mancha,
13071 Ciudad Real, Spain
nelly.ple@insa-rouen.fr; julian.rodriguez@uclm.es
ReceiVed January 16, 2009
A series of V-shaped 4,6-bis(arylvinyl)pyrimidines have been efficiently prepared by aldol condensation
between 4,6-dimethylpyrimidine and the appropriate aromatic aldehyde. The methodology also proved
successful when dendritic first generation poly(phenylenevinylene) aldehydes were used. Moreover,
asymmetrically functionalized molecules were also obtained by the stepwise incorporation of arms in a
controlled manner. The optical absorption and emission properties of these systems were studied in different
solvents and media. The materials display strong emission solvatochromism that is reflected by a large
red shift in their fluorescence emission maxima on increasing the solvent polarity. This change is
accompanied by a successive decrease in fluorescence intensity. This behavior suggests a highly polar
emitting state, which is characteristic of compounds that undergo an internal charge transfer upon excitation.
The abilities of these molecules to function as colorimetric and luminescence pH sensors were demonstrated
with dramatic color changes and luminescence switching upon the introduction of acid.
Introduction
devices have been developed that incorporate this type of
material. Poly(p-phenylenevinylenes) (PPVs)2 are the most
commonly used family in light-emitting or light-sensitive
devices as their electro-optical properties are known to depend
on the chain length and the presence of substituents. Thus, PPVs
dominate the field of PLEDs (polymer light-emitting diodes)
and similar applications, despite their moderate stability and
the sensitive process needed to obtain good-quality devices.
Organic polymers with good photoluminescent responses are
promising candidates for photonic, electronic, and optoelectronic
applications.1 These systems usually have in common a fully
conjugated backbone with alternate double and single (or triple
and single) bonds along a chain. Numerous technological
† INSA et Universite´ de Rouen.
‡ Universidad de Castilla-La Mancha.
In this context, π-conjugated oligomers have also been
intensively studied, and there are numerous reasons for working
with such short molecules. For example, they are more soluble
(1) (a) Seminario, J. M.; Tour, J. M. In Molecular Electronics-Science and
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P. F., Eds. Acc. Chem. Res. 1999, 32 (special issue). (d) Tour, J. M. Acc. Chem.
Res. 2000, 33, 791–804. (e) Mu¨llen, K., Scherf, U., Eds. Organic Light Emitting
DeVices; Wiley-VCH: Weinheim, Germany, 2006. (f) Skotheim, T. A., Reynolds,
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10.1021/jo900107u CCC: $40.75
Published on Web 04/21/2009
2009 American Chemical Society
J. Org. Chem. 2009, 74, 3711–3717 3711

Achelle et al.
in common organic solvents than homologous polymers, which
makes it possible to analyze them readily by standard spectro-
scopic techniques. The energies for optical transitions of short
oligophenylenevinylenes are higher than those for longer
chains,3 and moreover, the incorporation of meta linkages in
the phenylenevinylene backbone lowers the molecular conjuga-
tion. Both of these effects allow the system to emit blue light,3,4
the primary color that is the hardest to obtain with PLEDs.
phenylenevinylene dendritic arms can function as light-harvest-
ing antennae.13b
We describe here the synthesis and characterization of a novel
series of V-shaped molecules with a 4,6-divinylpyrimidine core
and phenylenevinylene arms bearing electron-donating and/or
electron-withdrawing groups. The influence of the nature of the
electroactive substituents on the optical absorption and emission
properties was examined along with the fluorosolvatochromism
and pH sensitivity. The incorporation of PPV dendritic structures
on the 4,6-divinylpyrimidine core was also assessed in order to
combine the properties of both types of chromophores.
Among these oligomers, molecules with a D-π-A or D-π-
A-π-D structure (where D is an electron-donating group, A an
electron-accepting group, and π a conjugating moiety) are of
high interest due to their fluorescence properties with internal
charge transfer (ICT) and as chromophores for second- and
third-order nonlinear optics (NLO).5 The pyrimidine ring is an
excellent candidate to be incorporated in such structures. Indeed,
this heterocycle has a high electron-withdrawing character,
significant aromaticity that can lead to highly conjugated
molecules, as well as basic and potential ligand properties that
can also be used to modulate the optoelectronic properties of
molecules. Oligomers that contain pyrimidine rings have been
used as liquid crystals,6 n-type semiconductors,7 components
of electroluminescent diodes,8 fluorescent molecules,9 and two-
photon absorption chromophores.10
Results and Discussion
Preparation of Divinylpyrimidines. Two main methods have
been described for the synthesis of (E)-vinylpyrimidines: the
Suzuki cross-coupling reaction of potassium alkenyltrifluorobo-
rates with chloropyrimidines15 and the condensation of alde-
hydes with methylpyrimidines.16 The latter approach has the
advantages of a wide range of commercially available aldehydes
and the use of environmentally friendly conditions in most cases.
In this way, a large variety of dicondensation products could
be readily obtained by aldol condensation between 4,6-dimeth-
ylpyrimidine and the corresponding aromatic aldehyde (Table
1). Reactions were carried out in boiling aqueous 5 M NaOH
(1 M for 2e) using Aliquat 336 as a catalyst, according to a
previously reported procedure.16a-c The experimental protocol
is straightforward and rather economical since it does not require
any organic solvent. All of the products were obtained in good
yields after recrystallizationsexcept for the naphthalene and
anthracene derivatives, which suffered from difficulties in the
purification process.
On the other hand, dendritic poly(phenylenevinylenes), also
called stilbenoid dendrimers, represent an important group within
this class of material. Several different studies have been
published to date concerning the synthesis and properties of
these materials.11 For example, such compounds have been used
successfully as charge transporting,12 light-emitting,13 and
electron-transfer materials.14 It has also been demonstrated that
In order to increase the electronic delocalization along each
arm, pentacyclic V-shaped oligomers 3a,b and 4 were also
synthesized in good yields by Suzuki cross-coupling of bromo
derivatives 2d and 2h, respectively, with the appropriate boronic
acids (Scheme 1).
(4) Halls, J. J. M.; Cornil, J.; dos Santos, D. A.; Silbey, R.; Wang, D.-H.;
Colmes, A. B.; Brédas, J. L.; Friend, R. H. Phys. ReV. B 1999, 60, 5721–5727.
(5) (a) Albota, M.; Beljonne, D.; Bre´das, J.-L.; Ehrlich, J. E.; Fu, J.-Y.; Heikal,
A. A.; Hess, S. E.; Kogej, T.; Levin, M. D.; Marder, S. R.; McCord-Maughon,
D.; Perry, J. W.; Ro¨ckel, H.; Rumi, M.; Subramaniam, G.; Webb, W. W.; Wu,
X.-L.; Xu, C. Science 1998, 281, 1653–1656. (b) Marder, S. R.; Beratan, D. N.;
Cheng, L.-T. Science 1991, 252, 103–106.
(6) Petrov, V. P. Mol. Cryst. Liq. Cryst. 2006, 457, 121–149. (b) Lin, Y. C.;
Lai, C. K.; Chang, Y. C.; Liu, K. T. Liq. Cryst. 2002, 29, 237–242.
(7) Kambara, T.; Kushida, T.; Saito, N.; Kuwayiama, I.; Kubota, K.;
Yamamoto, T. Chem. Lett. 1992, 583–587.
(8) Wang, C.; Jung, G. Y.; Batsanov, A. S.; Bryce, M. R.; Petty, M. C. J.
Mater. Chem. 2002, 12, 173–181. (b) Wong, K. T.; Hung, T. S.; Lin, Y.; Wu,
C. C.; Lee, G. H.; Peng, S. M.; Chou, C. H.; Su, Y. O. Org. Lett. 2002, 4,
513–516.
(9) (a) Achelle, S.; Ramondenc, Y.; Dupas, G.; Ple´, N. Tetrahedron 2008,
64, 2783–2791. (b) Achelle, S.; Ramondenc, Y.; Marsais, F.; Ple´, N. Eur. J.
Org. Chem. 2008, 3129–3140. (c) Itami, K.; Yamazaki, D.; Yoshida, J.-i. J. Am.
Chem. Soc. 2004, 126, 15396–15397. (d) Pascal, L.; Vanden Eynde, J. J.; Van
Haverbeke, Y.; Dubois, P.; Michel, A.; Rant, U.; Zojer, E.; Leising, G.; Van
Dorn, L. O.; Gruhn, N. E.; Cornil, J.; Bre´das, J. L. J. Phys. Chem. B 2002, 106,
6442–6450. (e) Pascal, L.; Vanden Eynde, J. J.; Van Haverbeke, Y.; Dubois, P.;
Michel, A.; Rant, U.; Zojer, E.; Leising, G.; Lazzaroni, R.; Cornil, J.; Bre´das,
J. L. Synth. Met. 2001, 119, 183–184. (f) Wong, K.-T.; Lu, Y.-R.; Liao, Y.-L.
Tetrahedron Lett. 2001, 42, 6341–6344.
Methods for the preparation of this class of oligomer in an
asymmetrically functionalized fashion (i.e., bearing differently
substituted phenylenevinylene arms) have not been described
in the literature to date. In our case, this goal was achieved by
the stepwise incorporation of arms in a controlled manner. In
the aforementioned procedure, the reaction with only 1 equiv
of aldehyde proved unsuccessful in giving the corresponding
monocondensation derivatives; however, when KButO was used
as the base in refluxing THF, vinylpyrimidines 5a,b could be
obtained. Although the corresponding dicondensation products
were also present in the crude mixtures, the desired compounds
could be separated by column chromatography in moderate
yields. However, it was not possible to obtain a pure sample of
5b, which was always accompanied by ca. 10% of the
dicondensation derivative 2b. As a result, this material was used
in the next step without further purification. This step involved
a second condensation reaction with a different aldehyde to
afford the asymmetric divinylpyrimidines 6a-c in good isolated
yields (Scheme 2).
(10) (a) Liu, Z.; Chen, T.; Liu, B.; Huang, Z.-L.; Huang, T.; Li, S.; Xu, Y.;
Qin, J. J. Mater. Chem. 2007, 17, 4685–4689. (b) Liu, B.; Hu, X.-L.; Liu, J.;
Zhao, Y.-D.; Huang, Z.-L. Tetrahedron Lett. 2007, 48, 5958–5962. (c) Liu, Z.;
Shao, P.; Huang, Z.; Liu, B.; Chen, T.; Qin, J. Chem. Commun. 2008, 2260–
2262.
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Synth. 2008, 5, 267–290.
(12) Lupton, J. M.; Samuel, I. D. W.; Beavington, R.; Burn, P. L.; Ba¨ssler,
H. AdV. Mater. 2001, 13, 258–261.
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N. J. Am. Chem. Soc. 2002, 124, 10875–10886. (b) Kwon, T. W.; Alam, M. M.;
Jenekhe, S. A. Chem. Mater. 2004, 16, 4657–4666.
(15) Molander, G. A.; Bernardi, C. R. J. Org. Chem. 2002, 67, 8424–8429.
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Commun. 2001, 31, 3167–3173. (b) Pascal, L.; Vanden Eynde, J. J.; Van
Haverbeke, Y.; Dubois, P. Lett. Org. Chem. 2004, 1, 112–118. (c) Mainil, M.;
Pascal, L.; Vanden Eynde, J.-J.; Van Haverbeke, Y.; Dubois, P. e-Polymers 2003,
056. (d) Brown, D. M.; Kon, G. A. R. J. Chem. Soc. 1948, 2147–2154.
(13) (a) Pillow, J. N. G.; Halim, M.; Lupton, J. M.; Burn, P. L.; Samuel,
I. D. W. Macromolecules 1999, 32, 5985–5993. (b) Halim, M.; Pillow, J. N. G.;
Samuel, I. D. W.; Burn, P. L. AdV. Mater. 1999, 11, 371–374. (c) Ma, D.; Lupton,
J. M.; Beavington, R.; Burn, P. L.; Samuel, I. D. W. AdV. Funct. Mater. 2002,
12, 507–511. (d) D´ıez-Barra, E.; Garc´ıa-Mart´ınez, J. C.; Merino, S.; del Rey,
R.; Rodr´ıguez-Lo´pez, J.; Sa´nchez-Verdu´, P.; Tejeda, J. J. Org. Chem. 2001, 66,
5664–5670. (e) Tolosa, J.; Romero-Nieto, C.; D´ıez-Barra, E.; Sa´nchez-Verdu´,
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3712 J. Org. Chem. Vol. 74, No. 10, 2009

V-Shaped 4,6-Bis(arylVinyl)pyrimidine Oligomers
TABLE 1. Condensation of 4,6-Dimethylpyrimidine with
Aromatic Aldehydes
SCHEME 1. Suzuki Cross-Coupling Reaction of Bromo
Derivatives 2d and 2h
decomposition and/or cis-trans isomerization when they were
allowed to stand in chloroform solution at room temperature
for several days, especially those compounds with strongly
electron-donating amino groups.
Preparation of Dendritic PPV Pyrimidines. Having verified
the general accessibility of divinylpyrimidines through simple
aromatic aldehydes, the corresponding dendritic materials were
synthesized by following a similar protocol in aqueous media.
The appropriate first generation dendritic PPV aldehydes 7a,b
were easily obtained by reaction of the corresponding iodo
derivatives17 with BunLi at -78 °C followed by quenching with
DMF.13e,18 These two compounds were then reacted with 4,6-
dimethylpyrimidine 1 to provide the expected dendritic pyri-
midines 8a,b (Scheme 3).
The long alkyl chain in dendrimer 8a led to a reasonable
level of solubility in a variety of organic solvents such as THF,
chloroform, and dichloromethane. In contrast, the presence of
the terminal CF3 groups lower the solubility of the system, and
dendrimer 8b is only sparingly soluble in THF. The 1H and 13
C
NMR spectra are consistent with these compounds having
double bonds with a trans configuration [3J(H,H) ∼ 16 Hz].
The MS and HRMS data also support the formation of the
desired dendrimers.
UV/Vis and PL Spectroscopy. The optical properties of the
synthesized divinylpyrimidines were investigated by UV/vis and
photoluminescence (PL) spectroscopy on CH2Cl2 solutions at
room temperature. The data obtained are summarized in Table
2. All compounds were photostable and did not undergo
cis-trans isomerization under the analysis conditions. The meta
arrangement through which the different units are linked
prevents efficient delocalization throughout the conjugated
backbone, and as a result, all of these compounds show
absorption wavelengths (λmax) in the UV or visible region
(330-492 nm). In some cases, a second or even a third
absorption band of higher energy can be observed, a situation
in agreement with calculated and experimental results for related
structures.9d
All new compounds were characterized using a variety of
analytical techniques. The overall purities of these compounds
were confirmed by elemental analysis. NMR experiments proved
very useful to confirm the structures of the compounds (see
Experimental Section and Supporting Information). The selec-
tivity of the aldol reactions was sufficiently high to generate
all-trans isomers within the limits of NMR detection. The
stereochemistry of the double bonds was unequivocally estab-
lished on the basis of the coupling constant for the vinylic
protons in the 1H NMR spectra (J ∼ 16 Hz). The low solubility
of compounds 2n, 3a, and 4 required the addition of TFA to
the NMR tube in order to acquire the spectra. Well-resolved
signals for the corresponding protonated species were obtained
in all cases. It is also worth noting the line broadening observed
at room temperature for compound 2m. The signals for these
compounds appeared well-resolved upon heating the sample at
60 °C, a phenomenon that is probably due to a tendency for
this kind of compound to aggregate.
The materials are also fluorescent and strongly emit light
when irradiated, showing a typical response in the visible region
(407-592 nm). Only compound 2o is nonemissive, and this
(17) D´ıez-Barra, E.; Garc´ıa-Mart´ınez, J. C.; Rodr´ıguez-Lo´pez, J. J. Org.
Chem. 2003, 68, 832–838.
(18) Garc´ıa-Mart´ınez, J. C.; Atienza, C.; de la Pen˜a, M.; Rodrigo, A. C.;
Tejeda, J.; Rodr´ıguez-Lo´pez, J. J. Mass Spectrom. early view DOI 10.1002/
jms.1534.
These materials are perfectly stable in the solid state and could
be stored without the need for any special precautions. However,
it should be noted that some samples underwent partial
J. Org. Chem. Vol. 74, No. 10, 2009 3713

Achelle et al.
SCHEME 2. Preparation of Asymmetric Divinylpyrimidines
a Compound 5b was isolated accompanied by ca. 10% of the dicondensation derivative 2b.
SCHEME 3. Synthesis of Dendritic PPV Pyrimidines
TABLE 2. UV/Vis and Photoluminescence (PL) Data
experimental values obtained from the UV/vis spectra (see
Supporting Information, Table S1).
UV/vis (CH2Cl2) λmax
nm (ε, M-1 cm-1
,
PL (CH2Cl2) λmax
,
Stokes shift,
cm-1
b
compda
)
nm
ΦF
Compounds 3a,b (with one biphenyl unit per arm) have
higher values of λabs, λem, and ε than analogous compounds 2a,b
(with only one benzene ring per arm) due to extension of the
conjugation. As far as the fluorescence quantum yield is
concerned, a dramatic increase was observed in the case of
compound 3a, whereas compound 3b had a lower ΦF value,
probably due to more effective aggregation.
A similar bathochromic tendency is observed on changing
from phenyl rings9d to naphthyl (2j,k) and then anthracyl (2m)
or phenanthryl (2n) derivatives. Comparison of compound 3a
with 4 reveals that the replacement of a benzene by a thiophene
ring in each arm of the oligomer causes a red shift of the
absorption and the emission as well as a decrease in ε and ΦF.
Surprisingly, thienyl compounds 2g and 2i have low fluo-
rescence, whereas similar molecules without vinyl bridges are
highly emissive.9b This finding might be related to the low
luminescence of oligothienylenevinylene.20
The UV/vis spectra for compounds 6 and 8 consist of a simple
superposition of the absorptions due to the independent chro-
mophores. As one would expect, the absorption becomes much
stronger in dendritic compound 8a owing to the exponential
increase in the number of light-absorbing phenylenevinylene
units.
In general, large Stokes shifts occur. This phenomenon is
particularly noteworthy in dendritic compound 8a, where a
Stokes shift up to 10 344 cm-1 is observed. The magnitudes of
the Stokes shifts indicate large (vibrational, electronic, geomet-
ric) differences between the excited state reached immediately
after absorption and the excited state from which the emission
starts. Charge-transfer processes should be fairly effective due
to the presence of a π-donor and a π-acceptor group (pyrimidine)
2a
2b
2c
2d
2e
2f
2g
2i
2j
296 (17000), 369 (35600)
429 (42100)
444
530
540
407
393, 419
496
444
410
451
0.06c
4578
4442
4901
4001
3880f
4349
4505
4263
5838
6536
5039
0.40c
301 (38900), 427 (47600)
290 (41300), 350 (26400)
280 (38500), 341 (30200)
289 (95200), 408 (29700)
304 (12700), 370 (31900)
287 (21800), 349 (33300)
256 (31600), 357 (27100)
276 (20300), 332 (26200)
263 (26500), 283 (27100),
380 (27100)
0.55c
0.01d
<0.01e
0.14c
<0.01c
0.03d
0.11c
2k
2l
424
470
0.08c
0.37c
2m 410 (27300)
525
457
0.02c
0.12c
5342
5440
2n
2o
366 (28500)
258 (15900), 329 (23400),
492 (5300)
3a
3b
4
6a
6b
6c
8a
373 (60500)
309 (49200), 407 (71600)
423 (47800)
483
592
529
543
545
586
508
0.59c
0.26c
0.14c
0.42c
0.45c
0.05c
0.10d
6106
7678
4737
5797
5012
6688
10344
413 (35900)
301 (19200), 428 (39500)
317 (25000), 421 (28400)
330 (105400)
a All spectra were recorded at room temperature at c ) 2 mg/L (1.8
× 10-6 to 6.8 × 10-6 M). b Fluorescence quantum yield ((10%) in
dichloromethane determined relative to quinine sulfate in 0.1 M H2SO4
as standard (ΦF ) 0.54).21 c Excitation at 382 nm. d Excitation at 330
nm. e Excitation at 350 nm. f Calculated using the fluorescence maxima
at 393 nm.
observation might be related to the poor luminescent properties
of ferrocene derivatives.19
A red shift of the absorption and emission bands, along with
an increase in the value of the fluorescence quantum yield (ΦF),
was observed on increasing electron-donating strength of the
aromatic groups. Phenyl derivatives 2a-e and naphthyl deriva-
tives 2k,l provide good examples of this trend. The calculated
HOMO-LUMO energy gaps are in good agreement with the
(20) (a) Hwang, I.-W.; Xu, Q.-H.; Soci, C.; Chen, B.; Jen, A. K.-Y.; Moses,
D.; Heeger, A. J. AdV. Funct. Mater. 2007, 17, 563–568. (b) Samuel, I. D. W.;
Meyer, K. E.; Bradley, D. D. C.; Friend, R. H.; Murata, H.; Tsutsiu, T.; Saito,
S. Synth. Met. 1991, 41, 1377–1380. (c) Apperloo, J. J.; Martineau, C.; van Hal,
P. A.; Roncali, J.; Janssen, R. A. J. J. Phys. Chem. A 2002, 106, 21–31.
(19) Fery-Forgues, S.; Delavaux-Nicot, B. J. Photochem. Photobiol. A: Chem.
2000, 132, 137–159.
3714 J. Org. Chem. Vol. 74, No. 10, 2009

V-Shaped 4,6-Bis(arylVinyl)pyrimidine Oligomers
TABLE 3. Optical Properties of Several Divinylpyrimidines in Different Solvents
hexane (∆f ) -0.001)b
THF (∆f ) 0.209)b
CH2Cl2 (∆f ) 0.217)b
methanol (∆f ) 0.308)b
compda
λ
absmax emmax
λ
λ
absmax
λ
emmax
λ
absmax
λ
emmax
λabsmax
λ
emmax
2a
2c
2f
2l
296, 359, 377
298, 408, 426
315, 394, 416
275, 338, 361
328
388, 409
444, 467
431, 451
397, 425, 451
410, 430
366
419
318, 406
262, 282, 377
330
430
517
485
450
491
296, 369
301, 427
289, 408
263, 283, 380
330
444
540
496
470
508
297, 373
298, 428
318, 420
262, 283, 374
325
489
583
531
519
407
8a
a All spectra were recorded at room temperature at c ) 2 mg/L (1.8 × 10-6 to 6.8 × 10-6 M). b Solvent orientation polarizabilities (∆f).23
FIGURE 2. Changes in color of CH2Cl2 solutions of several vinylpy-
rimidines (c ) 2 mg/L) in the presence of 10-2M TFA.
FIGURE 1. Normalized emission of compound 2f in various solvents
(c ) 2 mg/L).
a complete quenching of the fluorescence of the pyrimidine
moiety in this polar protic solvent. This quenching can be
attributed to the hydrogen bond interaction between the molecule
and surrounding solvent, which results in an additional nonra-
diative decay. Indeed, two bands at 424 and 532 nm can be
observed in a nonprotic polar solvent such as DMSO (∆f )
0.263), where only a partial energy transfer from the phenyle-
nevinylene units should occur (see Supporting Information for
PL spectra of 2a, 2c, 2l, and 8a).
directly connected by the phenylenevinylene system. This effect
also leads to a very low degree of self-absorption of emitted
light. Hence, these compounds are expected to perform well in
organic light-emitting diodes and as laser dyes.
In an effort to gain further insight into the photophysical
process within these V-shaped molecules, we investigated the
absorption and emission behaviors of 2a, 2c, 2f, 2l, and 8a in
different solvents. The results of these investigations are
summarized in Table 3. The absorption spectra are nearly
independent of solvent polarity, except for a slight, insignificant
red shift that indicates a negligible intramolecular interaction
between donor and acceptor groups in the ground state. In
contrast, the emission spectra exhibit distinct solvent depen-
dence. Broad structureless emission and larger Stokes shifts were
observed on increasing the solvent polarity along with a
successive decrease in the fluorescence intensity. This solva-
tochromic behavior, which results from the stabilization of the
highly polar emitting state by polar solvents, is typical for
compounds that undergo an internal charge transfer upon
excitation and has been fully documented for numerous fluo-
rophores containing donor-acceptor units.22 As an example,
the PL spectra for compound 2f are shown in Figure 1.
Interestingly, the emission of dendritic compound 8a appears
again narrow and strongly blue-shifted in methanol, with a
maximum wavelength of 407 nm. Whereas in THF and CH2Cl2
the charge-separated state can be formed with energy transfer
from the peripheral phenylenevinylene chromophores, in metha-
nol the energy transfer should not take place. Thus, emission is
only obtained from the peripheral phenylenevinylene units with
The nitrogen atoms of all the prepared pyrimidines are basic
centers that can be protonated. Thus, the effect of protonation
on the optical properties of CH2Cl2 solutions of vinylpyrimidines
2a, 2c, 2e, 2f, 2l, 2o, 4, and 8a was also studied. Dichlo-
romethane solutions of these compounds underwent a significant
color change in the presence of TFA (10-2 M), with the
exception of 2e (Figure 2). This color change was found to be
fully reversible by neutralization with a base such as Et3N or
KButO.
The change in the UV-vis absorption spectra of 2a upon
addition of acid is illustrated in Figure 3. The spectra show the
progressive attenuation of the absorption band for the neutral
compound on increasing the concentration of TFA from 10-5
to 10-2 M, whereas a new, more intense red-shifted band
corresponding to the protonated species appeared (see Support-
ing Information for spectra of the other compounds described
here).
As far as the emission properties are concerned, the proto-
nated forms of compounds 2a, 2f, 2e, 2l, and 4 remain emissive
with a red-shifted fluorescence (Table 4 and Figure 4). In
contrast, pyrimidine 2c becomes nonemissive upon addition of
TFA. In this case, not only the pyrimidine ring but also the
diphenylamino group can be protonated, and therefore, the
(21) Melhuish, W. H. J. Phys. Chem. 1961, 65, 229–235.
(22) (a) Bordeau, G.; Lartia, R.; Metge, G.; Fiorini-Debuisschert, C.; Charra,
F.; Teulade-Fichou, M.-P. J. Am. Chem. Soc. 2008, 130, 16836–16837. (b) Chew,
S.; Wang, P.; Hong, Z.; Kwong, H. L.; Tang, J.; Sun, S.; Lee, C. S.; Lee, S.-T.
J. Lumin. 2007, 124, 221–227. (c) Katan, C.; Terenziani, F.; Mongin, O.; Werts,
M. H. V.; Porre`s, L.; Pons, T.; Mertz, J.; Tretiak, S.; Blanchard-Desce, M. J.
Phys. Chem. A 2005, 109, 3024–3037. (d) Detert, H.; Schmitt, V. J. Phys. Org.
Chem. 2004, 17, 1051–1056.
(23) (a) Mys´liwa-Kurdziel, B.; Solymosi, K.; Kruk, J.; Böddi, B.; Strzałka,
K. Eur. Biophys. J. 2008, 37, 1185–1193. (b) Berezin, M. Y.; Lee, H.; Akers,
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J. Org. Chem. Vol. 74, No. 10, 2009 3715

Achelle et al.
Conclusions
In conclusion, we have successfully synthesized and char-
acterized a series of new 4,6-bis(arylvinyl)pyrimidines in a
straightforward manner by aldol condensation between 4,6-
dimethylpyrimidine and the appropriate aromatic aldehyde. The
protocol permits not only the preparation of dendritic derivatives
but also the stepwise incorporation of different aldehydes,
resulting in molecules that are functionalized in an asymmetric
fashion. The meta-substitution pattern causes all chromophores
to be independent, and all of the compounds present absorption
wavelengths in the UV or visible region, emitting light with
large Stokes shifts. The materials exhibit strong emission
solvatochromism, and red-shifted broad structureless bands are
obtained in polar solvents, an observation characteristic of
intramolecular charge transfer at excited states. The pH sensing
properties of these compounds were also studied. These
compounds display a dramatic and reversible color change and
luminescence switching upon addition of acid as a result of the
protonation of the nitrogen atoms of the pyrimidine ring. These
findings suggest that suitable design of the molecules and a
sound understanding of their spectroscopic properties could
enable promising colorimetric and luminescence pH sensors to
be developed.
FIGURE 3. UV/vis absorption changes of 2a in CH2Cl2 (c ) 2 mg/L)
with increasing TFA concentration.
TABLE 4. UV/Vis and PL Data for Several Prepared
Divinylpyrimidines upon Addition of TFA
UV/vis (TFA 10-2
in CH2Cl2) λmax, nm
(ε, M-1 cm-1
M
PL (CH2Cl2)
Stokes
Experimental Section
compda
)
λmax, nm
ΦF shifts cm-1
General Procedure for the Dicondensation of Aldehydes
and 4,6-Dimethylpyrimidine. Preparation of Compounds 2
and 8.9d A stirred mixture of 4,6-dimethylpyrimidine 1 (108 mg,
1.0 mmol) and the corresponding aldehyde (2.0 mmol) in aqueous
sodium hydroxide (5 M, 15 mL) containing Aliquat 336 (43 mg,
0.1 mmol) was heated under reflux for 1 h (1 M NaOH and 8 h
under reflux was used for compound 2e). The mixture was allowed
to cool, and the precipitate was filtered off, washed with water,
and purified by recrystallization from the indicated solvent.
Representative examples are described below.
2a
2c
2e
2f
2l
2o
4
462 (42000)
297 (24100), 581 (54300)
380 (31200)
546
0.33b
2164
445
596
590
0.06c
0.17c
0.19b
3844
1809
2075
538 (61200)
315 (15000), 482 (39600)
407 (26000), 629 (12400)
354 (27000), 553 (255900)
331 (86000), 409 (29800)
666
0.03b
1808
8a
a All spectra were recorded at room temperature at c ) 2 mg/L (1.8
× 10-6 to 6.8 × 10-6 M). b Fluorescence quantum yield ((10%) in
dichloromethane determined relative to fluorescein in 0.1 M NaOH as
(E,E)-4,6-Bis(4-hexyloxystyryl)pyrimidine (2a): Colorless solid;
standard (ΦF
)
0.79).24 c Fluorescence quantum yield ((10%) in
1
yield 81%; mp 126.9-128.3 °C (EtOH); H NMR (CDCl3, 500
dichloromethane determined relative to quinine sulfate in 0.1 M H2SO4
MHz) δ 0.91 (t, 6H, J ) 7.0 Hz), 1.33-1.37 (m, 8H), 1.47 (m,
4H), 1.80 (m, 4H), 3.99 (t, 4H, J ) 7.0 Hz), 6.92 (A of ABq, 4H,
J ) 8.5 Hz), 6.93 (A of ABq, 2H, J ) 16.0 Hz), 7.21 (d, 1H, J )
1.0 Hz), 7.54 (B of ABq, 4H, J ) 8.5 Hz), 7.85 (B of ABq, 2H, J
) 16.0 Hz), 9.05 (d, 1H, J ) 1.0 Hz); 13C NMR and DEPT (CDCl3,
125 MHz) δ 14.0 (CH3), 22.6 (CH2), 25.7 (CH2), 29.2 (CH2), 31.6
(CH2), 68.1 (CH2), 114.9 (CH), 115.8 (CH), 123.4 (CH), 128.3 (C),
129.1 (CH), 136.6 (CH), 158.6 (CH), 160.3 (C), 162.9 (C). Anal.
Calcd for C32H40N2O2: C, 79.30; H, 8.32; N, 5.78. Found; C, 78.99;
H, 8.15; N, 5.74.
as standard (ΦF ) 0.54),21 excitation at 382 nm.
(E,E,E,E,E,E)-4,6-Bis[3,5-bis(4-hexyloxystyryl)styryl]pyrimi-
dine (8a): Pale yellow solid; yield 70%; mp 136.2-138.3 °C
1
(CHCl3/EtOH); H NMR (CDCl3, 500 MHz) δ 0.92 (t, 12H, J )
7.0 Hz), 1.33-1.38 (m, 16H), 1.48 (m, 8H), 1.80 (m, 8H), 3.99 (t,
8H, J ) 7.0 Hz), 6.91 (A of ABq, 8H, J ) 8.5 Hz), 6.99 (A of
ABq, 4H, J ) 16.0 Hz), 7.14 (A of ABq and B of ABq, 6H, J )
16.0 Hz), 7.35 (d, 1H, J ) 1.0 Hz), 7.47 (B of ABq, 8H, J ) 8.5
Hz), 7.57 (br s, 2H), 7.58 (s, 4H), 7.92 (B of ABq, 2H, J ) 16.0
Hz), 9.13 (d, 1H, J ) 1.0 Hz); 13C NMR and DEPT (CDCl3, 125
MHz) δ 14.1 (CH3), 22.6 (CH2), 25.7 (CH2), 29.2 (CH2), 31.6
(CH2), 68.1 (CH2), 114.7 (CH), 116.3 (CH), 124.2 (CH), 125.1
(CH), 125.7 (CH), 126.1 (CH), 127.8 (CH), 129.2 (CH), 129.6 (C),
136.3 (C), 137.0 (CH), 138.6 (CH), 158.7 (CH), 159.0 (C), 162.7
(C); MS (FAB+, m-NBA) m/z 1093.3 (MH+•); HRMS m/z calcd
for C76H89N2O4 1093.6822, found 1093.6841.
FIGURE 4. Normalized PL spectra of 2a in CH2Cl2 (c ) 2 mg/L)
with and without addition of TFA.
ability of the latter to donate electrons is significantly attenuated.
Ferrocenyl derivative 2o remains nonemissive after protonation.
General Procedure for the Monocondensation of Aldehydes
and 4,6-Dimethylpyrimidine. Preparation of Compounds 5. To
a stirred solution of 4,6-dimethylpyrimidine 1 (108 mg, 1.0 mmol)
(24) Umberger, J. Q.; LaMer, V. K. J. Am. Chem. Soc. 1945, 67, 1099–
1109.
3716 J. Org. Chem. Vol. 74, No. 10, 2009

V-Shaped 4,6-Bis(arylVinyl)pyrimidine Oligomers
and KButO (1 mmol) in refluxing THF (30 mL) under argon was
added a solution of the corresponding aldehyde (1.0 mmol) in THF
(30 mL) in four portions every 15 min. The mixture was heated
for 1 h. The mixture was allowed to cool, water was added, and
the THF was evaporated under vacuum. For 5a, the mixture was
extracted with EtAcO (×3), dried (MgSO4), and the solvent
evaporated. For 5b, the solid was filtered off, washed with water,
and purified. A representative example is described below.
(E)-4-(4-Hexyloxystyryl)-6-methylpyrimidine (5a). Purification
by column chromatography (Al2O3, hexanes/EtAcO, 7:3) gave the
title compound as a colorless solid: yield 32%; mp 77-78 °C; 1H
NMR (CDCl3, 500 MHz) δ 0.91 (t, 3H, J ) 7.0 Hz), 1.32-1.37
(m, 4H), 1.47 (m, 2H), 1.79 (m, 2H), 2.52 (s, 3H), 3.99 (t, 2H, J
) 7.0 Hz), 6.87 (A of ABq, 1H, J ) 16.0 Hz), 6.91 (A of ABq, 2H,
J ) 9.0 Hz), 7.14 (s, 1H), 7.53 (B of ABq, 2H, J ) 9.0 Hz), 7.82
(B of ABq, 1H, J ) 16.0 Hz), 9.00 (d, 1H, J ) 1.0 Hz); 13C NMR
(CDCl3, 125 MHz) δ 14.0, 22.6, 24.2, 25.7, 29.2, 31.6, 68.2, 114.9,
123.2, 128.2, 129.1, 136.7, 158.4, 160.3, 162.4, 167.1. Anal. Calcd
for C19H24N2O: C, 76.99; H, 8.16; N, 9.45. Found: C, 77.12; H,
8.14; N, 9.37.
General Procedure for Suzuki Cross-Coupling Reactions.
Preparation of Compounds 3 and 4. A stirred mixture of bromo
derivative (1 mmol), arylboronic acid (2.5 mmol), Pd(PPh3)4 (0.1
mmol), aqueous 2 M potassium carbonate (2.5 mmol, 1.25 mL),
and ethanol (1.5 mL) in degassed toluene (20 mL) was heated under
reflux under nitrogen for 40 h. The reaction mixture was cooled,
and the precipitate was filtered off. A representative example is
described below.
(E,E)-4,6-Bis[4-(4-dimethylaminophenyl)styryl]pyrimidine
(3b): Brown-gray solid; yield 80%; mp >260 °C; 1H NMR (CDCl3,
500 MHz) δ 3.02 (s, 12H), 6.81 (A of ABq, 4H, J ) 8.5 Hz), 7.09
(A of ABq, 2H, J ) 16.0 Hz), 7.31 (s, 1H), 7.56 (B of ABq, 4H, J
) 8.5 Hz), 7.62 (A of ABq, 4H, J ) 8.5 Hz), 7.65 (B of ABq, 4H,
J ) 8.5 Hz), 7.93 (B of ABq, 2H, J ) 16.0 Hz), 9.10 (s, 1H); 13C
NMR and DEPT (CDCl3, 125 MHz) δ 40.5 (CH3), 112.7 (CH),
116.2 (CH), 124.9 (CH), 126.4 (CH), 127.6 (CH), 128.0 (C), 128.2
(CH), 133.4 (C), 136.8 (CH), 142.2 (C), 150.2 (C), 158.7 (CH),
162.9 (C). MS (CI+, isobutane) m/z 523 (MH+). Anal. Calcd for
C36H34N4: C, 82.72; H, 6.56; N, 10.72. Found: C, 82.49; H, 6.41;
N, 10.48.
Acknowledgment. Financial support from the Spanish Mi-
nisterio de Educacio´n y Ciencia (Projects NAN2004-08843-
C05-02 and CTQ2006-08871) and the Junta de Comunidades
de Castilla-La Mancha (Project PCI08-0033) is gratefully
acknowledged. We thank Prof. Georges Dupas for performing
the DFT calculations. We also thank the Centre de Ressources
Informatiques de Haute-Normandie (CRIHAN) for software
optimization and technical support.
General Procedure for the Preparation of Asymmetric
Compounds 6. All operations were identical to those described
for the synthesis of compounds 2 and 8, except that monoconden-
sation products 5 were used as the starting materials. A representa-
tive example is described below.
(E,E)-4-(4-Trifluoromethylstyryl)-6-(4-dimethylaminostyryl)py-
rimidine (6c): Orange solid; purified by column chromatography
(Al2O3, hexanes/EtAcO, 7:3); mp 205-207 °C; yield 68%; 1H NMR
(CDCl3, 500 MHz) δ 3.03 (s, 6H), 6.72 (A of ABq, 2H, J ) 8.5
Hz), 6.87 (A of ABq, 1H, J ) 16.0 Hz), 7.12 (A of ABq, 1H, J )
16.0 Hz), 7.25 (s, 1H), 7.52 (B of ABq, 2H, J ) 8.5 Hz), 7.65 (A
of ABq, 2H, J ) 8.5 Hz), 7.70 (B of ABq, 2H, J ) 8.5 Hz), 7.86
(B of ABq, 1H, J ) 16.0 Hz), 7.91 (B of ABq, 1H, J ) 16.0 Hz),
9.06 (s, 1H); 13C NMR and DEPT (CDCl3, 125 MHz) δ 40.2
(NCH3), 112.1 (CH), 116.2 (CH), 120.5 (CH), 123.5 (C), 124.0
(q, J ) 270.2 Hz, CF3), 125.8 (q, J ) 3.9 Hz, CH), 127.6 (CH),
128.5 (CH), 129.3 (CH), 130.7 (q, J ) 32.6 Hz, C), 134.7 (CH),
138.0 (CH), 139.3 (C), 151.3 (C), 158.6 (CH), 161.4 (C), 164.1
(C).
Supporting Information Available: Characterization data
and copies of 1H NMR and 13C NMR spectra for all compounds;
calculated and experimental HOMO-LUMO energy gaps for
compounds 2; emission spectra for 2a, 2c, 2l, and 8a in various
solvents; absorption spectra of 2c, 2e, 2f, 2l, 2o, and 8a with
increasing TFA concentration; and emission spectra of 2f and
2l with and without addition of TFA. This material is available
free of charge via the Internet at http://pubs.acs.org.
JO900107U
J. Org. Chem. Vol. 74, No. 10, 2009 3717