A new series of pyrimidine-containing linear molecules: Their elegant crystal structures and intriguing photophysical properties

Article abstract of DOI:10.1021/jo048914h

A new series of aza-substituted analogues 3-5 based on the 1,4-bis(phenylethynyl)benzene moiety have been synthesized by the selective Pd-catalyzed Sonogashira coupling reaction from 5-bromo-2-iodopyrimidine (1). In these linear molecules, the dipolar pyr

Full text of DOI:10.1021/jo048914h

A New Series of Pyrimidine-Containing Linear Molecules: Their
Elegant Crystal Structures and Intriguing Photophysical
Properties
Ken-Tsung Wong,* Fu-Chuan Fang, Yi-Ming Cheng, Pi-Tai Chou,* Gene-Hsiang Lee, and
Yu Wang
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan
kenwong@ntu.edu.tw; chop@ntu.edu.tw
Received June 28, 2004
A new series of aza-substituted analogues 3-5 based on the 1,4-bis(phenylethynyl)benzene moiety
have been synthesized by the selective Pd-catalyzed Sonogashira coupling reaction from 5-bromo-
2-iodopyrimidine (1). In these linear molecules, the dipolar pyrimidine moiety is introduced as a
probe to investigate factors that control the intermolecular interactions over the crystal engineering.
The results reveal that the manner of packing changes both dipolar interactions between linear
pyrimidine-containing molecules and transition moments simultaneously, resulting in remarkably
different photophysical properties. Due to their versatile dipole-dipole and face-to-face π-π
interactions in a crystal motif, further applications on the design of ordered crystalline materials
for the field effect transistors are promising.
1. Introduction
a functional moiety have been developed. The resulting
materials may render useful sensing properties for proton
or metal ions. More recently, a phenyl ring bearing both
donor and acceptor groups has been introduced as the
central skeleton for a molecular wire, leading to an in-
triguing negative differential resistance.9 For most optical
applications based on arylethynyl materials, optimization
of the emission efficiency is the major challenge. In con-
centrated thin films as well as in bulk solids, the emission
intensities of the linear arylethynyl materials normally
tend to decrease, along with a bathochromic shift and
broadening of the electronic absorption and/or fluores-
cence spectra, the phenomena of which are primarily
attributed to the molecular aggregation.10 Thus, materi-
als functionalized by bulky substituents to suppress the
self-quenching effects via hindering or minimizing the
cofacial aggregations usually exhibit highly efficient
fluorescence in the solid state.11 Furthermore, recent
studies on the conformation-dependent photophysical
properties have revealed that the twisting angle for the
aryl versus the linear enthynyl groups is crucial to
account for the spectral variations,12 while the relative
orientation of the planar aromatic system along the
Linear arylethynyl molecules with extended π-conjuga-
tion are of considerable research interest due to their
potential application in optoelectronics,1 sensors,2 and
molecular devices.3 Significant progress has been made
to develop functional materials with ethynyl skeleton via
modifying the primary structure of the conjugated sys-
tem.4 For example, incorporating electroactive heteroare-
nes such as thiophene into the backbone gives rise to new
materials with interesting properties of electron conduc-
tivity and transport.5 Moreover, linear oligomers bearing
2,2′-bipyridine,6 terpyridine,7 or 1,10-phenanthroline8 as
(1) (a) Bunz, U. H. F. Chem. Rev. 2000, 100, 1605. (b) Kraft, A.;
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S.-M.; Lee, G.-H. Chem. Commun. 2000, 2259 and references therein.
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10.1021/jo048914h CCC: $27.50 © 2004 American Chemical Society
Published on Web 10/15/2004
8038
J. Org. Chem. 2004, 69, 8038-8044

Pyrimidine-Containing Linear Molecules
SCHEME 1. The Synthesis of
molecular axis plays an important role in modifying the
conductivity of a molecular wire.13 Along this line, prob-
ing the factors that affect the conformational changes as
well as the formation and characteristics of molecular
aggregation becomes fundamentally important. In one
aspect, the nature of interchromophore interactions can
be attained via X-ray structural analyses in combination
with the photophysical properties in the crystalline
form.14 Information provided from these investigations
may gain deep insights into the relationship between
molecular structure and physical properties. In another
aspect, to engineer efficient and applicable molecular
devices, control of the interbackbone interactions is an
important issue. Different intermolecular interactions
such as hydrogen bonding, π-π stacking, van der Waals,
and dipole-dipole, can be applied to fine-tune the mo-
lecular alignments. On this basis, many exquisite works
have been reported based on self-assembly techniques,
leading to great successes in the development of polymer-
based organic electronics.15
Aimed at the design and synthesis of efficient materials
for field effect transistors (FETs), we envisage taking
advantage of the moderately strong dipole-dipole inter-
actions as a crystal-engineering motif to control the
molecular crystal packing. For this purpose, it is reason-
able to select pyrimidine as a candidate because the
dipolar interactions between pyrimidine-containing mol-
ecules have been widely applied for the preparation of
liquid crystalline materials.16 To further explore this
intriguing type of intermolecular interaction in crystal
engineering, we have synthesized a series of aza-
substituted analogues of 1,4-bis(phenylethynyl)benzene.
Their elegant crystal structures along with interesting
photophysical properties in the crystalline forms are
reported in this contribution.
Pyrimidine-Containing Linear Molecules
tially introducing different alkynes at 2 and 5 positions
of pyrimidine.17 The control on the different arrange-
ments of the dipolar orientation of pyrimidine along a
well-defined conjugated system has previously been
achieved in our laboratory.18 The synthesis of phenyl-
ethynylene-pyrimidine linear molecules by the Pd-
catalyzed Sonogashira reaction starting from 5-bromo-
2-iodopyrimidine (1) is outlined in Scheme 1.
The selective Sonogashira coupling reaction of 1 on the
iodo-substituted carbon with phenylacetylene was carried
out under standard conditions with Pd(PPh3)4 as the
catalyst, CuI as the cocatalyst, and diisopropylamine as
the base in THF at room temperature. 5-Bromo-2-
phenylethynylpyrimidine (2) was obtained in 80% yield
after purification by chromatography. Further, 2 is
coupled with another equivalent of phenylacetylene at
80 °C to furnish 2,5-bis(phenylethynyl)pyrimidine (3) in
85% yield. Compound 3 can also be efficiently prepared
in one pot from 1 with the addition of an excess amount
of phenylacetylene at 80 °C by using the same catalytic
system. The isolated yields for both stepwise and one-
pot syntheses are comparable. Introducing a new conju-
gated skeleton on the phenyl group may further increase
the backbone conjugation length of 3. Accordingly, treat-
ment of 2 with 4-(trimethylsilylethynyl)phenylacetylene
in the presence of Pd catalyst and cocatalyst (CuI) at 80
°C afforded dissymmetric linear molecule 4 in 80%
isolated yield. The removal of the alkyne-protecting group
(trimethylsilyl group, TMS) was provided with K2CO3 in
MeOH/THF to afford 5 in 75% yield. Further extension
of the conjugation length is achievable by reacting 5 with
1 or other halogenated arenes.
2.2. X-ray Crystal Structures. A single crystal of 3
was obtained by carefully layering the THF solution
containing 3 with n-hexane at ambient temperature. A
clear identification of the exact position of nitrogen atoms
in the molecule is not possible due to the disordered
arrangement of the nitrogen atoms in the molecular
structure. The structure shown in Figure 1 is drawn with
a 50% probability of interexchanging nitrogen atoms
[N(1), N(1B)] and carbons [C(1A), C(1C)]. Obviously, the
aromatic rings of 3 are nearly coplanar with a dihedral
angle of 0.43° between the phenylene ring and the
2. Results and Discussion
2.1. Synthesis. The high chemoselectivity of 5-bromo-
2-iodopyrimidine (1) toward Pd-catalyzed coupling reac-
tions provides an effective synthetic strategy for sequen-
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J. Org. Chem, Vol. 69, No. 23, 2004 8039

Wong et al.
FIGURE 1. The thermal ellipsoid plot of the crystal structure of 3 at the 50% probability level.
FIGURE 3. The thermal ellipsoid plot of the molecular
structure of 4 at the 50% probability level.
FIGURE 2. (a) The crystal packing diagram of 3 as seen along
the c-axis. (b) The presentation of stepped plane-to-plane
interactions within the crystal packing.
4 and 5 were synthesized. Single crystals of 4 and 5
suitable for X-ray analyses were obtained from layering
the THF solution of 4 and 5 with MeOH at ambient
temperature. For crystal data of 4 and 5 please see the
Supporting Information for details. The crystal structure
of 4 on the main conjugated backbone resembles that of
3 (Figure 3) except for the coplanarity among the aryl
rings. The phenyl ring attached to trimethylsilylethynyl
group is slightly twisted from the molecular plane of
the pyrimidine ring with a dihedral angle of 4.30°,
whereas the dihedral angle between the terminal phe-
nylene ring and the pyrimidine ring is calculated to be
4.25°. The CtC-C(Ar) bond angle is as linear as 178.9°,
except for the terminal trimethylsiylethynyl group that
slightly bends away from the linear backbone. The
Si-C(22)-C(21) and C(22)-C(21)-C(18) bond angles are
estimated to be 175.1° and 176.1°, respectively. This
bending structure, which may be ascribed to the longer
C-Si bond and the crystal packing forces, is occasionally
observed in the silicon-containing alkynyl compounds.20
Apparently, the steric bulkiness of the TMS group has
pronounced effects on the crystal packing. The conjugated
backbones of neighboring molecules within the chain are
aligned in a plane-to-plane but antiparallel manner to
mitigate the steric interactions raised from the bulky
TMS group in 4.
pyrimidine ring, and the molecule is almost linear with
bond angles between triple bonds and adjacent atoms of
179.5° (C(2)-C(3)-C(4)) and 179.8° (C(3)-C(4)-C(5)).
The structure resembles that of 1,4-bis(phenylethynyl)-
benzene possessing a linear, coplanar structure in the
solid state.19 However, as shown in Figure 2, the crystal-
packing diagram of 3 is completely different from the
S-shaped crystal packing behavior of 1,4-bis(phenylethy-
nyl)benzene. The crystal packing of 3 is comprised of two
translationally nonequivalent infinite π-stacks, which are
perpendicular to each other, resulting in a cross-her-
ringbone-like packing pattern as viewed from the projec-
tion along the c axis (Figure 2a). Molecules within a given
π-stack are tilted by ca. 46° with respect to the stacking
axis (b-axis), and are stepped relative to one another
along the long axis of conjugation, rendering a zigzag-
staircase structure that allows the center of one aromatic
ring placed on top of the C-C triple bond of the next
molecule (Figure 2b). The perpendicular intermolecular
distance between the C(4) and the plane of the central
pyrimidine ring of the neighboring molecule is estimated
to be 3.51 Å, suggesting the existence of π-π interactions.
Although an unambiguous X-ray identification of the
position of nitrogen atoms is difficult at this stage, we
speculate that the moderate dipolar and, in part, the clear
π-π interactions between the neighboring molecules play
an important role on guiding compound 3 to an extremely
ordered crystal packing.
As shown in Figure 4a there are two pairs of step-type
molecular packing within the chain structure. The pair
of molecules B and C has a longer plane-to-plane contact,
in which the aromatic rings are located over the others
of the next molecule with a closest C(8)-C(13′) inter-
atomic distance of 3.64 Å, indicating a weak and possibly
negligible π-π stacking effect. Alternatively, the dipole-
dipole interaction between the neighboring pyrimidine
To further support the above viewpoint as well as to
probe the effects of possible intermolecular dipolar in-
teractions on the crystal packing, unsymmetric molecules
(20) (a) Anthony, J. E.; Eaton, D. L.; Parkin, S. R. Org. Lett. 2002,
4, 15. (b) Anthony, J. E.; Brooks, J. S.; Eaton, D. L.; Parkin, S. R. J.
Am. Chem. Soc. 2001, 123, 9482.
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1998, 31, 1093.
8040 J. Org. Chem., Vol. 69, No. 23, 2004

Pyrimidine-Containing Linear Molecules
FIGURE 4. (a) The presentation of different plane-to-plane interactions within the crystal packing of 4. (b) The crystal packing
diagram of molecule 4.
FIGURE 5. The thermal ellipsoid plot of the molecular
structure of 5 at the 50% probability level.
rings of molecule B and C is plausibly the dominant factor
that dictates the molecular alignment. For molecules A
and B, the plane-to-plane interactions are only re-
stricted to the pyrimidine ring and the phenyl ring
attached to a trimethylsilylethynyl group. The interpla-
nar C(13)-C(14′) distance is estimated to be 3.52 Å,
suggesting moderate face-to-face intermolecular interac-
tions among the aromatic systems in the crystalline form.
A similar packing pattern is also resolved in the pair of
molecules C and D. Thus, the entire chain structure is
comprised of these two alternative pairs of molecules. The
dipolar interactions and face-to-face π-π stacking ef-
ficiently guide the crystal packing of 4, leading to a
slightly tilted cross-herringbone-type or T-shaped/parquet-
type packing fashion (Figure 4b) analogous to that of 3.
This observation may support our speculation of possible
dipole-dipole interactions in the crystal packing of 3.
The crystal structure of 5 is shown in Figure 5. The
phenyl ring attached to an ethynyl group slightly
bends from the linear molecular backbone. The
C(13)-C(14)-C(15) bond angle is estimated to be 177.9°,
being comparable to CtC-C(Ar) with an average bond
angle of 178.7°. In comparison to that of crystal 3, the
linear 1-D chain with a less coplanar configuration for
aromatic rings becomes obvious. The phenyl ring at-
tached to an ethynyl group is twisted from the molecular
plane of the pyrimidine ring with a dihedral angle of
7.92°, whereas the dihedral angle between the terminal
phenylene ring and the pyrimidine ring is calculated to
be 5.80°. The crystal packing of molecule 5 is significantly
different from those of 3 and 4, which periodically pack
in piles on top of each other as viewed from the projection
along the a axis (Figure 6).
FIGURE 6. The crystal packing of molecule 5.
The molecular stacks are arranged in such a way that
constitutes a ribbonlike structure along the c axis. To
efficiently increase the attraction, hence the stabilization,
the neighboring molecular stacks within each individual
ribbon are arranged in an antiparallel manner. The two
neighboring ribbons are then tilted with respect to each
other, leading to a herringbone-like packing pattern.
Upon close inspection of the crystal structure from
various viewing angles, we found that, within the indi-
vidual band, there exist two types of short N-H distances
between neighboring molecules, indicating the possible
existence of weak, non-hydrogen-bonding interactions.
Type A with a N(2)‚‚‚H(20A) distance of 2.56 Å and Type
B with a N(1)‚‚‚H(16A) distance of 2.60 Å are illustrated
in Figure 7a. These two short N‚‚‚H interactions result
in the formation of an infinite wrinkled sheet (as shown
in the side view of Figure 7b).
In addition, the terminal ethynyl hydrogen H(3A) is
very close to the H(22A′) of the adjacent molecule with a
H(3A)-H(22A′) distance estimated to be 3.02 Å. This
nearly perpendicular head-to-tail arrangement of the
adjacent bands leads to a zigzag packing feature shown
in Figure 8a. Theoretically, the removal of the bulky TMS
group from 4, giving compound 5, should effectively
release the intermolecular steric interactions. Along this
line, a closer plane-to-plane stack is expected for the
crystal structure of 5. Upon careful inspection on the
crystal packing of 5, two types of plane-to-plane contact
J. Org. Chem, Vol. 69, No. 23, 2004 8041

Wong et al.
FIGURE 7. (a) The view of the crystal packing of 5 with two
different short N- -H distances between neighboring molecules.
(b) The side view of the wrinkled sheetlike structure.
FIGURE 9. The absorption and emission spectra of 3 (9), 4
(b), and 5 (2) in cyclohexane.
TABLE 1. The Photophysical Properties of 3, 4, and 5
λmax(abs)a
(nm)
ꢀmax
λmax(em)a
(nm)
λmax(em)b
(nm)
τfb
a
(M-1 cm-1
)
Φfa
Φfc (ns)
3
4
5
320
335
325
4 × 104
5.3 × 104
425
390
415
9.0 × 10-4
8.5 × 10-3
1.5 × 10-3
451
485
442
0.08 0.8
0.65 5.7
0.13 1.1
8.2 × 104
a Ca. 1.0 × 10-5 M in cyclohexane. b In the solid crystal. c In
the solid film (see Experimental Section).
luminescence intensity is linearly proportional to the
increase of concentration, eliminating the emission as-
sociated with any high-order aggregation. The entire
emission band originating from a common ground-state
species is ascertained by the same fluorescence excitation
spectra throughout the monitored wavelengths of
350-600 nm. The excitation spectra, within experimental
error, are also effectively identical with the absorption
spectrum, indicating that the entire emission results from
a common Franck-Condon excited state for 3-5.
FIGURE 8. (a) The zigzag-type packing pattern of molecule
5. (b) The two different plane-to-plane interactions within the
crystal of 5.
For the case of 3, despite the slightly structural
S0 f S1 (π, π*) absorption band, the corresponding
emission spectrum exhibits diffusive, broad spectral fea-
tures with a peak wavelength at 425 nm. The Stokes
shift, defined as the difference in peak frequencies
between absorption and emission bands, is as large as
∼7700 cm-1. As indicated by the similar Stokes shift
being ∼8100 cm-1 in a much stronger polar solvent such
as CH3CN, such a large shift clearly does not originate
from the solvent dipolar relaxation due to the large
dipolar change of the solute (i.e. compound 3) upon exci-
tation. Alternatively, the results, in combination with the
low emission yield of 9.0 × 10-4, can be more plausibly
rationalized by the existence of various rotational isomers
in the excited state. Unfortunately, the multiple degrees
of rotational freedom in a large molecule such as 3 make
quantitative analyses complicated. Although further rele-
vant approaches are not pursued, quenching of the fluor-
escence dominated by the corresponding torsional mo-
tions is obvious, giving rise to extremely weak emission
intensity. Similar dynamics of relaxation were applied
to rationalize the broad, weak, and large Stokes shifted
emission for compound 5 in solution. In contrast, the
emission of 4 revealed a vibronic-like spectral feature
that is a mirror image with respect to the corresponding
absorption spectrum. The results, in combination with
are identified, in which molecules B and C (Figure 8b)
with different alignment of the dipolar orientation of
pyrimidine are paired up as a group. Therefore, the three
aromatic rings of the individual molecule within the pair
(B and C) are superimposed with a closest N(2)-C(11′)
interatomic distance of 3.54 Å. Another plane-to-plane
contact between pairs also exists. In this case, the phenyl
ring (molecule A) attached by a terminal ethynyl group
is closer to the pryimidine ring (molecule B) with a closest
interatomic C(11)-C(16′) distance calculated to be 3.43
Å, indicating the possibility of strong π-π interactions
in this region. Therefore, similar to the case found in
molecule 4, the dipole-dipole interactions and significant
plane-to-plane contact again play important roles on the
crystal packing in 5.
2.3. Photophysical Properties. Figure 9 shows the
absorption and emission spectra of 3, 4, and 5 in room-
temperature cyclohexane. Detailed photophysical proper-
ties are listed in the Table 1. For 3-5, the longest
wavelength absorption peak with ꢀmax values of >104 M-1
cm-1 leads to the assignment of a characteristic π-π*
transition unambiguously. For all three compounds
studied, the corresponding room-temperature emission
in cyclohexane is very weak (Φf , 10-2, see Table 1). The
8042 J. Org. Chem., Vol. 69, No. 23, 2004

Pyrimidine-Containing Linear Molecules
that the large Stokes shifted emission for 3 in solution
mainly results from the rotational electrochromism.
Finally, it should be noted that in the quantum-yield
measurement of solid, a film sample had to be prepared
via either the spin coating or the vapor deposition method
to obtain reliable data (see the Experimental Section).
We have conducted an XRD analysis on a vacuum (5 ×
10-6 Torr) deposited film of compound 4. Unfortunately,
the diffraction pattern could not be resolved, indicating
the formation of a complicated packing manner during
the vacuum deposition. Despite the quantum yield mea-
surement, other photophysical parameters were obtained
in solid crystalline forms. Thus, the correlation between
solid-state quantum yield and crystal structure may only
be qualitative.
3. Conclusion
FIGURE 10. The emission spectra of 3 (9), 4 (b), and 5 (2)
in solid crystalline.
In conclusion, an efficient synthetic route to prepare a
new series of aza-substituted analogues 3-5 based on
the 1,4-bis(phenylethynyl)benzene moiety has been es-
tablished. The pyrimidine moiety is incorporated into
these linear molecules for probing the possibility of
dipolar interactions that can control the crystal packing.
Comprehensive investigations on the crystal structures
of these linear molecules have revealed that the dipole-
dipole interactions between proximal pyrimidine moieties
as well as plane-to-plane contact between adjacent aryl
rings play key roles in determining the crystal packing.
In low viscous solution, various degrees of rotational
freedom in 3 and 5 result in a large red-shifted emission
with rather low yields. Anchoring a terminal TMS group
drastically diminishes the rotational deactivation pro-
cesses and the emission behaves as normal Stokes shifts.
In a single crystal, factors such as steric hindrance,
conjugation, etc. fine-tune the intermolecular interactions
and hence the corresponding crystal engineering. The
manner of packing changes both dipolar interactions
between linear pyrimidine-containing molecules and
transition moments simultaneously, resulting in sub-
stantially different photophysical properties. Information
provided from these investigations, especially their un-
usually strong dipole-dipole interactions in a crystal
motif, may shed more light on the nature and molecular
design of materials for the highly efficient field effect
transistor (FET). Focus on 3-5 in the application of EET
is currently in progress.
the highest quantum yield among three studied mol-
ecules, indicate that the introduction of a bulky TMS
retards the internal rotational motions in 4 and hence,
to a certain extent, diminishes the radiationless deactiva-
tion. However, the variation of electronic density of the
conjugated chromophore perturbed by the TMS group21
may also play an important role on the higher photolu-
minescence efficiency for linear compound 4.
Drastic increase of the emission yield was observed for
three studied compounds in a single crystal (Figure 10).
The quantum efficiency was measured to be 0.08, 0.65,
and 0.13 for 3, 4, and 5, respectively. The results correlate
well with the time-dependent study, in which the lifetime
is beyond the system response limit of 200 ps for 3-5,
while it was measured to be 0.8, 5.7, and 1.1 ns for 3, 4,
and 5, respectively, in the solid crystalline form. In
comparison to the solution phase, the increase of ∼2
orders of magnitude of the emission efficiency in solid
supports the rotational motions as the dominant deac-
tivation process in solution. In the absence of large-
amplitude motions the quenching of the fluorescence may
be due to the S1 f Tn intersystem crossing. However,
since phosphorescence is rather small for all three
compounds in solid, the e0.65 fluorescence yield cannot
be solely attributed to the quenching by the intersystem
crossing. Alternatively, it may be more plausible to
propose that one radiationless channel is associated with
the π-stacking effect in solid. This viewpoint can be
supported by the highest quantum yield of 0.65 for 4
among 3-5 in solid, in which the slightly tilted cross-
herringbone-type or T-shaped/parquet-type packing fash-
ion may provide less stacking force for the exciton
formation. However, one cannot rule out that the specif-
ically high quantum yield in 4, to a certain extent, is due
to the different dipolar interaction in the antiparallel
packing manner, which results in a larger transition
moment than that of 3 and 5. Finally, for the case of 3,
although dipole-dipole interactions in the crystal pack-
ing are strong (vide supra), the bathochromic shift (∼25
nm) of the emission is rather small in comparison to that
in cyclohexane. The results further support the viewpoint
4. Experimental Section
4.1. General Experimental Details. All reactions were
performed under argon and were magnetically stirred. Sol-
vents were distilled from appropriate drying agent prior to
use: THF from sodium and benzophenone, diisopropylamine
from CaH2. Commercially available reagents were used with-
out further purification. All reactions were monitored by TLC
with precoated aluminum foil sheets (0.20 mm with fluorescent
indicator UV254). Compounds were visualized with UV light
at 254 and 365 nm. Melting points were measured by dif-
ferential scanning calorimetry (DSC). Infrared (IR) spectra
1
were recorded within KBr. H NMR and 13C NMR in CDCl3
were recorded with a spectrometer at 400 and 100 MHz,
respectively. Low- and high-resolution mass spectra were
recorded in the FAB mode.
(21) (a) Maeda, H.; Inoue, Y.; Ishida, H.; Mizuno, K. Chem. Lett.
2001, 1224. (b) Yamaguchi, S.; Xu, C.; Tamao, K. J. Am. Chem. Soc.
2003, 125, 13662.
4.2. Synthesis. (a) Synthesis of 5-Bromo-2-phenylethy-
nylpyrimidine (2).17a A mixture of phenylacetylene (0.13 mL,
J. Org. Chem, Vol. 69, No. 23, 2004 8043

Wong et al.
1.2 mmol), 5-bromo-2-iodopyrimidine (1) (285 mg, 1.0 mmol),
CuI (9.5 mg, 0.05 mmol), Pd(PPh3)4 (57.8 mg, 0.05 mmol), and
diisopropylamine (0.28 mL, 2.0 mmol) in dry THF (10 mL) was
placed in a sealed two-neck flask under argon. The mixture
was stirred at room temperature for 24 h. The resulting
reaction mixture was filtered through a pad of silica gel (or
Al2O3) and washed with Et2O (2 × 15 mL). The filtrate was
concentrated under reduced pressure. The resultant crude
product was purified by chromatography on silica gel, eluting
with EtOAc/hexane ) 1/9 to afford the cross-coupling product
added. After the mixture was stirred for 1 h at ambient
temperature, the residue was concentrated and purified by
chromatography on silica gel, eluting with CH2Cl2/hexane )
1/2 to give the product 5-(4-ethynyl-phenylethynyl)-2-phenyl-
ethynylpyrimidine (230 mg, 75%). The analytically pure
product was recrystallized from CH2Cl2 and hexane. Mp 210.8
°C (DSC analysis); IR (neat) ν 3283, 2230, 1496, 1427 cm-1
;
1H NMR (CDCl3, 400 MHz) δ 3.2 (s, 1H), 7.38 (m, 3H), 7.49 (s,
4H), 7.66 (d, 2H, J ) 6.4 Hz), 8.83 (s, 2H); 13C NMR
(CDCl3, 400 MHz) δ 79.6, 82.8, 84.4, 87.9, 90.2, 97.0, 117.6,
121.1, 122.0, 123.2, 128.5, 129.9, 131.6, 132.2, 132.6, 150.8,
158.9; MS (m/z, FAB+) 304 (3), 136 (60); HRMS (M+, FAB+)
calcd for C22H12N2 304.1000, found 304.1007. Anal. Calcd for
C22H12N2: C, 86.82; H, 3.97; N, 9.20. Found: C, 86.79; H, 4.05;
N, 9.17.
1
as a white solid (210 mg, 80%). H NMR (CDCl3, 400 MHz) δ
7.37 (m, 3H), 7.65 (d, 2H, J ) 8.1 Hz), 8.79 (s, 2H); 13C NMR
(CDCl3, 400 MHz) δ 87.1, 89.4, 118.9, 121.0, 128.4, 129.9,
132.6, 151.0, 158.0; MS (M + H+, m/z, FAB+) 258.9 (70); HRMS
(M + H+, FAB+) calcd for C12H879BrN2 258.9871, found
258.9868, and for C12H881BrN2 260.9850, found 260.9853.
(b) Synthesis of 2,5-Bis(phenylethynyl)pyrimidine (3).
A mixture of phenylacetylene (0.13 mL, 1.2 mmol), 5-bromo-
2-phenylethynylpyrimidine (259 mg, 1.0 mmol), CuI (9.5 mg,
0.05 mmol), Pd(PPh3)4 (57.8 mg, 0.05 mmol), and diisopropyl-
amine (0.28 mL, 2.0 mmol) in dry THF (10 mL) was placed
in a sealed two-neck flask under Argon. The mixture was
heated and stirred at 80 °C for 24 h and then cooled to room
temperature. The solution was filtered through a pad of silica
gel, and the filtrate was concentrated under reduced pressure.
The residue was chromatographed on silica gel, eluting with
EtOAc/hexane ) 1/7 to give 2,5-bis(phenylethynyl)pyrimidine
(238 mg, 85%). Mp 181.1 °C (DSC analysis); IR (neat) ν 3052,
2227, 2214, 2200, 1494, and 1428 cm-1; 1H NMR (CDCl3, 400
MHz) δ 7.40 (m, 6H), 7.56 (d, 2H, J ) 7.6 Hz), 7.68 (dd, 2H, J
) 1.6, 7.6 Hz), 8.85 (s, 2H); 13C NMR (CDCl3, 400 MHz) δ 82.6,
88.0, 90.0, 97.8, 117.9, 121.1, 121.7, 128.4, 128.5, 129.4, 131.7,
132.6, 150.6, 158.9; MS (m/z, FAB+) 281 (15), 147 (100), 74
(55); HRMS (M+, FAB+) calcd C20H12N2 280.1000, found
280.1007. Anal. Calcd.for C20H12N2: C, 85.69; H, 4.31; N, 9.99.
Found: C, 85.31; H, 4.64; N, 9.58.
4.3. Photophysical Measurements. Steady-state absorp-
tion and emission spectra were recorded with a corrected
excitation light source. In addition, the wavelength-dependent
characteristics of the monochromator and photomultiplier have
been calibrated by recording the scattered light spectrum of
the corrected excitation light from a diffused cell in the 220-
700 nm ranges. To obtain the precise extinction coefficient five
different concentrations ranging from 5 × 10-5 to 5 × 10-7
M
were performed. Quinine Sulfate in 1 N H2SO4 aqueous
solution was used as a reference, assuming a yield of 0.57 with
a 325-nm excitation, to determine the fluorescence quantum
yields of the studied compounds in solution. Lifetime studies
were performed by a photon-counting system with a hydrogen-
filled/or a nitrogen lamp as the excitation source. Data were
analyzed by using the nonlinear least-squares procedure in
combination with an iterative convolution method. The emis-
sion decays were analyzed by the sum of exponential functions,
which allows partial removal of the instrument time broaden-
ing and consequently renders a temporal resolution of ∼200
ps.
A configuration of front-face excitation was used to measure
the emission of the solid crystal. The solid crystal was fixed
by assembling two edge-polished quartz plates with various
Teflon spacers. A combination of appropriate filters was used
to avoid the interference from the scattering light. An inte-
grating sphere was applied to measure the quantum yield in
the solid state, in which the solid sample film was prepared
via either the spin coating or the vapor deposition method and
was excited by a 325-nm He-Cd laser line. The resulting
luminescence was acquired by an intensified charge-coupled
detector for subsequent quantum yield analyses.
(c) Synthesis of 2-Phenylethynyl-5-(4-trimethylsilyl-
ethynylphenylethynyl)pyrimidine (4). A mixture of 4-(tri-
methylsilylethynyl)phenylacetylene (238 mg, 1.2 mmol), 5-bro-
mo-2-phenylethynylpyrimidine (2) (259 mg, 1.0 mmol), CuI (9.5
mg, 0.05 mmol), Pd(PPh3)4 (57.8 mg, 0.05 mmol), and diiso-
propylamine (0.28 mL, 2.0 mmol) in dry THF (10 mL) was
placed in a sealed two-neck flask under argon. The mixture
was stirred at 80 °C for 24 h. Chromatography on silica gel
and eluting with EtOAc/hexane ) 1/7 afforded 4 (300 mg, 80%).
Mp 208.2 °C (DSC analysis); IR (neat) ν 2960, 2225, 2157,
1500, 1495, 1427, 1421 cm-1 1H NMR (CDCl3, 400 MHz) δ
;
0.25 (s, 9H), 7.40 (m, 3H), 7.48 (m, 4H), 7.67 (dd, 2H, J ) 1.7,
8.0 Hz), 8.84 (s, 2H); 13C NMR (CDCl3, 400 MHz) δ 84.3, 87.9,
90.2, 97.2, 97.3, 104.1, 117.6, 121.1, 121.5, 124.2, 128.4, 129.9,,-
131.5, 132.0, 132.6, 150.7, 158.8; MS (m/z, FAB+) 377 (25), 136
(60); HRMS (M+, FAB+) calcd for C25H20N2Si 376.1396, found
376.1396. Anal. Calcd for C25H20N2Si: C, 79.75; H, 5.35; N, 7.44.
Found: C, 79.54 H, 5.35 N, 7.39.
(d) Synthesis of 5-(4-Ethynylphenylethynyl)-2-phenyl-
ethynylpyrimidine (5). 2-Phenylethynyl-5-(4-trimethylsilyl-
ethynylphenylethynyl)pyrimidine (4) (377 mg, 1.0 mmol) was
dissolved in THF (10 mL), and a solution of potassium
carbonate (70 mg, 0.5 mmol) dissolved in methanol (5 mL) was
Acknowledgment. We thank the National Science
Council and Ministry of Education for financial support.
The authors thank Prof. Hsiu-Fu Hsu for his construc-
tive suggestions.
Supporting Information Available: Summarized crystal
data and crystallographic data (CIF) for 3, 4, and 5. This
material is available free of charge via the Internet at
http://pubs.acs.org.
JO048914H
8044 J. Org. Chem., Vol. 69, No. 23, 2004