Synthesis and optical/thermal properties of low molecular mass V-shaped materials based on 2,3-dicyanopyrazine

Article abstract of DOI:10.1016/j.tet.2007.01.045

A novel series of luminescent low molecular mass materials containing a 2,3-dicyanopyrazine central core were synthesized through an esterification reaction between diphenol 10 and different aromatic carboxylic acids 1-6, containing terminal long alkyl chains. They have a similar V-shaped geometry with lack of planarity between the two arms, confirmed by the X-ray structure of the central core. The optical and thermal properties of these compounds were evaluated. They show blue fluorescence in solution (λ_em^max 440-480 nm) with quantum fluorescence yields (Φ_F) from 0.003 to 0.1 and Stokes shifts of around 90 nm. In solid state, optical band gaps (E_g) were from 3.14 to 3.32 eV. Thin films of 11, 13, and 14 exhibited blue fluorescence (λ_em^max 430-456 nm), and 12, 15, and 16 (more bulky) displayed green fluorescence (λ_em^max 488-512 nm). Most of the materials exhibited good thermal stability, exhibiting an amorphous glassy state after melting. Transparent amorphous films were easily obtained through spin coating and characterized by AFM analysis.

Full text of DOI:10.1016/j.tet.2007.01.045

Tetrahedron 63 (2007) 2851–2858
Synthesis and optical/thermal properties of low molecular mass
V-shaped materials based on 2,3-dicyanopyrazine
Rodrigo Cristiano,a Eduard Westphal,a Ivan H. Bechtold,b Adailton J. Bortoluzzia
and Hugo Gallardoa,
*
aDepartamento de Quꢀımica, Universidade Federal de Santa Catarina, 88040-900 Florianoꢀpolis, SC, Brazil
bDepartamento de Fꢀısica, Universidade Federal de Santa Catarina, 88040-900 Florianoꢀpolis, SC, Brazil
Received 1 November 2006; revised 8 January 2007; accepted 22 January 2007
Available online 25 January 2007
Abstract—A novel series of luminescent low molecular mass materials containing a 2,3-dicyanopyrazine central core were synthesized
through an esterification reaction between diphenol 10 and different aromatic carboxylic acids 1–6, containing terminal long alkyl chains.
They have a similar V-shaped geometry with lack of planarity between the two arms, confirmed by the X-ray structure of the central core.
max
em
The optical and thermal properties of these compounds were evaluated. They show blue fluorescence in solution (l
quantum fluorescence yields (FF) from 0.003 to 0.1 and Stokes shifts of around 90 nm. In solid state, optical band gaps (Eg) were from
440–480 nm) with
3.14 to 3.32 eV. Thin films of 11, 13, and 14 exhibited blue fluorescence (lmemax 430–456 nm), and 12, 15, and 16 (more bulky) displayed green
max
em
fluorescence (l
488–512 nm). Most of the materials exhibited good thermal stability, exhibiting an amorphous glassy state after melting.
Transparent amorphous films were easily obtained through spin coating and characterized by AFM analysis.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
despite possessing a small chromophoric system, they pres-
ent strong fluorescence even in solid state.9 In this context,
our research is focused on fluorescent molecular materials
containing electron deficient N-heterocycles that can exhibit
either liquid crystalline phases10 or a stable glassy state for
opto-electronic applications.
Organic compounds that exhibit a variety of interesting
optical, electrical, photoelectrical, and magnetic properties
in the solid state have found widespread use in a growing
number of practical applications, including their utility as
emitters in light-emitting diodes,1 laser dyes,2 photoconduc-
tors,3 optical switches,4 and optical data storage devices.5
Low molecular mass materials that are able to form stable
films are particularly interesting in such functional systems
due to their well-defined molecular structures and molecular
weights, and also because they can be easily purified.
Among these materials and their organization states, two
classes in particular have been extensively exploited: fluo-
rescent liquid crystals,6 as the self organizing properties of
these materials can lead to defect-free layers with high car-
rier mobility for electroluminescence devices, and various
types of amorphous molecular glass, because of their good
processability, transparency, and homogeneity.7
In this paper, we report on the synthesis of a series of fluores-
cent 2,3-dicyanopyrazine derivatives (11–16) possessing
a similar V-shaped geometry11 with some lack of planarity,
in order to achieve a mesomorphic or film forming behavior.
The photophysical properties in solution and in thin film,
along with the thermal behavior of these compounds, were
also evaluated.
2. Results and discussion
2.1. Synthesis
Dicyanopyrazine derivatives have been synthesized and are
valuable in a broad range of chemistry fields due to their
applications as functional dyes, nonlinear optical materials,
etc.8 They have strong electron withdrawing ability and,
Scheme 1 outlines the synthesis of the series of 2,3-dicyano-
pyrazine derivatives. Anisil 7 was prepared from 4-methoxy-
benzaldehyde through benzoin condensation followed by
hydroxyl group oxidation using CuSO4/pyridine. A con-
densation reaction of anisil 7 with diaminomaleonitrile
(DAMN) in the presence of a catalytic amount of p-toluene-
sulfonic acid (p-TsOH) gave the 2,3-dicyanopyrazine deriv-
ative 8 in good yield (90%). An attempt to achieve methyl
Keywords: 2,3-Dicyanopyrazine; Luminescent materials; X-ray structure;
Functional dyes.
* Corresponding author. Fax: +55 48 3721 6850; e-mail: hugo@qmc.ufsc.br
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.01.045

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R. Cristiano et al. / Tetrahedron 63 (2007) 2851–2858
O
O
H
a, b
H3CO
H3CO
OCH3
O
7
c
d
NC
N
CN
N
O
HO
OH
O
9
8
H3CO
OCH3
c
NC
N
CN
N
HO
e
OH
f
10
1
,
2, 3
4
,
5
,
6
O
O
OC10H21
OC12H25
OC12H25
OC12H25
R1 = OC10H21
R2 = H
R1
=
NC
N
CN
N
11
12
14
R2 = H
O
R
1 = R2 = OC12H25
R1
=
O
15
16
OC10H21
R1
=
R2 = H
13
R2 = H
O
O
O
O
R1 = R2 =
O
OC12H25
R2
R2
11-16
R1 R2
R2 R1
Scheme 1. Synthetic route for final compounds 11–16. Conditions (yield): (a) KCN, EtOH/H2O, reflux (38%); (b) CuSO4, pyridine/H2O, 60 ꢀC (93%);
(c) diaminomaleonitrile, TsOHcat., MeOH, reflux (90%); (d) HBr(aq), AcOH, reflux (69%); (e) (i) respective carboxylic acid (1, 2, or 3), SOCl2, CH2Cl2, reflux
(100%); (ii) TEA, CH2Cl2, rt (70–85%); (f) respective carboxylic acid (4, 5, or 6), DCC, DMAPcat., CH2Cl2, rt (70–85%).
group elimination was firstly carried out using BBr3, how-
ever, this gave poor yields of compound 10. Thus, another
approach was performed, in which anisil 7 was demethyl-
ated by refluxing it in a 1:1 HBr(aq)/acetic acid mixture.
This procedure was found to be better, giving diphenol 9,
which was reacted with DAMN and p-TsOHcat. affording
compound 10. The final compounds 11–16 were synthesized
through esterification of diphenol 10 with different, previ-
ously prepared, aromatic carboxylic acids 1–6 shown in
Figure 1. Two approaches were used: for carboxylic acids
1–3 the reaction was carried out using the freshly prepared
acid chloride in the presence of triethylamine in dichloro-
methane; and for carboxylic acids 4–6, to avoid ester or
benzyl ether cleavage of these acids by thionyl chloride, the
esterification was performed with dicyclohexylcarbodiimide
(DCC) in the presence of catalytic amount of dimethyl-
aminopyridine (DMAP).
of the solid state nature of the final molecules, since these
materials have the same central core. Slightly yellow crystals
of 8 suitable for X-ray analysis were obtained by recrystalli-
zation from methanol. The crystal structure and selected
bond lengths and angles of 8 are presented in Figure 2.
O
O
H21C10
O
H21C10
O
OH
OH
1
3
H25C12
H25C12
H25C12
O
O
O
O
O
O
H21C10
O
OH
O
OH
2
4
H25C12
H25C12
O
O
O
The structures of all the compounds were characterized
O
O
H25C12
O
1
by IR, H, and 13C NMR spectra and elemental analysis,
including X-ray diffraction analysis of compound 8.
OH
H25C12
O
5
O
O
O
H25C12
O
2.2. X-ray diffraction analysis of the central core
O
OH
Considering the enormous difficulty in crystallizing the final
compounds 11–16, the well-crystalline intermediate 2,3-di-
cyanopyrazine derivative 8 was crystallized and analyzed
by single crystal X-ray diffraction for a better understanding
H25C12
O
6
Figure 1. Carboxylic acid intermediates 1–6 synthesized.

R. Cristiano et al. / Tetrahedron 63 (2007) 2851–2858
2853
pyrazine and 4-methoxyphenyl rings due to the C1–C11
˚
and C2–C21 bond distances (1.475(3) and 1.480(3) A,
respectively), which are a little shorter than typical C–C s
˚
bonds (1.53 A), and this is in agreement with a similar struc-
ture published in the literature.12 Intermolecular packing of
this molecule is based on van der Waals forces and the pres-
ence of an intermolecular p,p-stacking between the rings
C21–C26 and C1–N6. The centroid–centroid distance
˚
between them is 3.684 A.
2.3. Absorption and fluorescence properties
The UV–vis and fluorescence spectroscopic data in chloro-
form solution and in thin film for the 2,3-dicyanopyrazine
derivatives are summarized in Table 1. In dilute solution,
all the compounds exhibit two maxima absorption wave-
lengths peaking at around 270–310 and 350–380 nm,
respectively. The highest and lowest energy absorption
bands are assigned to p–p* transitions due to their high mo-
Figure 2. Molecular structure of compound 8 with atom labeling scheme.
Selected bond distances and angles: C(1)–N(6) 1.341(2), C(1)–C(2)
1.434(3), C(1)–C(11) 1.475(3), C(2)–N(3) 1.335(3), C(2)–C(21) 1.480(3),
N(3)–C(4) 1.345(3), C(4)–C(5) 1.386(3), C(5)–N(6) 1.341(3), C(1)–C(2)–
N(3) 120.76(19), C(2)–N(3)–C(4) 117.66(18), N(3)–C(4)–C(5) 121.6(2),
C(4)–C(5)–N(6) 121.45(19), C(5)–N(6)–C(1) 117.99(18), N(6)–C(1)–
C(2) 120.1(2).
lar absorption coefficients. These compounds display blue
max
em
emission in solution (l
440–480 nm) with quantum fluo-
rescence yields (FF) from 0.003 to 0.1 and Stokes shifts of
around 90 nm. The luminescence of these compounds may
be attributed to p–p* fluorescence of the 2,3-dicyanopyr-
azine ring.
As shown in Figure 2, the two 4-methoxyphenyl groups con-
nected to positions 5 and 6 of 2,3-dicyanopyrazine are
twisted with respect to each other and they are not coplanar
with the central pyrazine ring. The planes of both 4-methoxy-
phenyl rings are rotated with respect to the mean plane of
the pyrazine ring by 40.39(6)ꢀ and 48.36(5)ꢀ, and with re-
spect to each other by 48.95(6)ꢀ. The nonplanarity of this
compound is clearly caused by the steric hindrance between
C16–H and C22–H. Despite the lack of planarity, some lim-
ited forms of p-conjugation may be occurring between the
Figure 3 shows the UV–vis absorption and fluorescence
spectra in solution of the intermediate compounds 8 and
10. The presence of free hydroxyl groups in compound 10
led to a hypsochromic shift of around 10 nm in the spectra
and also led to a lower quantum yield. The final compounds
11–16 exhibit similar absorption and fluorescence patterns
in the curves (Fig. 4). Compound 13 showed a bathochromic
shift of around 80 nm in the highest energy absorption band
due to its additional phenyl conjugated ring, but this had no
effect on the Stokes shift.
Table 1. Optical absorption and emission spectroscopic data for 2,3-dicyanopyrazine derivatives
lambasx/nma (3/104)b
l
/nma,c
Stokes shift/nma
99
FF
lmabasx/nmf
lmemax/nmf,c
Stokes shift/nmf
107
Eogpt/eV
d
max
em
Compd
8
383 (2.4)
314 (1.9)
482
470
444
437
445
438
438
440
0.101
0.053
0.008
0.003
0.010
0.007
0.007
0.005
385
317
492
2.99
g
—
10
11
12
13
14
15
372 (2.3)
305 (2.0)
98
94
89
86
91
91
92
388
320
—
2.92
3.14
3.30
3.23
3.25
3.32
3.28
350 (2.1)
268 (4.6)
372
255
430
512
456
432
498
488
58
348 (1.9)
283 (3.2)
359e (2.5)
309 (5.9)
342
174
89
367e
305
347 (2.0)
276 (7.2)
353
273
79
347 (2.5)
282 (6.1)
347
282
151
140
16
348 (2.4)
281 (5.1)
348
280
2,3-Dicyanopyrazine derivatives of 10ꢁ5 M in chloroform.
.
Excitation wavelength was at lowest energy maxima of absorption.
a
Units¼molꢁ1 cmꢁ1
b
c
d
e
f
Quantum yield of fluorescence using quinine sulfate in 1 M H2SO4 as standard (FF¼0.546).
Shoulder peak.
In solid thin film.
Diphenol 10 is not fluorescent in solid state.
g

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R. Cristiano et al. / Tetrahedron 63 (2007) 2851–2858
The solid state optical absorption and emission spectra of
compounds 11–16 are shown in Figure 5. The lowest energy
absorption bands relating to the 2,3-dicyanopyrazine ring,
observed in solution, are also seen in the solid state. The
solid state optical absorption bands are, however, red shifted
for most of the compounds, as in 11, 13, and 14, showing an
increase of 22, 8, and 6 nm, respectively, compared to the
corresponding bands in solution. This may be due to a slight
increase in planarity in the solid state leading to an increase
in the conjugation.13 On the other hand, 12 exhibits a blue
shift of 6 nm in the lowest energy absorption band in the
solid state, and 15 and 16 do not show changes in the spectra.
The optical band gaps (Eg) of all compounds were deter-
mined by their corresponding absorption in thin films, using
the same method applied in the literature14 (Table 1). The
optical band gap varies from 2.92 eV in 10 to 3.32 eV in
15. The Eg wavelengths were obtained from the derivative
of the UV curves, giving the midpoint between the threshold
energy at which the lowest energy absorption band starts
increasing and that at which it starts decreasing (Supplemen-
tary data).
Figure 3. Normalized absorption and fluorescence spectra in chloroform
solution for compounds 8 and 10.
Compounds 11–16 also present fluorescence in the solid
state. Optical absorption and emission of these materials in
the solid state were measured in thin films, obtained by
spin coating from chloroform solution onto a quartz plate.
Thin films of 11, 13, and 14 exhibit blue fluorescence
(Fig. 5b). Of these compounds only 13, containing a biphenyl
Figure 4. Optical absorption (a) and fluorescence (b) spectra of 11–16 in
chloroform solution.
Figure 5. Optical absorption (a) and fluorescence (b) spectra of 11–16 in
solid state.

R. Cristiano et al. / Tetrahedron 63 (2007) 2851–2858
2855
unit, shows a red shift (11 nm), as 11 and 14 show a slight
blue shift (14 and 6 nm, respectively) relative to their emis-
sion in chloroform solution. However, for bulky compounds
containing more alkyl chains, such as 12, 15, and 16, the
fluorescence is strongly red shifted from 50 to 75 nm com-
pared to that observed in solution, with large Stokes shifts
(174, 151, and 140 nm, respectively). These compounds
exhibit a green emission in solid state. A broadening in the
fluorescence spectra is also observed for these compounds,
especially for 15.
The thermal stability was analyzed by TGA. In most cases
the onset of decomposition is in the range of 410–430 ꢀC,
showing a good thermal stability for electro-optic device
applications. Compound 16, containing semi-flexible benzyl
units, shows a lower thermal stability, decomposition start-
ing at 295 ꢀC.
2.5. AFM of thin films
Spin-coated films of 14 and 15 showed quite diverse super-
ficial structures, in topographic images obtained from
atomic force microscopy (AFM) (Fig. 6). Both the films
presented a homogeneous deposition on the glass substrate,
however, the surface of the film of compound 14 had a mor-
phology with grain boundaries typical of crystallized layers,
consistent with a more crystalline structure, also confirmed
previously by DSC measurements. On the other hand, the
2.4. Thermal properties
The thermal behavior of 11–16 was investigated by DSC,
optical microscopy, and thermogravimetry, and the results
are summarized in Table 2. Some of the materials (11, 12,
and 15) exhibit a glassy state on the second heating after
melting, with no or little crystallization on cooling the iso-
tropic liquid. On heating sample 11 there are two crystal–
crystal transitions at 56 and 97 ꢀC, and the material melts
at 114 ꢀC to an isotropic liquid. On cooling, no crystalliza-
tion is observed, even when the material is allowed to stand
for over a week. A second heating gives only a glass transi-
tion at 87 ꢀC. Compound 12 has a similar profile, exhibiting
only a glass transition at a lower temperature (6.2 ꢀC) on
a second heating of the sample.
Compounds 13, 14, and 16 show partial crystallization on
cooling of the isotropic liquid. On cooling, a monotropic
smectic C phase is observed just before the crystallization
of compound 13, detected only by optical microscopic anal-
ysis. This may be taken as evidence for a certain degree
of order in this material, due to the possibility of intermolec-
ular packing by van der Waals forces and intermolec-
ular p,p-stacking, which are observed in the X-ray
analysis of the central core (intermediate 8, see Section
4.3). Compound 14 shows partial crystallization on cooling
of the isotropic liquid, being in a semi-crystalline state. On
a second heating, the sample starts to recrystallize at around
97 ꢀC and then melts at 135 ꢀC. No glass transition temper-
ature was observed. Compound 15 is a wax that does not
have a melting point, exhibiting only a glass transition
temperature at 32 ꢀC.
Table 2. Thermal properties of compounds 11–16
T (DH)a
heating
T (ꢁDH)a
Tg
Tdec
a
b
Compd
Transitions
cooling
11
Cr0–Cr0
Cr –Cr00
Cr00–I
56 (8.13)
97 (13.5)
114 (37.3)
—
—
—
87
416
12
Cr–Cr0
Cr0–I
19 (2.62)
52 (40.1)
—
—
6.2
431
13c
14
Cr–I
171 (38.1)
147 (36.2)
—
—
411
425
SCr–Cr
Cr–Cr0
Cr0–I
97 (ꢁ26.0)
100 (13.4)
112 (16.1)
135 (61.2)
—
15
g–I
—
32
—
434
295
16
Cr–I
93 (16.7)
76 (15.4)
a
Determined by DSC 10 ꢀC/min.
b
c
By TGA, onset of decomposition in nitrogen, 10 ꢀC/min.
Optical microscopy shows monotropic smectic C. Cr¼crystal, SCr¼semi-
Figure 6. AFM images showing the surface morphology of solid films of
14 (a) and 15 (b).
crystalline state, I¼isotropic liquid, g¼glassy state.

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R. Cristiano et al. / Tetrahedron 63 (2007) 2851–2858
topography of the film of compound 15 is similar to that of
amorphous systems. This trend is in agreement with the dif-
ferences observed in their luminescent and thermal proper-
ties. From the AFM images, we obtained the superficial
roughness of the films, which is around 16 nm for 14 and
7 nm for 15, indicating that the film of sample 15 is flatter.
compounds were purified by column chromatography on sil-
ica gel (70–230 mesh), and crystallization from analytical
grade solvents. Carboxylic acids 1, 2, 3, and 4 were prepared
according to published procedures.15–18 40-(3,4,5-Trido-
decyloxybenzoyloxy)benzoic acid 5 was obtained from the
iterative esterification of 3,4,5-tridodecyloxybenzoic acid
2 with benzyl-4-hydroxybenzoate followed by deprotection
of the benzyl ester through a modified version of a procedure
given in the literature.19 Acid 6 was synthesized through
Williamson etherification of methyl 3,4,5-trihydroxy-
benzoate with 4-n-dodecyloxybenzyl bromide, followed by
alkaline ester cleavage according to a method described in
detail elsewhere.20 Anisil 7 was synthesized through benz-
oin condensation of anisaldehyde21 followed by hydroxyl
oxidation using copper(II) sulfate/pyridine.22 Compound
9 was synthesized according to a procedure given in the
literature.22
3. Conclusions
In summary, a series of luminescent V-shaped low molecular
mass materials containing a 2,3-dicyanopyrazine central
core were synthesized and characterized. The photophysical
and thermal properties of these compounds, in solution and
in solid state, were evaluated. These compounds displayed
fluorescence in the solid state in a wide range of the spectra
(from 430 to 512 nm). Most of the materials achieved good
thermal stability, exhibiting an amorphous glassy state after
melting. Consequently, fluorescent transparent amorphous
films can be easily obtained from these compounds by spin
coating.
4.3. X-ray crystallographic analysis
Suitable crystals of 8 were obtained through recrystallization
from methanol followed by filtration of the crystals and
washing with cold methanol. Crystallographic analysis
was carried out at room temperature with a selected pris-
matic yellow crystal, which was mounted on a CAD-4
Enraf–Nonius diffractometer using graphite monochromated
4. Experimental
4.1. General
˚
Mo Ka radiation (l¼0.71073 A). Cell parameters were
determined from 25 centered reflections in the q range of
4.96ꢀ–16.99ꢀ. Intensities of 3130 were collected using the
uꢁ2q scan technique with a q angle ranging from 1.65 to
25.07ꢀ. The structure was solved using direct methods
with SIR-9723 and refined by full-matrix least-squares pro-
cedures on F2 using SHELXL-97.24 H atoms bonded to C
atoms were added at their calculated positions and included
in the structure factor calculations, with C–H distances and
Ueq taken from the default of the refinement program.
Selected crystal data: C20H14N4O2, M¼342.35, monoclinic,
Infrared spectra were recorded on a Perkin–Elmer model
283 spectrometer in KBr discs or films. H NMR spectra
1
were obtained with a Varian Mercury Plus 400-MHz instru-
ment using tetramethylsilane (TMS) as the internal standard.
13C NMR spectra were recorded on a Varian Mercury Plus
100-MHz spectrometer. Elemental analyses were carried
out using a Perkin–Elmer model 2400 instrument. Low-
resolution mass spectra were recorded on a triple quadrupole
mass spectrometer, GCMS-QP5050A Shimadzu. The melt-
ing points, thermal transitions, and mesomorphic textures
were determined using an Olympus BX50 microscope
equipped with a Mettler Toledo FP-82 hot stage and a PM-
30 exposure control unit. DSC measurements were carried
out using Shimadzu equipment with a DSC-50 module. An
HP UV–Vis model 8453 spectrophotometer was used to
record absorption spectra. Fluorescence spectra were re-
corded on a Hitachi-F-4500. The relative quantum yields
of fluorescence (FF) for all compounds are determined
according to Eq. 1:
˚
˚
˚
P21/n, a¼9.498(1) A, b¼7.38(1) A, c¼24.664(2) A, b¼
93.72(1) , V¼1726.0(3) A , Z¼4, Dcalcd¼1.317 Mg/m3,
m¼0.089 mmꢁ1, F(000)¼712, unique 3056 (Rint¼0.0147),
refined parameters¼237, Goof (F2)¼1.032, R1[I>2s(I)]¼
0.0422, wR2 (all data)¼0.1207.
3
ꢀ
˚
Crystallographic data (excluding structure factors) for the
structure reported in this paper have been deposited at the
Cambridge Crystallographic Data Centre under supplemen-
tary publication number CCDC 623033. Copies of the data
can be obtained, free of charge, on application to CCDC,
12 Union Road, Cambridge, CB2 1EZ, UK [fax: +44(0)
1223 336033, e-mail: deposit@ccdc.cam.ac.uk or http://
www.ccdc.cam.ac.uk)].
2
Funk ¼ FstdðIunk=AunkÞðAstd=IstdÞðhstd=hunkÞ
ð1Þ
where Fstd is the fluorescence yield of standard quinine sul-
fate (Fstd¼0.546 in 1 M H2SO4 at 298 K); Iunk and Istd are
the integrated emission intensities of the sample and stan-
dard, respectively; Aunk and Astd are the absorbance of the
sample and standard, respectively, at the desired wavelength
lexc (380 nm) such that absorbances were less than 0.10; and
hunk and hstd are the refractive indexes of the sample and
standard solutions.
4.4. Film preparation and characterization
The films were deposited on quartz plates for the optical
absorption and emission measurements. Before deposition,
the plates were first carefully cleaned by washing with
neutral detergent and then, followed by a sequence of
20-min sonications in acetone, alcohol, and water, and
finally dried in an oven. The studied compounds were
dissolved in chloroform, at 2% (wt), and then deposited
by spin coating at 4000 rpm for 30 s, at room temperature
(24 ꢀC).
4.2. Materials
All the reagents were obtained from commercial sources
and used without further purification. In general, all the

R. Cristiano et al. / Tetrahedron 63 (2007) 2851–2858
2857
The quality of solid films obtained from samples 14 and 15
was observed with an atomic force microscope (AFM),
using MI (Molecular Imaging Model IC 301) in contact
mode, at a 2.60 Hz scanning rate and 256ꢂ256 lines.
signal, Ar–H), 7.67 (d, J¼8.8 Hz, 4H, Ar–H), 8.12 (d,
J¼8.8 Hz, 4H, Ar–H). 13C NMR (CDCl3) d ppm: 14.1,
22.6, 25.9, 29.1, 29.3, 29.4, 29.6, 31.9, 68.4, 113.1, 114.4,
120.8, 122.5, 129.7, 131.2, 132.3, 132.4, 153.6, 154.3, 163.9,
164.3. Elemental analysis for C52H58N4O6, calcd: C, 74.79;
H, 7.00; N, 6.71. Found: C, 74.33; H, 7.04; N, 6.60%.
4.5. Synthesis of 2,3-dicyanopyrazine ring
A mixture of compound 7 or 9 (10 mmol), diaminomaleo-
nitrile (1.3 g, 12 mmol), and catalytic amount of p-toluene-
sulfonic acid in methanol (20 mL) was heated under reflux
overnight. After cooling, the precipitate was filtered and
washed with cold methanol to give the crude product as a
yellow powder.
4.6.2. 2,3-Dicyano-5,6-bis-4-(3,4,5-tridodecyloxybenzoyl-
oxy)phenyl pyrazine (12). The crude product was purified
by column chromatography (eluant dichloromethane) to
give a yellow wax. Yield: 67%; mp 52.5–55.4 ꢀC. IR (film)
nmax cmꢁ1: 2920, 2854, 1724 (C]O), 1591, 1504, 1433,
1
1331, 1179, 1111, 945. H NMR (CDCl3) d ppm: 0.87 (m,
18H, –CH3), 1.25–1.48 (br, 108H, –CH2–), 1.81–1.85 (m,
12H, –CH2CH2O–), 4.02–4.08 (m, 12H, –CH2O–), 7.27
(d, J¼8.4 Hz, 4H overlapped with CDCl3 signal, Ar–H),
7.38 (s, 4H, Ar–H), 7.68 (d, J¼8.4 Hz, 4H, Ar–H). 13C
NMR (CDCl3) d ppm: 14.4, 22.9, 26.3, 29.5, 29.6, 29.9,
30.6, 32.2, 69.5, 73.9, 108.8, 113.3, 122.8, 123.4, 129.9,
131.5, 132.7, 143.5, 153.2, 153.7, 154.5, 164.7. Elemental
analysis for C104H162N4O10, calcd: C, 76.71; H, 10.03; N,
3.44. Found: C, 76.24; H, 10.35; N, 3.22%.
4.5.1. 2,3-Dicyano-5,6-bis(4-methoxyphenyl)pyrazine
(8). The solid was recrystallized from acetonitrile. Yield:
85%; mp 190.2–190.6 ꢀC. IR (KBr pellet) nmax cmꢁ1
2967, 2838, 2233 (C^N), 1603, 1504, 1376, 1260, 1175,
1024, 840. H NMR (CDCl3) d ppm: 3.85 (s, 6H, CH3O),
:
1
6.87 (d, J¼6.8 Hz, 4H, Ar–H), 7.54 (d, J¼6.8 Hz, 4H,
Ar–H). 13C NMR (CDCl3) d ppm: 55.7, 113.7, 114.5,
127.9, 128.9, 131.7, 154.5, 162.2. Elemental analysis for
C20H14N4O2, calcd: C, 70.17; H, 4.12; N, 16.37. Found: C,
69.89; H, 4.15; N, 16.18%.
4.6.3. 2,3-Dicyano-5,6-bis-4-(4-decyloxy-4-phenylben-
zoyloxy)phenyl pyrazine (13). The crude product was puri-
fied by heating in acetonitrile (40 mL) and the solid filtered
off as a light yellow powder. Yield: 70%; mp 170.0–
172.5 ꢀC. IR (film) nmax cmꢁ1: 2921, 2851, 1734 (C]O),
1600, 1501, 1377, 1266, 1186, 1063, 825. 1H NMR (CDCl3)
d ppm: 0.89 (t, 6H, CH3), 1.28–1.48 (br, 28H, –CH2–), 1.81–
1.85 (m, 4H, –CH2CH2O–), 4.02 (t, 4H, –CH2O–), 7.00 (d,
J¼8.4 Hz, 4H, Ar–H), 7.29 (d, J¼8.4 Hz, 4H, Ar–H), 7.59
(d, J¼8.4 Hz, 4H, Ar–H), 7.70 (d, J¼8.0 Hz, 8H, Ar–H),
8.22 (d, J¼8.4 Hz, 4H, Ar–H). 13C NMR (CDCl3) d ppm:
14.4, 22.9, 26.3, 29.5, 29.6, 29.7, 29.8, 32.1, 68.4, 113.3,
115.2, 122.8, 126.9, 127.0, 128.6, 129.9, 131.0, 131.5,
131.9, 132.6, 146.6, 153.7, 154.5, 159.9, 164.7. Elemental
analysis for C64H66N4O6, calcd: C, 77.86; H, 6.74; N,
5.68. Found: C, 77.48; H, 6.80; N, 5.43%.
4.5.2. 2,3-Dicyano-5,6-bis(4-hydroxyphenyl)pyrazine
(10). The crude product was purified by column chromato-
graphy (eluant hexane/ethyl acetate 1:1). Yield: 78%; mp
185 ꢀC (dec). IR (KBr pellet) nmax cmꢁ1: 3407 (OH), 2250
(C^N), 2213, 1607, 1590, 1497, 1373, 1274, 1173, 1106.
1H NMR (CDCl3) d ppm: 6.86 (d, J¼8.8 Hz, 4H, Ar–H),
7.50 (d, J¼8.8 Hz, 4H, Ar–H), 9.10 (s, 2H, ArO–H). 13C
NMR (CDCl3) d ppm: 114.3, 115.7, 127.6, 129.0, 131.9,
154.8, 160.2. Elemental analysis for C18H10N4O2, calcd:
C, 68.79; H, 3.21; N, 17.83. Found: C, 68.70; H, 3.45; N,
17.49%. MS (EI, 70 eV) m/z (%): [M+] 314 (88%),
[M++1] 315 (19%), [M++2] 316 (2%), [M+ꢁ1] 313 (49%),
297 (28%), 119 (100%).
4.6. Esterification procedure for final compounds 11–13
4.7. Esterification procedure for final compounds 14–16
The corresponding carboxylic acid 1, 2, or 3 (2 mmol) and
thionyl chloride (0.19 mL, 2.6 mmol) in dichloromethane
(20 mL) were heated under reflux for 4 h. The solvent and
excess of thionyl chloride were evaporated under vacuum
affording the acid chloride, which was used without further
purification. To a three-necked round bottomed flask with
argon inlet–outlet containing compound 10 (0.314 g,
1 mmol) dissolved in dichloromethane (30 mL) and triethyl-
amine (5 mL), the respective acid chloride in 5 mL of
dichloromethane was added dropwise. The reaction mixture
was then stirred at room temperature for 20 h. The solvents
were evaporated to give the crude products.
A mixture of 10 (0.314 g, 1 mmol), the corresponding
carboxylic acid 4, 5, or 6 (2 mmol), DCC (0.494 g,
2.4 mmol), and a catalytic amount of DMAP in dichlorome-
thane (40 mL) was stirred at room temperature under argon
atmosphere for 24 h. The resulting precipitate was filtered
off and washed with dichloromethane (50 mL). The solvent
was evaporated and the crude product was purified by col-
umn chromatography (eluant dichloromethane).
4.7.1. 2,3-Dicyano-5,6-bis-4-[4-(4-decyloxybenzoyloxy)-
benzoyloxy]phenyl pyrazine (14). White solid. Yield:
74%, mp 134.3–136.7 ꢀC. IR (film) nmax cmꢁ1: 2924,
2852, 2375 (C^N, weak), 1736 (C]O), 1602, 1508, 1376,
4.6.1. 2,3-Dicyano-5,6-bis-4-(4-decyloxybenzoyloxy)-
phenyl pyrazine (11). The crude product was recrystallized
from acetonitrile as a light yellow powder. Yield: 89%; mp
113.6–115.0 ꢀC. IR (KBr pellet) nmax cmꢁ1: 2918, 2850,
2240 (C^N, weak), 1736 (C]O), 1602, 1507, 1377,
1
1260, 1202, 1160, 1052. H NMR (CDCl3) d ppm: 0.89 (t,
6H, CH3), 1.28–1.48 (br, 28H, –CH2–), 1.79–1.86 (m, 4H,
–CH2CH2O–), 4.05 (t, 4H, –CH2O–), 6.99 (d, J¼8.4 Hz,
4H, Ar–H), 7.32 (d, J¼8.4 Hz, 4H, Ar–H), 7.38 (d,
J¼8.4 Hz, 4H, Ar–H), 7.70 (d, J¼8.4 Hz, 4H, Ar–H), 8.15
(d, J¼8.4 Hz, 4H, Ar–H), 8.27 (d, J¼8.4 Hz, 4H, Ar–H).
13C NMR (CDCl3) d ppm: 14.4, 22.9, 26.2, 29.3, 29.5,
29.6, 29.8, 32.1, 68.6, 113.3, 114.6, 121.0, 122.5, 122.7,
1
1256, 1202, 1159, 1058. H NMR (CDCl3) d ppm: 0.88
(t, 9H, –CH3), 1.28–1.47 (br, 28H, –CH2–), 1.82 (m, 4H,
–CH2CH2O–), 4.04 (t, 4H, –CH2O–), 6.97 (d, J¼8.8 Hz,
4H, Ar–H), 7.28 (d, J¼8.8 Hz, 4H overlapped with CDCl3

2858
R. Cristiano et al. / Tetrahedron 63 (2007) 2851–2858
122.8, 126.4, 130.0, 131.5, 132.1, 132.6, 132.7, 153.5, 154.5,
155.9, 164.1, 164.5. Elemental analysis for C66H66N4O10,
calcd: C, 73.72; H, 6.19; N, 5.21. Found: C, 73.67; H,
6.19; N, 5.09%.
References and notes
1. (a) Shirota, Y. J. Mater. Chem. 2000, 10, 1–25; (b) Yao, Y.-S.;
Xiao, J.; Wang, X.-S.; Deng, Z.-B.; Zhang, B.-W. Adv. Funct.
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A. S. A.; Ebeling, G.; Gonc¸alves, R. S.; Costam, V. E. U.;
Quina, F. H.; Dupont, J. Tetrahedron 2005, 61, 10975–10982;
(d) Han, M.; Lee, S.; Jung, J.; Park, K.-M.; Kwon, S.-K.;
Ko, J.; Lee, P. H.; Kang, Y. Tetrahedron 2006, 62, 9769–9777;
(e) Chan, L.-H.; Lee, Y.-D.; Chen, C.-T. Tetrahedron 2006,
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2. (a) Valverde-Aguilar, G. Opt. Mater. 2006, 28, 1209–1215; (b)
Serwadczak, M.; Kucharski, S. J. Sol.-Gel Sci. Technol. 2006,
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Ed. 2006, 45, 2016–2035.
4.7.2. 2,3-Dicyano-5,6-bis-4-[4-(3,4,5-tridodecyloxyben-
zoyloxy)benzoyloxy]phenyl pyrazine (15). Yellow solid.
Yield: 70%; Tg 32 ꢀC. IR (film) nmax cmꢁ1: 2923, 2853,
2358 (C^N, weak), 1737 (C]O), 1594, 1502, 1462, 1431,
1
1380, 1335, 1261, 1164, 1061. H NMR (CDCl3) d ppm:
0.88 (m, 18H, CH3), 1.26–1.49 (broad, 108H, –CH2–),
1.75–1.86 (m, 12H, CH2CH2O–), 4.05 (t, 12H, –CH2O–),
7.32 (d, J¼8.4 Hz, 4H, Ar–H), 7.37 (d, J¼8.8 Hz, 4H, Ar–
H), 7.41 (s, 4H, Ar–H), 7.70 (d, J¼8.8 Hz, 4H, Ar–H),
8.28 (d, J¼8.4 Hz, 4H, Ar–H). 13C NMR (CDCl3) d ppm:
14.4, 22.9, 26.3, 29.5, 29.6, 29.9, 30.0, 30.6, 32.1, 69.5,
73.9, 108.8, 113.3, 122.5, 122.7, 123.4, 126.6, 130.0, 131.6,
132.2, 132.8, 143.6, 153.3, 153.5, 154.5, 155.8, 164.0, 164.7.
Elemental analysis for C118H170N4O14, calcd: C, 75.84; H,
9.17; N, 3.00. Found: C, 75.78; H, 9.11; N, 3.09%.
6. (a) Cristiano, R.; Ely, F.; Gallardo, H. Liq. Cryst. 2005, 32, 15–
25; (b) Gimenez, R.; Pin˜ol, M.; Serrano, J. L. Chem. Mater.
2004, 16, 1377–1383.
4.7.3. 2,3-Dicyano-5,6-bis-4-[3,4,5-tris-(4-dodecyloxy-
benzyloxy)benzoyloxy]phenyl pyrazine (16). Yellow
7. Shirota, Y. J. Mater. Chem. 2005, 15, 75–93.
solid. Yield: 59%; mp 92.9–96.0 ꢀC. IR (film) nmax cmꢁ1
:
8. (a) Jaung, J. Y.; Lee, B. H. Dyes Pigments 2003, 59, 135–142.
9. Kin, J. H.; Shin, S. R.; Matsuoka, M.; Fukunish, K. Dyes
Pigments 1999, 41, 183–191.
10. (a) Cristiano, R.; Vieira, A. A.; Ely, F.; Gallardo, H. Liq. Cryst.
2006, 33, 381–390; (b) Cristiano, R.; Santos, D. M. P. O.;
Gallardo, H. Liq. Cryst. 2005, 32, 7–14.
11. Yelamaggad, C. V.; Shashikala, I.; Rao, D. S. S.; Prasad, S. K.
Liq. Cryst. 2004, 31, 1027–1036.
12. Donzello, M. P.; Ou, Z.; Monacelli, F.; Ricciardi, G.;Rizzoli, C.;
Ercolani, C.; Kadish, K. M. Inorg. Chem. 2004, 43, 8626–8636.
13. Tonzola, C. J.; Alam, M. M.; Kaminsky, W.; Jenekhe, S. A.
J. Am. Chem. Soc. 2003, 125, 13548–13558.
2923, 2856, 1721 (C]O), 1593, 1508, 1464, 1428, 1380,
1244, 1183, 1111. 1H NMR (CDCl3) d ppm: 0.88 (m,
18H, CH3), 1.26–1.44 (br, 108H, –CH2–), 1.78 (br, 12H,
CH2CH2O–), 3.92–3.96 (m, 12H, –CH2O–), 5.04 and 5.07
(2 s, 12H, ArCH2O–), 6.76 (d, J¼8.4 Hz, 4H, Ar–H), 6.89
(d, J¼8.4 Hz, 8H, Ar–H), 7.25–7.28 (m, 8H overlapped
with CDCl3 signal, Ar–H), 7.33 (d, J¼8.4 Hz, 8H, Ar–H),
7.49 (s, 4H, Ar–H), 7.68 (d, J¼8.4 Hz, 4H, Ar–H). 13C
NMR (CDCl3) d ppm: 14.4, 22.9, 26.3, 29.6, 29.7, 29.9,
32.1, 68.3, 71.4, 75.0, 110.0, 113.3, 114.4, 114.7, 114.9,
122.8, 123.7, 128.5, 129.5, 130.2, 130.5, 131.5, 132.7,
143.7, 153.0, 153.6, 154.5, 159.3, 164.5. Elemental analysis
for C146H198N4O16, calcd: C, 77.41; H, 8.81; N, 2.47. Found:
C, 76.93; H, 9.05; N, 2.16%.
14. Joshi, A.; Manasreh, M. O.; Davis, E. A.; Weaver, B. D. Appl.
Phys. Lett. 2006, 89 111907-1–111907-3.
15. Merlo, A. A.; Braun, J. E.; Vasconcelos, U. B.; Ely, F.;
Gallardo, H. Liq. Cryst. 2000, 27, 657–660.
16. Hersmis, M. C.; Spiering, A. J. H.; Waterval, R. J. M.;
Meuldijk, J.; Vekemans, J. A. J. M.; Hulshof, H. A. Org.
Process Res. Dev. 2001, 5, 54–60.
17. Ely, F.; Conte, G.; Merlo, A. A.; Gallardo, H. Liq. Cryst. 2004,
31, 1413–1425.
Acknowledgements
The authors thank Dr. Frank Quina from Universidade de
~
Sao Paulo (USP) and Dr. Faruk Nome from Universidade
Federal de Santa Catarina (UFSC) for the free access to
ꢀ
the photophysical equipment, the Laboratorio de Sistemas
Nanoestruturados (LabSiN) for the film deposition, and to
18. Murthy, H. N. S.; Sadashiva, B. K. Liq. Cryst. 2004, 31,
1347–1356.
19. Lehmann, M.;Gearba, R. I.;Koch, M. H. J.;Ivanov, D. A. Chem.
Mater. 2004, 16, 374–376. With modifications: DMAP was
used instead of DMPS. For benzyl deprotection 1,4-dioxane
was used as the solvent and the reaction temperature was 40 ꢀC.
20. Percec, V.; Schlueter, D.; Kwon, Y. K.; Blackwell, J.; Moller,
M.; Slangen, P. J. Macromolecules 1995, 28, 8807–8818.
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39–41.
ꢀ
the Laboratorio de Filmes Finos e Superfıcies (LFFS) for
the AFM facilities, both located at Departamento de Fısica,
ꢀ
UFSC. The authors thank Dr. Andre A. Pasa for helpful
discussions. This work was supported by Conselho Nacional
ꢀ
ꢀ
ꢀ
ꢀ
de Desenvolvimento Cientıfico e Tecnologico (CNPq,
Brazil), FAPESC (Brazil), FINEP (Brazil), and Merck
(Germany).
22. Gilman, H.; Broadbent, H. S. J. Am. Chem. Soc. 1948, 70,
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Supplementary data
23. Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori,
G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115–119.
24. Sheldrick, G. M. SHELXL-97, Program for Crystal Structures
The complete set of the 1H and 13C NMR, UV–vis and fluo-
rescence spectra in solid phase, as well as DSC and TGA
curves of the new compounds are available. Supplementary
data associated with this article can be found in the online
version, at doi:10.1016/j.tet.2007.01.045.
€
Analysis; University of Gottingen: Gottingen, Germany,
1997.