2,5-Di(aryleneethynyl)pyrazine derivatives: Synthesis, structural and optoelectronic properties, and light-emitting device

Article abstract of DOI:10.1039/b401867m

A series of 2,5-di(aryleneethynyl)pyrazine derivatives has been synthesised in 23-41% yields by two-fold reaction of 2,5-dibromo-3,6-dimethylpyrazine 3 with ethynylarenes (arene = phenyl, 2-pyridyl, 4-ethylphenyl, 4-chlorophenyl, 4-biphenyl) under standard Sonogashira conditions [CuI, Pd(PPh₃) ₂CI₂, NEt₃, THF]. Compound 3 has been converted into 2,5-diethynyl-3,6-dimethylpyrazine, which reacts with 2-iodothiophene to yield 2,5-bis(thien-2-ylethynyl)-3,6-dimethylpyrazine. In the X-ray crystal structure of 2,5-di(phenylethynyl)-3,6-dimethylpyrazine 4 the two phenyl rings are parallel and the pyrazine ring is inclined to their planes by 14.2°. Quantum chemical calculations establish that the HOMO-LUMO gap for 4 (3.56 eV) is lower than that of di(phenylethynyl)benzene 12 (3.72 eV). The nitrogen atoms of 4 serve to localise the HOMO on the central ring's carbon atoms, resulting in a quinoidal-type population, in contrast to 12. Cyclic voltammetric studies establish that 4 undergoes a reduction to the radical anion at ca. -1.9 V (vs. Ag/Ag⁺ in MeCN), which is almost reversible at high scan rates (500 mV s^-1). The UV-vis absorption and photoluminescence profiles of 4 in cyclohexane are similar to those of 12; the emission for 4 (λ _max 379 and 395 nm) is red-shifted compared to 12. Single-layer OLEDs using MEH-PPV as the emissive polymer show significantly enhanced external quantum efficiencies (up to 0.07%) when 20% by weight of 2,5-di(biphenyl-4- ethynyl)-3,6-dimethylpyrazine 8 is added as a dopant: this is ascribed to the enhanced electron-transporting properties of the pyrazine system.

Full text of DOI:10.1039/b401867m

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P A P E R
2,5-Di(aryleneethynyl)pyrazine derivatives: synthesis, structural and
optoelectronic properties, and light-emitting devicey
a
Liang Zhao,a Igor F. Perepichka,z Figen Tu¨rksoy,ab Andrei S. Batsanov,a Andrew Beeby,a
Karen S. Findlaya and Martin R. Bryce*a
a
Department of Chemistry, University of Durham, Durham, UK DH1 3LE.
E-mail: m.r.bryce@durham.ac.uk; Fax: þ44 191 334 2018; Tel: þ44 191 384 4737
b
¨
MCTRI Polymer and Synthesis Technologies, TUBITAK-Marmara Research Centre,
P.B. 21, 41470, Gebze, Kocaeli, Turkey
Received (in Montpellier, France) 5th February 2004, Accepted 18th March 2004
First published as an Advance Article on the web 6th July 2004
A series of 2,5-di(aryleneethynyl)pyrazine derivatives has been synthesised in 23–41% yields by two-fold
reaction of 2,5-dibromo-3,6-dimethylpyrazine 3 with ethynylarenes (arene ¼ phenyl, 2-pyridyl, 4-ethylphenyl,
4-chlorophenyl, 4-biphenyl) under standard Sonogashira conditions [CuI, Pd(PPh3)2Cl2 , NEt3 , THF].
Compound 3 has been converted into 2,5-diethynyl-3,6-dimethylpyrazine, which reacts with 2-iodothiophene to
yield 2,5-bis(thien-2-ylethynyl)-3,6-dimethylpyrazine. In the X-ray crystal structure of 2,5-di(phenylethynyl)-
3,6-dimethylpyrazine 4 the two phenyl rings are parallel and the pyrazine ring is inclined to their planes by
14.2ꢀ. Quantum chemical calculations establish that the HOMO–LUMO gap for 4 (3.56 eV) is lower than that
of di(phenylethynyl)benzene 12 (3.72 eV). The nitrogen atoms of 4 serve to localise the HOMO on the central
ring’s carbon atoms, resulting in a quinoidal-type population, in contrast to 12. Cyclic voltammetric studies
establish that 4 undergoes a reduction to the radical anion at ca. ꢁ1.9 V (vs. Ag/Agþ in MeCN), which is
almost reversible at high scan rates (500 mV sꢁ1). The UV-vis absorption and photoluminescence profiles of 4 in
cyclohexane are similar to those of 12; the emission for 4 (lmax 379 and 395 nm) is red-shifted compared to 12.
Single-layer OLEDs using MEH-PPV as the emissive polymer show significantly enhanced external quantum
efficiencies (up to 0.07%) when 20% by weight of 2,5-di(biphenyl-4-ethynyl)-3,6-dimethylpyrazine 8 is added as
a dopant: this is ascribed to the enhanced electron-transporting properties of the pyrazine system.
Herein we describe our work on the synthesis, structure
Introduction
determination and optoelectronic properties of novel 2,5-di-
(aryleneethynyl)pyrazines. We targeted these molecules because
there is considerable current interest in the structural, opto-
electronic and photophysical properties of highly conjugated
oligoaryleneethynylenes.11 For example, 1,4-bis(phenylethynyl)-
benzene has been thoroughly studied12 as a model for poly-
(phenyleneethynylene) derivatives that are known emitters in
There is widespread interest in the synthesis and optoelectronic
properties of new conjugated oligoarylenes containing elec-
tron-deficient heterocycles in the backbone or as pendant
groups.1 1,3,4-Oxadiazole derivatives have been prominent in
this respect2 and it has also been established that pyridine,3
pyrimidine,4 quinoline5 and 1,3,5-triazine6 units improve elec-
tron transport characteristics compared to their phenylene
analogues. This has led to their use as new materials with tai-
lored photoluminescence and electroluminescence properties,
for example as the emitting layer or as an electron-transport,
hole-blocking (ETHB) layer in organic light emitting devices
(OLEDs).7 Pyrazine derivatives have been somewhat neglected
in this context, although the symmetry of 2,5-disubstituted
pyrazines has obvious attractions. Orange-red luminescence
has been reported from thin films of polyimides containing dis-
tyrylypyrazine units in the backbone.8 Electron injection is sig-
nificantly improved in OLEDs made with a PPV–copolymer
(PPV ¼ p-phenylenevinylene) that contains pyrazine units in
the backbone,9 and we have shown that p-phenylene-2,5-
dimethylpyrazine co-oligomers are efficient blue emitters in
photoluminescence and electroluminescence studies.10
OLEDs.13
Oligo(9,9-dihexyl-2,7-fluoreneethynylene)s
have
also been used as the emitting layer in blue OLEDs.14 Hitherto
2,5-diethynylpyrazine units have not been studied.
Results and discussion
Synthesis
Although 2,5-dibromopyrazine is known,15 in our hands
the literature synthesis (namely, a Sandmeyer reaction on
2-amino-5-bromopyrazine)15a did not routinely provide
sufficient material for further reactions. We, therefore, used
2,5-dibromo-3,6-dimethylpyrazine 3 as our starting reagent.
An outline synthesis has been reported for this compound,
although details are lacking.16 The reaction of alanine anhy-
dride 1 with a mixture of phosphoryl chloride and phosphorus
pentachloride at 105 ꢀC is reported to give 2,5-dichloro-3,6-
dimethylpyrazine 2 in 12% yield.17 With minor modifications
to this procedure we obtained 2 in 31% yield. Conversion of
compound 2 into the dibromo analogue 3 using phosphorus
tribromide at 170 ꢀC proceeded in 35% yield. In a typical reac-
tion we obtained ca. 3 g of analytically pure compound 3, and
this appears to be the optimum scale for this reaction: larger-
scale reactions resulted in incomplete conversion to 3.
y Electronic supplementary information (ESI) available: tables of the
optimised geometry atom coordinates; orbital contour plots for the
HOMOs and LUMOs of 4 and 12; spectra of 6. See http://
www.rsc.org/suppdata/nj/b4/b401867m/
z On leave from the L. M. Litvinenko Institute of Physical Organic
and Coal Chemistry, National Academy of Sciences of Ukraine, R.
Luxemburg Street 70, Donetsk 83 114 Ukraine
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
912
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 9 1 2 – 9 1 8

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Two-fold Sonogashira reactions18 on 3 using the appropriate
ethynylarene derivative under standard conditions [copper
iodide, triethylamine, Pd(PPh3)2Cl2 in refluxing THF] gave
products 4–8 in 23–41% yields (Scheme 1). The analogous
reaction of 3 with 2-methyl-3-butyn-2-ol gave product 9, which
was deprotected under standard basic conditions19 with loss of
acetone to afford 10 (76% yield for the two steps from 3). As an
alternative route to 2,5-di(aryleneethynyl)pyrazine derivatives,
we explored the Sonogashira reactions of compound 10 with
iodobenzene and 2-iodothiophene. The reactions were not
clean and products 4 and 11 were isolated in ca. 20% yields.
(2-Bromothiophene gave 11 in only 10% yield). It is preferable,
therefore, to use compound 3 rather than compound 10 as the
pyrazine reagent in these syntheses.
methods at the B3LYP/6-31G(d) level of theory, and for com-
pound 4 also at the B3LYP/6-311G(2d,p) level (for optimised
geometries see Tables S1–S9 in the Electronic supplementary
information). For reference the known phenylene analogue
1212 (Chart 1) was calculated at the same levels and the results
of B3LYP/6-311G(2d,p) calculations for 4 and 12 were com-
pared with X-ray diffraction single crystal structures for these
compounds.20 The calculated structures of both 4 and 12 are
essentially flat and all three aromatic rings lie in the plane.
There is generally a good agreement between the calculated
and the X-ray experimental bond distances: mean discrepan-
cies are 0.006 and 0.007 A, respectively (Fig. 2). The calculated
˚
triple bond lengths are somewhat longer (by 0.003–0.008 A)
and the single bonds (between the aromatic rings and sp-
hybrid carbon atoms) are somewhat shorter (by 0.008–0.012
˚
A) than the experimental data, but these are expected differ-
˚
X-Ray crystal structure of 4
ences because the calculated values are inter-nuclear distances
whereas X-ray gives interatomic distances based on mean elec-
The molecule of 4 (Fig. 1) is located at a crystallographic inver-
sion centre. Thus, the two phenyl rings are parallel with the
pyrazine ring inclined to their planes by 14.2ꢀ. Molecules in
the crystal form a layer of a brickwork pattern (Fig. 1) parallel
to the (0 0 1) plane. The long axes of all molecules belonging to
the same layer are strictly parallel, while those in the adjacent
layers are inclined to the latter by ca. 45ꢀ. Packing within the
layer is not particularly close due to the non-planar conforma-
tion of the molecule. The separation between the mean planes
=
tronic density, which for the fragment C– is shifted towards
the triple bond.
=
Orbital energy level analysis for compounds 4–8, 11 and 12
at the B3LYP/6-31G(d) level is summarised in Table 1, indi-
cating that all pyrazine derivatives are stronger electron
acceptors than compound 12 (their LUMO energies are lower
than that in 12 by 0.26–0.60 eV). Although their HOMO
energy levels are higher than that in 12, in total all pyrazine
derivatives show a narrower HOMO–LUMO gap as com-
pared to 12.
Calculations performed for compounds 4 and 12 at the
higher B3LYP/6-311G(2d,p) level for more precise estimation
of orbital energies showed ca. 0.21–0.26 eV lower energies as
compared to B3LYP/6-31G(d) calculations, although the gen-
eral trend remains the same and HOMO–LUMO gap energies
calculated by both methods are very close (3.53 and 3.56 eV
for 4, 3.70 and 3.72 eV for 12, respectively). For pyrazine
˚
of contacting molecules (3.49 A) and the shortest intramolecu-
˚
lar Cꢂ ꢂ ꢂC distances (3.46–3.48 A) correspond to normal Van
der Waals’ contacts. There is partial overlap between the phe-
nyl ring of one molecule and the pyrazine ring of another.
Quantum chemical calculations
We performed quantum mechanical calculations of the geome-
try and electronic structure of compounds 4–8 and 11 by DFT
Scheme 1 Synthesis of 2,5-(diaryleneethynyꢀl)pyrazines. Reagents and conditions: (i) POCl3/PCl5 , 105 ꢀC, 2ꢀ4 h; (ii) PBr3 , 170 ꢀC, 24 h; (iii)
ꢀ
=
=
=
ArC CH, CuI, Et N, Pd(PPh ) Cl , THF, 20 C/1 h, 65 C/1.5 h; (iv) Me C(OH)–C CH, CuI, Et N, THF, 20 C, 1 h, reflux; (v) NaOH, toluene,
=
3
3 2
2
2
3
reflux; (vi) 2-iodothiophene, CuI, Et3N, Pd(PPh3)2Cl2 , THF, 20 ꢀC/1 h, 65 ꢀC/1.5 h.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 9 1 2 – 9 1 8
913

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Fig. 1 (top) Molecular structure of 4. Thermal ellipsoids are drawn
at 50% probability level. Atoms generated by the inversion centre
are primed. (bottom) Layer of molecules in the crystal structure;
projection on the (0 0 1) plane.
derivative 4 (as compared to 12) the energies of both HOMO
and LUMO orbitals decrease (by 0.17 and 0.33 eV, respec-
tively) with, as expected, more pronounced changes in the
LUMO energy due to the electronegative nitrogen atoms
(Fig. 3). As a result the HOMO–LUMO gap contracts by
0.16 eV, from 3.72 eV for 12 to 3.56 eV for 4. For compound
12 the HOMO is located on the C–C triple bonds as well as on
the central 1,4-phenylene ring, with only minor population on
the outer phenyl moieties. The main population at the central
ring is on C–1 and C–4, which form bonding orbitals with
adjacent carbons. In 4, the HOMO is also located on the triple
C–C bonds and on the central ring. The electronegative nitro-
gen atoms in 4 result in localisation of the HOMO on the cen-
tral ring’s carbon atoms, giving a quinoidal-type population, in
contrast to 12. The LUMO in 12 populates the exocyclic C–C
single bond and C–2,3, C–5,6 bonds of the central phenylene
ring, resulting in a quinoidal structure. Similar localisation of
the LUMO on the exocyclic C–C single bonds is observed
for 4, whereas for the pyrazine ring the LUMO is localised
on the nitrogen atoms (Fig. 4).
The population of the unoccupied molecular orbitals
LUMO þ 1 to LUMO þ 4 is similar for both molecules 4
and 12 (Fig. S1 in ESI): LUMO þ 1 is shifted to the outer
phenyl rings, which become essentially quinoidal; LUMO þ 2
is localised on the central ring, forming antibonding orbitals
between C2–C3, C5–C6 and N1–C6, C3–N4 for 12 and 4,
respectively. Structures LUMO þ 3 and LUMO þ 4 for both
compounds are nearly the same, with population on the ter-
minal phenyl rings. For occupied HOMO ꢁ 1 to HOMO ꢁ 4
orbitals, the structures of HOMO ꢁ 1 and HOMO ꢁ 3 orbitals
for both compounds are quite similar, whereas a large differ-
ence is observed for HOMO ꢁ 2, which is quinoidally localised
on the end phenyls in 12 and on the central pyrazine ring in 4,
involving participation of methyl groups and the nitrogen lone
pairs.
Fig. 2 B3LYP/6-311G(2d,p) calculated bond distances in phenylene
12 and dimethylpyrazine 4 derivatives. Bond distances from X-ray ana-
lysis are given in parentheses for comparison (distances for the two
independent molecules from the X-ray crystal structure of phenylene
derivative 12 are given in parentheses and square brackets, respec-
tively). The structures at the top show bond distances in the rings
and while those at the bottom show acyclic bond distances.
Solution electrochemical properties
The redox properties of compound 4, studied by cyclic voltam-
metry (CV) in dry deoxygenated acetonitrile solution, show an
irreversible oxidation to the radical cation at þ1.62 V vs. Ag/
Agþ (Fig. 5). Reduction of 4 to its radical anion occurs at quite
high negative potentials of ca. ꢁ1.87 V (vs. Ag/Agþ). At slow
scan rates (20–50 mV sꢁ1) the reduction is an irreversible pro-
cess (not shown on Fig. 5). On increasing the scan rate to 100
mV sꢁ1 it becomes partly reversible (ic ꢃ ia) and, remarkably,
at a high scan rate of 500 mV sꢁ1 the process is almost rever-
sible (ic ꢄ ia ; DEpa–pc ¼ 77 mV). The electrochemical band
gap, estimated from the oxidation and reduction onsets
(þ1.45 V and ꢁ1.78 V) is EgCV ¼ 3.23 eV, which is in reason-
able agreement with the optical band gap estimated from the
red edge of the longest wavelength absorption (l ¼ 382 nm,
Eogpt ¼ 3.25 eV) and close to the calculated value of the
HOMO–LUMO gap (3.56 eV). No clear reduction peak was
observed in the CV of compound 12 within the acetonitrile sol-
vent window, under the same experimental conditions, demon-
strating the enhanced electron-accepting ability of compound
4, due to the presence of the pyrazine ring.
Table 1 HOMO/LUMO energy levels for compounds 4–8, 11 and 12
from B3LYP/6-31G(d) calculations
Compound
HOMO/eV
LUMO/eV
DEHOMO–LUMO/eV
4
5
ꢁ5.62
ꢁ5.86
ꢁ5.48
ꢁ5.81
ꢁ5.48
ꢁ5.47
ꢁ5.43
ꢁ2.09
ꢁ2.30
ꢁ2.00
ꢁ2.34
ꢁ2.15
ꢁ2.20
ꢁ1.74
3.53
3.56
3.48
3.47
3.33
3.27
3.70
6
7
8
11
12
Chart 1
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
914
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 9 1 2 – 9 1 8

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Fig. 3 Frontier orbitals of compounds 12 and 4 calculated by B3LYP/6-311G(2d,p) DFT method.
Absorption and photoluminescence studies on 4 and 6
explored by blending the compound as a dopant into thin films
of poly[2-(2-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene]
(MEH-PPV), following the precedent set by Heeger’s group.21
Analogous experiments using the phenylene analogues 12 and
13 (Chart 1) were performed for comparison. Preliminary data
establish that the electroluminescence (EL) characteristics of
single-layer devices in the configuration ITO/MEH-PPV þ
dopant/Al were unchanged compared to an MEH-PPV refer-
ence device (i.e., no added dopant) by addition of compounds
4, 5, 12 and 13. On the other hand, using compound 8 as the
dopant led to a marked increase in the external quantum effi-
ciency of the device (0.07% at a current density of 10 mA
cmꢁ3) compared to the reference device (ca. 2 ꢅ 10ꢁ3%)
although more rapid deterioration in the EL output was
observed for the MEH-PPV þ 8 layers. The EL spectra of all
the device structures are very similar, which suggests that, as
The absorption and steady state fluorescence spectra in cyclo-
hexane were recorded at ambient temperature for compounds
4 and 6 (Fig. 6 and S2 in ESI). The major absorption, emission
and excitation maxima are given in Table 2, along with data
for compound 12.12 The absorbances of these compounds were
recorded between 230 and 400 nm and are characterised by
two broad bands, with the longer wavelength bands at lmax
354 and 359 nm for 4 and 6, respectively, showing a similar
profile to that observed for 12 at ambient temperature. It is
believed that these spectral profiles represent the time-averaged
spectra of the various rotamers present at these tempera-
tures.12
The emission spectra of 4 and 6 were independent of excita-
tion wavelength. The fluorescence profiles and the excitation
spectra coincide well with the corresponding absorption spec-
tra. There is a small Stokes shift between the excitation and
emission spectra. The emission spectra have some vibrational
fine structure and resemble that of 12, although the maxima
for 4 and 6 are red-shifted compared to 12. The difference in
the excitation and emission profiles is attributed to differences
in the geometric arrangement of the nuclei in the ground and
excited states, resulting from the low barrier to rotation about
the alkynyl–aryl single bond.11b,12 The vibrational structure in
the emission spectra is indicative of an S1 state with a larger
barrier to rotation from the lowest energy planar orientation
of the rings. The ethyl substituents in 6 only slightly lower
the S1 excited state.
expected, emission is due to the MEH-PPV polymer
[l
EL
max
¼ 575, 640 (sh) nm]. The non-planar biphenyl units of
8 probably serve to reduce crystallisation (and hence segrega-
tion) of the dopant in the blend. A comparison between 8
and 13 demonstrates the benefit of the electron-deficient pyra-
zine unit. We have previously reported similar levels of
enhancement of the external quantum efficiencies of MEH-
PPV devices doped with oxadiazole-pyridine hybrid com-
pounds as the electron-transport material.22 Further studies
on single blended-layer OLEDs will be published shortly.
Conclusions
2,5-Di(aryleneethynyl)pyrazine derivatives have been studied
for the first time. In particular, the structural, electrochemical
Electroluminescent device studies
The potential of 2,5-di(aryleneethynyl)pyrazine derivatives 4, 5
and 8 to act as electron-transport materials in OLEDs has been
Fig. 5 Cyclic voltammogram for derivative 4 in acetonitrile/0.1 M
¼
Bu4NPF6 (20 ꢀC) at different scan rates: (—) 100 mV sꢁ1 (E1re=d2
ꢁ1.863 V; DEpa-pc ¼ 115 mV); (ꢂ ꢂ ꢂ) 200 mV sꢁ1 (E1=2 ¼ ꢁ1.874 V;
red
Fig. 4 B3LYP/6-311G(2d,p) orbital energy level diagrams for
phenylene 12 and dimethylpyrazine 4 derivatives.
DEpa-pc ¼ 97 mV); (---) 500 mV sꢁ1 (E1=2 ¼ ꢁ1.876 V; DEpa–pc
¼
red
77 mV).
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 9 1 2 – 9 1 8
915

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Device fabrication
For OLED studies a mixed solution containing MEH-PPV
and 20% by weight of the dopant was prepared using p-xylene
and chloroform as solvents (1:1 v/v) and films of ca. 100 nm
thickness were produced by spin-coating. ITO-coated glass
was used as the anode, and an Al top electrode was evaporated
onto the organic film.
X-Ray crystallography
The experiment was carried out on a Bruker 3-circle diffracto-
meter with a SMART 6K CCD area detector, using graphite-
ꢀ
˚
monochromated Mo-Ka radiation (l ¼ 0.71073 A). The crys-
tal was maintained at low temperature (T ¼ 120 K), using a
Cryostream (Oxford Cryosystems) open-flow N2 cryostat.
The structure was solved by direct methods and refined by
full-matrix least squares against F2 of all data, using
SHELXTL software. Compound 4 crystal data: C22H16N2 ,
MW ¼ 308.37, monoclinic, space group P21/c (No. 14),
Fig. 6 Normalised absorbance, excitation (for 432 nm emission
wavelength) and emission (for 354 nm excitation wavelength) spectra
of compound 4 in cyclohexane.
ꢀ
˚
a ¼ 9.059(1), b ¼ 7.515(1), c ¼ 12.102(4) A, b ¼ 92.33(1) ,
and photophysical properties of derivative 4 have been com-
pared with those of di(phenylethynyl)benzene 12. The presence
of the pyrazine ring leads to a significant enhancement of elec-
tron-accepting properties, as revealed by cyclic voltammetric
data. Quantum chemical calculations confirm these observa-
tions. Preliminary experiments establish the potential for
diethynylpyrazine derivative 8 to act as an electron-transport-
ing material as a blend with MEH-PPV in single-layer OLEDs.
The compounds reported herein are attractive for photophysi-
cal studies and compound 10 should be a very useful and ver-
satile building block for the synthesis of new, more highly
functionalised pyrazine derivatives, including conjugated
oligomers for optoelectronic device applications.
3
U ¼ 823.2(3) A , Z ¼ 2, Dc ¼ 1.244 g cmꢁ3, m ¼ 0.07 mmꢁ1
.
˚
Full sphere of reciprocal space was covered by four sets of 0.3ꢀ
o scans, each set with different j and/or 2y angles, yielding
10,887 reflections with 2y ꢆ55ꢀ, of which 1891 were indepen-
dent (Rint ¼ 0.039). All non-hydrogen atoms were refined in
anisotropic approximation; all H atoms were located in a
difference Fourier map and refined in isotropic approximation.
The refinement of 240 parameters converged at R ¼ 0.038
[for 1465 reflections with F2 ꢇ s(F2)] and wR(F2) ¼ 0.115
(for all data).x
Electrochemistry
Cyclic voltammetry experiments were performed on a BAS
CV50W electrochemical analyser with iR compensation. Plati-
num wire, platinum disk (ø 1.6 mm) and Ag/Agþ (0.01
AgNO3 in acetonitrile) were used as counter, working, and
reference electrodes, respectively. CV experiments were per-
formed in dry acetonitrile with 0.1 M Bu4NPF6 as supporting
Experimental
General
1H NMR spectra were recorded on Varian VXR 400s and Var-
ian Inova 500 instruments, operating at 400 and 500 MHz,
respectively. Chemical shifts are quoted in ppm, relative to tet-
ramethylsilane (TMS), using TMS or the residual solvent as
internal reference. 13C NMR spectra were recorded using
broad band decoupling, on the Varian VXR 400s or Varian
Inova 500 at 100 MHz and 125 MHz, respectively. Electron
impact (EI) mass spectra were recorded on a Micromass Auto-
Spec spectrometer operating at 70 eV with the ionisation mode
as indicated. An Ati Unicam UV/VIS UV2 spectrometer
was used to record the absorption spectra of the samples,
which were held in 1 cm path length cuvettes. Emission and
excitation spectra were recorded using a Jobin–Yvon Horiba
Fluorolog 3-22 Tau-3 spectrofluorimeter. The spectra of
dilute solutions with absorbance of less than 0.1 in the 200
to 400 nm range were recorded using conventional 90 degree
geometry.
electrolyte. The scan rate was varied from 50 to 2000 mV sꢁ1
.
The potentials were referred to the Fc/Fcþ couple as the inter-
nal reference, which showed a potential of þ0.08 V vs. Ag/
Agþ in our conditions.
Computational procedures
The ab initio computations of compounds 4–9 and 12 were car-
ried out with the Gaussian 9823 package of programs at the
density functional theory (DFT) level using Pople’s 6-31G
and 6-311G split valence basis sets supplemented by one or
two d-functions on heavy atoms and p-polarisation functions
on hydrogens (or without them). DFT calculations were
carried out using Becke’s three-parameter hybrid exchange
functional24 with Lee–Yang–Parr gradient-corrected correla-
tion functional (B3LYP).25 Thus, the geometries were opti-
mised with B3LYP/6-31G(d) and for compounds 4 and 12
also with B3LYP/6-311G(2d,p); electronic structures were cal-
culated at the same level. Contours of the five highest occupied
and five lowest unoccupied molecular orbitals were visualised
using the Molekel v.4.2 program.26 No constraints to bonds/
angles/dihedral angles were applied in the calculations and
all the atoms were free to optimise.
Table 2 The absorption and steady state emission and excitation
spectra of 4, 6 and 12a
Compound
labs/nm
l
PL/nm
l
ex/nm
4
6
286 (br), 354 (br) 379, 395 (br)
283 (br), 357 (br)
291, 359 (br),
376 (sh)
320, 345 (sh)
384, 403, 418 287, 360, 374 (sh)
346, 362, 375 319, 340 (sh)
12
x CCDC reference number 231332. See http://www.rsc.org/
suppdata/nj/b4/b401867m/ for crystallographic data in .cif or other
electronic format.
a
br – broad, sh – shoulder.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
916
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 9 1 2 – 9 1 8

View Article Online
Syntheses
2,5-Bis(biphenyl-4-ylethynyl)-3,6-dimethylpyrazine 8. 24%
yield, m.p. 188.0–188.7 ꢀC. MS (EI): m/z 460, (Mþ 100%).
HRMS (EI): (Mþ) calcd. for C34H24N2 460.19395; found
2,5-Dichloro-3,6-dimethylpyrazine 2. To alanine anhydride 1
(25.0 g, 0.176 mol) in POCl3 (150 cm3), PCl5 (9.0 g) was added
and the mixture was heated (oil bath temperature 105 ꢀC) for
24 h. The mixture was cooled to room temperature and excess
POCl3 was removed by vacuum distillation. Cold brine was
added slowly with stirring and a white solid formed after
standing for 12 h. Then the mixture was extracted with dichloro-
methane and the organic layer was dried with MgSO4 . The
solvent was removed by vacuum evaporation and the residue
was chromatographed on silica eluted with dichloromethane,
followed by crystallisation from hexane to yield compound 2
as a white solid (9.3 g, 31%), m.p. 73–74 ꢀC (lit.17 m.p.
72 ꢀC). Found C, 40.12; H, 3.78; N, 15.63%. C6H6Cl2N2
requires C, 40.71; H, 3.42; N, 15.82%. MS (EI): m/z 176
(Mþ, 100%). 1H NMR (CDCl3): d 2.61 (6H, s, CH3). 13C
NMR (CDCl3): d 21.24, 145.60, 149.77.
1
460.19385. H NMR (CDCl3): d 2.62 (6H, s, CH3), 7.18 (2H,
s, CH), 7.29 (4H, s, CH), 7.38 (8H, s, CH), 7.61 (4H, s, CH).
13C NMR (CDCl3): d 14.68, 74.66, 81.84, 120.63, 127.07,
127.14, 127.88, 128.93, 132.97, 140.10, 141.97, 156.91.
4-[5-(3-Hydroxy-3-methyl-but-1-ynyl)-3,6-dimethylpyrazin-2-
yl]-2-methylbut-3-yn-2-ol 9. Compound 3 (810 mg, 3.04 mmol),
2-methylbut-3-yn-2-ol (1.54 g, 18.24 mmol), CuI (58 mg),
Pd(PPh3)2Cl2 (213 mg) and triethylamine (10 cm3) were mixed
in dry THF (15 cm3) and stirred at room temperature for 1 h,
then at reflux under Ar to obtain a brownish suspension. The
mixture was filtered and the precipitate was washed with
diethyl ether. The filtrate was evaporated under reduced pres-
sure. The residue was purified by column chromatography on
silica [eluent: ethyl acetate–petroleum ether (b.p. 40–60 ꢀC), 7:3
(v/v)] to give compound 9 as white solid (780 mg, 95%), m.p.
170–171 ꢀC. Found: C, 70.43; H, 7.67; N, 10.02%. C16H20N2O2
requires C, 70.56; H, 7.40; N, 10.29%. MS (EI): m/z 272 (Mþ,
2,5-Dibromo-3,6-dimethylpyrazine 3. A mixture of 2 (5.37 g
30 mmol) and PBr3 (15 cm3) was stirred at reflux (oil bath tem-
perature 170 ꢀC) for 24 h then cooled to 20 ꢀC and excess PBr3
removed by vacuum distillation. Work up and purification as
described for 2 gave compound 3 (2.8 g, 35%) as a white solid,
m.p. 84.4–85.8 ꢀC. Found C, 26.76; H, 2.52; N, 10.35%.
C6H6Br2N2 requires C, 27.10; H, 2.27; N, 10.53%. MS (EI):
m/z 266 (Mþ 100%). 1H NMR (CDCl3): d 2.58 (6H, s,
CH3). 13C NMR (CDCl3): d 22.97, 138.80, 152.4.
1
100%). H NMR (CDCl3): d 1.59 (12H, s, CH3), 2.10 (2H, s,
OH), 2.55 (6H, s, CH3). 13C NMR (CDCl3): d 21.77, 31.34,
65.85, 79.40, 101.48, 136.18, 153.10.
2,5-Diethynyl-3,6-dimethylpyrazine 10. Compound 9 (200
mg, 0.735 mmol) was dissolved in dry toluene (15 cm3).
Sodium hydroxide powder (freshly ground from pellets; 64
mg) was added and the mixture was refluxed under Ar until
9 was completely consumed (TLC monitoring). The mixture
was cooled to room temperature then suction filtered through
a celite pad. The filtrate was evaporated in vacuo and residue
was purified by column chromatography on silica [eluent: ethyl
acetate–petroleum ether (b.p. 40–60 ꢀC), 5:4 (v/v)] to give
compound 10 as a pale-yellow solid (88 mg, 80%), m.p.
175–176 ꢀC. Found: C, 76.67; H, 5.34; N, 17.74%. C10H8N2
requires C, 76.90; H, 5.16; N, 17.94%. MS (EI): m/z 156
(Mþ, 100%). 1H NMR (CDCl3): d 2.59 (6H, s, CH3), 3.48
(2H, s, CH). 13C NMR (CDCl3): d 21.92, 80.32, 85.74,
136.08, 153.64.
Preparation of 4-8: general procedure. To the solution of 2,5-
dibromo-3,6-dimethylpyrazine 3 (0.51 g, 2.0 mmol) and the
alkynylarene (4.8 mmol) in dry THF (7.0 cm3), CuI powder
(10.6 mg) was added. Triethylamine (0.9 cm3) was added and
the solution was stirred at 20 ꢀC for 10 min to obtain a clear
solution. Pd(PPh3)2Cl2 (32 mg) was added in one portion
and the mixture was stirred at 20 ꢀC for 1 h, followed by reflux
at 65 ꢀC for another 1.5 h. The mixture was cooled to room
temperature and the solid that formed during the reaction
was removed by vacuum filtration. The filtrate was chromato-
graphed on silica, eluted with DCM, followed by crystallisa-
tion from hexane. The compounds were thereby obtained as
yellow solids.
2,5-Dimethyl-3,6-bis(phenylethynyl)pyrazine 4. 41% yield, m.p.
169–170 ꢀC. Found: C, 85.44; H, 5.21; N, 9.15%. C22H16N2
requires C, 85.69; H, 5.23; N, 9.08%. MS (EI): m/z 308,
(Mþ, 100%). 1H NMR (CDCl3): d 2.69 (6H, s, CH3) 7.32
(6H, s, CH) 7.58 (4H, s, CH). 13C NMR (CDCl3): d 21.75,
86.45, 96.72, 121.85, 128.51, 129.49, 132.06, 136.24, 153.02.
2,5-Dimethyl-3,6-bis(pyridin-2-ylethynyl)pyrazine 5. 31% yield,
m.p. 201–202 ꢀC. Found: C, 77.24; H, 4.57; N, 17.89%.
C20H14N4 requires C, 77.40; H, 4.55; N, 18.05%. MS (EI):
m/z 310 (Mþ, 100%). 1H NMR (CDCl3): d 2.60 (6H, s,
CH3), 7.31 (2H, s, CH), 7.48 (2H, s, CH), 8.56 (2H, s, CH).
13C NMR (CDCl3): d 20.13, 72.46, 123.31, 127.26, 135.34,
137.42, 149.41, 153.97.
2,5-Bis(thien-2-ylethynyl)-3,6-dimethylpyrazine 11. Com-
pound 10 (200 mg, 1.28 mmol), 2-iodothiophene (1.08 mg,
5.12 mmol), CuI (24 mg), Pd(PPh3)2Cl2 (89 mg) and triethyl-
amine (10 cm3) were mixed in dry THF (15 cm3) and reacted
according to the general procedure above. Column chroma-
tography [silica, dichloromethane–hexane, 1:1 (v/v)] gave
compound 11 as a yellow solid (81 mg, 20% yield), m.p.
145–147 ꢀC (dec). MS (EI): m/z 319.98 (Mþ, 100%). HRMS
(EI): (Mþ) calcd. C18H12N2S2 320.0450; found 320.0441. 1H
NMR (CDCl3): d 7.36 (2H, d, J ¼ 4.8 Hz), 7.34 (2H, d,
J ¼ 6.4 Hz), 6.99 (2H, t, J ¼ 8.8 Hz), 2.61 (6H, s). 13C
2,5-Bis(4-ethylphenylethynyl)-3,6-dimethylpyrazine þ6. 38%
yield, m.p. 196.5–198.3 ꢀC. MS (EI): m/z 364, (M , 100%).
HRMS (EI): (Mþ) calcd. for C26H24N2 364.19395; found
364.19400. 1H NMR (CDCl3): d 1.19 (6H, t, J ¼ 7.6 Hz,
CH3), 2.62 (4H, q, J ¼ 7.6 Hz, CH2), 2.68 (6H, s, CH3), 7.15
(4H, d, J ¼ 8 Hz, CH), 7.48 (4H, d, J ¼ 8 Hz, CH). 13C
NMR (CDCl3): d 15.89, 22.15, 29.78, 86.20, 97.81, 119.73,
128.22, 132.31, 136.23, 148.34, 152.68.
NMR (CDCl3):
d 22.17, 90.47, 110.00, 121.91, 127.66,
129.48, 134.10, 136.21, 153.04.
Compounds 12 and 13 were obtained analytically pure using
standard procedures.12
2,5-Bis(4-chlorophenylethynyl)-3,6-dimethylpyraziþne 7. 23%
yield, m.p. 190.5–191 ꢀC. MS (EI): m/z 376 (M , 58%), 270
(100%). HRMS (EI): (Mþ) calcd. for C22H14Cl2N2
Acknowledgements
We thank the Royal Society and the Royal Society of Chem-
istry Journals Grant for International Authors for funding
visits to Durham (I.F.P.), and One North-East via the County
Durham Sub-regional Partnership project SP/082 for funding
the purchase of equipment used in this work.
1
376.05340; found 376.05329. H NMR (CDCl3): d 2.69 (6H,
s, CH3), 7.31 (4H, s, CH), 7.48 (4H, s, CH). 13C NMR
(CDCl3): d 20.93, 74.66, 81.84, 120.63, 127.20, 133.21,
141.56, 156.45.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 9 1 2 – 9 1 8
917

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13 (a) A. Montali, P. Smith and C. Weder, Synth. Met., 1998, 97,
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T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
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918
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 9 1 2 – 9 1 8