Structure-property relationships and nonlinear optical effects in donor-substituted dicyanopyrazine-derived push-pull chromophores with enlarged and varied π-linkers

Article abstract of DOI:10.1002/ejoc.201101226

Thirteen new, stable, push-pull systems featuring dimethylamino and pyrazine-2,3-dicarbonitrile moieties as the donor and acceptor, respectively, and systematically extended and varied π-linkers were prepared and investigated. Evaluation of the measured UV/Vis spectra, electrochemical data (cyclic voltammetry, rotating disc voltammetry, and polarography), X-ray data, and experimentally determined and calculated hyperpolarizability values enabled structure-property studies; these revealed some important structural features that affected the efficiency of intramolecular charge-transfer and nonlinear optical properties in this class ofheterocyclic push-pull chromophores. The charge-transfer transition was most significantly affected by structuralfeatures such as π-linker length, chromophore planarity, and the number of 1,4-phenylene/ethynylene subunits in the π-linker. Linear and nonlinear optical properties of push-pull chromophores featuring a pyrazine-2,3- dicarbonitrile acceptor moiety and dimethylamino donors were modulated by systematic extension and variation of the π-linker. The length and character of the π-linker used significantly influenced the efficiency of the donor-acceptor conjugation and nonlinear responses. Copyright

Full text of DOI:10.1002/ejoc.201101226

FULL PAPER
DOI: 10.1002/ejoc.201101226
Structure–Property Relationships and Nonlinear Optical Effects in
Donor-Substituted Dicyanopyrazine-Derived Push–Pull Chromophores with
Enlarged and Varied π-Linkers
[a]
[a]
[a]
[a]
ˇ
ˇ
ˇ
Filip Bures,* Hana Cermáková, Jirí Kulhánek, Miroslav Ludwig,
Wojciech Kuznik,[b,c] Iwan V. Kityk, Tomás Mikysek, and Ales Ruzicka
[d]
[e]
[f]
ˇ
ˇ
˚ˇ ˇ
Dedicated to Professor Oldrˇich Pytela on the occasion of his 60th birthday
Keywords: Charge transfer / Donor–acceptor systems / Nonlinear optics / Electrochemistry / Pyrazine
Thirteen new, stable, push–pull systems featuring dimeth-
ylamino and pyrazine-2,3-dicarbonitrile moieties as the do-
nor and acceptor, respectively, and systematically extended
and varied π-linkers were prepared and investigated. Evalu-
ation of the measured UV/Vis spectra, electrochemical data
property studies; these revealed some important structural
features that affected the efficiency of intramolecular charge-
transfer and nonlinear optical properties in this class of
heterocyclic push–pull chromophores. The charge-transfer
transition was most significantly affected by structural
(cyclic voltammetry, rotating disc voltammetry, and polar- features such as π-linker length, chromophore planarity, and
ography), X-ray data, and experimentally determined and
calculated hyperpolarizability values enabled structure–
the number of 1,4-phenylene/ethynylene subunits in the π-
linker.
rage devices.[2] Conversely, many organic materials (mainly
polyenes and polyynes) suffer from low thermal stability.
Therefore, various five- and six-membered heterocycles
were recently used to strengthen the thermal and chemical
robustness of these materials.[3] Moreover, heteroaromatic
moieties incorporated into the chromophore backbone may
act as an auxiliary electron donor (D) or acceptor (A) and
further improve the optical nonlinearity of the chromo-
phore.[4] A typical organic NLOphore consists of strong
electron donors and acceptors appended to a π-conjugated
system. This D–π–A arrangement assures efficient intramo-
lecular charge transfer (ICT) between the donor and ac-
ceptor moieties and generates a dipolar push–pull system.[5]
The polarizability of such systems depends on the electronic
nature of the appended donors and acceptors and the
length of the π-conjugated linker.[6] In this respect, N,N-
dialkylamino (NR2) and alkoxy (OR), as well as nitro
(NO2) and cyano (CN), functionalities were utilized as the
most powerful electron donors and acceptors.
Over the last five years, we have synthesized several series
of heterocyclic chromophores with imidazole-4,5-dicarb-
onitrile as an acceptor unit and an dimethylamino donor
group appended to a systematically extended π-linker (1).[7]
Linear and nonlinear optical and fluorescence properties
in solution or in polymer matrices have been investigated
further.[8] Herein, we focus on donor-substituted di-
cyanopyrazines 2–9 as a related class of organic D–π–A
chromophores (Figure 1).
Introduction
Over the past two decades, great progress has been made
in the development of new organic systems for nonlinear
optical (NLO) applications.[1] In contrast to inorganic mate-
rials, the advent of organic molecules for NLO applications
has been stimulated by their relative ease of synthesis, well-
defined structure, facile property tuning, and large optical
nonlinearities and responses. Hence, organic molecules have
been heavily investigated as active components of optoelec-
tronic devices, organic light-emitting diodes (OLEDs),
photovoltaic cells, semiconductors, switches, and data sto-
[a] Institute of Organic Chemistry and Technology, University of
Pardubice, Faculty of Chemical Technology,
Studentská 573, Pardubice, 53210, Czech Republic
Fax: +420-46-603-7068
E-mail: filip.bures@upce.cz
[b] Physical Chemistry Department, Silesian University of
Technology,
Strzody 9, 44-100 Gliwice, Poland
[c] Institute of Physics, Tartu University,
Riia 142, 51014 Tartu, Estonia
[d] Electrical Engineering Department, Czestochowa University of
Technology,
Armii Krajowej 17, Czestochowa, 42201, Poland
[e] Department of Analytical Chemistry, University of Pardubice,
Faculty of Chemical Technology,
Studentská 573, Pardubice, 53210, Czech Republic
[f] Department of General and Inorganic Chemistry, University of
Pardubice, Faculty of Chemical Technology,
Studentská 573, Pardubice, 53210, Czech Republic
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201101226.
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F. Buresˇ et al.
FULL PAPER
benzils 11 and 12 with DAMN afforded chromophore 3a
and dibromo derivative 15 in yields of 97 and 45%, respec-
tively (Scheme 2). It was recently reported that a twofold
cross-coupling reaction on 15 was possible.[12] However, we
found this reaction was very sluggish (see below). Therefore,
we decided to synthesize the diiodo analogue 16. Whereas
compounds 11 and 12 were commercially available, α-diket-
ones 13 (see ref.[13]) and 14 were synthesized by the method-
ology of Faust and Weber.[14] Treatment of 4-ethynyl-N,N-
dimethylaniline or 1,4-diodobenzene with nBuLi at –78 °C
followed by transmetallation with CuBr and subsequent
quenching of the copper species with oxalyl chloride pro-
vided 4,4Ј-diiodobenzil (13) and extended α-diketone 14 in
48% yield for both compounds. The α-diketones 13 and 14
were further condensed with DAMN to give pyrazines 16
and 3b. Whereas 4,4Ј-disubstituted benzils 11–13 underwent
condensation with DAMN in AcOH at 150 °C within 5 h,
the extended α-diketone 14 could only be treated with
DAMN at 25 °C for 1 h; increased temperature or reaction
time led to decomposition (Scheme 2).
Figure 1. Imidazole Y-shaped versus pyrazine X-shaped push–pull
chromophores.
Whereas the imidazole series 1 possesses two cyano ac-
ceptor functions at the imidazole C4/C5 positions and one
dimethylamino donor group at C2 separated by an ad-
ditional π-linker (Y-shaped chromophores), the pyrazine
series 2–9 feature two cyano acceptors and two dimeth-
ylamino donors appended at the pyrazine C2/C3 and C5/
C6, respectively (X-shaped chromophores). The π-conju-
gated path separating NMe2 donors from the parent pyr-
azine-2,3-dicarbonitrile acceptor moiety was systematically
elongated by the insertion of 1,4-phenylene and ethynylene
subunits. Chromophores 3–9 were prepared as two series,
labeled a and b. The π-linker in series a is connected directly
to the pyrazine (n = 0), whereas the chromophores in series
b are separated by the π-linker and an additional triple
bond (n = 1). Pyrazine-2,3-dicarbonitrile derivatives were
recently utilized as luminescent materials or precursors of
pyrazinoporphyrazines and phthalocyanines.[9] We report
herein the synthesis, full spectral characterization, three X-
ray structures, electrochemical behavior, and linear and
nonlinear properties, as well as the structure–property rela-
tionships, for chromophores 2–9.
Results and Discussion
Synthesis
The synthesis of the simplest chromophore, 2a, with di-
methylamino donors appended directly to pyrazine began
with the condensation of diaminomaleonitrile (DAMN)
with oxalyl chloride and subsequent halogenation of the re-
sulting dioxopyrazine with thionyl chloride.[10] Resulting
5,6-dichloropyrazine-2,3-dicarbonitrile (10) was subjected
to an SNAr reaction with dimethylamine to give target com-
pound 2a in 81% yield (Scheme 1).
Scheme 2. Synthetic route leading to α-diketones 13–14 and 5,6-
disubstituted pyrazine-2,3-dicarbonitriles 3a/b, 15, and 16.
Attempted twofold Suzuki–Miyaura and Sonogashira
cross-coupling reactions of 15 with 4-(dimethylamino)phen-
ylboronic acid or 4-ethynyl-N,N-dimethylaniline were slug-
gish, low-yielding, and gave products of both mono- and
disubstitution, although an excess of boronic acid or ter-
minal acetylene was used. Moreover, separation of both
products by column chromatography was tedious. Accord-
ingly, we turned our attention to the cross-coupling reac-
tions on the diiodo derivative 16, which underwent the de-
Scheme 1. Synthesis of chromophore 2a.
The condensation of an α-diketone with an 1,2-diamino sired cross-coupling reactions smoothly with only 2.1 equiv.
compound is one of the most versatile methods used for the of the coupling partner. Purification of products was car-
construction of pyrazines.[11] Hence, an initial reaction of ried out by means of standard column chromatography. We
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Donor-Substituted Dicyanopyrazine-Derived Push–Pull Chromophores
Scheme 3. Cross-coupling reactions on 16 leading to target chromophores 4a–9a.
Scheme 4. Synthesis of chromophores 3b–5b and 8b–9b.
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F. Buresˇ et al.
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have recently reported a convenient synthesis for preparing repulsion of the appended dimethylamino and dimethylanil-
π-linkers as building blocks for modular chemistry.[15] ino moieties.[18] Hence, the dihedral angles φ1, φ2, φ3, and
Among others, dimethylamino-substituted π-linkers with φ4 have the following values: 16, 19, 11, and 4° for 2a and
phenyl, biphenyl, and phenylethynylphenyl backbones and 25, 18, 13, and 7° for 3a. The descending order clearly dem-
terminal boronic acid/ester or acetylene moieties were syn- onstrates that the deformation of the pyrazine ring is caused
thesized in a straightforward manner. With such π-linkers by the substituents at C5 and C6. The reduction in bond
in hand, we were able to perform [PdCl2(PPh3)2]-catalyzed length alternation in the N,N-dimethylanilino rings ex-
twofold cross-coupling reactions on 16 to yield target chro- pressed by the quinoid character (δr) can be used as a good
mophores 4a–9a in yields of 62–94% (Scheme 3).
indication of the charge-transfer (CT) efficiency from the
All of the chromophores in series a, except 2a, possess dimethylamino donors to the pyrazine-2,3-dicarbonitrile
the π-linker connected to the pyrazine C5/C6 through the acceptor.[19] Whereas in benzene δr equals 0, quinoid rings
1,4-phenylene moiety. In our recent work, we have shown have values of 0.08–0.1. The δr values of the two N,N-di-
that this nonplanar arrangement represents a barrier to ef- methylanilino rings in 3a, derived from X-ray analysis, were
ficient D–A conjugation (see also the X-ray structure of 3a 0.026 and 0.028. A comparison of these values with those
below).[6c,7] In chromophore 3b, prepared from the ex- measured for the N,N-dimethylanilino-substituted, fully
tended α-diketone 14, the 1,4-phenylene moiety was sepa- planar, push–pull systems (δr ca. 0.046–0.072),[6c] revealed
rated by an additional ethynylene subunit. This arrange- that the two N,N-dimethylanilino rings in 3a were forced
ment assured planarization of the entire π-conjugated sys- out of the pyrazine mean plane significantly and were less
tem. Therefore, we focused on the synthesis of chromo- engaged in the ground-state ICT.
phores 3–9 with an additional triple bond separating the π-
linker and the pyrazine moiety (Figure 1, series b; n = 1).
We found that the chlorine atoms in dichloropyrazine 10
could be replaced not only with an SNAr reaction (see
Scheme 1, 2a), but also with a palladium-catalyzed Sonoga-
shira cross-coupling reaction. The Sonogashira reaction of
10 with the aforementioned π-linkers (terminal acetylenes),
such as 4-ethynyl-N,N-dimethylaniline, 4Ј-ethynyl-N,N-di-
methylbiphenyl-4-amine, and 4-[(4-ethynylphenyl)ethynyl]-
N,N-dimethylaniline, afforded chromophores 3b–5b in
yields of 62–79% (Scheme 4).
Chromophore 3b was also prepared by the condensation
of 14 with DAMN (Scheme 2). Extended dimethylamino-
substituted π-linkers 19, 21, 23, and 25 (see ref.[16]), which
were required for the synthesis of chromophores 6b, 8b, and
9b, were prepared in a modular manner, as outlined in the
Supporting Information. In this respect, [(4-iodophenyl)-
ethynyl]trimethylsilane (17; see ref.[16b] and the Supporting
Information; also commercially available) was utilized as a
suitable precursor. Whereas the Sonogashira cross-coupling
reactions of 10 with acetylenes 23 and 25 provided target
chromophores 8b and 9b in modest yields of 34 and 42%,
similar cross-couplings with acetylenes 19 and 21 afforded
only traces of desired chromophores 6b and 7b. Although
the reaction conditions were optimized, these two reactions
were dominated by homocoupling of the starting terminal
acetylenes.[17]
Figure 2. ORTEP representations and spatial arrangements of
compound 10 (a) and chromophores 2a (b) and 3a (c) measured at
150 K. The thermal ellipsoids are shown at the 50% probability
with arbitrary spheres for H atoms.
Electrochemistry
Electrochemical measurements were carried out in DMF
containing 0.1 m Bu4NPF6 in a three-electrode cell by
means of cyclic voltammetry (CV), rotating disc voltamme-
Crystallography
Crystals suitable for X-ray analysis were prepared by try (RDV), and polarography. The working electrode was a
slow diffusion of hexane into solutions of pyrazines 10, 2a, platinum disc (2 mm in diameter) for CV and RDV experi-
and 3a in CH2Cl2. The ORTEP plots in Figure 2 confirm ments and a dropping mercury electrode (DME) for polaro-
the proposed molecular structures. Front and side views are graphic measurements. A saturated calomel electrode
provided. In contrast to the almost planar spatial arrange- (SCE), separated by a bridge filled with supporting electro-
ment of dichloropyrazine 10, in which the dihedral angles lyte, and Pt wire were used as the reference and auxiliary
φ1–φ4 are within the range of 0.09–0.66°, pyrazines 2a and electrodes, respectively. The acquired data is summarized in
3a adopt an angular arrangement as a result of the steric Table 1 and Tables S01–S02 in the Supporting Information.
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Donor-Substituted Dicyanopyrazine-Derived Push–Pull Chromophores
Representative CV and polarographic curves for com- chemical point of view, the dimethylamino donor groups in
pounds 2a, 3a/b, 5a/b, and 9a/b are shown in Figures S01– chromophores 4–9 are similarly engaged in ICT.
S08 in the Supporting Information.
The first reduction was probably localized on the pyr-
azine-2,3-dicarbonitrile acceptor moiety as a main re-
duction centre, whereas subsequent reductions involved the
π-linker. The observed first reductions were one-electron,
mostly reversible, processes. The first reduction potentials
were consistent when measured on Pt and DME electrodes,
which implied that neither electrode was directly involved
in the first reduction processes. The first reduction poten-
tials of chromophore series a and b ranged from –1.66 to
–0.92 V and from –0.87 to –0.67 V and were clearly depend-
ent upon the length and structure of the π-linker. The intro-
duction of one 1,4-phenylene subunit shifted the first re-
duction potential by 0.42 V from 2a [E1/2(red1) = –1.66 V] to
3a [E1/2(red1) = –1.24 V]. The introduction of an additional
1,4-phenylene unit, as in 4a [E1/2(red1) = –1.05 V], caused a
shift in the first oxidation potential of 0.19 V. However, sim-
ilar to the first oxidation, the first reduction potentials be-
came steady at about –1.00 V for 5a and 6a and –0.92 V for
7a–9a, respectively. As a result, chromophores 7a–9a
showed a constant difference between the first oxidation
and reduction potentials [E1/2(ox1) – E1/2(red1) ≈ 1.74 V].
Hence, a combination of three 1,4-phenylene subunits and
one acetylene subunit caused the complete isolation of the
dimethylamino donor and the pyrazine-2,3-dicarbonitrile
acceptor moieties. A similar trend was observed in chromo-
phore series b, which contains an additional triple bond
separating the π-linker and the pyrazine-2,3-dicarbonitrile
moiety. In this series, starting from chromophore 5b
[E1/2(red1) = –0.69 V, phenylethynylphenyl as a main π-
linker] onward, the first reduction potentials steadied at
about –0.68 V with a constant difference of E1/2(ox1) – E1/
Table 1. Electrochemical data, absorption maxima (λmax), and
melting points for chromophores 2a–9a and 3b–5b, 8b, and 9b.
E1/2(ox1) E1/2(red1) E1/2(ox1)
E1/2(red1)
–
EHOMO EHOMO λmax
M.p.
[V][a]
[V][a]
[V]
[eV][b]
[eV][b]
[nm]
[°C][d]
([eV])[c]
2a +1.14
3a +1.02
4a +0.81
5a +0.83
6a +0.78
7a +0.80
8a +0.83
9a +0.83
3b +1.04
4b +0.84
5b +0.85
8b +0.84
9b +0.83
–1.66
–1.24
–1.05
–0.97
–1.02
–0.93
–0.92
–0.92
–0.87
–0.74
–0.69
–0.70
–0.67
2.80
2.26
1.86
1.80
1.80
1.73
1.75
1.75
1.91
1.58
1.54
1.54
1.50
–5.49
–5.37
–5.16
–5.18
–5.13
–5.15
–5.18
–5.18
–5.39
–5.19
–5.20
–5.19
–5.18
–2.69
–3.11
–3.30
–3.38
–3.33
–3.42
–3.43
–3.43
–3.48
–3.61
–3.66
–3.65
–3.68
355
(3.49)
471
(2.63)
425
(2.92)
433
(2.86)
402
(3.08)
404
168–
170
272–
275
221
(dec.)
270–
271
268
(dec.)
310–
315
(3.07)
400, sh Ͼ350
(3.10)
402, sh 300–
(3.08)
499
(2.48)
465
(2.67)
466
(2.66)
354
305
220
(dec.)
Ͼ350
301
(dec.)
321
(dec.)
307
(dec.)
(3.50)
371
(3.34)
[a] E1/2(ox1) [E1/2(red1)] are half-wave potentials of the first oxidation
(reduction) measured by RDV or polarography; the potentials are
given versus SCE. [b] EHOMO/LUMO = E1/2(ox1/red1) + 4.350 (ref.[20]).
[c] sh indicates shoulder. [d] dec. indicates decomposition.
≈ 1.52 V. This electrochemical behavior clearly dem-
2(red1)
onstrates the “insulating” character of acetylene in conjunc-
tion with 1,4-phenylene linkers.[6c,7,21] However, a compari-
son of analogous chromophores and their first reduction
and oxidation potentials across the series a and b, for
example, 3a/3b, 4a/4b, and 5a/5b (Table 1) demonstrated the
beneficial role of the additional triple bond between the π-
linker and the pyrazine-2,3-dicarbonitrile. Whereas the first
oxidation potentials are similar, the first reduction poten-
tials are shifted to more positive values as a result of the
partial planarization of the chromophore backbone.
Generally, the first oxidation occurred on the two N,N-
dimethylanilino donor chromophore units, whereas the first
reduction involved the pyrazine-2,3-dicarbonitrile acceptor
moiety and the π-linker, similar to previous investigations
of imidazole push–pull systems.[7] All studied chromo-
phores showed two close, mostly reversible, one-electron
oxidation processes (differences of about 100 mV measured
by CV) that were, in most cases, merged into one two-elec-
tron wave, as indicated by RDV. This electrochemical be-
havior reflected the presence of two equivalent dimeth-
ylamino donor groups appended to pyrazine. The first oxi-
dation potentials of the chromophore series a and b range
from +1.14 to +0.78 V and from +1.04 to +0.83 V, respec-
tively. In contrast to the first reduction (see below), the first
oxidation potentials were affected only negligibly by the
length and structure of the π-linker. However, the first oxi-
UV/Vis Spectroscopy
Chromophores 2a–9a, 3b–5b, 8b, and 9b are colored
dation potentials decrease with increasing π-linker length, compounds (see Figure S09 in the Supporting Infor-
as a result of the isolation of the dimethylamino oxidation mation). Their absorption spectra, measured in CH2Cl2,
center from the chromophore acceptor moiety. From chro- showed variably intense CT absorption bands (Figure 3 and
mophores 4a and 4b onward, the first oxidation potentials Table 1). The CT character of the longest-wavelength ab-
were concentrated in the range of about +0.80 V and sorption bands was confirmed by protonation/neutraliza-
+0.84 V, respectively. This means that, from the electro- tion experiments with trifluoroacetic acid/triethylamine (for
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F. Buresˇ et al.
FULL PAPER
compounds 3a, 3b, and 6a see Figures S10–S12 in the Sup- tion of D and A chromophore parts. Weak CT bands were
porting Information). Whereas chromophore 2a showed observed as more or less distinguishable shoulders. The
only an absorption band with λmax appearing at 355 nm, most intensive bands, appearing at around 350 nm,
chromophores 3a and 3b showed two particularly devel- probably correspond to the transition between the dimeth-
oped and bathochromically shifted CT bands at 398/471 ylamino donors and the electronegative π-linkers. This ob-
and 442/499 nm, respectively. This observation reflects two servation confirms the results seen in the electrochemical
possible CT transitions from the dimethylamino peripheral data, in which chromophores 6a–9a and 8b–9b also showed
donors to the pyrazine-2,3-dicarbonitrile acceptor moiety “constant” electrochemical behavior.
and elongation of the π-linker with 1,4-phenylene (3a) or
phenylethynyl (3b) units. Compared with series a, the long-
est-wavelength absorption bands of chromophores in series
b (e.g., 3a vs. 3b, 4a vs. 4b, and 5a vs. 5b) showed a signifi-
Calculations and NLO Properties
cant redshift, which demonstrated the beneficial role of the
The NLO properties of the new push–pull chromophores
additional triple bond between the π-linker and the pyr- were determined by measuring the second-order suscep-
azine-2,3-dicarbonitrile moiety, as already observed tibilities of chromophores 2a–9a, 3b–5b, 8b, and 9b incor-
through electrochemical measurements (see above). In con- porated into oligoetheracrylate photopolymers.[22] These
trast to 3a and 3b, chromophores 4a/5a and 4b/5b showed polymers were solidified by external UV light with a power
only average, hypsochromically shifted CT bands at 425/433 density up to 60 WcmϪ2 and an applied dc-electric field
and 465/466 nm, respectively. This hypsochromic shift was that was successively enhanced up to 2.3–2.8 kVcmϪ1
.
even more pronounced in the absorption spectra of chro- Photo UV solidification was carried out within 3–5 min. In
mophores 6a–9a and 8b/9b. Thus, further insertion of either this way, a correlated, macroscopically aligned non-centro-
1,4-phenylene or acetylene units led to the complete isola- symmetry of the investigated molecules was achieved. This
corresponded to the degree of chromophore alignment
equal to about 85%. This ratio was determined on the basis
of monitoring the ratio of the principal polarized absorp-
tion bands, which unambiguously determined the degree of
chromophore orientation. Solutions of the chromophore in
chloroform oriented by an external dc electrical field were
used as the reference spectra. This methodology was de-
scribed in our earlier work.[23] A 10 ns Nd:YAG laser at a
wavelength of 1064 nm with a frequency repetition of about
12 Hz and pulse energy of about 30 mJ was used as a light
source. Detection was performed with a fast-response pho-
tomultiplier connected to a 200 MHz oscilloscope. The sec-
ond-harmonic generation (SHG) signal was detected by
using a monochromator with a spectral resolution of about
0.15 nm. The doubled-frequency intensity was measured for
a wavelength of 532 nm by using a fast-response photode-
tector, which was connected to a 1 GHz Tectronics oscillo-
scope. As reference samples, doped polyaniline samples
with known values of the second-order susceptibilities were
used. Due to statistical averaging over the different point of
surface (measured for at least 30 points), the uncertainty of
the second-order susceptibility determination was not
larger than 0.15 pmVϪ1. All of the quantum chemical cal-
culations and simulations of NLO properties of the target
chromophores were performed at the semiempirical level of
theory with AM1. The molecular geometries were opti-
mized by using the Gaussian 09 program. Hyperpolarizabil-
ities were computed by a time-dependent Hartree–Fock
(TDHF) calculation in PC GAMESS (see the Supporting
Information for more details). The experimental data and
the results of calculations are presented in Table 2.
Ground-state dipole moments and the HOMO and
LUMO energies of all target compounds were also calcu-
lated (Table 2). Representative HOMO and LUMO local-
izations for compounds 3a/b and 9a/b are shown in the Sup-
porting Information (Figures S13–S20). Whereas the
Figure 3. UV/Vis spectra of chromophores 2a–9a, 3b–5b, 8b, and
9b (2ϫ 10–5 m solutions in CH2Cl2).
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Donor-Substituted Dicyanopyrazine-Derived Push–Pull Chromophores
Table 2. Calculated electronic parameters and experimental hyper- SHG measurements, and quantum chemical calculations.
polarizabilities for chromophores 2a–9a, 3b–5b, 8b, and 9b embed-
ded into a photopolymer.
X-ray crystallographic analysis showed that 5,6-diphenyl
pyrazine substituents in series a were forced out of the mean
μ
[D]
EHOMO ELUMO ΔEHOMO–LUMO βav (AM1) βav (exp.)
chromophore plane and, therefore, were less engaged in the
ground-state ICT (assessed by the quinoid character of the
appended 1,4-phenylenes). CV, RDV, and polarography
revealed two close one-electron oxidations centered on the
dimethylamino donors, as well as one-electron reduction
processes occurring on the pyrazine-2,3-dicarbonitrile
moiety. Whereas the first oxidation potentials across the
whole series of chromophores did not show significant
changes, the first reduction potentials changed substan-
tially, as a result of π-linker elongation. The electrochemical
gaps in series a/b decreased from 2.80/1.91 V (2a/3b) and
steadied at 1.74/1.52 V (7a/5b onward), as a direct conse-
quence of the reduction in the D–A conjugation between
the D and A chromophore units. These observations were
further confirmed by UV/Vis spectroscopy in which chro-
mophores with larger π-linkers (7a/8b onward) showed
weak or hypsochromically shifted CT bands when com-
pared with chromophores 3a–5a/3b–5b. However, an ad-
ditional triple bond in series b demonstrated a beneficial
role for overall chromophore planarity on the efficiency of
D–A conjugation. In series a, the experimental and calcu-
lated β values showed a similar trend, as seen in the electro-
chemical and UV/Vis data. The less-evaluated series b
showed a typical trend in rising hyperpolarizability with in-
creased π-linker length of up to 3.12 pmVϪ1 (9b). We be-
lieve that these structure–property relationships will serve
as guidelines in the design and development of heterocyclic
chromophores with prospective application in optoelec-
tronic components or electrochemical sensors, particularly
in light of their considerable thermal and chemical robust-
ness.
[eV]
[eV]
[eV]
[esuϫ10–30
]
[pmVϪ1
]
2a
3a
4a
5a
6a
7a
8a
9a
3b
4b
5b
8b
9b
6.06
9.91
9.07
–9.39
–8.40
–8.28
–8.23
–8.19
–8.35
–8.15
–8.28
–8.33
–8.32
–8.38
–8.15
–8.29
–1.45
–1.43
–1.78
–1.69
–1.65
–1.75
–1.70
–1.78
–1.75
–1.92
–2.01
–2.00
–2.07
7.94
6.97
6.51
6.54
6.55
6.60
6.45
6.51
6.58
6.39
6.38
6.15
6.23
66
349
408
787
590
788
832
334
942
956
1444
1484
1912
0
0.5
0.6
1.1
1.0
1.0
1.0
0.7
1.1
1.2
1.4
3.0
3.1
10.69
10.67
10.46
10.79
9.92
11.89
11.49
11.30
11.38
11.27
HOMO is localized on the terminal N,N-dimethylanilino
donor moieties and partially spread over the π-conjugated
system, the LUMO is localized on the pyrazine-2,3-dicarbo-
nitrile acceptor moiety. In the chromophore series from
3a/b to 9a/b, the HOMO and the LUMO became clearly
separated. This trend further confirms the results from elec-
trochemistry and absorption spectroscopy. Moreover, the
AM1-calculated and electrochemically measured HOMO–
LUMO gaps show a good linear correlation, as shown in
Figure S21 in the Supporting Information. The THDF-cal-
culated βav (AM1) and experimental βav (exp.) hyperpolariz-
abilities are in good agreement and match the same trend.
In series a, the measured hyperpolarizabilities increase up
to 1.12 pmVϪ1 (5a) and became steady at about
0.95 pmVϪ1 for 7a/8a (9a was an outlier) similar to that
seen in the electrochemical measurements. Chromophore
series b shows a clear trend in rising β as the length of the
π-conjugated path increase. A comparison of series a and b
demonstrates the impact of the overall chromophore plan-
arity on hyperpolarizability. In general, chromophores in
series b, with the additional triple bond between the π-
linker and the pyrazine-2,3-dicarbonitrile, showed higher
calculated and experimental hyperpolarizability values. The
highest β values were calculated and measured for the fully
Experimental Section
Materials and General Methods: Reagents and solvents were rea-
gent grade and were purchased from Penta, Aldrich, and Acros
and used as received. Starting dimethylamino-substituted π-link-
ers[15] and compounds 10,[10] 14,[14b] and 15[12a] were synthesized
planar chromophore 9b, which featured three alternating according to literature procedures. For the synthesis and full spec-
tral characterization of compounds 13, 16, and 17–25, see the Sup-
porting Information. Pyrazines 2a,[24] 3a,[9b] and 4a[9b] were pre-
viously reported in the literature; however, their full spectral char-
acterization can be found in the Supporting Information. The gene-
ral method for the condensation reaction of 1,2-diketones and
1,4-phenylene and ethynylene subunits.
Conclusions
Two series of new D–π–A chromophores, in which two DAMN can be found in the Supporting Information. Column
chromatography was carried out with silica gel 60 (particle size
0.040–0.063 mm, 230–400 mesh; Merck) and commercially avail-
able solvents. TLC was conducted on aluminum sheets coated with
silica gel 60 F254, obtained from Merck, with visualization by a
UV lamp (254 or 360 nm). Melting points were measured on a
Büchi B-540 melting-point apparatus in open capillaries. 1H and
13C NMR spectra were recorded at 400 and 100 MHz, respectively,
with a Bruker AVANCE 400 instrument at 25 °C. Chemical shifts
are reported in ppm relative to the signal of Me4Si. The residual
solvent signal in the 1H and 13C NMR spectra was used as an
dimethylamino donors and the pyrazine-2,3-dicarbonitrile
acceptor moieties were separated by unsaturated π-linkers
of different size and shape, were prepared. The π-linker con-
sisted of 1,4-phenylene and ethynylene subunits. In contrast
to series a, series b possessed an additional triple bond,
which separated the π-linker and the pyrazine-2,3-dicarb-
onitrile moiety, further planarized the entire π-conjugated
system, and improved D–A conjugation. The properties of
newly synthesized, CT chromophores were studied by X-
ray analysis, electrochemistry, absorption spectroscopy, internal reference (CDCl3: δ = 7.25 and 77.23 ppm; [D6]DMSO: δ
Eur. J. Org. Chem. 2012, 529–538
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
535

F. Buresˇ et al.
FULL PAPER
= 2.55 and 39.51 ppm). Apparent resonance multiplicities are de-
scribed as s (singlet), br. s (broad singlet), d (doublet), and m (mul-
tiplet). Due to the limited solubility of chromophores 8a, 4b, 5b,
8b, and 9b in common deuterated solvents, some 13C NMR spectra
were not (8a) or partially measured (4b–9b, CH and CH3 carbon
atoms were only observed). IR spectra were recorded on a Perkin–
Elmer FTIR Spectrum BX spectrometer. Mass spectra were
measured on a GC-MS configuration comprised of an Agilent
Technologies 6890N gas chromatograph equipped with a 5973 Net-
work MS detector (EI 70 eV, mass range 33–550Da) or on an LC-
MS Micromass Quattro Micro API (Waters) instrument with a di-
rect input (ESI, CH3OH, mass range 200–1000 Da). Elemental
analyses were performed on an EA 1108 Fisons instrument. UV/
Vis spectra were recorded on a Hewlett–Packard 8453 spectropho-
tometer in CH2Cl2.
1590, 1523, 1505, 1356, 1133, 935, 839, 811, 733 cm–1. UV/Vis
(CH2Cl2): λmax (ε) = 433 (30400), 316 nm (48800 mol–1 dm3 cm–1).
MS (ESI): m/z (%) = 591 [M+ + 23]. C38H28N6 (568.67): calcd. C
80.26, H 4.96, N 14.78; found C 80.44, H 5.01, N 14.85.
Chromophore 6a: The title compound was synthesized from 16
(100 mg, 0.187 mmol) and 4Ј-(dimethylamino)biphenyl-4-ylboronic
acid pinacol ester (127 mg, 0.393 mmol) by following general
method A; yield 78 mg (62%); red solid; Rf = 0.20 (SiO2; CH2Cl2/
hexane, 3:1); m.p. 268 °C (dec.). 1H NMR (400 MHz, CDCl3,
25 °C, TMS): δ = 7.72–7.62 (m, 16 H, Ar), 7.54 [d, 3J(H,H) =
3
8.8 Hz, 4 H, Ar], 6.81 [d, J(H,H) = 9.0 Hz, 4 H, Ar], 3.00 [s, 12
H, N(CH3)2] ppm. 13C NMR (100 MHz, CDCl3, 25 °C. TMS): δ
= 155.1, 150.4, 143.9, 137.0, 134.0, 130.6, 129.6, 128.4, 127.8, 127.6,
127.4, 126.9, 122.5, 114.6, 112.9, 40.8 ppm. IR (neat): ν = 2223
˜
(CN), 1597, 1502, 1360, 1208, 947, 810, 731 cm–1. UV/Vis
(CH2Cl2): λmax (ε) = 402 (27300), 319 nm (49900 mol–1 dm3 cm–1).
MS (ESI): m/z (%) = 695 [M+ + 23]. C46H36N6 (672.82): calcd. C
82.12, H 5.39, N 12.49; found C 82.28, H 5.44, N 12.56.
X-ray Analysis: CCDC-823674 (for 10), -823672 (for 2a), and
-823673 (for 3a) contain the supporting crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. See also the Supporting Information.
Chromophore 7a: The title compound was synthesized from 16
(100 mg, 0.187 mmol) and 4Ј-ethynyl-N,N-dimethylbiphenyl-4-
amine (87 mg, 0.393 mmol) by following general method B; yield
89 mg (66%); orange solid; Rf = 0.35 (SiO2; CH2Cl2/hexane, 3:1);
General Method A: Suzuki–Miyaura Cross-Coupling on 16 (Chro-
mophores 4a, 6a, and 8a): Pyrazine 16 (100 mg, 0.187 mmol), the
corresponding boronic acid pinacol ester (0.393 mmol), and THF/
H2O (25 mL, 4:1) were placed in a flame-dried Schlenk flask. Ar-
gon was bubbled through the resulting solution for 10 min where-
upon [Pd(PPh3)2Cl2] (5 mg, 0.007 mmol) and Na2CO3 (42 mg,
0.4 mmol) were added and the reaction was stirred at 65 °C for
12 h. The reaction was cooled to 25 °C, diluted with water, and
extracted with CH2Cl2 (3ϫ50 mL). The combined organic extracts
were washed with water, dried (Na2SO4), and the solvent was evap-
orated. The crude product was purified by column chromatography
(SiO2; indicated solvent system).
1
m.p. 310–315 °C. H NMR (400 MHz, CDCl3, 25 °C, TMS): δ =
3
7.57–7.51 (m, 20 H, Ar), 6.78 [d, J(H,H) = 8.4 Hz, 4 H, Ar], 3.00
[s, 12 H, N(CH3)2] ppm. 13C NMR (100 MHz, CDCl3, 25 °C,
TMS): δ = 154.7, 150.5, 142.0, 134.4, 132.4, 132.2, 130.0, 129.9,
128.0, 127.8, 127.2, 126.2, 120.0, 113.3, 112.9, 93.6, 88.9, 40.7 ppm.
IR (neat): ν = 2210 (CN), 1608, 1593, 1505, 1355, 1138, 1060, 1014,
˜
941, 838, 808 cm–1. UV/Vis (CH2Cl2): λmax (ε) = 404 (37800),
336 nm (64200 mol–1 dm3 cm–1). MS (ESI): m/z (%) = 743 [M+
+
23]. C50H36N6 (720.68): calcd. C 83.31, H 5.03, N 11.66; found C
82.98, H 5.01, N 11.58.
General Method B: Sonogashira Cross-Coupling on 16 (Chromo-
phores 5a, 7a, and 9a): Pyrazine 16 (100 mg, 0.187 mmol), the corre-
sponding terminal acetylene (0.393 mmol), THF (10 mL), and tri-
ethylamine (5 mL) were placed in a flame-dried Schlenk flask. Ar-
gon was bubbled through the resulting solution for 10 min where-
upon [Pd(PPh3)2Cl2] (5 mg, 0.007 mmol) and CuI (3 mg,
0.015 mmol) were added and the reaction was stirred at 50 °C for
12 h. The solvent was evaporated in vacuo and the crude product
was purified by column chromatography (SiO2; indicated solvent
system).
Chromophore 8a: The title compound was synthesized from 16
(100 mg, 0.187 mmol) and 4-{[4-(dimethylamino)phenyl]ethynyl}-
phenylboronic acid pinacol ester (136 mg, 0.393 mmol) by follow-
ing general method A. The crude reaction mixture was filtered and
the crude product was washed with CH2Cl2; yield 98 mg (73%);
poorly soluble orange solid; Rf = 0.30 (SiO2; CH2Cl2/hexane, 3:1);
m.p. Ͼ350 °C. 1H NMR (400 MHz, [D6]DMSO, 25 °C, TMS): δ =
7.89 [d, J(H,H) = 8.4 Hz, 4 H, Ar], 7.83 [d, J(H,H) = 8.4 Hz, 4
H, Ar], 7.71 [d, 3J(H,H) = 8.4 Hz, 4 H, Ar], 7.61 [d, 3J(H,H) =
8.4 Hz, 4 H, Ar], 7.42 [d, 3J(H,H) = 8.4 Hz, 4 H, Ar], 6.76 [d,
3J(H,H) = 8.4 Hz, 4 H, Ar], 3.00 [s, 12 H, N(CH3)2] ppm. 13C
NMR: could not be measured due to the low solubility of 8a in
3
3
General Method C: Sonogashira Cross-Coupling on 10 (Chromo-
phores 3b–9b): Dichloropyrazine 10 (100 mg, 0.503 mmol), the cor-
responding terminal acetylene (1.005 mmol), and THF (25 mL)
were placed in a flame-dried Schlenk flask. Argon was bubbled
through the resulting solution for 10 min whereupon [Pd(PPh3)2-
Cl2] (14 mg, 0.02 mmol), CuI (8 mg, 0.04 mmol), and triethylamine
(1.0 mL) were added and the reaction was stirred at 60 °C for 2 h.
The solvent was evaporated in vacuo and the crude product was
purified by column chromatography (SiO2; indicated solvent sys-
tem).
available deuterated solvents. IR (neat): ν = 2205 (CN), 1598, 1523,
˜
1365, 1196, 1135, 1064, 944, 825, 812 cm–1. UV/Vis (CH2Cl2): λmax
(ε) = 400 (sh, 14300), 341 nm (24000 mol–1 dm3 cm–1). MS (ESI):
m/z (%) = 743 [M+ + 23]. C50H36N6 (720.68): calcd. C 83.31, H
5.03, N 11.66; found C 83.56, H 5.05, N 11.67.
Chromophore 9a: The title compound was synthesized from 16
(100 mg, 0.187 mmol) and 4-[(4-ethynylphenyl)ethynyl]-N,N-di-
methylaniline (96 mg, 0.393 mmol) by following general method B;
yield 122 mg (85%); orange solid; Rf = 0.80 (SiO2; CH2Cl2); m.p.
Chromophore 5a: The title compound was synthesized from 16
(100 mg, 0.187 mmol) and 4-ethynyl-N,N-dimethylaniline (57 mg,
0.393 mmol) by following general method B; yield 91 mg (85%); 7.51 (m, 8 H, Ar), 7.47 (br. s, 8 H, Ar), 7.39 [d, J(H,H) = 8.8 Hz,
1
300–305 °C. H NMR (400 MHz, CDCl3, 25 °C, TMS): δ = 7.57–
3
red solid; Rf = 0.30 (SiO2; CH2Cl2/hexane, 1:1); m.p. 270–271 °C.
4 H, Ar], 6.65 [d, 3J(H,H) = 8.8 Hz, 4 H, Ar], 2.99 [s, 12 H,
3
1H NMR (400 MHz, CDCl3, 25 °C, TMS): δ = 7.52 [d, J(H,H) = N(CH3)2] ppm. 13C NMR (100 MHz, CDCl3, 25 °C, TMS): δ =
8.8 Hz, 4 H, Ar], 7.46 [d, 3J(H,H) = 8.8 Hz, 4 H, Ar], 7.39 [d,
154.6, 150.5, 142.9, 134.7, 133.1, 132.3, 131.9, 131.5, 130.1, 130.0,
3
3J(H,H) = 8.8 Hz, 4 H, Ar], 6.64 [d, J(H,H) = 8.8 Hz, 4 H, Ar], 126.9, 125.1, 121.5, 113.3, 112.0, 91.7, 90.0, 87.8, 87.4, 40.4 ppm.
3.00 [s, 12 H, N(CH3)2] ppm. 13C NMR (100 MHz, CDCl3, 25 °C, IR (neat): ν = 2201 (CN), 1606, 1593, 1521, 1510, 1358, 1261, 1121,
˜
TMS): δ = 154.7, 150.7, 133.8, 133.2, 131.7, 129.9, 129.7, 129.0, 1061, 1015, 936, 837, 812 cm–1. UV/Vis (CH2Cl2): λmax (ε) = 402
113.4, 112.0, 109.3, 95.0, 87.1, 40.4 ppm. IR (neat): ν = 2194 (CN), (sh, 49400), 361 nm (68400 mol–1 dm3 cm–1). MS (ESI): m/z (%) =
˜
536
www.eurjoc.org
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2012, 529–538

Donor-Substituted Dicyanopyrazine-Derived Push–Pull Chromophores
791 [M+ + 23]. C54H36N6 (768.90): calcd. C 84.35, H 4.72, N 10.93;
found C 83.89, H 4.68, N 10.86.
general method C; yield 172 mg (42%); red solid; Rf = 0.30 (SiO2;
CH2Cl2/hexane, 3:1); m.p. 307 °C (dec.). 1H NMR (400 MHz,
CDCl3, 25 °C, TMS): δ = 7.64 [d, 3J(H,H) = 8.8 Hz, 4 H, Ar], 7.58
[d, 3J(H,H) = 8.8 Hz, 4 H, Ar], 7.48 (br. s, 8 H, Ar), 7.40 [d,
Chromophore 3b: The title compound was synthesized from 10
(100 mg, 0.503 mmol) and 4-ethynyl-N,N-dimethylaniline (146 mg,
1.005 mmol) by following general method C; yield 165 mg (79%);
dark-red metallic solid; Rf = 0.95 (SiO2; CH2Cl2); m.p. 220 °C
(dec.). 1H NMR (400 MHz, [D6]DMSO, 25 °C, TMS): δ = 7.59 [d,
3
3J(H,H) = 8.8 Hz, 4 H, Ar], 6.65 [d, J(H,H) = 8.8 Hz, 4 H, Ar],
3.00 [s, 12 H, N(CH3)2] ppm. 13C NMR (100 MHz, CDCl3, 25 °C,
TMS): δ = 133.4, 132.4, 131.7, 131.8, 131.7, 112.2, 40.4 (15 signals
missing) ppm. IR (neat): ν = 2190 (CN), 1591, 1520, 1484, 1388,
˜
3
3J(H,H) = 8.8 Hz, 4 H, Ar], 6.87 [d, J(H,H) = 8.8 Hz, 4 H, Ar],
1363, 1166, 1123, 1100, 945, 836, 809 cm–1. UV/Vis (CH2Cl2): λmax
(ε) = 371 (sh, 93300), 297 nm (sh, 48200 mol–1 dm3 cm–1). MS (ESI):
m/z (%) = 817 [M+ + 1]. C58H36N6 (816.95): calcd. C 85.27, H 4.44,
N 10.29; found C 84.98, H 4.39, N 10.15.
3.10 [s, 12 H, N(CH3)2] ppm. 13C NMR (100 MHz, [D6]DMSO,
25 °C, TMS): δ = 154.4, 151.9, 145.0, 134.2 133.5, 112.2, 104.5,
95.4, 81.8, 45.8 ppm. IR (neat): ν = 2165 (CN), 1602, 1529, 1472,
˜
1368, 1152, 1059, 945, 826, 811 cm–1. UV/Vis (CH2Cl2): λmax (ε) =
499 (24700), 442 (25000), 315 (sh, 12200), 285 nm
(22600 mol–1 dm3 cm–1). MS (ESI): m/z (%) = 439 [M+ + 23].
C26H20N6 (416.48): calcd. C 74.98, H 4.84, N 20.18; found C 75.16,
H 4.88, N 20.25.
Supporting Information (see footnote on the first page of this arti-
cle): Synthesis and characterization of compounds 10, 13, 15–25,
2a, 3a/b, and 4a; full electrochemical data including representative
CV and polarographic curves; UV/Vis spectra, including proton-
ation/neutralization experiments; further crystallographic data;
NLO measurements and calculations including representative
HOMO/LUMO localizations; and correlation of calculated and
electrochemical HOMO–LUMO gaps.
Chromophore 4b: The title compound was synthesized from 10
(100 mg, 0.503 mmol) and 4Ј-ethynyl-N,N-dimethylbiphenyl-4-
amine (222 mg, 1.005 mmol) by following general method C; yield
177 mg (62%); dark-red metallic solid; Rf = 0.45 (SiO2; CH2Cl2/
hexane, 3:1); m.p. Ͼ 350 °C. 1H NMR (400 MHz, CDCl3, 25 °C,
3
3
TMS): δ = 7.69 [d, J(H,H) = 8.8 Hz, 4 H, Ar], 7.64 [d, J(H,H) =
8.8 Hz, 4 H, Ar], 7.56 [d, 3J(H,H) = 8.8 Hz, 4 H, Ar], 6.79 [d,
3J(H,H) = 8.8 Hz, 4 H, Ar], 3.02 [s, 12 H, N(CH3)2] ppm. 13C
NMR (100 MHz, CDCl3, 25 °C, TMS): δ = 133.7, 128.1, 126.5,
Acknowledgments
This research was supported by the Ministry of Education, Youth
and Sport of the Czech Republic (grant numbers MSM
0021627501, LA 09041, and LC 06035). Calculations have been
carried out in the Wroclaw Centre for Networking and Supercom-
puting, http://www.wcss.wroc.pl (grant number 135).
112.8, 40.7 (9 signals missing) ppm. IR (neat): ν = 2182 (CN), 1590,
˜
1499, 1485, 1392, 1362, 1190, 1163, 1063, 943, 850, 809 cm–1. UV/
Vis (CH2Cl2): λmax (ε)
=
465 (35500), 322 nm (sh,
48300 mol–1 dm3 cm–1). MS (ESI): m/z (%) = 569 [M+ + 1].
C38H28N6 (568.67): calcd. C 80.26, H 4.96, N 14.78; found C 80.65,
H 5.03, N 14.80.
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Chromophore 5b: The title compound was synthesized from 10
(100 mg, 0.503 mmol) and 4-[(4-ethynylphenyl)ethynyl]-N,N-di-
methylaniline (246 mg, 1.005 mmol) by following general method
C; yield 211 mg (68%); dark-violet metallic solid; Rf = 0.45 (SiO2;
CH2Cl2/hexane, 3:1); m.p. 301 °C (dec.). 1H NMR (400 MHz,
CDCl3, 25 °C, TMS): δ = 7.61 [d, 3J(H,H) = 8.8 Hz, 4 H, Ar], 7.53
[d, 3J(H,H) = 8.8 Hz, 4 H, Ar], 7.41 [d, 3J(H,H) = 8.8 Hz, 4 H,
3
Ar], 6.65 [d, J(H,H) = 8.8 Hz, 4 H, Ar], 3.01 [s, 12 H, N(CH3)2]
ppm. 13C NMR (100 MHz, CDCl3, 25 °C, TMS): δ = 133.3, 132.9,
131.8, 112.0, 40.3 (11 signals missing) ppm. IR (neat): ν = 2189
˜
(CN), 1590, 1525, 1487, 1389, 1366, 1166, 1130, 1062, 943, 832,
812 cm–1. UV/Vis (CH2Cl2): λmax (ε) = 466 (36800), 339 nm (sh,
61200 mol–1 dm3 cm–1). MS (ESI): m/z (%) = 617 [M+ + 1].
C42H28N6 (616.71): calcd. C 81.80, H 4.58, N 13.63; found C 81.95,
H 4.62, N 13.71.
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Chromophore 8b: The title compound was synthesized from 10
(100 mg, 0.503 mmol) and 23 (323 mg, 1.005 mmol) by following
general method C; yield 131 mg (34%); red solid; Rf = 0.30 (SiO2;
CH2Cl2/hexane, 3:1); m.p. 321 °C (dec.). 1H NMR (400 MHz,
CDCl3, 25 °C, TMS): δ = 7.77 [d, 3J(H,H) = 8.8 Hz, 4 H, Ar], 7.70
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Chromophore 9b: The title compound was synthesized from 10
(100 mg, 0.503 mmol) and 25 (347 mg, 1.005 mmol) by following
Eur. J. Org. Chem. 2012, 529–538
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Received: August 22, 2011
Published Online: November 30, 2011
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