Diaryldistyrylpyrazines: Solvatochromic and acidochromic fluorophores

Article abstract of DOI:10.1002/ejoc.201300463

Diaryldimethylpyrazines are the starting materials for the synthesis of C₂-symmetric donor- or acceptor-substituted distyrylpyrazines. The optical properties of these cruciform-shaped dyes are dominated by the distyrylpyrazine units; the photop

Full text of DOI:10.1002/ejoc.201300463

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FULL PAPER
Pyrazine Fluorophores
V. Schmitt, S. Moschel,
H. Detert* ...................................... 1–16
Diaryldistyrylpyrazines:
Solvatochromic
and Acidochromic Fluorophores
Centrosymmetric diaryldistyrylpyrazines
with terminal acceptor and donor groups
have been prepared. The electronic spectra
are highly sensitive towards changes in the
environment: solvatochromism and huge
Stokes shifts result from large dipole mo-
ments of the C2-symmetric dyes. Pro-
tonation induces multiple alterations of ab-
sorption and emission, thus allowing op-
tical sensing of the pH and polarity.
Keywords: Solvatochromism / Acidochro-
mism / Fluorescence / Nitrogen hetero-
cycles / Conjugation
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V. Schmitt, S. Moschel, H. Detert
FULL PAPER
DOI: 10.1002/ejoc.201300463
Diaryldistyrylpyrazines: Solvatochromic and Acidochromic Fluorophores
Volker Schmitt,[a] Sebastian Moschel,[a] and Heiner Detert*[a]
Dedicated to Professor Wolfgang Liptay on the occasion of his 85th birthday
Keywords: Solvatochromism / Acidochromism / Fluorescence / Nitrogen heterocycles / Conjugation
Diaryldimethylpyrazines are the starting materials for the
synthesis of C2-symmetric donor- or acceptor-substituted
distyrylpyrazines. The optical properties of these cruciform-
shaped dyes are dominated by the distyrylpyrazine units; the
photophysics is controlled by the styryl substitution, the di-
aryl substituents on the central pyrazine only having a small
effect. Protonation occurs on the pyrazine and/or lateral
amines or azines, thereby altering the absorption and
emission properties. Hypso- and bathochromism as well as
fluorescence quenching depend on the nature of the terminal
substituent. This, and a significant positive solvatochromism
of the fluorescence, allow optical sensing of the pH and
polarity of the environment.
tical transparency of tissues from 650 to 900 nm allows exci-
tation of the fluorescent probe by two-photon absorption.[9]
The replacement of a benzene ring in a DSB by pyridine
or related azines results, as an EPA substituent, in enhanced
electron affinity.[10] Because these rings can act as bases or
ligands, they provide a further pathway to influence optical
properties.[11] Similarly, amines act as bases and acceptors
for hydrogen bridging. Whereas the protonation or quat-
ernization of amines changes their character from donor
to acceptor, the acceptor strength of pyridine and related
moieties is greatly enhanced upon protonation or quaterniz-
ation.[12] Complexation with Lewis acids or protonation has
been shown to cause significant changes in the electronic
spectra of π-conjugated chromophores with basic sites at
terminal positions.[11] As triarylamines are very poor bases
they are protonated only under extreme forcing conditions,
thus preserving their electron-donating effect even in the
presence of strong acids.
As part of our interest in fluorophores with switchable
optical properties and efficient two-photon absorption,[2b,5]
we report herein the synthesis of C2-symmetrical 2,5-distyr-
ylpyrazines (DSPs) with central diaryl substitution. A few
donor-substituted distyrylpyrazines are known,[13] and
some of their photophysical[14] and solid-state properties[15]
have been studied. The generally applied synthetic routes
to styrylpyrazines are based on the Lewis acid catalyzed
condensation of aromatic aldehydes[16] and on the depro-
tonation of methylpyrazines and aldol condensation with
aromatic aldehydes. Variations like the Siegrist reaction[17]
or phase-transfer conditions[18] proved to be advantageous
in specific preparations.
Introduction
Organic semiconductors of the distyrylbenzene type
(DSB) have great potential as active materials in optoelec-
tronic devices,[1] as nonlinear optical materials,[2] and as
sensors.[3] Two strategies have been successfully applied to
adjust the (nonlinear) optical and electronic properties of
the fundamental chromophore. Although the extension of
the conjugated system is of limited effect,[4] substitution
with electron-pair donating (EPD) and/or withdrawing
(EPA) groups[5] allows marked alterations of the absorption
and fluorescence maxima, quantum yield, two-photon
cross-section, ionization potential, and electron affinity.
Furthermore, the substituents can be designed to allow in-
teraction with external stimuli.[6] Electronic coupling be-
tween these EPA/EPD groups and the π system can result
in optical responses like shifts of absorption and emission
maxima or fluorescence quantum yields.
The optical sensing of polarity, ions, pH, and other sol-
utes with fluorescent dyes has become an important tool
for the analysis of local properties and in imaging.[7] Appli-
cations range from the fuel industry to the investigation of
biological tissues.[8] Fluorescent probes can be used in solu-
tion[7c] or, immobilized in polymeric matrices,[7a,8a] in flow
cells. Dyes with an emission above 550 nm are preferred for
biological sensing because they do not interfere with the
autofluorescence of tissues. Furthermore, the enhanced op-
[a] Institute for Organic Chemistry, Johannes Gutenberg-
University Mainz,
Duesbergweg 10–14, 55099 Mainz, Germany
E-mail: detert@uni-mainz.de
Homepage: http://www.blogs.uni-mainz.de/fb09ak-detert/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300463.
In these DSP chromophores, pyrazine acts as an electron
acceptor resulting in a donor–acceptor–donor electronic
2
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Diaryldistyrylpyrazines
structure. The internal charge transfer depends on the rela- Scheme 2, Table 1). No DSP was obtained in the reactions
tive strength of the EPA/EPD substituents and is tuned by with the electron-deficient p-cyanobenzaldehyde. Oxidation
external stimuli. To study the impact of polar and protic of the dimethylpyrazines 17a,c with SeO2 led to the corre-
environments on the electronic transitions, the substitution sponding pyrazinedicarbaldehydes 19a,c (56%) and subse-
on the termini has been systematically varied from (basic) quent two-fold Horner olefination allowed the direct con-
acceptor to basic and nonbasic donor as has the donor sub- densation to be bypassed resulting in DSPs 2, 11, and 12
stitution on the central diphenylpyrazine unit. Interactions with cyano- and α-quinolyl end-groups.
of these di- or multipolar compounds with the environment
can result in optical responses that are useful for sensing
purposes.[19]
Structures of Distyrylpyrazines
The structures 1–12 were determined by spectroscopic
methods, including 2D NMR spectroscopy. After
Results
chromatography and recrystallization, all the compounds
were isolated as (E,E)-DSPs without detectable traces of
Synthesis
The general strategy for the construction of centrosym- other isomers.
metric diaryldistyrylpyrazines 1–12 started with the synthe-
Single crystals of compound 5 were obtained by slow
sis of 2,5-dimethylpyrazines 17 with appropriate 3,6-diaryl evaporation of a solution of dichloromethane/methanol. In
substitution. A base-induced condensation of the methyl contrast to the centrosymmetry of the molecular structure
groups with suitable benzaldehydes 18 yielded the title com- in solution, the molecule adopts a noncentrosymmetric
pounds. For the synthesis of dimethyldiarylpyrazines 17, conformation in the crystal (Figure 1). Steric crowding due
ethyl acetoacetate was first nitrosated in aqueous solution. to four substituents on the pyrazine results in large dihedral
A copper-catalyzed Sandmeyer-like coupling with diazo- angles of –129.2° (N1–C2–C7–C8) and –131.4° (N4–C5–
tized anilines yielded the 1-aryl-substituted hydroxyimi- C13–C18) in the teraryl axis. These distortions are slightly
noacetones 14a–c (Scheme 1). These compounds can also larger than those reported for tetraphenylpyrazine (41.1 and
be prepared by nitrosation of arylacetones 13a,b,d in non- 48.8°).[20] Compared with the terphenyl axis, the distyryl-
aqueous solution. Reduction of the hydroxyimino ketones pyrazine system is more planar. Nevertheless, the dihedral
14 in alkaline solution led to the 2,5-diaryl-substituted het- angles between the pyrazine and vinylene moieties of
erocycles 16 by condensation of the initially formed amino –164.5° (C2–C3–C19–C20) and –175.3° (C5–C6–C34–C35)
ketones 15. Aromatization to 17 was achieved by the oxi- and those between the vinylene and aniline ring of –162.8°
dation of the dihydropyrazines 16 with air in moderate (C19–C20–C21–C26) and –171.8° (C34–C35–C36–C37) are
yields. Pyrazine 17e with terminal carbazole units was ob- significantly larger than those found in a similar compound
tained in 80% yield by Ullmann condensation of the bromo with central 3,6-dimethyl (Ͻ3.5°) or distyryl substitution
compound 17b with carbazole.
(0.3°).[21] The sums of angles around the aniline N atoms
A variety of base/solvent systems were studied for the of 358.8° (N27) and 359.5° (N42), the dihedral angles be-
two-fold base-induced condensation of the dimethylpyr- tween the dialkylamino groups and benzene rings of nearly
azines 17 with benzaldehydes 18; the best results were ob- 179° (C40–C39–N42–C43: –173.8°; C25–C24–N27–C28:
tained with potassium tert-butoxide in DMF. Nevertheless, 173.9), and the short aniline C–N bonds of 1.381 Å (C39–
even under these optimized conditions, the yields of the de- N42) and 1.377 Å (C24–N27) indicate a strong electronic
sired DSPs 1–10 varied from poor to good (19–78%; coupling between the peripheral donor groups and the cen-
Scheme 1. Synthesis of 2,5-dimethyl-3,6-diarylpyrazines 17. a: Ar = phenyl; b: Ar = 4-bromophenyl; c: Ar = 4-methoxyphenyl; d: Ar =
1-naphthyl.
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V. Schmitt, S. Moschel, H. Detert
FULL PAPER
Scheme 2. Synthesis of 2,5-distyryl-3,6-diarylpyrazines 1–12.
Table 1. Substitution pattern of DSPs 1–12.
tral pyrazine. All the structural information indicates that
the main conjugation path extends along the bis(amino-
styryl)pyrazine axis.
R1
R2
R3
Yield [%] Color
1
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
CN
64
71
67
32
57
19
29
21
39
78
48
85
yellow
yellow
yellow
yellow
orange
red
yellow
orange
red
2
3
OC8H17
9-Carbaz.[a]
N(C3H7)2
N-Crown[b]
Optical Properties
4
5
The fluorophores 1–12 form yellow, orange, or red solids,
and also colored and fluorescent solutions. Increasing do-
nor strength correlates with a more intense color in solu-
tion. Diphenyldistyrylpyrazine 1 represents the fundamen-
tal chromophore of the series 1–12. In cyclohexane, the
6
7
naphth.[c] OC6H13
naphth.[c] N(C3H7)2
8
9
OCH3
9-Carbaz.
H
H
H
H
H
N(C3H7)2
N(CH3)2
10
11
12
orange
orange
yellow
long-wavelength absorption band of 1 peaks at λmax
=
OCH3
398 nm and emission occurs at λFmax = 442 nm. A compari-
son of the optical data of 1 with those of simple 2,5-distyr-
[a] 9-carbaz. = 9-carbazolyl. [b] N-crown = 1,4,7,10-tetraoxa-13-
aza-13-cyclopentadecyl. [c] Phenyl replaced by α-naphthyl.
ylpyrazine lacking the diphenyl substitution (λmax
=
Figure 1. Molecular structure of 5.
4
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Diaryldistyrylpyrazines
382 nm, λF
= 424 nm in cyclohexane[14e]) reveals a sig- deficient quinoline and pyrazine units show that methoxy
max
nificant influence of these phenyl rings: Both transitions in donors on the central teraryl axis have a significant effect
1 are redshifted by about 17 nm (ΔνAbs = 1052 cm–1, ΔνF = on the spectra. Whereas the electronic transitions of 11 oc-
˜
˜
960 cm–1; Table 2).
cur at λmax = 408 nm and λFmax = 456 nm, methoxy substi-
tution on the teraryl axis (12) shifts these maxima by about
Δλmax = 13 nm (ΔνAbs = 757 cm–1) and ΔλF
= 23 nm
Table 2. Optical properties and solvatochromism of 1–12.
˜
max
(ΔνF = 1053 cm–1) to lower energies.
˜
Comp.: λ [nm] CH Tol DCM AN
Φ
EtOH
logεmax
[m–1 cm–1] in DCM
The fluorescence quantum yields of DSPs 1, 2, and 11
without donor substitution are low (Φ = 0.02–0.05 in cyclo-
hexane), but even weak donor groups significantly enhance
the quantum yield (Φ = 0.08–0.16 for 3, 7 and 12 in cyclo-
hexane). With Φ = 0.24–0.34, amino-substituted DSPs 4, 5,
and 10 are the most efficient emitters.
1: λmax
max
398 402 401 397
442 447 452 450
0.02 0.03 0.02 0.005
403 408 406 402
453 454 459 460
0.05 0.07 0.06 0.04
413 419 418 414
456 466 477 479
0.16 0.19 0.15 0.09
416 422 419 414
463 475 490 502
0.24 0.31 0.26 0.05
456 468 479 477
502 524 561 570
0.34 0.25 0.11 0.11
455 466 470 468
501 522 536 561
0.19 0.22 0.29 0.17
413 419 417 412
453 464 474 475
0.10 0.15 0.13 0.08
459 474 486 484
505 526 561 575
0.13 0.11 0.06 0.03
455 468 477 475
502 523 556 566
0.12 0.14 0.08 0.06
452 467 471 466
505 527 554 571
0.28 0.27 0.17 0.12
408 412 411 407
456 463 463 464
0.03 0.06 0.06 0.02
421 426 423 421
479 492 493 491
0.08 0.17 0.16 0.13
398
453
0.02
402
458
0.05
414
494
0.13
412
493
0.02
474
584
0.09
467
574
0.10
414
492
0.08
486
583
0.03
474
584
0.04
464
580
0.08
407
463
0.04
422
497
0.16
4.58
λF
Φ
2: λmax
4.64
4.63
4.79
4.82
4.78
4.57
4.73
4.49
4.76
4.76
4.80
λF
max
Φ
Semi-empirical calculations at the INDO/S level[22] gave
absorption bands with significantly higher energies. But the
correlation of substituent effects within the experimental
data and within the theoretical data is striking: Although
diphenyl substitution on the pyrazine ring of distyrylpyraz-
ine causes a redshift (exp./calcd.: Δλ = 16/24 nm), the posi-
tion of the absorption maximum is independent of central
phenyl (3) or naphthyl (7) substitution, whereas exchange
of the central naphthyl groups in 8 by p-anisyl (9) causes a
blueshift (exp./calcd.: Δλ = –7/–4 nm).
3: λmax
λF
max
Φ
4: λmax
λF
max
Φ
5: λmax
λF
max
Φ
6: λmax
λF
max
Φ
7: λmax
λF
max
Φ
Solvatochromism
8: λmax
λF
max
Solvatochromism of the fundamental chromophore 1 as
well as of 2 and 11 is very weak, both in absorption and
emission. The effect of electron-pair donating groups of
moderate strength on the solvatochromic displacement of
the absorption maxima of 3 and 4 is small, but their fluores-
cence is more sensitive: Positive solvatochromism shifts the
Φ
9: λmax
λF
max
Φ
10: λmax
λF
max
Φ
emission to the red (ΔλF
= 38 and 30 nm, ΔνF = 1687
˜
max
11: λmax
and 1314 cm–1, for cyclohexane and ethanol, respectively).
With strong donors on the terminal rings (5, 6, 8–10) a
remarkable increase in the solvatochromic sensitivity can be
λF
max
Φ
12: λmax
λF
max
noted: Displacements of Δλ = 12–19 nm (Δν = 572–
˜
˜
Φ
881 cm–1) in absorption and of Δλ = 73–82 nm (Δν = 2538–
2797 cm–1) in emission separated the maxima recorded in
cyclohexane from those in ethanol (Table 2, Figure 2).
Replacing the central phenyl rings by α-naphthyl groups
(3, 5 and 7, 8) or by donor-substituted anisyl rings (5, 9
and 11, 12) only slightly modifies the solvatochromic char-
acteristics. The fluorescence efficiencies of 1–12 are higher
in solvents of low polarity like cyclohexane and toluene.
The fluorescence efficiencies are reduced with increasing
polarity. This sensitivity is poor for DSPs with terminal ac-
ceptors (1, 2, 11, 12: ca. 75–90%). Dyes 3 and 7 with alkoxy
donors are less emissive in highly polar solvents (ca. 50%)
and the residual fluorescence of amino-substituted DSPs 4–
6 and 8–10 in ethanol or acetonitrile drops to values of 15–
30%.
Symmetrical substitution by cyano groups (2) causes
weak redshifts of absorption (Δλ = 5 nm, ΔνAbs = 250 cm–1)
˜
˜
and fluorescence maxima (Δλ = 11 nm, ΔνF = 549 cm–1),
and the larger π system of the α-quinolyl moiety (11) results
in a slightly greater shift. Groups with a moderate electron-
donating effect (3: O-hexyl; 4: N-carbazolyl) on the styryl
units shift the absorption and fluorescence maxima by 15–
25 nm (ΔνF = 695–1087 cm–1) to the red, but with the
˜
stronger dialkylamino donors (5 and 6), the displacements
in cyclohexane are about three times larger (λmax = 456/
455 nm, Δλ = 58/57 nm, ΔνAbs = 3196/3148 cm–1; λF
=
˜
max
502/501 nm, ΔλF = 60/59 nm, ΔνF = 2704/2664 cm–1). In-
˜
creasing the size of the central aryl moieties from a phenyl
substituent (3 and 5) to the sterically more demanding α-
naphthyl group (7 and 8) or to phenyl groups bearing elec-
tron-donating groups (9 and 10) has only a negligible effect Acidochromism
on the position of the maxima in the electronic spectra
Due to the nitrogen atoms in the central ring, distyrylpy-
(ΔλՅ 4 nm). This is valid as long as the DSP unit carries razine 1 is a weak base. Addition of TFA to solutions of 1
donor substituents. DSPs 11 and 12 composed of electron- in dichloromethane results in the protonation of 1. In
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V. Schmitt, S. Moschel, H. Detert
FULL PAPER
Figure 2. Electronic spectra of 8 in different solvents (fluorescence normalized to identical optical density at λexc = 345 nm; cps =
countss–1).
10–2 m TFA, the UV/Vis spectrum is nearly a 1:1 superposi- 0.2–0.4). Even the second protonated species exhibit signifi-
tion of the spectra of neutral 1 and 1 + H+; in 0.1 m TFA cant fluorescence quantum yields (Φ = 0.14–0.25).
only 1 + H+ is visible, as indicated by a significant redshift
Uniquely, 1 m TFA is required to protonate the carbazole
to λmax = 470 nm (Δλ = 69 nm, Δν = 3661 cm–1; Table 3). analogue 4, with only traces of protonated 4 visible in 0.1 m
˜
The impact of acid on the fluorescence is similar. In 10–2
m
TFA. The fluorescence is more sensitive: The efficiency of
TFA the spectrum is composed of emission mainly from 1 the emission of 4 in 0.1 m TFA is reduced to 30%, and
+ H+ together with the residual emission from 1; in 0.1 m protonated 4 is not fluorescent. In contrast to 4, the nitro-
TFA the fluorescence peaks at 578 nm, a redshift of about gen atoms in the terminal quinolines in DSP 11 are basic
126 nm (Δν = 4823 cm–1) compared with the emission of sites and 11 shows two distinct species (Figure 3). Proton-
˜
the neutral form. Higher concentrations of TFA (1 m) pro- ation starts in 10–4 m TFA and is complete in 10–2 m TFA.
voke further shifts of the absorption up to λ = 484 nm (Δν The absorption maxima are separated by Δλ = 43 nm
˜
= 615 cm–1) and the emission shifts to λF = 579 nm. Proton- (ΔνAbs = 2304 cm–1) and the spectra show an isosbestic
˜
ation results in a reduced extinction coefficient (ca. 50%) point at λ = 430 nm. A second protonation in 1 m TFA
and fluorescence efficiency (ca. 30%). The cyano groups in causes a weak blueshift and hypsochromism. In the emis-
2 reduce the basicity. For both, absorption and emission, sion spectra an isostilbic point (λ = 504 nm) was observed
1 m TFA was needed to observe protonated 2, and the ba- in the transformation of neutral (λF
= 463 nm) to pro-
max
thochromic shifts are very small (Δλ = 10 nm, ΔλF = 34 nm; tonated 11 (λF
= 531 nm, ΔνF = 2766 cm–1). In 10–4
m
˜
max
ΔνAbs = 592 cm–1, ΔνF = 1503 cm–1). The basicity of the TFA, a nearly equimolar equilibrium of both species is ob-
˜
˜
pyrazine is weakly affected by alkoxy donors (3, 7). TFA served. The spectra show that a second protonated form of
(10–2–10–1 m) shifts the absorption maxima to the red (Δλ excited 11 appears in 1 m TFA with an emission maximum
F
= 126–137 nm, Δν = 5541–5930 cm–1) and, because proton- (λF
= 476 nm, Δν = = 590 cm–1) close to that of the
˜
˜
max
ated 3 and 7 are not emissive, quenches the fluorescence. neutral molecule (Figure 3). The methoxy derivative 12
The relative sensitivity of their absorption spectra towards shows slightly enhanced basicity and similar effects of TFA
TFA is 3Ͻ1Ͻ7.
on the absorption, but the fluorescence behavior is com-
Like the absorption maxima, the fluorescence quantum pletely different. In 10–3 m TFA, the emission of the neutral
yields are dependent on the concentration of TFA. The im- form has vanished in favor of a weak band at around λF =
pact of TFA is small for 1, 2, and 11 without donor groups, 600 nm (ΔνF = 3836 cm–1) that decreases at higher concen-
˜
but the quantum yields of DSPs 4 and 7 with terminal do- trations.
nor groups are reduced with increasing TFA concentration,
Contrary to the protonation-induced bathochromism of
with 1 m TFA totally quenching the emission. On the other the absorption and emission of the dyes 1–4 and 7, those
hand, the protonation of the terminal amino groups in 5, with dialkylaminostyryl groups (5, 6, 8–10) are more easily
6, and 9 results first in a decrease in the quantum yield protonated, with the absorption and emission spectra
of the emission of the neutral compound followed by the of the protonated dyes exhibiting peaks at shorter wave-
appearance of a strongly emissive protonated species (Φ = lengths.
6
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Diaryldistyrylpyrazines
Table 3. Acidochromism of 1–12 in dichloromethane/TFA.
Comp.: λ [nm]
TFA [m]
Φ
DCM
10–5
10–4
10–3
10–2
10–1
1
1: λmax
max
Φ
401
452
0.02
406
459
0.05
418
477
0.16
419
490
0.24
479
561
0.11
470
536
0.29
417
474
0.13
486
561
0.06
477
556
0.12
471
554
0.17
411
463
0.03
423
493
0.16
401
452
0.02
406
459
0.04
418
478
0.17
419
491
0.25
479
561
0.11
470
589
0.15
417
474
0.10
486
561
0.05
477
556
0.12
469
563
0.15
411
467
0.03
425
493
0.17
401
452
0.02
406
459
0.04
418
478
0.16
419
492
0.24
479
562
0.10
466
450/589
0.15
417
474
0.11
477
583/446
0.03
477
557
0.12
467
574
0.08
413
464/524
0.03
434
494
0.10
401
452
0.02
406
459
0.04
418
478
0.16
419
493
0.25
473
576
0.05
398
439
0.05
417
474
0.10
395
446
0.05
472
574
0.09
409
582/485
0.10
437
530
0.04
467
590
0.03
416
569
0.015
406
459
0.04
418
478
0.72
419
493
0.24
399
443
0.20
396
455
0.21
519
476
0.02
393
442
0.04
414
472
0.40
398
490
0.10
449
530
0.04
485
603
0.01
470
578
0.01
406
459
0.04
513
479
0.11
419
493
0.08
398
453
0.22
422
514
0.19
534
–
484
579
0.005
416
493
0.05
544
–
λF
2: λmax
λF
max
Φ
3: λmax
λF
max
Φ
0
549
–
4: λmax
λF
max
Φ
0
5: λmax
411
482
0.25
425
517
0.14
554
–
λF
max
Φ
6: λmax
λF
max
Φ
7: λmax
λF
max
Φ
0
0
8: λmax
435
528
0.02
413
481
0.34
402
–
442
532
0.01
454
524
0.15
420
423
0.04
439
476
0.04
497/386
–
λF
max
Φ
9: λmax
λF
max
Φ
10:λmax
λF
max
Φ
0
11:λmax
454
531
0.05
492
608
0.01
λF
max
Φ
12:λmax
λF
max
Φ
0
The electronic properties of compounds 5, 6 and 8–10 basicities were found. Dye 6 with aza-crown end-groups is
are dominated by the 2,5-bis(p-aminostyryl)pyrazine unit, significantly more prone to protonation in its excited state
substituents on the diphenylpyrazine axis only slightly mod- than its congeners: Even in 10–5 m TFA, the spectrum is
ulating the acidochromic behavior. Protonation of DSP 5 dominated by the emission of protonated 6 with residual
starts in 10–3 m TFA, in 10–2 m TFA protonated 5 dominates emission of neutral 6. This maximum disappears only at
the absorption spectrum (ca. 80%), and protonation is TFAՆ10–3 m and the redshift of the maximum (6: Δν =
˜
complete in 0.1 m TFA, 1 m TFA provoking a further pro- 1679 cm–1) is significantly larger than that observed for 5
tonation. In contrast to the previously discussed dyes, pro- and 8–10.
tonation causes a severe blueshift in the absorption maxi-
The changes in the electronic spectra of 1–12 result from
mum (Δλ = 81 nm, Δν = 3984 cm–1), but this shift is re- protonation of the amines or azines. These dyes can also
˜
versed in highly concentrated TFA (λmax = 411 nm, Δν = act as ligands for metal ions. Two preliminary results may
˜
795 cm–1). Acid-induced changes in the fluorescence spectra illustrate the effect of cations on the electronic spectra.
occur at the same concentrations. The long-wavelength Upon addition of Fe3+ (0.13–1.25 equiv.) to 3 in CH2Cl2,
emission at λmax = 561 nm is first weakened and slightly the band at λ = 418 nm vanishes gradually and a new band
redshifted (10–3 m TFA: λmax = 576 nm, ΔνF = 464 cm–1) at λ = 549 nm appears; the emission is simultaneously
˜
and in 10–2 m TFA is transformed into a more hypso- quenched. The absorption spectra show an isosbestic point
chromic emission with λmax = 443 nm (ΔνF = 4784 cm–1). at λ = 461 nm. Crown ether 6 is capable of chelating Na+
˜
A further increase in TFA concentration leads to a redshift and Ca2+: The absorption maxima are hypsochromically
of this intense emission (1 m TFA: λmax = 482 nm, ΔνF
=
shifted from λ = 470 to 412 nm (NaClO , Δν = –2995 cm–1)
˜
4
˜
1826 cm–1). The impact of TFA on the absorption spectra and λ = 404 nm [Ca(ClO ) , Δν = –3496 cm–1], and the
˜
4 2
of 8–10 is very similar, only minor variations of shifts and emission shifts from λ = 536 to 530 nm (NaClO , ΔνF
=
7
˜
4
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V. Schmitt, S. Moschel, H. Detert
FULL PAPER
Figure 3. Electronic spectra of 11 in DCM with increasing concentrations of TFA (upper spectra: absorption; lower spectra: emission,
normalized to identical optical density at λexc = 345 nm; cps = countss–1).
–211 cm–1) and 523 nm [Ca(ClO ) , ΔνF = –464 cm–1]. The able charge transfer from the donors to the central pyrazine
˜
4 2
metal-sensing properties of these fluorophores are currently and a significant stabilization of the excited state. The influ-
under investigation.
ence of aryl groups on the pyrazine is less pronounced:
Electron-donating anisyl strengthens the solvatochromism
of a quinolyl-DSP (11 vs. 12) but weakens that of amino-
DSPs (5 vs. 9).
Discussion
Changes in the charge distribution and geometry on go-
ing from the ground to the excited state are reflected in
Solvatochromism
In accord with the formal C2 symmetry of 1–12, the sol- the Stokes shifts (ΔνSt) of the fluorophores. As the polarity
˜
˜
vatochromism of the absorption is only weak. Nevertheless, increases, so the ΔνSt increases if the excited-state dipole
Table 4 reveals that the strong amine donors in DSPs 5, 6 moment is larger than that of the ground state.[23] DSPs 1–
and 8–10 significantly enhance the solvatochromic response 12 are formally centrosymmetric, but judged on the basis
ΔνAbs of the DSPs in the absorption spectra. Fluorescence of geometric parameters, conjugation through one of the
˜
gives a more refined picture: ΔνF is small for DSPs with two arms is preferred. Although this may be due to pack-
˜
terminal acceptors (1, 2, 11, 12: ΔνF = 336–510 cm–1), in- ing, electro-optical absorption measurements on related dis-
˜
creases with moderate donors (3, 4, 7: ΔνF = 1022– tyrylpyrazines[24] in solution revealed that these molecules
˜
1668 cm–1), and amino groups cause the largest shifts (5, 6, have a dipole moment that increases strongly upon exci-
8–10: ΔνF = 2134–2411 cm–1). This corresponds to a favor- tation. These dipole moments result from the breaking of
˜
8
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Diaryldistyrylpyrazines
Table 4. Solvatochromic shifts Δν (cyclohexane – acetonitrile), correlation of ΔνF (ET ) and ΔνSt (Δf), and changes in the dipole moments
˜
˜
˜
N
Δμ of 1–12.
DSP
Solvatochromic shifts
νF (ET
)
Lippert–Mataga: ΔνSt (Δf)
˜
˜
N
ΔνAbs [cm–1]
ΔνF [cm–1]
ΔΔν [cm–1]
m [cm–1]
m [cm–1]
y [cm–1]
Δμge [D]
˜
˜
˜
1
–63
–62
58
402
336
465
398
995
–891
–805
1527
1557
3067
5325
4282
4014
3275
5965
4075
4864
1109
1481
2493
2593
2322
2843
2120
2084
2201
1466
2123
2322
2653
4899
7.6
2
12.8
14.1
26.2
16.2
15.7
14.5
19.2
15.8
17.3
9.6
3
1053
1668
2376
2135
1022
2411
2253
2289
378
–2266
–3555
–5269
–4268
–2167
–5293
–4950
–4863
–649
4
–116
965
610
–59
1125
925
665
–60
0
1784
1411
1525
1081
1286
1328
1624
438
5
6
7
8
9
10
11
12
510
510
–881
11.1
the formal centrosymmetry[25] in their ground states and
from localized excitations in their Franck–Condon states.
The order of increasing solvatochromic sensitivity of the
A = A0 + bSA + cSB + dSP + eSdP
(1)
By using this approach to elucidate the solvatochromism,
fluorescence of 1–12 can be obtained from a correlation of we can conclude that the solvent polarizability SP is by far
the emission maxima with a polarity parameter. “Solvent the most important contribution to the solvatochromism of
polarity” is a property that results from different influences the excitation (|d| ϾϾ |b|, |c|, |e|). The vibrational relaxation
of the individual solvent. Among the large number of po- of the Franck–Condon state to the emitting state results
larity scales, Reichardts EN is probably most estab- in geometrical reorganization of the solute and the solvent
T
lished.[26] To estimate the relationship ΔνF(ENT), ethanol shell.[30] Solvent dipoles are reorganized according to the
˜
was omitted because it is a hydrogen-bond donor (HBD). charge distribution in the excited solute molecule thus stabi-
The solvatochromic shifts of the DSPs observed in protic lizing the emitting state. Accordingly, the multiparameter
solvents deviate significantly from those obtained in non- analysis of the emission reveals that in addition to the po-
HBD solvents.[27] The results of a linear regression of the larizability SP, the solvent acidity SA and the solvent di-
emission maxima νF and EN are collected in Table 4: The polarity SdP significantly contribute to the solvatochro-
˜
T
slope m reflects the polarity-induced stabilization of the ex- mism. The impact of solvent acidity on the fluorescence
cited states of the chromophores. The fluorescence of the increases with donor substitution on the distyryl segment
acceptor-substituted DSPs is less influenced by polarity (11Ͻ1Ͻ4Ͻ10). The same sequence is valid for the impact
than the fluorescence of DSPs with alkoxy donors, with of solvent dipolarity SdP on the emission. Note that the
amino-substituted dyes showing the highest sensitivity. impact of dipolarity exceeds the contribution of solvent po-
The variation in m (11Ͻ2Ͻ12Ͻ1Ͻ7Ͻ3Ͻ4Ͻ6Ͻ10 larizability only when the DSP is substituted with the very
Ͻ9Ͻ5Ͻ8) also reflects the influence of substitution on the strong EPD dialkylamino group (10).
teraryl axis: Donors on acceptor-substituted DSPs enhance
The Lippert–Mataga equation [Equation (2)][31] relates
the sensitivity (11Ͻ12) but reduce it if the π system is do- solvatochromic shifts ΔνSt, the orientation polarizability Δf,
˜
nor-substituted (10Ͻ9Ͻ5).
and the cavity radius a, and allows calculation of the
A more detailed study of the different influences of sol- change in the dipole moment Δμge on going from the
vent properties on the solvatochromic shifts of this class of ground to the excited states. To estimate Δμge, the diameter
dyes was performed on representative chromophores: The 2a of the cavity was approximated to the length of the π
fundamental dye 1, 4 with terminal carbazole donors, the system,[22] thus reflecting the lower limit of the diameter of
cruciform 10 with amino and carbazolyl groups, and 11 a spherical solvent cage. The results are collected in Table 4.
with quinoline end-groups. The solvatochromism of the Like the solvatochromic sensitivity of 1–12, their dipole
electronic spectra results from the interplay of different sol- moments in the excited state μe increase with enhanced do-
vent characteristics with the initial and Franck–Condon nor strength on the terminal positions. Alkoxy donors (3,
states of the chromophore. Catalán[29] developed a multipa- 7) are significantly less effective than amino donors (4–6,
rameter scale to analyze the contributions of solvent prop- 8–10). The largest value for Δμge was obtained for 4, in part
erties, which is given by Equation (1) in which A is the sol- due to the larger value of a = 10.9 Å. Recently we obtained
vent-dependent property of a solute in a given solvent and the dipole moments μg = 4.4 D and μe = 47.0 D for 5 from
results from the sum of this property of the solute in the electro-optical absorption measurements.[24] The different
gas phase A0 and the contributions of the solvent effects results obtained by the solvatochromic (Δμge = 16.2 D) and
caused by solvent acidity SA, solvent basicity SB, solvent EOAM methods (Δμge = 42.6 D) can be accounted for by
dipolarity SdP, and solvent polarizability SP weighted by the following two reasons. First, EOAM compares ground
the regression coefficients b to e describing the sensitivity and Franck–Condon states in the absorption process
of property A to the different solute–solvent interactions.
whereas the Lippert–Mataga method (Δμge = 16.2 D) in-
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V. Schmitt, S. Moschel, H. Detert
FULL PAPER
cludes the relaxation from the Franck–Condon state to the electronic transitions result from protonation on the hetero-
emitting state and the emission process. Secondly, the On- cycle (pyrazine: pKB = 13.4[37]), thus enhancing its acceptor
sager radius of the cavity a is a crucial value:[32] To obtain strength. Accordingly, cyano-substitution (2) reduces the
the minimal values for Δμge we approximated a spherical basicity and it was expected that EPD substituents would
cavity with the radius a being half the length of the π system enhance the basicity. But the protonation of 3 and 4 re-
2a. If the Onsager radius of the more ellipsoidal cavity is quires higher TFA concentrations than 1; the main effects
approximated as 0.7ϫ2a,[33] much larger values for Δμge of donor substitution are a largely enhanced redshift of the
result, for example, Δμge = 26.8 D for 5. Furthermore, 2a absorption band and a quenching of the fluorescence be-
was approximated as the length of the π system, the entire cause these protonated DSPs are not emissive. An exchange
molecule is longer, thus resulting in even greater changes in of the central phenyl substitution (3) to α-naphthyl (7) re-
the dipole moment. Besides the Onsager radius, two more sults in similar redshifts but enhanced basicity: 10–2 m TFA
aspects should be mentioned: 1) The evaluation of the sol- is sufficient to protonate the majority of 7. The pyrazine N
vatochromism by the Lippert–Mataga method is based on in 3 acts as a ligand for Lewis acidic Fe3+, and even with
the orientation polarizability Δf, but solvent dipolarity also concentrations as low as 10–7 m, a new band at λ = 549 nm
contributes to the solvatochromic shifts (compare eSdP in appears.
Table 5) and 2) although significant dipole moments of cen-
Because quinoline has a higher basicity (pKB = 9.13),[37]
trosymmetrical chromophores in their excited states have protonation of DSPs 11 and 12 starts in 10–4 m TFA. Pro-
been reported,[34] their solvatochromic shift has not yet be- tonation of the quinoline is supported by NMR spec-
come a standard tool.[35] Nevertheless, even with these im- troscopy. The protons of the quinoline and 1-H of the vinyl-
ponderabilities, these dyes exhibit large dipole moments in ene bridge are strongly shifted (Δδ Յ 0.6 ppm) to lower
the excited states in spite of their formal centrosymmetry.
fields upon addition of TFA. The initial redshifts of the
absorption bands of dipolar protonated 11 and 12 (Δλ = 43
and 69 nm) are reversed upon transfer of a proton to the
second quinoline unit in 1 m TFA. With methoxy as EPD
substituents on the central phenyl rings (12), the electronic
transitions are shifted to lower energies and, whereas pro-
tonated 11 is highly emissive, the fluorescence in 12 is
quenched.
(2)
Table 5. Solvatochromic shifts of 1, 4, 10, and 11, and data from
Cataláns multiparameter analysis.[28]
Similarly to these dyes, the terminal amino groups in 5,
6, and 8–10 are the preferred sites for protonation. Proton-
ation results in large hypsochromic shifts of the absorption
(Δλ = 73–89 nm) followed by small redshifts in 1 m TFA
(Δλ = 29–90 nm). Protonation transforms strong EPD
amines into weak electron-withdrawing ammonium ions.
The large blueshifts indicate that both amino groups in 5, 6,
and 8–10 are simultaneously protonated and the transition
energies (λmax = 393–414 nm) are similar to that of the fun-
damental dye 1 (λmax = 401 nm in DCM). The redshifts in
1 m TFA result from further protonation of the pyrazine.
Although the absorption spectra show the neutral and two
protonated forms of 5, 6, and 8–10, fluorescence shows four
species: Small concentrations of TFA lead to a redshifted
emission, best visible in the spectra of aza-crown substi-
tuted 6. This accounts for a monoprotonated dye. The sec-
ond protonation of the excited dyes or excitation of the two-
fold protonated dyes gives rise to a strongly blueshifted
emission (Δλ = 84–119 nm). Cations like Na+ and Ca2+ are
chelated by the aza-crown of 6 but the hypsochromic shifts
are less pronounced because these ions interact with several
donor atoms in the crown. The transition energies of dipro-
tonated 5, 6, and 8–10 are similar to those of the neutral
fundamental dye 1. In 1 m TFA, the fourth emitting species
of 5, 6, and 8–10 result from protonation of the pyrazine.
With ΔλF = 29–90 nm, these bathochromic shifts adopt an
intermediate position between those observed by proton-
ation of cyano-substituted 2 (ΔλF = 34 nm) and 1, which
Solvent
1
4
10
11
λAbs [nm]; λAbs [nm]; λAbs [nm]; λAbs [nm];
λF [nm]
λF [nm]
λF [nm]
λF [nm]
n-Heptane
396; 441
397; 443
398; 442
399; 445
398; 441
402; 447
402; 439
401; 452
398; 442
397; 450
402; 450
403; 452
398; 451
398; 453
397; 456
414; 461
412; 464
416; 463
417; 476
409; 466
422; 475
422; 477
419; 490
413; 476
414; 502
419; 501
420; 500
414; 488
412; 493
411; 516
451; 501
453; 505
452; 505
462; 528
456; 495
467; 527
469; 509
471; 554
463; 513
466; 571
476; 551
480; 562
463; 551
464; 580
467; 570
408; 458
408; 460
408; 456
410; 461
409; -
412; 463
413; 463
411; 463
408; 460
407; 464
413; 465
414; 468
407; 462
407; 463
406; 464
Methylcyclohexane
Cyclohexane
1,4-Dioxane
Triethylamine
Toluene
Benzene
Dichloromethane
Ethyl acetate
Acetonitrile
Dimethylacetamide
Dimethyl sulfoxide
2-Propanol
Ethanol
Methanol
Abs ν [cm–1]
26631
–144
–36
–2189
–15
23110
–783
116
–726
–417
26049
–101
262
–2894
–92
22853
–1626
398
–2135
–1365
24790
–417
29
–4143
–625
20959
–1959
665
26005
–45
34
–2252
3
22684
–250
66
˜
0
Abs SA: b [cm–1]
Abs SB: c [cm–1]
Abs SP: d [cm–1]
Abs SdP: e [cm–1]
EM: νF [cm–1]
˜
0
EM SA: b [cm–1]
EMSB: c [cm–1]
EM SP: d [cm–1]
EM SdP: e [cm–1]
–1835
–2128
–1300
–256
Acidochromism
The weak basicity of the pyrazine unit in 1 increases lacks a terminal acceptor (ΔλF = 127 nm).
upon charge transfer in the excited state.[36] The redshifted
10
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Diaryldistyrylpyrazines
0.61 mmol) and benzaldehyde (196 mg, 1.84 mmol) in DMF
(30 mL) under N2. After 30 min at 0 °C and 4 h at 25 °C, water
(80 mL) was added and 1 was extracted with CHCl3 (3ϫ50 mL).
The combined organic solutions were washed with brine, dried
(MgSO4), concentrated, and purified by chromatography to give
170 mg (0.39 mmol, 64%) of a yellow solid with m.p. 268–269 °C.
Conclusions
A series of centrosymmetric diaryldistyrylpyrazines have
been prepared, and the optical properties of these fluoro-
phores have been studied. The influence of diaryl substitu-
tion the central pyrazine on absorption and fluorescence
spectra is only small because the conjugation through the
distyrylpyrazine is favored. A positive solvatochromism of
the emission indicates a highly dipolar excited state. Ac-
cording to the solvatochromic method, excitation leads to
an increase in the dipole moments of about 9–16 D. DSPs
with or without non-basic donors on the styryl termini are
protonated on the central pyrazine ring resulting in ba-
thochromism of the absorption and emission and gradually
quenched fluorescence. Two kinds of basic sites on the styr-
yl axis were studied: Quinolyl end-groups are more basic
than the pyrazine, protonation resulting in bathochromism
of the absorption and emission, but with dialkylamino
groups an initial strong hypsochromism of the two maxima
is followed by bathochromism. These fluorophores are
interesting for sensing applications because their absorption
and emission properties are strongly affected by environ-
mental conditions such as polarity, pH, and the presence of
cations.
3
1H NMR (CDCl3, 300 MHz, 25 °C): δ = 7.97 (d, J = 15.7 Hz, 2
3
H, 2-H vin), 7.85 (d, J = 6.9 Hz, 4 H, 2-H, 6-H Ph-vin), 7.6–7.5
(m, 10 H, 3-H, 4-H, 5-H Ph-vin, 2-H, 6-H Ph-p), 7.38–7.26 (m, 8
H, 3-H vin, 3-H, 4-H, 5-H Ph-p) ppm. 13C NMR (CDCl3, 75 MHz,
25 °C): δ = 150.1 (C-2 Pz), 145.0 (C-1 Pz), 138.0 (C-1 Ph-p), 136.8
(C-1 Ph-vin), 134.8 (C-2 vin), 130.0 (C-2, C-6 Ph-p), 129.0 (C-4
Ph-vin), 128.7 (C-3, C-5 Ph-vin), 128.5 (C-3, C-5 Ph-p), 128.4 (C-
4 Ph-p), 127.3 (C-2, C-6 Ph-vin), 124.0 (C-1 vin) ppm. IR (ATR):
ν = 3056, 1625, 1495, 1449, 1382, 1203, 1134, 1024, 963, 765,
˜
752 cm–1. MS (FD): m/z (%) = 436.6 (100) [M]+. HRMS: calcd. for
C32H25N2 437.2018 [M + H]+; found 437.2023.
+
(E,E)-2,5-Bis[2-(4-cyanophenyl)ethenyl]-3,6-diphenylpyrazine
(2):
Phosphonate 20a (148 mg, 0.58 mmol) and 18a (70 mg, 0.24 mmol)
were dissolved in THF (30 mL) in a flame-dried flask. While stir-
ring at 0 °C, KOtBu (100 mg, 0.89 mmol) was added and after
30 min at 0 °C and 4 h at 25 °C, water (70 mL) was added, 2 was
extracted with ethyl acetate (4ϫ40 mL), and the combined organic
solutions were washed with brine (2ϫ30 mL), dried (Na2SO4), and
concentrated. The residue was recrystallized from methanol/
CH2Cl2 to yield 85 mg (0.17 mmol, 71%) of a yellow solid with
1
m.p. 305 °C (dec.). H NMR (CDCl3, 400 MHz, 25 °C): δ = 7.97
(d, 3J = 15.7 Hz, 2 H, 2-H vin), 7.80 (m, 4 H, 2-H, 6-H Ph-p), 7.64–
3
Experimental Section
7.56 (m, 14 H, 2-H, 3-H, 5-H, 6-H Ph-p), 7.43 (d, J = 15.7 Hz, 2
H, 1-H vin) ppm. 13C NMR (CDCl3, 100 MHz): δ = 150.8 (C-3,
C-6 Pz), 144.7 (C-2, C-5 Pz), 141.0 (C-1 Ph), 137.5 (C-1 Ph-p),
133.3 (C-2 vin), 132.5 (C-2, C-6 Ph), 129.9 (C-2, C-6 Ph-p), 129.5
(C-4 Ph-p), 128.7 (C-3, C-5 Ph-p), 127.7 (C-3, C-5 Ph), 127.3 (C-1
General: All the reactions were carried out under dry nitrogen un-
less otherwise indicated. Commercially available reagents were used
without further purification; solvents were dried by standard pro-
cedures, yields refer to chromatographically and spectroscopically
vin), 118.8 (CN), 111.5 (C-4 Ph) ppm. IR (KBr): ν = 3058, 2225,
˜
pure compounds unless otherwise stated. H and 13C NMR spec-
1
1624, 1601, 1504, 1412, 1381, 1323, 1202, 1140, 1026, 970, 823, 701,
553 cm–1. MS (FD): m/z (%) = 486.3 (100) [M]+. HRMS: calcd. for
C34H23N4 487.1923 [M + H]+; found 487.1934.
tra: Bruker AC 300 (300 MHz), AV 400 (400 MHz), and ARX 400
(400 MHz) spectrometers in CDCl3 and [D6]DMSO. 1H and 13C
NMR signals were assigned on the basis of DEPT, COSY 45,
HMQC, and HMBC experiments. Chemical shifts: δ values are in
ppm, coupling constants are in Hz. Abbreviations: Pz = pyrazine,
Ph = phenyl, Ph-p = phenyl attached to pyrazine, Cb = carbazole,
vin = vinyl, Np = naphthyl, Qu = quinolyl, br. = broad signal, sup.
= superimposed signals. Melting points: Tottoli apparatus (Büchi).
IR spectra: Beckman Acculab 4 or JASCO 4100 FT-IR (ATR).
MS (FD) spectra: Finnigan Mat 95 spectrometer. HR-ESI-MS: Q-
TOF-ULTIMA 3 spectrometer with Lock Spray device (Waters-
Micromass) and NaICsI Standard as reference. UV/Vis spectra:
Perkin–Elmer Lambda 16 spectrophotometer; fluorescence spectra:
Perkin–Elmer LS 50B spectrophotometer. Fluorescence spectra are
normalized to identical optical density at the excitation wavelength
of 350 nm. Fluorescence quantum yields (+/– 20%) were obtained
by comparison with quinine sulfate in 0.1 m H2SO4 (Φ = 0.577).[38]
Elemental analyses: Vario EL.
(E,E)-2,5-Bis[2-(4-octyloxyphenyl)ethenyl]-3,6-diphenylpyrazine (3):
According to the procedure for 1, a solution of 17a (150 mg,
0.50 mmol), 18b (272 mg, 1.16 mmol), and KOtBu (250 mg,
2.23 mmol) in DMF (20 mL) gave 250 mg (0.39 mmol, 67%) of a
yellow solid with m.p. 170 °C. 1H NMR (CDCl3, 400 MHz, 25 °C):
3
δ = 7.90 (d, J = 15.6 Hz, 2 H, 2-H vin), 7.85–7.83 (m, 4 H, 2-H,
3
6-H Ph-p), 7.59–7.50 (m, 6 H, 3-H, 4-H, 5-H Ph-p), 7.42 (d, J =
3
8.7 Hz, 4 H, 2-H, 6-H, Ph), 7.21 (d, J = 15.6 Hz, 2 H, 1-H vin),
3
6.85 (d, J = 8.7 Hz, 4 H, 3-H, 5-H Ph), 3.96 (t, 4 H, OCH2), 1.78
(m, 4 H, β-CH2), 1.45 (m, 4 H, γ-CH2), 1.30 (m, 16 H, CH2), 0.89
(t, 6 H, CH3) ppm. 13C NMR (CDCl3, 75 MHz): δ = 159.5 (C-4
Ph), 149.8 (C-1 Pz), 145.0 (C-2 Pz), 138.4 (C-1 Ph-p), 134.2 (C-2
vin), 130.0 (C-2, C-6 Ph-p), 129.5 (C-3, C-5 Ph-p), 128.8 (C-4 Ph-
p), 128.7, 128.4 (C-1, C-2, C-6 Ph), 121.8 (C-1 vin), 114.7 (C-3, C-
5 Ph), 68.0 (OCH2), 31.8 (β-CH2), 29.4 (CH2), 29.3 (2 CH2), 26.0
(CH ), 22.6 (CH ), 14.1 (CH ) ppm. IR (KBr): ν = 3060, 2923,
˜
2
2
3
The starting materials were prepared according to literature meth-
ods: 1-(4-Bromophenyl)propan-2-one[39] (13b), 1-naphthylpropan-
2-one[40] (13d), 1-hydroxyimino-1-phenylpropan-2-one[41] (14a), p-
octyloxybenzaldehyde[42] (18b), 4-(9-carbazolyl)benzaldehyde[14e]
2853, 1625, 1604, 1385, 1285, 1246, 1173, 1135, 966, 819, 702 cm–1.
MS (FD): m/z (%) = 347.0 (1.8) [M]2+, 692.2 (100) [M]+.
C48H56N2O2 (696.47): calcd. C 81.19, H 8.15, N 4.04; found C
80.86, H 7.99, N 3.92.
(18c),
4-(N,N-dipropylamino)benzaldehyde[43]
(18d),
4-
(4Ј,7Ј,10Ј,13Ј-tetraoxa-1-azacyclopentadecyl)benzaldehyde[44] (18e),
4-hexyloxybenzaldehyde[45] (18f), diethyl 4-cyanobenzylphos-
phonate[46] (20a) and diethyl quinolylmethylphosphonate[47] (20b).
(E,E)-2,5-Bis{2-[4-(9-carbazolyl)phenyl]ethenyl}-3,6-diphenyl-
pyrazine (4): According to the procedure for 1, a solution of 17a
(100 mg, 0.38 mmol), 18c (230 mg, 0.85 mmol), and KOH (100 mg,
1.78 mmol) in DMF (30 mL) gave 90 mg (0.12 mmol, 32%) of a
bright-yellow solid that decomposed above 360 °C. 1H NMR
(E,E)-2,5-Distyryl-3,6-diphenylpyrazine (1): KOtBu (205 mg,
1.84 mmol) was added to a stirred solution of 17a (160 mg,
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
11

Job/Unit: O30463
/KAP1
Date: 16-07-13 18:43:51
Pages: 16
V. Schmitt, S. Moschel, H. Detert
FULL PAPER
3
3
(CDCl3, 400 MHz, 25 °C): δ = 8.15 (d, J = 7.7 Hz, 4 H), 8.10 (d,
H vin), 6.61 (d, J = 8.4 Hz, 4 H, 3-H, 5-H Ph), 3.76–3.60 (m, 40
H, CH2) ppm. 13C NMR (CDCl3, 75 MHz): δ = 149.4, 147.7, 145.0
3J = 15.7 Hz, 2 H), 7.91 (m, 4 H, 3-H), 7.74 (d, 3J = 7.74 Hz, 4
H), 7.63 (m, 4 H), 7.60–7.56 (m, 6 H), 7.49–7.45 (m, 6 H), 7.42 (dt, (C-4 Ph, C-3, C-6, C-2, C-5 Pz), 138.7 (C-1 Ph-p), 134.2 (C-2 vin),
3J = 8.2, 4J = 1.2 Hz, 4 H), 7.30 (dt, 3J = 7.7, 4J = 1.3 Hz, 4
129.9 (C-2, C-6 Ph-p), 128.8, 128.6, 128.3 (C-2, C-6 Ph, C-3, C-4,
H) ppm. 13C NMR (CDCl3, 75 MHz, 25 °C): δ = 150.4, 145.0, C-5 Ph-p), 124.9 (C-1 Ph), 119.5 (C-1 vin), 111.4 (C-3, C-5 Ph),
140.6, 138.0, 137.7, 135.9, 133.9, 130.0, 129.2, 128.7, 128.6, 127.1, 71.3, 70.2, 70.1 (OCH2), 68.4 (N-CH2CH2), 52.6 (NCH2) ppm. IR
126.0, 124.8, 123.5, 120.3, 120.1, 109.8 ppm. IR (KBr): ν = 3055,
(ATR): ν = 2853, 1595, 1517, 1381, 1349, 1182, 1116, 958, 807,
˜
˜
1624, 1600, 1514, 1478, 1451, 1361, 1316, 1228, 1169, 1138, 825,
748, 724, 700, 623 cm–1. MS (FD): m/z (%) = 383.2 (6) [M]2+, 766.2
(100) [M]+. C56H38N4 (766.93): calcd. C 87.79, H 4.99, N 7.31;
found C 87.46, H 4.71, N 7.04.
762, 699 cm–1. MS (FD): m/z (%) = 870.3 (100) [M]+. C52H62N4O8
(871.08): calcd. C 71.70, H 7.17, N 6.43; found C 71.66, H 7.07, N
6.34.
(E,E)-2,5-Bis[2-(4-hexyloxyphenyl)ethenyl]-3,6-di(1-naphthyl)-
pyrazine (7): According to the procedure for 1, a solution of 17d
(150 mg, 0.42 mmol), 18f (190 mg, 0.92 mmol), and KOtBu
(190 mg, 1.69 mmol) in DMF (30 mL) gave 85 mg (0.12 mmol,
29 %) of bright-yellow needles with m.p. 156–158 °C. 1H NMR
(E,E)-2,5-Bis{2-[4-(dipropylamino)phenyl]ethenyl}-3,6-diphenyl-
pyrazine (5): According to the procedure for 1, a solution of 17a
(150 mg, 0.58 mmol), 18d (260 mg, 1.27 mmol), and KOtBu
(250 mg, 2.23 mmol) in DMF (25 mL) gave 210 mg (0.33 mmol,
57%) of bright-orange crystals with m.p. 184–185 °C. 1H NMR
3
(CDCl3, 400 MHz, 25 °C): δ = 8.04 (d, J = 8.2 Hz, 2 H, 4-H Np),
3
7.99 (d, 3J = 8.0, 4J = 1 Hz, 2 H, 5-H Np), 7.86 (br. d, 3J = 8.0 Hz,
(CDCl3, 400 MHz, 25 °C): δ = 7.86 (d, J = 15.5 Hz, 2 H, 2-H, 2-
3
3
H vin), 7.88–7.86 (m, 4 H, 2-H, 6-H Ph-p), 7.58–7.49 (m, 6 H, 3-
2 H, 8-H Np), 7.75 (d, J = 15.7 Hz, 2 H, 2-H vin), 7.71 (br. d, J
3
3
H, 4-H, 5-H Ph-p), 7.37 (d, J = 8.9 Hz, 4 H, 2-H, 6-H Ph), 7.14
= 7.4 Hz, 2 H, 2-H Np), 7.67 (t, J = 7.4 Hz, 2 H, 3-H Np), 7.55
3
3
3
4
3
(d, J = 15.5 Hz, 2 H, 1-H vin), 6.60 (d, J = 8.9 Hz, 4 H, 3-H, 5-
H Ph), 3.27 (t, 8 H, NCH2), 1.63 (m, 8 H, CH2), 0.94 (t, 12 H,
CH3) ppm. 13C NMR (CDCl3, 75 MHz, 25 °C): δ = 149.3 (C-4 Ph),
148.3 (C-3, C-6 Pz), 144.9 (C-2, C-5 Pz), 138.8 (C-1 Ph-p), 134.3
(C-2 vin), 129.9 (C-2, C-6 Ph-p), 128.7 (C-3, C-5 Ph-p), 128.5 (C-
2,C-6 Ph), 128.3 (C-4 Ph-p), 124.6 (C-1 Ph), 119.1 (C-1 vin), 111.4
(C-3, C-5 Ph), 52.8 (NCH2), 20.5 (CH2), 11.4 (CH3) ppm. IR
(dt, J = 8.0, J = 1.2 Hz, 2 H, 6-H Np), 7.50 (br. t, J = 7.0 Hz,
3
3
2 H, 7-H Np), 7.13 (d, J = 8.8 Hz, 4 H, 2-H, 6-H Ph), 6.76 (d, J
3
= 15.7 Hz, 2 H, 1-H vin), 6.69 (d, J = 8.8 Hz, 4 H, 3-H, 5-H Ph),
3.98 (t, 4 H, OCH2), 1.70 (m, 4 H, β-CH2), 1.38 (m, 4 H, CH2),
1.27 (m, 8 H, CH2), 0.86 (t, 6 H, CH3) ppm. 13C NMR (CDCl3,
75 MHz, 25 °C): δ = 159.4 (C-4 Ph), 150.4 (C-3, C-6 Pz), 147.0 (C-
2, C-5 Pz), 135.9 (C-1 Np), 134.4 (C-2 vin), 133.9 (C-4a Np), 132.0
(C-8a Np), 129.3 (C-4 Np), 129.2 (C-1 Ph), 128.6 (C-2, C-6 Ph),
128.4 (C-2 Np), 128.3 (C-5 Np), 126.7 (C-7 Np), 126.2 (C-8 Np),
126.0 (C-6 Np), 125.4 (C-3 Np), 121.5 (C-1 vin), 114.5 (C-3, C-5
Ph), 67.9 (OCH2), 31.5 (β-CH2), 29.1, 25.6, 22.6 (CH2), 14.0
(KBr): ν = 2960, 2931, 2872, 1598, 1519, 1380, 1365, 1238, 1183,
˜
1136, 809, 700 cm–1. MS (FD): m/z (%) = 633.9 (100) [M]+. HRMS
(ESI): calcd. for C44H51N4 635.4114; found 635.4095.
X-ray Structure Determination of 5: Performed with an Enraf–Non-
ius Turbo-Cad 4 diffractometer equipped with a rotating anode
(CH ) ppm. IR (ATR): ν = 2923, 2852, 1603, 1572, 1245, 1174,
˜
3
1025, 970, 823, 799, 771, 667 cm–1. MS (FD): m/z (%) = 737.1 (100)
[M]+. C52H52N2O2 (736.98l): calcd. C 84.75, H 7.11, N 3.80; found
C 84.40, H 7.69, N 3.51.
using a red block. Crystal data: C44H50N4, M 635 g mol–1
,
3
¯
0.1ϫ0.36ϫ0.56 mm , triclinic, space group: P1, Mo-Kα, graphite-
monochromated 1.54180 Å, T = 193 K, unit cell dimensions: a =
9.2287(10), b = 13.058(14), c = 16.610(2) Å, α = 100.300(2), β =
(E,E)-2,5-Bis{2-[4-(dipropylamino)phenyl]ethenyl}-3,6-di(1-
naphthyl)pyrazine (8): According to the procedure for 1, a solution
of 17d (150 mg, 0.42 mmol), 18d (190 mg, 0.92 mmol), and KOtBu
(186 mg, 1.66 mmol) in DMF (40 mL) gave 65 mg (0.088 mmol,
21 %) of an amorphous orange solid with m.p. 202–203 °C. 1H
105.007(2), γ = 102.174(2)°, V = 1831.3(6) Å3, z = 2, dcalcd.
=
1.151 gcm–3, absorption μ = 0.07 mm–1, the θ range for data collec-
tion was 2–26°, index ranges: –11Յ hՅ 10, –16Յ kՅ 15, –20Յ lՅ
19. Number of reflections collected: 13155; independent reflections:
6871 (Rint = 0.0216). The structure was solved by direct methods
(program SIR92, refinement by SHELXL97).[48] Structure refine-
ment was performed by full-matrix least-squares methods on 437
3
NMR (CDCl3, 400 MHz, 25 °C): δ = 8.03 (d, J = 8.2 Hz, 2 H, 4-
3
4
H Np), 8.00 (dd, J = 8.1, J = 1.1 Hz, 2 H, 5-H Np), 7.92 (br. d,
3J = 8.0 Hz, 2 H, 8-H Np), 7.74 (br. d, J = 6.7 Hz, 2 H, 1-H vin),
3
parameters, weighted refinement: w = 1/[σ2(Fo2) + (0.0631P)2
+
3
7.70 (d, J = 15.6 Hz, 2 H, 2-H vin), 7.67 (t, 2 H, 3-H Np), 7.56
0.32P] with P = [max(Fo2,0) + 2Fo2]/3 and hydrogen atoms located
from difference Fourier synthesis and refined isotropically as-
suming a riding motion model, non-hydrogen atoms improved by
anisotropic refinement. Goodness-of-fit on S = 1.026, maximal
range of parameters 0.001ϫe.s.d, final R indices: R1 = 0.0490, wR2
= 0.1378, the final difference Fourier map showed minimum and
maximum values of 1.09 and –0.81 eÅ–3, respectively.
(dt, 3J = 8.6, J = 1.2 Hz, 2 H, 6-H Np), 7.50 (br. t, 2 H, 7-H Np),
4
3
3
7.07 (d, J = 8.9 Hz, 4 H, 2-H, 6-H Ph), 6.66 (d, J = 15.6 Hz, 2
H, 1-H vin), 6.43 (d, J = 8.9 Hz, 4 H, 3-H, 5-H Ph), 3.17 (t, 8 H,
3
NCH2), 1.54 (m, 8 H, CH2), 0.87 (t, 12 H, CH3) ppm. 13C NMR
(CDCl3, 100 MHz, 25 °C): δ = 150.0 (C-3, C-6 Pz), 148.2 (C-4 Ph),
146.9 (C-2, C-5 Pz), 136.4 (C-1 Np), 134.4 (C-2 vin), 133.8 (C-4a
Np), 132.0 (C-8a Np), 128.9 (C-4 Np), 128.7 (C-2, C-6 Ph), 128.3
(C-2 Np), 128.2 (C-5 Np), 126.5 (C-7 Np), 126.2 (C-8 Np), 126.0
(C-6 Np), 125.4 (C-3 Np), 124.0 (C-1Ph), 118.9 (C-1 vin), 111.2 (C-
CCDC-918792 (for 5) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/datarequest/cif.
3, C-5 Ph), 52.7 (NCH ), 20.4 (CH ), 11.3 (CH ) ppm. IR (ATR): ν
˜
2
2
3
= 2922, 2852, 1596, 1518, 1390, 1363, 1178, 1135, 968, 801,
776 cm–1. MS (FD): m/z (%) = 735.2 (100) [M]+. HRMS: calcd. for
C52H54N4 734.4348 [M]+; found 734.4332.
(E,E)-2,5-Bis{2-[4-(4Ј,7Ј,10Ј,13Ј-tetraoxa-1-azacyclopentadecyl)-
phenyl]ethenyl}-3,6-diphenylpyrazine (6): According to the pro-
cedure for 1, a solution of 17a (150 mg, 0.58 mmol), 18e (175 mg,
(E,E)-2,5-Bis{2-[4-(dipropylamino)phenyl]ethenyl}-3,6-bis(4-meth-
1.56 mmol), and KOtBu (100 mg, 1.78 mmol) in DMF (30 mL) oxyphenyl)pyrazine (9): According to the procedure for 1, a solution
gave 95 mg (0.11 mmol, 19%) of a red solid with m.p. 229–230 °C. of 17c (100 mg, 0.31 mmol), 18d (155 mg, 0.76 mmol), and KOtBu
1H NMR (CDCl3, 400 MHz, 25 °C): δ = 7.86–7.82 (m, 6 H, 2-H (170 mg, 1.52 mmol) in DMF (30 mL) gave 80 mg (0.12 mmol,
vin, 2-H, 6-H Ph-p), 7.56–7.47 (m, 6 H, 3-H, 4-H, 5-H Ph-p), 7.36
(d, J = 8.4 Hz, 4 H, 2-H, 6-H Ph), 7.12 (d, J = 15.6 Hz, 2 H, 1-
39 %) of a red solid with m.p. 205–208 °C. 1H NMR (CDCl3,
3
3
400 MHz, 25 °C): δ = 7.82 (d, 3J = 15.6 Hz, 2 H, 2-H vin), 7.79 (d,
12
www.eurjoc.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 0000, 0–0

Job/Unit: O30463
/KAP1
Date: 16-07-13 18:43:51
Pages: 16
Diaryldistyrylpyrazines
3
1
3J = 8.8 Hz, 4 H, 2-H, 6-H Ph-p), 7.36 (d, J = 8.9 Hz, 4 H, 2-H,
poorly soluble yellow solid that decomposed at 330 °C. H NMR
6-H Ph), 7.11 (d, 3J = 15.6 Hz, 2 H, 1-H vin), 7.06 (d, 3J = 8.8 Hz,
4 H, 3-H, 5-H Ph-p), 6.58 (d, J = 8.9 Hz, 4 H, 3-H, 5-H Ph), 3.92
(CDCl3, 400 MHz): δ = 8.25 (d, J = 15.6 Hz, 2 H, 1-H vin), 8.12
3
(d, 3J = 8.6 Hz, 2 H, 4-H Qu), 8.08 (d, J = 8.6 Hz, 2 H, 8-H Qu),
3
3
3
3
(s, 6 H, OCH3), 3.26 (t, 8 H, NCH2), 1.61 (m, 8 H, CH2), 0.93 (t,
7.96 (d, J = 15.6 Hz, 2 H, 2-H vin), 7.88 (d, J = 8.8 Hz, 4 H, 2-
6 H, CH3) ppm. 13C NMR (CDCl3, 75 MHz, 25 °C): δ = 160.0 (C- H, 6-H Ph), 7.78 (d, J = 8.1 Hz, 2 H, 5-H Qu), 7.71 (dt, J = 8.6,
3
3
4 Ph-p), 148.6, 148.2 (C-2, C-3, C-5, C-6 Pz), 144.6 (C-1 Ph-p),
133.9 (C-2 vin), 131.3 (C-2, C-6 Ph-p), 128.7 (C-2, C-6 Ph), 124.4 7.51 (dt, J = 8.1, J = 1.1 Hz, 2 H, 6-H Qu), 7.12 (d, J = 8.8 Hz,
4J = 1.4 Hz, 2 H, 7-H Qu), 7.65 (d, 3J = 8.6 Hz, 2 H, 3-H Qu),
3
4
3
(C-1 Ph), 119.5 (C-1 vin), 113.8 (C-3, C-5 Ph-p), 111.4 (C-3, C-5 4 H, 3-H, 5-H Ph), 3.95 (s, 6 H, OCH ) ppm. IR (KBr): ν = 3057,
˜
3
Ph), 1 C sup., 55.4 (OCH3), 52.8 (NCH2), 20.5 (CH2), 11.4 (CH3). 2929, 2836, 1654, 1608, 1595, 1517, 1505, 1426, 1409, 1384, 1303,
IR (KBr): ν = 2960, 2931, 2873, 1600, 1519, 1379, 1364, 1295, 1249,
1251, 1204, 1175, 1136, 1030, 980, 838, 823, 756, 620 cm–1. MS
˜
1183, 1137, 1032, 837, 810 cm–1. MS (FD): m/z (%) = 347.5 (1) (FD): m/z (%) = 598.4 (100) [M]+. HRMS (ESI): calcd. for
[M]2+, 694.1 (100) [M]+. HRMS: calcd. for C46H54N4O2 694.4247
[M]+; found 694.4225.
599.2447 C40H31N4O2; found 599.2473.
1-Hydroxyimino-1-(4-bromophenyl)propan-2-one (14b): Similar to
(E,E)-2,5-Bis[4-(9-carbazolyl)phenyl]-3,6-bis{2-[4-(dimethylamino)- the procedure for 14c (see below), ethyl acetoacetate (20 g,
phenyl]ethenyl}pyrazine (10): 4-Dimethylaminobenzaldehyde 0.15 mol), 4-bromoaniline (25.0 g, 0.15 mol), NaNO2 (2ϫ10.0 g,
(1.00 g, 6.7 mmol) and KOtBu (750 mg, 6.7 mmol) were dissolved
in DMF (30 mL) under N2 and 17e (120 mg, 0.2 mmol) was added
slowly to the stirred mixture. Stirring was continued for 4 h, water
(100 mL) was added, and the product extracted with CHCl3
(4ϫ50 mL). The combined organic solutions were washed with
brine (3ϫ30 mL), dried (MgSO4), concentrated, and recrystallized
2ϫ0.15 mol), and KOH (8.6 g, 0.15 mol) gave 20.5 g (84.7 mmol,
55%) of light-brown crystals with m.p. 190 °C. The same product
was obtained from 13b (6.5 g, 30.5 mmol) in diethyl ether (50 mL),
HCl (1 mL), and isoamyl nitrite (4.5 g, 38 mmol) following the pro-
1
cedure given for 14d (see below), yield 4.0 g (16.5 mmol, 54%). H
NMR (CDCl3, 300 MHz, 25 °C): δ = 11.97 (s, 1 H, OH), 7.33 (d,
3
from CH2Cl2/methanol, yield 135 mg (0.16 mmol, 78%) of orange 3J = 8.6 Hz, 2 H, 3-H, 5-H), 7.02 (d, J = 8.6 Hz, 2 H, 2-H, 6-H),
crystals with m.p. 347 °C. 1H NMR (CDCl3, 400 MHz, 25 °C): δ
2.32 (s, 3 H, CH3) ppm. 13C NMR (CDCl3, 75 MHz, 25 °C): δ =
3
3
= 8.20 (d, J = 7.8 Hz, 4 H, 4-H, 5-H Cb), 8.16 (d, J = 8.4 Hz, 4
196.5 (C=O), 154.5 (C=N), 131.0, 130.6 (C-2, C-3, C-5, C-6), 127.9
3
H, 2-H, 6-H Ph-p), 8.01 (d, J = 15.5 Hz, 2 H, 2-H vin), 7.80 (d,
(C-1), 122.8 (C-Br), 25.8 (CH ) ppm. IR (KBr): ν = 3032, 2924,
˜
3
3J = 8.4 Hz, 4 H, 3-H, 5-H Ph-p), 7.63 (d, J = 8.2 Hz, 4 H, 1-H,
2860, 1661, 1591, 1490, 1435, 1396, 1367, 1309, 1298, 1187, 1073,
3
8-H Cb), 7.52–7.48 (m, 8 H, 2-H, 6-H Ph, 2-H, 7-H Cb), 7.36 (t, 4 995, 935, 819, 631 cm–1. MS (FD): m/z (%) = 240.9 (100) [M]+.
3
3
H, 3-H, 6-H Cb), 7.32 (d, J = 15.5 Hz, 2 H, 1-H vin), 6.72 (d, J
= 8.9 Hz, 4 H, 3-H, 5-H Ph), 3.00 (s, 12 H, NCH3) ppm. 13C NMR
(CDCl3, 75 MHz): δ = 150.7 (C-2, C-5 Pz), 148.6 (C-4 Ph), 145.1
(C-3, C-6 Pz), 140.7 (C-1 Ph-p), 138.2 (C-4a, C-4b Cb), 137.6 (C-
4 Ph-p), 135.2 (C-2 vin), 131.5 (C-3, C-5 Ph-p), 128.8 (C-2, C-6
Ph), 126.8 (C-2, C-6 Ph-p), 126.1 (C-2, C-7 Cb), 125.1 (C-1 Ph),
123.6 (C-8a, C-9a Cb), 120.4 (C-4, C-5 Cb), 120.1 (C-3, C-6 Cb),
119.2 (C-1 vin), 112.2 (C-3, C-5 Ph), 110.0 (C-4 Ph), 40.3
1-Hydroxyimino-1-(4-methoxyphenyl)propan-2-one (14c): A solu-
tion of 4-methoxyphenyldiazonium sulfate was prepared from
NaNO2 (5.1 g, 73.9 mmol) in water (20 mL) and p-anisidine (9.0 g,
73.1 mmol) in H2SO4 (20%, 40 mL). A second solution was pre-
pared from ethyl acetoacetate (9.5 g, 0.073 mol) and KOH (4.1 g,
0.077 mol) in water (45 mL). This was stirred for 3 h before NaNO2
(5.1 g, 0.074 mol) was added. The mixture was then cooled to 0 °C
and H2SO4 (30%, 25 mL) was added dropwise. After vigorous stir-
ring for 30 min, CuSO4·5H2O (1.9 g, 7.7 mmol) in water (20 mL)
and Na2S2O5 (4.4 g, 0.023 mol) in water (10 mL) were added fol-
lowed by the solution of the diazonium salt. Vigorous stirring was
continued for 3 h at ambient temperature. The mixture was satu-
rated with NaCl and the precipitate collected by filtration through
a Büchner funnel. The residue was dissolved in NaOH (2.5 m), fil-
tered, and precipitated by addition of glacial acetic acid. Suction
filtration, dissolution in dichloromethane, drying (MgSO4), evapo-
ration, and recrystallization from toluene gave 3.8 g (19.7 mmol,
27 %) of an off-white solid with m.p. 146–148 °C. 1H NMR
(CDCl3, 400 MHz, 25 °C): δ = 11.43 (s, 1 H, OH), 7.28 (d, 3J =
(CH ) ppm. IR (ATR): ν = 2920, 2851, 1601, 1517, 1449, 1354,
˜
3
1222, 1186, 1165, 1073, 807, 742, 721 cm–1. MS (FD): m/z (%) =
426.2 (7) [M]2+, 852.0 (100) [M]+, 1572.5 (1.2) [M2]+. C60H48N6
(853.08): calcd. C 84.48, H 5.67, N 9.85; found C 84.14, H 5.83, N
9.59.
(E,E)-2,5-Bis[2-(2-quinolyl)ethenyl]-3,6-diphenylpyrazine (11): Ac-
cording to the procedure for 1, a solution of 19a (200 mg,
0.69 mmol), 20b (419 mg, 1.5 mmol) in abs. THF (50 mL), and
KOtBu (233 mg, 2.08 mmol) yielded 180 mg (48%) of orange need-
1
3
les with m.p. 301 °C. H NMR (CDCl3, 400 MHz): δ = 8.27 (d, J
3
= 15.6 Hz, 2 H, 1-H vin), 8.09 (d, J = 8.6 Hz, 2 H, 4-H Qu), 8.07
(d, 3J = 8.6 Hz, 2 H, 8-H Qu), 7.95 (d, 3J = 15.6 Hz, 2 H, 2-H vin),
8.9 Hz, 2 H, 3-H, 5-H), 6.88 (d, J = 8.9 Hz, 2 H, 2-H, 6-H), 3.78
3
3
4
7.90 (m, 4 H, 2-H, 6-H Ph), 7.77 (dd, J = 8.1, J = 1.4 Hz, 2 H,
(s, 3 H, OCH3), 2.47 (s, 3 H, CH3) ppm. 13C NMR (CDCl3,
3
4
5-H Qu); 7.68 (dt, J = 8.6, J = 1.4 Hz, 2 H, 7-H Qu), 7.64–7.55
75 MHz, 25 °C): δ = 197.5 (C=O), 159.9 (C-4), 155.4 (C=N), 131.0
(m, 8 H, 3-H Qu, 3-H, 4-H, 5-H Ph), 7.49 (dt, 3J = 8.1, 4J = 1.1 Hz, (C-3, C-5), 121.0 (C-1 Ph), 113.2 (C-2, C-6), 55.2 (OCH3), 26.2
2 H, 6-H Qu) ppm. 13C NMR (CDCl3, 75 MHz): δ = 155.5 (C-2 (CH ) ppm. IR (KBr): ν = 3235, 3186, 3018, 2961, 1663, 1610,
˜
3
Qu), 150.8 (C-2, C-5 Pz), 148.4 (C-8a Qu), 144.9 (C-3, C-6 Pz),
137.7 (C-1 Ph), 136.3 (C-4 Qu), 135.5 (C-2 vin), 130.2 (C-2, C-6
Ph), 129.8, 129.7, 129.5, 129.2 (C-1 vin, C-7, C-8 Qu, C-4 Ph),
128.5 (C-3, C-5 Ph), 127.6, 127.5, 126.5 (C-4a, C-5, C-6 Qu), 120.0
1519, 1435, 1362, 1298, 1258, 1179, 1024, 999, 950, 822, 729,
619 cm–1. MS (FD): m/z (%) = 192.9 (100) [M]+.
1-Hydroxyimino-1-(1-naphthyl)propan-2-one (14d): Compound 13d
(8.6 g, 46.7 mmol) was dissolved in diethyl ether (50 mL) and conc.
hydrochloric acid (1 mL) was added. Isoamyl nitrite (6.6 g,
56.3 mmol) was added dropwise and the mixture heated at reflux
for 2 h. After cooling (0 °C), the precipitate was collected, dissolved
in NaOH (aq., 2.5 m), filtered, acidified with glacial acetic acid,
cooled to 0 °C), and filtered. Recrystallization from toluene gave
7.1 g (33.3 mmol, 71%) of off-white crystals with m.p. 158–159 °C.
(C-3 Qu) ppm. IR (KBr): ν = 3057, 1628, 1594, 1505, 1426, 1384,
˜
1205, 1134, 974, 822, 754, 699, 620 cm–1. MS (FD): m/z (%) = 598.4
(100) [M]+. HRMS (ESI): calcd. for C38H27N4 539.2236; found
539.2230.
(E,E)-2,5-Bis[2-(2-quinolyl)ethenyl]-3,6-bis(4-methoxyphenyl)-
pyrazine (12): Similar to the preparation of 11, 19c (30 mg,
0.086 mmol), and 20b (60 mg, 0.21 mmol) gave 44 mg (85%) of a 1H NMR (CDCl3, 400 MHz, 25 °C): δ = 11.67 (s, 1 H, OH), 7.86–
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Pages: 16
V. Schmitt, S. Moschel, H. Detert
FULL PAPER
7.81 (m, 2 H, 4-H, 5-H), 7.51–7.47 (m, 1 H, 8-H), 7.45–7.37 (m, 3 126.1, 125.4 (C-5, C-6, C-7 Np), 125.1 (C-3 Np), 21.9 (CH3) ppm.
H, 2-H, 3-H, 6-H), 7.24 (d, 1 H, 7-H), 2.60 (s, 3 H, CH3) ppm. 13
IR (ATR): ν = 1507, 1404, 1369, 1154, 1124, 950, 800, 774,
NMR (CDCl3, 25 °C, 100 MHz): δ = 196.6 (C=O), 156.0 (C=N), 732 cm–1. MS (FD): m/z (%) = 359.9 (100) [M]+. C26H20N2
C
˜
132.9, 130.2, 129.3, 128.7, 128.3, 126.6, 126.4, 126.0, 125.3, 26.1
(360.45): calcd. C 86.64, H 5.59, N 7.77; found C 86.28, H 5.50, N
7.78.
(CH ) ppm. IR (ATR): ν = 3368, 1671, 1388, 1358, 1300, 1258,
˜
3
1225, 1160, 1091, 1021, 991, 937, 793, 773, 723 cm–1. MS (FD):
2,5-Dimethyl-3,6-bis[4-(9-carbazolyl)phenyl]pyrazine (17e): CuI
(20 mg, 0.11 mmol), copper powder (200 mg, 3.15 mmol), and
K2CO3 (2.5 g, 18.1 mmol) were added to a solution of 14b (3.0 g,
7.17 mmol) and carbazole (2.7 g, 15.8 mmol) in 1,2-dichlorobenz-
ene (30 mL) and heated at 190 °C for 4 d. The mixture was cooled
and diluted with CHCl3, filtered, and concentrated in vacuo. The
product was recrystallized from CHCl3/toluene to yield 3.4 g
(5.7 mmol, 80%) of an off-white solid with m.p. 324–326 °C. 1H
m/z (%) = 212.8 (100) [M]+.
General Procedure for the Synthesis of 2,5-Dimethyl-3,6-diarylpyr-
azines 17 from Hydroxyimino Ketones 14: The 1-hydroxyimino-1-
arylpropan-2-one 14 (1 equiv.) was dissolved in NaOH (5 n) and
the solution added dropwise to a vigorously stirred suspension of
zinc dust (4 equiv.) in aqueous NaOH (5 n) and toluene. The mix-
ture was stirred for 2 h at 25 °C by external cooling. The organic
layer was removed and replaced by pure toluene. After stirring for
1 h, the organic layer was separated, combined with the first tolu-
ene phase, and heated at reflux while bubbling air through the solu-
tion. The reaction was complete after 1 h (TLC), the toluene was
removed in vacuo and the colorless residue recrystallized from
CH2Cl2/methanol.
3
NMR (CDCl3, 400 MHz, 25 °C): δ = 8.18 (d, J = 7.6 Hz, 4 H, 4-
3
3
H, 5-H Cb), 7.95 (d, J = 8.4 Hz, 4 H, 2-H, 6-H Ph), 7.76 (d, J =
8.4 Hz, 4 H, 3-H, 5-H Ph), 7.55 (d, 3J = 8.2 Hz, 4 H, 1-H, 8-H
Cb), 7.46 (t, 4 H, 2-H, 7-H Cb), 7.34 (t, 4 H, 3-H, 6-H Cb), 2.85
(s, 6 H, CH ) ppm. IR (ATR): ν = 1603, 1518, 1448, 1392, 1220,
˜
3
1167, 1069, 750, 739, 719, 676 cm–1. MS (FD): m/z (%) = 294.2 (7)
[M]2+, 588.7 (100) [M]+. C42H30N4 (590.71): calcd. C 85.40, H 5.12,
N 9.48; found C 84.98, H 5.03, N 9.13.
2,5-Dimethyl-3,6-diphenylpyrazine (17a): Yield: 38 %, m.p. 125–
126 °C (ref.[33] 126 °C). 1H NMR (CDCl3, 400 MHz, 25 °C): δ =
7.65–7.63 (m, 4 H, 2-H, 6-H Ph-p), 7.52–7.48 (m, 4 H, 3-H, 5-H
Ph-p), 7.46–7.42 (m, 2 H, 4-H Ph-p), 2.65 (s, 6 H, CH3) ppm. 13C
NMR (CDCl3, 75 MHz, 25 °C): δ = 151.0 (C-2 Pz), 147.8 (C-1 Pz),
138.8 (C-1 Ph-p), 129.0 (C-2, C-6 Ph-p), 128.5 (C-3, C-5 Ph-p),
3,6-Diphenylpyrazine-2,5-dicarbaldehyde (19a): SeO2 (382 mg,
3.44 mmol), 17a (450 mg, 1.72 mmol) and 1,4-dioxane (0.3 mL)
were added to diphenyl ether (25 mL). The mixture was heated at
180 °C and stirred for 4 d under nitrogen. The cooled mixture was
filtered and purified by column chromatography on silica gel using
gradient elution of diphenyl ether/petroleum ether changing to tol-
uene/ethyl acetate (5:1). The product was recrystallized from tolu-
ene/petroleum ether, yield 280 mg (56%) of a yellow solid with m.p.
180 °C. Rf = 0.5 (toluene/ethyl acetate = 5:1). 1H NMR (CDCl3,
400 MHz, 25 °C): δ = 10.27 (s, 2 H, CHO), 7.80–7.78 (m, 4 H, 2-
H, 6-H Ph-p), 7.59–7.57 (m, 6 H, 3-H, 4-H, 5-H Ph-p) ppm. 13C
NMR (CDCl3, 75 MHz, 25 °C): δ = 190.2 (CHO), 153.0 (C-1 pyr),
143.9 (C-2 pyr), 134.3 (C-1 Ph-p), 130.7 (C-2, C-6 Ph-p), 130.1 (C-
128.4 (C-4 Ph-p), 22.7 (CH ) ppm. IR (ATR): ν = 3048, 1675, 1448,
˜
3
1392, 1226, 1162, 1064, 1025, 963, 917, 795, 754, 695, 670 cm–1.
MS (FD): m/z (%) = 261.1 (100) [M]+.
2,5-Dimethyl-3,6-bis(4-bromophenyl)pyrazine (17b): According to
the general procedure, 14b (19 g, 75.8 mmol), Zn (20.0 g, 0.31 mol),
NaOH (150 mL), and toluene (100 mL) yielded 6.8 g (16.3 mmol,
42%) of a colorless solid with m.p. 220–222 °C. 1H NMR (CDCl3,
400 MHz, 25 °C): δ = 7.64 (d, 3J = 8.4 Hz, 4 H, 2-H, 6-H Ph), 7.52
(d, 3J = 8.4 Hz, 4 H, 3-H, 5-H Ph), 2.63 (s, 6 H, CH3) ppm. 13C
NMR (CDCl3, 100 MHz, 25 °C): δ = 150.1 (C-3, C-6 Pz), 147.8
(C-2, C-5 Pz), 137.4 (C-1 Ph), 131.6 (C-2, C-6 Ph), 130.7 (C-3, C-
3, C-5 Ph-p), 128.8 (C-4 Ph-p) ppm. IR (KBr): ν = 3056, 2873,
˜
1704, 1448, 1421, 1353, 1200, 1170, 1081, 1065, 1023, 847, 769,
702 cm–1. MS (FD): m/z (%) = 288.2 (100) [M]+. HRMS: calcd.
C18H12O2N2Na 311.0800; found 311.0797.
5 Ph), 123.1 (C-4 Ph), 22.6 (CH ) ppm. IR (ATR): ν = 3021, 1441,
˜
3
1396, 1384, 1160, 1073, 1007, 966, 827, 814, 715, 669 cm–1. MS
(FD): m/z (%) = 417.7 (100) [M]+. C18H14Br2N2 (418.13): calcd. C
51.71, H 3.37, N 6.70; found C 51.17, H 3.38, N 6.67.
3,6-Bis(4-methoxyphenyl)pyrazine-2,5-dicarbaldehyde (19c): Pre-
pared from 17c according to the procedure for 19a, yield 100 mg
(21 %) of an orange solid with m.p. 167 °C. 1H NMR (CDCl3,
2,5-Dimethyl-3,6-bis(4-methoxyphenyl)pyrazine (17c): According to
the general procedure, 14c (3.6 g, 18.6 mmol), Zn (5.0 g,
76.5 mmol), NaOH (50 mL), and toluene (50 mL) yielded 0.7 g
(12%) of a colorless solid with m.p. 189–191 °C. 1H NMR (CDCl3,
400 MHz, 25 °C): δ = 7.59 (d, 3J = 8.8 Hz, 4 H, 2-H, 6-H Ph), 7.01
3
300 MHz): δ = 10.24 (s, 2 H, CHO), 7.78 (d, J = 8.8 Hz, 4 H, 2-
3
H, 6-H ph), 7.08 (d, J = 8.8 Hz, 4 H, 3-H, 5-H, ph), 3.93 (s, 6 H,
OCH3) ppm. 13C NMR (CDCl3, 75 MHz): δ = 190.6 (CHO), 161.9
(C4 ph), 151.8 (C2, C5 pz), 143.2 (C3, C6 pz), 131.7 (C1 ph), 126.7
3
(C2, C-6 ph), 114.4 (C3, C-5 ph), 55.5 (OCH ) ppm. IR (KBr): ν
˜
(d, J = 8.8 Hz, 4 H, 3-H, 5-H Ph), 3.88 (s, 6 H, OCH3), 2.64 (s, 6
3
= 2932, 2841, 1706, 1606, 1518, 1417, 1255, 1176, 1026, 840 cm–1.
MS (FD): m/z (%) = 348.2 (100) [M]+. HRMS: calcd. for
C20H17O4N2 349.1189; found 349.1193.
H, CH3) ppm. 13C NMR (CDCl3, 75 MHz, 25 °C): δ = 159.8 (C-4
Ph), 150.2 (C-3, C-6 Pz), 147.5 (C-2, C-5 Pz), 131.3 (C-1 Ph), 130.4
(C-2, C-6 Ph), 113.8 (C-3, C-5 Ph), 55.4 (OCH3), 22.8 (CH3) ppm.
IR (KBr): ν = 2999, 2963, 2939, 1608, 1514, 1451, 1394, 1294,
˜
Supporting Information (see footnote on the first page of this arti-
cle): 1H and 13C NMR spectra and UV/Vis/fluorescence spectra of
compounds 1–12.
1253, 1170, 1029, 835 cm–1. MS (FD): m/z (%) = 320.0 (100) [M]+.
C20H20N2O2 (320.39): calcd. C 74.98, H 6.29, N 8.74; found C
74.58, H 6.14, N 8.64.
2,5-Dimethyl-3,6-di(1-naphthyl)pyrazine (17d): According to the ge-
neral procedure, 14d (6.0 g, 28.1 mmol), Zn (6.0 g, 91.8 mmol),
NaOH (50 mL, 5 n), and toluene (50 mL) yielded 1.8 g (5.0 mmol,
36%) of a colorless solid with m.p. 240–241 °C. 1H NMR (CDCl3,
400 MHz, 25 °C): δ = 8.01–7.96 (m, 4 H, 2-H, 4-H Np), 7.67–7.60
(m, 6 H, 3-H, 5-H, 8-H Np), 7.28–7.51 (m, 4 H, 6-H, 7-H Np),
2.44 (s, 6 H, CH3) ppm. 13C NMR (CDCl3, 75 MHz): δ = 151.6
(C-2 Pz), 149.5 (C-1 Pz), 136.2 (C-1 Np), 133.8 (C-8a Np), 131.3
(C-4a Np), 129.0 (C-4 Np), 128.5 (C-2 Np), 127.0 (C-8 Np), 126.6,
Acknowledgments
The authors thank the Deutsche Forschungsgemeinschaft (DFG)
for financial support, H. Kolshorn for NMR spectra, and the refer-
ees of this article for important advice.
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