Conjugated compounds based on vinylthiazole units

Article abstract of DOI:10.1515/znb-2007-1209

Conjugated compounds based on vinylthiazole units show a strong polarization along the π chain, when NR₂ groups as electron donors are attached in the terminal position. The effect can be even more enhanced by a CHO group as electron acceptor in the opposite terminal position. This property makes such oligomers (n = 1, 2, . . .) interesting for applications in linear and nonlinear optics.

Full text of DOI:10.1515/znb-2007-1209

Conjugated Compounds Based on Vinylthiazole Units
Herbert Meier, Frank Nicklas, and Ralf Petermann
Institute of Organic Chemistry, University of Mainz, Duesbergweg 10 – 14, 55099 Mainz, Germany
Reprint requests to Prof. Dr. H. Meier. Fax: 0049-(0)6131-3925396.
E-mail: hmeier@mail.uni-mainz.de
Z. Naturforsch. 2007, 62b, 1525 – 1529; received July 2, 2007
Conjugated compounds based on vinylthiazole units show a strong polarization along the π chain,
when NR2 groups as electron donors are attached in the terminal position. The effect can be even
more enhanced by a CHO group as electron acceptor in the opposite terminal position. This property
makes such oligomers (n = 1, 2, . . . ) interesting for applications in linear and nonlinear optics.
Key words: Absorption, Conjugation, Fluorescence, Polarization, Push-pull Effect
Introduction
X
X
+
S
S
A
A
Conjugated oligomers attract increasing attention
because of their interesting properties in materials sci-
ence [1]. Moreover, they are model compounds for
conjugated polymers. Molecules polarized by elec-
tronic effects, in particular push-pull systems having
D
D
–
S
S
X
X
n
n
1
2
CH
X
N
terminal donor-acceptor substitution represent a spe- Fig. 1. Push-pull compounds with conjugated 2,5-thienylene-
vinylene or thiazole-2,5-diylvinylene building blocks (n = 1,
2, . . . ).
cial class of such oligomers [2]. Their major applica-
tions are in the field of nonlinear optics (NLO), two-
photon absorption (TPA), two-photon induced fluo-
rescence (TPIF), and photorefractive materials (PR).
Results and Discussion
Fig. 1 illustrates known 2,5-thienylenevinylenes with
zwitterionic resonance structures 1 (D = NR2; A =
NO2, CHO, SO2R; n = 1, 2) [3, 4]. The corresponding
compounds 2 with thiazole instead of thiophene rings
are unknown.
Scheme 1 shows the synthetic route to the tar-
get compounds 2a – c. 2-Bromo-1,3-thiazole (3) was
subjected to a Bouveault formylation to thiazole-2-
carbaldehyde (4). Reduction of 4 with NaBH4 yielded
alcohol 5, which was transformed with PBr3 to 2-
(bromomethyl)thiazole (6). The variant [15] of the Ar-
buzov reaction using NaOP(OC2H5)2 afforded phos-
phonate 7. The reaction of 6 with triethyl phosphite
led to the formation of C2H5Br, which attacked the ni-
trogen atom of the thiazole ring and lowered the yield
of 7.
On the other side, thiazole 3 was reacted with di-
hexylamine to yield 2-dihexylaminothiazole (8) [16].
Vilsmeier formylation led to aldehyde 9, which in a
Wittig-Horner reaction with 7 gave the target com-
pound 2a. Formylation of 2a with BuLi/DMF af-
forded the aldehyde 2b. The reactivity in 5-position
of the thiazole ring is much higher than in the two
free 4-positions of both thiazole rings. The final
step 2b + 7 → 2c was achieved by another Wittig-
Horner reaction.
On the whole, very few 1,2-di(thiazolyl)ethenes
were reported. Among the three symmetric (2,2ꢀ;
4,4ꢀ; 5,5ꢀ) and the three unsymmetric (2,4ꢀ; 2,5ꢀ;
4,5ꢀ) linkings of two thiazole rings by a vinylene
bridge, only one parent system, namely 1,2-di(2-
thiazolyl)ethene has been described [5]. Whereas
some derivatives of 1,2-di(4-thiazolyl)ethene [6 –
8], 1,2-di(5-thiazolyl)ethene [9 – 13] and 1-(2-thi-
azolyl)-2-(4ꢀ-thiazolyl)ethene [14] are known, 2,5ꢀ-
and 4,5ꢀ-linkings are – to our best knowledge –
unknown.
We report here on 1-(2-thiazolyl)-2-(5ꢀ-thiazol-
yl)ethenes (2). The electron donor D is represented by
a dihexylamino group, the electron acceptor A can be
a thiazole ring (2a) or an thiazol bearing a carbonyl
group (2b).
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H. Meier et al. · Conjugated Compounds Based on Vinylthiazole Units
Fig. 2. 1H and 13C NMR data of 2a (δ values in CD3COCD3,
TMS as internal standard) [18]. The olefinic protons show
a
3Jtrans coupling of 15.6 Hz. The AB spin system of the
thiazole ring has a 3J coupling constant of 3.2 Hz.
Fig. 3. Absorption (···) and fluorescence (—, normalized in-
tensity) of 2a and 2b in CH2Cl2.
1
the Experimental Section. The H and 13C chemical
shifts of 2a are shown as an example in Fig. 2.
The polarization along the conjugated chains in
compounds 2a – c is revealed by the polarization of
the olefinic C=C double bonds. The ∆δ values of their
13C chemical shifts provide a reliable indication for
this effect [2]. We found for 2a a ∆δ value of 126.2 –
117.7 = 8.5 ppm. The strong push-pull effect in 2b cor-
responds to ∆δ = 14.1 ppm. The electron-releasing ef-
fect of the dihexylamino group is extended along the
chain of the dimer 2c as well; however, its decrease
with increasing distance from the NR2 group is docu-
mented by the fact, that the ∆δ value for the olefinic
double bond on the side of the amino group is much
higher than on the other side, namely 10.1 compared
to 1.3 ppm.
Scheme 1. Preparation of the target compounds 2a – c.
The trans selectivity of the double bond formation
Formylation of 2c on the outer, mono-substituted
thiazole ring would lead again to a strong push-pull
effect. We tried the conditions used for 2a → 2b, but
2c yields a non-uniform product mixture.
1
in a Wittig-Horner olefination is high. The H NMR
spectra of crude reaction mixtures showed only small
amounts (≤ 5 %) of the cis isomers [17].
A full spectroscopic characterization of all com-
Fig. 3 exhibits the absorption and emission spectra
pounds by FT IR, FD MS, and NMR data is given in of 2a and 2b. The strong push-pull effect in 2b shifts
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H. Meier et al. · Conjugated Compounds Based on Vinylthiazole Units
1527
Table 1. Comparison of the thiazole systems 2a – c with the
corresponding (E)-stilbenes having the same endgroupsa.
Thiazole-2-carbaldehyde (4)
The preparation of 4 was performed according to the liter-
Thiazole
system 2
(this work)
390
Stilbenoid
system
[22, 23]
366
ature [19, 20]. Pale yellow oil, yield 85 % (lit. [19]: 88 %). –
1H NMR (CDCl3): δ = 7.75 (dd, J = 2.9 Hz, 6J = 1.0 Hz,
3
1H, 5-H), 8.12 (d, 3J = 2.9 Hz, 1H, 4-H), 10.06 (d, 6J =
1.0 Hz, 1H, CHO). – 13C NMR (CDCl3): δ = 126.3 (C-5),
145.8 (C-4), 166.2 (C-2), 183.9 (CHO).
Absorption: λmax (CH2Cl2)
2a
2b
2c
451
452
423
403
2a
2b
2c
8.5
14.1
10.1
1.3
6.4
10.6
6.8
Chemical shift differences ∆δ
(in CD3COCD3) of the 13C nu-
clei of the C=C double bonds
Thiazol-2-yl-methanol (5)
The preparation was carried out according to the liter-
ature [5, 19]. Colorless solid, yield 89 % (lit. [19]: 81 %),
m. p. 63 ◦C (lit. [19]: 66 – 67 ◦C). – 1H NMR (CDCl3):
δ = 4.42 (br. s, 1H, OH), 4.91 (s, 2H, CH2), 7.28 (d,
3J = 3.0 Hz, 1H, 5-H), 7.67 (d, 3J = 3.0 Hz, 1H,
4-H). – 13C NMR (CDCl3): δ = 61.7 (CH2), 119.2 (C-5),
142.3 (C-4), 171.8 (C-1).
0.7
a
λ in nm, δ in ppm.
the long-wavelength absorption maximum from 390
for 2a to 451 nm and the fluorescence maximum
from 458 to 540 nm. The extension of the conju-
gated chromophore in 2c has almost the same effect.
Dimer 2c in CH2Cl2 has λmax values of 452 nm (A)
and 541 nm (F), respectively. The Stokes shift amounts
to 3807, 3654 and 3640 cm−1 for 2a, 2b and 2c.
2-Bromomethyl-thiazole (6)
To a solution of 5 (1.06 g, 9.2 mmol) in CHCl3 (8 mL),
PBr3 (0.29 mL, 0.835 g, 3.1 mmol) was carefully added.
After 1 h refluxing, the mixture was treated with aque-
ous Na2CO3 solution till the precipitate initially formed
was dissolved. The neutralized water phase was then ex-
tracted with CHCl3 (2×25 mL). The organic layer was dried
with MgSO4 and evaporated. Filtration over a SiO2 column
(7 × 10 cm) with ethyl acetate yielded 1.27 g (77 %) of a
light green-yellow oil. (It is advisable to use the compound
directly for the next step, since it decomposes fast in the neat
form. Storage in diluted solution in CHCl3 in the refrigera-
Conclusions
The compounds 2a – c, which consist of vinyl-
thiazole building blocks, represent a new class of
conjugated oligomers. Terminal dihexylamino groups
as electron donors induce a polarization along the
π chain, which can be further enhanced when a formyl
group as electron acceptor is attached to the oppo-
site terminal position. Compared to the corresponding
oligo(1,4-phenylenevinylene)s [OPV], which contain
benzene instead of thiazole rings but the same end-
groups (D, A), 2a – c exhibit red-shifted UV/vis ab-
sorptions. The polarization of the π chains in 2a – c is
stronger than in the corresponding OPVs, as indicated
by the ∆δ shift differences of the C=C double bonds.
Table 1 summarizes this comparison. The extension of
the conjugated chain in 2 from n = 1, 2 to higher num-
bers of repeat units (n = 3, 4, ... ) will be of special
interest for applications in materials science.
1
tor is possible.) [21]. – H NMR (CDCl3): δ = 4.74 (s, 2H,
3
CH2), 7.37 (d, J = 2.9 Hz, 1H, 5-H), 7.74 (d, 3J = 2.9 Hz,
1H, 4-H). – 13C NMR (CDCl3): δ = 26.4 (CH2), 121.3 (C-5),
143.0 (C-4), 165.7 (C-2).
Diethyl thiazol-2-ylmethylphosphonate (7)
To NaH (60 %, 0.45 g, 11.2 mmol) in THF (37 mL),
diethyl phosphite (1.55 g, 11.2 mmol) dissolved in THF
(37 mL) was added dropwise within 5 min at 0 ◦C. Af-
ter further 30 min stirring at this temperature, 6 (1.95 g,
10.9 mmol) dissolved in THF (8 mL) was added dropwise.
After stirring overnight, the reaction was stopped by addi-
tion of crushed ice. Extraction with CHCl3 (4 × 50 mL),
drying (MgSO4) and evaporation furnished a residue, which
Experimental Section
FT-IR spectra were collected on a Perkin-Elmer GX/2000 was purified by column chromatography (4 × 40 cm silica
1
spectrometer. H and 13C NMR spectra were recorded on a gel, ethyl acetate / ethanol 10 : 1). Viscous oil, yield 1.24 g
Bruker AM 400 spectrometer. For FD MS and HRMS mea- (48 %). – FT IR (neat): ν = 3081, 2984, 2910, 1499, 1393,
surements a Finnigan MAT 95 spectrometer was used. The 1256, 1166, 1029, 971, 790, 764 cm−1. – 1H NMR (CDCl3):
UV/vis and the fluorescence spectra were obtained on a Zeiss δ = 1.27 (t, 3J = 7.3 Hz, 6H, CH3), 3.63 (d, 2J (P,H) =
MCS 320/340 and a Perkin-Elmer LS 50 B spectrometer, re- 21.0 Hz, 2H, PCH2), 4.10 (quint., 3J (P,H) ≈ 3J (H,H) =
spectively. The elemental analyses were performed in the mi- 7.3 Hz), 7.27 (dd, 3J = 3.4 Hz, 6J = 1.5 Hz, 1H, 5-H), 7.70 (d,
croanalytical laboratory of the Chemistry Department of the 3J = 3.4 Hz, 1H, 4-H). – 13C NMR (CDCl3): δ = 16.3 (d, 3J
1
University of Mainz.
(P,C) = 6.4 Hz, CH3), 31.9 (d, J (P,C) = 140.5 Hz, PCH2),
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H. Meier et al. · Conjugated Compounds Based on Vinylthiazole Units
62.7 (d, 2J (P,C) = 6.4 Hz, OCH2), 120.1 (d, 5J (P,C) = to an organic phase which was dried with MgSO4 and
3.2 Hz, C-5), 142.3 (d, 4J (P,C) = 3.2 Hz, C-4), 160.1 (d, evaporated. Purification of the residue was achieved by
2J (P,C) = 9.6 Hz, C-1). – FD MS: m/z ( %) = 236 (100)
column chromatography (4 × 24 cm, Al2O3, petroleum
[M+H]+. – C8H14NO3PS (235.2): calcd. C 40.85, H 6.00,
N 5.95, S 13.63; found C 40.63, H 6.12, N 6.15, S 13.78.
ether (b. p. 40 – 70 ◦C) / ethyl acetate 8 : 1). Yellow solid,
yield 320 mg (41 %), m. p. 75 ◦C. – FT IR: ν = 3931,
2858, 1668, 1606, 1534, 1408, 1346, 1235, 1145, 953,
813 cm−1. – 1H NMR (CD3COCD3): δ = 0.88 (t, 6H,
CH3), 1.34 (m, 12H, CH2), 1.70 (m, 4H, β-CH2), 3.53 (m,
4H, α-CH2), 6.67 (d, 3J = 15.6 Hz, 1H, olefin. H, car-
bonyl side), 7.51 (s, 1H, 4-H, amino-substituted thiazole
ring), 7.84 (d, 3J = 15.6 Hz, 1H, olefin. H, amino side),
8.47 (s, 1H, 4-H, formyl-substituted thiazole ring), 10.01 (s,
1H, CHO). – 13C NMR (CD3COCD3): δ = 14.2 (CH3),
23.2, 27.2, 27.8, 32.2 (CH2), 52.3 (NCH2), 116.3 (olefin.
CH, carbonyl side), 124.5 (C-5, amino-substituted thiazole
ring), 130.4 (olefin. CH, amino side), 138.0 (C-5, formyl-
substituted thiazole ring), 147.8 (C-4, formyl-substituted thi-
azole ring), 154.0 (C-4), 171.8 (C-2, amino-substituted thia-
zole ring), 174.6 (C-2), 183.0 (CHO). – FD MS: m/z ( %) =
406 (100) [M+H]+. – C21H31N3OS2 (405.6): calcd. C 62.18,
H 7.70, N 10.36, S 15.81; found C 62.11, H 7.65, N 10.21,
S 15.83.
N,N-Dihexyl-(thiazol-2-yl)-amine (8)
Dihexylamine (6.90 g, 37.2 mmol), 2-bromothiazole
(3) [24] (6.10 g, 37.2 mmol) and K2CO3 (1.03 g, 7.5 mmol)
were refluxed in DMF (22 mL) for 10 h. Evaporation of
the solvent in the vacuum led to a residue, which was
washed with 75 mL of 10 % NaOH and extracted with di-
ethyl ether (4 × 25 mL). The organic phase was dried with
MgSO4 and evaporated. Purification by column chromatog-
raphy (4 × 45 cm, silica gel, ethyl acetate / petroleum ether
◦
(b. p. 40 – 70 C) 1 : 20) yielded 6.90 g (69 %) of a color-
less oil, which was identical with an authentic probe ob-
tained by the reaction of dihexylthiourea and 2-chloro-1,1-
dimethoxyethane [8].
2-Dihexylamino-thiazole-5-carbaldehyde (9)
The aldehyde 9 was prepared according to ref. [8]. Pale
yellow oil; yield 63 % (lit. [8]: 63 %).
N,N-Dihexyl-(5-{(E)-2-[5-((E)-2-thiazol-2-yl-vinyl)thiazol-
2-yl]vinyl}thiazol-2-yl)amine (2c)
N,N-Dihexyl-[5-((E)-2-thiazol-2-yl-vinyl)thiazol-2-yl]amine
(2a)
The preparation of 2c was performed as discribed above
for 2a. Phosphonate 7 (74 mg, 0.31 mmol) and aldehyde 9
yielded after the described column chromatography 96 mg
(64 %) of a viscous, yellow oil. – IR (neat): ν = 2927, 2865,
1714, 1601, 1537, 1424, 1334, 1231, 1133, 1079, 941, 717,
631 cm−1. – 1H NMR (CD3COCD3): δ = 0.88 (t, 6H, CH3),
1.33 (m, 12H, CH2), 1.68 (m, 4H, β-CH2), 3.50 (t, 4H,
Phosphonate 7 (493 mg, 2.1 mmol) was treated in DME
(4 mL) with NaH (60 %, 100 mg, 2.5 mmol). At the end of
the evolution of H2, aldehyde 9 (620 mg, 2.0 mmol) was
added in DME (4 mL). After TLC control (silica gel, diethyl
ether) had indicated the consumption of 9, H2O (50 mL)
was slowly added. Extraction with diethyl ether (5×20 mL)
gave an organic phase which was dried (MgSO4) and evap-
orated. Purification by column chromatography (3 × 27 cm,
Al2O3, petroleum ether (b. p. 40 – 70 ◦C) / ethyl acetate 8 : 1)
yielded 520 mg (69 %) of a yellow oil. – FT IR (neat):
ν = 2955, 2857, 1611, 1538, 1478, 1132, 936 cm−1. –
FD MS: m/z ( %) = 378 (100) [M+H]+. – HRMS: m/z =
377.1940 (calcd. 377.1946 for C20H31N3S2, [M]+).
3
NCH2), 6.59/7.58 (AB, J = 15.6 Hz, 2H, olefin. H, amino
3
side), 7.05/7.69 (AB, J = 15.6 Hz, 2H, olefin. H), 7.40 (s,
1H, 4-H, thiazole ring on the amino side), 7.53/7.81 (AB,
3J = 3.1 Hz, 2H, terminal thiazole ring), 7.88 (s, 1H, 4-H,
thiazole ring in the middle). – 13C NMR (CD3COCD3):
δ = 14.2 (CH3), 23.2, 27.2, 27.9, 32.2 (CH2), 52.3 (NCH2),
117.4, 127.5 (olefin. CH, amino side), 119.9 (HC-5, terminal
thiazole ring), 123.3, 124.6 (olefin. CH), 124.8 (C-5, amino
substituted thiazole ring), 135.5 (C-5, middle thiazole ring),
144.7, 145.8, 146.1 (C-4 of thiazole rings), 166.3, 167.1,
171.0 (C-2 of thiazole rings). – FD MS: m/z ( %) = 487 (100)
[M+H]+. – HRMS: m/z = 486.1944 (calcd. for C25H34N4S3:
486.1931 [M]+).
2-[(E)-2-(2-Dihexylamino-thiazol-5-yl)vinyl]thiazole-5-
carbaldehyde (2b)
Compound 2a (520 mg, 1.◦4 mmol), dissolved in dry THF
(5 mL), was treated at −78 C with 0.51 mL (1.38 mmol)
of a 2.7 M solution of BuLi in n-heptane and then after
about 40 min with 110 mg (1.5 mmol) DMF in dry THF
(2.5 mL). Stirring was continued for 1 h at −78 ◦C and
Acknowledgement
We are grateful to the Deutsche Forschungsgemeinschaft
overnight at r. t. A saturated NH4Cl solution was added and the Fonds der Chemischen Industrie for financial sup-
to the mixture. Extraction with CH2Cl2 (4 × 30 mL) led port.
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H. Meier et al. · Conjugated Compounds Based on Vinylthiazole Units
1529
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◦
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