Synthesis of new, highly luminescent bis(2,2'-bithiophen-5-yl) substituted 1,3,4-oxadiazole, 1,3,4-thiadiazole and 1,2,4-triazole

Article abstract of DOI:10.3762/bjoc.10.165

A new synthetic approach towards the preparation of functionalised, soluble, donor-acceptor (DA) alkylbithiophene derivatives of oxadiazole, thiadiazole and triazole is reported. Taking advantage of the Fiesselmann reaction, reactive bithiophene synthons having alkyl or alkoxy substituents at designated positions are prepared. Following a synthetic strategy, featuring the bottom-up approach, sequential structural elements are built, starting from a simple thiophene compound, until the target molecule is obtained, all in good yield. Supplementing the well established methods of oxadiazole and thiadiazole synthesis, efficient ring closure reaction affording a 4H-1,2,4-triazole unit is presented. All target ambipolar compounds display strong photoluminescence with measured quantum yields up to 0.59. Modification of the demonstrated synthetic routes may be exploited for the preparation of longer, specifically functionalised oligothiophenes, coupled to other heteroaromatic cores.

Full text of DOI:10.3762/bjoc.10.165

Synthesis of new, highly luminescent
bis(2,2’-bithiophen-5-yl) substituted 1,3,4-oxadiazole,
1,3,4-thiadiazole and 1,2,4-triazole
Anastasia S. Kostyuchenko1,2, Vyacheslav L.Yurpalov1, Aleksandra Kurowska3,
Wojciech Domagala3, Adam Pron4 and Alexander S. Fisyuk*1,2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 1596–1602.
1Department of Organic Chemistry, Omsk F. M. Dostoevsky State
University, 55a Mira Ave, 644077 Omsk, Russia, 2Laboratory of New
Organic Materials, Omsk State Technical University, Mira Ave, 11,
Omsk 644050, Russia, 3Department of Physical Chemistry and
Technology of Polymers, Silesian University of Technology, Marcina
Strzody 9, 44-100 Gliwice, Poland and 4Faculty of Chemistry Warsaw
University of Technology, Noakowskiego 3, 00-664 Warszawa,
Poland
doi:10.3762/bjoc.10.165
Received: 30 March 2014
Accepted: 16 June 2014
Published: 14 July 2014
Associate Editor: J. P. Wolfe
© 2014 Kostyuchenko et al; licensee Beilstein-Institut.
License and terms: see end of document.
Email:
Alexander S. Fisyuk* - fisyuk@chemomsu.ru
* Corresponding author
Keywords:
bithiophene; donor–acceptor; luminescence; 1,3,4-oxadiazole;
1,3,4-thiadiazole; 4H-1,2,4-triazole
Abstract
A new synthetic approach towards the preparation of functionalised, soluble, donor–acceptor (DA) alkylbithiophene derivatives of
oxadiazole, thiadiazole and triazole is reported. Taking advantage of the Fiesselmann reaction, reactive bithiophene synthons
having alkyl or alkoxy substituents at designated positions are prepared. Following a synthetic strategy, featuring the bottom-up ap-
proach, sequential structural elements are built, starting from a simple thiophene compound, until the target molecule is obtained,
all in good yield. Supplementing the well established methods of oxadiazole and thiadiazole synthesis, efficient ring closure reac-
tion affording a 4H-1,2,4-triazole unit is presented. All target ambipolar compounds display strong photoluminescence with
measured quantum yields up to 0.59. Modification of the demonstrated synthetic routes may be exploited for the preparation of
longer, specifically functionalised oligothiophenes, coupled to other heteroaromatic cores.
Introduction
In the past two decades oligo- and polythiophenes gained a effect transistors (FETs), electrochemical and chemical sensors
significant research interest due to their wide application as and other organic electronic devices [1-5]. This family of
organic semiconductors in light emitting diodes (OLED’s), field organic semiconductors is especially interesting because they
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Results and Discussion
can be rendered solution processable, yielding strictly
controlled supramolecular organisation, provided that appro- Synthetic toolbox
priate deposition techniques are applied, such as zone casting There are several approaches to the synthesis of 2,5-
[6-8], mechanical rubbing or directional epitaxial crystallisa- bis(2,2 '-bithiophen-5-yl)-1,3,4-oxadiazoles and 1,3,4-thiadia-
tion [9]. For many applications, however, the HOMO and zoles. Transition metal-catalysed cross-coupling reactions, such
LUMO levels of oligo- and polythiophenes and by conse- as Stille, Suzuki etc. are typically used for the formation of thio-
quence their electron affinity (EA) and ionization potential (IP) phene–thiophene or azole–thiophene bonds [14,16,17].
are not appropriate [2]. Since these two parameters determine However, the best results (higher yields, more facile purifica-
the redox, spectroscopic, electronic and optoelectronic prop- tion, shorter reaction times) are obtained by the formation of the
erties of oligothiophenes, their tuning is of crucial importance in azole ring via appropriately functionalized bithiophenes [14,22].
any design of new organic semiconductors. One of the possible Recently, we have developed a new method for the preparation
ways of the preparation of low molecular mass semiconductors, of 3-alkyl(aryl) substituted esters of 2,2'-bithiophene-5-
showing higher than oligothiophene IP values, is to synthesize carboxylic acids, which is based on a modification of the
molecules in which a central electron accepting group separates Fiesselmann reaction [11,23]. Ethyl 3-decyl-2,2'-bithiophene-5-
two bi-, ter- or quaterthiophene units. Such compounds carboxylate (3) was obtained by this method in 80–85% yields
containing thiadiazole [10-15], oxadiazole [14-17] or tetrazine at each step [24] (Scheme 1).
units [13,18-20] have been reported. Moreover, luminescence
properties of these derivatives are superior to those of the Ethyl 3-oxo-3-(2-thienyl)propanoate (4), obtained by a known
corresponding penta-ring oligothiophenes. For example, the method [25], was used as starting compound for the synthesis of
central heterocycle ring replacement in substituted quinquethio- ethyl 4-hydroxy-2,2'-bithiophene-5-carboxylate (6). Reaction of
phene by 1,3,4-oxadiazole leads to a substantial increase of the 4 with POCl3 led to the formation of ethyl 3-chloro-3-(2-
fluorescence quantum yield and a hypsochromic shift of the thienyl)acrylate (5) which was then converted to ethyl
emission band [14], both features being technologically advan- 4-hydroxy-2,2'-bithiophene-5-carboxylate (6) by reacting with
tageous. They are, however, difficult to solution process due to ethyl mercaptoacetate in a basic medium. The yields of 5 and 6
strong intermolecular interactions, associated with their were 76% and 49%, respectively. Ethyl 4-(hexyloxy)-2,2'-
extended conjugation and frequently have to be rendered bithiophene-5-carboxylate (7) was obtained by the alkylation of
soluble by introducing long alkyl- or alkoxy-type side 6 with hexyl iodide in the presence of potassium tert-butoxide
substituents. This functionalisation opens up an additional route in 81% yield (Scheme 2). Compounds 5–7 were chromato-
to modifying their redox, electronic and optical properties by graphically purified using a silica gel column.
using side substituents with different electron accepting/elec-
tron donating properties [11].
Esters 3 and 7 were converted to the corresponding carboxylic
acids 8 and 9 by heating in an alcoholic solution of sodium
In this article, we are expanding our prior syntheses of alkyl- hydroxide. The hydrolysis of 7 to the corresponding acid runs
bithiophene substituted thiadiazoles [11] to evaluate the effect more difficult than in the case of 3 and requires longer reaction
of other heteroaromatic rings at the central position on the times due to lower electrophilicity and a bigger steric hindrance
resulting properties of these five-ring compounds. DA com- at the carbethoxy group. The hydrazide derivative 10 was
pounds with a 1,3,4-oxadiazole or 1,2,4-triazole central ring are obtained by refluxing 3 with hydrazine monohydrate in alcohol.
reported herein. The synthesized derivatives are very interest- Hydrazinolysis of 7 under these conditions runs slow and leads
ing new semiconductors of possible use as electroluminophores to a mixture of products (Scheme 3).
since preliminary studies showed that they could be used as
active components of host–guests organic light emitting diodes Diacylhydrazine 11 was prepared by the reaction of carboxylic
[21].
acid 8 and hydrazide 10 in the presence of dicyclohexylcarbodi-
Scheme 1: Synthesis of ethyl 3-decyl-2,2'-bithiophene-5-carboxylate (3).
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Beilstein J. Org. Chem. 2014, 10, 1596–1602.
Scheme 2: Synthesis of ethyl 4-(hexyloxy)-2,2'-bithiophene-5-carboxylate (7).
Scheme 3: Hydrolysis and hydrazinolysis of esters 8–10.
imide (DCC) in 78% yield. Compound 12 was obtained in 60% 2,2'-bithiophen-5-yl)-1,3,4-oxadiazole (13) and 2,5-bis[4-
yield by reacting hydrazine monohydrate and the carboxylic (hexyloxy)-2,2'-bithiophen-5-yl]-1,3,4-oxadiazole (14) by the
acid chloride which was prepared in situ from 9 with oxalyl reaction with phosphorus oxychloride, in 78 and 83% yields,
chloride (Scheme 4).
respectively. The reaction of 11 with a Lawesson’s reagent led
to the formation of 2,5-bis(3-decyl-2,2'-bithiophen-5-yl)-1,3,4-
It is known that 1,3,4-oxadiazoles [26], 1,3,4-thiadiazole [27] tiadiazole (15, 70% yield) (Scheme 4). To the contrary, 3,5-
and 1,2,4-triazoles [28-30] can be obtained from diacyl- bis(3-decyl-2,2'-bithiophen-5-yl)-4-phenyl-4H-1,2,4-triazole
hydrazines. Thus, 11 and 12 were converted to 2,5-bis(3-decyl- (18) could not be obtained by reacting 11 with O=P(NHPh)3
Scheme 4: Synthesis of 2,5-bis(2,2'-bithiophen-5-yl)-1,3,4-oxadiazoles 13 and 14 and 1,3,4-thiadiazole 15.
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[30]. The major reaction product was the corresponding oxadia-
zole derivative 13. An attempt to replace the oxygen atom by
the nitrogen one upon refluxing 13 with aniline [31,32] was also
unsuccessful. For these reasons, a different approach [33] had to
be used for the synthesis of 3,5-bis(3-decyl-2,2'-bithiophen-5-
yl)-4-phenyl-4H-1,2,4-triazole (18). Anilide 16 was used as a
building block for the synthesis of 18. It was prepared from 8
by a consecutive reaction with oxalyl chloride and then aniline.
In the next step 16 was converted to 3-decyl-N-phenyl-2,2'-
bithiophene-5-carboximidoyl chloride (17) by the reaction with
phosphorus pentachloride, and then, without additional purifica-
tion, used at once in the reaction with acyl hydrazide 10. As a
Table 1: Absorption spectral parameters of investigated
donor–acceptor–donor molecules.
Compound λmax [nm]
λonset [nm]a HOMO–LUMO
transition [eV]a
13
14
15
18
376
395
406
365
432
455
469
421
2.87
2.72
2.64
2.94
aPertaining to onset of the π–π* absorption peak.
result, 3,5-bis(3-decyl-2,2'-bithiophen-5-yl)-4-phenyl-4H-1,2,4- the two 1,3,4-oxadiazole derivatives 13 and 14, is caused by the
triazole (18) was obtained in 62% yield (Scheme 5).
electron-donating alkoxy chain, increasing the molecule HOMO
level resulting in narrowing of the frontier orbital gap. For the
All intermediate and final products were identified by elemental three targets with invariant bithiophene units (18, 13 and 15),
analysis, NMR and IR spectroscopy (see Supporting Informa- the smallest absorption band energy is observerd for 15, i.e., the
tion File 1).
compound with the thiadiazole central unit. Such a decrease of
the absorption band energy is typical of all sulfur containing
five membered heterocycles [14,15,34].
Spectroscopic and luminescent properties of
the synthesized compounds
The target molecules 13, 14, 15 and 18 feature light absorption Upon excitation with the UV light, all investigated compounds
in the near ultraviolet, tailing into high energy visible spectral feature strong photoluminescence, emitting blue and green light
range, giving them yellow to orange-tinted colouration in the (Table 2). Their emission energy maxima arrange themselves in
solid state. When dissolved, they produce faint yellow coloured the very same order as did the absorption energy maxima, i.e.:
solutions exhibiting clear fluorescence properties, perceivable 18 > 13 > 14 > 15. Compared to the featureless absorption
as a blue to bluish-green hue when exposed to direct sunlight. spectra, structured fluorescence peaks are observed with two
The absorption and maximum emission fluorescence spectra of clearly discernible components and a third one concealed in the
their solutions in dichloromethane are collected in Figure 1, low energy tail (Figure 1). This fine structure appears to be a
while their representative spectral parameters have been manifestation of enhanced vibronic coupling, which clearly
compiled in Table 1.
suggests rigidification of the excited state molecular geometry
of the compounds. This can be anticipated, considering the
All four compounds display an intense absorption band at the quinoid bond configuration of the excited state populated
visible range edge, ascribable, on account of its energy, to a LUMO energy level of each molecule, in which the order of
π–π* transition. The wavelength maximum of this band clearly bonds connecting the heteroaromatic units increases. The
changes with the azole unit, as well as with the thiophene solu- energy dissipated upon geometric relaxation of the excited
bilising substituent, from 365 nm for 18, 376 nm for 13, 395 nm molecule following the vertical HOMO–LUMO transition, prior
for 14 up to 406 nm for 15. The difference of 19 nm between to the return transition from S1 to S0 state, is manifested as a
Scheme 5: Synthesis of 3,5-bis(3-decyl-2,2'-bithiophen-5-yl)-4-phenyl-4H-1,2,4-triazole (18).
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Beilstein J. Org. Chem. 2014, 10, 1596–1602.
Figure 1: UV–vis absorption and fluorescence spectra of diluted solutions of a) 13, b) 14, c) 15, and d) 18 in dichloromethane. Optimum fluorescence
excitation wavelengths (given in the legends) have been selected based on fluorescence emission profiles.
sequence 15 > 18 > 13 ≈ 14, and scaling well with the atomic
Table 2: Fluorescence spectral parameters of investigated
donor–acceptor–donor molecules.
radii of the azole completive heteroatom. Moderate to high fluo-
rescence quantum efficiency yields are also observed, ranging
from 0.59 for 18 to 0.21 for 15. Here, a clear heteroatom effect
is observed, while the influence of the thiophene substituent
seems negligible. The observed trend of decreasing quantum
yield with increasing atomic number of the heteroatom of the
azole unit is a clear manifestation of the heavy atom effect [35].
The proximity and availability of higher energy 3d levels in
sulfur facilitates radiationless relaxation modes of the excited
electron. Enhanced spin–orbit coupling opens up intersystem
crossing channels as well, making the thermal decay pathway
Compound λex [nm]a
λem [nm]b
Δ [nm]c
Фfd
13
14
15
18
365
382
373
363
438
461
484
502
454
62
60
75
66
0.42
0.45
0.21
0.59
455
481
431
aExcitation; bemission; cminimum Stokes shift; dquantum yield deter-
mined relative to 9,10-diphenylanthracene standard.
Stokes shift. Since 0−0 transitions are rarely observed in accessible. The hindering of these decay modes in the oxadia-
room temperature solution spectra, it is acceptable to use zole derivatives 13 and 14 doubles their quantum yield. For the
Δ = λem − λmax (see Table 1 and Table 2) as the Stokes shift triazole derivative 18 the quantum yield is even nearly triple
magnitude index [15]. Moderate values of Stokes shifts are that of 15, but this enhancement should also be traced to the
observed for the investigated compounds indicating that the impaired conjugation of the triazole moiety with the bithio-
geometric reconstruction takes place upon their optical excita- phene arms.
tion, the dominant reason being the bond order switching
accompanying the benzenoid–quinoid transition. Small differ- Comparing the obtained results with those reported for similar
ences in the magnitude of this shift are observed arranging in a donor–acceptor–donor compounds, we can see that both the
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Beilstein J. Org. Chem. 2014, 10, 1596–1602.
number of donor groups and the chemical nature of the central Acknowledgements
electron accepting ring exert strong influence on their fluores- This work was supported by the Russian Foundation for Basic
cence performance. With an increasing number of thiophene Research (12-03-98013-p_sibiria_a) (A. S .K., V. L. Y and A.
units higher quantum yield materials are obtained. For example, S. F). W. D. acknowledges the support of Polish National
the quantum yield of 2,5-bis(3-methylthien-2-yl)-1,3,4-oxadia- Science Centre grant no.: 2011/03/D/ST5/06042. A. K. is a
zole is 0.13 as compared to 0.46 reported for 2,5-bis[5-(3- scholar supported by the “DoktoRIS – scholarship program for
octylthien-2-yl)-3-methylthien-2-yl]-1,3,4-oxadiazole [16]. an innovative Silesia”, co-financed by European Union within
Similarly, the position of the substitution with solubilising European Social Fund. A. P. acknowledges the support of the
groups and the length of the side chains lead to a significant Foundation for the Polish Science through the project entitled
change of the quantum yield [11]. The effect of the central ring “New solution processable organic and hybrid (organic/inor-
nature on the optical properties of 13–15 and 18 is clearly ganic) functional materials for electronics, optoelectronics and
observed. In Figure 2 photoluminescence quantum yields are spintronics” (Contract no. TEAM/2011–8/6) co-financed by the
indicated together with the formulae of the corresponding com- EU European Regional Development Fund.
pounds. In the case of 18 (4-phenyl-4H-1,2,4-triazole ring) the
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Supporting Information
Supporting Information File 1
Experimental part.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-165-S1.pdf]
Figure 2: Photoluminescence quantum yields measured for 13, 15 and 18 together with their formulae.
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