Synthesis and chemosensing properties of cinnolinecontaining poly(arylene ethynylene)s

Article abstract of DOI:10.3762/bjoc.11.43

Novel poly(arylene ethynylene)s comprising a cinnoline core were prepared in high yields via a three-step methodology. A Richtertype cyclization of 2-ethynyl-and 2-(buta-1,3-diynyl)aryltriazenes was used for cinnoline ring formation, followed by a Sonogas

Full text of DOI:10.3762/bjoc.11.43

Synthesis and chemosensing properties of cinnoline-
containing poly(arylene ethynylene)s
Natalia A. Danilkina, Petr S. Vlasov, Semen M. Vodianik, Andrey A. Kruchinin,
Yuri G. Vlasov and Irina A. Balova*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2015, 11, 373–384.
Institute of Chemistry, Saint Petersburg State University,
Universitetskiy pr. 26, Saint Petersburg, 198504, Russia.
Fax: +7(812)4286733
doi:10.3762/bjoc.11.43
Received: 19 November 2014
Accepted: 02 March 2015
Published: 20 March 2015
Email:
Irina A. Balova* - balova.irina@chem.spbu.ru
Associate Editor: H. Ritter
* Corresponding author
© 2015 Danilkina et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
cinnolines; fluorescence quenching; Pd2+ detection; poly(arylene
ethynylene)s; Sonogashira coupling
Abstract
Novel poly(arylene ethynylene)s comprising a cinnoline core were prepared in high yields via a three-step methodology. A Richter-
type cyclization of 2-ethynyl- and 2-(buta-1,3-diynyl)aryltriazenes was used for cinnoline ring formation, followed by a Sono-
gashira coupling for the introduction of trimethylsilylethynyl moieties and a sila-Sonogashira coupling as the polycondensation
technique. The fluorescence of the cinnoline-containing polymers in THF was highly sensitive to quenching by Pd2+ ions.
Introduction
Optical sensors provide an efficient and sensitive approach for ular wire" concept [2-9]. Binding of a single analyte molecule
the rapid detection and identification of a wide range of chem- to a single recognition site completely quenches the fluores-
ical substrates by measuring the changes of optical properties cence of the whole polymer chain, giving the increase of the
such as absorbance, reflectance, luminescence, or fluorescence sensitivity (amplification). This effect is also known as
[1]. Among them a type of sensors based on conjugated poly- superquenching and comes from the delocalization of the
mers (CPs) are of crucial importance for biological, environ- charge carriers in the polymeric structures. Transport of exci-
mental and clinical applications because of the remarkably tons along the polymer backbone in a separate molecule as well
amplified response of CPs to an analyte, compared with the as the transport of excitons between adjacent molecules makes
respective monomer unit [2-7]. Since the change in fluores- possible a non-radiative relaxation of many excitons at a single
cence is the most sensitive and the most commonly used type of trap created as a result of an analyte binding to a receptor inte-
response, conjugated polymers for sensing are termed as ampli- grated into the polymer chain [10]. As long as the trap and the
fying fluorescent polymers (AFPs) [4]. The mechanism of the generation sites of the exciton are spatially separated, the
response amplification can be explained in terms of the "molec- quenching amplification factor is only limited by the diffusion
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Beilstein J. Org. Chem. 2015, 11, 373–384.
distance of the exciton. In practice, a single bound analyte sensing [12,15] and pyridine-containing polymers for other
molecule can deactivate excitons within 130 neighboring applications [16-18]. Moreover, various species of` PAEs with
phenylene ethynylene repeating units [4].
quinoline [19,20], quinoxaline [21,22], thiadiazole [17,23],
carbazole [18,24], 1,2,4-triazoles [25], thiazole [26] and
Sensors based on CPs can be classified either by the nature of azametallocyclic [27] units incorporated into a polymer chain
the polymer backbone, e.g., polyphenylenes, polythiophenes, have been described.
poly(phenylene vinylene)s, poly(phenylene ethynylene)s etc, or
by their chemosensing properties [3,4]. The latter depends on Since ligands based on pyridazine rings can form complexes
the nature of receptors (recognizing units) incorporated into a with transition metal ions (e.g. Cu+, Cu2+, Ag+, Co2+, Ni2+)
polymer chain that bind an analyte. Such materials find wide- [28-35], the introduction of this structural fragments into PAEs
spread use in sensing of metal ions and anions, detection of might give rise to a new family of chemosensors. Here we
explosives and various biomolecules [2−4]. Among the great describe the first synthesis of PAEs consisting of altering cinno-
variety of known conjugated polymers, poly(arylene ethyny- line, ethynyl and aryl units and investigate their chemosensing
lene)s (PAEs) are one of the most promising materials in the ability towards transition metal ions.
search for new chemical sensors [7].
Results and Discussion
Synthesis and structure of cinnoline-
containing PAEs
Cinnoline-containing PAEs with two types of repeating units
that differ by substituents at the C-3 carbon atom of a cinnoline
ring (alkyl or alkynyl moiety) were chosen as target structures
(Figure 2).
A wide range of PAEs has been developed in the last decade
[2-4], for example polymers with ether, ester or carboxylic acid
functions [11] and N-containing heterocyclic scaffolds [12] for
metal ion sensing; polymers with polycyclic aromatics and rigid
groups for the detection of explosives [13]; and PAEs with
ionogenic groups and specific functions for different kinds of
biosensing [14] (Figure 1).
They were chosen in order to study the influence of an addition-
There are several known examples of PAEs having bi-, tri- and al triple bond on the photochemical properties of cinnoline-
terpyridyl scaffolds that are capable of transition metal ion containing PAEs. It should be noted that PAEs with an addi-
Figure 1: Recent examples of PAEs and their application for the detection of Hg2+ (a) [11], Ni2+ (b) [12], explosives (c) [13] and trypsin (d) [14].
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Beilstein J. Org. Chem. 2015, 11, 373–384.
Figure 2: Target structures of PAEs.
tional ethynyl moiety are almost unknown and only few exam- symmetrically substituted diarylacetylenes [49,50]. Mild condi-
ples of dimers and trimers were studied to date [36]. The tions allowed carrying out both reactions chemoselectively
O-alkylated hydroquinone scaffold was chosen as a commonly leaving the bromine atom for further synthetic steps. 4,6-
used spacer between the diethynylcinnoline units [2-4] that Dibromo-3-butylcinnoline (4a) and 4,6-dibromo-3-(hept-1-yn-
prevents the aggregation of polymer chains.
1-yl)cinnoline (4b) were obtained from corresponding triazenes
3a,b by Richter cyclization under known conditions [43] in
An efficient modification of the Richter type cyclization [37,38] satisfactory and good yields.
based on mono- [39,40] and diacetylene [41-43] aryltriazene
derivatives as starting materials was chosen over the approach Although the Sonogashira coupling of 4-bromocinnolines has
that uses unstable o-ethynyl- or o-(buta-1,3-diynyl)arenedi- been recently reported [41,51], the double Sonogashira coupling
azinium salts. This reaction was a key step for the synthesis of for dibromocinnolines has not been investigated. To optimize
cinnoline-containing structural units – 4,6-dibromocinnolines the coupling conditions, several test experiments were carried
4a,b (Table 1).
out using the 4,6-dibromo-3-butylcinnoline (4a, Table 2).
Triazenes 3a,b were synthesized from 4-bromo-2-iodophenyl- Taking into account that the substitution of a bromine atom in
triazene 1 by the Sonogashira coupling [44,45] with hexyne Pd-catalyzed coupling typically requires heating to 80–100 °C
(2a) and TMS-protected diacetylene 2b using conditions for the [45], we first tried heating in DMF at 80 °C over 24 h in the
one-pot TMS group removal and the Sonogashira coupling presence of diisopropanolamine (DIPA) as a base (Table 2,
developed recently [46]. The last reaction could also be defined entry 1). Under those conditions the reaction yielded an unex-
as a type of sila-Sonogashira coupling [47,48], which is an pected 4-methylcinnoline 7a that could be explained in terms of
extremely convenient method for the one-step synthesis of non- unusual Hiyama coupling [52-54] of trimethylsilylacetylene
Table 1: Synthesis of 4,6-bis(trimethylsilylethynyl)cinnolines 4.
Entry
R1
R2
Acetylene
Conditionsa
Triazene, (yield, %)
Conditionsa
Cinnoline, (yield, %)
1
2
C4H9
H
2a
2b
A
B
3a (76)
3b (90)
C
C
4a (87)
4b (58)
≡−C5H11
TMS
aConditions: A – Pd(PPh3)4, CuI, Et3N, 35 °C; B – Pd(PPh3)4, CuI, KF, MeOH, DMF, rt; C – aqueous HBr (48%, 20 equiv), acetone, 20 °C.
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Beilstein J. Org. Chem. 2015, 11, 373–384.
Table 2: Conditions optimization for the double Sonogashira coupling.
Entry
Dibromocinnoline
R
Conditionsa
Reaction product (yield, %)b
1
2
3
4
5
4a
4a
4a
4a
4b
Bu
A
B
C
C
C
7a (50)
Bu
5a:8a (or 8’a) (34:23)c
Bu
5a
Bu
5a (88)
5b (91)
≡−C5H11
aA – trimethylsilylacetylene 6 (5 equiv), Pd(PPh3)4, CuI, DIPA, DMF, 80 °C, 24 h; B – trimethylsilylacetylene 6 (3 equiv), Pd(PPh3)4, CuI, Et3N, 50 °C,
20 h; C – trimethylsilylacetylene 6 (3 equiv), Pd(PPh3)4, CuI, Et3N, 50 °C, 3.5 h; bEntries 1,2,4,5 – isolated yields, entry 3 – the full conversion of 4a to
5a was estimated by 1H NMR spectroscopy, the isolated yield of 5a was not determined; cThe distinguishing between 8a and 8’a was not done.
with the more reactive C-4 carbon atom of the cinnoline ring. Having starting compounds 5a,b in hand, we were ready to
The substitution of the bromine atom at C-4 (not at C-6) of the carry out the polycondensation with 1,4-diiodo-2,5-bis(octyl-
cinnoline ring was proved by an 1H NMR NOESY experiment. oxy)benzene (9). There are two main reactions of polyconden-
Switching to Et3N as a solvent and a base as well as decreasing sation that have been reported as synthetic approaches towards
the temperature gave a mixture of the desired product 5a along PAEs: the acyclic diyne metathesis polymerization (ADIMET)
with the monodesilylated compound 8a or 8’a (Table 2, entry [55] and the Sonogashira coupling [56]. Both of them have their
2). Using 1H NMR spectroscopy to monitor the reaction own advantages and drawbacks [57]. The ADIMET approach
allowed figuring out that heating in Et3N at 50 °C for 3.5 hours usually gives products with higher molecular weights and fewer
is enough for a full conversion of the starting cinnoline 4a into defects than the Pd-catalyzed polycondensation by the Sono-
the desired bis(trimethylsilylethynyl)cinnoline 5a (Table 2, gashira reaction. On the other hand, the Sonogashira coupling is
entry 3). The optimized conditions were used in the synthesis of more tolerant to different functional groups and allows the
bis(trimethylsilylethynyl)cinnolines 5a,b, so that the starting polymer structure to be varied significantly. We chose the
materials for the synthesis of PAEs were obtained in good Sonogashira coupling of diethynylcinnolines and alkylated
isolated yields (Table 2, entries 4 and 5). These experiments diiodohydroquinone as a method for the formation of PAE’s
revealed that both bromine atoms at pyridazine and benzene backbone.
rings are highly reactive in the Sonogashira coupling due to the
strong activation of the electron deficient nature of the hetero- It also should be mentioned that despite the fact that sila-Sono-
cycle.
gashira coupling have been widely applied for the synthesis of
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Beilstein J. Org. Chem. 2015, 11, 373–384.
different acetylenes with aryl- and hetaryliodides, -bromides ~2200 cm−1 are observed whereas there are no bands of
-tosylates and -triflates [47-50], there are only a few examples stretching vibration for monosubstituted triple bonds
where polycondensation was carried out under these conditions Ar−C≡C−H (≈2100 cm−1) and Ar−C≡C−TMS (≈2150 cm−1)
[58-60]. In our case this approach seemed to be more conveni- [61].
ent than the common Sonogashira coupling, because it allows
TMS-protected diethynylcinnolines to be used as a starting ma- The results of the elemental analysis were in agreement with the
terial instead of unstable terminal alkynes. It also avoids an oligomer structures possessing alkylated iodohydroquinone
extra synthetic step.
units as the end groups. The presence of iodine in oligomers
was also confirmed by the Beilstein test [62].
When we tried polycondensation of bis(trimethylsilyl)cinno-
lines 5a,b with diiodoarene 9 in the presence of Pd(PPh3)4/CuI Regarding the NMR data, the chemical shift values of H and C
as a catalytic system and KF/MeOH as a deprotecting source in atoms in both PAEs synthesized do not significantly differ from
Et3N, it afforded compounds 10a,b in quantitative yields even the chemical shift values in staring cinnolines 5a,b and
at 40 °C (Scheme 1).
diiodoarene 9 that allowed all 1H and 13C signals in NMR
spectra of PAEs to be assigned based on 1D and 2D NMR
Both products were readily soluble in CHCl3, DCM, THF, and spectra of starting compounds 5a,b, 9 and 1,4-diethynyl-2,5-
MeCN. This allowed for a convenient removal of copper and bis(octyloxy)benzene (for copies of all NMR spectra see
palladium ion traces by successive washing of the DCM solu- Supporting Information File 2). Moreover, all signals in 1H and
tions of the products 10a,b with dilute aqueous ammonia. It also 13C NMR spectra are rather sharp that corresponds to the
allowed us to determine the molecular mass using gel-perme- oligomer structure of both PAEs 10a,b. In 1H NMR spectra of
ation chromatography (GPC). The GPC analysis showed mass- both compounds 10a,b the ratio of integral intensities within
average molecular weights (
9.2 kDa (10b), mass average degree of polymerization ( ) of hydrogen atoms) and aromatic and alkyl pendant groups of
9 (10a) and 6 (10b) and molar-mass dispersity (ĐM = hydroquinone structural units matches the oligomers structure
) of 13.3 kDa (10a) and signals of H atoms from cinnoline (aromatic and alkyl chain
)
of 2.5 for both compounds. In accordance with this data, com- (Figure 3).
pounds 10a and 10b should be defined as oligomers. However
the common accepted abbreviation for this class of compounds The 13C NMR spectra also confirm the presence of both units in
is PAEs (poly(arylene ethynylene)s). This name has also been PAE molecules 10a,b. It is important to note that there are four
applied before to compounds with a similar range of mass- types of Csp atom signals (A–D) in the spectrum of PAE 10a,
average molecular weights (
) [17,19,25]. Therefore in order which come from the oligomer chain while in the 13C NMR
to avoid using of new unusual terms the oligomers obtained spectrum of PAE 10b two additional Csp signals (E, F) of
were named PAEs.
ethynyl pendant group are observed (Figure 4). The splitting of
H atoms (2', a') and C atoms (1'–3', a') signals of hydroquinone
The structure of oligomers 10a,b was confirmed by 1H and structural units of PAEs 10a,b in NMR spectra confirmed that
13C NMR spectroscopy, FTIR spectra and CHN-elemental the orientation of cinnoline units in both oligomer chains is not
analysis. In FTIR spectra of compounds 10a,b the specific regular. Thus there are three types of hydroquinone spacers
bands for disubstituted triple bonds (R−C≡C−R) stretching at between head-to-head (red), tail-to-tail (blue) and head-to-tail
Scheme 1: Synthesis of cinnoline-containing PAEs 10a,b.
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Beilstein J. Org. Chem. 2015, 11, 373–384.
Figure 3: 1H NMR spectra of PAEs 10a,b solutions in CDCl3.
Figure 4: 13C NMR spectra of PAEs 10a,b solutions in CDCl3.
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Beilstein J. Org. Chem. 2015, 11, 373–384.
Figure 5: Irregular chain structure (nonequivalent structural units are marked in different colors).
(orange–purple) oriented cinnoline units in the irregular PAEs
chains (Figure 5). Moreover for the hydroquinone moiety that
connects head-to-tail oriented cinnoline units (orange–purple)
the additional splitting was observed (for zoom copies of NMR
spectra see Supporting Information File 2).
Optical properties and metal-sensing ability
of cinnoline-containing PAEs
The absorption spectrum in the UV–vis range of THF solutions
of the PAE 10b with an additional acetylene fragment differed
from the absorption spectrum of the PAE 10a by a small
bathochromic shift (≈7 nm) of two absorption bands and an ad-
ditional absorption band at 266 nm (Figure 6). The fluores-
cence emission spectra of the same solutions (Figure 7) were
recorded at the optimal excitation wavelength of 425 nm which
corresponds to the absorption bands at ≈425 nm in UV–vis
Figure 6: Optical absorption spectra of PAEs 10a,b in THF solutions.
spectra of both samples (Figure 6). An additional ethynyl
moiety in the repeating unit of olygomer 10b led to the increase
in quantum yield from 1% for 10a to 2% for 10b, while qualita-
tive similarity of both emission spectra remained unchanged.
Then the metal-sensing ability of the PAEs 10a,b was studied
by the screening of the influence of different transition metal
ions (Pd2+, La3+, Ag+) on the UV–vis absorption of THF solu-
tions of oligomers. A significant red shift of all absorption
bands was observed after the addition of a PdCl2/HCl solution
to both PAE samples. The influence of Ag+ and La3+ ions was
much weaker and almost did not change the absorption spectra
at all (Figure 8 and Figure 9).
It was also found that Pd2+ cations noticeably quenched the
fluorescence of oligomers 10a,b (Figure 10 and Figure 11). For
quantitative evaluation of the fluorescence quenching the para-
meter Q was used (Q = Io/I – 1, where Q, Io and I are the
quenching efficiency, the initial fluorescence intensity and the
Figure 7: Emission spectra of PAEs 10a,b in THF solutions.
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Beilstein J. Org. Chem. 2015, 11, 373–384.
Figure 8: Optical absorption spectra of PAE 10a in THF before and
Figure 10: Emission spectra of PAE 10a in THF before and after the
after the addition of metal analytes.
addition of metal ions.
Figure 9: Optical absorption spectra of PAE 10b in THF before and
Figure 11: Emission spectra of PAE 10b in THF before and after the
after the addition of metal analytes.
addition of metal ions.
fluorescence intensity after the interaction with a quencher, res- PAE 10b with shorter chains and an additional ethynyl moiety
pectively).
in the cinnoline ring means that the conjugation extension in the
repeating unit that comes from the triple bond is more impor-
Figure 8 and Figure 9 illustrate that changes in the absorption tant for the collective quenching effect than the degree of poly-
spectra of compounds 10a,b resulting from the interaction of merization.
oligomers with Pd2+ are similar for both samples. The simi-
larity means that neither the additional ethynyl moiety of the In order to prove that the observed changes in optical prop-
repeating unit of 10b nor the degree of polymerization (n = 9 erties of PAEs 10a,b were induced by Pd2+ and are not related
for 10a and n = 6 for 10b) contribute considerably to the inter- to the hydrochloric acid which was essential for the preparation
action between the palladium and oligomer molecules in the of an aqueous solutions of PdCl2, the influence of HCl on the
ground (non-excited) state. However, at the same time, the fluo- optical properties of the oligomer 10a was studied. The
rescence quenching efficiency exhibited almost a two fold obtained data revealed that changes in the absorption and fluo-
increase for the PAE 10b (Q = 47) in comparison with the PAE rescence emission spectra for the PAE 10a under 10-fold excess
10a (Q = 26). This illustrates the significant contribution of the of HCl are opposite to the ones induced by Pd2+ ions. Thus
aforementioned triple bond to the interaction of Pd2+ with instead of a "red" shift of absorption bands and the fluores-
oligomer molecules in the excited state (Figure 10 and cence quenching, a slight "blue" shift and the increase in the
Figure 11). Moreover, the increased quenching efficiency of the fluorescence emission were observed (Figure 12 and Figure 13).
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Beilstein J. Org. Chem. 2015, 11, 373–384.
Figure 12: Optical absorption spectra of PAE 10a in THF before and
Figure 14: Optical absorption spectra of PAE 10b in THF before after
after the addition of HCl (10 equiv).
the addition of methanol solution of PdCl2.
Figure 13: Emission spectra of PAE 10a in THF before and after the
Figure 15: Emission spectra of PAE 10b in THF before and after the
addition of HCl (10 equiv).
addition of methanol solution of PdCl2.
The additional evidence for the Pd2+-promoted changes in the conjugated triple bonds (and/or the additional ethynyl
absorption and the fluorescence of PAEs 10a,b was obtained by moiety in the case of compound 10b) and a cinnoline ring. In
measuring the UV–vis and fluorescence emission spectra for the order to determine the role of the cinnoline fragment
THF solution of 10b in the presence of a methanol solution of in this process, UV–vis and fluorescence emission spectra
PdCl2 (Figure 14 and Figure 15). Thus in both spectra changes for the dibromocinnoline 4a, the compound without
similar to the ones depicted in Figure 9 and Figure 11 were any triple bonds, were measured. The observed "red"
observed.
shift of absorption bands in the UV spectrum of the cinnoline
4a in the presence of Pd2+ ions (Figure 16) suggested
Previously palladium(II)-alkyne π-complexes have been the interaction of Pd2+ with the cinnoline moiety. This
proposed as intermediates in Pd(II)-catalyzed diamination allowed us to conclude that the cinnoline fragment is playing
of alkynes [63], enyne coupling [64], hydroarylation of alkynes the role of a recognition site of Pd2+ cations. On the other hand
[65], inramolecular carbocyclization [66] and other the fluorescence quenching for compound 4a (Figure 17) is
reactions. Some types of these complexes have been isolated much less effective than for PAE 10a (Figure 10). This could
[67]. Therefore, two structural features in the obtained oligomer be explained by a known collective effect in the conjugated
chains can be responsible for the interaction with Pd 2+ cations: chains [8,9].
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Beilstein J. Org. Chem. 2015, 11, 373–384.
Supporting Information
Supporting Information File 1
General information and methods, all synthetic procedures
and analytical data, procedures for the investigation of
cation sensing ability for PAEs 10a,b.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-11-43-S1.pdf]
Supporting Information File 2
Copies of 1H and 13C NMR spectra for all compounds,
copies of DEPT and 2D NMR spectra for compounds 5a,b
and 7, copies of IR spectra for compounds 4a,b; 5,b; 7 and
10a,b, copies of GPC chromatograms for PAEs 10a,b.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-11-43-S2.pdf]
Figure 16: Optical absorption spectra of cinnoline 4a in THF before
and after the addition of aqueous solution of PdCl2.
Acknowledgements
This study was supported by Saint Petersburg State University
(research project, 12.38.195.2014). The research was carried out
using equipment at the resource centers of Saint Petersburg
State University: Center for Magnetic Resonance, Center for
Chemical Analysis and Materials Research. N.A.D. acknowl-
edges the President of the Russian Federation for the research
grant for young scientists (MK-3322.2014.3). The authors are
grateful to Dr. Nikolay Samusik (Stanford University) for the
helpful comments and additions to the text.
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