Synthesis and chemosensing properties of cinnoline-containing poly(arylene ethynylene)s.

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 Sonogashira 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 Pd(2+) ions.

Introduction

Optical sensors provide an efficient and sensitive approach for the rapid detection and identification of a wide range of chemical substrates by measuring the changes of optical properties such as absorbance, reflectance, luminescence, or fluorescence [1]. Among them a type of sensors based on conjugated <span class="Chemical">polymersn> (<span class="Chemical">CPs) <span class="Chemical">are of crucial importance for biological, environmental and clinical applications because of the remarkably amplified response of CPs to an analyte, compared with the respective monomer unit [2-7]. Since the change in fluorescence is the most sensitive and the most commonly used type of response, conjugated polymers for sensing are termed as amplifying fluorescent polymers (AFPs) [4]. The mechanism of the response amplification can be explained in terms of the "molecular wire" concept [2-9]. Binding of a single analyte molecule to a single recognition site completely quenches the fluorescence of the whole polymer chain, giving the increase of the sensitivity (amplification). This effect is also known as superquenching and comes from the delocalization of the charge carriers in the polymeric structures. Transport of excitons along the polymer backbone in a separate molecule as well as the transport of excitons between adjacent molecules makes possible a non-radiative relaxation of many excitons at a single trap created as a result of an analyte binding to a receptor integrated into the polymer chain [10]. As long as the trap and the generation sites of the exciton are spatially separated, the quenching amplification factor is only limited by the diffusion distance of the exciton. In practice, a single bound analyte molecule can deactivate excitons within 130 neighboring phenylene ethynylene repeating units [4].

Sensors based on <span class="Chemical">CPsn> can be classified either by the nature of the <span class="Chemical">polymer backbone, e.g., <span class="Chemical">polyphenylenes, polythiophenes, poly(phenylene vinylene)s, poly(phenylene ethynylene)s etc, or by their chemosensing properties [3-4]. The latter depends on the nature of receptors (recognizing units) incorporated into a polymer chain that bind an analyte. Such materials find widespread use in sensing of metal ions and anions, detection of explosives and various biomolecules [2−4]. Among the great variety of known conjugated polymers, poly(arylene ethynylene)s (PAEs) are one of the most promising materials in the search for new chemical sensors [7].

A wide range of <span class="Chemical">PAEsn> has been developed in the last decade [2-4], for example <span class="Chemical">polymers with <span class="Chemical">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).

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].

Recent examples of <span class="Chemical">PAEs and their application for the detection of <span class="Chemical">Hg2+ (a) [11], <span class="Chemical">Ni2+ (b) [12], explosives (c) [13] and trypsin (d) [14].

There are several known examples of PAEs having bi-, tri- and terpyridyl scaffolds that are capable of transition metal ion sensing [12,15] and pyridine-containing polymers for other applications [16-18]. Moreover, various species of` PAEs with quinoline [19-20], quinoxaline [21-22], thiadiazole [17,23], carbazole [18,24], 1,2,4-triazoles [25], thiazole [26] and azametallocyclic [27] units incorporated into a polymer chain have been described.

Since ligands based on pyridazine rings can form complexes with transition metal ions (e.g. Cu+, Cu2+, Ag+, Co2+, Ni2+) [28-35], the introduction of this structural fragments into PAEs might give rise to a new family of chemosensors. Here we describe the first synthesis of PAEs consisting of altering cinnoline, ethynyl and aryl units and investigate their chemosensing ability towards transition metal ions.

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).

Figure 2

Target structures of PAEs.

T<span class="Chemical">arget structures of <span class="Chemical">PAEs.

They were chosen in order to study the influence of an additional triple bond on the photochemical properties of <span class="Chemical">cinnolinen>-containing <span class="Chemical">PAEs. It should be noted that <span class="Chemical">PAEs with an additional ethynyl moiety are almost unknown and only few examples of dimers and trimers were studied to date [36]. The O-alkylated hydroquinone scaffold was chosen as a commonly used spacer between the diethynylcinnoline units [2-4] that prevents the aggregation of polymer chains.

An efficient modification of the Richter type cyclization [37-38] based on mono- [39-40] and <span class="Chemical">diacetylenen> [41-43] <span class="Chemical">aryl<span class="Chemical">triazene derivatives as starting materials was chosen over the approach that uses unstable o-ethynyl- or o-(buta-1,3-diynyl)arenediazinium salts. This reaction was a key step for the synthesis of cinnoline-containing structural units – 4,6-dibromocinnolines 4a,b (Table 1).

Table 1

Synthesis of 4,6-bis(trimethylsilylethynyl)cinnolines 4.

Entry	R1	R2	Acetylene	Conditionsa	Triazene, (yield, %)	Conditionsa	Cinnoline, (yield, %)
1	C4H9	H	2a	A	3a (76)	C	4a (87)
2	≡−C5H11	TMS	2b	B	3b (90)	C	4b (58)

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.

Synthesis of <span class="Chemical">4,6-bis(trimethylsilylethynyl)cinnolines 4.

Triazenes 3a,b were synthesized from 4-bromo-2-iodophenyltriazene 1 by the Sonogashira coupling [44-45] with hexyne (2a) and TMS-protected diacetylene 2b using conditions for the one-pot TMS group removal and the Sonogashira coupling developed recently [46]. The last reaction could also be defined as a type of sila-Sonogashira coupling [47-48], which is an extremely convenient method for the one-step synthesis of non-symmetrically substituted diarylacetylenes [49-50]. Mild conditions allowed carrying out both reactions chemoselectively leaving the bromine atom for further synthetic steps. 4,6-Dibromo-3-butylcinnoline (4a) and 4,6-dibromo-3-(hept-1-yn-1-yl)cinnoline (4b) were obtained from corresponding triazenes 3a,b by Richter cyclization under known conditions [43] in satisfactory and good yields.

Although the Sonogashira coupling of 4-bromocinnolines has been recently reported [41,51], the double Sonogashira coupling for dibromocinnolines has not been investigated. To optimize the coupling conditions, several test experiments were carried out using the 4,6-dibromo-3-butylcinnoline (4a, Table 2).

Table 2

Conditions optimization for the double Sonogashira coupling.

Entry	Dibromocinnoline	R	Conditionsa	Reaction product (yield, %)b
1	4a	Bu	A	7a (50)
2	4a	Bu	B	5a:8a (or 8’a) (34:23)c
3	4a	Bu	C	5a
4	4a	Bu	C	5a (88)
5	4b	≡−C5H11	C	5b (91)

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.

Conditions optimization for the double <span class="Chemical">Sonogashira coupling.

Taking into account that the substitution of a <span class="Chemical">brominen> atom in Pd-catalyzed coupling typically requires heating to 80–100 °C [45], we first tried heating in <span class="Chemical">DMF at 80 °C over 24 h in the presence of <span class="Chemical">diisopropanolamine (DIPA) as a base (Table 2, entry 1). Under those conditions the reaction yielded an unexpected 4-methylcinnoline 7a that could be explained in terms of unusual Hiyama coupling [52-54] of trimethylsilylacetylene with the more reactive C-4 carbon atom of the cinnoline ring. The substitution of the bromine atom at C-4 (not at C-6) of the cinnoline ring was proved by an 1H NMR NOESY experiment. Switching to Et3N as a solvent and a base as well as decreasing the temperature gave a mixture of the desired product 5a along with the monodesilylated compound 8a or 8’a (Table 2, entry 2). Using 1H NMR spectroscopy to monitor the reaction allowed figuring out that heating in Et3N at 50 °C for 3.5 hours is enough for a full conversion of the starting cinnoline 4a into the desired bis(trimethylsilylethynyl)cinnoline 5a (Table 2, entry 3). The optimized conditions were used in the synthesis of bis(trimethylsilylethynyl)cinnolines 5a,b, so that the starting materials for the synthesis of PAEs were obtained in good isolated yields (Table 2, entries 4 and 5). These experiments revealed that both bromine atoms at pyridazine and benzene rings are highly reactive in the Sonogashira coupling due to the strong activation of the electron deficient nature of the heterocycle.

Having st<span class="Chemical">arn>ting compounds 5a,b in hand, we were ready to c<span class="Chemical">arry out the polycondensation with <span class="Chemical">1,4-diiodo-2,5-bis(octyloxy)benzene (9). There are two main reactions of polycondensation that have been reported as synthetic approaches towards PAEs: the acyclic diyne metathesis polymerization (ADIMET) [55] and the Sonogashira coupling [56]. Both of them have their own advantages and drawbacks [57]. The ADIMET approach usually gives products with higher molecular weights and fewer defects than the Pd-catalyzed polycondensation by the Sonogashira reaction. On the other hand, the Sonogashira coupling is more tolerant to different functional groups and allows the polymer structure to be varied significantly. We chose the Sonogashira coupling of diethynylcinnolines and alkylated diiodohydroquinone as a method for the formation of PAE’s backbone.

It also should be mentioned that despite the fact that sila-<n class="Chemical">span class="Chemical">Sonogashira coupling have been widely applied for the synthesis of different <spn>an class="Chemical">acetylenes with aryl- and hetaryliodides, -bromides -tosylates and -triflates [47-50], there are only a few examples where polycondensation was carried out under these conditions [58-60]. In our case this approach seemed to be more convenient than the common Sonogashira coupling, because it allows TMS-protected diethynylcinnolines to be used as a starting material instead of unstable terminal alkynes. It also avoids an extra synthetic step.

When we tried polycondensation of bis(trimethylsilyl)cinnolines 5a,b with diiodoarene 9 in the presence of Pd(PPh3)4/CuI as a catalytic system and KF/MeOH as a deprotecting source in Et3N, it afforded compounds 10a,b in quantitative yields even at 40 °C (Scheme 1).

Scheme 1

Synthesis of cinnoline-containing PAEs 10a,b.

Synthesis of <span class="Chemical">cinnoline-containing <span class="Chemical">PAEs 10a,b.

Both products were readily soluble in CHCl3, DCM, THF, and MeCN. This allowed for a convenient removal of copper and palladium ion traces by successive washing of the DCM solutions of the products 10a,b with dilute aqueous ammonia. It also allowed us to determine the molecular mass using gel-permeation chromatography (GPC). The GPC analysis showed mass-average molecular weights () of 13.3 kDa (10a) and 9.2 kDa (10b), mass average degree of polymerization () of 9 (10a) and 6 (10b) and molar-mass dispersity (ĐM = ) of 2.5 for both compounds. In accordance with this data, compounds 10a and 10b should be defined as oligomers. However the common accepted abbreviation for this class of compounds is PAEs (poly(arylene ethynylene)s). This name has also been applied before to compounds with a similar range of mass-average molecular weights () [17,19,25]. Therefore in order to avoid using of new unusual terms the oligomers obtained were named PAEs.

The structure of oligomers 10a,b was confirmed by <span class="Chemical">1Hn> and <span class="Chemical">13C NMR spectroscopy, FTIR spectra and CHN-elemental analysis. In FTIR spectra of compounds 10a,b the specific bands for disubstituted triple bonds (R−C≡C−R) stretching at ~2200 cm−1 <span class="Chemical">are observed whereas there are no bands of stretching vibration for monosubstituted triple bonds Ar−C≡C−H (≈2100 cm−1) and Ar−C≡C−TMS (≈2150 cm−1) [61].

The results of the elemental analysis were in agreement with the oligomer structures possessing alkylated <span class="Chemical">iodohydroquinone units as the end groups. The presence of <span class="Chemical">iodine in oligomers was also confirmed by the Beilstein test [62].

Reg<span class="Chemical">arn>ding the NMR data, the chemical shift values of H and C atoms in both <span class="Chemical">PAEs synthesized do not significantly differ from the chemical shift values in st<span class="Chemical">aring cinnolines 5a,b and diiodoarene 9 that allowed all 1H and 13C signals in NMR spectra of PAEs to be assigned based on 1D and 2D NMR spectra of starting compounds 5a,b, 9 and 1,4-diethynyl-2,5-bis(octyloxy)benzene (for copies of all NMR spectra see Supporting Information File 1). Moreover, all signals in 1H and 13C NMR spectra are rather sharp that corresponds to the oligomer structure of both PAEs 10a,b. In 1H NMR spectra of both compounds 10a,b the ratio of integral intensities within signals of H atoms from cinnoline (aromatic and alkyl chain hydrogen atoms) and aromatic and alkyl pendant groups of hydroquinone structural units matches the oligomers structure (Figure 3).

Figure 3

1H NMR spectra of PAEs 10a,b solutions in CDCl3.

The <span class="Chemical">13Cn> NMR spectra also confirm the presence of both units in PAE molecules 10a,b. It is important to note that there are four types of Csp atom signals (A–D) in the spectrum of <span class="Chemical">PAE 10a, which come from the oligomer chain while in the 13C NMR spectrum of PAE 10b two additional Csp signals (E, F) of ethynyl pendant group are observed (Figure 4). The splitting of H atoms (2', a') and C atoms (1'–3', a') signals of hydroquinone structural units of PAEs 10a,b in NMR spectra confirmed that the orientation of cinnoline units in both oligomer chains is not regular. Thus there are three types of hydroquinone spacers between head-to-head (red), tail-to-tail (blue) and head-to-tail (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 1).

Figure 4

13C NMR spectra of PAEs 10a,b solutions in CDCl3.

Figure 5

Irregular chain structure (nonequivalent structural units are marked in different colors).

Irregul<span class="Chemical">ar chain structure (nonequivalent structural units <span class="Chemical">are m<span class="Chemical">arked in different colors).

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 additional absorption band at 266 nm (Figure 6). The fluorescence 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 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 qualitative similarity of both emission spectra remained unchanged.

Figure 6

Optical absorption spectra of PAEs 10a,b in THF solutions.

Figure 7

Emission spectra of PAEs 10a,b in THF solutions.

Optical absorption spectra of <span class="Chemical">PAEs 10a,b in <span class="Chemical">THF solutions.

Emission spectra of <span class="Chemical">PAEs 10a,b in <span class="Chemical">THF solutions.

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 solutions 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).

Figure 8

Optical absorption spectra of PAE 10a in THF before and after the addition of metal analytes.

Figure 9

Optical absorption spectra of PAE 10b in THF before and after the addition of metal analytes.

It was also found that <span class="Chemical">Pd2+ cations noticeably quenched the fluorescence of oligomers 10a,b (Figure 10 and Figure 11). For quantitative evaluation of the fluorescence quenching the p<span class="Chemical">arameter Q was used (Q = I/I – 1, where Q, I and I <span class="Chemical">are the quenching efficiency, the initial fluorescence intensity and the fluorescence intensity after the interaction with a quencher, respectively).

Figure 10

Emission spectra of PAE 10a in THF before and after the addition of metal ions.

Figure 11

Emission spectra of PAE 10b in THF before and after the addition of metal ions.

Figure 8 and Figure 9 illustrate that changes in the absorption spectra of compounds 10a,b resulting from the interaction of oligomers with <n class="Chemical">span class="Chemical">Pd2+ <spn>an class="Chemical">are similar for both samples. The similarity means that neither the additional ethynyl moiety of the repeating unit of 10b nor the degree of polymerization (n = 9 for 10a and n = 6 for 10b) contribute considerably to the interaction between the palladium and oligomer molecules in the ground (non-excited) state. However, at the same time, the fluorescence quenching efficiency exhibited almost a two fold increase for the PAE 10b (Q = 47) in comparison with the PAE 10a (Q = 26). This illustrates the significant contribution of the aforementioned triple bond to the interaction of Pd2+ with oligomer molecules in the excited state (Figure 10 and Figure 11). Moreover, the increased quenching efficiency of the PAE 10b with shorter chains and an additional ethynyl moiety in the cinnoline ring means that the conjugation extension in the repeating unit that comes from the triple bond is more important for the collective quenching effect than the degree of polymerization.

In order to prove that the observed changes in optical properties of <span class="Chemical">PAEsn> 10a,b were induced by <span class="Chemical">Pd2+ and <span class="Chemical">are not related to the hydrochloric acid which was essential for the preparation of an aqueous solutions of PdCl2, the influence of HCl on the optical properties of the oligomer 10a was studied. The obtained data revealed that changes in the absorption and fluorescence emission spectra for the PAE 10a under 10-fold excess of HCl are opposite to the ones induced by Pd2+ ions. Thus instead of a "red" shift of absorption bands and the fluorescence quenching, a slight "blue" shift and the increase in the fluorescence emission were observed (Figure 12 and Figure 13).

Figure 12

Optical absorption spectra of PAE 10a in THF before and after the addition of HCl (10 equiv).

Figure 13

Emission spectra of PAE 10a in THF before and after the addition of HCl (10 equiv).

The additional evidence for the Pd2+-promoted changes in the absorption and the fluorescence of PAEs 10a,b was obtained by measuring the UV–vis and fluorescence emission spectra for the THF solution of 10b in the presence of a methanol solution of PdCl2 (Figure 14 and Figure 15). Thus in both spectra changes similar to the ones depicted in Figure 9 and Figure 11 were observed.

Figure 14

Optical absorption spectra of PAE 10b in THF before after the addition of methanol solution of PdCl2.

Figure 15

Emission spectra of PAE 10b in THF before and after the addition of methanol solution of PdCl2.

Previously palladium(II)-alkyne π-complexes have been proposed as intermediates in Pd(II)-catalyzed diamination of alkynes [63], enyne coupling [64], hydroarylation of alkynes [65], inramolecular carbocyclization [66] and other reactions. Some types of these complexes have been isolated [67]. Therefore, two structural features in the obtained oligomer chains can be responsible for the interaction with Pd 2+ cations: conjugated triple bonds (and/or the additional ethynyl moiety in the case of compound 10b) and a cinnoline ring. In order to determine the role of the cinnoline fragment in this process, UV–vis and fluorescence emission spectra for the dibromocinnoline 4a, the compound without any triple bonds, were measured. The observed "red" shift of absorption bands in the UV spectrum of the cinnoline 4a in the presence of Pd2+ ions (Figure 16) suggested the interaction of Pd2+ with the cinnoline moiety. This allowed us to conclude that the cinnoline fragment is playing the role of a recognition site of Pd2+ cations. On the other hand the fluorescence quenching for compound 4a (Figure 17) is much less effective than for PAE 10a (Figure 10). This could be explained by a known collective effect in the conjugated chains [8-9].

Figure 16

Optical absorption spectra of cinnoline 4a in THF before and after the addition of aqueous solution of PdCl2.

Figure 17

Emission spectra of cinnoline 4a in THF before and after the addition of aqueous solution of PdCl2.

Conclusion

Cinnoline-containing poly(arylene ethynylene)s 10a,b were synthesized for the first time via polycondensation based on one-pot removal of the TMS group and CuI/Pd0-catalyzed Sonogashira coupling of bis(trimethylsilylethynyl)cinnolines and dioctylated diiodohydroquinone. The Pd2+-sensing ability of PAEs obtained was demonstrated by Pd2+-dependent fluorescence quenching of their THF solutions. This effect is likely due to the binding of Pd2+ cations to the cinnoline moiety of oligomers. The comparison of photochemical properties and Pd2+-sensing ability of PAEs obtained suggested that the incorporation of an additional triple bond into a cinnoline ring offers a more significant improvement to the fluorescence quantum yield and the Pd2+dependent quenching efficiency than increasing of the polymerization degree.

General information and methods, all synthetic procedures and analytical data, procedures for the investigation of cation sensing ability for <span class="Chemical">PAEsn> 10a,b.

Copies of <span class="Chemical">1H and <span class="Chemical">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 <span class="Chemical">PAEs 10a,b.