Synthesis and properties of polymer chemosensors based on 1-vinylimidazole

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Introduction
9][20][21] Polymers bearing pendant heterocyclic moieties containing chemically active centers are promising macromolecular frameworks for the creation of chemosensor materials.
Herein, we have chosen poly-(1-vinylimidazole) (PVI) containing highly basic nitrogen atoms of the pyridine rings prone to quaternization reactions.Substituted aminoalkyl chlorides and chlorocarboxylic acid amides with anthracene substituent were used as alkylating agents.3][24] Thus, the goals of this study were the synthesis of polymers containing functionalized imidazolium groups and investigation of their interaction with ions.

Results and Discussion
Alkylating agents 2-7 possessing the anthracene fluorophore were synthesized according to Scheme 1.

Scheme 1. Synthesis of aminoalkyl chlorides and chlorocarboxylic acid amides 2-7.
Compounds 1a-d were prepared by condensation of the corresponding amines with anthracene-9carbaldehyde and the subsequent reduction of the resulting azomethines by NaBH 4 .The reaction was monitored by the appearance in the 1 H NMR spectra of characteristic proton signals of the CH 2 groups linking the anthracene fragment and NH groups, as well as by the disappearance of the methine proton signals of the intermediate azomethines.Amino alcohols 1a and 1b were chlorinated by SOCl 2 to form chloroalkyl amines 2 and 3, respectively.Interaction of amines 1c and 1d with 2-chloroacetyl and 3-chloropropanoyl chlorides gave rise to the corresponding chloroamides 4-7.Acylation of amines 1c and 1d is accompanied by the appearance of additional proton signals of CH 2 group detected by the 1 H NMR spectra and characteristic carbonyl absorption bands in the IR spectra.
The macromolecular structures 8-13 were synthesized by quaternization of poly-(1-vinylimidazole) with an excess of the obtained alkylating agents 2-7 (Scheme 2).According to the intensity ratios for the proton signals of the (CHCH 2 ) n fragments of the polymer chain and the (CH 2 ) n groups connected with the anthryl fluorophores, as registered by 1 H NMR, quaternization of the imidazole moieties was a fully exhaustive process.

Scheme 2. Synthesis of poly-(1-vinylimidazolium) derivatives 8-13.
The electronic absorption spectra of polymers 8-13 measured in DMSO solution are represented by the structured bands in the region of 350-390 nm.When excited with the light λ exc = 370 nm the solutions display low-intensity anthracene-type fluorescence (three individual maxima at 400-470 nm and a shoulder at ~ 480 nm).The fluorescence quantum yields for amines 8 and 9 were 0.008 and 0.007, respectively, and for amides 10-13 ~ 0.005.Previously, we have found that chemosensors containing a 9-aminomethylanthracene fragment, manifest the PET effect (Photoinduced Electron Transfer) in the excited state due to the effective interaction of the unshared electron pair of the nitrogen atom with the anthracene moiety.As a rule, the emission intensity in such systems did not exceed the value of φ 0.001. 23,24The described polymer polymers 8-13 possess noticeably more intense emission.Tentatively, we attributed this to two competing processes acting in different directions -PET effect and EET effect caused by the electron density transfer from the donor (anthryl) part of the molecule to the acceptor (imidazolium) fragment occurring in the excited state (Scheme 2). 25,26nvestigation the interaction of polymers 8-13 in DMSO with Zn 2+ , Cd 2+ , Cu 2+ , Ni 2+ , Pb 2+ , Hg 2+ cations (in the form of perchlorates) showed that for aminoalkyl polymers 8 and 9, the most significant changes in the fluorescence intensity were caused by zinc (II) cations, which produce a 21 and 14 fold enhancement of the emission of solutions of 8 and 9, respectively (Table 1, Figure 1).It was shown earlier that even one NH group is enough for effective binding of d-metal ions, inhibition of the PET effect and a significant increase in the emission intensity. 23The result was also supported by DFT calculations that predicted additional cation coordination with anthracene π system and the highest energy of complex formation specifically for Zn 2+ . 27able 1.Relative changes in fluorescence intensities of polymers 8-13 (с 5.0•10 -6 M) in DMSO upon addition of cations (in the form of perchlorates) (с 2.5•10 -5 M) By contrast, polyamides 10-13 exhibited a decrease in both sensitivity and selectivity of the cation detection.This observation supported the important role played by the NH groups in the process of complexation. 23,27,28ddition of tetra-butylammonium salts (TBAX: X = F, CN, AcO, NO 3 , HSO 4 ) to solutions of polymers 8-13 in DMSO leads to changes in absorption spectra (Fig. 2a).However, the appearance of a new intense absorption band in the region of 290-320 nm was observed exclusively for F -, CN -and, in particular, AcO -anions.According to preliminary DFT calculations, this process is associated with the substitution of chloride and the formation of much more stable (at 60-75 kcal/mol) salt structures.Apparently, the corresponding redistribution of the electron density is the cause of the observed chromogenic "naked-eye" effect. 29,30,31Unlike cations, F -, CN -, AcO -and, to a lesser extent, some other anions cause only CHEQ effect (CHelation-Enhanced fluorescence Quenching) (Fig. 2b, Table 2). 3Emission quenching is insufficiently suited for fluorimetric sensing, because it is unspecific and may be caused by several different external factors. 22

Polymer
Relative change in fluorescence intensity,

Experimental Section
General.The IR spectra were recorded on a Varian Excalibur 3100 FT-IR instrument using the attenuated total internal reflection technique (ZnSe crystal).The 1 H NMR spectra in DMSO-d 6 were recorded on a Varian Unity 300 spectrometer (300 MHz), the signals were referenced with respect to the signals of residual protons of deutero-solvent (2.50 ppm).DMSO of the spectroscopic grade, d-metal perchlorates and tetrabutylammonium salts (Aldrich) were used to prepare solutions.Melting points were determined on a PTP (M) instrument.Elemental analysis was performed by classical procedure. 32Poly(1-vinylimidazole) was synthesized according to the literature. 33

Figure 1 .
Figure 1.Changes in fluorescence intensity of polymers 8 and 9 in DMSO solutions in the presence of metal cations.