Novel photo-switchable polymers based on calix[4]arenes

Abstract

5,17-Dinitrocalix[4]arenes with varying lower rim substituents (butyl, dodecyl chains), locked in the cone conformation, were synthesized and further subjected to reductive coupling reactions to yield a new type of photoswitchable polymers—poly(azocalix[4]arenes). These rigid polymers can undergo reversible photoswitching between the trans and cis form upon irradiation with specific wavelengths, as well as slow thermal isomerisation back to the preferred trans form.

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The preparation of polymeric systems, that respond to external stimuli with change in physicochemical properties has been a trend in the scientific community. The pursuit of smart materials has not been limited to temperature¹ or pH-responsive polymers,² but also a large amount of attention has been focused on photo-switchable systems.³

Over the course of time research on calixarenes was predominantly directed towards understanding their role in host–guest complexes with various metal ions,⁴ biomolecular recognition,⁵ as well as small organic molecules. However, the cross-over to polymer science has been slow, with the majority of the studies involving the preparation of star polymers from calixarene cores,⁶ using them as templates for ‘click’ reactions,⁷ attachment of calixarene moieties as pendant groups in the chain,⁸ or embedding into a macromolecular backbone.⁹ Rigid, alternating copolymers¹⁰ of calixarenes and tetrathiophenes have been synthesized in the past. Using the upper rim of the calix[4]arene moieties as the linking point in the preparation of oligomers has also been reported.¹¹

Numerous azocalix[4]arene derivatives have been prepared by introducing the azo group into either the upper¹² or the lower rim¹³ of the molecules, thus yielding units in the system capable of photoisomerisation. However only limited examples exist of more complex structures¹⁴ bearing a larger amount (n > 1) of azocalix[4]arene moieties.

A vast library of architectures of main¹⁵ or side¹⁶ chain azobenzene-containing polymers has been obtained in the last two decades. The influence of these photo-switchable units on the properties of the polymeric materials before and after irradiation has been the primary subject of interest.¹⁷ In the majority of synthetic approaches, the azobenzene derivatives are preformed prior to coupling or polymerisation steps.

In our study, we have prepared 5,17-dinitrocalix[4]arenes substituted with aliphatic chains of different length at the lower rim (butyl and dodecyl). These calix[4]arene species were fixed in the cone conformation, to ensure that the overall shape of the molecule was persistent and the interconversion was restricted. In a subsequent step, reductive coupling of the 5,17-dinitrocalixarene monomers (1 and 2—Scheme 1) was performed to directly yield poly(azocalix[4]arenes). In this novel approach, the end product should be a rigid polymer, in which any conformational changes of the backbone are the result of isomerisation of the azo group used as the linkage between the calix[4]arene units. Furthermore, as a result of using calix[4]arenes, which are known cavitands in supramolecular chemistry, the polymers may exhibit potential interactions with low molar mass guests.

Scheme 1 Top: preparation of poly(azocalix[4]arenes) from monomers 1 (n-butyl chains) and 2 (n-dodecyl chains); a—LAH, various solvents, r.t.; b—Red-Al, toluene, 0 °C–r.t. Bottom: graphical illustration of coupling reaction on monomer 2.

Because of the specificity of functional group placement (5,17-positions at the upper rim), as well as the urge to obtain conformationally non-flexible calixarenes two separate synthetic protocols were used to prepare the monomers. In the first trials (ESI†, Scheme S1), tetrahydroxycalix[4]arene was o-alkylated (butyl or dodecyl chains) at the 25,27-positions (opposing hydroxyl groups at the lower rim) leaving the 26,28-positions unreacted. This induces a higher rate of electrophilic substitution on the phenolic units and conventional nitration under mild conditions gave good yields to produce 5,17-dinitrocalix[4]arenes with free hydroxyl groups at positions 26,28. However, it seems that in the subsequent o-alkylation step, steric hindrance (distorted conformation) and cooperative strong hydrogen bonding led to long reaction times and yield loss. Numerous reports in the literature state that ipso nitration is highly probable for calix[4]arenes with tert-butyl groups at the upper rim.¹⁸ The preparation of 5,17-di(tert-butyl)calix[4]arenes was done according to the literature procedure¹⁹ (ESI†, Scheme S2). The following synthetic step of o-alkylation gave very good yields of cone conformers. Because of the possibility to monitor the progress of the nitration through the ipso mechanism (strong purple coloration of the reaction mixture), we could obtain the 5,17-dinitrocalix[4]arene monomers, 1 and 2, in good yield and high purity (ESI†, Scheme S3).

Preliminary attempts at reductive coupling of the monomer 2 were done using lithium aluminium hydride (LAH) suspensions in tetrahydrofuran (THF). An intense colour change to dark orange in the reaction mixture indicated the formation of an azo bridge between the calix[4]arene species. Size exclusion chromatography (SEC) calibrated against PS standards in THF was done on the samples after work-up to reveal that various oligomers (n < 10) were readily formed in rather dilute conditions (10% w/v monomer/solvent). A slightly better result (n < 15) was obtained, when monomer 1 was used. This was at first attributed to a difference in size of the substituents (monomer 1 is sterically less crowded than monomer 2), however, preliminary molecular modelling studies revealed that steric hindrance should not influence the degree of polymerisation (n) to a large extent. Varying time and temperature, as well as concentration, did not significantly affect the outcome of the reaction and hence optimisation of solvent needed to be done. Whilst using diphenyl ether or monoglyme did not substantially alter the degree of polymerisation of monomer 1, when the reaction was performed in diethyl ether (higher solubility of LAH than in THF) with a minimal amount of THF (solubilisation of the monomer), the development of higher molecular weight species (n < 40, majority at n = 20) could be observed. However, despite longer reaction times and highly concentrated conditions the SEC eluograms indicated the existence of oligomers in all samples. This led us to believe that a side reaction of reduction towards aminocalix[4]arenes occurs and thus renders the oligomers and the end groups of the polymers non-reactive under the applied reaction conditions. Comparative FTIR measurements on monomer 1 and resultant oligomers extracted from the reaction mixture (n < 4) clearly showed the disappearance of strong Ar–NO₂ signals (ESI†, Fig. S1) at 1524, 1347 and 745 cm⁻¹. This was not at all surprising and in the next step two different procedures for coupling of aniline derivatives²⁰ were used to determine whether larger molecular weight polymers could be produced from the oligomers extracted from crude reaction mixtures of the first coupling reactions. Using cetyltrimethylammonium dichromate (CTADC) in chloroform, or iron(II)sulfate heptahydrate with potassium permanganate in chloroform or dichloromethane clearly promoted the formation of larger polymers, while simultaneously decreasing the oligomer content (ESI†, Fig. S2 and S3).

Furthermore, during the course of our study, we observed that while the monomers, 1 and 2, were only partially soluble in toluene, upon increase in molecular weight (oligomers) a complete solubilisation could be induced. This difference in solution behaviour triggered another pathway for the synthesis of poly(azocalix[4]arenes). The more stable industrial equivalent of LAH—sodium bis(2-methoxyethoxy)aluminium hydride in toluene—was used for the coupling reactions. The evolution from a cloudy suspension to a clear solution with time indicated the formation of larger molecular weight species. While short oligomers could still be observed in the crude reaction mixture (Fig.1), the content of higher molecular weight fractions was much larger. Upon precipitation, we could easily extract species of M_n = 32 000 g mol⁻¹ (n ≈ 50). The improved results obtained in this coupling procedure may be attributed to better solubility of the coupling agent with respect to the reactions involving LAH. The procedure proved to be reproducible and was thus determined to be the most suitable for the preparation of poly(azocalix[4]arenes). Different precipitated fractions from other reaction mixtures, information on molecular weights and the degrees of polymerisation are listed in Table S1 (ESI†).

Fig. 1 Comparison of SEC traces of monomer 1 (black line), crude reaction mixture (red line) and precipitate (blue line).

The polymers derived from monomer 1 (n-butyl chains in the lower rim) were then analyzed by ¹H NMR (500 MHz, CDCl₃). The first difference in the spectrum was the clear downfield shift of the signals from protons adjacent to nitro groups (Ar–NO₂, a in Fig. 2) from 7.45 to 7.82 ppm (a′ in Fig. 2). This value corresponds well to the formation of an azo-linkage (Ar–N = N–Ar) between the aromatic moieties.²¹ Furthermore, an upfield shift is observed for unsubstituted aromatic protons (b–b′) from 6.73 to 6.29 ppm. This is attributed to an induced pinched cone conformation (C_2v symmetry) as the opposite, unsubstituted aromatic units are fixed in closer proximity to each other.²²

Fig. 2 ¹H NMR spectra (500 MHz, CDCl₃) of monomer 1 (above) and polyazocalix[4]arene with n-butyl chains (below).

This change in conformation is further supported by the enhanced difference in chemical shifts of signals (d′, e′ and also h′, i′), which refer to the protons in the alkyl chains, as the distortion of the conformation induces a distinction in spatial positioning of the chains located at the 25,27-positions (unsubstituted aromatic units) and 26,28-positions (para to the azo-linkage). Broadening of all signals indicates reduced mobility and is a further proof of polymeric structure of the species. While proton integration for the aromatic region (a′, b′) and methylene (c′, f′) bridge is correct, a slight overestimation of the integration for proton signals of the lower rim chains is visible. This, however, can be explained by increased mobility of the chains with respect to the calix[4]arene cores and is more pronounced for polymers derived from monomer 2 (ESI†, Fig. S4).

In order to see whether the system can undergo trans–cis isomerisation, samples of poly(azocalix[4]arene) with n-butyl chains (c = 0.025 mg mL⁻¹ in THF) were irradiated with 365 nm and UV-Vis absorbance spectra were collected at different time intervals (0–145 min). It could clearly be seen that the photoisomerisation process is not as fast as reported for other less sterically restricted species.¹⁷ Irradiation with 365 nm induced isomerisation from trans to cis form of the azo-linkage, which is visualized through a decrease in absorbance maximum value at 365 nm (π–π* transition), with a simultaneous increase of maximum value at 450 nm (n–π* transition). The system could reach its photostationary state (lowest trans content) within 145 minutes of irradiation (Fig. 3).

Fig. 3 UV-Vis absorbance spectra for poly(azocalix[4]arene) with n-butyl chains (M_n = 32 000 g mol⁻¹, n = 50, c = 0.025 mg ml⁻¹) upon irradiation with 365 nm measured at different times.

Upon completion of the first experiment, the sample was subjected to irradiation with 450 nm wavelength (cis to trans isomerisation). Simultaneous increase of maximum absorbance at 365 nm and decrease at 450 nm could be observed. The rate of photoisomerisation was higher (the trans form is preferred) and the primary state (highest trans content) could be reached within 75 min of irradiation. Because the cis to trans isomerisation process can also be induced by temperature, a sample of polyazocalix[4]arene with n-butyl chains (M_n = 8000 g mol⁻¹, n = 12, c = 0.01 mg ml⁻¹) was irradiated with 365 nm wavelength until the photostationary state was achieved. The sample was then heated to 40 °C and UV-Vis absorbance spectra were taken at 5 min intervals (ESI†, Fig. S5). Thermal isomerisation back to the trans form occurred and after 40 minutes the sample reached approximately 40% of the primary trans content (Fig. 4).

Fig. 4 UV-Vis absorbance spectra for poly(azocalix[4]arene) with n-butyl chains (M_n = 32 000 g mol⁻¹, n = 50, c = 0.025 mg ml⁻¹) upon irradiation with 450 nm measured at different times.

Conclusions

In summary, we have prepared a new type of polymer in which the backbone comprises exclusively of calix[4]arenes locked in their cone conformation with different aliphatic chains (n-butyl and n-dodecyl) linked by azo bridges. The synthetic pathways have been described and the best procedure was chosen for the preparation of high molecular weight species (M_n = 32 000 g mol⁻¹). ¹H NMR investigations showed the formation of the azo-linkage between the aromatic units and that the calix[4]arene cores adopt a pinched cone conformation. Upon exposure to irradiation with specific wavelengths (365 nm for trans-to-cis and 450 nm for cis-to-trans) the system underwent reversible photoisomerisation in solution. The induced cis species could also return to the preferred trans form upon increase in temperature. This novel approach towards preparation of polymers containing units capable of hosting small molar mass guests has potential applicability as photoswitchable nanoscale templates for multi-responsive polymeric systems.

Acknowledgements

We would like to thank the Academy of Finland (project no. 127329) for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthesis, ¹H and ¹³C NMR, MALDI-ToF MS and FTIR data, and molecular weight characteristics of polymers from different reactions. Instruction on instrumentation. See DOI: 10.1039/c2py20020a

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