Heterogeneous

Letter pubs.acs.org/macroletters

Fullerene-Attached Polymeric Homogeneous/Heterogeneous Photoactivators for Visible-Light-Induced CuAAC Click Reactions Omer S. Taskin,† Gorkem Yilmaz,† and Yusuf Yagci*,†,‡ †

Department of Chemistry, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Kingdom of Saudi Arabia

‡

S Supporting Information *

ABSTRACT: In this work, a visible-light-induced copper(I)-catalyzed azide− alkyne cycloaddition (CuAAC) click reaction via an electron transfer reaction employing fullerene (C60) containing linear polystyrene and network polymer as homogeneous and heterogeneous activators, PS-C60 and Gel-C60, respectively, is described. Various low molar mass organic and polymeric azide and alkyne compounds are used to conduct the click reaction, and almost quantitative yields are attained. Compared to the bare C60, the polymers with C60 units exhibited a much higher promoting effect to catalyze CuAAC reactions.

D

generation of the corresponding low oxidation state complexes (Cu(I)), which was attributed to a single electron transfer from the π-orbitals of the ligand to the guest copper ion. Thus, generated Cu(I) species were shown to prepare polymers with controlled molecular weight characteristics through the ATRP mechanism. Addition of photolabile compounds in the system, on the other hand, yielded radicals upon irradiation at appropriate wavelengths, which in turn reduced Cu(II) more efficiently. This so-called indirect photolysis mechanism was also applied to the ATRP chemistry in numbers of studies. A similar approach was also utilized for CuAAC reactions, where deliberately selected photoinitiators assisted the simultaneous generation of Cu(I) species. In another approach, we also applied polynuclear aromatic compounds as sensitizers for the photochemical reduction of Cu(II) species. Thus, employed perylene, anthracene, and phenothiazine were shown to efficiently induce photoCuAAC between molar mass compounds as well as polymeric structures.25−29 The simultaneous photochemical reduction of Cu(II) compounds in the media circumvents commonly encountered problems associated with the air sensitivity of Cu(I) catalysis and facilitates the application of these processes in the presence of air. Moreover, the photochemical catalytic system specifically offers significant advantages such as temporal and spatial control, sustainability, low-energy requirements, and environmentally friendly processing.30 Recent efforts also demonstrated the possibility of employing heterogeneous systems for similar catalytic reactions, which provides easy product

ue to its excellent photochemical, electrochemical, magnetic, and mechanical properties, Buckminster fullerene (C60) has received intensive research efforts since its discovery in 1985.1−8 Having all carbon atoms connected to each other by three σ-bonds with their single unpaired electron at the outer surface, C60 is a highly conjugated aromatic-like structure with interesting redox properties. Its excellent electrophilic characteristics enable its reduction easily by multiple single electron transfer steps from 0 to a maximum of −6 oxidation states.9−12 However, C60 has a poor tendency to be oxidized (Eox = 1.7 V (SCE)) plausibly due to the dominant s-character in the π-system facilitated by the three dimensionality. In an effort to demonstrate the possibility of oxidizing fullerenes, stable C60•+ and HC60+ cations were tamed in solution by treatment with super acids.13 On the other hand, we recently developed a photochemical protocol to oxidize C60 under visible-light irradiation using silver hexafluorophosphate and diphenyliodonium hexafluorophosphate as oxidants, which allows efficient initiation of cationic polymerization of appropriate monomers.14 The application of photochemical redox reactions to the synthetic polymer chemistry brought significant advantages compared to the traditional strategies. For example, free radical promoted cationic polymerizations were successfully carried out in the presence of radical photoinitiators in extended wavelength regions and thus lower energies.15−17 Similar photoinduced strategies were also applied to the other macromolecular synthesis methods, such as atom transfer radical polymerization (ATRP)18−24 and copper-catalyzed azide−alkyne cycloaddition (CuAAC) reactions. Direct photochemical reduction of high oxidation state copper (Cu(II)) complexes under UV irradiation were shown to facilitate the © XXXX American Chemical Society

Received: December 4, 2015 Accepted: December 28, 2015

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ACS Macro Letters separation, reusability, and higher stability.31,32 Due to the above-described advantages, many other light-induced ligation methods have been developed and successfully combined with various polymerization processes.33,34 In the present work, we exploit the possibility of utilizing fullerene-based structures as photoreducing agents, for the generation of Cu(I) species, and apply it to visible-lightinduced CuAAC reactions for the synthesis of low molar-mass organic compounds, telechelic polymers, and block copolymers. Since Cu(II) complexes have a strong absorption in the nearUV region, the possibility of conducting photoCuAAC reaction would yield lower efficiencies. The C60-functional catalytic system offers the potential to conveniently generate Cu(I) species under homogeneous and heterogeneous conditions. Initial experiments performed with bare fullerene as a photoactivator failed to induce CuAAC reactions between model clickable compounds, phenyl acetylene and octyl azide (see vide inf ra). Thus, based on our previous investigations using fullerene derivatives for the photosensitized cationic polymerization, we decided to use fullerene end-capped polystyrene (PS-C60) rather than bare C60. Typical results for photoinduced CuAAC reaction between phenyl acetylene and various antagonist click compounds in the presence of PS-C60 and CuCl2 are collected in Table 1. While poly(ethylene

As can be seen, photoinduced CuAAC reactions between low molar mass organic compounds (Runs 1−3) resulted in almost quantitative yields as found by NMR analyses. Although prolonged irradiation times were required, polymer chain end functionalization in identical conditions gave similar yields (runs 4 and 5). The progress of the photoinduced CuAAC reaction between phenyl acetylene and benzyl azide was monitored by real-time FT-IR studies (Figure 1). The azide stretching band at around

Table 1. Click Reactions between Various Azides and Phenylacetylene in DMSO with PS-C60a

Figure 1. Model click reaction between benzyl azide and phenylacetylene in the presence of PS-C60-CuCl2/PMDETA and typical presentation of decreasing intensity of the azide peak of benzyl azide as a result of clicking with phenylacetylene monitored by FT-IR.

2100 cm−1 dramatically diminished during the process and completely disappeared after 2 h of irradiation, suggesting that the reaction went to completion (Figure 1). However, the use of high molecular weight compounds as photosensitizers for such processes generally gives rise to difficulties in the separation of the products particularly in macromolecular applications. In order to overcome this consequence, we designed a heterogeneous fullerene-based activation system. This way, removing the insoluble activator from the media would give the click products without complicated purification steps. Thus, we prepared a crosslinked material by copolymerizing azidoethyl methacrylate, hydroxyethyl methacrylate, and ethylene glycol dimethacrylate by photopolymerization using 2,2-dimethoxy-2-phenylacetophenone as initiator (Supporting Information, SF3−5). The obtained network was subjected to postmodification in a similar manner to that described above to yield Gel-C60 (see Supporting Information). The incorporation of fullerene moieties on the Gel surface was evidenced by UV−vis and fluorescence spectroscopy. Since the cross-linked structure is methacrylate based, photolytic degradation at this operation wavelength is not likely to occur.35 The results of the photoinduced CuAAC reactions using GelC60 as photoactivator are depicted in Table 2. It can be seen that Gel-C60 operated as efficiently as PS-C60 in the induction of photoinduced CuAAC reactions. We decided to explore the possibility of expanding the scope of the described photoinduced process to the click reaction between polymers to form block copolymers (Table 2, run 5).

a

All reactions were carried out under irradiation at 400−500 nm with a light intensity of 45 mW cm−2 at room temperature in d6-DMSO. [azide] = [alkyne] = 0.5 mM, [PS-C60] = 1 mM, [CuCl2] = [PMDETA] = 0.08 mM. bDetermined by 1H NMR spectroscopy (the course of all reactions is monitored by 1H NMR and given in the Supporting Information). cConducted in the presence of bare C60 rather than PS-C60 under identical conditions.

glycol)-azide (PEG-N3), benzyl azide, anthracene azide, and 1azidooctane were obtained by simple azidation processes, PSC60 was synthesized in three steps. First, polystyrene was obtained by ATRP, followed by substitution of the bromide chain end group to azido functionality. Finally, the resulting polystyrene-azide (PS-N3) was heated in the presence of C60 in chlorobenzene to afford PS-C60 (see Supporting Information for the experimental details, characterization, SS1, and SF1−3). 104

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ACS Macro Letters Table 2. Click Reactions between Various Azides and Alkynes in DMSO with Gel-C60a

a

All reactions were carried out under irradiation at 400−500 nm with a light intensity of 45 mW cm−2 at room temperature in d6-DMSO. [azide] = [alkyne] = 0.5 mM, [CuCl2] = [PMDETA] = 0.08 mM, MGel‑C60 = 30 mg. bDetermined by 1H NMR spectroscopy (the course of all reactions is monitored by 1H NMR and given in the Supporting Information).

By reacting PS-N3 with alkyne-terminated poly(ethylene glycol) (PEG-alkyne) in the presence of Gel-C60 and CuCl2/ PMDETA, poly(styrene-b-polyethylene glycol) (PS-b-PEG) was readily formed. The structure of the block copolymer was confirmed by 1H NMR analysis as characteristic protons of both segments clearly appeared (Figure 2a). The signal of benzylic proton at the junction point adjacent to the triazole moiety at 5.01 ppm and the peak corresponding to the triazole proton appearing at 7.46 ppm further evidence the expected structure. By comparing the integrated ratio of the triazole proton to the total aromatic protons of the polystyrene segment, the yield of this photoinduced CuAAC reaction was calculated as 94%. Figure 2b shows the GPC chromatograms of the precursors PS-N3 and PEG-alkyne as well as the resulting PS-b-PEG copolymer. The GPC trace of the block copolymer displayed a unimodal distribution, proving the absence of any side reactions during the click process. The shift to the higher molecular weight region further demonstrates the success of the photoinduced blocking process. To assess the reusability of the heterogeneous catalyst, GelC60 was removed from the reaction media, washed with various solvents, dried, and reused in further cycles of click reactions between benzyl azide and phenyl acetylene under identical conditions (SI). Since the electron-rich nature of the fullerene moiety was diminished in each electron transfer step during the process, the photoreducing ability of the activator gradually reduced. Thus, after each cycle, although still high, the yield of the coupling processes decreased to some extent (Supporting Information, SF6). Any process in nature is thermodynamically favorable only if its free energy change (ΔG) corresponds to a negative value. At this point, the feasibility of a photoinduced electron transfer process to occur between a polyaromatic compound and an

Figure 2. 1H NMR spectra of PS-b-PEG (a) and GPC traces of PS-N3, PEG-alkyne, and PS-b-PEG (b).

oxidant is determined by the Rehm−Weller equation (eq 1).36,37 On the basis of the oxidation potential (Eox) and the active excitation energy (E*) of fullerene species and the reduction potential (Ered) of Cu(II), ΔG for the photoinduced electron transfer processes were estimated by eq 1 where fc is the Faraday constant. ΔG = fc [Eox (C60) − Ered(Cu(II))] − E*(C60)

(1)

Table 3 depicts the ΔG values of the electron transfer processes between the excited-state bare C60 and PS-C60 and the yields of the CuAAC reactions between clickable low-molar mass compounds: benzyl azide and phenyl acetylene. Initial experiments, using bare C60 as the photocatalyst, failed to produce sufficient concentrations of Cu(I) catalyst due to Table 3. Free Energy Changes of the Electron Transfer Processes between Fullerenes and Cu(II) and the Yields of the CuAAC Reactions Catalyzed by the Generated Cu(I)a fullerene

Eoxb (V)

ΔG (kcal/mol)

yield (%)c

C60 PS-C60

1.7 1.2

−35.8 −47.3

4 98

a

Carried out under irradiation at 400−500 nm with a light intensity of 45 mW cm−2 at room temperature in d6-DMSO, [azide] = [alkyne] = 0.5 mM, [C60] = 1 mM, [CuCl2] = [PMDETA] = 0.08 mM. bAs found by cyclic voltametry measurements cDetermined by 1H NMR spectroscopy. 105

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ACS Macro Letters Notes

the less favorable thermodynamic conditions and limited solubility. The relatively higher oxidation potentials might be due to the three-dimensional symmetry of C60. However, when the electron symmetry of the system was simply disrupted in the case of PS-C60, a more favorable oxidation potential (1.2 V) was attained leading to higher yields in CuAAC. On the basis of the experimental and spectroscopic investigations, Scheme 1 provides an outline of the possible pathways for photoinduced CuAAC reactions.

The authors declare no competing financial interest.

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Scheme 1. Proposed Mechanism of Photoinduced CuAAC Reaction Using Fullerenes

In this scenario, the fullerene derivative undergoes photoexcitation to afford an excited state that engages in an electrontransfer reaction with Cu(II) to furnish Cu(I) species, which next catalyze the CuAAC reaction. We have shown for the first time that fullerene derivatives can be employed in photoinduced CuAAC reactions. Both C60 incorporated linear and networked polymer structures, PS-C60 and Gel-C60, respectively, are highly efficient activators for visible-light click coupling reactions between the low molecular weight compounds and polymers with unprecedented efficiency. The process is based on the excellent reducing power of photoexcited C60 derivatives by one electron transfer to reduce Cu(II) ions to form Cu(I) species, which are ideal photoactivators for CuAAC reactions. The approach is noteworthy for its visible-light sensitivity under therefore mild reaction conditions, carried out at room temperature without interfering with the absorption of the click components. The protocol described for a networked catalyst system is particularly attractive as it provides easy separation of the products. Further studies on the application of the described activating system for photoinduced ATRP are now in progress and will be reported elsewhere.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.5b00885. All experimental procedures and additional spectral data (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

All authors contributed equally. 106

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ACS Macro Letters (37) Dossow, D.; Zhu, Q. Q.; Hizal, G.; Yagci, Y.; Schnabel, W. Polymer 1996, 37, 2821.

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