Mesoporous Graphitic Carbon Nitride as a Heterogeneous Visible

Apr 11, 2012 - The use mesoporous graphitic carbon nitride (mpg-C3N4) in conjunction with tertiary amines as initiators in visible-light-induced free ...
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Mesoporous Graphitic Carbon Nitride as a Heterogeneous Visible Light Photoinitiator for Radical Polymerization Baris Kiskan,†,‡ Jinshui Zhang,†,§ Xinchen Wang,*,†,§ Markus Antonietti,† and Yusuf Yagci*,†,‡ †

Department of Colloid Chemistry, Max Planck Institute of Colloids and Interfaces, D-14424, Potsdam, Germany Department of Chemistry, Istanbul Technical University, 34469, Maslak, Istanbul, Turkey § Research Institute of Photocatalysis, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou University, 350002, Fuzhou, China ‡

S Supporting Information *

ABSTRACT: The use mesoporous graphitic carbon nitride (mpgC3N4) in conjunction with tertiary amines as initiators in visible-lightinduced free radical polymerization is described. The initiation mechanism involves photoinduced free radical generation by scavenging holes with amines and subsequent hydrogen abstraction. The efficiency of the photoinitiation is controlled by the nature of the amines and specific surface area of the carbon nitride powder. Apparently, amines with higher basicity and available hydrogens provide more favorable conditions for the photoinitiation process. Due to its heterogeneous nature, the photoinitiator preserves its photoinitiation activity after the polymerization and can easily be separated and used for further polymerizations.

S

Scheme 1. Representation of the Idealized Structure of gC 3N 4

olar energy is the source of all natural chemistry, but is increasingly used to drive various organic reactions. Indeed, such processes are characterized by important advantages meeting the actual targets of green chemistry. Since the most common organics are transparent in the visible radiation (400− 800 nm), which accounts for about 43% of the incoming solar spectrum, various inorganic and polymeric visible light catalysts have been developed and used. In the context of organic synthesis mediated by photoredox catalysis, dye molecules, CdS, and dye-sensitized TiO2 have been extensively employed as energy transducers,1−3 but there is a concern about the sustainability of the systems due to the corrosion/degradation of these sensitizers and the involvement of precious metals. A recently introduced polymeric semiconductor on the basis of a defective, slightly disordered graphitic carbon nitride (g-C3N4) is a metal-free photocatalyst and seems to fulfill the basic requirements for a wide range of applications (Scheme 1). For example, g-C3N4 was found to be an active visible light catalyst for water splitting,4 Friedel−Crafts reaction,5 reduction of CO2,6 and oxidation of benzene.7 The carbon nitride polymer can be “activated” for heterogeneous (photo)catalysis by increasing its specific surface. Mesoporosity with the correct pore diameter for liquid processes was introduced into the bulk g-C3N4 by using silica nanoparticles as templates.8 The obtained mesoporous g-C3N4 (mpg-C3N4) provided the combination of a high mesopore surface area (∼200 m2/g), accessible surface sites for interfacial (photo) reactions, and a wall thickness sufficient to build up semiconductor behavior. © 2012 American Chemical Society

Photopolymerization is one of the most important forthcoming technological processes in polymer science congregating a wide range of economic and ecological anticipations. For many years, it has been the basis of numerous conventional applications in coatings, adhesives, inks, printing plates, optical waveguides, and microelectronics.9−13 Less traditional but illustrative applications14−17 of photopolymerization are the production of laser video discs, curing of acrylate dental fillings, and fabrication of 3D objects. As playing the most important role in the polymerization process, a variety of photoinitiators have been developed to meet industrial requirements. An Received: March 14, 2012 Accepted: April 9, 2012 Published: April 11, 2012 546

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Compare to the nonmesoporous structure, mpg-C3N4 exhibits enhanced activity in the polymerization process due to the large external surface. It is also seen that oxygen totally inhibits the polymerization, indicating that the reactive species playing a role in the polymerization, such as initiating and propagating radicals, are quenched by O2. As shown in Figure 1, conversion of the polymerization at fixed low light intensity increased almost linearly with time.

efficient photoinitiator should possess suitable spectral characteristics matching with the emission spectrum of the irradiation source to attain high initiating quantum yield. Many cleavage18−22 and hydrogen abstraction23−29 type photoinitiators with different absorption ranges were proved to be efficient in free radical polymerization, however, mainly with UV absorption, due to the energetic processes involved. However, there has been, in the past decade, great interest on the development of new photoinitiators5,12,18−24,30−32 acting at the near UV and visible ranges because of the lower energy requirements and less material damage during photolysis. Such sensitivity for longer wavelengths is particularly useful for highly pigmented coatings, rapid setting injet inks, coatings for wood preservation, and for three-dimensional imaging processes such as stereolithography (enabling cheaper and much stronger lasers). Despite their high initiation efficiency, many low molecular weight photoinitiators show drawbacks such as relatively strong odor and migration from cured films. One way to combat such problems is to use macromolecular photoinitiators.33−37 However, again, problems including low efficiency, higher viscosity, and reduced solubility encumber the industrial applications of macrophotoinitiators. Ultrasmall semiconductor particles were proposed to be alternative photoinitiators with no migration.38 Here we report the first successful use of mpg-C3N4 as a visible light photoinitiator in free radical polymerization of vinyl monomers such as methyl methacrylate (MMA). We show that the mesoporous material exhibits excellent performance in the initiation and can easily be separated from the polymerization mixture and reused without a significant loss of the activity. Due to the insoluble colloidal structure, the initiating system overcomes the odor, toxicity, and migration problems generally encountered with low molar mass photoinitiators. On the basis of previous knowledge on the photoredox chemistry of mpg-C3N4, the photopolymerization of MMA was conducted by exposing the monomer mixture in visible light at different experimental conditions. Typical results are collected in Table 1.

Figure 1. Photopolymerization of methyl methacrylate in argon saturated bulk solution containing mpg-C3N4 and triethylamine at room temperature: λ > 420 nm, [MMA] = 9.35 mol·L−1.

In a comparative study at different wavelengths using a monochromatic light, it was observed that the conversion decreased with decreasing absorbance at longer wavelengths indicating the crucial role of the solid photoinitiator in the initiation process (Figure 2).

Table 1. Visible Light Initiated Bulk Polymerization of Methyl Methacrylate Using mpg-C3N4 in the Presence of Amine Co-Initiators at Room Temperaturea co-initiator

conversion (%)

Mn × 10−3 (g mol−1)

Mw/Mn

triethylamine triethylamineb ethanolamine N,N-dimethylaniline benzyl alcohol

13 4 8 10

39.5

2.79

55.3 91.0

3.68 2.38

a

λ > 420 nm; tirr= 2 h; [MMA] = 9.35 mol·L−1, [co-initiator] = 0.256 mol·L−1. bWith bulk g-C3N4.

Figure 2. The absorption spectrum of mpg-C3N4 and photo polymerization of methyl methacrylate in Argon saturated bulk solution containing mpg-C3N4 and triethylamine at room temperature at different wavelengths.

In all cases, rather moderate light intensities were chosen to adjust low radical concentrations and enable a liquid final product and simple polymer separation. The initiating capability of mpg-C3N4 was analyzed in conjunction with three different amines as co-initiators. As can be seen from the table, all amines were effective in initiating the polymerization of MMA, and the presence of two components of the initiating system are indispensable for the polymerization to occur; no polymer is formed in the absence of either compound under our reaction conditions. Notably, triethylamine was most efficient in accordance with the fact that it is the most basic amine and contains higher number of abstractable hydrogens.

The efficiency of the photoinitiating system in the photocuring of formulations containing multifunctional monomers was also studied. The heat released during the photocuring of the formulations was followed by photo-DSC. Kinetic profile referring to the polymerization of poly(ethylene oxide)diacrylate (PEO−DA) under visible light emitting at λ = 430 nm is shown in Figure S1. The shape of the curves indicates the existence of two stages: a rapid first stage followed by a slow stage. At the second stage, gelation and vitrification of the polymerizing difunctional acrylate most likely render the diffusion of the components more difficult. The initiation mechanism of mpg-C3N4 is presumably based on the reaction 547

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ACS Macro Letters

Letter

polymerization, the mesoporous compound retains its activity. This behavior was also confirmed by the unchanged overall and surface structures of the photoinitiator after use, as confirmed by XRD and FTIR (Figure S4). In conclusion, photoinitiation of free radical polymerization using mpg-C3N4 could be efficiently achieved at wavelengths of visible region, using tertiary amines as a reaction coupler. In the process, the generation of initiating radicals occurs via oxidation of amines formed by the visible light photocatalysis of mpgC3N4, followed by hydrogen abstraction. The effect of the type of amine, oxygen, surface area, and irradiation wavelength was evaluated, supporting the proposed mechanism. The low migration and reuse due to the insolubility and excellent light absorption properties in the visible range facilitated by the large surface area ensuring efficient light absorption make these types of photoinitiators valuable for many targeted applications involving photopolymerization. Moreover, because the initiating radicals are stemming from the amine compounds, block and graft copolymers may be prepared by using precursor polymers possessing terminal and side chain amines, respectively. Further studies along this line are now in progress.

of holes with amine, in analogy to the usual behavior of mpgC3N4 as photocatalyst in the oxidation of amines and alcohols (Scheme 2).39 Scheme 2. Proposed Initiation Mechanism of the Free Radical Polymerization by Using Mesoporous Graphitic Carbon Nitride in the Presence of Amines

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In this mechanism photochemically formed holes oxidize amine to the corresponding radical cation which abstracts hydrogen from another amine leading to the formation of initiating radicals. Although at present the fate of electrons is not known, it is very likely that they undergo redox reaction to form hydrogen radical and neutral amine as depicted in Scheme 3.

ASSOCIATED CONTENT

S Supporting Information *

Experimental details, materials and methods, XRD patterns, and FT-IR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

Scheme 3. Electron Transfer Reactions of Electrons Expelled by the Irradiation of mpg-C3N4

*Phone: +49-(0) 331-567-9515 (X.W.); +90 212 285 32 41 (Y.Y.). Fax: +49-(0)331-5679502 (X.W.); +90 212 285 63 86 (Y.Y.). E-mail: [email protected] (X.W.); yusuf@ itu.edu.tr (Y.Y.). Notes

The authors declare no competing financial interest.

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To investigate further if the amine compound is the reactive component of the initiating system and to determine the actual initiating radicals, the experiments using an excess of a stable radical, such as 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), in the absence of an added monomer was performed under similar experimental conditions. Indeed, the 1H NMR spectrum of the product (Figure S2) formed from the photolysis of mpgC3N4 and TEA in the presence of TEMPO in CDCl3 presents a weak resonance of the proton at 4.45 ppm indicative of the coupling product and confirms that the hydrogen abstraction occurs from the amine compound. To gain more insight into the reaction mechanism, we have tested the use of other hole scavengers such as benzyl alcohol in the photoinitiation system. This compound is also known to be a strong hydrogen donor for triplet state photosensitizers such as camphorquinone.40 Interestingly, the experiments using benzyl alcohol at the same concentration under identical conditions failed to produce any precipitable polymer favoring the mechanism proposed in Scheme 2. Notably, the dominating process in the case of benzyl alcohol is the oxidation reaction leading to the formation of corresponding aldehyde. An additional advantage of the initiating system described is connected to the simple separation of the main component, mpg-C3N4 from the mixture after the polymerization, which can be purified and reused as photoiniator in further polymerizations. As can be seen from Figure S3, after several uses in the photo-

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