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7,thioxanthen-14-one (TX-DPA) was synthesized and characterized as a potential visible light photoinitiator for radical polymerization. TX-DPA has an excellent absorption character in the visible region and the photophysical properties of TX-DPA were investigated by fluorescence and laser flash photolysis studies and the photopolymerization of methyl methacrylate in air atmosphere helped to understand the initiation mechanism of TX-DPA. Moreover, it is possible that the production of singlet oxygen from the quenching of 3 TX-DPA by molecular oxygen resulted in the reaction of singlet oxygen with TX-DPA to form an endoperoxide: decomposition of endoperoxide leads to initiating radicals. ■ INTRODUCTIONIn recent years, many attempts have been made to synthesize photoinitiators that operate in the visible region of the spectrum. The reason for this is that visible light is cheap, safe and possesses high penetration ability in the presence of UV absorbing monomers, pigments and substrates. 1−7 Thioxanthone (TX) and its derivatives are some of the most widely used type II photoinitiators in various UV curing applications because of their excellent light absorption characteristics. 8 In most cases, the photoinitiating free radicals are generated by hydrogen abstraction of triplet excited states of TX from hydrogen donors such as amines or thiols (see Scheme 1). 9−11 However, low molecular-weight amines, particularly when used at high concentrations, have several intrinsic disadvantages such as odor, toxicity 12 and migration in UV-curing technology and cause a decrease in the pendulum hardness of the cured film. 13 One way to overcome these problems is to chemically incorporate the hydrogen donating sites into TX chromophores. 14−16 Amine, 17−20 alkyl amino, 21,22 acid, 23−27 or thiol 28 linked photoinitiators are seen in the literature and they may find various applications in the Radiation Curing Industry because of the great advantages of their one-component nature. Because of the one-component structure of the photoinitiator, it can serve as both a triplet photosensitizer and as a hydrogen donor during photopolymerization. Thus, these photoinitiators do not require an additional co-initiator.An alternative approach concerns the attachment of both chromophoric and hydrogen donating groups into polymer chains. 29−35 This way the odor and toxicity problems perceived with the conventional photoinitiators and amine hydrogen donors have been overcome.We have also developed a novel thioxanthone based photoinitiator possessing an anthracene group, thioxanthone− anthracene 36 (TX-A, Chart 1), which exhibits completely different photochemical properties compared to conventional TX type photoinitiators.In contrast to TX type photoinitiators, TX-A is an efficient photoinitiator for free radical polymerization of both acrylic and styrenic type monomers in the presence of oxygen without an additional hydrogen donor. 36−38 Moreover, for other TX type photoinitiators oxygen inhibits the polymerization because of the quenching of...

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