Recent Advances in Photosensitizer and Photosensitive System

Dear Colleagues,

For the last 50 years, photopolymerization and photoimaging science have been fields of central importance in polymer science and UV–vis technology.

A photopolymerization reaction starts upon the light exposure of the reactants. Light is one of the non-invasive stimuli which can be used in the spatiotemporal control of chemical reactions. Over the past decade, light has found wide applications in polymer science; for example, in polymer synthesis, the release of small molecules from polymers, polymeric photosensors, etc.

Inherent to these technologies is the use of a photoactivator system which is capable of absorbing the incident UV and/or visible radiation wavelengths used for converting a monomer or prepolymer system into a linear polymer or crosslinked network. Notably, the safety concerns related to the use of UV light, the high energy consumption resulting from the use of UV irradiation setups, and the low light penetration of UV light within the photocurable resins are three major concerns successfully addressed by visible light photoinitiators.

Light-initiated polymerization processes are currently an important tool in various industrial fields. The advancement of technology has resulted in the use of photopolymerization in various biomedical applications, such as the production of 3D hydrogel structures, the encapsulation of cells, and drug delivery systems.

The development of new procedures aimed at reducing the environmental impact of polymerization processes is a major societal issue. Light-assisted polymerization, and in particular visible light photopolymerization, can address this issue in the future. The polymer material itself and its composition are also facing numerous questions concerning their environmental and human health impacts.

The migration of low-molecular-weight photoproducts is currently an important drawback in many application fields—for example, in food packaging materials, leading to unpleasant secondary features such as odor, blooming and contamination.

Beyond monomers, the different additives introduced in the resin can also be potentially toxic, so a great deal of effort is currently being invested in developing new photoinitiators with a toxicity assessed as being insignificant.

Photoinitiating systems (PSIs) are sometimes complex combinations of several compounds that require at least a photoinitiator (I). Other components of a photoinitiating system may include a photosensitizer, a coinitiator, and added compounds.

Polymerization photoinitiators which are activable under low light intensity and in the visible range are being actively researched by both the academic and industrial communities.

Over the years, a wide range of Type I and Type II photoinitiators have been developed, ranging from D-π-A push–pull dyes to polyaromatic structures, indane-1,3-dione, ferrocene, ferrocenium, ketone derivatives, xanthene dyes, porphyrins, metalated porphyrins, benzophenone hybrid structures, naphthalic anhydrides and 1,8-naphthalimide-derivatives, anthraquinone derivatives, phthalocyanines, anthraquinone functional phthalocyanines, quinizarin derivatives, carbazole derivatives, pyrene, triphenylamine-based hexaarylbiimidazole, yellow triazine, silyl glyoxylates, chromones or acridinediones, as well as methal complexes (e.g., copper). Over the years, a family of compounds has been identified as being highly promising due to its remarkable photoinitiating ability, photochemical stability, low cost, and ease of functionalization.

Depending on the applications, the use of synthetic photoinitiators can constitute a severe limitation for future applications of photopolymers, and so the use of natural products has been identified as a promising alternative to address the issues of toxicity and biocompatibility.

Therefore, the design and development of photoinitiating systems applicable to UV or even visible light for free radical and cationic polymerizations, and the hybrid polymerization of cationic and radical monomers, has been attracting increasing attention due to their great potential applications in various fields.

In this Issue, the representative trends in investigations of light-induced polymerization and an overview of the photoinitiators based on synthetic or/and natural products, as well as their potentials for various future industrial applications (e.g., printing, enabling an unprecedented access to structures of incredible complexity, medical applications, photocomposite synthesis, manufacturing of fiber-reinforced polymers, protective coatings, dental fillings, adhesives, inks, rapid prototyping, and advanced high-technology purposes (micro and nano-fabrication, optoelectronics, holographic data storage, etc.)), will be highlighted and discussed.

Dr. Janina Kabatc
Guest Editor

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• photopolymerization
• photosensitizer
• initiator
• application