Recent Advances on Visible Light Metal-Based Photocatalysts for Polymerization under Low Light Intensity

Abstract

In recent years, polymerization processes activated by light have attracted a great deal of interest due to the wide range of applications in which this polymerization technique is involved. Parallel to the traditional industrial applications ranging from inks, adhesives, and coatings, the development of high-tech applications such as nanotechnology and 3D-printing have given a revival of interest to this polymerization technique known for decades. To initiate a photochemical polymerization, the key element is the molecule capable to interact with light, i.e., the photoinitiator and more generally the photoinitiating system, as a combination of several components is often required to create the reactive species responsible for the polymerization process. With the aim of reducing the photoinitiator content while optimizing the polymerization yield and/or the polymerization speed, photocatalytic systems have been developed, enabling the photosensitizer to be regenerated during the polymerization process. In this review, an overview of the photocatalytic systems developed for polymerizations carried out under a low light intensity and visible light is provided. Over the years, a wide range of organometallic photocatalysts has been proposed, addressing both the polymerization efficiency and/or the toxicity, as well as environmental issues.

High‐performance photocatalysts for the selective oxidation of alcohols to carbonyl compounds

“…Since the redox potential that is obtained from visible light is in the order of −2.4 eV/SCE for reduction and + 2.4 eV/SCE for oxidations, [191] there is a need to identify robust catalytic materials that can extend this range in order to effectively replace stoichiometric oxidizing reagents with photocatalysis. This has provided the motivation to search for novel non-metallic organic compounds [192,193] and earthabundant transition metal complexes [194,195] as photocatalysts. Nevertheless, since most photocatalytic oxidation involves radical intermediates, its usage is limited to reactions that do not make use of stoichiometric oxidizing agents.…”

Catalyst Halogenation Enables Rapid and Efficient Polymerizations with Visible to Near-Infrared Light

“…The recent widespread availability of inexpensive visible light emitting diodes (LEDs) offers an alternative method to UV photocuring that provides 1) milder and more cost-effective reaction conditions, 2) larger penetration depths due to reduced scattering and background absorption, and 3) wavelengthselectivity (i.e., the ability to activate different chemical pathways with individual wavelengths/colors of electromagnetic radiation). [12][13][14][15][16][17] As such, visible light photocuring holds the potential to promote the preparation of, inter alia, biocompatible materials containing UV-absorbing or -sensitive components, strong and lightweight composite structures, and multimaterial objects having predefined functionality embedded in discrete domains. For example, in dentistry photocurable coatings have shifted to blue irradiation to mitigate risks associated with UV exposure by employing visible light absorbing photocatalysts (e.g., camphorquinone and acylgermanes).…”

“…18,19 However, photocuring with longer wavelengths of light (green to near infrared, NIR) is an ongoing challenge that, to date, has been restricted to long exposure times (> 60 s) and/or high intensity irradiation (> 50 mW/cm 2 ), precluding their utility in photocuring applications. [12][13][14][15][16][17] To address the grand challenge of efficient photocuring with visible to NIR light, a number of metal and metal-free photocatalysts have been examined for their ability to induce polymerization. 12,16 These photocatalysts fall into one of two categories: Type I or Type II.…”

“…[12][13][14][15][16][17] To address the grand challenge of efficient photocuring with visible to NIR light, a number of metal and metal-free photocatalysts have been examined for their ability to induce polymerization. 12,16 These photocatalysts fall into one of two categories: Type I or Type II. Type I photosystems consist only of a photoinitiator (PI) that degrades upon light absorption to yield reactive fragments capable of initiating polymerization (e.g., radicals, cations, or anions).…”

Rapid High Resolution Visible Light 3D Printing

“…As an alternative, visible light offers numerous benefits, including reduced cost and energy of irradiation from readily available and modular light emitting diodes (LEDs), improved biocompatibility and functional group tolerance, greater depth of penetration, and reduced scattering (Figure 1). 10,[12][13][14][15][16] As such, visible light photocuring has the potential to enable next generation designer material fabrication, including, hydrogels containing live cells, 17 opaque composites, 16 and wavelength-selective multi-material structures [18][19][20][21][22][23] that promise to advance a range of applications, from structural plastics to tissue engineering and soft robotics.…”