Photopolymerization of Coumarin-Containing Reversible Photoresponsive Materials Based on Wavelength Selectivity

Double-Wavelength-Switchable Molecular Self-Assembly of a Photoacid and Spirooxazine in an Aqueous Solution

Quadruple-switchable nanoscale assemblies are built by combining two types of water-soluble molecular photoswitches through dipole-dipole interaction. Uniting the wavelength-specific proton dissociation of a photoacid and ring-opening of an anionic spirooxazine results in an assembly that can be addressed by irradiation with two different wavelengths: pH and darkness.

Recent Advances in Monocomponent Visible Light Photoinitiating Systems Based on Sulfonium Salts

During the last decades, multicomponent photoinitiating systems have been the focus of intense research efforts, especially for the design of visible light photoinitiating systems. Although highly reactive three-component and even four-component photoinitiating systems have been designed, the complexity to elaborate such mixtures has incited researchers to design monocomponent Type II photoinitiators. Using this approach, the photosensitizer and the radical/cation generator can be combined within a unique molecule, greatly simplifying the elaboration of the photocurable resins. In this field, sulfonium salts are remarkable photoinitiators but these structures lack absorption in the visible range. Over the years, various structural modifications have been carried out in order to redshift their absorptions in the visible region. In this work, an overview of the different sulfonium salts activable under visible light and reported to date is proposed.

Light-activated cellulose nanocrystals/fluorinated polyacrylate-based waterborne coating: Facile preparation, mechanical and self-healing behavior

The development of environmental-friendly self-healing nanocomposites has attracted much attention. In this paper, the light-activated cellulose nanocrystals/ fluorinated polyacrylate-based waterborne coating based on the reversible cycloaddition reaction of the coumarin groups was prepared via Pickering emulsion polymerization. The cellulose nanocrystals (CNCs) modified by the PDMAEMA-b-PGMA-b-P(HFBA-co-VBMC) copolymer were studied via FT-IR and TGA. In addition, the dispersity and interface behavior of CNCs before and after modification were investigated by DLS and interfacial tension measurements. Afterwards, we focused on the influence of modified CNCs, PDMAEMA-g-CNC-g- P(HFBA-co-VBMC) (MCNC) dosage on the Pickering emulsion, emulsion polymerization and properties of latex film. The droplet diameter of Pickering emulsion gradually reduced with the increase of MCNC dosage. The MCNC dosage for the minimum average size and optimum stability of latex particles was 1.0 wt%. Moreover, the latex film comprising 1.0 wt% MCNC presented not only high tensile stress (6.0 MPa), large elongation at break (567.70 %) and superior oil/water repellency but also excellent self-healing properties. The outstanding self-healing capability of latex film was attributed to the reversible light-activated dimerization of coumarin groups. The preparation method for the advanced performance waterborne cellulose nanocrystals/fluorinated polyacrylate will provide valuable guidance for the development of versatile materials.

Realizing Vat-Photocycloaddition 3D Printing with Recyclable Synthetic Photorheological Silicone Fluids

Synthetic silicone rubbers are finding a broad spectrum of applications, yet there has been a demand for developing greener silicone rubbers with processability, recyclability, and reversible tunability in their mechanical properties. Here, a recyclable photorheological silicone fluid (RPSF) is developed, which realizes completely reversible wavelength-selective liquid-rubber conversion upon photoirradiation, relying on the reversible photocycloaddition of coumarin upon alternating irradiation of light with wavelengths of 365 nm (UV₃₆₅ ) and 254 nm (UV₂₅₄ ). Rheological studies demonstrate that the storage modulus of the developed RPSF increases by a factor of more than 100,000 upon UV₃₆₅ irradiation to reach 20-50 kPa, while it decreases to ca. 0.01 kPa upon UV₂₅₄ irradiation. The reversibility of the photocycloaddition of coumarin enables the application of RPSF as a photodismantlable adhesive. Furthermore, unprecedented vat-photocycloaddition 3D printing of silicone rubber is realized by taking advantage of the excellent photocurability, i.e., dramatic increase in viscoelasticity upon UV₃₆₅ irradiation. This article is protected by copyright. All rights reserved.

Oxidative formation of bis-N-methylquinolinone from anti-head-to-head N-methylquinolinone cyclodimer

The light-driven formation and cleavage of cyclobutane structural motifs resulting from [2 + 2]-pericyclic reactions, as found in thymine and coumarin-type systems, is an important and intensively studied photochemical reaction. Various applications are reported utilizing these systems, among others, in cross-linked polymers, light-triggered drug release, or other technical applications. Herein coumarin is most frequently used as the photoactive group. Quite often, a poor quantum yield for dimerization and cyclobutane-cleavage and a lack of reversibility are described. In this work, we present the identification of a heterogeneous pathway of dimer cleavage found in a rarely studied coumarin analog molecule, the N-methyl-quinolinone (NMQ). The monomer was irradiated in a tube flow-reactor and the reaction process was monitored using online HPLC measurements. We found the formation of a pseudo-equilibrium between monomeric and dimeric NMQ and a continuous rise of a side product via oxidative dimer splitting and proton elimination which was identified as 3,3'-bis-NMQ. Oxidative conversion by singlet oxygen was identified to be the cause of this non-conventional cyclobutane cleavage. The addition of antioxidants suppressing singlet oxygen enables achieving a 100% photochemical conversion from NMQ to the anti-head-to-head-NMQ-dimer. Using dissolved oxygen upon light activation to singlet oxygen limits the reversibility of the photochemical [2 + 2]-cycloaddition and cycloreversion of NMQ and most likely comparable systems. Based on these findings, the development of highly efficient cycloaddition-cycloreversion systems should be enabled.

Recent Advances in bis-Chalcone-Based Photoinitiators of Polymerization: From Mechanistic Investigations to Applications

Over the past several decades, photopolymerization has become an active research field, and the ongoing efforts to develop new photoinitiating systems are supported by the different applications in which this polymerization technique is involved-including dentistry, 3D and 4D printing, adhesives, and laser writing. In the search for new structures, bis-chalcones that combine two chalcones' moieties within a unique structure were determined as being promising photosensitizers to initiate both the free-radical polymerization of acrylates and the cationic polymerization of epoxides. In this review, an overview of the different bis-chalcones reported to date is provided. Parallel to the mechanistic investigations aiming at elucidating the polymerization mechanisms, bis-chalcones-based photoinitiating systems were used for different applications, which are detailed in this review.

Recent Advances in Functional Polymers Containing Coumarin Chromophores

Natural and synthetic coumarin derivatives have gained increased attention in the design of functional polymers and polymer networks due to their unique optical, biological, and photochemical properties. This review provides a comprehensive overview over recent developments in macromolecular architecture and mainly covers examples from the literature published from 2004 to 2020. Along with a discussion on coumarin and its photochemical properties, we focus on polymers containing coumarin as a nonreactive moiety as well as polymer systems exploiting the dimerization and/or reversible nature of the [2πs + 2πs] cycloaddition reaction. Coumarin moieties undergo a reversible [2πs + 2πs] cycloaddition reaction upon irradiation with specific wavelengths in the UV region, which is applied to impart intrinsic healability, shape-memory, and reversible properties into polymers. In addition, coumarin chromophores are able to dimerize under the exposure to direct sunlight, which is a promising route for the synthesis and cross-linking of polymer systems under "green" and environment-friendly conditions. Along with the chemistry and design of coumarin functional polymers, we highlight various future application fields of coumarin containing polymers involving tissue engineering, drug delivery systems, soft robotics, or 4D printing applications.

Visible-light reversible photopolymerisation: insights via online photoflow – electrospray ionisation – mass spectrometry

Herein, we introduce a scalable photopolyaddition polymerisations using the pyrene-chalcone [2+2]-cycloaddition and monitor the photodepolymerisation process via an online photoflow – electrospray ionisation mass spectrometry setup.