Photothermal Nanoparticle Initiation Enables Radical Polymerization and Yields Unique, Uniform Microfibers with Broad Spectrum Light

Molecular Photothermal Conversion Catalyst Promotes Photocontrolled Atom Transfer Radical Polymerization

Photothermal conversion is a growing research area that promotes thermal transformations with visible light irradiation. However, few examples of dual photothermal conversion and catalysis limit the power of this phenomenon. Here, we take inspiration from nature's ability to use porphyrinic compounds for nonradiative relaxation to convert light into heat to facilitate thermal polymerization catalysis. We identify the photothermal conversion catalytic activity of a vitamin B12 derivative, heptamethyl ester cobyrinate (HME-Cob), to perform atom transfer radical polymerization (ATRP) under irradiation. Rapid polymerization are obtained under photothermal activation while maintaining good control over polymerization with the aid of a photoinitiator to enable light-induced catalyst regeneration. The catalyst exhibits exquisite temporal control in photocontrolled thermal polymerization. Ultimately, the activation of this complex is accessed across a broad range of wavelengths, including near-IR light, with excellent temporal control. This work showcases the potential of developing photothermal conversion catalysts.

Photothermal Conversion by Carbon Black Facilitates Aryl Migration by Photon-Promoted Temperature Gradients

The Newman Kwart Rearrangement (NKR) offers an efficient and high-yielding method for producing substituted thiophenols from phenols. While an industrially important protocol, it suffers from high activation energy barriers (35-43 kcal/mol), requiring the use of extreme temperatures (>200 °C) and specialty equipment. This report details a highly efficient and straightforward method for facilitating the NKR using photothermal conversion. This underused, unique reactivity pathway arises from the irradiation of nanomaterials that relax via a non-radiative decay pathway to generate intense thermal gradients. We show carbon black (CB) can be an inexpensive and abundant photothermal agent under visible light irradiation to achieve a facile NKR under mild conditions. The scope includes a wide array of stereo- and electronically diverse substrates with increasing difficulty of rearrangement, including BHT and BINOL as effective substrates. Furthermore, we demonstrate the unique application for temporal control in a thermal reaction and tunability of thermal gradients by modulating light intensity. Ultimately, photothermal conversion enables high-temperature reactions with simple, visible light irradiation.

Intrinsic Optical Properties and Emerging Applications of Gold Nanostructures

The collective oscillation of free electrons at the nanoscale surface of gold nanostructures is closely modulated by tuning the size, shape/morphology, phase, composition, hybridization, assembly, and nanopatterning, along with the surroundings of the plasmonic surface located at a dielectric interface with air, liquid, and solid. This review first introduces the physical origin of the intrinsic optical properties of gold nanostructures and further summarizes stimuli-responsive changes in optical properties, metal-field-enhanced optical signals, luminescence spectral shaping, chiroptical response, and photogenerated hot carriers. The current success in the landscape of nanoscience and nanotechnology mainly originates from the abundant optical properties of gold nanostructures in the thermodynamically stable face-centered cubic (fcc) phase. It has been further extended by crystal phase engineering to prepare thermodynamically unfavorable phases (e.g., kinetically stable) and heterophases to modulate their intriguing phase-dependent optical properties. A broad range of promising applications, including but not limited to full-color displays, solar energy harvesting, photochemical reactions, optical sensing, and microscopic/biomedical imaging, have fostered parallel research on the multitude of physical effects occurring in gold nanostructures.

Two-in-One: Photothermal Ring-Opening Copolymerization of CO2 and Epoxides

A two-in-one strategy for the photothermal ring-opening copolymerization (PROCOP) of carbon dioxide (CO₂) and epoxides was developed by using visible light as an external stimulus. This strategy bridges two processes involving light-to-heat conversion and the alternating copolymerization of CO₂ and epoxides. As a proof-of-concept, aluminum porphyrin complexes were explored as photothermal catalysts to afford the copolymerization of CO₂/epoxides under a 635 nm laser irradiation. We demonstrated photothermally enhanced polymerization activity, in which the polymerization initiated by the photothermal effect showed a much higher turnover frequency than in the thermal system. Moreover, the PROCOP demonstrated a spatial-temporal control by a light on/off switch. This study provides a fascinating photothermal strategy not only for the CO₂/epoxides copolymerization but also for the ring-opening (co)polymerization of other cyclic monomers.

Photothermal polymerization of benzoxazines

Visible irradiation of mixtures of benzoxazine monomers and metal salt catalysts leads to extensive photothermal polymerization, which allows the preparation of complex polybenzoxazine features via photolithography.

Asymmetries in the Electronic Properties of Spheroidal Metallic Nanoparticles, Revealed by Conduction Electron Spin Resonance and Surface Plasmon Resonance

Using electron spin resonance spectroscopy, we demonstrate that the morphological asymmetries present in small spheroidal metallic nanoparticles give rise to asymmetries in the behavior of electrons held in states near the metal's Fermi energy. We find that the effects of morphological asymmetries for these spheroidal systems are more important than the effects of size distributions when explaining the asymmetry in electronic behavior. This is found to be true for all the particles examined, which were made from Cu, Ag, Pd, Ir, Pt, and Au, bearing dodecanethiolate ligands. In the case of the Ag particles, we also demonstrate that the same model used to account for morphological effects in the electron spin resonance spectra can be used to account for small asymmetries present in the plasmon spectrum. This result demonstrates that the electronic properties of even small particles are tunable via morphological changes.

Ultrathin Near-Infrared Light Activated Nano-Hotplate Catalyst

A combined photothermal-catalytic system that contains a single active element, without using different entities for separate roles (catalytic vs photothermal), is designed here for efficient catalytic reactions. Herein, ultrathin (sub-6 nm) rectangular-like KNdF₄ nanoplates consisting of 3-4 unit cell layers are prepared where the Nd³⁺ ions act as a Lewis acid catalyst. In addition, the nanoplates undergo light-to-heat conversion when irradiated with NIR light due to cross-relaxation and nonradiative relaxation processes from excited Nd³⁺ . The cyanosilylation of a series of ketones is performed using the nano-hotplate catalysts to give near quantitative yields of the cyanohydrin trimethylsilyl ethers. This is because of the high surface area-to-volume ratio of the thin nanoplates that provides a large number of surface Nd³⁺ catalytic sites for reaction. The reaction kinetics are enhanced by the photothermal effect, leading to the observed > 10-fold increase in product yields.

Fillers as Heaters for Photothermal Polymerization upon NIR Light

Photo-induced thermal polymerization upon near-infrared (NIR) light irradiation has been reported in the literature. In this approach, a component able to convert the NIR light into heat must be used in combination with a thermal initiator to initiate the free-radical polymerization of (meth)acrylates. In recent studies, some absorbers have been presented as very efficient heat generators (called heaters). In the present work, different fillers are investigated as heaters and compared to organic NIR absorbers. An alkoxyamine (e.g., BlocBuilder-MA) is used as thermal initiator and is dissociated by the heat generated by the NIR photoexcitation of the fillers. In the present work, several fillers are examined: graphene oxide, graphene nanoplatelets, multi-walled carbon nanotubes, and silicon carbide. Due to the energy of the photon delivered, NIR light curing is challenging but offers several advantages compared to visible light. The most interesting feature is the deeper penetration of the light inside the photocurable resin, enabling the polymerization of thick samples. Parallel to this, incorporation of fillers in resins allows unique access to composites through photothermal polymerization of (meth)acrylates. Three different wavelengths of irradiation have been studied: 785, 940, and 1064 nm.