Overcoming the challenge of metal contamination in traditional ATRP systems, a metal-free ATRP process, mediated by light and catalyzed by an organic-based photoredox catalyst, is reported. Polymerization of vinyl monomers are efficiently activated and deactivated with light leading to excellent control over the molecular weight, polydispersity, and chain ends of the resulting polymers. Significantly, block copolymer formation was facile and could be combined with other controlled radical processes leading to structural and synthetic versatility. We believe that these new organic-based photoredox catalysts will enable new applications for controlled radical polymerizations and also be of further value in both small molecule and polymer chemistry.

A class of tunable visible and near-infrared donor−acceptor Stenhouse adduct (DASA) photoswitches were efficiently synthesized in two to four steps from commercially available starting materials with minimal purification. Using either Meldrum's or barbituric acid "acceptors" in combination with aniline-based "donors", an absorption range spanning from 450 to 750 nm is obtained. Additionally, photoisomerization results in complete decoloration for all adducts, yielding fully transparent, colorless solutions and films. Detailed investigations using density functional theory, nuclear magnetic resonance, and visible absorption spectroscopies provide valuable insight into the unique structure−property relationships for this novel class of photoswitches. As a final demonstration, selective photochromism is accomplished in a variety of solvents and polymer matrices, a significant advantage for applications of this new generation of DASAs.

Here we report the use of 10-phenylphenothiazine (PTH) as an inexpensive, highly reducing metal-free photocatalyst for the reduction of carbon-halogen bonds via the trapping of carbon-centered radical intermediates with a mild hydrogen atom donor. Dehalogenations were carried out on various substrates with excellent yields at room temperature in the presence of air.In recent years, photoredox chemistry has enabled the development of a wide variety of synthetic transformations. 1 These methods are based on photocatalysts which, upon absorption of light, enter either a highly reducing or oxidizing excited state capable of facilitating redox-based transformations. In particular, the reduction of activated carbon-halide (C-X) bonds has generated wide interest, largely because of the broad synthetic utility of resulting carbon-centered radical intermediates. [1][2][3][4][5][6][7][8][9][10] One example includes subsequent trapping of these intermediates with a mild H-atom source to achieve radical dehalogenations. 3,5,6,9 In this case, the power of using a photoredox approach is that it offers a more efficient and safer alternative to traditional dehalogenation protocols involving metal-halogen exchange, 11,12 stoichiometric tin hydride, 13 and various other highly toxic reagents. [14][15][16] However, despite the notable advantages of photoredox catalysis, 1 a number of major challenges still exist. This includes the use of catalysts based on rare-earth transition metals such as Ru and Ir, which have inherent limitations due to the cost of the catalyst itself (B$1 mg À1 for Ir(ppy) 3 ), 17 as well as the expense associated with the removal of trace metals from the desired products -critical for applications from pharmaceuticals to micro-electronics. In addition, although an assortment of activated carbon-halogen bonds have been accessed using these catalysts, 1 higher energy unactivated halides are a significantly more challenging task, with only unactivated iodides being explored to date. 5,18 To this end, a more affordable gold-based photocatalyst has been developed, 10 and although offering broader substrate scope, the disadvantages of metal-based systems remain. In addressing this, the use of an organic perylene diimide (PDI)-based photocatalyst was recently reported, and while providing a metal-free alternative, it requires elevated temperatures and has a scope limited to activated aryl-halides. 8 In this context, we envisioned the development of a highly reducing, inexpensive, metal-free photocatalyst that could offer access to a wide range of carbon-halogen substrates under markedly mild conditions (Fig. 1).Our groups previously employed 10-phenylphenothiazine (PTH) as a metal-free catalyst for photomediated atom transfer radical polymerizations (ATRP). 19 In this system, PTH acts as a photoreductant in a similar manner to Ir(ppy) 3 with a reduction potential (E 1/2 * = À2.1 V vs. SCE) significantly higher than Ir(ppy) 3 (E 1/2 * = À1.7 V vs. SCE). Based on our interest in metalfree ATRP, we envisioned that th...

ABSTRACT:The development of an operationally simple, metal-free surface-initiated atom transfer radical polymerization (SI-ATRP) based on visible-light mediation is reported. The facile nature of this process enables the fabrication of well-defined polymer brushes from flat and curved surfaces using a "benchtop" setup that can be easily scaled to four-inch wafers. This circumvents the requirement of stringent air-free environments (i.e., glovebox), and mediation by visible light allows for spatial control on the micron scale, with complex three-dimensional patterns achieved in a single step. This robust approach leads to unprecedented access to brush architectures for nonexperts.