Effects of the Chalcogenide Identity in N‐Aryl Phenochalcogenazine Photoredox Catalysts

Abstract

Phenochalcogenazines such as phenoxazines and phenothiazines have been widely employed as photoredox catalysts (PCs) in small molecule and polymer synthesis. However, the effect of the chalcogenide in these catalysts has not been fully investigated. In this work, a series of four phenochalcogenazines is synthesized to understand how the chalcogenide impacts catalyst properties and performance. Increasing the size of the chalcogenide is found to distort the PC structure, ultimately impacting the properties of each PC. For example, larger chalcogenides destabilize the PC radical cation, possibly resulting in catalyst degradation. In addition, PCs with larger chalcogenides experience increased reorganization during electron transfer, leading to slower electron transfer. Ultimately, catalyst performance is evaluated in organocatalyzed atom transfer radical polymerization and a photooxidation reaction for C(sp 2 )À N coupling. Results from these experiments highlight that a balance of PC properties is most beneficial for catalysis, including a long-lived excited state, a stable radical cation, and a low reorganization energy.

Chalcogenide‐Doped Anthracenes as Organophotocatalysts for Metal‐Free Atom Transfer Radical Polymerization

“…Nevertheless, the Se catalyst 4a showed the best control among the five catalysts in the polymerization of MMA ( M n = 20 100 Da, Đ = 1.44, Entry 4), but the calculated molecular weight was lower than those measured by GPC, which might be due to the low initiator efficiency or some side reactions. [ 58 ] Of note, compared with the corresponding O and S analogues [ 57 ] ( N ‐phenyl phenoxazine [ 49 ] and phenothiazine [ 36 ] ), 4a ·+ possesses a similar oxidizing ability (0.7–0.8 V vs SCE), but with a less reductive (−1.9 V vs −2.1 V) excited state ( 4a *), which may account for the observed dispersity control and a lower initiator efficiency.…”

“…[ 56 ] In this way, we became particularly interested to revisit anthracene as a parental arene for doping with other heteroatoms, such as oxygen used in ODA, as well as other chalcogenide heteroatoms like Se and Te, which remains unexplored. [ 57 ] This idea encouraged us to compare the property and performance of these heteroatom‐doped anthracenes with the different integration of O, S, Se, or Te atoms (Figure 1B), which unveiled a significant effect of these heteroatoms on the photophysical and electrochemical properties. To our delight, upon further structural optimization, we could identify N ‐aryl phenoselenazines, the selenium analogues of phenothiazines, as effective photocatalysts for the organocatalytic ATRP in the end, which could enable the O‐ATRP of methyl methacrylate to proceed under control and with additional temporal regulation by light.…”

Structure and Photophysics of N‐Tolanyl‐phenochalcogenazines and their Radical Cations

“…Concerning the structural parameters of the heterocyclic unit, the PZXtolan compounds are very similar to the parent PZX-phenyl derivatives (see Table S9 in the SI). [16] We also calculated the structures of the pa conformer which allows a comparison of the relative ground state energies for the two conformers (see Table 2). While for PZO-tolan the pe structure is clearly preferred, the pa structure is lower in energy for PZTe-tolan, in agreement with the X-ray structure.…”

Impact of Alkyl Core Substitution Kinetics in Diaryl Dihydrophenazine Photoredox Catalysts on Properties and Performance in O-ATRP

“…For deactivation, the PC •+ must have an oxidation potential positive enough to rapidly oxidize the propagating polymer radical, forming a halide-capped polymer ( E °( 2 PC •+ / 1 PC) > −0.8 V vs SCE) and regenerating the ground-state PC. Photophysical properties including molar absorptivity (ε max,abs ), wavelength of maximum absorption (λ max,abs ), charge transfer (CT) character, and excited-state lifetimes (τ) have also been found to be catalytically relevant and influence polymerization control. ,, Since the seminal reports of O-ATRP in 2014 with the use of perylene and 10-phenyl phenothiazine as PCs for O-ATRP, several new families of highly reducing organic PCs have been used, including phenoxazines, dihydrophenazines, dihydroacridines, and other phenochalcogenazines . This work focuses on dihydrophenazine PCs, which are notable for their highly reducing excited state and good polymerization control ( Đ < 1.3) …”

“…6,19,27 Since the seminal reports of O-ATRP in 2014 with the use of perylene 28 and 10phenyl phenothiazine 21 as PCs for O-ATRP, several new families of highly reducing organic PCs have been used, including phenoxazines, 13 dihydrophenazines, 12 dihydroacridines, 29 and other phenochalcogenazines. 30 This work focuses on dihydrophenazine PCs, which are notable for their highly reducing excited state and good polymerization control (Đ < 1.3). 12 O-ATRP with dihydrophenazine PCs has proven to be a robust method for producing polymers with low Đ, but in early work, I* was typically 60−80%, which indicates an inability to achieve targeted molecular weights.…”