Structure-Dependent Electron Transfer Rates for Dihydrophenazine, Phenoxazine, and Phenothiazine Photoredox Catalysts Employed in Atom Transfer Radical Polymerization
Structure-Dependent Electron Transfer Rates for Dihydrophenazine, Phenoxazine, and Phenothiazine Photoredox Catalysts Employed in Atom Transfer Radical Polymerization
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原子转移自由基聚合中使用的二氢吩嗪、吩恶嗪和吩噻嗪光氧化还原催化剂的结构依赖性电子转移速率
DOI：
10.1021/acs.jpcb.1c05069
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发表时间：
2021
期刊：
影响因子：
--
通讯作者：
Sneha M
中科院分区：
文献类型：
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作者：
Sneha M
有机光催化剂（PC）在光氧化还原催化中的应用越来越广泛，但很少有研究探讨其工作方式。我们报告了一个详细的有机催化原子转移自由基聚合（O-ATRP）的电子转移活化步骤的机制调查涉及电子激发的有机PC和自由基引发剂，甲基2-溴丙酸酯（MBP）。本研究比较了具有二氢吩嗪，吩恶嗪，或吩噻嗪核心发色团的九个N-芳基修饰的PC。瞬态电子和振动吸收光谱在亚皮秒到纳秒和微秒的时间间隔，分别跟踪光谱特征的反应物和产品的光诱导电子转移inN，N-二甲基甲酰胺，二氯甲烷和甲苯的解决方案。电子转移速率系数的范围可达1010 M ~（-1）s ~（-1），受PC结构的系统影响。这些速率系数是一个数量级较小的催化剂的电荷转移字符在其第一激发单重态（S1）或三重态（T1）比光催化剂的局部激发字符。后者的物种显示出近扩散限制的电子转移到MBP的速率系数。推导出的动力学参数被用来模拟电子转移的贡献，从S1状态的每个PC为不同浓度的MBP。对其中一种吩恶嗪PC的单重态和三重态反应性的比较表明，从T1态转移电子的速率系数kET（T1）=（2.7 ± 0.3）× 107 M-1 s-1比从S1态转移电子的速率系数kET（S1）=（2.6 ± 0.4）× 109 M-1 s-1低2个数量级.双分子电子转移速率系数的趋势占使用修改后的马库斯理论的解离电子转移。
Organic photocatalysts (PCs) are gaining popularity in applications of photoredox catalysis, but few studies have explored their modus operandi. We report a detailed mechanistic investigation of the electron transfer activation step of organocatalyzed atom transfer radical polymerization (O-ATRP) involving electronically excited organic PCs and a radical initiator, methyl 2-bromopropionate (MBP). This study compares nineN-aryl modified PCs possessing dihydrophenazine, phenoxazine, or phenothiazine core chromophores. Transient electronic and vibrational absorption spectroscopies over subpicosecond to nanosecond and microsecond time intervals, respectively, track spectroscopic signatures of both the reactants and products of photoinduced electron transfer inN,N-dimethylformamide, dichloromethane, and toluene solutions. The rate coefficients for electron transfer exhibit a range of values up to ∼1010M–1s–1influenced systematically by the PC structures. These rate coefficients are an order of magnitude smaller for catalysts with charge transfer character in their first excited singlet (S1) or triplet (T1) states than for photocatalysts with locally excited character. The latter species show nearly diffusion-limited rate coefficients for the electron transfer to MBP. The derived kinetic parameters are used to model the contributions to electron transfer from the S1state of each PC for different concentrations of MBP. Comparisons of singlet and triplet reactivity for one of the phenoxazine PCs reveal that the rate coefficientkET(T1) = (2.7 ± 0.3) × 107M–1s–1for electron transfer from the T1state is 2 orders of magnitude lower than that from the S1state,kET(S1) = (2.6 ± 0.4) × 109M–1s–1. The trends in bimolecular electron transfer rate coefficients are accounted for using a modified Marcus theory for dissociative electron transfer.