Department of Chemistry, University of Colorado, Boulder, Colorado 80309, United States.
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States.
J Phys Chem A. 2021 Apr 22;125(15):3109-3121. doi: 10.1021/acs.jpca.1c00855. Epub 2021 Apr 7.
Organocatalyzed ATRP (O-ATRP) is a growing field exploiting organic chromophores as photoredox catalysts (PCs) that engage in dissociative electron-transfer (DET) activation of alkyl-halide initiators following absorption of light. Characterizing DET rate coefficients () and photochemical yields across various reaction conditions and PC photophysical properties will inform catalyst design and efficient use during polymerization. The studies described herein consider a class of phenoxazine PCs, where synthetic handles of core substitution and -aryl substitution enable tunability of the electronic and spin characters of the catalyst excited state as well as DET reaction driving force (Δ). Using Stern-Volmer quenching experiments through variation of the diethyl 2-bromo-2-methylmalonate (DBMM) initiator concentration, collisional quenching is observed. Eight independent measurements of are reported as a function of Δ for four PCs: four triplet reactants and four singlets with values ranging from 1.1 × 10 M s, where DET itself controls the rate, to 4.8 × 10 M s, where diffusion is rate-limiting. This overall data set, as well as a second one inclusive of five literature values from related systems, is readily modeled with only a single parameter of reorganization energy under the frameworks of the adiabatic Marcus electron-transfer theory and Marcus-Savéant theory of DET. The results provide a predictive map where can be estimated if Δ is known and highlight that DET in these systems appears insensitive to PC reactant electronic and spin properties outside of their impact on the driving force. Next, on the basis of measured values in selected PC systems and knowledge of their photophysics, we also consider activation yields specific to the reactant spin states as the DBMM initiator concentration is varied. In -naphthyl-containing PCs characterized by near-unity intersystem crossing, the T is certainly an important driver for efficient DET. However, at DBMM concentrations common to polymer synthesis, the S is also active and drives 33% of DET reaction events. Even in systems with low yields of ISC, such as in -phenyl-containing PCs, reaction yields can be driven to useful values by exploiting the S under high DBMM concentration conditions. Finally, we have quantified photochemical reaction quantum yields, which take into account potential product loss processes after electron-transfer quenching events. Both S and T reactant states produce the PC radical cation with a common yield of 71%, thus offering no evidence for spin selectivity in deleterious back electron transfer. The subunity PC yields suggest that some combination of solvent (DMAc) oxidation and energy-wasting back electron transfer is likely at play and these pathways should be factored in subsequent mechanistic considerations.
有机催化原子转移自由基聚合（O-ATRP）是一个快速发展的领域，它利用有机发色团作为光氧化还原催化剂（PC），在吸收光后进行卤代烷引发剂的离解电子转移（DET）活化。在各种反应条件和 PC 光物理性质下，表征 DET 速率系数（）和光化学产率将为催化剂设计和聚合过程中的有效利用提供信息。本文所描述的研究考虑了一类苯并恶嗪 PC，其中核心取代和 -芳基取代的合成处理能够调节催化剂激发态的电子和自旋特性以及 DET 反应驱动力（Δ）。通过改变二乙基 2-溴-2-甲基丙二酸盐（DBMM）引发剂浓度的 Stern-Volmer 猝灭实验，观察到碰撞猝灭。报告了四个 PC 中八个独立的的测量值作为 Δ 的函数：四个三重态反应物和四个单重态，值范围从 1.1×10 M s，其中 DET 本身控制速率，到 4.8×10 M s，其中扩散是限速步骤。这个整体数据集，以及一个包含来自相关系统的五个文献值的第二个数据集，可以在绝热 Marcus 电子转移理论和 DET 的 Marcus-Savéant 理论框架下，仅使用一个重组能参数进行很好的拟合。结果提供了一个预测图，如果知道 Δ，就可以估计，如果在这些系统中，PC 反应物的电子和自旋性质对驱动力没有影响，那么 DET 就不敏感。接下来，基于在选定的 PC 系统中测量的值和对其光物理性质的了解，我们还考虑了在改变 DBMM 引发剂浓度时特定于反应物自旋态的激活产率。在具有近乎完全系间窜越的 -萘基 PC 中，T 肯定是高效 DET 的重要驱动力。然而，在聚合合成中常见的 DBMM 浓度下，S 也很活跃，并驱动 33%的 DET 反应事件。即使在 ISC 产率较低的系统中，如 -苯基 PC 中，也可以通过在高 DBMM 浓度条件下利用 S 来驱动反应产率达到有用的值。最后，我们已经量化了光化学反应量子产率，它考虑了电子转移猝灭事件后潜在的产物损失过程。S 和 T 反应物状态都产生 PC 自由基阳离子，共同产率为 71%，因此没有证据表明在后电子转移中有自旋选择性。低于单位的 PC 产率表明，一些溶剂（DMAc）氧化和能量浪费的后电子转移的组合可能在起作用，这些途径应该在随后的机制考虑中加以考虑。
