Brugh Alexander M, Forbes Malcolm D E
Department of Chemistry , Center for Photochemical Sciences , Bowling Green State University , Bowling Green , OH 43403 , USA . Email:
Chem Sci. 2020 May 28;11(24):6268-6274. doi: 10.1039/d0sc02691c. eCollection 2020 Jun 28.
Time-resolved electron paramagnetic resonance (TREPR) spectroscopy has been used to study the proton coupled electron transfer (PCET) reaction between a ruthenium complex (Ru(bpz)(bpy)) and several substituted hydroquinones (HQ). After excitation at 355 nm, the HQ moiety forms a strong hydrogen bond to the exposed N atoms in the bpz heterocycle. At some point afterwards, a PCET reaction takes place in which an electron from the O atom of the hydrogen bond transfers to the metal center, and the proton forming the hydrogen bond remains on the bpz ligand N atom. The result is a semiquinone radical (HQ˙), whose TREPR spectrum is strongly polarized by the triplet mechanism (TM) of chemically induced dynamic electron spin polarization (CIDEP). Closer examination of the CIDEP pattern reveals, in some cases, a small amount of radical pair mechanism (RPM) polarization. We hypothesize that when the HQ moiety has electron donating groups (EDGs) substituted on the ring, S-T RPM polarization is observed in HQ˙. These anomalous intensities are accounted for by spectral simulation using polarization from S-T mixing. The generation of S-T RPM is attributed to slow radical separation after PCET due to stabilization of the positive charge on the ring by EDGs. Results from a temperature dependence support the hypothesis.
时间分辨电子顺磁共振（TREPR）光谱已被用于研究钌配合物（Ru(bpz)(bpy)）与几种取代对苯二酚（HQ）之间的质子耦合电子转移（PCET）反应。在355nm处激发后，HQ部分与bpz杂环中暴露的N原子形成强氢键。此后的某个时刻，发生PCET反应，其中来自氢键O原子的一个电子转移到金属中心，形成氢键的质子保留在bpz配体N原子上。结果是一个半醌自由基（HQ˙），其TREPR光谱通过化学诱导动态电子自旋极化（CIDEP）的三重态机制（TM）强烈极化。对CIDEP模式的进一步检查在某些情况下揭示了少量的自由基对机制（RPM）极化。我们假设当HQ部分在环上有供电子基团（EDG）取代时，在HQ˙中观察到S-T RPM极化。这些异常强度通过使用S-T混合极化的光谱模拟来解释。S-T RPM的产生归因于PCET后由于EDG对环上正电荷的稳定作用导致的自由基缓慢分离。温度依赖性结果支持这一假设。
