Kosaka Maria, Miyokawa Katsuki, Kurashige Yuki
Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku Kyoto 606-8502, Japan.
FOREST, JST, Honcho 4-1-8, Kawaguchi, Saitama 332-0012, Japan.
Phys Chem Chem Phys. 2024 Dec 4;26(47):29449-29456. doi: 10.1039/d4cp03386h.
Efficient formation of excited triplet states on metal-free photosensitizer dyads, bodipy-phenoxazine (BDP-PXZ) and tetramethylbodipy-phenoxazine (TMBDP-PXZ), was investigated using calculations. We revealed the reason why two different triplet transient species, CT and BDP, can co-exist only for BDP-PXZ as observed in the previous study with the TR-EPR measurements. It was found that the state mixing of CT enables the transition from CT to CT and BDP states only for BDP-PXZ. This mixing effect is commonly seen in the singlet states of twisted intermolecular charge transfer molecules, though the key factor that determines the mixing of the excited states of the dyes was found to be the electron-donating ability of the substituents rather than their steric hindrance. This mechanism was corroborated by comparing the spin polarization ratio of the triplet spin-sublevels measured by TR-EPR with the theoretical predictions. The spin polarization ratio of the triplets should contain information about the transition intersystem crossing, the twisted angle of two chromophores of the dyad, and thus it can be a powerful tool to analyze the molecular mechanism of photochemical processes at the electronic structure level. These insights on the molecular structures' effect provided by this theoretical study would be a compass to molecular design of metal-free triplet photosensitizers.
利用计算方法研究了无金属光敏剂二元体系，即硼二吡咯 - 吩恶嗪（BDP - PXZ）和四甲基硼二吡咯 - 吩恶嗪（TMBDP - PXZ）中激发三重态的有效形成。我们揭示了为何在之前的瞬态电子顺磁共振（TR - EPR）测量研究中观察到的两种不同的三重态瞬态物种，即电荷转移态（CT）和硼二吡咯态（BDP），仅在BDP - PXZ中能共存。研究发现，CT态的态混合仅使BDP - PXZ能从CT态跃迁到CT态和BDP态。这种混合效应在扭曲分子间电荷转移分子的单重态中普遍存在，不过发现决定染料激发态混合的关键因素是取代基的给电子能力而非其空间位阻。通过将TR - EPR测量得到的三重态自旋子能级的自旋极化率与理论预测值进行比较，证实了这一机制。三重态的自旋极化率应包含有关系间窜越跃迁、二元体系中两个发色团的扭曲角度的信息，因此它可以成为在电子结构层面分析光化学过程分子机制的有力工具。这项理论研究提供的关于分子结构效应的这些见解将为无金属三重态光敏剂的分子设计指明方向。
