Internal Heavy-Atom Effect on Visible-Light-Induced Cyclization Reaction in Diarylethene–Perylenebisimide Dyads

All-visible-light switchable diarylethene–perylenebisimide (DAE-PBI) dyads having bromine heavy atoms in the molecule were designed and synthesized. Very recently, we found a unique visible-light-induced cyclization reaction in a DAE-PBI dyad. The dyad exhibited reversible cyclization and cycloreversion reactions upon alternate irradiation with green (500–550 nm) and red (>600 nm) light. From the experimental results, it was suggested that the triplet state of DAE unit was generated via multiplicity conversion based on intramolecular energy transfer from the singlet excited state of PBI unit and that the cyclization reaction of DAE unit proceeded from the triplet state. In addition, it was revealed that the reactivity remarkably increased in a solvent containing heavy atoms such as carbon tetrachloride and iodoethane (i.e., external heavy-atom effect). Based on such results, in this study, we attempted to design and synthesize novel DAE-PBI dyads introducing bromine heavy atoms at different positions in the molecule. The synthesized dyads exhibited higher quantum yields of photocyclization reaction under visible-light irradiation even in a heavy-atom-free solvent compared to the previous dyad having no heavy atoms. The magnitude of enhancement well correlated to the contribution ratio of atomic orbital of bromine to the molecular orbital in LUMOs. These results indicated that the internal heavy atom effectively contributed to the visible-light-induced cyclization reaction in DAE-PBI dyads. Such an internal heavy-atom effect will pave the way for new molecular design to develop all-visible-light-activatable molecular switches.

本研究设计并合成了分子中引入溴重原子的全可见光可切换二芳基乙烯-苝二酰亚胺（diarylethene–perylenebisimide, DAE-PBI）二元体。近期，我们在DAE-PBI二元体中发现了一种独特的可见光诱导环化反应。该二元体在交替辐照绿光（500–550 nm）与红光（>600 nm）时，可发生可逆的环化与逆环化反应。实验结果表明，DAE单元的三重态是通过苝二酰亚胺（PBI）单元单重激发态的分子内能量转移实现多重态转换而生成的，且DAE单元的环化反应正是从该三重态路径进行。此外，研究还发现，在四氯化碳、碘乙烷等含重原子的溶剂中，反应活性显著提升，即外重原子效应（external heavy-atom effect）。基于上述结果，本研究尝试设计并合成了在分子不同位置引入溴重原子的新型DAE-PBI二元体。相较于此前不含重原子的二元体，所合成的二元体即使在无重原子溶剂中，可见光照射下的光环化反应量子产率也更高，且增强幅度与溴原子原子轨道对最低未占据分子轨道（Lowest Unoccupied Molecular Orbital, LUMOs）的贡献占比高度相关。上述结果表明，内重原子效应（internal heavy-atom effect）可有效促进DAE-PBI二元体中的可见光诱导环化反应。这类内重原子效应将为开发全可见光激活型分子开关的新型分子设计策略开辟新的路径。