Remarkable Acceleration for Back-Reaction of a Fast Photochromic Molecule

We demonstrate that photochromism based on light-driven reversible C−N single bond cleavage can enable rapid coloration upon UV light irradiation and successive fast thermal back-reaction within tens of microseconds at room temperature. According to Marcus theory, the thermal back-reaction would be accelerated with increasing ΔG0, which is closely linked to the decrease in ΔG‡. We have considered that the ΔG0 of the thermal back-reaction could be enlarged by destabilizing the colored species and designed pseudogem-DPI-PI[2.2]PC, with a [2.2]paracyclophane moiety that couples diphenylimidazole and phenanthroimidazole groups. The present study demonstrates that controlling the stability of the biradical state is effective in accelerating the thermal back-reaction for the photochromic [2.2]paracyclophane-bridged imidazole dimer.本研究证实，基于光驱动C-N单键可逆断裂的光致变色体系，可在紫外光辐照下快速显色，并在室温条件下于数十微秒内完成快速热逆反应。 根据马库斯理论（Marcus theory），热逆反应速率将随ΔG⁰的增大而提升，而ΔG⁰与活化吉布斯自由能ΔG‡的降低密切相关。 我们提出可通过使发色物种失稳来增大热逆反应的ΔG⁰，并设计了伪gem-DPI-PI[2.2]PC，该化合物带有连接二苯基咪唑与菲并咪唑基团的[2.2]对环芳烷（[2.2]paracyclophane）结构单元。 本研究表明，调控双自由基态的稳定性，可有效加速光致变色的[2.2]对环芳烷桥联咪唑二聚体体系的热逆反应。