Enhancing the Versatility and Functionality of Fast Photochromic Bridged Imidazole Dimers by Flipping Imidazole Rings

The widely tunable optical properties and the visible sensitivity have been required for fast photochromic molecules whose coloration–decoloration cycle completes in μs to ms time scale not only for practical applications such as full-color holographic displays but also for fundamental researches in biochemistry. However, the so far developed [2.2]­paracyclophane-bridged imidazole dimers, which are one of the best candidates for fast photochromic molecules, have their weaknesses for these requirements. Herein, we overcome the issues with sustaining fast photochromism and high durability by flipping the two imidazole rings (the head-to-tail and tail-to-tail forms). The alteration in the relative configuration of the imidazole rings suppresses the broad absorption band resulting from the radical–radical interaction. The substitution to the 2-position of the imidazole ring of the tail-to-tail form gives the drastic changes in the steady-state and the transient absorption spectra. The pyrene-substituted tail-to-tail form demonstrates that the transient absorption spectrum is featured by the inherent spectrum of the imidazolyl radical. This molecular framework is easy to functionalize fast photochromic molecules such as sensitizations to the red light, chirality, and biological tagging, and therefore it is versatile for various fast photochromic applications.对于着色-褪色循环在微秒至毫秒级完成的快速光致变色分子而言，其需具备可大范围调控的光学特性与可见光敏感性，这一需求不仅存在于全彩全息显示等实际应用场景，也涵盖生物化学领域的基础研究。然而，作为快速光致变色分子的最优候选体系之一，迄今开发的[2.2]对环芳烷桥联咪唑二聚体（[2.2]­paracyclophane-bridged imidazole dimers）仍无法满足上述需求。本研究通过翻转两个咪唑环的相对构型（分为头-尾型（head-to-tail）与尾-尾型（tail-to-tail）两种异构体），在维持快速光致变色性能与高耐久性的同时，解决了上述问题。咪唑环相对构型的改变，可抑制由自由基-自由基相互作用引发的宽吸收带。对尾-尾型咪唑环的2位进行取代修饰，可使该体系的稳态与瞬态吸收光谱产生显著改变。经芘基修饰的尾-尾型异构体的瞬态吸收光谱，呈现出咪唑自由基（imidazolyl radical）的本征光谱特征。该分子骨架易于对快速光致变色分子进行功能化修饰，例如实现红光敏化、手性调控以及生物标记等，因此可广泛适配各类快速光致变色应用场景。