Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee (Hong Kong)) and Department of Chemistry, The University of Hong Kong , Pokfulam Road, Pokfulam, Hong Kong, P. R. China.
Department of Chemistry, City University of Hong Kong , Tat Chee Avenue, Kowloon, Hong Kong, P. R. China.
Acc Chem Res. 2018 Jan 16;51(1):149-159. doi: 10.1021/acs.accounts.7b00426. Epub 2017 Dec 21.
Photochromic compounds are well-known for their promising applications in many areas. In this context, many different photochromic families have been developed. As the early study of these photochromic compounds was mainly focused on the organic system, their photochromic reactivity was mainly derived from the singlet excited state. We hypothesized that the incorporation of the photochromic ligand to the transition metal complex and coordination complex systems would not only render the triplet state of the organic photochromic system more readily accessible due to the large spin-orbit coupling of the heavy metal center but also would lead to ready extension of the excitation wavelength to less destructive longer wavelength low-energy excitation. On the other hand, the long-lived triplet excited states of the metal complexes are also suitable for energy or electron transfer processes, which should lead to new photochromic behavior and photoswitchable functional properties. Through the incorporation of the stilbene-, azo-, spirooxazine-, and dithienylethene-containing ligands to transition metal complex systems with heavy metal centers and suitable excited states, triplet state photosensitized photochromism has been achieved. With the triplet state photosensitization, the photochromism of these compounds could be extended from the high energy UV region to the visible region. In the development of dithienylethene-containing ligands, we have adopted an alternative strategy, which involves the incorporation of nitrogen and sulfur heterocycles that directly form part of the dithienylethene framework as ligands to exert a much stronger perturbation and influence on the excited state properties of the photochromic unit by the metal center. On the basis of the new design, wide ranges of dithienylethene-containing ligands, including phenanthrolines, 2-pyridylimidazoles, N-pyridylimidazol-2-ylidenes, cyclometalating thienylpyridines, β-diketonates, and β-ketoiminates have been designed and incorporated into various coordination systems. Apart from the photosensitization, tuning of the closed form absorption and photochromic behavior based on the perturbation of the metal center, coordination-assisted planarization, modification of the ancillary ligands and introduction of various electronic excited states derived from the coordination system have been successfully demonstrated. This strategy can be used for developing NIR photochromic dithienylethenes. With the above effects observed upon the coordination to different transition metal centers and central atoms, this strategy offers a simple and effective way for the modification of the photochromic characteristics. Moreover, the emission and other functional properties of the coordination systems could also be photoswitched by the photochromic reactions.
光致变色化合物因其在许多领域的应用前景而备受关注。在这方面，已经开发出了许多不同的光致变色家族。由于早期对这些光致变色化合物的研究主要集中在有机体系上，因此它们的光致变色反应主要源于单重激发态。我们假设将光致变色配体引入过渡金属配合物和配位化合物体系中，不仅由于重金属中心的大自旋轨道耦合作用会使有机光致变色体系的三重态更容易获得，而且还会导致激发波长扩展到破坏性较小的长波长低能量激发。另一方面，金属配合物的长寿命三重态激发态也适合能量或电子转移过程，这应该会导致新的光致变色行为和光可切换功能特性。通过将含有螺恶嗪、二噻吩乙烯的配体与含有重金属中心和合适激发态的过渡金属配合物体系结合，实现了三重态光敏光致变色。通过三重态敏化，这些化合物的光致变色可以从高能 UV 区域扩展到可见区域。在开发二噻吩乙烯配体时，我们采用了一种替代策略，即将氮和硫杂环直接作为配体并入二噻吩乙烯骨架中，通过金属中心对光致变色单元的激发态性质产生更强的干扰和影响。基于新的设计，设计并合成了包括菲咯啉、2-吡啶基咪唑、N-吡啶基咪唑-2-亚基、环金属化噻吩吡啶、β-二酮和β-酮亚胺在内的广泛的二噻吩乙烯配体，并将其引入各种配位体系中。除了光致敏化之外，还基于金属中心的扰动、配位辅助平面化、辅助配体的修饰以及引入来自配位体系的各种电子激发态，成功地调谐了闭环吸收和光致变色行为。这种策略可用于开发近红外光致变色二噻吩乙烯。通过与不同过渡金属中心和中心原子配位观察到上述效应，该策略为修饰光致变色特性提供了一种简单有效的方法。此外，配位体系的发射和其他功能特性也可以通过光致变色反应进行切换。
