Achieving UV and X-ray Dual Photochromism in a Metal-Organic Hybrid via Structural Modulation
AUTHORS: Lu, HJ; Zheng, ZF; Li, ZJ; Bao, HL; Guo, XJ; Guo, XF; Lin, J; Qian, Y; Wang, JQ 
Rational design and synthesis of new photochromic sensors have been active research areas of inquiry, particularly on how to predict and tailor their properties and functionalities. Herein, two thulium 2,2':6',2 ''-terpyridine-4'-carboxylate (TPC)-functionalized metal-organic hybrids, Tm-(TPC)(2)(HCOO)(H2O) (TmTPC-1) and Tm(TPC)(HCOO)(2)(Tm-TPC-2) with different photochromic response behaviors, have been successfully prepared, allowing for straightforward investigations of the structure-property correlation. Single-crystal X-ray diffraction and electron paramagnetic resonance analyses revealed that the incorporation of a unique dangling decorating TPC unit in TmTPC-1 offers a shorter and more accessible pi-pi interaction pathway between the adjacent TPC moieties than that in TmTPC-2. Such a structural feature leads to the production of radical species via a photoinduced intermolecular electron-transfer (IeMCT) process upon UV or X-ray irradiation, which ultimately endows TmTPC-1 with a rather unusual UV and X-ray dual photochromism. A linear relationship between the change of UV-vis absorbance intensity and X-ray dose was established, making TmTPC-1 a promising dosimeter for X-ray radiation with an extremely high energy threshold (30 kGy). To advance the development for real-world application, we have fabricated polyvinylidene fluoride (PVDF) membranes incorporating TmTPC-1 for functioning either as a UV imager or as an X-ray radiation indicator. Lastly, TmTPC-1 exhibits high thermal stability (up to 400 degrees C) and radioresistance (at least 900 kGy), and also excellent reversibility of photochromic transformation (at least 5 cycles).