Achieving UV and X‑ray Dual Photochromism in a Metal–Organic Hybrid via Structural Modulation

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 (TmTPC-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 π–π
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 °C) and radioresistance
(at least 900 kGy), and also excellent reversibility of photochromic
transformation (at least 5 cycles).