Reversible single-crystal to single-crystal photoreaction between a coordination comb and a ladder displays photo-switchable fluorescence†

Photoreactive coordination polymers are important platforms for the implementation of tailored solid-state photochemical reactions. These crystalline compounds can also be employed as photo-controlled intelligent materials for the design and manufacture of advanced devices. Herein, a comb-like photoreactive coordination polymer, formulated as {[Zn(5-Cl-1,3-bdc)(H₂O)(2,3-ppe)]·H₂O}_n (1), was prepared based on the hydrothermal reaction between Zn(NO₃)₂·6H₂O, 5-chlorobenzene-1,3-dicarboxylic acid (5-Cl-1,3-H₂BDC) and 1-(2-pyridyl)-2-(3-pyridyl)-ethylene (2,3-ppe). Upon irradiation with sunlight, the ¹H NMR spectroscopy, UV-vis absorption spectroscopy and single crystal X-ray diffraction analyses results indicated that 1 can undergo a [2 + 2] photocycloaddition reaction and thus generate a unique ladder-like coordination chain {[Zn(5-Cl-1,3-bdc)(H₂O)(2,3-bpbpcb)_0.5]·H₂O}_n (1a, 2,3-bpbpcb = 1,3-bis(2-pyridyl)-2,4-bis(3-pyridyl)cyclobutane) via single-crystal to single-crystal (SCSC) transformation. Upon irradiation of UV light with a wavelength of 254 nm, the newly formed coordination chain 1a can undergo a reversible cycloreversion reaction and return to 1. The reversible photo-controllable cycloaddition–cycloreversion reaction between 1 and 1a also exhibits an interesting behavior of photo-switchable fluorescence. The reversible structural transformation and fluorescence switching behavior of 1 make it a potential photo-controlled intelligent material for optical information storage, optical anti-counterfeiting, fluorescence sensors and other fields.