First Observation of Unusual Domino Effect and Triggering Mechanism of Sequential Single-Crystal-to-Single-Crystal Photochemical Reactions in a Metal–Organic Framework with Multiple Photoreactive Centers

A metal–organic framework (MOF), {[Co 3 (ip) 3 (1,3-bpeb) 4 ]·H 2 O} n ( 1 ), with multiple photoreactive centers, was synthesized via hydrothermal reaction of Co(II) ion with isophthalate and 1,3-bis[2-(4-pyridyl)ethenyl]benzene (1,3-bpeb) at 180 °C. The MOF 1 was characterized by single-crystal X-ray crystallography and spectroscopic techniques. Its photochemical response under UV irradiation was investigated in detail. An unusual sequence of single-crystal-to-single-crystal reactions was observed as a function of irradiation time. The consecutive reactions were characterized as a [2 + 2] dimerization ( D 1 ) of a particular pair of C═C bonds of 1,3-bpeb ligands (Pair A ) followed by a conformational isomerization ( I ) and [2 + 2] dimerization ( D 2 ) of an adjacent pair of 1,3-bpeb ligands (Pair B ). The isomerization ( I ) was instantaneous. The demarcation point for the D 1 - ID 2 sequence was the completion of D 1 and the commencement of D 2 at 0.644 h of irradiation. The most striking feature observed is the domino effect triggered by Pair A at the demarcation point, resulting in the isomerization and dimerization of Pair B . The triggering mechanism is believed to be stereochemical in nature.