Structural Tuning of Anion‐Templated Motifs with External Stimuli through Crystal‐to‐Crystal Transformation

Protonation of trans ‐1,2‐bis(4‐pyridyl)ethylene (4,4′‐bpe) with dilute sulfuric acid (33 %) afforded a protonated adduct [{4,4′‐bpe⋅2 H + } 2 {HSO 4 } −2 {SO 4 } −2 {H 2 O} 2 ] ( 1 ). The neighboring olefinic bond in 1 is in a suitable range (3.931–4.064 Å) to undergo a photochemical [2+2] cycloaddition reaction. Upon irradiation with UV light (365 nm), 1 undergoes a molecular sliding involving the 4,4′‐bpe⋅2 H + units, affording 2 , stabilized through O SO4 ⋅⋅⋅π interactions. Heating 1 to 50° C leads to a 3D hydrogen‐bonded organic framework (HOF) ( 3 ). This process occurs through thermal dissociation of the bisulfate anion. Diffusion of iodine through the crystal lattice of 1 and 3 enables the reduction of sulfate to bisulfate, affording a 1D hydrogen‐bonded chain ( 4 ). Solid‐state 13 C CPMAS NMR, IR, DSC, and powder XRD studies further support stimuli‐responsive structural tuning through crystal‐to‐crystal transformation. All these conversions occur with significant translational and rotational movements along with a series of bond‐breaking and bond‐forming processes.