Stepwise Host–Guest [2 + 2] Photoreaction in a Hydrogen-Bonded One-Dimensional Coordination Polymer to a Two-Dimensional Layered Structure

Two hydrogen-bonded interdigitated
one-dimensional (1D) coordination
polymers (CPs) were synthesized from Cd­(II) nitrate and 1,4-benzenedicarboxylate
(bdc) with two 4-styrylpyridine (4spy) derivatives. In the solid state
structure of 1D CP containing 2′-fluorostyrylpyridine (2F-4spy),
a guest 2F-4spy was sandwiched between the terminal 2F-4spy ligands
and the CC bonds of all the 2F-4spy are well-aligned in host–guest-host–host-guest–host- sequence
infinitely. From time-dependent photoreaction studies, it has been
found that the [2 + 2] photocycloaddition occurs between host–guest molecules first followed by host–host pairs
in a two-step process, which resulted in the quantitative conversion
of CC bond pairs to cyclobutane rings. It is a rare quantitative
photoreaction between the host and guest molecules. Further, solvothermal crystallization of the final photoproduct
furnished an interesting three-dimensional CP with bcg topology. Interestingly, 2NO2-4spy also furnished a similar
hydrogen-bonded 1D CP with parallel orientation of olefin bonds. Although
the olefin bonds satisfy the Schmidt’s criteria for photoreactivity,
it was found to be photoinert under UV light. This has been attributed
to the steric hindrance caused by NO2 substituent or the
electron withdrawing effect.