Light-induced self-assemblies suitable for photodimerization and their light-induced [2+2] cycloaddition: Light as a control factor of crystal growth

The reaction of Cd(NO3)2 and CF3COONH4 with bpe (trans-1,2-(bis(4-pyridyl)ethene)) ligands was investigated for a light-induced solid-state [2+2] cycloaddition reaction. The three structures (1 [Cd(bpe)2(CF3COO)2]n, 2 [Cd(bpe)2(NO3)2]n, and 3 [Cd(NO3)(μ-NO3)(H2O)(bpe)1.5]n) formed in light while only compound 1 (or 1-dark) was formed in the dark. All three compounds were completely transformed to the final [2+2] cycloaddition products containing rctt-tetrakis(4-pyridyl)cyclobutane (rctt-tpcb; rctt means regional cis, trans, and trans of the four pyridyl groups attached to the cyclobutane ring) in solution under a natural light after 4 weeks. Among the final products, only one 3-D structure (4 [{Cd(rctt-tpcb)2(NO3)2}{Cd(rctt-tpcb)2(OH)2}]n) with a good crystallinity was determined by the X-ray diffraction technique. These observations were also in line with the spectroscopic results such as 1H NMR. Meanwhile, 3 was also prepared separately and exposed to natural light. X-ray and NMR studies showed that 3 was completely transformed to 4 containing rctt-tpcb by a light-induced cycloaddition reaction for the 4 weeks. The reaction progress was further monitored by fluorescent spectroscopy. 1-dark also underwent the solid-state [2+2] cycloaddition under the natural light over four weeks.