Observance of a large conformational change associated with the rotation of the naphthyl groups during the photodimerization of criss-cross aligned C=C bonds within a 2D coordination polymer

Five coordination polymers [Zn(oba)(4-spy)](2)center dot(4-spy)(0.25)center dot(H2O) (1), [Zn-2(oba)(2)(HT-ppcb)](2)center dot(4-spy)(0.25) (1a), [Zn(oba)(4-(2-npy))](2)center dot(4-(2-npy))(0.25)center dot(H2O)(1.25) (2), [Zn(oba)(4-npy)](2)center dot(4-npy)(0.25) (3) and [Zn-2(oba)(2)(HT-pncb)]center dot(4-npy)(0.25) (3a) (H(2)oba = 4,4'-oxydibenzoic acid; 4-spy = (E)-4-styrylpyridine; HT-ppcb = 1,3-bis(4-pyridyl)-2,4-bis(phenyl)cyclobutane; 4-(2-npy) = (E)-4-(2-(naphthalen-2-yl)vinyl)pyridine; 4-npy = (E)-4-(2-(naphthalen-1-yl)vinyl)pyridine; HT-pncb = rctt-1,3-bis(4-pyridyl)-2,4-bis(1-naphthyl)cyclobutane) were structurally determined. In each case, pairs of Zn(II) centres are linked by oba(2-) anions to form a 2D network. Olefinic pyridyl ligands bound to Zn(II) extend above and below the mean plane of the network and are located within the windows of adjacent parallel networks. Each window contains a pair of vinyl-pyridyl ligands that are tethered to networks that lie above and below and arranged in a head-to-tail manner. 1-3 show different alignments of the C=C bond pair achieved by introducing olefinic ligands with different sizes of their terminal groups. In 1, the C=C bonds are parallel and close enough for a [2 + 2] cycloaddition reaction to occur upon exposure to UV light. The C=C bonds in 2 are too far apart to combine and no reaction occurs upon UV irradiation. In 3, the C=C bonds are less than 4.0 angstrom apart and has a criss-cross arrangement. Exposure of 3 to UV light leads to a [2 + 2] cycloaddition reaction. Analysis of the before and after structures shows a large conformational change associated with the rotation of the olefinic bonds and/or the naphthyl groups when the photodimerization of criss-cross C=C bonds is undergone.