r-1,t-3-Bis[4-(dimethylamino)phenyl]-c-2,t-4-bis(pyridin-4-yl)cyclobutane

The title compound, C30H32N4, was synthesized by the photodimerization of trans-4-{2-[4-(dimethylamino)phenyl]ethenyl}pyridine in benzene upon irradiation with UV light. This photodimer has a puckered cyclobutane ring with the four aryl substituents in an r-1,t-2,c-3,t conformation. The puckering angle of the cyclobutane ring is 32.22 (7)°, which is the largest among reported tetraaryl-substituted cyclobutanes. In the crystal, the molecules form a hollow, one-dimensional structure extending parallel to the c axis via two different pairs of C—H⋯π interactions.

The title compound, C 30 H 32 N 4 , was synthesized by the photodimerization of trans-4-{2-[4-(dimethylamino)phenyl]-ethenyl}pyridine in benzene upon irradiation with UV light. This photodimer has a puckered cyclobutane ring with the four aryl substituents in an r-1,t-2,c-3,t conformation. The puckering angle of the cyclobutane ring is 32.22 (7) , which is the largest among reported tetraaryl-substituted cyclobutanes. In the crystal, the molecules form a hollow, one-dimensional structure extending parallel to the c axis via two different pairs of C-HÁ Á Á interactions.
Cg1 and Cg2 are the centroids of the C8-C13 and N1/C1-C5 rings, respectively. This work was supported by the Youth Foundation of Beijing University of Chemical Technology.
The structure of 2A shows that the four aryl substituents adopt the r-1,t-2,c-3,t-4 conformation, whereas the styrylpyridine photodimers synthesized in acidic aqueous solution adopt the head-to-tail r-1,c-2,t-3,t-4 conformation. In addition, the photodimer of 2-[2-(4-dimethylaminophenyl)ethenyl]benzoxazole synthesized in acetonitrile also adopts the head-to-tail r- 1,c-2,t-3,t-4 conformation (Li et al., 2007). This difference is ascribed to solvent effects. The steric hindrance of the dimethylamino groups prevents A to align in a parallel manner in the non-polar benzene solvent, resulting in the all trans conformation of the adjacent aryl groups in 2A.
The dimethylamino plane and the phenyl ring (C8-C9-C10-C11-C12-C13) are not coplanar, the torsion angle (Li et al., 2007). The distances of the two methyl groups (C14 and C15) from the mean plane of the phenyl ring are 0.2629 (22)  As shown in Figure 2, the molecules of 2A pack with each other to form a hollow, one-dimensional structure along the c axis. This arrangement appears to be directed by two sets of C-H···π interactions with that involving H15B stronger than that using H12 (Table 1).

Experimental
A was synthesized according to the literature (Wang et al., 2005) and 1.97 g (8.78 mmol) was dissolved in 200 mL of benzene and irradiated with a water-cooled 125 W medium-pressure mercury lamp which was immersed in the solution.
After irradiation for about 30 h, the solvent was evaporated to dryness and the crude product was separated by column chromatography (ethyl acetate: dichloromethane = 1: 1) to give 0.73 g of colorless crystals of 2 A. Yield, 37%; 1    Packing diagram of 2A viewed along c axis.

Figure 3
Packing diagram of 2A viewed along b axis.  (17) Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.