Photodimerization of heteroarylethenes and crystal structure of the photodimers

r-1, c-2, t-3, t-4-Tetra (2-benzoxazolyl) cyclobutane (BBC) and r-1, c-2, t -3, t-4-bis (2-benzimidazolyl)-2,4-bis(2-benzoxazolyl)cyclobutane(BMC) were synthesized with a high stereoselectivity by the photodimerization of tran s-1,2-bis(2-benzoxazolyl) ethene and trans-1-(2-benzimidazolyl)-2-(2-benzox azolyl)ethene respectively. The structures of BBC and BMC were identified b y elemental analysis, IR, UV, H-1 NMR, C-13 NMR, MS as well as X-ray diffra ction method. The crystal of BBC is triclinic, space group: <P(1)over bar>. Unit cell parameters: a = 0. 646 84(13) nm, b = 1. 324 2(3) nm, c = 1. 624 5(3) nm; alpha = 74. 66(3)degrees, beta = 78. 77(3)degrees, gamma = 76. 06 (3)degrees, V = 1. 289 6(4) nm(3), Z = 2, D-c = 1. 351 g . cm(-1). There ar e two kinds of molecules with the same stereostructure but different orient ations in the crystal cells. The pi-stacking between the benzoxazolyl plane s of different molecules was found. The distance of the pi-stacking planes is 0. 330 similar to 0. 376 nm, quite similar with that of graphite(0. 335 nm). Both of BBC and BMC have head to tail configuration. It was found that the dimerization benefit by increasing the polarity of the solvents and in creasing the conjugate system. The photodimerization has a high stereoselec tivity and isn't affected by the presence of oxygen, which demonstrated the reaction proceeding through an excited singlet state. The excimer fluorese nce of BBC and BMC was detected.