Photodimerization of Heteroarylethenes and Crystal Structure of the Photodimers
ZHANG Wen-Qin, LI Song-Lin, ZHANG Zhi-Ming, QI Xin, ZHENG Yan, ZHUANG Jun-Peng, Thomas-C. W. Mak
2000, 21(4):
556-561.
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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 trans 1,2-bis(2-benzoxazolyl)ethene and trans-1-(2-benzimidazolyl)-2-(2-benzoxazolyl)ethene respectively. The structures of BBCand BMCwere identified by elemental analysis, IR, UV, 1H NMR, 13C NMR, MSas well as X-ray diffraction method. The crystal of BBCis triclinic, space group: P1. Unit cell parameters: a = 0. 64684 (13) nm, b = 1. 3242(3) nm, c = 1.6245(3) nm; α= 74. 66(3)° , β= 78.77(3)°, γ= 76.06(3)°, V = 1.2896(4) nm3 , Z=2, Dc= 1.351 g·cm-1 . There are two kinds of molecules with the same stereostructure but different orientations in the crystal cells. The π-stacking between the benzoxazolyl planes of different molecules was found. The distance of the π-stacking planes is 0.330 ～ 0.376 nm, quite similar with that of graphite(0.335 nm). Both of BBCand BMChave head to tail configuration. It was found that the dimerization benefit by increasing the polarity of the solvents and increasing the conjugate system. The photodimerization has a high stereoselectivity and isn't affected by the presence of oxygen, which demonstrated the reaction proceeding through an excited singlet state. The excimer fluoresence of BBCand BMCwas detected.
