Synthesis of quinoxaline-benzoxale conjugates and mesomorphic properties

A new series of non-discotic heterocyclic compounds 1a-e derived from quinoxaline was prepared and their mesomorphic properties investigated. The crystal and molecular structures of nonmesogenic 2,3-bis(3,4-didodecyloxyphenyl)quinoxaline-6-carboxylic acid 4-(4-butoxy2-hydroxyphenylimino)methyl]phenyl ester 2a (n=4, m=12) were determined by means of X-ray structural analysis. It crystallizes in a monoclinic space group P2(1)/c, with a=21.9193(13) Å, b=8.3693(4) Å, c=30.896(2) Å, and Z=4. The molecule was considered as an elongated or tapered triangle. Both inter- and intra-molecular H-bonds were observed in the crystal lattice, which was attributed to the formation of columnar mesophase in compounds 2. The mesomorphic behavior of compounds 1-2 was studied by thermal analysis and polarized optical microscopy. All compounds 1-2 exhibited hexagonal columnar phases (Col_h), which were also confirmed by powder XRD diffractometer. A N_cell and R_ar value equal to 4.74 and 4.34 within a slice of 9.0 Å thick were obtained for 1b and 2b, indicating that a more disc-like correlated structure by two molecules lying side-by-side was formed in Col_h phases. The fluorescent properties of the compounds 1-4 in CH₃Cl were also examined.