title: Structural characterization and molecular order of rodlike mesogens with three- and four-ring core by XRD and 13C NMR spectroscopy.
abstract: Structural characterizations using XRD and (13)C NMR spectroscopy of two rodlike mesogens consisting of (i) three phenyl ring core with a polar cyano terminal and (ii) four phenyl ring core with flexible dodecyl terminal chain are presented. The three-ring-core mesogen with cyano terminal exhibits enantiotropic smectic A phase while the four-ring mesogen reveals polymesomorphism and shows enantiotropic nematic, smectic C, and tilted hexatic phases. The molecular organization in the three-ring mesogen is found to be partial bilayer smectic Ad type, and the interdigitation of the molecules in the neighboring layers is attributed to the presence of the polar terminal group. For the four-ring mesogen, the XRD results confirm the existence of the smectic C and the tilted hexatic mesophases. A thermal variation of the layer spacing across the smectic C phase followed by a discrete jump at the transition to the tilted hexatic phase is also observed. The tilt angles have been estimated to be about 45° in the smectic C phase and about 40° in tilted hexatic phase. (13)C NMR results indicate that in the mesophase the molecules are aligned parallel to the magnetic field. From the (13)C-(1)H dipolar couplings determined from the 2D experiments, the overall order parameter for the three-ring mesogen in its smectic A phase has been estimated to be 0.72 while values ranging from 0.88 to 0.44 have been obtained for the four-ring mesogen as it passes from the tilted hexatic to the nematic phase. The orientations of the different rings of the core unit with respect to each other and also with respect to the long axis of the molecule have also been obtained.
authors.affiliation: Department of Chemistry, S V University, Tirupati, India.
authors.forename: M Kesava
authors.initials: MK
authors.lastname: Reddy
authors.forename: K Subramanyam
authors.initials: KS
authors.lastname: Reddy
authors.forename: K
authors.initials: K
authors.lastname: Yoga
authors.forename: M
authors.initials: M
authors.lastname: Prakash
authors.forename: T
authors.initials: T
authors.lastname: Narasimhaswamy
authors.forename: A B
authors.initials: AB
authors.lastname: Mandal
authors.forename: Nitin P
authors.initials: NP
authors.lastname: Lobo
authors.forename: K V
authors.initials: KV
authors.lastname: Ramanathan
authors.forename: D S Shankar
authors.initials: DS
authors.lastname: Rao
authors.forename: S Krishna
authors.initials: SK
authors.lastname: Prasad
mesh_terms: D001713:Biphenyl Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D002247:Carbon Isotopes; D050866:Liquid Crystals / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D009570:Nitriles / Q000138:chemical synthesis / Q000737:chemistry*; D011789:Quantum Theory; D014961:X-Ray Diffraction
chemical_list: D001713:Biphenyl Compounds; D002247:Carbon Isotopes; D009570:Nitriles