Liquid Crystal Structure of a Schiff Base by Combined Room

SMCr. VIII Congreso Nacional de Cristalografía.
II Reunión Latinoamericana de Cristalogfrafía
VI Reunión de Usuarios de Luz Sincrotrón
Mérida, Yucatán, México 23-27 de octubre del 2016.
Liquid Crystal Structure of a Schiff Base by
Combined Room Temperature Single Crystal
and Variable-Temperature Powder X-ray
Diffraction
Oscar E. Piro
Departamento de Física, Facultad de Ciencias Exactas, Universidad Nacional de La Plata e Instituto IFLP
(CONICET, CCT-La Plata), C. C. 67, 1900 La Plata, Argentina.
*e-mail: piro@fisica.unlp.edu.ar
Abstract. Structurally, liquid crystals are intermediate phases between the highly ordered crystal and the
totally disordered liquid phase that combine the anisotropic physical properties of the former with the
fluidity exhibited by the latter. Because of their partial order, the molecular structure of this condensed
phase can only be modelled based on X-ray (and neutron) diffraction combined with, among other
techniques, optical and thermal analyses. In this presentation we show the capability of X-ray diffraction
methods to infer the detailed molecular structure of a Schiff base in its liquid crystal phase.
Key words: Single-crystal and powder X-ray diffraction, liquid crystal, smectic phase
The liquid crystals (LC), discovered in 1888 by botanist T. Reinitzer in cholesteric substances, is a mesogenic
(intermediate) phase, structurally located between the well-ordered crystal and the completely disordered isotropic
liquid [1]. Their unique physical properties are the subject of an all host of macroscopic studies, including optical,
magnetic, dielectric, elastic, thermodynamics, flow and viscosity, etc, and have important technological applications
as monitoring screens (displays) and other electro-optical devices.
Because them can be considered either as disordered crystals or ordered liquids, the CL molecular structure cannot
be fully investigated only by diffraction methods and we must resort to further characterization techniques,
including, among others, Differential Scanning Calorimetry (DSC) and Polarized Optical Microscopy (POM).
According to the constituting molecular shapes, the CL can be roughly catalogued into calamitic (rod-shaped),
discotic (disk-shaped) and sanidic (brick-shaped) mesogens. Concerning the molecular structure, the CL can be
grouped into nematic (only orientation order) and smectic (both orientation and layered positional ordering)
mesophases.
We show here in the case of a Schiff base, namely 1-(4-((4-dimethylaminobenzylidene)-amino) phenyl) ethanone,
the capability of room temperature single-crystal X-ray diffraction (XRD) and powder XRD vs temperature,
combined with DSC measurements and POM observations, to develop a molecular model for the calamitic CL
mesogenic phase exhibited by the Schiff base in the 150-175ºC temperature range.
The Schiff base crystallizes at room temperature in the triclinic P-1 space group with a=9.9357(4) Å, b=17.1016(9)
Å, c=18.1945(9) Å, α=78.347(4)°, β=77.169(4)°, γ=76.996(4)°, and Z=8 molecules per unit cell. The structure was
determined by intrinsic phasing with SHELXT and refined with SHELXL of the SHELX package [2]. There are
four independent molecules per asymmetric unit that mainly differ from one another in rotations around the σ-bond
of the azomethine N-atom with the phenyl ring, hence reflecting the conformational degree of freedom of the
molecule. At the same time, the molecules are organized into columns along the crystal b-axis that are loosely bound
to each other trough van der Waals and CH3…O interactions and packed together as a distorted hexagonal, centered
wasp net-like, arrangement [3]. This defines a embedded quasi-hexagonal sub-cell of 1/8 the volume of the triclinic
cell that reflects itself in a corresponding weighted reciprocal space quasi-hexagonal super-cell exhibiting a high
degree of D6h Laue symmetry, an unexpected feature to be found in a nominally triclinic lattice. This strongly
SMCr. VIII Congreso Nacional de Cristalografía
II Reunión Latinoamericana de Cristalogfrafía
VI Reunión de Usuarios de Luz Sincrotrón
Mérida, Yucatán, México 23-27 de octubre del 2016