Effect of the Magnetic Field on the Supramolecular Structure of Chiral Smectic C Phases: 2 H NMR Studies

We present a theoretical and experimental 2 H NMR study of the effect of external magnetic fields on the supramolecular organization of chiral smectic liquid‐crystalline mesophases, such as SmC* and re‐entrant SmC* . Three experimental cases in which the supramolecular helical structure of the smectic C* phase is unwound by a magnetic field (H) , parallel to the helical axes of this phase, are discussed in detail. Unwinding of the helical structure is described by using a theoretical model based on the Landau‐de Gennes theory, which allows us to explain the transition temperatures among the SmA, SmC* , and uSmC* phases. The energy‐density behavior in the vicinity of the transitions and the value of the critical magnetic field H C for unwinding the helical structure are discussed by applying this model to three ferroelectric smectogens ( MBHB , 11EB1M7 , ZLL7/* ), which are studied by 2 H NMR spectroscopy at different magnetic fields (from 2.4 to 9.4 Tesla). Furthermore, the tilt angle of the three smectogens in the SmC* phase has been directly evaluated, for the first time, by comparing the quadrupolar splittings at different magnetic fields. In one case, 2 H NMR angular measurements are used to obtain the tilt angle in the re‐entrant smectic C phase.