Prodotti
Articoli
From the SmA to the Hexatic, Including the SmC*, SmC*A and SmC*re Phases: A 2H NMR Relaxation Study
- autori
- DOMENICI VALENTINA , Marini A. , VERACINI CARLO ALBERTO , MALANGA CORRADO , MENICAGLI RITA
- anno
- 2009
- tipo prodotto
- Articolo su rivista scientifica Index Medicus, Science Citation Index
- numero autori totale
- 5
- rivista
- CHEMPHYSCHEM: A EUROPEAN JOURNAL OF CHEMICAL PHYSICS AND PHYSICAL CHEMISTRY
- numero pagine
- 13
- pagina iniziale
- in press
- isbn-issn
- 1439-4235
- tipo indicizzazione
- ISI
- lingua
- Inglese
- NOTA
- The molecular dynamics of a ferroelectric liquid crystal, denoted as ZLL 7/*, has been investigated by means of 2H NMR relaxation. The spin-lattice, T1Q and T1Z, and spin-spin, T2, relaxation times of two isotopomers of ZLL 7/*, labeled on the phenyl and biphenyl fragments, have been measured and their behavior passing from the SmA to the hexatic phase, through the ferroelectric SmC*, antiferroelectric SmC*A and re-entrant ferroelectric SmC*re phases, is discussed. The comparison between the measured T2 and the T2*, directly related to the experimental linewidth, provides information on the heterogeneity of the system allowing us to confirm previous hypotheses J. Phys. Chem. B 2006, 110, 16459 concerning the structural and ordering properties of the SmC*A and SmC*re phases. The possibility to look at different sites of the core of the ZLL 7/* smectogen revealed a peculiar sensitivity of the phenyl moiety, with respect to the biphenyl one, which may be justified by its vicinity to the chiral centers. Interestingly, the trend of the longitudinal relaxation times is characterized by a flat minimum in correspondence of the SmC*A and SmC*re phases, which is reproducible for the two isotopomers and at several Larmor frequencies. A quantitative analysis of T1Q and T1Z was performed in the SmA and SmC* phases, where the narrowing regime approximation is valid. Here, a multi-frequency approach was applied to self-consistently determine the diffusion coefficients for the overall molecular motions, namely spinning and tumbling ones, and the internal rotations around the para axes of the phenyl and biphenyl fragments. The effect of the magnetic field in unwinding the helical structure of the SmC* phase (for H > 9 T) allowed us to observe a sensitive change in the rotational diffusion coefficients in the frustrated unwound SmC* phase (uSmC*) with respect to the SmC* one.
- parole chiave
- liquid crystals ,ferroelectric ,dynamics