Dipartimento di Chimica e Chimica Industriale, Università di Pisa, via Risorgimento 35, 56126 Pisa, Italy.
Chemphyschem. 2009 Oct 19;10(15):2679-91. doi: 10.1002/cphc.200900385.
The molecular dynamics of a ferroelectric liquid crystal, denoted ZLL 7/, is investigated by means of (2)H NMR relaxation. The spin-lattice (T(1Q) and T(1Z)) and spin-spin (T(2)) relaxation times of two isotopomers of ZLL 7/, labeled on the phenyl and biphenyl fragments, are measured and their behavior upon passing from the SmA to the hexatic phase, through the ferroelectric SmC*, antiferroelectric SmC*(A), and re-entrant ferroelectric SmC*(re) phases, is discussed. A comparison between the measured T(2) and T(2), directly related to the experimental linewidth, provides information on the heterogeneity of the system, thus allowing confirmation of previous hypotheses 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 reveals a peculiar sensitivity of the phenyl moiety with respect to the biphenyl fragment, which may be justified by its vicinity to the chiral centers. Interestingly, the trend of the longitudinal relaxation times is characterized by a minimum that corresponds to the SmC*(A) and SmC*(re) phases, which is reproducible for the two isotopomers and at several Larmor frequencies. A quantitative analysis of T(1Q) and T(1Z) is performed in the SmA and SmC* phases, for which the narrowing regime approximation is valid. A multifrequency approach is applied to self-consistently determine the diffusion coefficients for the overall molecular motions, namely spinning and tumbling, 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) allows observation of a sensitive change in the rotational diffusion coefficients in the frustrated unwound SmC* phase with respect to the SmC* phase.
采用（2）H NMR 弛豫研究了铁电液晶 ZLL 7/的分子动力学。测量了 ZLL 7/的两种同位素标记在苯环和联苯片段上的自旋晶格（T(1Q) 和 T(1Z)）和自旋-自旋（T(2)）弛豫时间，并讨论了它们在从 SmA 相通过铁电 SmC、反铁电 SmC(A)和再入铁电 SmC*(re)相到六方相的过程中的行为。测量的 T(2)和 T(2)之间的比较，直接与实验线宽相关，提供了关于系统非均质性的信息，从而证实了先前关于 SmC(A)和 SmC*(re)相的结构和有序性质的假设。观察 ZLL 7/的向列核不同部位的可能性揭示了苯环部分相对于联苯片段的特殊敏感性，这可以通过其与手性中心的接近来解释。有趣的是，纵向弛豫时间的趋势具有一个对应于 SmC(A)和 SmC*(re)相的最小值，该最小值对于两种同位素和几个拉莫尔频率都是可重复的。在 SmA 和 SmC相，对 T(1Q)和 T(1Z)进行了定量分析，其中窄化 regime 近似有效。应用多频方法自洽地确定了整体分子运动，即旋转和翻滚，以及苯环和联苯片段的para 轴的内部旋转的扩散系数。磁场在解开 SmC相的螺旋结构（对于 H>9 T）的作用允许观察到在受挫的解开 SmC相中相对于 SmC相的旋转扩散系数的敏感变化。
