Chakrabarti Dwaipayan, Bagchi Biman
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, India.
J Phys Chem B. 2007 Oct 11;111(40):11646-57. doi: 10.1021/jp079516w. Epub 2007 Sep 20.
The orientational dynamics of thermotropic liquid crystals across the isotropic-nematic phase transition have traditionally been investigated at long times or low frequencies using frequency domain measurements. The situation has now changed significantly with the recent report of a series of interesting transient optical Kerr effect (OKE) experiments that probed orientational relaxation of a number of calamitic liquid crystals (which consist of rod-like molecules) directly in the time domain, over a wide time window ranging from subpicoseconds to tens of microseconds. The most intriguing revelation is that the decay of the OKE signal at short to intermediate times (from a few tens of picoseconds to several hundred nanoseconds) follows multiple temporal power laws. Another remarkable feature that has emerged from these OKE measurements is the similarity in the orientational relaxation behavior between the isotropic phase of calamitic liquid crystals near the isotropic-nematic transition and supercooled molecular liquids, notwithstanding their largely different macroscopic states. In this article, we present an overview of the understanding that has emerged from recent computational and theoretical studies of calamitic liquid crystals across the isotropic-nematic transition. Topics discussed include (a) single-particle as well as collective orientational dynamics at a short-to-intermediate time window, (b) heterogeneous dynamics in orientational degrees of freedom diagnosed by a non-Gaussian parameter, (c) fragility, and (d) temperature-dependent exploration of underlying energy landscapes as calamitic liquid crystals settle into increasingly ordered mesophases upon cooling from the high-temperature isotropic phase. A comparison of our results with those of supercooled molecular liquids reveals an array of analogous features in these two important classes of soft matter systems. We further find that the onset of growth of the orientational order in the parent nematic phase induces translational order, resulting in smectic-like layers in the potential energy minima of calamitic systems if the parent nematic phase is sandwiched between the high-temperature isotropic phase and the low-temperature smectic phase. We discuss implications of this startling observation. We also discuss recent results on the orientational dynamics of discotic liquid crystals that are found to be rather similar to those of calamitic liquid crystals.
传统上，热致液晶在各向同性-向列相转变过程中的取向动力学是通过频域测量在长时间或低频下进行研究的。随着最近一系列有趣的瞬态光学克尔效应（OKE）实验的报道，情况发生了显著变化。这些实验在时域中直接探测了多种棒状分子构成的 calamitic 液晶的取向弛豫，时间窗口范围从亚皮秒到数十微秒。最引人注目的发现是，在短至中等时间（从几十皮秒到几百纳秒）内，OKE 信号的衰减遵循多个时间幂律。这些 OKE 测量中出现的另一个显著特征是，尽管 calamitic 液晶在各向同性-向列相转变附近的各向同性相和过冷分子液体的宏观状态有很大不同，但它们的取向弛豫行为具有相似性。在本文中，我们概述了从最近关于 calamitic 液晶在各向同性-向列相转变的计算和理论研究中得出的认识。讨论的主题包括：（a）在短至中等时间窗口内的单粒子以及集体取向动力学；（b）通过非高斯参数诊断的取向自由度中的非均匀动力学；（c）脆性；（d）随着 calamitic 液晶从高温各向同性相冷却进入越来越有序的中间相，对潜在能量景观的温度依赖性探索。将我们的结果与过冷分子液体的结果进行比较，揭示了这两类重要软物质系统中的一系列类似特征。我们进一步发现，母向列相中取向序的增长开始会诱导平移序，如果母向列相夹在高温各向同性相和低温近晶相之间，则会在 calamitic 系统的势能极小值处形成类似近晶相的层。我们讨论了这一惊人观察结果的意义。我们还讨论了关于盘状液晶取向动力学的最新结果，发现它们与 calamitic 液晶的非常相似。
