Probing columnar discotic liquid crystals by EPR spectroscopy with a rigid-core nitroxide spin probe.
Probing columnar discotic liquid crystals by EPR spectroscopy with a rigid-core nitroxide spin probe.
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DOI：
10.1002/anie.201303194
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发表时间：
2013-08-19
影响因子：
16.6
通讯作者：
Oganesyan, Vasily S.
中科院分区：
文献类型：
--
作者：
Gopee, Hemant;Cammidge, Andrew N.;Oganesyan, Vasily S.
在过去的十年中，柱状dispersible liquid crystals（DLC）吸引了相当大的兴趣，不仅仅是因为它们在传感器，场效应晶体管和光伏太阳能电池中作为一维导体的潜在技术应用。[2-6]堆叠的聚芳族核内的偶联提供了用于电荷沿着柱传输的有效结构，从而提供了用于电荷和能量转移的一维路径，其效率取决于π-延伸的核的重叠的程度和稳定性。[7]在分子尺度上理解柱状毒品的动力学和微观相行为特别具有挑战性，但对于具有所需功能的新系统和设备之间的联系至关重要。最广泛研究的柱状LC是基于六取代的苯并菲，其中六（正己氧基）-苯并菲（HAT 6）是代表性实例。HAT 6和HATn系列的其他成员已经通过许多方法进行了研究，包括宽带介电谱、DSC、X射线衍射、μ子谱、氘NMR谱和准电中子散射。[8-11]氮氧自旋探针（SPs）的EPR光谱是研究蛋白质及其复合物、DNA/RNA、生物膜、纳米颗粒和软物质等复杂的部分无序系统的结构和动力学的一种有价值的方法。EPR光谱具有通过谱线形状的变化直接解析10 μ 11 - 10 μ 7s时间尺度上的分子再取向动力学的能力。[12]由于EPR技术的高灵敏度，实验上只需要很低浓度的探针，大约100 μm，这样主系统基本上不受干扰。连续波（CW）EPR谱提供了关于部分有序流体状态的三种类型的重要信息，即分子动力学、在小体积上平均的分子的局部有序以及多域系统中的全局或长程有序。的光谱是非常敏感的动态（相关时间）和顺序（序参数）的LC系统内的SP的变化。例如，胆甾烷衍生物等棒状EPR SP已成功应用于研究棒状4-n-烷基-4 о-氰基联苯（nCB）向列型液晶。[23-28]然而，目前可用的自旋探针分子与分散系统不相容。在这里，我们报告的第一个应用程序的EPR光谱与专门设计的顺磁SP兼容的柱状dispersants LC。SP设计需要两个重要特性。探针本身应该尽可能接近地类似于主体基质分子，以有利于嵌入并对相造成最小的破坏，并且SP片段本身必须相对于分散核心具有定向刚性。设计Dispensing SP 1以满足这些标准，并与HAT 6一起显示在方案1中。其合成如方案2所示，并在支持性信息中进行了描述。我们在这里证明，新的探头，结合变温CW EPR，是一个sensitiveScheme 1。a和B）HAT 6和分散的刚性核氮氧化物自旋探针1的结构。氮氧头基的磁轴由箭头表示，其中磁坐标系的x沿着NH 4 O方向，z轴垂直于氮氧平面。c）磁场B存在下HAT 6分子的柱状畴分布示意图。自旋探针显示为红色。
Within the last decade columnar discotic liquid crystals (DLCs) have attracted considerable interest, not least for their potential technological applications as one-dimensional conductors,[1] in sensors, field-effect transistors, and photovoltaic solar cells.[2–6] The coupling within stacked polyaromatic cores provides an efficient structure for charge transport along the columns thus providing one-dimensional pathways for charge and energy transfer with an efficiency that depends on the extent and stability of the overlap of the π-extended cores.[7] Understanding the dynamics and microscopic phase behavior of columnar discotics at the molecular scale is particularly challenging but fundamental for the link between new systems and devices with desired functionalities. The most extensively studied columnar LCs are based on hexasubstituted triphenylenes, of which hexakis (n-hexyloxy)-triphenylene (HAT6) is a representative example. HAT6 and other members of the HATn series have been studied by many methods including broad-band dielectric spectroscopy, DSC, X-ray diffraction, Muon spectroscopy, deuteron NMR spectroscopy, and quasi-electric neutron scattering.[8–11] EPR spectroscopy of nitroxide spin probes (SPs) is a valuable method for the study of both structure and dynamics of complex partially disordered systems such as proteins and their complexes, DNA/RNA, biological membranes, nanoparticles, and soft matter.[12–22] EPR spectroscopy has the ability to resolve directly molecular re-orientational dynamics over time scales of 10À11–10À7 s through the variation in spectral line shapes.[12] Because of the high sensitivity of the EPR technique only very low concentrations of the probe, about 100 μm, are required experimentally so that the host system is essentially unperturbed. Continuous-wave (CW) EPR spectra provide three types of important information about the partially ordered fluid state, namely, molecular dynamics, local order of molecules averaged over a small volume, and global or long-range order in a multi-domain system. The spectra are very sensitive to changes in both dynamics (correlation times) and the order (order parameter) of the SP within the LC system. For instance, rod-shaped EPR SPs such as cholestane derivatives have been successfully applied to study calamitic 4-n-alkyl-4о-cyanobiphenyl (nCB) nematic liquid crystals.[23–28] However, currently available spin probe molecules are incompatible with discotic systems. Here we report the first application of EPR spectroscopy with a purpose-designed paramagnetic SP compatible with columnar discotic LCs. The SP design requires two important features. The probe itself should resemble the host matrix molecules as closely as possible to favour intercalation and cause minimal disruption to the phase, and the SP fragment itself must be orientationally rigid with respect to the discotic core. Discotic SP 1 was designed to meet these criteria and is shown in Scheme 1 together with HAT6. Its synthesis is shown in Scheme 2 and described in the Supporting Information. We demonstrate herein that the novel probe, when combined with variable-temperature CW EPR, is a sensitiveScheme 1. a and b) Structures of HAT6 and the discotic rigid-core nitroxide spin probe 1. Magnetic axes of the nitroxide head group are indicated by arrows, where x of the magnetic frame lies along the NÀO direction and the z axis is perpendicular to the nitroxide plane. c) A schematic diagram of the columnar domain distribution of HAT6 molecules in the presence of a magnetic field, B. Spin probes are shown in red.
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影响因子：
3.3
作者：
Oganesyan, Vasily S.
通讯作者：
Oganesyan, Vasily S.
影响因子：
3.4
作者：
Chami, F.;Wilson, M. R.;Oganesyan, V. S.
通讯作者：
Oganesyan, V. S.
影响因子：
8
作者：
Guo, Zhefeng;Cascio, Duilio;Hubbell, Wayne L.
通讯作者：
Hubbell, Wayne L.
影响因子：
4.4
作者：
BERGGREN, E;TARRONI, R;ZANNONI, C
通讯作者：
ZANNONI, C
影响因子：
4.3
作者：
Kuprusevicius, Egidisus;Edge, Ruth;Oganesyan, Vasily S.
通讯作者：
Oganesyan, Vasily S.