Sokolowsky Kathleen P, Bailey Heather E, Fayer Michael D
Department of Chemistry, Stanford University , Stanford, California 94305, United States.
J Phys Chem B. 2014 Jul 17;118(28):7856-68. doi: 10.1021/jp500144p. Epub 2014 Feb 21.
Nematogen liquids in the isotropic phase are macroscopically homogeneous but on multinanometer length scales have pseudonematic domains with correlation lengths that grow as the isotropic to nematic phase transition temperature (TNI) is approached from above. Orientational relaxation of nematogens in the isotropic phase manifests as two fast power laws and a slow exponential decay when measured by optical heterodyne detected optical Kerr effect (OHD-OKE) experiments. The long time exponential relaxation is associated with complete randomization of pseudonematic domains. We examine the effect of local orientational correlation on spectral diffusion (structural evolution) experienced by a vibrational probe molecule within the pseudonematic domains of 4-cyano-4'-pentylbiphenyl (5CB) using two-dimensional infrared (2D IR) vibrational echo spectroscopy. The addition of low concentration 4-pentyl-4'-thiocyanobiphenyl (5SCB) as a long-lived vibrational probe to 5CB is shown to lower TNI of the sample slightly, but the fast power law dynamics and exponential decays observed by OHD-OKE spectroscopy are unchanged. We compare the complete orientational relaxation and spectral diffusion for samples of 5SCB in 5CB to 5SCB in 4-pentylbiphenyl (5B) at four temperatures above TNI. 5B has a molecular structure similar to 5CB but is not a nematogen. At all but the lowest temperature, the spectral diffusion in 5CB and 5B is described well as a triexponential decay with very similar time constants. The results demonstrate that the presence of local orientational order at temperatures well above TNI does not affect the spectral diffusion (structural evolution) within pseudonematic domains when the correlation lengths are short. However, when the temperature of the sample is held very close to TNI, the spectral diffusion in 5CB slows dramatically while that in 5B does not. It is only as the correlation length becomes very long that its presence impacts the spectral diffusion (structural fluctuations) sensed by the vibrational probes located in pseudonematic domains. The orientational relaxation is modeled with schematic mode coupling theory (MCT). Fitting with MCT provides density and orientational correlation functions. The density correlation decays are similar for 5B and 5CB, but the orientational correlation decays are much slower for 5CB. Additionally, the time dependence of the spectral diffusion in 5CB is strikingly similar to that of the density correlation function decay, while the orientational correlation function decay is far too slow to contribute to the spectral diffusion. Therefore, density fluctuations are likely the source of spectral diffusion at temperatures at least 5 K above TNI.
各向同性相中的向列型液体在宏观上是均匀的，但在多纳米长度尺度上具有伪向列畴，其关联长度随着从上方接近各向同性向向列相转变温度（TNI）而增长。当通过光学外差检测光学克尔效应（OHD - OKE）实验测量时，各向同性相中的向列型分子的取向弛豫表现为两个快速幂律和一个缓慢的指数衰减。长时间的指数弛豫与伪向列畴的完全随机化有关。我们使用二维红外（2D IR）振动回波光谱研究了局部取向相关性对振动探针分子在4 - 氰基 - 4'- 戊基联苯（5CB）的伪向列畴内经历的光谱扩散（结构演化）的影响。向5CB中添加低浓度的4 - 戊基 - 4'- 硫氰基联苯（5SCB）作为长寿命振动探针，结果表明样品的TNI略有降低，但OHD - OKE光谱观察到的快速幂律动力学和指数衰减不变。我们比较了在高于TNI的四个温度下，5CB中5SCB样品与4 - 戊基联苯（5B）中5SCB样品的完全取向弛豫和光谱扩散。5B具有与5CB相似的分子结构，但不是向列型分子。除了最低温度外，5CB和5B中的光谱扩散都可以很好地描述为具有非常相似时间常数的三指数衰减。结果表明，在远高于TNI的温度下，当关联长度较短时，局部取向有序的存在不会影响伪向列畴内的光谱扩散（结构演化）。然而，当样品温度非常接近TNI时，5CB中的光谱扩散急剧减慢，而5B中的光谱扩散则没有。只有当关联长度变得非常长时，其存在才会影响位于伪向列畴中的振动探针所感知的光谱扩散（结构波动）。取向弛豫用示意性模式耦合理论（MCT）进行建模。用MCT拟合可提供密度和取向相关函数。5B和5CB的密度相关衰减相似，但5CB的取向相关衰减要慢得多。此外，5CB中光谱扩散的时间依赖性与密度相关函数衰减的时间依赖性惊人地相似，而取向相关函数衰减太慢，对光谱扩散没有贡献。因此，在至少比TNI高5 K的温度下，密度涨落可能是光谱扩散的来源。
