Department of Chemistry, Stanford University , Stanford, California 94305, United States.
J Phys Chem B. 2013 Dec 5;117(48):15060-71. doi: 10.1021/jp4071955. Epub 2013 Nov 21.
The long time scale orientational relaxation of nematogens in the isotropic phase is associated with the randomization of pseudonematic domains, which have a correlation length that grows as the isotropic-to-nematic phase transition temperature is approached from above. Here we begin to address the fast dynamics of the nematogen molecules within the domains using two-dimensional infrared (2D IR) vibrational echo experiments. The problems of performing ultrafast IR experiments in pure liquids are discussed, and solutions are presented. In addition, the issue of short vibrational lifetimes, which limit the ability of 2D IR experiments to examine dynamics over a wide range of times, is addressed. The experiments were performed on the nematogen 4-cyano-4'-pentylbiphenyl (5CB), with the CN stretch initially used as the vibrational probe. Although the CN stretch has a small transition dipole, because the sample is a pure liquid it is necessary to use an exceedingly thin sample to perform the experiments. The small sample volume leads to massive heating effects that distort the results. In addition, the high concentration in the pure liquid can result in vibrational excitation transfer that interferes with the measurements of structural dynamics, and the CN vibrational lifetime is very short (3.6 ps). These problems were overcome by performing the experiments on the natural-abundance (13)CN stretch (5(13)CB), which greatly reduced the absorbance, eliminating the heating problems; also, this stretch has a longer lifetime (7.9 ps). Experiments were also performed on benzonitrile, which showed that the heating problems associated with pure liquids are not unique to 5CB. Again, the problems were eliminated by conducting measurements on the (13)CN stretch, which has an even longer lifetime (20.2 ps) compared with the (12)CN stretch (5.6 ps). Finally, to extend the range of the dynamical measurements, 4-pentyl-4'-thiocyanobiphenyl (5SCB) was synthesized and studied as a dilute solute in 5CB. The CN stretch of 5SCB has a vibrational lifetime of 103 ps, which permits dynamical measurements to 200 ps, revealing the full range of fast structural dynamics in the isotropic phase of 5CB. It is shown that the 5SCB probe reports essentially the same dynamics as 5(13)CB on the short time scale that is observable with the 5(13)CB vibrational probe.
各向同性相中向列相的长弛豫时间与伪向列畴的随机化有关，当接近各向同性-向列相转变温度时，其关联长度会增长。在此，我们开始使用二维红外（2D IR）振动回波实验研究畴内向列相分子的快速动力学。讨论了在纯液体中进行超快红外实验的问题，并提出了解决方案。此外，还解决了限制 2D IR 实验在较宽时间范围内研究动力学的短振动寿命问题。实验是在向列相 4-氰基-4'-戊基联苯（5CB）上进行的，最初使用 CN 伸缩作为振动探针。尽管 CN 伸缩的跃迁偶极矩很小，但由于样品是纯液体，因此需要使用极薄的样品进行实验。小的样品体积会导致大量的热效应，从而扭曲结果。此外，纯液体中的高浓度可能会导致振动激发转移，从而干扰结构动力学的测量，并且 CN 振动寿命非常短（3.6 ps）。通过在天然丰度（13）CN 伸缩（5（13）CB）上进行实验克服了这些问题，这大大降低了吸光度，消除了加热问题；此外，该伸缩的寿命更长（7.9 ps）。还在苯甲腈上进行了实验，表明与纯液体相关的加热问题并非仅存在于 5CB 中。同样，通过在（13）CN 伸缩上进行测量消除了这些问题，与（12）CN 伸缩（5.6 ps）相比，（13）CN 伸缩的寿命更长（20.2 ps）。最后，为了扩展动力学测量的范围，合成了 4-戊基-4'-硫代氰基联苯（5SCB）并将其作为 5CB 中的稀溶质进行了研究。5SCB 的 CN 伸缩的振动寿命为 103 ps，这允许在 200 ps 的时间尺度上进行动力学测量，从而揭示了 5CB 各向同性相的快速结构动力学的全范围。结果表明，5SCB 探针在 5（13）CB 振动探针可观察到的短时间尺度上报告的动力学与 5（13）CB 基本相同。
