Dynamics in the Isotropic Phase of Nematogens Using 2D IR Vibrational Echo Measurements on Natural-Abundance ¹³CN and Extended Lifetime Probes

Abstract

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.

New divergent dynamics in the isotropic to nematic phase transition of liquid crystals measured with 2D IR vibrational echo spectroscopy

“…We have previously shown that temperature dependent ultrafast two dimensional infrared (2D IR) vibrational echoes and polarization selective pump-probe experiments combined with the findings from OHD-OKE experiments provide an expanded picture of nematogen dynamics in the isotropic phase. 19,20 The 2D IR experiments yield information on the dynamics experienced by non-perturbative probe molecules within a pseudonematic domain, while the complete reorientation dynamics over a very wide range of time scales are reported by the complementary OHD-OKE experiments.…”

“…19,22 Nitrile stretches have previously been shown to be useful vibrational probes for 2D IR experiments, although their utility is frequently limited by the short CN vibrational lifetime. [23][24][25][26] 2D IR spectroscopy provides information on structural dynamics through the measurement of spectral diffusion of a vibrational oscillator, like the CN stretch.…”

“…19 The lifetime of the CN stretch is ∼4 ps, which would permit spectral diffusion data to be collected to ∼16 ps under ideal conditions. 19 Large, heat induced peaks caused by vibrational relaxation in the very thin sample (∼3 μm) necessary to study the pure liquid distort the 2D IR spectra by 8 ps. Thus, spectral diffusion measurements on 5CB itself were limited to <8 ps.…”

“…Thus, spectral diffusion measurements on 5CB itself were limited to <8 ps. 19 Difficulties arising from heating were mitigated by examining the carbon-13 nitrile stretch of 5CB, which permitted a thicker sample. 19 However, the short vibrational lifetime (∼8 ps) prevents measurement of complete spectral diffusion, even at the highest temperatures.…”

The Influence of Cholesterol on Fast Dynamics Inside of Vesicle and Planar Phospholipid Bilayers Measured with 2D IR Spectroscopy

“…64,65 In the isotropic phase, the liquid has nematic order (pseudonematic domains) over a correlation length that increases as the I−N transition is approached. 66 Detailed analysis using mode coupling theory of the results of the 2D IR experiments and optical heterodyne detected optical Kerr effect experiments showed that orientational relaxation of the nematogens was far too slow to be responsible for the CN stretch spectral diffusion.…”