One- and Two-Dimensional Infrared Time-Resolved Spectroscopy Using a Step-Scan FT-IR Spectrometer: Application to the Study of Liquid Crystal Reorientation Dynamics

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Abstract
This paper describes the advances in step-scan FT-IR time-resolved spectroscopy (TRS) and its application to the study of liquid crystal reorientation dynamics. The most important advantage of step-scan interferometry lies in the fact that the optical retardation of the interferometer is held constant during the sampling of interferogram elements, and consequently the spectral multiplexing is decoupled from the time dependence of data collection. This feature of step-scan interferometry allows us to perform both time-domain (one-dimensional time-resolved spectroscopy: 1D TRS) and frequency-domain (two-dimensional frequency correlation spectroscopy: 2D IR) dynamic experiments without the need to deconvolute the time dependence of the sample response from that of the data collection process. The design of the step-scan FT-IR spectrometer used in this study (Bio-Rad FTS60A/896), the experimental setup for 1D and 2D TRS measurements, and the results of a performance test are detailed. The FT-IR TRS techniques applied to the dynamic analysis of liquid crystals have revealed new information that enables us to penetrate into detailed sub-molecular mechanisms of the electrically induced liquid crystal reorientation. The results include the following: (1) 1D FT-IR TRS with microsecond time resolution has been able to follow the real-time transition dynamics of each individual functional group in the molecule; (2) 2D FT-IR TRS, capable of analyzing spatial and temporal correlations between reorientational motions of different sub-molecular segments, has shown that a flexible chain appended to a rigid core of the liquid crystalline molecule undergoes a fast local motion in addition to the rotational relaxation motion of the entire molecule; and (3) 2D frequency correlation analyses have been able to isolate a hidden absorption band and have suggested a possible assignment of this new band. It is emphasized that all these results have been obtained by taking the advantage of time-resolved spectroscopy that provides both temporal and spectral information simultaneously. The results presented in this paper should illustrate the potential applicability of FT-IR TRS to the study of a wide variety of time-dependent phenomena.