Watching molecules reorient in liquid crystal droplets with multiphoton-excited fluorescence microscopy

Abstract

A new form of time-resolved multiphoton-excited fluorescenceimaging is described and used to study electric-field-induced reorientation dynamics in polymer-dispersed liquid crystal(LC)films. This method provides information on the static and dynamic LC orientation via polarization-dependent three-photon excitation of the nematic ultraviolet chromophores in these materials. Static fluorescenceimages are obtained with ≈235 nm resolution in all three dimensions. In dynamics studies, the three-photon-excited fluorescence is recorded as a function of time and position over individual LCdroplets, as an applied electric field is switched on and off. Time-resolvedimages with ≈235 nm spatial resolution and 200 μs time resolution are obtained. Movies depicting the local reorientation dynamics are prepared from these data and are presented for common ellipsoidal LCdroplets and for novel toroidal droplets. The field-induced reorientation dynamics within the ellipsoidal droplets are shown to be more complex (i.e., spatially variable) than in the toroidal droplets. Dynamical complexity is concluded to arise from LC organizational complexity in the droplets. The bipolar configuration found in ellipsoidal droplets incorporates bend and splay deformations of the nematic phase and two disclination points. In contrast, toroidal droplets incorporate a simpler toroidal configuration in which only bend deformations occur.