Optical studies of molecular rotational dynamics in isotropic liquid crystalline materials

Abstract

Molecular rotational dynamics is an old problem that continues to be relevant today. In the continuing search for new optical materials and phenomena, both from an application and scientific standpoint, a detailed understanding of molecular rotational dynamics is often of crucial importance. An example is the so-called Janossy effect, where it was found that the already large optical reorientation of liquid crystals (LC) could be greatly enhanced by doping a small amount of absorbing dye. We report here our recent effort studying the Janossy effect in isotropic-phase LC. The current explanation of the Janossy effect assumes a change in guest-host interaction upon photoexcitation of the guest molecules, which would manifest as a difference in rotational dynamics between solute ground and excited states. While there are many available experimental techniques to selectively probe the solute excited state, probing the solute ground state is often difficult, due to interferences from the excited solute and solvent molecules. We have developed an optical pump/probe technique that employs two successive pump pulses that are adjusted to allow selective measuring of the solute ground state rotational dynamics. Application of the technique to the dye-LC system that exhibits the Janossy effect shows a large difference between rotational diffusion rates of the ground and the excited state of the dye molecules. Combined with results from optical pump/probe of the LC host, information on the rotational dynamics of the dye yields a better understanding of the mechanism of the Janossy effect.