This journal is © The Royal Society of Chemistry 2019 Soft Matter Cite this: DOI: 10.1039/c9sm01705d Interaction of supramolecular aggregates and the enhanced optical torque on the director in a dye doped nematic liquid crystal V. M. Pergamenshchik,* V. V. Multian, V. Ya. Gayvoronsky, V. A. Uzunova, S. V. Kredentser and V. G. Nazarenko There has been strong experimental evidence that molecules of some dyes in an anisotropic solvent, nematic liquid crystal, form aggregates. We present a detailed experimental analysis of the light-induced director reorientation (DR) in a dye-doped nematic liquid crystal (known as the Ja ´ nossy effect) and a theoretical model of its strong enhancement based on the aggregates’ interaction. The DR transition is found to be very different from the Frederiks effect. If the light polarization is normal to the director, the transition is jump-like first order. Moreover, light polarization along the director also induces a DR which is a smooth second order transition with a very low threshold intensity. The theoretical model which explains these effects is based on the idea that dye molecules form rodlike supramolecular aggregates. The aggregates interact via the director distortions and their effective diameter gets certain field- dependence. As a result, the related entropy depletion depends on the light intensity and polarization and can be decreased by a certain DR along with the aggregate subsystem. This entropy gain is proportional to the square of light intensity which is a two-photon effect: the first resonance photon excites the dye molecule and the second photon polarizes the aggregate. This is in line with the experimental dependence of the critical intensity on the sample thickness. A special experiment shows that the effect is not connected with a possible heat-induced isotropic phase and hydrodynamic motion. 1 Introduction The anisotropy axis n of a nematic liquid crystal (NLC), called the director, can be reoriented by the electric field of light which is known as optical Frederiks transition. 1,2 In the Frederiks transition, the torque induced by the electric field of light on the director has a dielectric origin: the field E induces polarization of the nematic molecules, and the inter- action of this polarization with the field gives rise to the (Frederiks) torque G F p DeE 2 sin 2y en where y en is the angle between E and n, and De is anisotropy of the dielectric constant. This torque results in the following director reorientation (DR): for De 4 0, a second order reversible threshold Frederiks transition if initially E>n, no DR if E8n, and thresholdless DR towards the field in a tilted geometry and for De o 0, a second order reversible threshold Frederiks transition in the geometry E8n, no DR if E>n, and thresholdless DR away from the field in the tilted geometry. The threshold field is inversely proportional to the thickness h of the NLC cell, E F p 1/h, and the critical light intensity I F p 1/h 2 . In 1991, Ja ´nossy and coworkers 3–6 discovered that adding a small amount ( B1%) of a light absorbing dye to a nematic host can strongly enhance the optical reorienting torque so that the critical light intensity of the optical DR transition drops by two orders of magnitude. The intrigue of this discovery was that the polarizability, size, and other obviously pertinent parameters of the dye molecule were rather similar to those of the LC molecule and thus could not explain the huge effect. Soon after the discovery Ja ´nossy 7 proposed an explanation to the anomalously strong light action which, having passed through modifications and further development, has become known as the mechanism of molecular motors (see, e.g. , ref. 7–11). This theory assumes that the ground (g) state and excited (e) state dye molecules differ in their interaction with the director 4 and/or angular diffusion coefficients. 8 The light absorption produces a depletion in the distribution function of g-molecules and creates e-molecules. Relaxation of the g- and e-distributions results in the angular flows that pull the director in opposite directions and, by assumption, with different efficiencies which can result in a DR. The torque on n induced by noninteracting dye monomers is predicted to be linear both in the dye concentration and the light intensity, so that the threshold of light-induced DR I c p 1/ c and cI c ( c) = const. The torque-angle dependence of the mechanism of molecular motors was predicted to be similar to G F . Institute of Physics, Prospect Nauki 46, Kiev 03039, Ukraine. E-mail: victorpergam@yahoo.com Received 22nd August 2019, Accepted 29th September 2019 DOI: 10.1039/c9sm01705d rsc.li/soft-matter-journal Soft Matter PAPER Published on 16 October 2019. Downloaded by KENT STATE UNIVERSITY on 10/16/2019 3:30:06 PM. View Article Online View Journal