Reversible photocyclization of achiral dithienylperfluorocyclopentene dopants in a ferroelectric liquid crystal: bistable SSFLC photoswitching

Abstract

The spontaneous polarization (P_S) of a ferroelectric liquid crystal is modulated reversibly by photocyclization (λ = 313 nm) of dithienyl- and dibenzothienylperfluorocyclopentene dopants with relatively high aspect ratios. Photocyclization of these dopants causes a decrease of P_S due to a photomechanical effect which may originate from a loss of conformational flexibility of the dopant upon photocyclization. The degree of P_S photomodulation increases with dopant concentration up to the solubility limit of 3 mol%. The maximum polarization photomodulation, ΔP_S = 16.2 nC cm⁻², is achieved at 6 K below the Curie point with the dopant 1,2-bis(5-(4-heptyloxyphenyl)-2-methylthien-3-yl)perfluorocyclopentene (3c, 3 mol%). The resulting P_S photoswitch is fatigue resistant and photochemically bistable. Polarized spectroscopy experiments suggest that the dopants form homogeneous solutions with the FLC host and are integrated within the layer structure of the SmC* phase, with their molecular long axes approximately coincident with the director. To the best of our knowledge, this is the first example of a bistable ferroelectric liquid crystal photoswitch to be reported in the literature.