Thermochromic liquid crystalline medium with smectic-nematic phase transition based on chiral dopant with high helical twisting power

摘要

A thermochromic liquid crystalline medium is provided which comprises a liquid crystalline host mixture and an optically active component, characterized in that the liquid crystalline host mixture has a smectic-nematic phase transition temperature in the range from 0 to 60 {C and the optically active component comprises at least one chiral dopant with a helical twisting power (HTP) of at least 20 žmSP-1/SP. Such a medium may be encapsulated in a light transmissive polymeric material to provide a thermochromic ink. The thermochromic medium or ink may be used in decorative applications such as pigments, inks and paints, cosmetics, or in thermodiagnostic applications like medical thermography, thermometry, or in optical and electrooptical applications, or in security applications (eg documents of value) and devices. A security marking or device may comprise two thermochromic media or inks. Using chiral compounds with a high HTP in small amounts, it is possible to obtain a thermochromic medium with a wide temperature range of the thermochromic colour play, the width and position of which can easily be controlled by careful selection of the amount and HTP of the chiral compounds and of the smectic-nematic phase transition temperature (TSBS-N/SB) of the host mixture. Since amounts are small, the dopants do not necessarily have to be liquid crystalline or mesogenic themselves, which allows a broader choice of suitable chiral materials. Furthermore, problems occurring due to low solubility of the chiral compounds in the host mixture are reduced or even circumvented. Thus, the present media are more stable against partial or total crystallization or precipitation of the chiral components especially at low temperatures.