Effects of fluorine atoms introduced into flexible chains or mesogenic structures on their liquid-crystalline and photoluminescence characteristics

Abstract

Introducing semifluoroalkoxy-type flexible chains into π-conjugated mesogens induces the formation of smectic A (SmA) liquid-crystalline (LC) phases, which are higher-order LC phases. Although tolanes are frequently used as π-conjugated mesogens, tolane and its derivatives are generally non-emissive in the solution and crystalline (Cr) phases. Tetrafluorinated tolanes, wherein four F atoms are introduced into the minor molecular axes, exhibited significantly increased photoluminescence (PL) efficiencies. However, even when a semifluoroalkoxy-type flexible chain was introduced into the tetrafluorinated tolane-based mesogen, no LC-phase formation was observed due to the increased Cr-phase stability. Therefore, we developed new semifluoroalkoxy-substituted difluorinated tolanes with two F atoms within the short molecular axes of the mesogens. The length of the fluoroalkyl segment of the semifluoroalkoxy chain and the positions of the F atoms introduced into the short molecular axis of the mesogen affected the LC and PL characteristics. Remarkably, introducing flexible chains with long fluoroalkyl segments decreased the PL intensity in the Cr phase, whereas after the phase transition to the SmA phase, the PL intensity was increased compared to that of the Cr phase in the case of difluorinated tolane with a 3,5-difluorobenzonitrile unit. With appropriate molecular design, strong PL may be realized, even in the LC phase, and new tolanes should be applied as useful photoluminescent temperature sensors in the future.