2-Chloroalkoxy-Substituted Pentafluorinated Bistolanes as Novel Light-Emitting Liquid Crystals

Shigeyuki Yamada,Tsutomu Konno,Toshio Kubota,Kazuya Miyano,Tomohiro Agou

doi:10.3390/cryst9040195

Shigeyuki Yamada, Tsutomu Konno + Show 3 more

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https://doi.org/10.3390/cryst9040195

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Abstract

In this study, we designed and synthesized novel pentafluorinated bistolane derivatives with 2-chloropentyl or 2-chlorohexyl flexible units as novel light-emitting liquid crystals (LELCs). By measuring the phase-transition behaviors, all derivatives were found to display liquid-crystalline (LC) phases during both heating and cooling processes. Among the novel bistolanes, the S- and R-configured derivatives exhibited a chiral nematic (N*) phase with a typical Grandjean optical texture. Interestingly, the chiral derivatives also exhibited a blue phase with a typical platelet texture in a narrow temperature range (2–4 °C). Photophysical measurements revealed that the 2-chloroalkoxy-substituted pentafluorinated bistolanes exhibited intense photoluminescence (PL) both in solution and in crystalline phases. The PL characteristics, especially the maximum PL wavelength, were found to switch sensitively during the heating and cooling cycles depending on the molecular aggregates through the crystal (Cr) ⇄ N* phase transition. The 2-chloroalkoxy flexible units induced dynamic changes in the LC and PL properties, providing valuable insight into the potential of various LELCs as PL sensing materials.

Highlights

IntroductionMost LE molecules have extended π-conjugations and emit intense luminescence in dilute solution, which is essential for the first two of the aforementioned applications [6,7]
Organic light-emitting (LE) materials have become widely used in our lives, for example, in biomedical imaging, optical probes, light-emitting diodes, and electroluminescent displays [1,2,3,4,5].Most LE molecules have extended π-conjugations and emit intense luminescence in dilute solution, which is essential for the first two of the aforementioned applications [6,7]
That π-conjugated LE molecules cannot be applied in the last two applications because their luminescence smoothly quenches with the formation of molecular aggregates due to the aggregation-caused quenching effect [8,9]

Summary

Introduction

Most LE molecules have extended π-conjugations and emit intense luminescence in dilute solution, which is essential for the first two of the aforementioned applications [6,7]. It is known, that π-conjugated LE molecules cannot be applied in the last two applications because their luminescence smoothly quenches with the formation of molecular aggregates due to the aggregation-caused quenching effect [8,9]. Since Tang et al discovered an intriguing luminescence phenomenon in which the luminescence intensity of the aggregated forms dramatically increases through restriction of intermolecular motion [10], significant efforts have been devoted to the development of organic LE molecules showing photoluminescence (PL) in condensed phases [11,12]. It was of great interest that the bistolane-type LE molecules showed PL, and liquid-crystalline (LC) properties, namely light-emitting liquid crystals

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