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A molecular design rule for dichroic dyes with a high solubility in fluorinated liquid crystals has been investigated. Molecular interactions and a concept of motional segments proportional to molecular weight were introduced to explain the deviation from an ideal solution. A semi-empirical equation was obtained: log C =-20.75 (ΣΔ H f / T - ΣΔ S f )- 0.013( E h - E p )+2.32, where C represents a saturated solubility in wt% at T (K). ΣΔ H f (kJ kg -1 ) represents the sum of the enthalpies of fusion and of phase-transitions from T to melting point. ΣΔ S f represents the sum of the entropies. Calculated E h represents a hydration energy. E p is the hydrophobic energy determined from calculated 1-octanol log P . Log C is mainly determined by the interaction between dye molecules. Bulky, rigid and asymmetric structures give a high solubility. Large electrostatic interaction between a dye molecule and liquid crystal molecules increases solubility.
Naito et al. (1998) studied this question.