Reversible control of the pitch of cholesteric liquid crystals by photochromism of chiral fulgide derivatives

When an enantiomer of resolved indolylfulgide (1) or either of the (R)-binaphthol-condensed indolylfulgides (2, 3) was mixed with a nematic liquid crystal (5CB), the cholesteric phase was induced. Their cholesteric pitches were reversibly changed by photoirradiation. Whereas the twisting ability of the cholesteric pitch (beta(M)) was larger for the colorless form than for the colored form in the case of 1, it was larger for the colored form than for the colorless form in the cases of 2 and 3. In addition, beta(M) of 1 is generally smaller than that of 2 and 3. These phenomena were explained in terms of the helical conformation of the hexatriene moiety of fulgides and the chirality of the binaphthol moiety. The cholesteric sense of a fulgide-doped liquid crystal was changed between plus and minus reversibly by the pre-doping of the chiral formaldehyde acetal of 1,1'-bi-2-naphthol ((S)-4).