We report the occurrence of a reentrant twist grain boundary phase, which we designate as Re- ${\mathrm{TGB}}_A$. Microscopic observations on a nonsymmetric dimer showed the phase sequence Iso– $N^{*}$– ${\mathrm{TGB}}_A$–Sm- $A$–Re– ${\mathrm{TGB}}_A$– ${\mathrm{TGB}}_{C^{*}}$. Here $N^{*}$ and Sm- $A$ stand for the chiral nematic and smectic- $A$ phases, ${\mathrm{TGB}}_A$ is the twist grain boundary phase with smectic- $A$ blocks, and ${\mathrm{TGB}}_{C^{*}}$ that with smectic- $C^{*}$ blocks and exhibiting features of both the smectic- $C^{*}$ and TGB phases. The reentrance of the ${\mathrm{TGB}}_A$ phase is unambiguously demonstrated using x-ray diffraction, selective reflection, and optical rotation data.

• Received 3 April 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.085504