Significant roles of lateral alkoxy and alkylthio groups on the ferroelectric nematic phase and phase transition behavior

Abstract

The discovery of the ferroelectric nematic (N_F) phase has opened a new era in the soft matter research. Herein, we report two homologous series of ferroelectric RM734-based analogs, which have only the lateral ortho alkoxy or alkylthio groups in their alkyl chains named o-On and o-Sn (n = 1–10, which is the carbon atom number). Remarkably, all the o-On and o-Sn homologs exhibit a direct isotropic (Iso) to N_F phase transition, and no conventional nematic (N) phase is observed. Most of these N_F phases are supercooled and vitrified below room temperature. The Iso–N_F phase-transition temperatures of the o-Sn series are always higher than those of the o-On counterparts at the same n. This trend is opposite to the general trend, where normal para-alkoxy-substituted molecules exhibit higher Iso–N phase transition temperatures than the alkylthio counterparts. Single-crystal structural analysis reveals that the directions of the ortho-alkoxy and alkylthio chains of the molecules can be both roughly lateral and parallel outside their molecular long axes. The formation of the N_F phase is discussed in terms of the lateral chains filling space. This study offers new room-temperature ferroelectric nematogens and reveals the significant effects of lateral alkoxy and alkylthio chains on both the N_F phase formation and phase transition behavior.