A new series with smectic blue phases and SmC*-BPSm2 direct transition

A series of trifluoro-substituted benzoate derivatives: (R)-1-methylheptyl 4-[4-(4-alkyloxy-3-fluorobenzoyloxy)-3-fluorobenzoyloxy]-3-fluorobenzoates is reported. The short chain members (n = 8 to 12) display the phase sequen ce Cr-SmC*-TGBC-TGBA-BPsm-I (except for n = 8, 9 where there is no TGBC pha se), whereas for the longer ones (n = 13, 14, 16) a direct SmC*-BPSm2 trans ition is observed for the first time. We observe for n = 8 to 10, BPSm1 and BPSm3; for n = 11, the full set of three BPSm phases, and for n > 11, only BPSm2 and BPSm3 phases. The mesomorphic properties were studied by optical microscopy, DSC, and electrooptical, optical rotatory power and X-ray scat tering measurements. The effect of the positions of fluorine atoms and thei r influence over mesomorphic behaviour are discussed.