NJC
The dimers belonging to the third series are different from
Paper
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those of the parent series or the two series of materials described
above. That is, they possess an elongated chalcone where one of
the aryl rings is substituted with a 4-n-octyloxybiphenyl instead
of a phenyl ring. Interestingly, all of them are mesomorphic;
stabilization of BPIII, BPI/II and N* phases by the dimer posses-
sing an odd-parity (5-oxypentanoyl) spacer is noteworthy in view
of the fact that the compounds of the other series with an
analogous spacer are crystalline. The other three dimers having
4-oxybutanoyl or 6-oxyhexanoyl or 8-oxyoctanoyl spacers exhibit
N*, TGB and SmA phases. The occurrence of the intercalated
SmA phase for the dimer consisting of an 8-oxyoctanoyl spacer
was established by XRD measurements. The increase in the
spacer length seems to reduce the Cr–SmA/N*, N*–I or I–N*
transition temperatures. In general, the thermal behaviour of
these compounds agrees with that of the parent dimers.
5 (a) S. T. Lagerwall, in Ferroelectric and Antiferroelectric Liquid
Crystals, ed. S. T. Lagerwall, Wiley-VCH, Weinheim, Germany,
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The fourth series of dimers, which are the fluoro analogues
of dimers of the third series, display mesomorphic behavior
with the expected lower transition temperatures. In this series
also, the dimer with an odd-parity spacer is mesomorphic
exhibiting BP and N* phases. For the members with even-
parity spacers, the number of mesophases increases on length-
ening the spacer; the dimers with 4-oxybutanoyl, 6-oxyhexanoyl
and 8-oxyoctanoyl spacers show respectively the sequences:
N*–TGBC*, N*–TGB–SmA and BP–N*–TGB–SmA.
In essence, the study clearly illustrates the complex interplay
of the different molecular sub-units of the dimers in stabilizing
mesophases such as blue phase(s), chiral nematic, twist grain
boundary, smectic A and chiral smectic C phases.
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Acknowledgements
CVY sincerely thanks the Science and Engineering Board
(SERB), DST, Govt. of India for funding this work through the
project No. SR/S1/OC-04/2012.
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