Abstract
Two novel series of unsymmetrically substituted 1,2,4-oxadiazole viz., R.Ox.C*Cn compounds are synthesized and characterized. An optically active, (S)-(+)-methyl 3-hydroxy-2-methylpropionate is used to introduce a chiral center in the molecule. A biphenyl moiety prepared by Suzuki coupling reaction is directly attached to the oxadiazole core at C-5 position. Investigations for the phase behavior revealed that the series with a benzyl group on one end of the oxadiazole core exhibits an 1D orthogonal smectic-A phase while the second series with dodecyl flexible end chain shows orthogonal smectic-A and tilted chiral smectic-C (SmC*) phases over a wide range of temperatures. The smectic-C phase exhibits ferroelectric (FE) polarization switching. The mesomorphic thermal stabilities of these compounds are discussed in the domain of the symmetry and the flexibility of the alkyloxy end chain length attached to the chiral center.
Highlights
The 1,2,4-oxadiazole derivatives are prevalently reported compounds with promising biological [1,2,3,4,5] and physiological [6,7,8] activity such as antiinflammatory, antibacterial, antimicrobial, antifungal, anticancer, anticonvulsant, growth hormone secretogogues, antispasmodics, antithrombotic, etc. These compounds received [9] considerable attention for the last two decades in new drug invention programs. These compounds are exploited for various technical applications [10,11,12,13,14,15] due to their electroluminescent, non-linear optical, electron transport and liquid crystalline (LC) properties
A moiety with a chiral center is attached to the phenyl ring of oxadiazole in the molecular structure
The esters containing the benzyl ether group are deprotected to give the corresponding alcohols (11p–11s). They are treated with 4-bromobenzoic acid to give products 12p–12s with two ester groups and a chiral center. This bromosubstituted chiral molecule is treated with boronate esters 6a/6b under Suzuki coupling conditions to get the final products with a biphenyl moiety directly attached to the oxadiazole core at C-5 position, 13ap–13as (Ph.Ox.C*Cn) / 13bp–13bs (C12.Ox.C*Cn)
Summary
The 1,2,4-oxadiazole derivatives are prevalently reported compounds with promising biological [1,2,3,4,5] and physiological [6,7,8] activity such as antiinflammatory, antibacterial, antimicrobial, antifungal, anticancer, anticonvulsant, growth hormone secretogogues, antispasmodics, antithrombotic, etc. The optical texture of the SmA phase exhibited by 13ar (Ph.Ox.C*C10) is given in Figure 1a as a representative of the series. The compounds of the 13a (Ph.Ox.C*Cn) series exhibiting a SmA phase are found to get transformed into crystal phase upon further cooling.
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