Synthesis and characterization of some novel tetrazole liquid crystals

The synthesis and characterization of new 4-[(2-alkyl)-2H-tetrazol-5-yl] phenyl 4-alkyloxybenzoates (5a-i) and 5-{4-[(4-substituted)benzyloxy]phenyl}-2-alkyl-2H-tetrazoles (7a-e) are reported. The reported tetrazoles contain two alkyl chains (six to ten carbon atoms) except for 5h and 7e which have a chiral citronellyl group linked at the N-2 nitrogen atom of the tetrazole ring and 5i having a bromine atom at the para position of benzoate. The regiochemistry in the alkylation step was established unequivocally by single-crystal X-ray analysis for 5a and 5f. The regioselectivity in the alkylation step was also confirmed by analysis of C-13 NMR chemical shifts of the C-5 carbon atom in the tetrazole ring and the methylene carbon atom vicinal to the nitrogen atom. The mesogenic behavior of the liquid crystals was characterized by polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) techniques. The tetrazolyl benzoates 5a-h show a nematic mesophase. The lengthening of alkyl chains favors the appearance of the smectic mesophase. For instance, 5g displayed nematic and smectic C mesophases, while 5h which has the chiral citronellyl group displayed nematic and smectic A mesophases. Tetrazoles 7a-e did not show mesomorphic behavior mainly due to replacement of the carbonyl by a methylene group.