E. Melendez

Synthesis and Mesomorphic Properties of Three Homologous Series of 4,4′-Dialkoxy-α,α′-Dimethylbenzalazines A Comparative Study (I

Abstract Three homologous series of α,α′-Dimethylbenzalazines, have been synthesized: 4,4′-Dialkoxy-α,α′-Dimethylbenzalazines, 4,4′-dialkoxy-2,2′-dihydroxy-α,α′-Dimethylbenzalazines, and 4,4′-dialkoxy-2-hydroxy-α,α′-Dimethylbenzalazines. The influence of molecular structure on the mesomorphic properties was studied. Mesomorphic properties and phase transitions were determined using a polarizing hotstage microscope and a differential scanning calorimeter. The introduction of one or two hydroxyl groups in position 2- or 2-and 2′- of the aromatic rings (4,4′-dialkoxy-2-hydroxy-α,α′-Dimethylbenzalazines and 4,4′-dialkoxy-2,2′-dihydroxy-α,α′-Dimethylbenzalazines respectively) gives molecules with an excellent mesogenic quality, and all the prepared compounds of these series exhibit mesomorphism. Compounds of the series with two hydroxyl groups exhibit smectic polymorphism which is not so in the case of the compounds of the other series. The melting temperatures are systematically lower for the 4,4′-dialkoxy-2-...

Chemoselectivity in the Oxidation of unsymmetrical Thioureas with NBS/sulfuric Acid: Benzothiazoles vs. 1,2,4-Thiadiazoles

Abstract Exclusive formation of 2-heteroarylaminobenzothiazoles is observed when N-aryl-N′-heteroarylthioureas are treated with NBS/H2SO4. A possible explanation to account for the lack of 1,2,4-thiadiazole formation is offered, and a previous report3 concerning the oxidation of N-phenyl-N′-(2-pyridyl)thiourea is shown to be erroneous.

Structural studies of mesogen benzalazine derivatives: Part I. Semiempirical MO calculations (CNDO/2,MNDO) on intramolecular H-bond and vicinal interactions

Abstract The molecular geometry of benzalazine derivatives was studied by spectroscopic and semiempirical MO (MNDO, CNDO/2) methods. The existence of a strong intramolecular H-bond (7–10 kcal mol−1) was determined by 1H NMR spectroscopy in molecules having a hydroxy group in the ortho position with reference to the diazine central bridge. A study of the H-bonds using the MNDO method proved to be unsatisfactory, whereas the results obtained by the CNDO/2 method were in excellent agreement with experimental values. Both theoretical calculations (CNDO/2) and experimental evidence (UV and 1H NMR) show that the existence of a first H-bond weakened the force of a second in benzalazines.

Relation Between Structure and Mesogenic Activity in α,α′-Dimethylbenzalazines Influence of Intramolecular H-Bonding (II)

Abstract The central molecular geometry of three series of benzalazines: 4,4′-Dialkoxy-α,α′-Dimethylbenzalazines, 4,4′-dialkoxy-2,2′-dihydroxy-α,α′-Dimethylbenzalazines, and 4,4′-dialkoxy-2-hydroxy-α,α′-Dimethylbenzalazines was studied by U.V., I.R., and 1H.N.M.R. spectroscopy. All the compounds studied show the E-E configuration. The intramolecular H-bond between the nitrogen of the azine central group and the H of the OH group in position 2 of the aromatic ring is “exceptionally” strong in accordance with the calculations made using Schaefers equation. This bond conditions the geometry of the molecules, as well as their mesogenic properties. The intramolecular H-bonding favors the formation of mesophases whose properties are also related to the number of bonds of this type which the molecules contain.

6-n-Alkoxy-3-Pyridinecarboxaldehydes: New Intermediates to Liquid Crystals. Properties of Some Derivatives

Abstract The synthesis of new intermediates to liquid crystals, a series of 6-n-alkoxy-3-pyridinecarboxaldehydes is described. The mesomorphic properties of their precursors, the 6-n-alkoxy-3-pyridinecarboxylic acids are reported. Some derivatives of the carboxaldehydes, the 6,6′-di-n-alkoxy-3,3′-dipyridylmethylenehydrazines and the 6′,6”- di-n-n-alkoxy-3′,3”-dipyridylmethylene-l,4-phenylenediamines, are described and their mesomorphic properties reported. The 1,4-phenylenediamine derivatives show greater mesomorphic stability than the azines, most of the azines being non-mesomorphic while the 1,4-phenylenediamine derivatives show smectic polymorphism with wide mesophase ranges.

New chiral smectic C materials. Introduction of chirality in SC phases by means of mixtures

Abstract Three new series of derivatives of benzalazines have been synthesized and the mesomorphism has been studied. Most of the compounds exhibit mesogenic properties. The S-4′-alkoxy-2,2′-dihydroxy-4-(2-methyl-butoxy)benzalazines show cholesteric (Ch) and chiral smectic C (S′C*) phases. The S-4′-alkoxy-2,2′-dihydroxy-4-(2-methylbutoxy)-α,α′-dimethylbenzalazines and the S-4′-alkoxy-2-hydroxy-4-(2-methylbutoxy)-α,α′-dimethylbenzalazines show cholesteric and smectic A (SA) phases. Mixtures of the latter compounds with compounds of similar structure that exhibit an achiral smectic C phase have been made, and their mesomorphism studied. The mixtures show chiral smectic C phases in wide temperature ranges.

Influence of Polar End Groups on Induced Smectic Phase Appearance

Abstract Twenty binary mixtures of a polar and a non-polar α,α-dimethylbenzalazine were prepared in order to study the influence of the polar end group on smectic phase induction. No EDA-complex formation was detected, and only strongly polar groups like ‒CN, ‒NO2, ‒NCS, ‒CF3 ‒Cl, ‒Br, ‒COCH3, which increase the dipole moment along the molecular axis of the polar component, gave rise to induced smectic phases (always SA phases). Voluminous polar end groups and/or those with a weak dipole moment or that increase the component of the molecular dipole moment perpendicular to the molecular axis (i.e., ‒H, ‒F, ‒CH3, ‒OCH3, ‒NH2, ‒OH,‒SH, ‒COOCH3, OCOCH3, ‒OCOC6H5, ‒N(CH3)2) make it more difficult for suitable molecular interactions that give rise to smectic phases to occur, and no induced smectic mesophase appears.

New Liquid Crystals: 6-n-Alkoxy-3-Pyridinecarboxaldehyde Derivatives

Abstract Several members of a new mesogenic family of Schiff s bases: 6-n-aIkoxy-3-pyridyl-methylene-4′-n-alkoxyanilines, were prepared and their mesomorphic properties studied. Nematic mesomorphism was obtained when the alkoxy group in the carbocyclic ring was an ethoxy group, while the rest of the compounds studied showed smectic polymorphism (A, C, and B types). Comparative studies with the corresponding 4-n-alkoxybenzylidene-4′-n-alkoxyanilines and 4-n-alkoxybenzylidene-2′-n-alkoxy-5′-aminopyridines were carried out.

Mesomorphic Properties of Some Series of 5-n-Alkoxy-2- Pyridinecarboxaldehyde Derivatives

Abstract A new type of mesogen intermediate, the 5-n-alkoxy-2-pyridinecarboxaldehydes, has been synthesized. Three homologous series of derivatives were prepared and their mesomorphic properties studied: Schiffs monobases (5-n-alkoxy-2-pyridylmethylene-4′-n-alkoxyanilines), Schiffs dihases (bis-(5′-n-alkoxy-2′-pyridylmethylene)-1,4-phenylenediamines) and azines (his-(5-n-alkoxy-2-pyridylmethylene)-hydrazines). These derivatives are all mesogenic. The principal phases formed are nematic but, to a lesser extent, smectic C phases occur also. In some cases smectic A, B, G and H phases were detected.

Tetrhmethvl Bis[N-(2-benzothiazolyl)dithiocarbonimidates]: Novel Reagents for Heterocyclic Polymer Synthesis

Abstract The title compounds, prepared in a one-pot procedure, can be reacted with a variety of dinucteophiles to obtain molecules with four benzoazole rings, thus allowing a new, promising approach to the synthesis of heterocyclic polymers.