Synthesis And Characterization Of Symmetrical Dimeric Nematogens And Smectogens With Phenylenediamine And 3,3'-Dimethoxybenzidine Core System
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Date
2006-02
Authors
TIANG CHUAN, HENG
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Abstract
Four series of dimeric liquid crystal compounds comprising of thirty homologues
were synthesized and characterized. Various spectroscopy techniques namely Fourier
transform infrared (FTIR) and nuclear magnetic resonance CH-NMR, 13C-NMR, DEPT 90,
DEPT 135 and 2D-NMR) spectroscopy were employed to elucidate the molecular structure
of the dimers. Whilst the various phase transitions and textures were observed via polarizing
optical microscopy, the respective transition temperatures and enthalpies were determined
using differential scanning calorimetry. The first series consists of eight N,N'-bis[(4-
alkoxyphenyl)methylene ]benzene-1 ,4-diamine homologues, and were synthesized via
condensation reaction between 4-hydroxybenzaldehyde and p-phenylenediamine in the first
step. Subsequently, this intermediate compound was reacted with various even-parity
brooalkanes ranging from C4H9Br to C1sH37Br to yield the desired homologues. The lowest
member in the series exhibited the nematic phase whilst hexyl, octyl and decyl homologues
exhibited the nematic phase as well as SmC, SmF and SmG or SmJ phases. The remaining
four highest members however were purely smectogenic. The second series of N,N' -bis[(2-
hydroxy-4-alkoxyphenyl)methylene]benzene-1 ,4-diamine consists of another eight
homologues. These dimers with ortho hydroxyl groups were synthesized via condensation
and alkylation reactions involving 2,4-dihydroxybenzaldehyde, p-phenylenediamine and
Similar bromoalkanes as used in the first series. All members were smectogenic with the
lower members exhibited the SmA phase whilst the higher members exhibited the SmC
phase. For C12H25 homologue, the appearance of the unique SmC filamentary texture was
also observed prior to transformation to SmC compact domain for both heating and cooling
cycles. The high clearing temperatures, above 200°C of all the compounds were associated
with the presence of two intramolecular hydrogen bonding which occur between the
azomethine and hydroxyl groups at the core center. The third series consists of eight N,N'-
bis[(3-methoxy-4-alkoxyphenyl)methylene]benzene-1 ,4-diamine homologues, and were
synthesized via similar methods, but replacing 2,4-dihydroxybenzaldehyde with vanillin for
the condensation reaction. The title compounds were then obtained via alkylation of the
intermediate compound with even-parity bromoalkanes in a similar manner to the first two
series. All members were nematogenic except butyl, hexyl and octadecyl homologues. The
fourth series of dimeric liquid crystal compounds consists of 3,3'-dimethoxy(biphenylimino)-
4,4' -bis[4-phenyl alkanoate] and 3,3' -dimethoxy(biphenylimino)-4,4' -bis[2-bromo-4-phenyl
alkanoate1 esters. The three brominated homologues were smectogenic in contrast to the
nematogenic properties of the non-brominated homologues. For the former, the presence of
the polar bromine atoms at the lateral position has induced attractive intermolecular
interactions which are essential for the molecules to be arranged in the smectic layers.
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Keywords
Synthesis And Characterization Of Symmetrical Dimeric Nematogens And Smectogens With Phenylenediamine , And 3,3'-Dimethoxybenzidine Core System