Synthesis and properties of potentially liquid crystalline phosphorus compounds

Abstract

Traditionally it was considered necessary for molecules to be either rod or disc shaped to full] the requirements necessary for liquid crystalline phase formation. However, recent moves away from these traditional theories have led to the discovery of molecules possessing geometries such as tetrahedral and even octahedral producing mesogenic phases. One molecular geometry not yet shown to display liquid crystalline behaviour is trigonal pyramidal. Group V elements exhibit such geometry.
This thesis reports the synthesis of a number of novel phosphorus(IIl) molecules and an investigation into their possible liquid crystalline phase behaviour. Trigonal pyramidal phosphines together with the corresponding tetrahedral phosphine oxides, with three alkoxybiphenyl groups attached to a central phosphorus atom, were synthesised with varying alkyl chain lengths between C1-C22. Investigation into liquid crystalline behaviour by both sets of compounds failed to demonstrate any mesophase formation. However, tri(nonyloxybiphenyl)phosphine oxide did show evidence of crystal to crystal transitions, often seen as a precursor to liquid crystaffine behaviour.
Conversion of the phosphorus molecules into a more traditional linear shape by the replacement of an alkoxybiphenyl group with a smaller alkyl or phenyl group was also undertaken to investigate possible liquid crystalline behaviour, although none
was observed.
Attempts were also made to extend the length of the groups attached to the phosphorus atom to a tri-phenyl ring system incorporating an azo linking unit, which has often been shown to demonstrate mesophase behaviour. This led to the synthesis of a new series of compounds, phenyl (4-alkoxyphenylazo)benzoates, which demonstrated both nematic and smectic C liquid crystalline phases. Unfortunately, synthesis of the desired phosphorus was not achieved. Di(alkoxybiphenyl)phosphine oxides separated by an alkyl spacer group, were synthesised in an attempt to produce H-shaped molecules, an arrangement that would be intermediate between rod and disc shaped molecules. Again, liquid crystalline behaviour was not observed.
A series of mixed butyl and methyl phosphonium nonadecafluorodecanoates were shown to produce lyotropic liquid crystalline phases in accordance with those observed for the equivalent ammonium compounds.