Synthesis And Characterization Of The Mesomorphism In High-taper Minidendrons

Taper-shape molecules that possess a degree of amphiphilicity often exhibit liquid crystal（LC）phase with 2-D or 3-D periodicity.The majority of such compound so far have been called dendrons and “minidendrons”,typically containing an aromatic core and flexible terminal alky or oligo（ethylene oxide）chain.Taper-shape mesogens,including dendrons,are some of the most fundamental and widespread building blocks in supramolecular chemisty.There are some interesting correlations between the molecular architecture and the phase type.When the tapered molecules are fan-shape,they can easily fit in a plane,and the compound is likely to bottom-up approaches from a columnar phase,assemble on a 2-D lattice.If the peripheral chains have a cross-section larger than those would fit in a plane,such tapered molecules adopt a conical shape.The cones then assemble into spheres which,in ture pack on 3-D lattices.Our thesis is based on the body centred cubic（BCC）phase and the structure of dendrons.Generally speaking,the BCC is well represented by the 3,4,5-tris（alkoxy）benzoate salts of alkali metals or their sulfonate equivalents,as well as some other such AB₃ “minidendrons”,including those attached to a polymer backbone.These can be regarded as 1st generation dendrons;when the alkyl chains are dodecyl or longer,they all display LC phases of which the BCC occurs at the highest temperatures.All of these salts also form other stable LC phases at lower temperatures.Based on this understanding,we designed and synthesized a series of AB₅ taper-shape dendron molecules and AB₄ double cone dendrons.The objective is to explore what happens when the terminal chains are not all of the same length and the replaced chains are not all of the same position.The main contents are as follows:In the first part,we have synthesized sodium tris（alkoxy）benzoates in which one of the three alkyl chains branches further into three C18H37 chains.These AB₅ hyperbranched minidendrons melt directly into a bodycentred cubic（BCC）mesophase formed by spherical ??micelles??.In contrast,their non-branched counterparts display various mesophases before they turn into BCC upon heating.This agrees with the predictions from a numerical geometric model that relates the shape of the molecular wedge to the type of mesophase they adopt.The spheres were found to shrink in volume upon heating and expand upon cooling,as molecules,in some cases nearly half of them,are ejected and reintegrated in the spheres.The ejection of dendrons is caused by their lateral thermal expansion.The BCC appears to be the ultimate mesophase for the extremely divergent wedges such as the hyperbranched minidendrons.In dendrons with chains of unequal length,the sphere size is fixed by the shorter chains,the longer ones back-folding or interdigitating to effectively widen the wedge.This new understanding of their assembly will help in designing new dendrons,e.g.for better encapsulation of guest molecules.In the second part,in order to explore more interesting dendrons and understand deeply the relationship between dendron structure and liquid crystalline mesophase,we designed a new type of hyperbranched molecules that has double tapered structure.By derivatizing AB₃ type dendrons 183 Na and 123 Na with two more octadecyl and dodecyl chains linked by decyl chain,AB₄ type dendrons 25 and 26 were designed.Through 9 steps synthesis,the target compounds were successfully obtained.1H NMR,13 C NMR,and elemental analysis experiments were carried out to confirm their structures.This work is helpful for the continuing investigation of their liquid crystalline properties,which can provide new understanding of the self-assembly of taper-shape liquid crystals.