Photoresponsive liquid-crystalline dendrimers based on a cyclotriphosphazene core

Abstract

A series of new liquid-crystalline dendrimers is synthesized from hexa(4-hydroxyphenyloxy)cyclotriphosphazene as the central core and bis(MPA) as a branching agent. The dendrimers are functionalized with azobenzene mesogenic moieties. The monodisperse nature of the dendrimers is confirmed by 1H and 31P{1H} NMR and by matrix assisted laser desorption/ionization (MALDI) mass spectrometry (MS). A study of the thermal properties reveals mesomorphic behavior. All of the materials exhibit a lamellar phase and this is confirmed to be smectic A in nature by X-ray diffraction. A nematic phase is also observed for the dendrimers that contain 4-cyanoazobenzene. A calamitic arrangement in which the mesogenic moieties point both upwards and downwards with respect to the cyclotriphosphazene core is proposed to explain the mesomorphic properties. The introduction of photochromic moieties leads to a photoresponse for these materials. The response to linearly polarized light is studied and an order parameter of about 0.5 is measured for dendrimer G1-Azo-3.