Macrocyclic discotic materials

Due to the disc-like geometry of the porphyrin macrocycle, discotic mesophases have generally been the most common for porphyrin liquid-crystalline materials. The first porphyrin liquid crystals featuring rod-like calamitic mesophases were described by Bruce and coworkers using substituted 5,15-di(phenyl) zinc porphyrin complexes as the basic molecular building block.The first materials... 

Pc macrocycle. Some of them are mesomorphic and show a Colh phase (Fig. 79 I-Gl G 115 Coin 270 I II-Gl G <-20 Colh > 320 h H-G2 G 115 Colh 250 I II-G3 94 Colh 108 I) for which the structure of the columnar mesophase is frozen at room temperature (anisotropic glasses). The functionalization of these dendrons in the 3 and 5 position of the terminal rings by oligo(ethyleneoxy) chains (0CH2CH2)30CH3, and their subsequent grafting onto the phthalocyanine of type I (Fig. 79) led to amphiphiUc materials [333]. In concentrated ethanol solutions ( 20-40% by mass), the compound with the fimctionaUzed G1 dendron behaves as a discotic amphiphUe forming a coliunnar nematic lyotropic phase. In addition, it possesses a columnar mesophase stable from room temperature up to 260 °C. Systems of type III are not mesomorphic. 

A recent observation has been that not all porphyrin LC phases are discotic. Thus 5,15-meso-substituted porphyrins have been prepared which show a variety of smectic phases that are not discotic. The porphyrin macrocycle could be acting to impose biaxial symmetry within the meso-phase, which makes the new porphyrins of great interest as new nematic liquid-crystal materials. 

The most obvious family of compounds able to induce columnar phases is that constituted by the macrocyclic compounds which often possess a discoid molecular shape. The ability of macrocyclic systems to stack and to self-assemble into columnar mesophases confers on these systems the properties required for potential utilization in molecular devices based on photonics, electronics, or low-dimensional ionic transport. The search for new discotic mesogens is thus still of current interest, in order to improve their chemical, thermal, and photophysical stabihty, and new families of discogenic materials appear regularly. 

An isomer of TTC 4.34, in which the macrocycle is built up from 3,4-substituted thiophene units, was synthesized by the same group with the aim of developing liquid crystalline materials, in particular discotic mesogens. 2,5-Alkynylated 3-ethynyM-iodothiophenes were cycUzed with the catalytic system Pd /Cu(I) to give colorless cyclotriynes 4.36 in yields up to 25 % (Scheme 1.46). An X-ray structure analysis of one derivative showed that in the crystal the cyclotriyne cores do not interact [411]. 

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