Supramolecular Structure, Physical Properties, and Langmuir-Blodgett Film Formation of an Optically Active Liquid-Crystalline Phthalocyanine

The structure and physical properties of optically active, metal-free 2,3,9,10,16,17,23,24-octa(S-3,7-dimethyloctoxy)phthalocyanine ((S)-Pc(8,2)) are reported and compared with those of the phthalocyanine with (R,S) side chains (mixture of 43 stereoisomers). Unlike the latter compound, (S)-Pc(8,2) lacks a crystalline phase. A freshly prepared sample is in a distorted mesophase and reorganizes irreversibly to a more ordered phase above 65 degrees C. X-ray diffraction and circular dichroism studies indicate that the molecules are stacked in columns which have a hexagonal arrangement and a left-handed helical superstructure, that is, a novel chiral D-h* mesophase. Solid state NMR measurements reveal that the phthalocyanine units in the columns begin to vibrate laterally when the temperature is increased. At 111 degrees C (D-h* --> D-r transition) they start to rotate around their columnar axes and at the same time the side chains become liquidlike. Energy migration is very efficient in the chiral D-h* phase and also in the frozen mesophase below 3 degrees C, as follows from luminescence spectroscopy. Intracolumnar charge transport, studied by the time-resolved microwave conductivity technique, turns out to be slower in the helically distorted columns than in linear columns. (S)-Pc(8,2) forms a very stable bilayer at the air-water interface, which can be transferred to give a high quality Langmuir-Blodgett film. The fact that this phthalocyanine is mesogenic at room temperature is thought to be responsible for this behavior.