Nineteen novel alkyl substituted porphyrins have been synthesized and
their thermal phase behaviour has been investigated in detail. Twelve
compounds showed a reversible phase transition below the isotropizatio
n temperature. From time resolved microwave conductivity (TRMC) measur
ements and powder X-ray diffractometry it was concluded that the molec
ular packing does not change significantly at the lower phase transiti
on temperature and that the porphyrin cores occupy isolated positions.
Single X-ray diffraction measurements showed that the porphyrins are
arranged in a layered structure and that the space between the layers
is occupied by the alkyl substituents of the pyrrole units. The phase
transitions at the lower temperature were therefore identified as chan
ges in the crystal ordering of the porphyrins.