Transition from free rotation of C _(70) molecules to static disorder in the molecular C _(70) complex with covalently linked porphyrin dimers: {(Fe ~(III)TPP)_ 2o}·C _(70)

摘要

Fullerene C _(70) and covalently linked porphyrin (Fe ~(III)TPP) _2O dimers (TPP: tetraphenyl-. porphyrin) form molecular complex {(Fe ~(III)TPP) _2O} C _(70) (1). Each C _(70) molecule is sandwiched between two (Fe ~(III)TPP) _2O dimers and is surrounded by phenyl groups of four other porphyrin dimers. At 295 K, C _(70) molecules freely rotate about the short axis of the molecule in the ab plane. As a result, the disordered C _(70) molecules form spheroids flattened at the poles. The rotation of C _(70) stops near 100 K, resulting in the transition from a tetragonal to a monoclinic unit cell with a four-fold increase in the unit cell volume. In monoclinic phase the C _(70) molecules are located on the axis which coincides with the short axis of the molecule and passes through the midpoints of two oppositely located 6-6 bonds. Since this position has higher symmetry than the own symmetry of C _(70), it results in statistic disorder of the C _(70) molecules between two orientations. It was shown that multiple Van der Waals contacts with a concave Fe ~(III)TPP macrocycle which appear below 100 K, play an important role in the ordering of C _(70).