Temperature-Dependent Structural Phase Transition in Rubrene Single Crystals: The Missing Piece from the Charge Mobility Puzzle?

Accurate
structural models for rubrene, the benchmark organic semiconductor,
derived from synchrotron X-ray data in the temperature range of 100–300
K, show that its cofacially stacked tetracene backbone units remain
blocked with respect to each other upon cooling to 200 K and start
to slip below that temperature. The release of the blocked slippage
occurs at approximately the same temperature as the hole mobility
crossover. The blocking between 200 and 300 K is caused by a negative
correlation between the relatively small thermal expansion along the
crystallographic b-axis and the relatively large
widening of the angle between herringbone-stacked tetracene units.
DFT calculations reveal that this blocked slippage is accompanied
by a discontinuity in the variation with temperature of the electronic
couplings associated with hole transport between cofacially stacked
tetracene backbones.