Phase transition and electronic properties of fluorene: A joint experimental and theoretical high-pressure study

We report a structural phase transition of fluorene and the resulting changes of its electronic and optical properties investigated by a combination of experimental and theoretical methods. Fluorene is a pi-conjugated organic compound that crystallizes in the orthorhombic space group Pnma with four molecules per unit cell. We probe the stability of the molecular arrangement by x-ray powder diffraction experiments under hydrostatic pressure up to 14 GPa. Our measurements reveal a fully reversible crystallographic phase transition at 3.6 +/- 0.3 GPa indicated by abrupt changes in the lattice constants, which are accompanied by a re-arrangement of the molecules. The orientation of the molecules relative to each other evolves from the familiar herringbone pattern towards pi-stacking. This results in dramatic modifications of the electronic structure and thus the optical response as revealed by density functional calculations. In particular, the effective hole masses in the high-pressure phase become comparable to those of conventional semiconductors.