DECAFLUOROQUATERPHENYL - CRYSTAL AND MOLECULAR STRUCTURE SOLVED FROM X-RAY POWDER DATA
B. Koppelhuber-Bitschnau1,
L. Smrcok2, D. Tunega2,
K. Shankland3, W.I.F. David3,
R. Resel4, G. Leising4
1Institute of Physical and Theoretical
Chemistry, Technical University Graz, Austria
2Institute of Inorganic Chemistry, Slovak
Academy of Science, Slovakia
3Rutherford Appleton Laboratory, UK
4Institute of Solid State Physics, Technical
University Graz, Austria
Light emitting diodes (LED) using conjugated organic molecules as electroluminescent material have high potential for future applications e.g. in flat panel displays or in flexible area light sources. These LED's are realised by a thin layer arrangement of the electroluminescent material between two electrodes. Decafluoroquaterphenyl (DFQP, C24H8F10), due to conjugation along its' four phenyl rings, is a electroactive material which is suitable as electron transport material in a multilayer LED's based on hexaphenyl [1]. Since layers of DFQP crystallise highly preferred oriented and the electrical conductivity depends strongly on this preferred growth, the knowledge of the crystal structure is essential for a detailed analysis of the function of electron transport layers in LED's.
Geometry optimisation of the DFQP molecule was performed with quantum-chemical package Gaussian94 [2] at the Hartree-Fock level. The basis set of SVP (split-valence + polarisation) quality [3] was used for all atoms, so that the total number of atomic orbitals was 516. Planarity and hexagonality of benzene rings were kept fixed during the optimisation procedure, whilst all interatomic distances, bond and interring dihedral angles were allowed to vary. Fig. 1 shows the final shape of the DFQP molecule. The bond length between central benzene rings is 1.495 A, and dihedral angle was optimised to 66.7 deg. The sum of interring dihedral angles is 181.2 deg, meaning that the first and fourth benzene rings lie almost in plane.
Fig.1:
Decafluoroquaterphenyl, DFQP, C24H8F10-
High-resolution powder diffraction patterns were taken with a Stoe STADI-P diffractometer in transmission mode and configured with a curved Ge(111) primary beam monochromator and a linear PSD. Strictly monochromatized CuKa1 and CoKa1 radiation was used to scan the patterns in the 2 angular ranges 6°-60° (Cu) and 7°-80° (Co) respectively. Cell parameters and I centering were found by ITO [4] program. As two possible space groups (Ia or I2/a) were derived from the systematic absences, the structure solution was performed as follows in Ia. Correlated integrated intensities were extracted from the Cu diffraction pattern via a Pawley refinement. A trial structure was built using an internal coordinate description of the molecule, utilising the geometry found by ab initio calculation. The three torsion angles between the rings were flagged as unknown. The trial structure was subjected to a simulated annealing procedure in which the three internal and six external degrees of freedom were optimised against the extracted intensities. A structure solution was obtained in ~40 seconds on a 233MHz DEC Alpha workstation, with subsequent fine-tuning of the solution taking ~60 seconds. The solution was clearly centrosymmetric and so Rietveld refinement was performed in space group I2/a with program GSAS [5]. Soft Constraints were used to keep the rings planar and hexagonal and the F- and H- atoms in plane, only the angles between rings were allowed to be refined (51 Positional Parameters, Rp 3.5, wRp 5, Fig 2). Refinements of different data sets are in good agreement, angle between two inner rings was refined to 52(2) deg, central torsion angle to 58(2) deg.
Fig. 2.: The final Rietveldplot with
observed, calculated and difference diffraction profiles for
DFQP, C24H8F10, (CuKa1 -data)