1-PERFLUOROPYRIDYL-4-PHENYLBENZENE A NEW MATERIAL FOR ELECTRON TRANS- PORT LAYFRS IN LIGHT EMITTING DIODES
R. Resel1, P. Thurner1, H. Kahlert1, H. Völlenklet3, B. Winkler2, F. Stelzer2, G.Leising1
SFB Elektroaktive Stoffe;
1Institut für Festkörperphysik,
Technische Universität Graz, Austria,
2Institut für Chemische Technologie
Organischer Stoffe, Technische Universität Graz, Austria
3Institut für Mineralogie,
Kristallographie und Strukturchemie, Technische Universität
Wien, Austria
Conjugated organic molecules are electroactive materials with high potential for future applications: such as electroluminescent materials in light emitting diodes (LED) which are applied in flexible flat panel displays. These displays are realised by thin film techniques. To increase the performance of LEDs additional layers between the electrodes and the electroluminescent layer are added. Electron transport layers increase the power efhciency drastically, fluorinated p-conjugated aromatics are suitable materials [1]. C6H5-C6H4-C5F4N was synthesized and tested for this purpose.
Single Crystal Diffraction
Single crystals were grown from a toluene solution. A plate like crystal of 0.7 x 0.4 x 0.03mm3 was analysed by x-ray diføraction. A triclinic unit cell with a=6.142, b=7.532, c=14.593 A, a=91.88o, b =101.71o, g=95.11o was obtained (space group P-1, Z=2).
Within the crystal structure the fluorinated pyridyl ring and the outer phenylring of one molecule are approximately in one plane (6o tilt), the central phenylring is twisted at about 46o relative to the pyridyl ring. This ring conformation seems to be caused by the repulsion of ortho-hydrogen and ortho-fluor atoms.
The molecules are packed in layers parallel to the (001) plane, within one layer the molecules are oriented perpendicular. Two layers are depicted in the Figure. The centrosymmetry of the crystal structure is expressed by alternating orientation of the molecules within one layer and by parallel orientation of the central phenylrings. The planes of the central phenylrings are shifted relative to each other, the closest intermolecular distance between two carbon atoms is 3.49 A.
The classification of crystal structures predicts herringbone or sandwich herringbone arrangement of long extended aroznatic molecules [2]. Replacement of hydrogen with fluorine yields in a break of the herringbone structure; parallel stacking of the aromatic rings caused by p-p interaction is favoured. Other crystal structures with partly fluorinated aromatics show the same parallel molecular stacking.
Thin Film Analysis
Thin films were cast from a chloroform solution on a glass substrate. The preferred orientation of the crystallites on the substrates was detected by q/2q scans and pole figure technique. A fiber texture with the (001) plane parallel to the substrate is observed, the molecules are oriented perpendicular to the substrate. Assuming weak interaction of the molecules with the glass substrate and following a theoretical model for van der Waals thin films [3] we conclude that the interaction within the molecular layers in the crystal structure is dominant. Weak forces act between the layers which are seperated by (001).