Scientific Service Group Diffraction


Isomers


Evidence for the facial isomer in the blue luminescent phase of tris(8-hydroxyquinoline)aluminum(III) (Alq3)

The following project is the result of a cooperation with Dr. Michael Cölle and Prof. Dr. Wolfgang Brütting (University of Bayreuth).

This page describes an educational exercise how to determine the crystal structure of a particular isomer from X-ray powder diffraction data. It is shown that the data quality in the high angle region of the powder pattern is crucial in order to distinguish between the different isomeric forms. All data sets are free for download and it is suggested that the interested reader tries out his favorite structure determination program.

The different crystalline phases of tris(8-hydroxyquinoline)aluminum(III) (Alq3) correspond to different isomeric forms with significantly different electro-optical properties. The molecular structure is given in Fig. 1.

Molecular structure of Alq3
Fig. 1: Molecular structure of Alq3


Train sublimation wih a gradient of about 60°C leads to different fractions of Alq3 (Fig.2 ) with slightly different X-ray powder diffraction patterns (Fig.3).

train sublimation diffraction patterns
Fig. 2: Alq3-phases obtained by train sublimation. Fig. 3: Laboratory powder diffraction patterns of different fractions of Alq3.
daylight / UV irradiated
wavelength shift
Fig. 4: Three different phases of Alq3 at daylight (a) and irradiated with UV light (b) Fig. 5: Wavelength shift of the three different phases of Alq3 irradiated with UV light


The most interesting phase of Alq3 is the blue luminescent δ-phase (A) (Figs. 4, 5).

Possible configurations
Fig. 6: Possible configurations of the Alq3 molecule (left: facial; middle: meridonal; right: trans-meridonal).


The main question is now which isomer (or isomeric mixture) (Fig. 6) corresponds to the δ-phase of Alq3 and if it is possible to deduce this information from powder diffraction data.

High resolution powder pattern
Fig. 7: High resolution Powder diffraction patterns of the δ-phase of Alq3 measuredin the laboratory and at the synchrotron.


For this reason, powder diffraction patterns of the δ-phase of Alq3 were recorded in high resolution mode the laboratory (primary beam monochromator) and at the synchrotron (analyzer crystal) (Fig. 7).

facial isomer meridonal isomer
Fig. 8: Rietveld plots of δ-Alq3 at ambient conditions The wavelength
was 1.15 Å. “Correct” structure with facial isomer (Rp= 5.0%, Rwp= 6.5%, R-F2 =10.5%) 		“False” structure with meridonal isomer (Rp= 7.3%, Rwp= 9.4%,
R-F2 =19.4%)


It turned out, that global optimization algortithms in direct space run into severe problems in localizing the global minimum except if the correct isomer is used as starting model allowing the torsion angles to vary within certain limits. Only the high angle part of the powder pattern contains the infomation which is necessary to distinguish between different isomers. Nevertheless, the correct solution (if found) can be clearly identified (Fig. 8).

crystal structure of delta Alq3 synchrotron diffraction
Fig. 9: Crystal structure of δ-Alq3 in a projection along the c-axis.
(a), (b) and (c) are projections perpendicular to the planes of the hydroxy-quinolineligands 1, 2 and 3, respectively, showing the
overlap between ligands of neighbouring Alq3 molecules. 		Fig. 10: Crystal structure of δ-Alq3 as obtaind from synchrotron powder diffraction (red curve) and single crystal diffraction (blue curve).


The existence of (exclusively) the facial isomer in the δ-phase of Alq3 is proven by structural analysis from synchrotron powder diffraction data, revealing that both the higher symmetry and the weaker overlap of the orbitals of hydroxyquinoline ligands belonging to neighboring Alq3 molecules (Fig. 9) as compared to other phases are likely to be the origin of the significantly different electro-optical properties of δ-Alq3.

References: Meanwhile, single crystals of sufficient size and quality for single crystal analysis could be obtained by Dr. Ching Tang and Dr. Manju
Rajeswaran from the Eastman-Kodak group, proving the correctnes of the “powder structure” (Fig. 10).

You may now download the zip file Alq3.zip consisting of:

  • Alq3.cif
CIF file from powder diffraction results obtained from the X3B1 data set.
  • alq3open.mol
    alq3open.zmatrix
Open conformation of the Alq3 molecule in MDL Mol and zmatrix format
  • stoe.xy
Powder diffraction patterns of the δ-phase of Alq3 measured with Stoe Stadi P diffractometer (Cu-K-α1 radiation, Ge-111 primary beam monochromator) in Debye-Scherrer geometry.
  • x3b1.gda
    x3b1.xye
Powder diffraction patterns of the δ-phase of Alq3 measured with Synchrotron radiation at beamline X3B1 at the NSLS (λ= 1.148917 Å, 97% polarized radiation) in Debye-Scherrer geometry.


The following procedures for data reduction of both data sets are suggested: