J. Phys. Chem. B, 2009, 113 (43), pp 14079–14086
Ignacio Hernández and William P. Gillin
We have studied the spectroscopic properties of OLED materials Alq3, Gaq3 and Inq3 (q = 8-hydroxyquinoline) under pressure. We discuss the results in terms of the influence of structural modifications, the isomeric state and the enhancement of the intermolecular interaction. As-grown Alq3, Gaq3, Inq3 containing meridional (mer) isomer experience a red shift of nearly 90 nm (2400 cm−1) in the 0−8 GPa range. Abrupt changes in the photoluminescence occur during compression at intermediate pressures for all materials. We assign them to a phase transition, its critical pressure depending on the central cation. All three samples experience an amorphization at P ~ 6 GPa, with associated changes in the spectroscopic properties. The pressure-induced phase transitions present hysteresis to ambient conditions. Photoluminescence lifetime decreases in all cases in the explored pressure range. In the case of facial isomer containing polymorphs of Alq3, luminescence does not change its energy significantly. The most significant spectroscopic change observed in fac-isomer containing materials corresponds to γ-Alq3, which presents a low energy component that gains relative importance when pressure is increased. We ascribe this phenomenon to the presence of sensitized mer isomer impurities.
I. Hernandez - A blog for research papers 
Have you got any idea of the geometry of the high pressure structure? Is it the same for the different chemical compositions?
We are currently investigating that. We can tell that the first high pressure phase is crystalline and that it is a denser phase in all cases (which is something normal for high pressure-induced transitions), but we don’t know yet which one it is or whether it is the same for all chemical compositions.
Thank you.
Was hysteresis to ambient conditions an unexpected finding?
High pressure hysteresis is always difficult to predict (and I believe that in organic compounds it is even more difficult) so I consider important not to rule it it out and I always emphasize the importance of taking the zero pressure measurements upon downstroke.
In this particular case, one would expect a pressure-induced amorphous material to return back to crystalline when the pressure is released, so in that sense it is somewhat unexpected. Part of the sample is, indeed, recovering the original structure.
Best wishes,
Ignacio