Crystalline organic thin films for crystalline OLEDs (I): orientation of phenanthroimidazole derivatives

Compared with traditional organic light-emitting diodes (OLEDs) based on amorphous thin films, crystalline OLEDs possess promising advantages in developing advanced display technologies, such as a longer lifetime and a lower driving voltage. In recently developed crystalline OLEDs, the crystalline thin films of 2-(4-(9H-carbazol-9-yl)phenyl)-1-(3,5-difluorophenyl)-1H-phenanthro [9,10-d]imidazole (2FPPICz) grown via the weak epitaxy growth (WEG) method determined the device performance and played a crucial role in achieving a superior photon output capability. However, the structural properties of such novel crystalline thin films remain to be investigated. Herein, we report the single-crystal structure of 2FPPICz and the WEG mechanism of the 2FPPICz crystalline thin film. The 2FPPICz molecules adopt an edge-on arrangement and the molecular long axis tends to be parallel to the inducing layer and substrate, indicating that the transition dipole moment is nearly horizontal. These unique orientation phenomena are attributed to the lattice matching that is established between the WEG and the inducing layers. This work on the 2FPPICz crystalline thin film provides a structural foundation for the further development of crystalline OLEDs.