Structure and infrared (IR) assignments for the OLED material:: N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB)

Organic light-emitting diodes (OLEDs) are currently under intense investigation for use in next-generation display technologies. Research into the fundamental properties of the materials used in OLEDs, such as structure and vibrational modes, will help provide experimental probes which are required to gain insight into the processes leading to device degradation and failure. Calculations using the hybrid B3LYP functional and the split-valence polarized 6-31G(d) basis set have been carried out to assign the IR bands of the OLED hole transport material N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'-biphenyl-4,4"-diamine (NPB). Excellent agreement was found between the computed and experimental wavenumbers allowing the reliable assignment of major IR bands. Comparison of the reflection absorption IR (RAIRS) spectra obtained from room temperature and thermally annealed NPB thin films indicates that, upon annealing, structural changes occur and the average orientation of the NPB naphthyl groups become predominately flat with respect to the surface.