Solid-state properties and spectroscopic analysis of thin-film TPBI

We characterized the prominent electron transport layer 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) via single-crystal X-ray diffraction, grazing incidence X-ray diffraction (GIXRD), infrared reflection absorption spectroscopy (IRRAS), and quantum mechanical calculations. The crystals generated via vapor diffusion are of the orthorhombic space group Pbca, with a unit cell [a = 19.3935(2) b = 12.81750(10) c = 28.5610(3) Å] containing eight TPBi molecules, and screw axes and glide planes along all three crystallographic axes. Thin-film analysis becomes viable with unit cell and symmetry data, and GIXRD measurements, which demonstrate that when the amorphous TPBi thin films are annealed, the molecules preferentially orient with the a−b crystallographic face exposed at the surface and with the central benzene rings oriented 29° from the surface normal. Changes in vibrational modes at the surface, studied via infrared reflection absorption spectroscopy (IRRAS), concur with the X-ray based assignments. A minor conformer of TPBi with C₃ symmetry was also identified via computational methods, appearing 0.95 kcal/mol higher in energy at the MP2/6-31G*//B3LYP/6-31G* level of theory. The combined structural insight allows fine-tuning of a device structure for organic light-emitting diodes (OLEDs) and organic photovoltaic applications.