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 C
3 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.