Stabilization of N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)benzidine thin film morphology with UV light

• We made variable thickness TPD films and exposed them to UV light under ambient conditions.
• Mass spectroscopy and proton NMR measurements of irradiated and pristine TPD films
• TPD molecules undergo oxidation process under UV light irradiation.
• Dipole–dipole interactions may be responsible for stabilization of morphological changes.

Owing to their low glass transition temperature, Tg, amorphous thin films of N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)benzidine (TPD) undergo morphological changes even at room temperature. It has been noticed previously that exposure to UV light can increase apparent Tg of TPD films and thus stabilize their morphology. However, the reason behind increase in structural stability was not examined at the time. Here we present evidence that TPD molecules undergo photo-oxidation in air when exposed to λ ≈ 350 nm radiation and that less than 5% of the photo-oxidized species are needed to prevent dewetting of thin TPD films. We propose that photo-oxidized TPD species bind strongly to both ordinary TPD molecules and to terminal hydroxyl groups at the substrate surface, which decreases mobility of TPD molecules and makes thin TPD film less prone to morphology changes.