Primary photodegradation pathways of an exciplex-forming A–D molecular system

Abstract

The photodegradation process of pyrene–(CH₂)₁₂–O–(CH₂)₂-N,N-dimethylaniline (Py-DMA), serving as a model molecular system for exciplex-forming A–D systems, is meticulously examined in solution. The alkyl chain-linker ensures efficient electron transfer between Py and DMA, enabling exciplex formation at concentrations as low as ∼5 μM, free from the interferences dominant in solid-state devices (domain–electrode interface, domain morphological change, accumulation of defects, and so on). The photodegradation mechanism of Py-DMA is proposed for the first time based on chemical identification using steady-state spectroscopy and LC-UV-MS techniques. The mechanism predicts Py-MMA (N-monomethylaniline) and Py-MFA (N-methylformanilide) as primary products and is verified by crosschecking experimental data from FT-IR and ¹H NMR, as well as quantum mechanical calculation data. The heavy involvement of molecular oxygen (O₂) predicted in the mechanism is confirmed by a series of deoxygenated condition experiments. Although we focus on the two primary photodegradation products, secondary, tertiary, and subsequent photodegradation products are also reported, such as PyOH-MPCA (methylphenylcarbamic acid), Py-FA (formanilide), and even unspecified black carbon precipitates. With recent emerging evidence of a close correlation between the stabilities of optoelectronic devices and their active molecules, the molecular photodegradation pathways of Py-DMA will shed light on the molecular design for exciplex-based optoelectronic devices with longer lifespans.