Reductive Transformation of Imine Covalent Organic Frameworks into Emissive Polymers: Insights into Emission Quenching

Abstract

Although imine-linked covalent organic frameworks (COFs) are readily synthesized, their lack of intrinsic emission constrains their potential in optoelectronic applications. Moreover, the underlying mechanisms governing excited-state energy dissipation remain poorly understood. To overcome these limitations, we present a straightforward strategy to transform nonemissive, anthracene-based imine-COFs (im-COF) into highly emissive amine-based covalent organic polymers (am-COP) through NaBH₃CN/benzoic acid reduction, achieving solid-state emission quantum yields of up to 30%. To clarify the excited-state energy dissipation mechanisms in COFs and COPs, we synthesized molecular reference compounds featuring either imine or amine functionalities. Photophysical studies, supported by density functional theory (DFT) calculations, reveal that the excited states of both amines and imines exhibit weak charge transfer (CT) properties. Furthermore, a comparative analysis of the photophysical behavior of molecular references, im-COFs, and am-COPs demonstrates that the polymer emission arising from charge-transfer processes is modulated by the polymer’s intrinsic polarity. These findings provide fundamental insights into the photophysical behavior of COFs, thus paving the way for the development of emissive porous materials with promising advanced applications.