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

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.