Critical-Thickness-Governed Assembly of Nonamphiphilic Molecular Monolayers at the Air-Water Interface for...

Nanoarchitectonics highlights layer-by-layer assembly as a versatile strategy for constructing thin films with precisely defined nanoscale structures. To date, most functional molecules are intrinsically nonamphiphilic and have long been considered unsuitable for reliably forming defect-free monolayers at the air-water interface, thereby limiting their integration into molecular-scale devices. Here, we identify a reversible-irreversible critical thickness that governs molecular homogenization at the interface and develop a general approach to produce fully covered, uniform monolayers from a broad range of nonamphiphilic molecules, including fully conjugated aromatic systems without hydrophilic groups or long alkyl chains, such as bare copper phthalocyanine, fullerene C₆₀, and coronene, with monolayer thicknesses of 1.3, 0.6, and 0.9 nm, respectively. The resulting monolayers adopt a glassy state that enables transfer and sequential layer-by-layer stacking with centimeter-scale uniformity. Their amorphous nature preserves strong photoluminescence and, combined with molecular-level control over donor-acceptor interfaces, allows precise tuning of charge retention dynamics essential for optoelectronic applications. This work provides experimental evidence that the air-water interface can be utilized to manipulate nonamphiphilic molecules with monolayer precision, thereby enabling bottom-up nanoarchitectonics of functional thin films using molecular building blocks not limited to conventional surface-active species.