Electron Confinement Effect in Single-Molecule Phenyl Clusters

Precise modulation of electronic structures is essential for designing materials with tunable optical and electronic properties. Through-space conjugation (TSC) offers a promising strategy for electron delocalization via spatial orbital overlap, without the need for covalent bonding. However, understanding whether increasing the number of interacting units can continuously enhance electron delocalization in TSC systems remains unresolved. This challenge stems from the difficulty of designing model systems with systematic structural variation and controlled spatial interactions. Here, we report the first discovery of an electron confinement effect in TSC systems. We synthesized a series of single-molecule phenyl clusters with 4 to 10 benzene rings and systematically investigated their photophysical behavior and electronic structure. Despite the increasing number of phenyl units, electron delocalization remained confined to just two rings, leading to emission saturation at ~450 nm. This finding stands in stark contrast to traditional π-conjugated systems, whose emission typically redshifts as the conjugation length increases. Our results reveal a fundamental size-independent behavior unique to TSC and demonstrate that spatial geometry, rather than molecular size, governs electron delocalization in such systems. This work establishes a mechanistic foundation for designing next-generation luminescent materials based on confined TSC.