Bilayer nanographenes: structure, properties, and synthetic challenges.

Molecular nanographenes (NGs)-graphene analogues at the nanoscale-exhibit atomically defined monodispersity in both size and shape. This synthetic precision enables fine control over their properties. Among the emerging strategies to modulate their electronic and optical properties, vertical π-π stacking between the graphitized layers has recently gained attention as a powerful design tool. In this review, we explore the synthesis, structural features, and functional implications of bilayer and multilayer nanographenes, with a particular focus on the bilayer effect-a through-space electronic communication arising from the interlayer overlap. We discuss how the degree of π-π overlap, rather than solely π-extension, governs key properties such as HOMO-LUMO gap, redox behavior, photoluminescence shifts and quatum yields, and chiroptical responses. Molecular architectures incorporating helicenes, spirocycles, or non-benzenoid motifs enable the deviation from planarity, ususally presented in nanographenes, allowing the precise synthesis of covalently π-π stacked topologies that amplify this effect. Furthermore, this concept also extends to other NGs such as multilayers, supramolecular assemblies, and donor-acceptor complexes, revealing the versatility of the bilayer approach. The first synthetic approaches to access enantiomerically pure bilayer NGs are also disclosed, opening new avenues for their use in advanced technological applications. Overall, the bilayer effect emerges as a novel structural parameter for tuning the properties and function of π-conjugated carbon-based materials, opening new frontiers in molecular chiral optoelectronics, spintronics, and quantum nanoscience.