Accessing Single-Molecule Properties of Heptacene Using a Metal-Organic Framework

Acenes, classic polycyclic aromatic hydrocarbons composed of linearly fused benzene rings, represent a model system for exploring the physical properties of 1D π-conjugated structures. Isolating individual molecules at the single-molecule level provides a means to investigate their intrinsic properties, which is typically done by dissolving in solutions. However, this approach becomes increasingly difficult to apply for higher acenes owing to their high insolubility and instability. Thus, the electronic structure of higher acenes has long been a subject of intense discussion. This paper introduces a method to encapsulate heptacene within a metal-organic framework (MOF) through in situ photochemical conversion of a precursor molecule. The transformation reaction of the precursor is significantly accelerated upon inclusion. This approach stabilizes otherwise unstable heptacene by suppressing undesirable side reactions through the spatial constraint. The bulk production of heptacene in an isolated state enables its exploration through various analytical techniques, providing insights into its single-molecule properties and leading to the first observation of its fluorescence. Moreover, experimental and theoretical studies reveal the electronic ground state of pristine heptacene.