Visualizing excitonic states in constructed cyclic molecular architectures by tip-enhanced photoluminescence

Natural photosynthesis inspires the design of artificial cyclic light-harvesting architectures, yet probing their excitonic behaviors-particularly optically dark states-remains challenging for far-field optics due to diffraction limits and dipole selection rules. Here, combining scanning tunneling microscope manipulation with tip-enhanced photoluminescence, we visualize discrete excitonic states in constructed cyclic zinc-phthalocyanine architectures with sub-nanometer resolution. By controlling intermolecular distances, we map excitonic evolution across weak, intermediate, and strong coupling regimes. We uncover a transition dipole rotation phenomenon arising from the molecules' orthogonal, degenerate dipoles, which facilitates coherent intermolecular coupling. Furthermore, by comparing hollow and solid architectures, we demonstrate that the molecular architectures determine the nature of the lowest-lying excitonic state: a hollow ring possesses an optically dark lowest-lying state, whereas a solid architecture features a bright one, highlighting a possible structural physical advantage of hollow cyclic architectures for suppressing radiative losses. These findings provide guidelines for designing organic architectures with high energy transfer efficiencies.