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"Luminescence induced by highly localized excitations that are produced by electrons tunnelling from the tip of a scanning tunnelling microscope is used to map the spatial distribution of the excitonic coupling in well-defined arrangements of a few zinc-phthalocyanine molecules and the dependence of this spatial distribution on the relative orientation and phase of the transition dipoles of the individual molecules. Excitonic couplings spanning several chromophores are crucial in many energy transfer and optical processes. Such couplings can be understood by simply considering the coherent intermolecular dipole–dipole interactions involved, but that can prove challenging in complex systems; moreover, the interactions cannot be observed directly with conventional optics. Zhen-Chao Dong and colleagues now show that the luminescence induced by highly localized excitations, caused by electrons tunnelling from the tip of a scanning tunnelling microscope, can be used to map both the spatial distribution of the excitonic coupling in well-defined arrangements of a few Zn-phthalocyanine molecules and to map how it depends on the relative orientation and phase of the transition dipole of the individual molecules. The approach is expected to deliver detailed insight into light-harvesting structures and molecular light sources. Many important energy-transfer and optical processes, in both biological and artificial systems, depend crucially on excitonic coupling that spans several chromophores1,2,3,4,5,6,7,8,9. Such coupling can in principle be described in a straightforward manner by considering the coherent intermolecular dipole–dipole interactions involved10,11. However, in practice, it is challenging to directly observe in real space the coherent dipole coupling and the related exciton delocalizations, owing to the diffraction limit in conventional optics. Here we demonstrate that the highly localized excitations that are produced by electrons tunnelling from the tip of a scanning tunnelling microscope, in conjunction with imaging of the resultant luminescence, can be used to map the spatial distribution of the excitonic coupling in well-defined arrangements of a few zinc-phthalocyanine molecules. The luminescence patterns obtained for excitons in a dimer, which are recorded for different energy states and found to resemble σ and π molecular orbitals, reveal the local optical response of the system and the dependence of the local optical response on the relative orientation and phase of the transition dipoles of the individual molecules in the dimer. We generate an in-line arrangement up to four zinc-phthalocyanine molecules, with a larger total transition dipole, and show that this results in enhanced ‘single-molecule’ superradiance from the oligomer upon site-selective excitation. These findings demonstrate that our experimental approach provides detailed spatial information about coherent dipole–dipole coupling in molecular systems, which should enable a greater understanding and rational engineering of light-harvesting structures and quantum light sources."
<https://www.nature.com/articles/nature17428?error=cookies_not_supported&code=f20a8117-43f8-44c9-b340-04c91eca32e9>