Wavelike electronic energy transfer in donor-acceptor molecular systems through quantum coherence

Quantum-coherent intermolecular energy transfer is believed to play a key role in light harvesting in photosynthesis and photovoltaics. So far, a direct, real-space demonstration of quantum coherence in donor-acceptor systems has been lacking because of the fragile quantum coherence in lossy molecular systems. Here, we precisely control the separations in well-defined donor-acceptor model systems and unveil a transition from incoherent to coherent electronic energy transfer. We monitor the fluorescence from the heterodimers with subnanometre resolution through scanning tunnelling microscopy induced luminescence. With decreasing intermolecular distance, the dipole coupling strength increases and two new emission peaks emerge: a low-intensity peak blueshifted from the donor emission, and an intense peak redshifted from the acceptor emission. Spatially resolved spectroscopic images of the redshifted emission exhibit a sigma antibonding-like pattern and thus indicate a delocalized nature of the excitonic state over the whole heterodimer due to the in-phase superposition of molecular excited states. These observations suggest that the exciton can travel coherently through the whole heterodimer as a quantum-mechanical wavepacket. In our model system, the wavelike quantum-coherent transfer channel is three times more efficient than the incoherent channel. Light harvesting in photosynthesis and photovoltaics may rely on quantum-coherent energy transfer, but experimental verification is hindered by the lossy nature of the molecular systems. Subnanometre-resolved electroluminescence spectroscopy now reveals wavelike intermolecular electronic energy transfer through quantum coherence in artificially constructed donor-acceptor heterodimers at the single-molecule level.

Automated summary

This study demonstrates a transition from incoherent to coherent electronic energy transfer in donor-acceptor model systems by precisely controlling the separations. Through scanning tunnelling microscopy induced luminescence, two new emission peaks were observed with decreasing intermolecular distance. The spatially resolved spectroscopic images of the redshifted emission reveal a delocalized nature of the excitonic state over the whole heterodimer, suggesting a coherent wave-like transfer of the excitons.