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 σ 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.

量子相干分子间能量转移被认为在光合作用和光伏发电中的光收集中起关键作用。到目前为止，由于有损分子系统中脆弱的量子相干性，供体-受体系统中量子相干性的直接、真实空间演示一直缺乏。在这里，我们精确控制了定义明确的供体-受体模型系统中的分离，并揭示了从非相干到相干电子能量转移的转变。我们通过扫描隧道显微镜诱导发光监测亚纳米分辨率的异二聚体的荧光。随着分子间距离的减小，偶极耦合强度增加，并出现两个新的发射峰：一个从供体发射蓝移的低强度峰，一个从受体发射红移的强峰。σ反键样模式，因此表明由于分子激发态的同相叠加，激发态在整个异二聚体上的离域性质。这些观察结果表明，激子可以作为量子力学波包相干地穿过整个异二聚体。在我们的模型系统中，波状量子相干传输通道的效率是非相干通道的三倍。