Engineering coherence among excited states in synthetic heterodimer systems

The design principles that support persistent electronic coherence in biological light-harvesting systems are obscured by the complexity of such systems. Some electronic coherences in these systems survive for hundreds of femtoseconds at physiological temperatures, suggesting that coherent dynamics may play a role in photosynthetic energy transfer. Coherent effects may increase energy transfer efficiency relative to strictly incoherent transfer mechanisms. Simple, tractable, manipulable model systems are required in order to probe the fundamental physics underlying these persistent electronic coherences, but to date, these quantum effects have not been observed in small molecules. We have engineered a series of rigid synthetic heterodimers that can serve as such a model system and observed quantum beating signals in their two-dimensional electronic spectra consistent with the presence of persistent electronic coherences. The discovery of long-lived coherence be-tween the singly excited electronic statesof photosynthetic antenna complexes has necessitated both a rethinking of energy transfer in biological systems and, more broadly, a recon-sideration of the role of the surrounding environ-ment and of coherent dynamics in the condense