Molecular Excitonic Seesaws.

Significance Transformation of light into electronic excitation energy and the spatial redistribution of this energy within a complex of molecular units constitute the processes central to photochemical energy conversion in photosynthesis. Excitations migrate downward in energy between molecules, ultimately localizing at a global energetic minimum. But, how does energy flow in a system with built-in bistability, a local and a global energetic minimum? A molecular excitonic seesaw structure probes such fundamental intramolecular bistability. Much like watching a pen balanced on its tip fall randomly, in single molecules, energy flows nondeterministically to one or the other energetic minimum. Such a labile excitonic seesaw makes energy transfer highly susceptible to external stimuli and can be controlled, for example, by the incident light polarization. The breaking of molecular symmetry through photoexcitation is a ubiquitous but rather elusive process, which, for example, controls the microscopic efficiency of light harvesting in molecular aggregates. A molecular excitation within a π-conjugated segment will self-localize due to strong coupling to molecular vibrations, locally changing bond alternation in a process which is fundamentally nondeterministic. Probing such symmetry breaking usually relies on polarization-resolved fluorescence, which is most powerful on the level of single molecules. Here, we explore symmetry breaking by designing a large, asymmetric acceptor–donor–acceptor (A1-D-A2) complex 10 nm in length, where excitation energy can flow from the donor, a π-conjugated oligomer, to either one of the two boron-dipyrromethene (bodipy) dye acceptors of different color. Fluorescence correlation spectroscopy (FCS) reveals a nondeterministic switching between the energy-transfer pathways from the oligomer to the two acceptor groups on the submillisecond timescale. We conclude that excitation energy transfer, and light harvesting in general, are fundamentally nondeterministic processes, which can be strongly perturbed by external stimuli. A simple demonstration of the relation between exciton localization within the extended π-system and energy transfer to the endcap is given by considering the selectivity of endcap emission through the polarization of the excitation light in triads with bent oligomer backbones. Bending leads to increased localization so that the molecule acquires bichromophoric characteristics in terms of its fluorescence photon statistics.