Excitation Power Modulates Energy-Transfer Dynamics in a Supramolecular RuII -FeII -RuII Triad.

Multichromophoric arrays are key to light harvesting in natural and artificial photosynthesis. A trinuclear, symmetric RuII -FeII -RuII triad may resemble a light-harvesting model system in which excitation energy from donor units (Ru-terpyridine fragments) is efficiently transferred to the acceptor (the Fe-terpyridine fragment). The photoinduced dynamics after simultaneous excitation of more than a single chromophoric unit (donor/acceptor) at varying excitation fluence is investigated in this contribution. Data suggests that energy transfer is decelerated if the acceptor states (on the FeII unit) are not depopulated fast enough. As a consequence, the lifetime of a high-lying excited state (centered on either of the RuII units) is prolonged. A kinetic model is suggested to account for this effect. Although the proposed model is specifically adopted to account for the experimental data reported here, it might be generalized to other situations in which multiple energy or electron donors are covalently linked to a single acceptor site, a situation of interest in contemporary artificial photosynthesis.