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 (an 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. While the model proposed 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.