In a model complex for an artificial light collection unit, in which one iron-polypyridine chromophore and two ruthenium-polypyridine chromophores are connected to each other, the energy transfer from the ruthenium center to the iron center was investigated in dependence on the intensity of the excitation light. It was demonstrated that the energy transfer process is delayed at high excitation intensities when both a ruthenium center and an iron center are excited.

By: Joachim Kübel // Maria Wächtler // Benjamin Dietzek 

Photosynthetically active organisms have developed unique strategies to efficiently collect the energy of light across a wide spectral range with the help of a sophisticated antenna system and transfer it to a reaction center where the light reaction of photosynthesis takes place. The key to success is the defined spatial and energetic arrangement of the individual light-absorbing chromophoric units. This principle can be successfully imitated in artificial systems by organizing several light-absorbing units in (supra)molecular structures. The high density of chromophoric units in such structures opens the possibility that at high light intensities more than one chromophoric unit per molecule can be excited by the absorption of light. Due to the small distances between the chromophoric units, specific interactions between the excited chromophores in multi-excited species can influence the photophysics compared to single-excited species. This can, for example, lead to the annihilation of excited states, which reduces the overall population of excited states and thus the efficiency of the antenna system, or highly reactive states may accumulate in the system. Natural systems possess safety strategies to prevent potential damage from the latter. To be able to establish such protective mechanisms in artificial systems, the first step is to identify processes that can occur at high light intensities.

In this context, scientists at IPHT have investigated the light-induced processes in a symmetrical trinuclear transition metal complex (RuFeRu, see Figure 1), which serves as a model for a light collection unit, depending on the excitation intensity. In this system, three chromophoric units contribute to light absorption in the visible spectral range: two peripheral ruthenium-polypyridine units and one central iron-polypyridine unit. At low excitation intensities, only a maximum of one of these units per molecule is excited. Excitation of one of the ruthenium chromophores leads to a downstream intramolecular energy transfer to the iron center, which acts as an energy sink. The excited iron chromophore returns to its original state without emission. If the excitation intensity is increased on the one hand, the probability of a double or even triple excitation increases, while on the other hand, the ratio of species containing only one excited chromophore decreases. The influence of the presence of multi-excited species on the observed light-induced dynamics was investigated using femtosecond time-resolved transient absorption spectroscopy. With increasing intensity of the excitation light, a significant delay in the temporal development of the system was observed, which became more pronounced with increasing intensity (see Figure 1). A simulation of the excitation process shows that under the experimental conditions used, the only multi-excited species formed in significant quantities is Ru*Fe*Ru with an excited ruthenium center and an excited iron center, the contribution to the overall signal of which increases with increasing excitation intensity. Therefore, an obvious explanation of the experimentally observed delay in the temporal evolution of the system is that a blockade of the energy transfer from the ruthenium center to the iron center is present as long as the energy-accepting iron center is in an excited state. Based on this assumption, a kinetic model was developed which describes the dynamic processes in Ru*Fe*Ru and contains an effective rate of energy transfer from the ruthenium center to the iron center. This effective rate depends on the occupation of the acceptor states and changes during relaxation.

This is the first report of such a kinetic blockade of energy transfer in a multi-nuclear transition metal complex. This hitherto unknown effect is of fundamental importance for the development of artificial light collection systems. Although the proposed kinetic model was developed specifically for the systems studied here, it can be generalized and is also applicable to other situations in which several energy or electron donors are covalently bound to a single acceptor – a common design in artificial structures imitating the principles of natural photosynthesis.

Funded by: EU, DFG, FCI