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Photophysical probes for multiple-redox and multiple-excited-state interactions in molecular aggregates.

Photosynthesis takes place through a highly efficient series of energy, electron, and proton transfers initiated by absorption of one or more photons within the visible region of the solar spectrum. Because of the presence of multiple chromophores needed for effective light harvesting, extinction coefficients must be very high. The absorbing multiunit array is held within a rigidly arranged structure that facilitates each electron hop. A fully artificial, yet biomimetic, alternative to photosynthesis that produces fuels directly and efficiently from sunlight and simple low molecular weight molecules would change the world. Achieving this goal requires a detailed understanding of the mechanisms of the key steps of the complex chemical and photochemical processes taking place in natural photosynthesis. One of these mechanisms relies on light harvesting to initiate multiple-step sequences to obtain combustible molecules suitable for burning. In particular, we are devising and testing photophysical models with characteristics that facilitate multiple electron transfers within a single aggregate and are directly relevant to light harvesting. We focus on structural features that promote photoinduced electron transfer at high dye densities, placed for optimal solar utilization and catalysis. The reaction producing oxygen is further complicated by the need for four electrons to complete the sequence, even though the first initiation step is presumably absorption of a single photon. Therefore we explore steps that accumulate charge or have the potential to do so. We also emphasize the synthesis of model systems that probe the complexity of individual steps. This Account examines the factors that influence the efficiency of electron redistribution in multiple-dye, multiple-excited-state, and multiple-redox equivalent arrays. Such knowledge will allow us to optimize the efficiency of electron migration and may contribute to a better understanding of multiple-equivalent light harvesting events by which photosynthetic energy storage takes place.