Unraveling Charge-Transfer States and Their Ultrafast Dynamics in Artificial Light-Harvesting Complexes

Photosynthesis relies on highly organized pigment-protein complexes in order to store sunlight energy as biochemical energy. These complexes capture light with remarkable efficiency and are responsible for ultrafast charge separation within a finely tuned energy landscape provided by the protein environments, producing one of nature's most sophisticated energy conversion systems. Inspired by nature, de novo designed proteins have been proven to be versatile platforms to emulate the function of natural light-harvesting complexes and reaction centers. With Stark and ultrafast transient absorption spectroscopies, we explored the exciton and charge-transfer (CT) mixing, as well as the excited-state dynamics, of a chlorophyll a analogue (Zn-pheophorbide a) in dimers formed within 4-α-helix bundles whose design was previously guided by molecular dynamics simulations. Due to dimerization, we observe an increase in the CT character of the excitonically coupled dimers' excited state in comparison to monomeric ZnP. Furthermore, additional nonradiative relaxation pathways, together with the formation of transient species absent in monomeric systems, were observed for the dimers. We demonstrate that de novo designed proteins can replicate key features of photosynthetic energy conversion, serving as tunable scaffolds for optimizing light-harvesting processes. Ultimately, these systems have promising applications including photovoltaic cells and biomedical treatments based on sustainable materials.