Engineering Excitonic and Charge-Transfer States in Bio-Inspired Chromophore-Protein Assemblies

Photosynthesis, a vital process for life on Earth, converts solar energy into chemical energy through pigment-protein complexes. Understanding the intricate interplay between exciton delocalization and charge transfer (CT) states within these complexes is crucial for developing bioinspired solar energy conversion devices. Photosynthetic charge separation involves structural, electronic, and vibrational factors, and insights from Photosystem II have revealed key design principles for efficient artificial systems: mixing of excitons and CT states, resonant vibrations, a smart protein matrix, and a balance between coherence and decoherence.

This thesis explores the development of artificial systems using de novo designed protein scaffolds to manipulate chromophore properties. Efficient excitonic and CT states require careful chromophore arrangement and interplay with protein matrices. The study presents a systematic approach to develop chromophore-protein assemblies for artificial photosynthesis, utilizing Zinc-pheophorbide-a and maquette proteins.

This thesis investigates chromophore-chromophore and chromophore-protein interactions using spectroscopic techniques, with a focus on excitonically coupled dimers. We explored spectroscopic and binding properties, as well as the excitonic states, thermal stability, and folding of chromophore-protein complexes. Additionally, The CT characteristics of the newly formed excitonic states was studied with Stark spectroscopy, revealing a significant increase in CT character for excitonically coupled dimers compared to monomeric complexes. Furthermore, ultrafast transient absorption spectroscopy (TAS) was employed to delve into the ultrafast dynamics of mixed excitonic and CT states, demonstrating that excitonic interactions influence excited state dynamics, leading to faster recovery of ground state bleach, enhanced stimulated emission, and additional ultrafast decay pathways.

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