Biological photosynthesis offers a tantalizing glimpse of the clean-energy opportunities at the interface of synthetic biology, chemical catalysis, molecular excitonics, and soft-matter physics. However, this sophisticated system is optimized in nature for biological objectives (competitive fitness) that are often at odds with human concerns such as energy-storage efficiency. While some progress has been made in reconfiguring native photosystems for biofuel production, advances are limited by gaps in our understanding of the “structure-spectrum-function” relationship, i.e., of the mechanisms by which protein structures tune pigment optical and excitonic properties and, in turn, how these properties translate into biological function.