Siphonein enables an effective photoprotective triplet-quenching mechanism in green algal light-harvesting complexes

Photostability is essential for the safe operation of photosynthetic systems under ever-changing light intensities. Although natural light-harvesting complexes (LHCs) use carotenoids to efficiently quench chlorophyll triplet states by triplet-triplet energy transfer (TTET) and prevent photo-oxidative damage, the electronic principles governing this process remain only partially understood. Here, we explore a class of algal LHCs featuring non-canonical carotenoids, such as siphonein and siphonaxanthin, which display remarkably enhanced TTET effectiveness when compared to their plant analogs. Using a combined experimental-theoretical approach, we resolve the quantum-chemical determinants of this effect, identifying how electronic coupling, spin density localization, and pigment geometry promote complete triplet quenching. These findings establish clear electronic design rules for TTET-based photoprotection, enabling the rational engineering of bioinspired LHC systems with an embedded protective function.