Heavy metal-free visible-to-UV photon upconversion with over 20% efficiency sensitized by a ketocoumarin derivative

Efficient triplet-triplet annihilation-based photon upconversion (TTA-UC) from visible to UV light without using heavy metals is still a challenging task. Here we achieve a record-high TTA-UC efficiency of 20.3% among 100% maximum by employing a ketocoumarin derivative as a triplet sensitizer, which shows strong visible absorption, weak UV absorption, and efficient intersystem crossing. The highest efficiency of heavy metal-free visible-to-UV photon upconversion is achieved by employing a ketocoumarin sensitizer with strong visible absorption, weak UV absorption, and high intersystem crossing efficiency.