Photochemical upconversion is a strategy for converting infrared light into more energetic, visible light, with potential applications ranging from biological imaging and drug delivery to photovoltaics and photocatalysis. Although systems have been developed for upconverting light from photon energies in the near-infrared, upconversion from below the silicon bandgap has been out of reach. Here, we demonstrate an upconversion composition using PbS semiconductor nanocrystal sensitizers that absorb photons below the bandgap of silicon and populate violanthrone triplet states below the singlet oxygen energy. The triplet-state violanthrone chromophores luminesce in the visible spectrum following energy delivery from two singlet oxygen molecules. By incorporating organic chromophores as ligands onto the PbS nanocrystals to improve energy transfer, we demonstrate that violanthrone upconverts in the absence of oxygen by the triplet–triplet annihilation mechanism. The change in mechanism is shown by exploiting the magnetic field effect on triplet–triplet interactions.

光化学上转换是一种将红外光转换为能量更高的可见光的策略，其潜在应用范围从生物成像和药物输送到光伏和光催化。尽管已经开发出用于上转换来自近红外光子能量的光的系统，但是从硅带隙下方进行上转换仍然遥不可及。在这里，我们演示了使用PbS半导体纳米晶体敏化剂的上转换成分，该敏化剂吸收硅带隙以下的光子，并在单线态氧能量以下填充紫蒽酮三重态。从两个单线态氧分子传递能量后，三重态紫罗兰酮发色团在可见光谱中发光。通过将有机发色团作为配体掺入PbS纳米晶体中以改善能量转移，我们证明了三重态-三重态mechanism灭机制中的紫罗兰素在无氧条件下上转换。通过利用磁场对三重态-三重态相互作用的作用来显示机理的变化。