Lanthanide-doped nanocrystals enable organic room-temperature phosphorescence in solution through direct triplet excitation

Organic triplet excitons are of great interest for applications in optoelectronics, photochemistry and theranostics. Due to spin-selection rules, triplets are 'dark states', rendering direct photoexcitation from the ground state and efficient phosphorescent emission nearly impossible. Overcoming these spin-dependent limitations is a long-standing challenge. Here we report a universal method to brighten organic triplet excitons by attaching chromophores onto the surface of lanthanide-doped nanocrystals, enabling spin-exchange coupling between the unpaired spins of lanthanide ions and organic molecules. This allows direct photoexcitation of the organic triplets, and room-temperature, nanosecond-timescale, oxygen-insensitive phosphorescence in both solution and film under ambient conditions. Different organic chromophores and lanthanide ions are combined to obtain phosphorescence in the visible and near-infrared range. Compared with common organic phosphorescence, which only exists in crystals or at low temperature, the triplet emission established here does not require crystallization, low temperatures or an inert atmosphere. Our approach could open avenues to the application of room-temperature organic phosphorescence in optoelectronic devices and biological labelling and imaging.