Tunable upconversion in a nanocrystal-organic molecule hybrid: reabsorption vs. resonant energy transfer

Organic semiconductors, such as polycyclic aromatic hydrocarbons, are typically not responsive to near infrared (NIR) light due to their relatively large bandgaps. Here, we show that the NIR light at 980 nm can be efficiently converted to broadband visible upconversion (UC) emission by rubrene molecules in a solution dispersed with upconversion nanoparticles (UCNPs). Spectroscopic characterizations indicate that the sensitization of emission by rubrene molecules strongly depends on the location of the 4f levels of Er³⁺ ions and the interplay between reabsorption and Förster-type energy transfer. Even for the solution with the highest rubrene concentration (>1 mg mL^-1), energy transfer by reabsorption of UC emission from the UCNPs is the dominant process, and the radiationless resonant energy transfer process is weak, as the separation between most rubrene molecules and NCs is far larger than the Förster distance. The results of this work could be of particular interest for the development of organic-inorganic hybrid systems for NIR light harvesting and detection.