Excitation Energy Transfer from Single Rare-Earth Upconversion Nanoparticles to Organic Conjugated Polymer Film: a Super-Resolved Microscopic Study

We have utilized an upconverting nanoparticle (UCNP) with thulium (Tm) ions as emitters and Ytterbium (Yb) ions as sensitizers (TmUCNP) for nanometric photoexcitation of a film of conjugated polymer, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV). The TmUCNPs surrounded by a thin layer of MEH-PPV were photoexcited at 976 nm using a continuous-wave (CW) near-infrared laser. The NIR light was selectively absorbed by the sensitizer of the TmUCNPs, leading to the formation of submillisecond-lifetime excited states of Tm ions in the UCNPs through stepwise multiphoton energy transfer between Yb and Tm. The visible emission from the Tm ions, coupled with their long excited-state lifetime, allowed for the production of locally excited states of the MEH-PPV through the excitation energy transfer (EET) from the individual TmUCNPs. This EET was confirmed by spectrally separated wide-field single-particle imaging and single-particle emission dynamics measurement with a hyperspectral confocal microscope. The spectrally separated wide-field images of the TmUCNPs and the MEH-PPV film, analyzed using a 2D localization method, provided a 2D spatial distribution of the emissive trap sites in the MEH-PPV surrounding individual TmUCNPs. From the 2D map of the emissive sites, the length of EET in the MEH-PPV solid was determined to be in the range from several to 50 nm. Thus, nanometric local photoexcitation using single UCNPs enabled direct evaluation of EET length in nano-space.