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

18 March 2025, Version 2
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

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.

Keywords

Rare-earth upconversion nanoparticles
Excitation energy transfer
Super-resolved measurement

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