Upconverted nanoparticle for 2D nanomapping of excitation energy transfer in conjugated polymer

Real-space evaluation of exciton migration in a nanodomain potentially provides substantial information in advanced material design, that is, however, still challenging as optical excitation and detection are diffraction limited to submicrometer resolutions. To explore the real-space evaluation, we used rare-earth core-shell upconverting nanoparticles, with thulium Tm³⁺ as emitters and Yb³⁺ as sensitizers (TmUCNP), for nanometric photoexcitation of conducting polymer, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV). The TmUCNPs in MEH-PPV solid were photoexcited with a near-infrared laser. The visible emission of the Tm ions allowed for the production of excited states of MEH-PPV through excitation energy transfer. The emission of MEH-PPV spectrally separable from Tm ions permitted individual imaging of their emission spots and single-particle emission dynamics measurement. The luminescence images of the TmUCNPs and MEH-PPV film, analyzed using the localization method, provided two-dimensional (2D) nanoscale distributions of the emissive sites in MEH-PPV surrounding the TmUCNPs. From the 2D map of the emissive sites, we determined the length of excitation energy transfer in the MEH-PPV solid to be ≦55 nanometers.