In this study, we demonstrate that
the localized surface plasmon
(LSP) resonance of metal nanoparticles, depending strictly on the
generating wavelength of LSP resonance, can have both beneficial enhancement
and harmful quenching effects on a triplet–triplet annihilation-based
upconversion (TTA-UC) emission. When the LSP resonance band of anisotropic
silver nanoprisms spectrally overlapped with the photoexcitation wavelength
of a sensitizer and the fluorescence of an emitter, an increase in
the photoexcitation efficiency and an acceleration of the radiative
decay rate were respectively induced, resulting in an effective enhancement
in the TTA-UC emission. Furthermore, the overlapping with the photoexcitation
wavelength led to a significant decrease (93%) in the threshold light
excitation intensity, which greatly enhances the figure-of-merit in
TTA-UC systems. However, when the LSP resonance band overlapped with
the phosphorescence band of the sensitizer, the TTA-UC emission was
extremely quenched, accompanied by the enhanced phosphorescence and
the decreased phosphorescence lifetime. These results suggest that
the decrease in the TTA-UC emission is a result of the competition
between the triplet–triplet energy transfer to the emitter
and the LSP-induced nonradiative energy transfer to the silver nanoprisms
from the triplet-excited sensitizer. This discovery of the conflicting
effects of LSP resonance provides an important guideline: a precise
adjustment of LSP resonance wavelengths is needed for the efficient
enhancement of TTA-UC emission. This requirement is different from
those of other fluorescence systems such as single downconverted fluorophores
and lanthanide-based upconversion nanoparticles.