The upconversion emission systems using triplet-triplet annihilation (TTA-UC emission) has attracted much attention because these systems can be driven by a noncoherent light with low intensity (e.g. sunlight). Namely, the achievement of efficient TTA-UC emission systems directly leads to the development of high-performance solar energy devices. On the other hand, since the Dexter energy transfer between the sensitizers and the emitters is contained in the process of TTA-UC emission, the emission efficiency is quite low in solid state. We have tried to enhance the emission in solid state TTA-UC systems by interacting with the localized surface plasmon (LSP) resonance of metal nanoparticles.

The anisotropic triangular silver nanoplates (silver nanoprisms: AgPRs) were employed as a plasmonic metal nanoparticle because of the generating wavelength tunability of their LSP resonance and the generation of strong local electromagnetic fields. The hybrids of AgPRs and TTA-UC systems were fabricated by spin-coating the polymer solution containing the sensitizer (palladium(II) tetraphenyltetrabenzoporphyrin: Pd-TPTBP) and the emitter (9,10-bis(phenylethynyl)anthracene: BPEA) onto the AgPRs-immobilized glass plates. When the LSP resonance band of AgPRs spectrally overlapped with both the photoexcitation wavelength of Pd-TPTBP and the fluorescence band of BPEA, the TTA-UC emission was efficiently enhanced. It is reasonable to consider that the former spectral overlap provides the enhancement of TTA-UC emission through the photoexcitation enhancement of Pd-TPTBP (Figure 1). In addition, the threshold excitation intensity (the figure-of-merit in TTA-UC systems) significantly decreased by overlapping the LSP resonance band with the photoexcitation wavelength. Also, it was suggested by the fluorescence lifetime measurements that the enhancement of TTA-UC emission caused by overlapping with the fluorescence band was induced by the efficient nonradiative energy transfer from the singlet excited BPEA to the LSP of AgPRs (Figure 1). On the other hand, when the LSP resonance band overlapped well with the phosphorescence band of Pd-TPTBP, the TTA-UC emission was rather quenched. These results suggest that the nonradiative energy transfer by the LSP resonance from the triplet excited Pd-TPTBP to the LSP became dominant, as compared with the triplet-triplet energy transfer from the Pd-TPTBP to the BPEA. Namely, it was demonstrated that the LSP resonance of AgPRs has both of the positive and negative effects on the solid state TTA-UC emission systems.[1] Furthermore, we investigated the optical interaction between the singlet excited palladium porphyrin and the LSP resonance of AgPRs. Consequently, we will discuss the detailed effects of LSP resonance of the AgPRs on the complicated TTA-UC emission processes.

[1] S. Jin, K. Sugawa et al., ACS Photon., 2018, 5, 5025-5037.