30 Single-Molecule Level Study and Control …
531
hotspot is an increased enhancement of UC emission compared to phosphorescence,
whereas enhancement of the triplet diffusion along the AgNW causes a larger spatial
dispersion of UC emission along the nanowires. We note that plasmon enhancement
of Dexter-type triplet–triplet energy transfer has not been reported, and the concrete
mechanism of such process is not completely understood. Further, experimental and
theoretical work will be needed to resolve this problem.
30.3.3 Nanoscale Triplet Exciton Diffusion in Molecular
Crystals Using Imaging of Single Hybrid
Nanoparticles
In the photon upconversion process based on triplet–triplet annihilation, triplet
exciton diffusion between the acceptor molecules in a molecular solid is a crucial
parameter. Large values of triplet diffusion length L T result in lowering of the irradiation intensity threshold required for the UC emission and are thus desirable for practical applications using conventional light sources. Consequently, accurate measurement of L T in UC crystals is very important in the development of the state-of-the-art
UC materials working under weak irradiation. However, such accurate measurements are generally difficult in UC crystals because the triplet donor molecules can
be dispersed inhomogeneously in the acceptor crystals [26]. Recently, advanced
microscopic techniques that use delayed fluorescence imaging or transient absorption imaging have been reported, but these still suffer from the limitation of optical
resolution.
For the study of truly localized nanoscale triplet diffusion process, microscopic
imaging using localized point-like triplet exciton source with sub-diffraction limited
size is one potential way forward. Here, we report direct observation of L T in UC
crystalline systems consisting of donor-functionalized nanoparticles and acceptor
molecules. In these systems, individual hybrid donor nanoparticles (HDPs) prepared
by chemically attaching donor molecules together with fluorescence dyes to an
alumina nanosphere were doped into acceptor molecular crystals to serve as localized point-like triplet exciton generators. In microscopic single-particle fluorescence
images under green laser excitation, the individual HDPs in the film generate red
emission due to fluorescence of the fluorescence dyes as well as blue UC emission due to fluorescence of the acceptor molecules (Fig. 30.7a). The emission was
spectrally separated into blue and red regions and detected simultaneously using a
CCD camera. This enabled an analysis of the different color emission patterns from
individual HDPs at the same time. Comparative analysis of the two images enabled
direct determination of L T in the acceptor UC crystals.
Pt (II) mesoporphyrin IX (D1) and mesoporphyrin IX-dihydrochloride (FD1)
were used as the donor and fluorescent molecules, respectively. Anthracene (AC) or
9,10-diphenylanthracene (DPA) were used as the acceptors. The D1 and FD1 were
chemically attached to the hydroxyl-functionalized alumina nanoparticles (size of
531
hotspot is an increased enhancement of UC emission compared to phosphorescence,
whereas enhancement of the triplet diffusion along the AgNW causes a larger spatial
dispersion of UC emission along the nanowires. We note that plasmon enhancement
of Dexter-type triplet–triplet energy transfer has not been reported, and the concrete
mechanism of such process is not completely understood. Further, experimental and
theoretical work will be needed to resolve this problem.
30.3.3 Nanoscale Triplet Exciton Diffusion in Molecular
Crystals Using Imaging of Single Hybrid
Nanoparticles
In the photon upconversion process based on triplet–triplet annihilation, triplet
exciton diffusion between the acceptor molecules in a molecular solid is a crucial
parameter. Large values of triplet diffusion length L T result in lowering of the irradiation intensity threshold required for the UC emission and are thus desirable for practical applications using conventional light sources. Consequently, accurate measurement of L T in UC crystals is very important in the development of the state-of-the-art
UC materials working under weak irradiation. However, such accurate measurements are generally difficult in UC crystals because the triplet donor molecules can
be dispersed inhomogeneously in the acceptor crystals [26]. Recently, advanced
microscopic techniques that use delayed fluorescence imaging or transient absorption imaging have been reported, but these still suffer from the limitation of optical
resolution.
For the study of truly localized nanoscale triplet diffusion process, microscopic
imaging using localized point-like triplet exciton source with sub-diffraction limited
size is one potential way forward. Here, we report direct observation of L T in UC
crystalline systems consisting of donor-functionalized nanoparticles and acceptor
molecules. In these systems, individual hybrid donor nanoparticles (HDPs) prepared
by chemically attaching donor molecules together with fluorescence dyes to an
alumina nanosphere were doped into acceptor molecular crystals to serve as localized point-like triplet exciton generators. In microscopic single-particle fluorescence
images under green laser excitation, the individual HDPs in the film generate red
emission due to fluorescence of the fluorescence dyes as well as blue UC emission due to fluorescence of the acceptor molecules (Fig. 30.7a). The emission was
spectrally separated into blue and red regions and detected simultaneously using a
CCD camera. This enabled an analysis of the different color emission patterns from
individual HDPs at the same time. Comparative analysis of the two images enabled
direct determination of L T in the acceptor UC crystals.
Pt (II) mesoporphyrin IX (D1) and mesoporphyrin IX-dihydrochloride (FD1)
were used as the donor and fluorescent molecules, respectively. Anthracene (AC) or
9,10-diphenylanthracene (DPA) were used as the acceptors. The D1 and FD1 were
chemically attached to the hydroxyl-functionalized alumina nanoparticles (size of