528
M. Vacha and S. Hirata
In summary, we designed novel hybrid nanoparticles composed of FRET pair
dyes covalently attached to GNR. Using single-particle optical microscopy, we have
shown that energy transfer from a donor to an acceptor in the presence of a GNR
and, consequently, the emission color from the hybrid nanostructure can be reversibly
modulated by the state of the polarization of the incident light.
30.3.2 Plasmon Enhancement of Triplet Exciton Diffusion
in Photochemical Photon Upconversion
Photon upconversion (UC) based on triplet–triplet annihilation (TTA) between chromophores is a photochemical mechanism that can convert low energy photons to
higher energy ones [25]. The TTA-UC process is a sequence of excitation of a singlet
state of a donor, intersystem crossing into a triplet state of the donor, triplet–triplet
energy transfer from the donor to an acceptor, and TTA between two acceptors
leading to upconverted fluorescence from the acceptor singlet state. The TTA-UC
mechanism can be optimized by material design to achieve high UC quantum yield,
relatively low threshold of the UC emission, and broad and well-tunable excitation
and emission characteristics. There is also a considerable potential for utilizing the
effect of localized plasmon resonances of metal nanoparticles on the TTA-UC process
to further optimize the UC mechanism toward practical applications. Our goal here
was to gain insight into the effect of localized plasmons of metal nanostructures on
the individual photophysical steps involved in the TTA-UC process. We used optical
microscopy to study the properties of plasmonic hybrid nanostructures consisting of
silver nanowires combined with a TTA-UC film [11]. We find fundamental differences in the way the localized plasmon influences phosphorescence on one hand and
TTA-UC on the other, both in terms of emission intensity and spatial extent of the
enhancement phenomena.
The samples, as schematically shown in Fig. 30.6a, are composed of a mesh of
silver nanowires (AgNW) randomly dispersed on a microscope cover glass substrate
(as seen in the dark-field image in Fig. 30.6b) and coated with a TTA-UC active material. The active material consists of donor molecules of platinum octaethylporphyrin
(PtOEP), and acceptor molecules of 9,10-diphenylanthracene (DPA) dispersed in a
matrix of poly(methyl methacrylate) (PMMA). Extinction and emission spectra of
the donor and acceptor molecules in solution are shown in Fig. 30.6c. The AgNW
extinction spectrum also shown in Fig. 30.6c has a maximum around 400 nm and
extends beyond 700 nm. This broadband absorption ensures that plasmon resonances
of AgNW couple with the absorption and emission of the chromophores dispersed
in the film and could potentially mediate and enhance the energy transfer processes.
Microscopic study of the photophysical processes of TTA-UC was carried out by
simultaneous imaging of phosphorescence from the donor molecules and UC fluorescence from the acceptor molecules using an image splitter. Microscopic phosphorescence image of the sample excited in the absorption Q-band of the donor PtOEP
M. Vacha and S. Hirata
In summary, we designed novel hybrid nanoparticles composed of FRET pair
dyes covalently attached to GNR. Using single-particle optical microscopy, we have
shown that energy transfer from a donor to an acceptor in the presence of a GNR
and, consequently, the emission color from the hybrid nanostructure can be reversibly
modulated by the state of the polarization of the incident light.
30.3.2 Plasmon Enhancement of Triplet Exciton Diffusion
in Photochemical Photon Upconversion
Photon upconversion (UC) based on triplet–triplet annihilation (TTA) between chromophores is a photochemical mechanism that can convert low energy photons to
higher energy ones [25]. The TTA-UC process is a sequence of excitation of a singlet
state of a donor, intersystem crossing into a triplet state of the donor, triplet–triplet
energy transfer from the donor to an acceptor, and TTA between two acceptors
leading to upconverted fluorescence from the acceptor singlet state. The TTA-UC
mechanism can be optimized by material design to achieve high UC quantum yield,
relatively low threshold of the UC emission, and broad and well-tunable excitation
and emission characteristics. There is also a considerable potential for utilizing the
effect of localized plasmon resonances of metal nanoparticles on the TTA-UC process
to further optimize the UC mechanism toward practical applications. Our goal here
was to gain insight into the effect of localized plasmons of metal nanostructures on
the individual photophysical steps involved in the TTA-UC process. We used optical
microscopy to study the properties of plasmonic hybrid nanostructures consisting of
silver nanowires combined with a TTA-UC film [11]. We find fundamental differences in the way the localized plasmon influences phosphorescence on one hand and
TTA-UC on the other, both in terms of emission intensity and spatial extent of the
enhancement phenomena.
The samples, as schematically shown in Fig. 30.6a, are composed of a mesh of
silver nanowires (AgNW) randomly dispersed on a microscope cover glass substrate
(as seen in the dark-field image in Fig. 30.6b) and coated with a TTA-UC active material. The active material consists of donor molecules of platinum octaethylporphyrin
(PtOEP), and acceptor molecules of 9,10-diphenylanthracene (DPA) dispersed in a
matrix of poly(methyl methacrylate) (PMMA). Extinction and emission spectra of
the donor and acceptor molecules in solution are shown in Fig. 30.6c. The AgNW
extinction spectrum also shown in Fig. 30.6c has a maximum around 400 nm and
extends beyond 700 nm. This broadband absorption ensures that plasmon resonances
of AgNW couple with the absorption and emission of the chromophores dispersed
in the film and could potentially mediate and enhance the energy transfer processes.
Microscopic study of the photophysical processes of TTA-UC was carried out by
simultaneous imaging of phosphorescence from the donor molecules and UC fluorescence from the acceptor molecules using an image splitter. Microscopic phosphorescence image of the sample excited in the absorption Q-band of the donor PtOEP