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Y. Ishibashi and T. Asahi
tens-nm to a few hundred-nm were prepared for various organic compounds, and the
nano-aggregation effects on the optical properties such as size-dependent absorption
and fluorescence spectra and aggregation-induced emission were examined [1, 2].
The mechanism has been discussed in terms of restricted intramolecular motions
and intermolecular electronic interactions in the confined space. Moreover, synergetic interactions between photo-excited molecules and/or the ground-state ones
have been attracting increasing interests since novel photochemical reactions hardly
realized in solution were induced in solid systems [3–5].
To elucidate the details of photochemical processes in nanoparticles, femtosecond
transient absorption spectroscopy is a fruitful technique, which makes it possible
to directly observe the ultrafast dynamics of excited states and transient intermediates. It is now recognized that pump-probe experiments for individual nanoparticles are important and indispensable for elucidating the ultrafast photochemical
dynamics. Thus, in the recent decade several pump-probe setups combining a farfield microscope and a femtosecond Ti:Sapphire oscillator have been reported and
applied to various kinds of nanomaterials such as metal nanoparticles and semiconductor nanowires, and organic nanocrystals [6–13]. In this chapter, we introduce our
femtosecond pump-probe microspectroscope using a femtosecond supercontinuum
as a probe pulse and compare transient signal sensitivity between the conventional
transmittance-mode detection and the back-scattering mode one of the probe light. It
was demonstrated that the detection of the back-scattered light from a single nanoparticle permitted 20 times higher gain of the transient signals in comparison with
the conventional absorption-based measurement. The high-sensitivity of the pumpprobe, back-scattering microspectroscopy allowed us to examine ultrafast excitedstate dynamics of single organic nanoparticles which in general are lower photodurability than inorganic ones. The size-dependent excimer formation dynamics of
perylene nanocrystals and the crystalline-phase dependence of the exciton decay
for copper phthalocyanine (CuPc) nanorods are demonstrated. Also, some applications of the femtosecond pump-probe microspectroscope to studies on the excited
state dynamics of μm-sized single crystals are demonstrated. We will discuss the
photoinduced charge separation dynamics in a ps-time scale for a weak chargetransfer complex and the effect of ultrafast transient laser heating on the singlet
fission dynamics of rubrene.
29.2 Setup of Femtosecond Pump-Probe
Microspectroscopic System
Figure 29.1a shows the schematic illustration of our femtosecond transient absorption microspectroscopic system [13]. A femtosecond Ti:Sapphire laser (Tsunami,
Spectra-Physics) pumped by a cw DPSS green laser (Millennia eV, Spectra-Physics)
was used as a light source. The output pulse (795 nm, 1 W, 90 fs fwhm, 82 MHz) was
Y. Ishibashi and T. Asahi
tens-nm to a few hundred-nm were prepared for various organic compounds, and the
nano-aggregation effects on the optical properties such as size-dependent absorption
and fluorescence spectra and aggregation-induced emission were examined [1, 2].
The mechanism has been discussed in terms of restricted intramolecular motions
and intermolecular electronic interactions in the confined space. Moreover, synergetic interactions between photo-excited molecules and/or the ground-state ones
have been attracting increasing interests since novel photochemical reactions hardly
realized in solution were induced in solid systems [3–5].
To elucidate the details of photochemical processes in nanoparticles, femtosecond
transient absorption spectroscopy is a fruitful technique, which makes it possible
to directly observe the ultrafast dynamics of excited states and transient intermediates. It is now recognized that pump-probe experiments for individual nanoparticles are important and indispensable for elucidating the ultrafast photochemical
dynamics. Thus, in the recent decade several pump-probe setups combining a farfield microscope and a femtosecond Ti:Sapphire oscillator have been reported and
applied to various kinds of nanomaterials such as metal nanoparticles and semiconductor nanowires, and organic nanocrystals [6–13]. In this chapter, we introduce our
femtosecond pump-probe microspectroscope using a femtosecond supercontinuum
as a probe pulse and compare transient signal sensitivity between the conventional
transmittance-mode detection and the back-scattering mode one of the probe light. It
was demonstrated that the detection of the back-scattered light from a single nanoparticle permitted 20 times higher gain of the transient signals in comparison with
the conventional absorption-based measurement. The high-sensitivity of the pumpprobe, back-scattering microspectroscopy allowed us to examine ultrafast excitedstate dynamics of single organic nanoparticles which in general are lower photodurability than inorganic ones. The size-dependent excimer formation dynamics of
perylene nanocrystals and the crystalline-phase dependence of the exciton decay
for copper phthalocyanine (CuPc) nanorods are demonstrated. Also, some applications of the femtosecond pump-probe microspectroscope to studies on the excited
state dynamics of μm-sized single crystals are demonstrated. We will discuss the
photoinduced charge separation dynamics in a ps-time scale for a weak chargetransfer complex and the effect of ultrafast transient laser heating on the singlet
fission dynamics of rubrene.
29.2 Setup of Femtosecond Pump-Probe
Microspectroscopic System
Figure 29.1a shows the schematic illustration of our femtosecond transient absorption microspectroscopic system [13]. A femtosecond Ti:Sapphire laser (Tsunami,
Spectra-Physics) pumped by a cw DPSS green laser (Millennia eV, Spectra-Physics)
was used as a light source. The output pulse (795 nm, 1 W, 90 fs fwhm, 82 MHz) was