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Y. Ishibashi and T. Asahi
two nanorods. The two nanorods showed different exciton lifetimes and spectral
shapes. Fig. 29.7b shows the relation between the lifetime and the peak ratio of
800 nm to 660 nm in the scattering spectra. When the peak ratio was larger than 1, the
lifetime was 200 ps. The nanorods of the peak ratio larger than 1 were assigned to be
β-phase and their lifetime was 200 ps. On the other hand, the lifetimes were scattered
from 200 ps to 600 ps for the nanorod having the ratio less than 1. These nanorods
could be considered to be α-phase or the mixture of α- and β-phases. We succeeded
in demonstrating the exciton relaxation dynamics of CuPc nanorods depending on
the crystalline phase.
29.4 Excited-State Relaxation Processes of µm-Sized Single
Organic Crystal
In the previous section, we described the results of the pump-probe microspectroscopy in the back-scattering mode and the excited-state dynamics of single organic
nanocrystals. Our microspectroscopic system, of course, is fruitfull to examine ultrafast excited state dynamics of μm-sized organic crystals. In the case of μm-sized
samples, the transmittance mode (absorption-based) observation permits to get
enough transient signals (>10
−3 OD) because the effect of stray light can be ignored
(see Sect. 29.2). Here, a boxcar average (SR205, Stanford Research) was used instead
of the lock-in amplifier, and a pocket cell (OG8/10, Avest) to reduce a repetition rate
of the oscillator to 10 kHz was inserted after the output of the femtosecond laser oscillator. The repetition rate of the optical chopper for pump pulses was set to 500 Hz.
Because the above setup also enables us to obtain time profile of transient absorbance
at single probe light wavelength, transient absorption spectra can be reconstructed
in the following manner. First, a kinetic trace of transient absorption at single wavelength by changing the delay time between the pump and probe pulses using the
optical delay is obtained. Second, time traces at various wavelengths are recorded
by considering the frequency chirp of the PCF-generated supercontinuum. Finally,
transient absorption spectra are constructed by obtained kinetic traces in arbitrary
wavelength range after correcting the frequency chirp at each probe wavelength. The
obtained data are analyzed by using commercial software (Igor 6) and homemade
programs. All the measurements can be performed under O 2 free condition.In the
following, we present the results of two organic solid systems; charge-transfer (CT)
complexes and rubrene investigated by using the transmittance mode femtosecond
pump-probe microspectroscopic system.
CT interaction in excited states plays fundamental and important roles in a lot of
photochemical and photophysical processes in condensed phases such as electron
and proton transfer, photoinduced charge transport of organic molecular crystals and
polymer films, and so on. Until now, there are enormous reports on synthesis of
various kinds of donor (D)-acceptor (A) systems and their excited-state dynamics by
Y. Ishibashi and T. Asahi
two nanorods. The two nanorods showed different exciton lifetimes and spectral
shapes. Fig. 29.7b shows the relation between the lifetime and the peak ratio of
800 nm to 660 nm in the scattering spectra. When the peak ratio was larger than 1, the
lifetime was 200 ps. The nanorods of the peak ratio larger than 1 were assigned to be
β-phase and their lifetime was 200 ps. On the other hand, the lifetimes were scattered
from 200 ps to 600 ps for the nanorod having the ratio less than 1. These nanorods
could be considered to be α-phase or the mixture of α- and β-phases. We succeeded
in demonstrating the exciton relaxation dynamics of CuPc nanorods depending on
the crystalline phase.
29.4 Excited-State Relaxation Processes of µm-Sized Single
Organic Crystal
In the previous section, we described the results of the pump-probe microspectroscopy in the back-scattering mode and the excited-state dynamics of single organic
nanocrystals. Our microspectroscopic system, of course, is fruitfull to examine ultrafast excited state dynamics of μm-sized organic crystals. In the case of μm-sized
samples, the transmittance mode (absorption-based) observation permits to get
enough transient signals (>10
−3 OD) because the effect of stray light can be ignored
(see Sect. 29.2). Here, a boxcar average (SR205, Stanford Research) was used instead
of the lock-in amplifier, and a pocket cell (OG8/10, Avest) to reduce a repetition rate
of the oscillator to 10 kHz was inserted after the output of the femtosecond laser oscillator. The repetition rate of the optical chopper for pump pulses was set to 500 Hz.
Because the above setup also enables us to obtain time profile of transient absorbance
at single probe light wavelength, transient absorption spectra can be reconstructed
in the following manner. First, a kinetic trace of transient absorption at single wavelength by changing the delay time between the pump and probe pulses using the
optical delay is obtained. Second, time traces at various wavelengths are recorded
by considering the frequency chirp of the PCF-generated supercontinuum. Finally,
transient absorption spectra are constructed by obtained kinetic traces in arbitrary
wavelength range after correcting the frequency chirp at each probe wavelength. The
obtained data are analyzed by using commercial software (Igor 6) and homemade
programs. All the measurements can be performed under O 2 free condition.In the
following, we present the results of two organic solid systems; charge-transfer (CT)
complexes and rubrene investigated by using the transmittance mode femtosecond
pump-probe microspectroscopic system.
CT interaction in excited states plays fundamental and important roles in a lot of
photochemical and photophysical processes in condensed phases such as electron
and proton transfer, photoinduced charge transport of organic molecular crystals and
polymer films, and so on. Until now, there are enormous reports on synthesis of
various kinds of donor (D)-acceptor (A) systems and their excited-state dynamics by