29 Femtosecond Pump-Probe Microspectroscopy …
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time-resolved spectroscopies [26]. Among them, we focus here on the weak groundstate CT complexes, such as methoxy-substituted benzene derivatives as a donor and
pyromellitic dianhydride (PMDA) as a acceptor. Picosecond and femtosecond diffuse
reflectance spectroscopies (fs-DRSs) has been reported for microcrystalline powders
of these complexes [26]. For example, on CT crystals of 1,2,4-trimethoxybenzene
(124TMB)-PMDA, a broad PMDA anion-like band around 700 nm appeared in the
temporal response of the spectroscopic system (a few picoseconds) with the excitation
of the ground-state CT absorption and decayed with a time constant of 6 ps. Because
the spectral shape and the lifetime of PMDA anion-like band are obviously proven,
measurements of the solid crystal CT interaction dynamics are appropriate to evaluate
the developed spectroscopic system.
As a representative example, we present here the results of 1,2,3trimethoxybenzene (123TMB)-PMDA crystals. The sample CT crystals on a glass
plate were grown from the acetonitrile solution containing equimolar amounts of
123TMB and PMDA by slowly evaporating the solvent. A smooth and flat crystal
having the size with a few tens of μm was selected under the optical microscope.
Figure 29.8b shows time profiles of transient absorbance of 123TMB-PMDA single
microcrystal at several probe wavelengths, excited with a femtosecond 395-nm laser
pulse. These time profiles are corrected by taking the frequency chirp into account.
At all probe wavelengths, an appearance of positive absorbance with the temporal
response (350 fs) was followed by the decay to zero in 50 ps. On the basis of
these profiles, we constructed transient absorption spectra of 123TMB-PMDA single
microcrystal and displayed the results in Fig. 29.8c. Immediately after the excitation, a broadband around 680 nm was observed, and the spectral shape became sharp
together with the peak shift to 690 nm with delay time. At 2 ps, the shape was similar
to that of the reported PMDA anion radicals [26]. After 2 ps, the transient absorption
spectra monotonously decayed without any appreciable spectral change. Precisely
to discuss the CT interaction, the time profile at 690 nm was analyzed with a doubleexponential function of rise and decay components. The obtained time constants are
1.0 ps for the rise and 8.5 ps for the decay in Fig. 29.8d. The decay component was
in good agreement with the reported value by fs-DRS, which was assigned to the
lifetime of the relaxed CT excited state. That is, the electronic excitation is localized
in one D-A pair at 2 ps after the excitation. However, because the temporal resolution of our spectroscopic system is superior to that of the reported fs-DRS, the rise
component of 1.0 ps was clearly observed in our developed system. Considering
slight changes in the spectral shape in 2 ps, the rise component was assigned to the
relaxation from Franck–Condon state to the relaxed CT excited state.
Figure 29.9 shows the transient absorption spectra and the time profiles of
CT complex crystals of 1,2,4-trimethoxybenzene (124TMB)-PMDA, and 1,3,5trimethoxybenzene (135TMB)-PMDA. In both cases, the PMDA anion radical was
observed at 2 ps after the excitation and decayed to the baseline. The time profiles
at the peak of the PMDA anion radical were analyzed with a double-exponential
function, and obtained results are as follows: 0.8 ps (rise) and 6.0 ps (decay) for
124TMB-PMDA, and 1.2 ps (rise) and >600 ps (decay) for 135TMB-PMDA. We
firstly found that the decay of the relaxed CT excited state in each CT crystal was
505
time-resolved spectroscopies [26]. Among them, we focus here on the weak groundstate CT complexes, such as methoxy-substituted benzene derivatives as a donor and
pyromellitic dianhydride (PMDA) as a acceptor. Picosecond and femtosecond diffuse
reflectance spectroscopies (fs-DRSs) has been reported for microcrystalline powders
of these complexes [26]. For example, on CT crystals of 1,2,4-trimethoxybenzene
(124TMB)-PMDA, a broad PMDA anion-like band around 700 nm appeared in the
temporal response of the spectroscopic system (a few picoseconds) with the excitation
of the ground-state CT absorption and decayed with a time constant of 6 ps. Because
the spectral shape and the lifetime of PMDA anion-like band are obviously proven,
measurements of the solid crystal CT interaction dynamics are appropriate to evaluate
the developed spectroscopic system.
As a representative example, we present here the results of 1,2,3trimethoxybenzene (123TMB)-PMDA crystals. The sample CT crystals on a glass
plate were grown from the acetonitrile solution containing equimolar amounts of
123TMB and PMDA by slowly evaporating the solvent. A smooth and flat crystal
having the size with a few tens of μm was selected under the optical microscope.
Figure 29.8b shows time profiles of transient absorbance of 123TMB-PMDA single
microcrystal at several probe wavelengths, excited with a femtosecond 395-nm laser
pulse. These time profiles are corrected by taking the frequency chirp into account.
At all probe wavelengths, an appearance of positive absorbance with the temporal
response (350 fs) was followed by the decay to zero in 50 ps. On the basis of
these profiles, we constructed transient absorption spectra of 123TMB-PMDA single
microcrystal and displayed the results in Fig. 29.8c. Immediately after the excitation, a broadband around 680 nm was observed, and the spectral shape became sharp
together with the peak shift to 690 nm with delay time. At 2 ps, the shape was similar
to that of the reported PMDA anion radicals [26]. After 2 ps, the transient absorption
spectra monotonously decayed without any appreciable spectral change. Precisely
to discuss the CT interaction, the time profile at 690 nm was analyzed with a doubleexponential function of rise and decay components. The obtained time constants are
1.0 ps for the rise and 8.5 ps for the decay in Fig. 29.8d. The decay component was
in good agreement with the reported value by fs-DRS, which was assigned to the
lifetime of the relaxed CT excited state. That is, the electronic excitation is localized
in one D-A pair at 2 ps after the excitation. However, because the temporal resolution of our spectroscopic system is superior to that of the reported fs-DRS, the rise
component of 1.0 ps was clearly observed in our developed system. Considering
slight changes in the spectral shape in 2 ps, the rise component was assigned to the
relaxation from Franck–Condon state to the relaxed CT excited state.
Figure 29.9 shows the transient absorption spectra and the time profiles of
CT complex crystals of 1,2,4-trimethoxybenzene (124TMB)-PMDA, and 1,3,5trimethoxybenzene (135TMB)-PMDA. In both cases, the PMDA anion radical was
observed at 2 ps after the excitation and decayed to the baseline. The time profiles
at the peak of the PMDA anion radical were analyzed with a double-exponential
function, and obtained results are as follows: 0.8 ps (rise) and 6.0 ps (decay) for
124TMB-PMDA, and 1.2 ps (rise) and >600 ps (decay) for 135TMB-PMDA. We
firstly found that the decay of the relaxed CT excited state in each CT crystal was