Masayuki Yoshizawa

Relaxation dynamics of photoexcitations in polydiacetylenes and polythiophene

The relaxation dynamics of photoexcitations in cast films of the polydiacetylenes, poly[4,6-decadiyne-1,10-diol bis([(n-butoxycarbonyl)methyl]urethane)] (PDA-3BCMU) and poly[5,7-dodecadiyne-1,12-diol bis([n-butoxycarbonyl)methyl]urethane)] (PDA-4BCMU), and in electrochemically prepared films of poly(3-methylthiophene) (P3MT) were investigated by femtosecond absorption and picosecond luminescence spectroscopies. 1Bu free excitons, generated by 100-fsec pump pulses, relaxed to the self-trapped (ST) state with time constants of 150 ± 50 fsec in PDA-3BCMU and 70 ± 50 fsec in P3MT. The difference was explained in terms of the relative sizes of the side chains. PDA-3BCMU has bulky side chains that are linked by hydrogen bonds, while P3MT has a repeating unit that contains a ring structure with a small methyl group. The lifetimes of the ST excitons at 10 K (290 K) were 2.0 ± 0.1 (1.5 ± 0.1) psec in PDA-3BCMU, 3.0 ± 0.3 (2.1 ± 0.2) psec in PDA-4BCMU, and 800 ± 100 (800 ± 100) fsec in P3MT. The decay kinetics of the ST exciton were explained by potential crossing and tunneling between two potential curves of the ST exciton and the ground state. The weak temperature dependence indicates that the activation process over the potential barrier between the ST exciton and the ground state is not dominant in the radiationless relaxation of the ST exciton. The relaxation dynamics of fluorescence from the PDA-4BCMU film cannot be represented by a single exponential decay; however, it can be described in terms of a random walk in the fractal dimension. By applying the fractal-dimension model, the spectral dimension was determined to be between 0.50 and 0.85. The spectral changes that are due to several nonlinear-optical processes, i.e., hole burning, Raman gain, and the dynamic Stark effect, were also observed in femtosecond-resolved spectra. The third-order susceptibility was determined for these nonlinear processes, such as absorption saturation at the exciton transition, Raman gain, and the resonant Kerr effect from the observed spectral change. The third-order susceptibility that corresponds to absorption saturation in PDA-3BCMU and in P3MT was obtained as Im[χ1111(3)(−ω;ω,ω,−ω)] equal to −2.6 × 10−9 and −3.5 × 10−10 esu, respectively, for ħω = 1.97 eV. The third-order susceptibility that corresponds to the Raman gain in PDA-3BCMU and in P3MT was determined to be Im[χ1111(3)(−ω2;ω2,ω1,−ω1)] equal to 5.8 × 10−10 and −1.5 × 10−10 esu, respectively, for ħω1 = 1.97 eV and ħω2 = 1.79 eV. From the resonant Kerr experiment in the P3MT film, |Δχ(3)|≡|χ1111(3)(−ω2;ω2,ω1,−ω1)−χ1122(3)(−ω2;ω2,ω1,−ω1)|=3.9×10−11 esu was determined for ħω1 = 1.97 eV and ħω2 = 1.88 eV.

Ultrafast optical response in polydiacetylenes and polythiophenes

Ultrafast optical response in the films of poly(3-dodecylthiophene) (P3DT) and blue-and red-phase polydiacetylenes (PDA-4BCMU) has been investigated by femtosecond absorption and picosecond luminescence spectroscopies. Several nonlinear optical processes, i.e., hole burning, Raman gain, inverse Raman scattering, and induced-frequency shift, have been observed. The relaxation processes from photoexcited free excitons to self-trapped excitons (STEs) has been observed. The time constant is estimated as 140±40 fs in the blue-phase PDA-4BCMU and 100±50 fs in P3DT. The generated unthermalized STEs thermalize with the time constant of about 1 ps. The STEs in the blue-phase PDA-4BCMU decay exponentially with lifetime of 1.6±0.1 ps at 290 K and 2.1±0.2 ps at 10 K. The decay curves in the red-phase PDA-4BCMU and P3DT are not single exponential but can be fitted to biexponential functions with time constants of slightly shorter than 1 ps and about 5 ps. These two decay time constants correspond to relaxations to the ground state, respectively, from the free exciton and unthermalized STE and from the thermalized STE.

The dependence of the ultrafast relaxation kinetics of the S2 and S1 states in β-carotene homologs and lycopene on conjugation length studied by femtosecond time-resolved absorption and Kerr-gate fluorescence spectroscopies

The ultrafast relaxation kinetics of all-trans-beta-carotene homologs with varying numbers of conjugated double bonds n(n=7-15) and lycopene (n=11) has been investigated using femtosecond time-resolved absorption and Kerr-gate fluorescence spectroscopies, both carried out under identical excitation conditions. The nonradiative relaxation rates of the optically allowed S(2)(1(1)B(u) (+)) state were precisely determined by the time-resolved fluorescence. The kinetics of the optically forbidden S(1)(2(1)A(g) (-)) state were observed by the time-resolved absorption measurements. The dependence of the S(1) relaxation rates upon the conjugation length is adequately described by application of the energy gap law. In contrast to this, the nonradiative relaxation rates of S(2) have a minimum at n=9 and show a reverse energy gap law dependence for values of n above 11. This anomalous behavior of the S(2) relaxation rates can be explained by the presence of an intermediate state (here called the S(x) state) located between the S(2) and S(1) states at large values of n (such as n=11). The presence of such an intermediate state would then result in the following sequential relaxation pathway S(2)-->S(x)-->S(1)-->S(0). A model based on conical intersections between the potential energy curves of these excited singlet states can readily explain the measured relationships between the decay rates and the energy gaps.

Relaxation dynamics of photoexcitations in polydiacetylene films

The relaxation dynamics of photoexcitations in a polydiacetylene (poly-3BCMU) film was investigated by femtosecond time-resolved absorption spectroscopy. Spectral change due to several nonlinear optical processes, i.e. hole burning, Raman gain, and dynamic Stark effect, was observed. The difference absorption spectra of singlet and triplet excitons minus ground state were measured. The photoexcited singlet excitons relaxed to the self-trapped state with the time constant of 150+or-50 fs. The lifetime of the self-trapped excitons was 2.0+or-0.1 ps at 10 K and 1.5+or-0.2 ps at 290 K. The lifetime of the triplet excitons was much longer than several tens of picoseconds and was consistent with the lifetime of 18 mu s obtained by nanosecond spectroscopy. >

Femtosecond study of S2 fluorescence in malachite green in solutions

Abstract By using femtosecond fluorescence spectroscopy, the relaxation kinetics of the second electronic excited state (S2) in malachite green has been investigated in several solvents with different viscosity. The decay time of the S2 fluorescence in water solution is obtained to be 0.27 ps and the internal conversion rate from the S2 state to the S1 state is estimated as (1.2 ps)−1. It has also been found that the S2 decay kinetics consists of fast and slow components in high-viscosity solvents. The S2 relaxation is explained in terms of the internal conversion and the torsional configuration change of molecules.

Exciton transition energy and temperature dependence of ultrafast relaxation of self-trapped excitons in polydiacetylenes

Abstract Exciton transition energies and temperature dependence of ultrafast relaxation processes have been investigated in polydiacetylene (PDA-(12,8) Langmuir-Blodgett films with several exciton transition energies using femtosecond absorption spectroscopy. The relaxation from self-trapped exciton to the ground state in PDA with 1.88 eV exciton energy is faster than that relaxation in PDA with larger exciton energy. The decay kinetics in PDAs can be explained in terms of tunneling in the configuration space from the self-trapped exciton to the ground state.

Ultrafast Hydrogen-Bonding Dynamics in the Electronic Excited State of Photoactive Yellow Protein Revealed by Femtosecond Stimulated Raman Spectroscopy

The ultrafast structural dynamics in the electronic excited state of photoactive yellow protein (PYP) is studied by femtosecond stimulated Raman spectroscopy. Stimulated Raman spectra in the electronic excited state, S(1), can be obtained by using a Raman pump pulse in resonance with the S(1)-S(0) transition. This is confirmed by comparing the experimental results with numerical calculations based on the density matrix treatment. We also investigate the hydrogen-bonding network surrounding the wild-type (WT)-PYP chromophore in the ground and excited states by comparing its stimulated Raman spectra with those of the E46Q-PYP mutant. We focus on the relative intensity of the Raman band at 1555 cm(-1), which includes both vinyl bond C═C stretching and ring vibrations and is sensitive to the hydrogen-bonding network around the phenolic oxygen of the chromophore. The relative intensity for the WT-PYP decreases after actinic excitation within the 150 fs time resolution and reaches a similar intensity to that for E46Q-PYP. These observations indicate that the WT-PYP hydrogen-bonding network is immediately rearranged in the electronic excited state to form a structure similar to that of E46Q-PYP.

Nanosecond time-resolved reflection spectrum of a polydiacetylene single crystal

Abstract A transient photoinduced reflection difference spectrum of a polymeric single crystal of 2,4-hexadiyn-1,6-di-p-toluenesulphonate was measured at room temperature. The recombination of the photoexcited species is dominated by one-dimensional random-walk kinetics which can be expressed by a single error function, erf[(τ/t)1/2]with τ = 0.75 ± 0.25 μs, over the delay time between 0 and 180 μs.

Confinement effect of photogenerated soliton-antisoliton pair on the ultrafast relaxation in a substituted polyacetylene

Femtosecond pump-probe experiments were performed on poly(o-(trimethylsilyl)phenylacetylene) thin films to investigate the effect of confinement of a photogenerated soliton-antisoliton pair on its ultrafast relaxation dynamics. The decay kinetics of transient photoinduced absorption due to the photoexcitations of the polymer were found to behave a erf (t/sup -n/) with n=0.65+or-0.05, implying a geminate recombination process of the soliton-antisoliton pair on a quasi-one-dimensional chain. The distance between the soliton and antisoliton could be estimated as about six repeat unit lengths, suggesting a fairly strong confinement of the pair. It is consistent with the size of a certain regular structure indicated to exist in this material by X-ray diffraction measurements. Creation of confined pairs of likely charged solitons due to interchain photoexcitation is also discussed in terms of the following energetically favored reaction between likely charged polarons on the same chain: P/sup +or-/+P/sup +or-/ to S/sup +or-/+S/sup +or-/. >

Fluorescence spectrum of a blue-phase polydiacetylene obtained by probe saturation spectroscopy

Abstract The dependence of the transient difference transmission spectrum on probe-light intensity was investigated for a blue-phase polydiacetylene with side groups of substituted urethanes. The spectra of stimulated emission and photoinduced absorption of the lowest 1 B u exciton were separately determined in the polymer, which exhibits extremely weak fluorescence with quantum efficiency −5 and ultrashort lifetime of 140 fs, by “probe saturation spectroscopy” newly proposed here. The fluorescence spectrum calculated from the stimulated emission spectrum has a peak near 1.9 eV and is an approximate mirror image to the stationary absorption spectrum. The relaxation time constant from the higher excited state m 1 A g exciton to the lowest 1 B u exciton is estimated to be shorter than 50 fs.