Ichiro Akai

Temperature effects on quasi-isolated conjugated polymers as revealed by temperature-dependent optical spectra of 16-mer oligothiophene diluted in a sold matrix

Temperature dependences (4-300 K) of photoluminescence (PL) and absorption spectra of 16-mer oligothiophene (16 T) extremely diluted in polypropylene (PP) have been investigated in order to clarify temperature effects on quasi-isolated conjugated polymers. The PL and absorption spectra are found to blueshift with increasing temperature. The reason for the blueshift is discussed by comparing models based on the refractive index of the solvent (PP) and on the thermal conformational change of 16 T. The blueshift is concluded to result from the thermal conformational change. Time-resolved PL spectra show a redshift of PL band following photoexcitation (spectral migration). The amount of the migration is shown to increase with increasing temperature. The increased migration is concluded to be due to the thermal conformational change. The temperature dependence of the effective conjugation length (ECL) of 16 T is calculated for the absorption and PL transitions. The calculation suggests that ECL is reduced at room temperature to two-thirds of the intrinsic chain length. The activation energy of the conformational change is estimated to be 22.4 meV from the temperature dependence of ECL. We demonstrate that the steady-state PL spectra are well reproduced by simple Franck-Condon analyses using a single Huang-Ryes factor over a wide temperature range. The analyses reveal features of temperature dependence in important spectral parameters such as the Stokes shift, linewidth, and Huang-Ryes factor.

Atomistic mechanisms of rapid energy transport in light-harvesting molecules

Synthetic supermolecules such as π-conjugated light-harvesting dendrimers efficiently harvest energy from sunlight, which is of significant importance for the global energy problem. Key to their success is rapid transport of electronic excitation energy from peripheral antennas to photochemical reaction cores, the atomistic mechanisms of which remains elusive. Here, quantum-mechanical molecular dynamics simulation incorporating nonadiabatic electronic transitions reveals the key molecular motion that significantly accelerates the energy transport based on the Dexter mechanism.

Direct optical probing of negative carriers from an operating [6,6]-phenyl C61 butyric acid methyl ester diode

We have performed spectroscopic measurements combining with the diode operation of [6,6]-phenyl C61 butyric acid methyl ester (PCBM) for directly characterizing the nature of its n-carriers. The measurements in the visible region reveal that electrons for the n-carrier generation are primarily accommodated into the t1g and t1u levels. The measurements in the near-infrared region indicate that the n-carriers of the PCBM diode are delocalized compared to the PCBM anions in solution. We also show that the frequency-dependence of the spectroscopic signals can estimate the lifetime of the n-carriers in the PCBM diode (23 μs).

Rapid energy transfer in a dendrimer having π-conjugated light-harvesting antennas

We investigate rapid energy transfer (ET) and its temperature dependence in a star-shaped stilbenoid phthalocyanine (SSS1Pc) dendrimer having π-conjugated light-harvesting (LH) antennas, and develop an appropriate model. In SSS1Pc, an intense core photoluminescence (PL) band appears under the selective excitation of the absorption bands of the LH antenna due to highly efficient ET at room temperature (RT). The transient response of core-absorption bleaching and the temporal behaviours of the PL intensities of the core and antenna reveal that ET from the LH antenna occurs rapidly prior to achieving quasi-equilibrium in the photoexcited state of the LH antenna. In addition, it is also clarified that the ET quantum efficiency in SSS1Pc degrades at temperatures lower than ~100 K. To understand these results, we develop an ET model based on a π-conjugating network between the LH antenna and the core that accounts for steric hindrance between the LH antenna and the torsional vibration of the LH-antenna subunit. This model reveals that highly efficient ET occurs at RT through the π-conjugated network mediated by the thermally activated torsional vibration of the LH-antenna subunit.

Stress effects on nP yellow excitons in Cu2O thin films recrystallized epitaxially in a sample gap between paired MgO substrates

We investigated the stress effects on nP yellow excitons in Cu2O thin films recrystallized epitaxially in a sample gap between paired MgO substrates. In such samples, it is expected that a two-dimensional compressive stress acts on Cu2O because of the slightly larger lattice constant of Cu2O than of MgO. To clarify such stress effects, we measured the X-ray diffraction and nP absorption transitions of the yellow excitonic system and analyzed the strain and stress effects. Although the detected lattice strain and the energy shift of the yellow excitonic band gap are smaller than the values expected from the lattice mismatch at the heterointerface, this can be explained self-consistently by considering strain and stress relaxations in Cu2O thin films with departing from the MgO heterointerface. Consequently, we can find that shallow trapping potentials for the yellow excitons are formed in the ∼1.3-µm-thick region interfaced with the MgO substrates.

Bayesian Analysis of an Excitonic Absorption Spectrum in a Cu2O Thin Film Sandwiched by Paired MgO Plates

We analyzed the absorption spectrum of a Cu2O thin film sandwiched by paired MgO plates with the Metropolis algorithm of Bayesian estimation to estimate the energy shift of a yellow excitonic band gap. In the absorption spectrum, discrete excitonic resonant transitions with homogeneous and inhomogeneous widths are superimposed upon the tail parts of excitonic continuum and band-to-band absorption bands. On the basis of the Metropolis samplings with 2.5 million steps after sufficient burn-in steps, the spectral parameters of the excitonic resonant transitions can be estimated satisfactorily with a high degree of accuracy. In particular, we demonstrated that the yellow excitonic band gap in the Cu2O thin film shifts to the lower energy side than that of Cu2O bulk crystals. This result is consistent with the stress relaxation model for such Cu2O thin films, which was reported in Eur. Phys. J. B 86, 194 (2013).

A wavelength modulation system for highly sensitive absorption spectroscopy.

We developed a newly designed wavelength modulation (WM) system for highly sensitive absorption spectroscopy. In our system, the WM is realized by yawing an output mirror in a monochromator. In order to control an amplitude Δλ of the WM in a wide range, we employed a forced vibration of a permanent magnet driven by a magnetic field of a solenoid. Our system has an advantage of that the WM amplitude Δλ can be adjusted in extensively wide range from 0.08 nm to 11 nm only by tuning a driving frequency of the applying current to the solenoid, because we utilize a resonance phenomenon of the forced vibration for adjustment of the WM amplitude. By using our system, we measured WM absorption spectra of a Cu(2)O thin film and found clearly spectral structures for weak 2-4P excitonic resonances in the WM absorption spectra.

OPTICAL STARK EFFECTS ON THE STACKING FAULT EXCITONS IN BiI3

A study is made of the optical Stark effects on quasi two-dimensional excitons localized at a stacking fault in layered crystal BiI3. The dynamic coupling of the driven ground states and driven multiple exciton states by the strong photon field occurs for a wide frequency range of light, from negative to positive detuning. For the positive detuning excitation, an additional nonlinear effect peculiar to condensed matter was also observed. Time-resolved measurements in the sub-nanosecond and picosecond time domains confirmed ultrafast switching between the optical Stark effect and the additional nonlinear effect. From the dependence of the optical Stark shift on the excitation intensity, it is shown that the quasi two-dimensional excition transition has a very large dipole matrix element and third-order nonlinear susceptibility χ(3).

Coherent Phonons in a 1,3,5-Tri-Phenylbenzene Crystal

We studied low frequency vibrational modes in a light-harvesting (LH) antenna component molecule (1, 3, 5-tri-phenylbenzene: tri-ph) of a LH dendrimer by measuring a coherent phonon (CP) signal and molecular vibration analyses with semi-empirical molecular orbital calculations. In a Fourier transform spectrum obtained from the transient CP signal, we found three vibrating components with the frequencies of 35 ± 13, 60 ± 10, and 84 ± 14 cm−1. In the lower frequency range than 100 cm−1, the molecular vibration analyses give the following three kinds of vibrational modes: torsional vibrations, a butterfly vibration and bending vibrations, in which peripheral aromatic rings librate against the central ring in different manners. We identified the lowest frequency vibrating component (35 ± 13 cm−1) as the torsional vibrational mode changing dihedral angles and co-planarities between the central and the peripheral aromatic rings. In addition, it was also found that the vibrational frequencies obtained by our measurements are higher than those of the vibrational mode analyses. These hardenings are considered to account for steric hindrance between the neighbor molecules after the crystallization.

Thickness Dependence of Excitonic Spectra in Cu2O thin Films Sandwiched by MgO Plates

We investigated lattice distortion effects and their thickness dependence on the excitonic transitions in Cu2O thin films recrystallized in a small gap between paired MgO plates. Recently, Yoshiokaet al. reported the importance of exciton trapping into shallow potential minima for realizing the excitonic Bose-Einstein Condensation (BEC) in Cu 2O and it was also reported that uniaxial stresses are very useful to form the exciton trapping potentials. [2] In our investigation, we adopt Cu 2O thin films sandwiched by MgO plates because a small lattice mismatch between Cu 2O (4.273Å) and MgO (4.210Å) is expected to introduce compressive stresses in the Cu 2O thin films, which form the trapping potentials for the Cu 2O excitons. Figure 1 shows a schematic diagram of the cross section in our samples and the variation of the compressive stresses (horizontal arrows). As shown in this figure, compressive stresses due to the lattice mismatch are considered to relax gradually departing from the interfaces of MgO plates. Since it is expected that the degree of the lattice distortion effects varies with the sample thickness, we investigated thickness dependence of the excitonic spectra in Cu 2O thin films to clarify the lattice distortion effects and the formation of the exciton trapping potentials. Figure 2 shows the thickness dependence of the band gap energy shifts of the yellow excitonic system in Cu2O thin films from that in bulk crystals. In thick films B, C, and D, the red-shifts of the band gap are rather small. On the other hand, in a thin film A, one can recognize a larger red-shift. Consequently, by controlling their thickness, we can form an operative trapping potential for the yellow excitons to realize the excitonic BEC in the Cu 2O thin films.