Shinichiro Iwai

Strong Optical Nonlinearity and its Ultrafast Response Associated with Electron Ferroelectricity in an Organic Conductor

We report experimental evidence for the generation of ferroelectric polarization in an organic conductor α-[bis(ethylenedithio)tetrathiafulvalene] 2 I 3 obtained by optical second-harmonic generation. The spontaneous polarization emerges along with a metal-to-insulator transition that is driven by the Wigner crystallization of electrons. The strong optical nonlinearity and its ultrafast photoresponse demonstrated by this study exemplify the nature of the ferroelectric polarization that originates from the electron ordering.

Ultrafast fluorescence quenching by electron transfer and fluorescence from the second excited state of a charge transfer complex as studied by femtosecond up-conversion spectroscopy

Photoinduced electron transfer dynamics between fluorescer (acceptor, A) and quencher (donor, D) was investigated by measuring the fluorescence quenching using femtosecond up-conversion spectroscopy. The measurements were made in a quencher concentration range of 0.15 M–1 M and also in a neat quencher solvent. Fluorescence decay at times longer than 5 ps can be explained by combining the diffusion equation with the Marcus equation of electron transfer. At higher quencher concentrations (>0.3 M), an additional component with a time constant of ∼250 fs appears. At these concentrations, the fluorescers (9-cyanoanthracene, CA and 9,10-dicyanoanthracene) and the quenchers (N,N-dimethylaniline, DMA) were found to form “weak” CT complexes. Fluorescence from the S1 state of the CA-DMA complex was detected by steady state spectroscopy. The excitation spectrum observed at the maximum intensity of this fluorescence indicates the existence of an excited S2 state of the CT complex near the energy of D⋅A* (the locally ...

Ultrafast interfacial charge separation processes from the singlet and triplet MLCT states of Ru(bpy)2(dcbpy) adsorbed on nanocrystalline SnO2 under negative applied bias

We have observed the stimulated emission from the 1MLCT state of Ru(bpy)2(dcbpy) adsorbed on SnO2 nanocrystal by femtosecond pump–probe spectroscopy under applied bias (Vex) for the first time. The luminescence from the 3MLCT state has been also observed by picosecond time-resolved measurement. Observed lifetimes of the stimulated emission and the luminescence are 70 fs and 130 ps, respectively, at zero applied voltage. Both these lifetimes continuously increase with increasing negative bias and reach 320 fs (stimulated emission) and 6 ns (luminescence) at Vex=−0.7 V. The change in the lifetime of luminescence and of stimulated emission under negative applied bias is considered to be due to the change in the interfacial CS rate, although the possibility of the bias dependent desorption of the dye cannot be completely excluded. We analyzed the result by assuming that the changes in the lifetimes of the stimulated emission and the luminescence are due to the change in the interfacial CS rate constant. This ...

Ultrafast Phase Control in One-Dimensional Correlated Electron Systems

One-dimensional (1D) correlated electron systems are good targets for the exploration of photoinduced phase transitions (PIPTs). This is because photocarrier generations and/or charge transfer (CT) excitations by lights can stimulate instabilities inherent to the 1D nature of electronic states through strong electron–electron interactions and electron(spin)–lattice interactions. In this paper, we review the ultrafast dynamics of three typical PIPTs observed in 1D correlated electron systems: 1) a photoinduced transition from a Mott insulator to a metal in a halogen-bridged Ni-chain compound, [Ni(chxn) 2 Br]Br 2 (chxn = cyclohexanediamine); 2) a photoinduced melting of a spin-Peierls phase in an organic CT compound, K-tetracyanoquinodimethane (TCNQ); 3) a photoinduced transition between neutral (N) and ionic (I) states in an organic CT compound, tetrathiafulvalene- p -chloranil (TTF-CA). The primary dynamics of these PIPTs are discussed on the basis of the results of femtosecond pump–probe spectroscopy.

Ultrafast charge separation and exciplex formation induced by strong interaction between electron donor and acceptor at short distances

We have measured the decay of fluorescence from the electron acceptors (A) dissolved in donor (D) solvent by femtosecond up-conversion spectroscopy with a time resolution of 75 fs. The measurements have been made for several acceptors (9-cyanoanthracene, CA; 9,10-dicyanoanthracene, DCA; and 1,2,9,10-tetracyanoanthracene, TCA) in the donor solvent (N,N-dimethylaniline, DMA, and aniline, ANL). The decay times obtained are between 150 fs (CA–ANL) and 280 fs (TCA–DMA). Observation of the decay of acceptor fluorescence and the concurrent rise of fluorescence from the exciplex state indicates that the excited acceptor (A*) state directly relaxes to the exciplex state. Charge separation rates (kCS) determined from the decay of the acceptor fluorescence lie between 3.6×1012–6.7×1012/s for six combinations of donors and acceptors. Very weak energy gap (ΔE) dependence of kCS was observed in the region of ΔE of 0.36–1.47 eV. This relation between kCS and ΔE is quite different from the charge recombination rate (kCR)...

Collective excitation of an electric dipole on a molecular dimer in an organic dimer-Mott insulator

The terahertz response in 10-100 cm(-1) was investigated in an organic dimer-Mott (DM) insulator κ-(ET)(2)Cu(2)(CN)(3) that exhibits a relaxorlike dielectric anomaly. An ~30 cm(-1) band in the optical conductivity was attributable to collective excitation of the fluctuating intradimer electric dipoles that are formed by an electron correlation. We succeeded in observing photoinduced enhancement of this ~30 cm(-1) band, reflecting the growth of the electric dipole cluster in the DM phase. Such optical responses in κ-(ET)(2)Cu(2)(CN)(3) reflect an instability near the boundary between the DM-ferroelectric charge ordered phases.

Charge, Lattice, and Spin Dynamics in Photoinduced Phase Transitions from Charge-Ordered Insulator to Metal in Quasi-Two-Dimensional Organic Conductors

To elucidate the different photoinduced melting dynamics of charge orders observed in the quasi-two-dimensional organic conductors θ-(BEDT-TTF) 2 RbZn(SCN) 4 and α-(BEDT-TTF) 2 I 3 [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene], we theoretically study the photoinduced time evolution of charge and spin correlation functions on the basis of exact many-electron wave functions coupled with classical phonons in extended Peierls–Hubbard models on anisotropic triangular lattices. In both salts, the so-called horizontal-stripe charge order is stabilized by nearest-neighbor repulsive interactions and electron–lattice interactions. In θ-(BEDT-TTF) 2 RbZn(SCN) 4 (abbreviated as θ-RbZn), the stabilization energy due to lattice distortion is larger, so that a larger quantity of energy needs to be absorbed to melt charge and lattice orders. The photoinduced charge dynamics shows a complex behavior owing to a substantial number of nearly degenerate eigenstates involved. This is related to the high structural symmetr...

Optical freezing of charge motion in an organic conductor

Dynamical localization, that is, reduction of the intersite electronic transfer integral t by an alternating electric field, E(ω), is a promising strategy for controlling strongly correlated systems with a competing energy balance between t and the Coulomb repulsion energy. Here we describe a charge localization induced by the 9.3 MV cm(-1) instantaneous electric field of a 1.5 cycle (7 fs) infrared pulse in an organic conductor α-(bis[ethylenedithio]-tetrathiafulvalene)(2)I(3). A large reflectivity change of >25% and a coherent charge oscillation along the time axis reflect the opening of the charge ordering gap in the metallic phase. This optical freezing of charges, which is the reverse of the photoinduced melting of electronic orders, is attributed to the ~10% reduction of t driven by the strong, high-frequency (ω ≧ t/ħ) electric field.

Contribution of the ultrafast, short-distance intermolecular electron transfer to the fluorescence quenching rate in solution

In a previous paper [S. Iwai et al., J. Chem. Phys. 112, 7111 (2000)] we have found ultrafast electron transfer (ET) which occurs between donor and acceptor molecules at short distances. We incorporate this ultrafast ET in the calculation of the second-order ET rate in order to explain the discrepancies between experimental [D. Rehm and A. Weller, Isr. J. Chem. 8, 259 (1970)] and theoretical [M. Tachiya and S. Murata, J. Phys. Chem. 96, 8441 (1992)] ET rates. The effect of the short-distance ET is significant in the Marcus normal region where the Marcus-type ET is not very fast. Compared to the case where the original Marcus equation is used as the first-order ET rate, the calculated second-order rate is found to increase by several orders of magnitude in the normal region. Thus the discrepancy between theory and experiment is potentially resolved and the importance of the short-distance ET in fluorescence quenching demonstrated.

Franz–Keldysh oscillation in the interband absorption spectrum of one-dimensional bis(dimethylglyoximato)platinum(II) complex

Abstract The electronic structure of a one-dimensional metal complex, bis(dimethylglyoximato)platinum(II), was investigated by measurements of photoconductivity and electroabsorption (EA). The EA spectrum exhibited a large oscillatory signal just above the excitonic absorption band. The position of the oscillatory EA signal was located near the onset of the photoconductivity, and the oscillation profile showed field-dependent broadening. These features can be explained as the characteristics of the Franz–Keldysh (FK) effect. The oscillatory EA signal is reproducible by a numerical calculation of the FK effect, in which the effect of the one-dimensionality of this electronic system is taken into account.