A large photoinduced change in reflectivity has been observed in the low-temperature charge separated ͑CS͒ phase of a dimeric radical anion salt, Et 2 Me 2 Sb͓Pd͑dmit͒ 2 ͔ 2 ͑dmit=1,3-dithiol-2-thione-4,5-dithiolate͒. Just after the photoexcitation, the reflectivity abruptly changed reflecting the appearance of a photoinduced metastable state, indicating occurrence of recrystallizing of the CS phase by intradimer photoexcitation within a picosecond. Quantitative analysis considering the linear combination of the dielectric functions of the CS and the dimer-Mott state suggests a rather high efficiency of the photoinduced phase transition. One photon can change the valence of about five dimers. This photoinduced metastable state relaxed to the initial CS state via two successive types of relaxation processes, a fast and a slow one. The relaxation time ͑͒ and the reflectivity of the fast process showed a clear excitation intensity and temperature dependence. In particular, and the estimated domain size were enhanced up to the transition temperature ͑T c ͒ with increasing temperature. This phenomenon, a sort of critical slowing down around T c , suggests that the density of the photoinduced state as well as the external temperature plays an important role in determining the relaxation dynamics of the photoinduced state. The results obtained indicate that this photoinduced phenomenon can be classified as a tuning of the charge in crystals via cooperative interaction between the degrees of freedom of "charge" and "molecular orbital" of the constituents, i.e., as a type of photoinduced phase transition.
The photoinduced phase formation in a strongly correlated crystal (EDO-TTF) 2 PF 6 (EDO-TTF: 4,5ethylenedioxytetrathiafulvalene) is investigated using a 12 fs laser pulse. The formation time is determined as 40 fs with observation of coherence of electron-phonon coupled excited states prior to formation. The temperature-independent dephasing time is determined as ß22 fs up to 180 K and the frequency of phonon oscillation is ß38 THz, corresponding to the intramolecular vibrations in EDO-TTF. The phase formation is coherently controlled by relative-phase-controlled two-pulse excitation.Strongly correlated materials exhibit intriguing phases such as superconductor, Mott insulator, spin liquid, and charge density wave [1]. Competitions between electrons, phonons, and spins in the materials play a key role in creating these phases. Photoexcitation controls this competition and initiates cooperative response resulting in a macroscopic ordering, called photoinduced phase transition (PIPT). In PIPT, electronphonon interactions lead to significant changes in dynamics. Thus, not only pure electronic but also electron-phonon dynamics have attracted considerable attention. Time-resolved optical spectroscopy has been widely used to study the electronic and vibrational dynamics and subsequent PIPT by monitoring the transient changes in optical constants on a femtosecond time scale [2][3][4][5][6]. However, the interactions dominating electronic phases are often inferred indirectly from these measurements.Coherent nonlinear spectroscopy utilizing third-order nonlinear response is a powerful tool to directly investigate interactions and has been applied to reveal ultrafast dynamics in semiconductors [7,8], light-harvesting protein complexes [9][10][11][12], and other functional materials [13,14]. However, this spectroscopy has been applied only to a few strongly correlated materials [15] because their electronic coherence is considered to be too short owing to their strong electron-electron interaction. In this Rapid Communication we have investigated 10-fs time scale dynamics of the strongly correlated organic chargetransfer (CT) complex (EDO-TTF) 2 PF 6 (EDO-TTF: 4,5ethylenedioxytetrathiafulvalene) [16], which exhibits unique PIPT due to strong electron-phonon interactions. We also revealed the formation process of the photoinduced phase * onda.k.aa@m.titech.ac.jp from the phonon-coupled excited state as well as the role of coherences in the excited state.(EDO-TTF) 2 PF 6 undergoes metal-insulator phase transition at 280 K because of its strong electron-electron and electron-phonon interactions and shows (0110)-type charge ordering below the transition temperature [16], where the number in parentheses represent the order of charge for EDO-TTF molecules, as shown in the upper panel of Fig. 1(a). This material exhibits a two-step phase transition triggered by photoexcitation in the low-temperature phase, that is, "insulator"-"photoinduced phase"-"metal" transition. The first step takes place within 100 fs and the photoinduced ph...
Radical polymerization of lactic acid‐based chiral and achiral methylene dioxolanones, a model for conformationally s‐cis locked acrylate, was carried out with AIBN to demonstrate an isospecific free radical polymerization controlled by chirality and conformation of monomer. Polymerization of the dioxolanones proceeded smoothly without ring opening to give a polymer with moderate molecular weight and 100% of maximum isotacticity. ESR spectrum indicated a twisted conformation of the growing poly(methylene dioxolanone) radical in contrast to an acyclic analogous radical, suggesting a restriction of the free rotation around main chain CαCβ bond of the growing radical center. Chirality as well as the polarity and bulkiness of monomer affected the polymer tacticity, and chiral alkyl substituent would afford a high isotactic polymer, in which higher the enantiomeric excess of the monomer was, higher the isotacticity of the polymer was. While, achiral or polar substituents including dibenzyl and trichloromethyl groups would afford an atactic polymer. In addition, glass transition temperature (Tg) of the resulting polymers was significantly high, ranging from 172.2 to 229.8 °C, and even for an isotactic polymer Tg was as high as 206.8 °C. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 2007–2016
We investigated the excited state of a ring-shapeddinuclear Re(I) diimine bis-carbonyl complex that shows promise with regard to efficient photosensitized reactions and multi-electron storage,using time-resolved infrared(TR-IR) vibrational spectroscopy and quantum chemical calculations. Anomalouspeaks in the CO stretching region of the TR-IR spectrum werewell reproduced by the calculations. It was found that metal-to-ligand charge transfer occurs only in one of the Re complex units, andthat the excited state geometry is deformed relative to the ground state in order to relax the associatedsteric tension.Thisstructural deformationgenerates the unique photophysical properties of the complex.
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