Photoinduced Phase Transition in Strongly Electron-Lattice and Electron–Electron Correlated Molecular Crystals

Abstract

Strongly electron-lattice-and electron-electron-correlated molecular crystals, such as charge transfer (CT) complexes, are often sensitive to external stimuli, e.g., photoexcitation, due to the cooperative or competitive correlation of various interactions present in the crystals. These crystals are thus productive targets for studying photoinduced phase transitions (PIPTs). Recent advancements in research on the PIPT of CT complexes, especially Et 2 Me 2 Sb[Pd(dmit) 2 ] 2 and (EDO-TTF) 2 PF 6 , are reviewed in this report. The former exhibits a photoinduced insulator-to-insulator phase transition with clearly assigned spectral change. We demonstrate how to find the dynamics of PIPT using this system. The latter exhibits a photoinduced hidden state as an initial PIPT process. Wide energy ranged time-resolved spectroscopy can probe many kinds of photo-absorption processes, i.e., intra-molecular and inter-molecular electron excitations and intramolecular and electron-molecular vibrations. The photoinduced spectral changes in these photo-absorption processes reveal various aspects of the dynamics of PIPT, including electronic structural changes, lattice structural changes, and molecular deformations. The complexities of the dynamics of the latter system were revealed by our measurements. OPEN ACCESSCrystals 2012, 2 1068

Time-Resolved Infrared Vibrational Spectroscopy of the Photoinduced Phase Transition of Pd(dmit)2 Salts Having Different Orders of Phase Transition

“…Among the materials exhibiting PIPT, we have intensively studied organic charge transfer (CT) complexes . CT complexes are single crystals consisting of small π-electron molecules and exhibit a wide variety of phases, such as metal, charge-ordered insulator, Mott insulator, and superconductor, by applying weak external stimuli, because they are essentially strongly correlated electron systems. − Moreover, CT complexes are suitable for applying time-resolved vibrational spectroscopy because their constituent molecules include the CC stretching vibrational modes whose energies and intensities are sensitive to the structure and charge of the molecules. − We have found that approximately 100 ps are required for the emergence of the high-temperature phase in some types of CT complexes. ,,, To elucidate the origin of this slow emergence of the high-temperature phase, it is useful to compare two samples that have the same characteristics except for one distinction.…”

“…16−19 Among the materials exhibiting PIPT, we have intensively studied organic charge transfer (CT) complexes. 22 CT complexes are single crystals consisting of small π-electron molecules and exhibit a wide variety of phases, such as metal, charge-ordered insulator, Mott insulator, and superconductor, by applying weak external stimuli, because they are essentially strongly correlated electron systems. 23−28 Moreover, CT complexes are suitable for applying time-resolved vibrational spectroscopy because their constituent molecules include the CC stretching vibrational modes whose energies and intensities are sensitive to the structure and charge of the molecules.…”