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Crystals 2017, 7(5), 132; doi:10.3390/cryst7050132

Ultrafast Electron and Molecular Dynamics in Photoinduced and Electric-Field-Induced Neutral–Ionic Transitions

1
Department of Advanced Materials Science, University of Tokyo, Chiba 277-8561, Japan
2
AIST-UTokyo Advanced Operando-Measurement Technology Open Innovation Laboratory, National Institute of Advanced Industrial Science and Technology, Chiba 277-8568, Japan
*
Author to whom correspondence should be addressed.
Academic Editors: Anna Painelli and Alberto Girlando
Received: 1 April 2017 / Revised: 30 April 2017 / Accepted: 8 May 2017 / Published: 11 May 2017
(This article belongs to the Special Issue The Neutral–Ionic Phase Transition)

Abstract

Mixed-stacked organic molecular compounds near the neutral–ionic phase boundary, represented by tetrathiafulvalene-p-chloranil (TTF-CA), show a unique phase transition from a paraelectric neutral (N) phase to a ferroelectric ionic (I) phase when subjected to decreasing temperature or applied pressure, which is called an NI transition. This NI transition can also be induced by photoirradiation, in which case it is known as a prototypical ‘photoinduced phase transition’. In this paper, we focus on the ultrafast electron and molecular dynamics in the transition between the N and I states induced by irradiation by a femtosecond laser pulse and a terahertz electric-field pulse in TTF-CA. In the first half of the paper, we review the photoinduced N-to-I transition in TTF-CA studied by femtosecond-pump-probe reflection spectroscopy. We show that in the early stage of the transition, collective charge transfers occur within 20 fs after the photoirradiation, and microscopic one-dimensional (1D) I domains are produced. These ultrafast I-domain formations are followed by molecular deformations and displacements, which play important roles in the stabilization of photogenerated I domains. In the photoinduced I-to-N transition, microscopic 1D N domains are also produced and stabilized by molecular deformations and displacements. However, the time characteristics of the photoinduced N-to-I and I-to-N transitions in the picosecond time domain are considerably different from each other. In the second half of this paper, we review two phenomena induced by a strong terahertz electric-field pulse in TTF-CA: the modulation of a ferroelectric polarization in the I phase and the generation of a large macroscopic polarization in the N phase. View Full-Text
Keywords: photoinduced phase transition; neutral to ionic transition; organic molecular compound; ultrafast laser spectroscopy; terahertz spectroscopy; ferroelectricity photoinduced phase transition; neutral to ionic transition; organic molecular compound; ultrafast laser spectroscopy; terahertz spectroscopy; ferroelectricity
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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Morimoto, T.; Miyamoto, T.; Okamoto, H. Ultrafast Electron and Molecular Dynamics in Photoinduced and Electric-Field-Induced Neutral–Ionic Transitions. Crystals 2017, 7, 132.

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