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Ultrafast Electron and Molecular Dynamics in Photoinduced and Electric-Field-Induced Neutral–Ionic Transitions
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Crystals 2017, 7(5), 144; doi:10.3390/cryst7050144

Modeling the Neutral-Ionic Transition with Correlated Electrons Coupled to Soft Lattices and Molecules

1
Institut Néel, CNRS and Grenoble Alpes University, F-38042 Grenoble, France
2
Department of Chemistry, Life Science and Environmental Sustainability, Università di Parma, 43100 Parma, Italy
3
Department of Chemistry, Princeton University, Princeton, NJ 08544, USA
*
Author to whom correspondence should be addressed.
Academic Editors: Anna Painelli and Alberto Girlando
Received: 3 April 2017 / Revised: 2 May 2017 / Accepted: 7 May 2017 / Published: 16 May 2017
(This article belongs to the Special Issue The Neutral–Ionic Phase Transition)
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Abstract

Neutral-ionic transitions (NITs) occur in organic charge-transfer (CT) crystals of planar π -electron donors (D) and acceptors (A) that form mixed stacks ... D+ρAρD+ρAρD+ρAρ ... with variable ionicity 0 < ρ < 1 and electron transfer t along the stack. The microscopic NIT model presented here combines a modified Hubbard model for strongly correlated electrons delocalized along the stack with Coulomb intermolecular interactions treated in mean field. It also accounts for linear coupling of electrons to a harmonic molecular vibration and to the Peierls phonon. This simple framework captures the observed complexity of NITs with continuous and discontinuous ρ on cooling or under pressure, together with the stack’s instability to dimerization. The interplay of charge, molecular and lattice degrees of freedom at NIT amplifies the nonlinearity of responses, accounts for the dielectric anomaly, and generates strongly anharmonic potential energy surfaces (PES). Dynamics on the ground state PES address vibrational spectra using time correlation functions. When extended to the excited state PES, the NIT model describes the early (<1 ps) dynamics of transient NIT induced by optical CT excitation with a fs pulse. Although phenomenological, the model parameters are broadly consistent with density functional calculations. View Full-Text
Keywords: neutral-ionic phase transition; charge transfer crystals; multistability; correlated electron models; structural instabilities; coupling to molecular and lattice vibrations; anharmonic vibrational spectra; polarization and polarizability; photoinduced phase transitions neutral-ionic phase transition; charge transfer crystals; multistability; correlated electron models; structural instabilities; coupling to molecular and lattice vibrations; anharmonic vibrational spectra; polarization and polarizability; photoinduced phase transitions
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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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D’Avino, G.; Painelli, A.; Soos, Z.G. Modeling the Neutral-Ionic Transition with Correlated Electrons Coupled to Soft Lattices and Molecules. Crystals 2017, 7, 144.

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