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+ρA−ρD+ρA−ρD+ρA−ρ ... 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.
Highlights
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+ρ A−ρ D+ρ A−ρ D+ρ A−ρ . . . with variable ionicity 0 < ρ < 1 and electron transfer t along the stack
We explicitly focus on the vibrational spectra across the continuous NIT of DMTTF-CA
The general situation discussed here with respect to modeling NIT is typical of the evolution of understanding of condensed phase systems
Summary
The Mulliken dimer illustrates CT in a simple two-level system. The neutral state | DAi describes an electron donor D and acceptor A molecule, both with closed shells. The energy difference between the ionic (I) and the neutral (N) states, 2z = I − A + V is a model parameter accounting for the ionization potential of D (I ), the electron affinity of A (A) and the Coulomb interaction V between the ions D+ and A−. The mean field treatment of a crystal of Mulliken dimers captures a central feature of NIT, the possibility of continuous or discontinuous ρ depending on ec /t. In this respect the electronic properties of ms-CT stacks mainly require a numerical analysis of the dependence of ρ on model parameters. DA dimers or dimer approximations to ms-CT stacks do not have inversion symmetry at D and A sites, which is a crucial property for the dimerization instability and for the spectroscopy of ms-CT crystals
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