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

Neutral-ionic transitions (NITs) occur in organic charge-transfer (CT) crystals of planar pi-electron donors (D) and acceptors (A) that form mixed stacks . . . D(+rho)A(-rho)D(+rho)A(-rho)D(+rho)A(-rho) . . . with variable ionicity 0 < rho < 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 rho 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.