Mixed stack charge transfer crystals: Crossing the neutral-ionic borderline by chemical substitution

We report extensive structural and spectroscopic characterization of four mixed stack charge-transfer (ms-CT) crystals formed by the electron donor 3,3^',5 ,5^' -tetramethylbenzidine (TMB) with Chloranil (CA), Bromanil (BA), 2,5-difluoro-tetracyanoquinodimethane (TCNQF₂), and tetrafluoro-tetracyanoquinodimethane (TCNQF₄). Together with the separately studied TMB-TCNQ [Phys. Rev. B 95, 024101 (2017), 10.1103/PhysRevB.95.024101] the TMB-acceptor series spans a wide range of degree of CT, from about 0.14 to 0.91, crossing the neutral-ionic interface, yet retaining similar packing and donor-acceptor CT integrals. First principle calculations of key phenomenological parameters allow us to get insight into the factors determining the degree of CT and other relevant physical properties.