No abstract

We report extensive structural and spectroscopic characterization of four mixed stack chargetransfer (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 tetrafluorotetracyanoquinodimethane (TCNQF4). Together with the separately studied TMB-TCNQ [Phys. Rev. B 95, 024101 (2017)] the TMB-acceptor series span 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 allows us to get insight into the factors determining the degree of CT and other relevant physical properties.

Organic charge-transfer (CT) crystals constitute an important class of functional materials, characterized by the directional charge-transfer interaction between π-electron Donor (D) and Acceptor (A) molecules, with the formation of one-dimensional ...DADAD... stacks. Among the many different and often unique phenomena displayed by this class of crystals, Neutral-Ionic phase transition (NIT) occupies a special place, as it implies a collective electron transfer along the stack. The analysis of such a complex yet fascinating phenomenon has required many years of investigation, and still presents some open questions and challenges. We present an updated and extensive summary of the phenomenology of the temperature induced NIT, with emphasis on the spectroscopic signatures of the transition. A much shorter summary is given for the NIT induced by pressure. Finally, we report on the exploration, by chemical substitution, of the phase space of ...DADAD... CT crystals, aimed at finding materials with important semiconducting or ferroelectric properties, and at understanding the subtle factors determining the crystal packing.

The combination of different nondestructive spectroscopic techniques (Raman, infrared, luminescence) is shown to provide identification of quinacridone polymorphs when this well known pigment is used in organic electronics devices.