Multiferroic materials represent a fascinating research field with enormous technological potential. Most of research is presently focused on inorganic multiferroics, whose physics and intrinsic limitations are fairly well understood. Comparatively, investigation on molecular multiferroics is still limited. Here [1] we introduce a new family of multifunctional molecular materials where new physics and new properties, including ferromagnetic and multiferroic behavior, emerge due to a subtle interplay between intramolecular electron transfer (IET) and intermolecular charge transfer (CT). Intermolecular CT interactions govern the behavior of CT salts, a large and widely investigated family of molecular materials, whose physics is driven by strongly correlated electrons on soft lattices in reduced dimensions, leading to multistability, photoinduced phase transitions, structural and electrical instabilities, charge ordering etc. CT salts span the whole range from insulators to metals and superconductors. Ferroelectricity was demonstrated and is actively investigated in several families of CT salts. IET is the key phenomenon in a variety of dyes and conjugated polymers that find use in OLED, organic solar cells and are exploited for their large nonlinear optical responses in bioimaging, nanofabrication, etc. The large nonlinearity of the responses of conjugated materials with low lying IET degrees of freedom shows up with a large and nontrivial sensitivity to the environment that eventually drives the system towards the rare, intriguing and technologically relevant phenomenon of multistability. Starting from chargetransfer crystals with a mixed stack motif, widely investigate for their ferroelectric behavior, we demonstrate via theoretical modelling that, decorating the crystal with stable organic radicals, it is possible to fine tune the CT and IET interplay as to obtain molecular materials with ferromagnetic as well as multiferroic behavior.

[1] Di Maiolo, F.; Sissa, C.; Painelli, A.; Combining intra- and intermolecular charge-transfer: a new strategy towards molecular ferromagnets and multiferroics. Sci. Rep. 6, 19682; doi: 10.1038/srep19682 (2016).