title: Bistability in Fc-PTM crystals: the role of intermolecular electrostatic interactions.
abstract: Fc-PTM is a valence tautomeric radical, where the ferrocene (Fc) group, a good electron donor, is linked by an ethylenic spacer to a perchlorotriphenylmethyl radical (PTM(*)), a good electron acceptor. In solution this compound exists mainly in the neutral Fc-PTM(*) form which can be photoexcited through an intramolecular electron transfer to the zwitterionic Fc(+*)-PTM(-) form. By contrast, in crystals of Fc-PTM at room temperature both the neutral and the zwitterionic forms coexist, pointing to a true bistability phenomenon. We rationalize these findings accounting for the role of intermolecular electrostatic interactions in Fc-PTM crystals. In fact the energy of the zwitterionic Fc(+*)-PTM(-) form is lowered in the crystal by attractive electrostatic intermolecular interactions and the cooperative nature of these interactions explains the observed coexistence of neutral Fc-PTM(*) and zwitterionic Fc(+*)-PTM(-) species. The temperature evolution of Mossbauer spectra of Fc-PTM is quantitatively reproduced adopting a bottom-up modeling strategy that combines a molecular model, derived from optical spectra of Fc-PTM in solution, with a model for intermolecular electrostatic interactions, supported by quantum-chemical calculations. Fc-PTM then offers the first experimental demonstration of bistability induced by electrostatic interactions in crystals of valence tautomeric donor-acceptor molecules.
authors.affiliation: Dipartimento Chimica GIAF, Parma University and INSTM-UdR Parma, I-43100 Parma, Italy.
authors.forename: Gabriele
authors.initials: G
authors.lastname: D'Avino
authors.forename: Luca
authors.initials: L
authors.lastname: Grisanti
authors.forename: Judith
authors.initials: J
authors.lastname: Guasch
authors.forename: Imma
authors.initials: I
authors.lastname: Ratera
authors.forename: Jaume
authors.initials: J
authors.lastname: Veciana
authors.forename: Anna
authors.initials: A
authors.lastname: Painelli