title: Bistability in Organic Magnetic Materials: A Comparative Study of the Key Differences between Hysteretic and Non-hysteretic Spin Transitions in Dithiazolyl Radicals.
abstract: Dithiazolyl (DTA)-based radicals have furnished many examples of organic spin-transition materials, some of them occurring with hysteresis and some others without. Herein, we present a combined computational and experimental study aimed at deciphering the factors controlling the existence or absence of hysteresis by comparing the phase transitions of 4-cyanobenzo-1,3,2-dithiazolyl and 1,3,5-trithia-2,4,6-triazapentalenyl radicals, which are prototypical examples of non-bistable and bistable spin transitions, respectively. Both materials present low-temperature diamagnetic and high-temperature paramagnetic structures, characterized by dimerized (⋅⋅⋅A-A⋅⋅⋅A-A⋅⋅⋅)nand regular (⋅⋅⋅A⋅⋅⋅A⋅⋅⋅A⋅⋅⋅A⋅⋅⋅)nπ-stacks of radicals, respectively. We show that the regular π-stacks are not potential energy minima but average structures arising from a dynamic inter-conversion between two degenerate dimerized configurations: (⋅⋅⋅A-A⋅⋅⋅A-A⋅⋅⋅)n↔(-A⋅⋅⋅A-A⋅⋅⋅A-)n. The emergence of this intra-stack dynamics upon heating gives rise to a second-order phase transition that is responsible for the change in the dominant magnetic interactions of the system. This suggests that the promotion of a (⋅⋅⋅A-A⋅⋅⋅A-A⋅⋅⋅)n↔(-A⋅⋅⋅A-A⋅⋅⋅A-)ndynamics is a general mechanism for triggering spin transitions in DTA-based materials. Yet, this intra-stack dynamics does not suffice to generate bistability, which also requires a rearrangement of the intermolecular bonds between the π-stacks via a first-order phase transition.
authors.affiliation: Departament de Ciència de Materials i Química Física, Universitat de Barcelona and IQTCUB, Av. Diagonal 645, 08028, Barcelona, Spain), E-mail.|Laboratoire de Chimie Quantique, Institut de Chimie, UMR 7177, CNRS-Université de Strasbourg, 1 Rue Blaise Pascal, 67000, Strasbourg, France.
authors.forename: Sergi
authors.initials: S
authors.lastname: Vela
authors.affiliation: Carlson School of Chemistry & Biochemistry, Clark University, 950 Main Street, Worcester, MA, 01610, USA.
authors.forename: Michael B
authors.initials: MB
authors.lastname: Reardon
authors.affiliation: Carlson School of Chemistry & Biochemistry, Clark University, 950 Main Street, Worcester, MA, 01610, USA.
authors.forename: Charles E
authors.initials: CE
authors.lastname: Jakobsche
authors.affiliation: Carlson School of Chemistry & Biochemistry, Clark University, 950 Main Street, Worcester, MA, 01610, USA.
authors.forename: Mark M
authors.initials: MM
authors.lastname: Turnbull
authors.affiliation: Departament de Ciència de Materials i Química Física, Universitat de Barcelona and IQTCUB, Av. Diagonal 645, 08028, Barcelona, Spain), E-mail.
authors.forename: Jordi
authors.identifier: http://orcid.org/0000-0003-4088-6187
authors.initials: J
authors.lastname: Ribas-Arino
authors.affiliation: Departament de Ciència de Materials i Química Física, Universitat de Barcelona and IQTCUB, Av. Diagonal 645, 08028, Barcelona, Spain), E-mail.
authors.forename: Juan J
authors.initials: JJ
authors.lastname: Novoa
keywords: computational chemistry; phase transitions; radicals; spin crossover