Reversible Spin Pairing in Crystalline Organic Radicals

Abstract

The concept of high spin and low spin configurations in d-block complexes is taught in every undergraduate introduction to the coordination chemistry of the transition metals. In this situation the interplay between interelectron repulsion or 'pairing energy' (P E ) and crystal (ligand) field splitting ( ) determines the electronic structure and provides an elegant example of how chemical tuning and modification of the ligand donor set can manipulate electronic structure. When is of a similar magnitude to P E then there is a fine balance between crystal field stabilisation energy (an enthalpic term which favours the low spin configuration) and maximising the number of microstates (an entropic term which favours the high spin configuration). In these cases spin-transitions typically occur between a low temperature enthalpically stabilised low spin configuration and a high temperature entropically favoured high spin configuration. The ability to drive spin-transitions thermally, through light-irradiation or pressure-induced transitions are the focus of the rest of this book. In this chapter we consider organic 'spin-transition' materials whose electronic structures can be manipulated in a conceptually similar but substantially different chemical fashion, that is by examining the fine interplay between inter-electron repulsion (P E ) and promotion energy to a low-lying vacant orbital ( ) within the context of organic chemistry.In order to understand the behaviour of such systems we shall begin with a discussion (Section 8.2) of stable free-radicals and their tendency to associate to form dimers, focusing on computational and experimental studies of the electronic structures of these dimers in the gas phase and in solution. In Section 8.3 we extend these discussions to the solid state and investigate examples in which we observe (i) thermal population of electronic excited states leading to a gradual thermal evolution of paramagnetism upon warming and (ii) firstorder solid state phase transitions in which bond cleavage can lead to abrupt diamagnetic-paramagnetic phase Spin-Crossover Materials: Properties and Applications, First Edition. Edited by Malcolm A. Halcrow.

Spin-state switches in molecular materials chemistry

“…As a result, magnetic transitions caused by radical dimerisations often exhibit thermal hysteresis. 27 Despite their underlying differences, these various types of spin-state switches exhibit very similar physical characteristics. They all lead to a change in magnetic moment, from a weakly paramagnetic (or diamagnetic) form at low temperature to a strongly paramagnetic state at high temperature.…”

“…26 Lastly, "on-off" switchable paramagnetism is also found in certain classes of conjugated radical, which can undergo reversible -dimerisation upon heating or cooling. 27 This results in strong antiferromagnetic pairing of the two radical centres, so the material becomes diamagnetic below the transition temperature; it also leads to a significant colour change. 28 Particularly large structural rearrangements are required to accommodate such a dimerisation in the solid state.…”

Radical–Radical Recognition: Switchable Magnetic Properties and Re-entrant Behavior

“…[36][37][38][39][40] These phase transitions are either the result of significant "plate slippage" of molecular/crystallographic planes 41 or of "tilting" of molecular planes in a "domino cascade". [42][43][44] Thus, although there is precedent for solid-state monomer-dimer phase transitions in related thiazyl species, the actual structural changes are only marginally comparable.…”

Breathing Some New Life into an Old Topic: Chalcogen-Nitrogen π-Heterocycles as Electron Acceptors

“…It is believed that magnetically bistable materials can find applications in molecular spintronic devices, i.e. , in magneto-thermal switching and information storage devices [30,34,51,52,53].…”