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R

ecent years have witnessed a growing interest in the

research of molecular materials that can be switched

between two different states through the application of an

external stimulus (e.g. heat, light) because these materials have a

great potential for application in sensors, displays and information

technology. The technological interest of switchable materials is

even greater when the corresponding phase transition between

the two different states occurs with an hysteresis loop, thereby

giving rise to a bistable material, i.e. a material that exists in two

interchangeable phases under identical conditions. Most of the

switchable compounds so far reported are based on transition

metal ions (especially Fe(II)-based spin-crossover complexes)

whose spin transitions (i.e. transitions between electronic states

with different numbers of unpaired electrons) bring about changes

in the material’s magnetic moment, colour and electrical

conductivity. However, remarkable work conducted on persistent

organic radicals over the past years strongly suggests that these

systems can also be at the forefront in the design and fabrication

of the next generation of sensors and electronic devices.

Switchable molecule-based magnets

In the Molecular Materials Structure Group at the University of

Barcelona we use different methods of theoretical and

computational chemistry in order to achieve a deep

understanding of the structure of ‘switchable molecule-based

magnets’ with potential technological interest and, in turn, of the

relationship between their structure and their magnetic properties.

Note that by switchable molecule-based magnets we mean those

materials that switch between two different spin states irrespective

of whether the spin transition occurs with or without hysteresis. In

particular, our group currently works on three different lines of

research which are intimately intertwined. The first line of research

focuses on the structural study of the different polymorphs

involved in a spin transition with the purpose of identifying the

mechanism of the spin-state transition, its feasibility (after

estimating the thermodynamic and kinetic parameters), and the

key factors that govern the transition temperature. The second line

of research deals with the study of the magnetic properties of all

polymorphs participating in the spin-state transition, in order to

determine which structural changes are responsible for the

variation of their observed magnetic properties. Finally, the third

line of research is aimed at developing efficient computational

strategies for the accurate prediction of the optimum geometrical

structure of polymorphs. It is worth mentioning that our efforts in

crystal structure prediction, which involves the theoretical study of

intermolecular interactions, are not only devoted to bistable

molecular materials but also to the study of polymorphism in

crystals that are relevant for the pharmaceutical industry. Overall,

the computational work carried out in our group should enable a

sound rationalisation of the properties of the currently known

molecule-based materials showing spin-state transitions, and

should ultimately lead to the rational design of new tailored

magnets with improved properties.

Dithiazolyl-based materials

Over the last five years, our group has intensively investigated the

spin transitions undergone by several Fe(II)-based spin-crossover

complexes and the spin transitions undergone by several organic

materials. Among the different systems studied, we would like to

highlight our work on dithiazolyl-based materials. Several

π-stacked 1,3,2 dithiazolyl (DTA) radicals have been shown to

feature spin-transition behaviour between a low temperature (LT)

diamagnetic distorted stack of π-radical pairs and a high

temperature (HT) paramagnetic regular π-stack. The neutral

radical 1,3,5-trithia-2,4,6-triazapentalenyl (T T TA), which belongs

to the family of DTA radicals, is one of the most prominent

materials in the field of molecular magnetism, not least because

it undergoes a magnetic phase transition that occurs with an

exceptionally wide thermal hysteresis loop that encompasses

room temperature (Fig. 1). Furthermore, the phase transition of

T T TA can also be driven by light irradiation. On heating above the

bistability temperature range of T T TA , only the HT paramagnetic

phase is observed. On cooling below the bistability range, in turn,

the LT diamagnetic phase is exclusively detected. The two

Studies at the Novoa Group of the Department of Physical Chemistry, University

of Barcelona, work towards uncovering the pathway to the rational design of

novel switchable materials

Bistable molecule-based materials

64

I S S U E S E V E N

H O R I Z O N 2 0 2 0 P R O J E C T S : P O R TA L

www.horizon2020projects.com

P R O F I L E

C H E M I S T R Y

Fig. 1 Temperature dependence of the magnetic susceptibility for TTTA

on cooling (downward triangles) and on heating (upward triangles).

The inset shows the molecular structure of a TTTA neutral radical