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polymorphs of T T TA belong to different space groups and their

crystal packings feature notable differences. The columns of

radicals present in the X-ray structure of the LT phase are

distorted π-stacks that comprise slipped pairs of nearly eclipsed

radicals. By contrast, the columns of the X-ray structure of the HT

phase are regular π-stacks of radicals where each molecule

exhibits a slipped overlap with its two adjacent molecules along

the stacking direction. Besides, the T T TA molecules of the LT

polymorph are all arranged in parallel planes, whereas the HT

polymorph includes two distinct molecular-plane orientations.

‘Pair-exchange dynamics’

In a recent article published in

Nature Communications

(2014,

5:4411 doi: 10.1038/ncomms5411), we have uncovered, by

means of large scale

ab initio

molecular dynamics simulations,

that the regular stack motif of the HT polymorph of T T TAdoes not

correspond to a minimum in the potential energy surface (PES) of

the system. Instead, it is the resulting average structure of fast

intrastack pair-exchange dynamics, whereby T T TA radicals

continually exchange the adjacent T T TA neighbour (upper or

lower) with which they form an eclipsed dimer. An alternative,

albeit equivalent, way of visualising these pair-exchange dynamics

is displayed in Fig. 2, where it is shown that the regular stack is

the result of a rapid interconversion between two distorted stacks.

At room temperature, the regular stack motif is a minimum in the

free energy surface (FES) of the system. Upon cooling, the pair-

exchange dynamics slow down and the HT polymorph of T T TA

gradually undergoes (at around 200K) a second-order phase

transition within the π-stacks from a disordered state featuring the

pair-exchange dynamics into an ordered (or dimerised) state.

Despite the intrastack dimerisation taking place in the HT

polymorph at low temperatures, the minimum-energy structure of

HT at 0K is different from the structure of the LT polymorph

because of the different spatial arrangement of the T T TA

molecules in these two phases. In contrast to HT, the LT

polymorph remains in an ordered state within the 0-310K

temperature range.

The markedly distinct dynamic behaviour of the LT and HT phases

of T T TA imply that the LT to HT phase transition at 310K is in fact

an LT (ordered) to HT (disordered) phase transition. In other words,

besides the cleavage/formation of intermolecular bonds between

stacks associated with the first-order phase transition, the LT to HT

transition brings about a transformation of ordered stacks into

disordered stacks. This important discovery sheds light on a

crucial facet of the phase transition, namely the driving force of

the transition. Indeed, our simulations have revealed that the pair-

exchange dynamics present in HT and absent in LT are the origin

of a significant vibrational entropic gain in the LT to HT transition,

and thereby they play a key role in driving the phase transition.

The studies we are currently performing on other members of the

family of switchable DTA radicals are aimed at examining whether

our discoveries on the T T TA material are of general applicability

within this family of compounds. The demonstration that the fast

pair-exchange dynamics uncovered for T T TA are a universal

feature for the family of DTA radicals would undoubtedly

constitute a milestone that would pave the way to the rational

design of new switchable materials.

Professor Juan Jose Novoa

Co-ordinator

Novoa Group

Department of Physical Chemistry (IQTC)

School of Chemistry

University of Barcelona

tel :

+34 93402 1228

juan.novoa@ub.edu www.ub.edu/gem2/ www.horizon2020projects.com

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

I S S U E S E V E N

65

P R O F I L E

C H E M I S T R Y

Fig. 2 The regular π-stacks of the HT phase of TTTA (centre) result

from the dynamic interconversion between two distorted stacks (left

and right-hand side)

School of Chemistry & Physics, University of Barcelona