MS34-P02 An isosymmetric supercritical-like transition in an organic semiconductor Arie van der Lee (Université de Montpellier, Montpellier, France) Gilles Roche (Institut Charles Gerhardt de Montpellier, Laboratoire AM2N, UMR-5253, Université de Montpellier, ENSCM, CNRS, Montpellier, France) Guillaume Wantz (Université de Bordeaux, IMS, CNRS, Bordeaux, France) Joël Moreau (Institut Charles Gerhardt de Montpellier, Laboratoire AM2N, UMR-5253, Université de Montpellier, ENSCM, CNRS, Montpellier, France) Olivier Dautel (Institut Charles Gerhardt de Montpellier, Laboratoire AM2N, UMR-5253, Université de Montpellier, ENSCM, CNRS, Montpellier, France) Jean-Sébastien Filhol (Institut Charles Gerhardt de Montpellier, Laboratoire CTMM, UMR-5253, Université de Montpellier, ENSCM, CNRS, Montpellier, France)email: arie.van-der-lee@umontpellier.fr

We show that a high mobility thiophene-based organic semiconductor, BHH-BTBT, has an exceptionally large negative non-linear thermal expansion (-216 < α2 <  -333 MK−1) between 95 and 295K along the crystallographic b-axis, being compensated by an even larger positive expansion in the perpendicular direction (287 < α1 <  634 MK−1) [1]. The temperature evolution of the angle between neighbouring thiophene moieties  and the optical band gap show a clear transition around 210 K but no space group change is detected, i.e. the transition is isosymmetric. A complete theoretical characterization of BHH-BTBT using ab initio molecular dynamics shows that below ~ 200 K two different domains exist of which one is dominant but which dynamically coexist around and above 210 K. This transition is reminiscent to the ‘diffuse’ or ‘crossover’ phase transition analyzed theoretically by Christy [2] that leads to solid-state supercritical behaviour. The origin of the extreme negative and positive thermal expansion is related to a steric hindrance between adjacent tilted thiophene units and strongly enhanced by attractive S∙∙∙S and S∙∙∙C interactions within the highly anharmonic dynamic mixed-domain phase.

References:

[1] Van der Lee, A, et al, Chem. Sci., accepted, DOI: 10.1039/C8SC00159F

[2] Christy, A. G. (1995), Acta Cryst. B51, 753-757.

Keywords: thermal expansion, phase transition, supercritical transition