Unraveling two distinct polymorph transition mechanisms in one n-type single crystal for dynamic electronics

Abstract

Cooperativity is used by living systems to circumvent energetic and entropic barriers to yield highly efficient molecular processes. Cooperative structural transitions involve the concerted displacement of molecules in a crystalline material, as opposed to typical molecule-by-molecule nucleation and growth mechanisms which often break single crystallinity. Cooperative transitions have acquired much attention for low transition barriers, ultrafast kinetics, and structural reversibility. However, cooperative transitions are rare in molecular crystals and their origin is poorly understood. Crystals of 2-dimensional quinoidal terthiophene (2DQTT-o-B), a high-performance n-type organic semiconductor, demonstrate two distinct thermally activated phase transitions following these mechanisms. Here we show reorientation of the alkyl side chains triggers cooperative behavior, tilting the molecules like dominos. Whereas, nucleation and growth transition is coincident with increasing alkyl chain disorder and driven by forming a biradical state. We establish alkyl chain engineering as integral to rationally controlling these polymorphic behaviors for novel electronic applications.

Trimodal operation of a robust smart organic crystal

“…47,48 In turn, the mode of temperature variation can be preset to control the expansion or the contraction of the crystal. Since the two polymorphs are expected to have different properties, 49–53 we suggest that this thermal control over the phase transition could be applied for continuous, bidirectional actuation of other objects.…”

Direct Laser Writing Crystal Polymorphs of Organic Semiconductors for Phase Change Electronics

“…Previous work showed that two-dimensional quinoidal terthiophene (2DQTT- o -B) (Figure a) exhibits a rich set of polymorph phase transitions with drastically different electronic properties. ,, Powders and films typically form a metastable polymorph I′ initially (depending on solvent and drying process), which are annealed to the stable polymorph I (Figure b). Additionally, the hysteresis for the II–III transition is substantial at 78 °C, suggesting a large energy barrier for the polymorph transition.…”

“…In fact, this led to kinetic trapping of a metastable polymorph in crystals at a relatively modest cooling rate of 5 °C min −1 in single crystals. 20 Additionally, polymorph I exhibited high performance as an organic semiconductor with mobilities of 0.22 ± 0.07 cm 2 V −1 s −1 , whereas polymorph III (stable above 220 °C) showed poor mobility, 5 orders of magnitude less than polymorph I at (5.63 ± 4.75) × 10 −5 cm 2 V −1 s −1 measured in organic thinfilm transistors. 3 This offers a compelling opportunity to modulate device behavior by controlling the local crystal structure via laser-induced polymorph transitions, given that polymorph I and III are enantiotropically related and therefore thermally accessible and reversible.…”

“…We further investigated the polymorph packing through Raman spectroscopy, which allows for local determination of the crystal polymorph. In previous work, 20 several key vibrational modes were identified related to the alkyl chain packing and conjugated core. The alkyl chain packing is reflected in the C−C stretching peaks for the trans(ν(C−C) T 1025 cm −1 , pink) and gauche (ν(C−C) G 1063 cm −1 , green) isomers.…”

“…Recently, two-dimensional quinoidal terthiophene (2DQTT- o -B) has shown promise for dynamic phase change electronics, exhibiting a diverse set of polymorphic transitions. ,− Our previous work detailed two thermally accessible phase transitions undergoing cooperative (I–II) and nucleation and growth (II–III) mechanisms. The polymorph II–III transition exhibited a large hysteresis between II-to-III and III-to-II transition temperatures, allowing for the potential of kinetic trapping by rapidly cooling polymorph III.…”