Visualization of Single-Crystal-to-Single-Crystal Phase Transition of Luminescent Molecular Polymorphs

Abstract

Single-crystal-to-single-crystal (SCSC) phase transition is an ideal model to study the structural correlation between the polymorphs at the molecular level. In this regard, a transition process with concomitant emission color change is in favor of direct visualization by fluorescence microscope. Here we report an SCSC transition on the luminescent single crystal of large conjugated molecules, which is accompanied by a drastic luminescence color change from red to orange upon heating. The transition process was clearly recorded under both fluorescence and polarized light. Combining with crystallographic analysis, the results indicate that the existence of molecular layers and the oriented motion of the interface between the daughter and the parent phase preserved the integrity of the single crystal, despite of remarkable changes of both conformational and supramolecular structure. Thus, the transition is rationalized to proceed by a nucleation-and-growth mechanism but not a martensitic one. This work on one hand delivers intuitive cognition about the polymorph-dependent optical properties and the mechanism of the phase transition between the polymorphs and, on the other hand, also proved the paramount importance of direct microscopy observation about the actual transition process.

Crossed 2D versus Slipped 1D π‐Stacking in Polymorphs of Crystalline Organic Thin Films: Impact on the Electronic and Optical Response

“…The device functionality of small‐molecule‐based organic optoelectronics depends strongly on the intermolecular arrangements as well as on morphological factors as intensively discussed in the past . This becomes apparent if the optical or electric response of different polymorphs of the same materials are considered . In particular, 2D arrangements—gathered via edge‐to‐face (herringbone) or slipped face‐to‐face (brickwork) packing motifs—were early realized as advantageous for efficient charge transport .…”

Robust Supramolecular Nano‐Tunnels Built from Molecular Bricks**

“…This indicates solvents in the voids of PCS S can be removed by heating to 100 °C. Notably, another single exothermic peak also appears at 220 °C which is ascribed to a crystalline phase transition [20] . Upon subsequent cooling, neither an exothermic peak nor an endothermic peak appears (blue curve in Figure 3 a), indicating that the phase transition at 220 °C is irreversible and the new form ( PCS T ) is thermodynamically more stable than PCS S .…”

“…Notably, another single exothermic peak also appears at 220 8 8Cw hich is ascribed to ac rystalline phase transition. [20] Upon subsequent cooling, neither an exothermic peak nor an endothermic peak appears (blue curve in Figure 3a), indicating that the phase transition at 220 8 8Cisirreversible and the new form (PCS T )i st hermodynamically more stable than PCS S .T of urther confirm this hypothesis,v ariable-temperature powder XRD (PXRD) data for PCS S were measured from 50 to 280 8 8C (Figure 3b). Its PXRD profile shows no obvious change below 140 8 8C.…”

A high contrast mechanochromic luminescent diacetylene-linked bis-benzothiadiazole derivative

“…Single crystals diffraction analysis is probably the most powerful tool to attain such understanding, as it provides accurate information on the relative arrangements of neighbouring molecule and the interactions involved among them. 9,10 However, although there are remarkable examples in the literature of molecular materials that retain their crystal integrity after a mechanical operation, [11][12][13][14] the emission switching between polymorphs is rather exceptional as it involves the transformation of the molecular structure in the constrained environment of the crystal. Notwithstanding, some design hints have been already proposed to induce a mechanochromic behaviour 15,16 among them the presence of weak interaction planes which allow the relative slippage of the molecules affecting the interactions with their next neighbours, and consequently their optical properties.…”