Synthesis of a Carbazole-[pi]-carbazole Molecular Rotor with Fast Solid State Intramolecular Dynamics and Crystallization-Induced Emission

Herein we report the synthesis of the highly stable crystalline carbazole-based rotor <b>3</b> with simultaneous rapid solid state internal rotation and good fluorescence emission. Single crystal and powder X-ray diffraction studies along with microscopy revealed a phase transition from a labile benzene solvate (phase <b>I</b>) to highly stable crystals (phase <b>II</b>) that feature fast intramolecular rotation in the megahertz regime at room temperature, according to variable temperature ²H solid state NMR experiments using isotopically enriched analogues. In addition to the megahertz rotation within its crystals, this crystal phase <b>II</b> displays enhanced solid state fluorescence with a higher quantum yield of ϕ = 0.28, relative to the emission of this compound in THF solution (ϕ = 0.06). These two solid state properties are significantly different from shorter compounds <b>1</b> and <b>2</b> (static and nonemissive) included here for comparison purposes

Transient Porosity in Densely Packed Crystalline Carbazole–(<i>p</i>‑Diethynylphenylene)–Carbazole Rotors: CO₂ and Acetone Sorption Properties

We report for the first time the high sorption properties of a molecular rotor with no permanent voids or channels in its crystal structure. Such crystalline phase originates from THF, DCM, or the irreversible desolvation of entrapped benzene molecules. From these, the benzene in its solvate form acts as rotation stopper, as supported by dynamic characterization using solid-state ²H NMR experiments. In the solvent-free form, the diffusion of small quantities of iodine vapors caused a significant change in the intramolecular rotation, increasing the known activation energy to rotation from 8.5 to 10.6 kcal mol^–1. Notably, those results paved the way for the discovery of the high CO₂ uptake (201.6 cm³ g^–1 at 196 K, under 1 atm) and acetone (5 wt %), a sorption property that was attributed to both, the restriction of the molecular rotation at low temperatures and the flexibility of the molecular axle made of conjugated <i>p</i>-(ethynylphenylene), surrounded by carbazole