Multifunctional Charge-Transfer Single Crystals through Supramolecular Assembly

Abstract

Centimeter-sized segregated stacking TTF-C60 single crystals are crystallized by a mass-transport approach combined with solvent-vapor evaporation for the first time. The intermolecular charge-transfer interaction in the long-range ordered superstructure enables the crystals to demonstrate external stimuli-controlled multifunctionalities and angle/electrical-potential-dependent luminescence.

Keywords: charge transfer; magnetoelectrics; materials design; self-assembly.

Figure 1.

Schematic illustration of the crystal growth methods, the morphology, structure, and packing of TTF-C₆₀ charge-transfer crystals (TCCTs). a) Crystal growth scheme. b) Photograph of TCCTs at the highest concentration of TTF. c,d) SEM images of TCCTs. TTF concentrations are 16.7 and 25 mg mL⁻¹, respectively. The C₆₀ concentration is maintained at 25 mg mL⁻¹. The scale bars for SEM images are 10 μm. e) Crystal structure of C₆₀-TTF-DCB complex in an asymmetric unit without disordered parts. The 50% probability ellipsoids of thermal vibrations are shown. The four C₆₀ molecules have different orientations and these are colored in red, cyan, blue, and purple. The crystallographic nonequivalent molecules are shown in different colors. f) Projection of C₆₀-TTF crystal packing along a axis. g) Double conformation of C₆₀ on the site close to the middle of b cell edge created in crystallographic object-oriented toolkit (COOT).^[46,48] The high occupancy (0.81) part is presented as blue line with the 2Fo-Fc electron density map in blue mesh at 2σ level. The low occupancy part (0.19) is presented as green line with Fo-Fc difference electron density map in green mesh at 5σ level. The density map was calculated by SHELXL with the minor part omitted and the major part at 0.81 occupacy fixed. Part (e) was drawn by Mercury,^[47] the C₆₀ molecules are shown as ellipsoids, TTF are shown as sticks, and DCBs are shown as wireframe.

Figure 2.

Optical, electrical properties, magnetic field effect on photoluminescence properties and MC of TCCTs. a) Absorption spectra of TCCTs. b) Potential-dependent photoluminescence spectra for emission polarized at 0° relative to the excitation polarization. The inset in (b) is the ratio of the emission intensities collected at 90° and 0° as a function of applied potential. c) Magnetic-field-dependent photoluminescence spectra at 0° of emission. The inset is the dark and photoexcited MC. d) Electric-field and magnetic-field-dependent MC change, respectively.

Figure 3.

Ferroelectric and dielectric properties of TCCTs. a,b) Piezoelectric force microscopy image. The square in the center is the polarized area under a voltage of 54 and −72 kV cm⁻¹, respectively. The scale bar is 5 μm. c,d) The phase and amplitude hysteresis loop for piezoelectric response. The inset in (c) is the PUND polarization measurement of TCCTs. e) Frequency-dependent permittivity at different temperatures. The inset is the temperature-dependent permittivity at 100 Hz. f) Magnetic-field and light-intensity-dependent permittivity change. The inset is the light-intensity-dependent polarization value change.

Figure 4.

Magnetism and magnetoelectric coupling properties of TCCTs. a) Electric field tuning of magnetization of TCCTs under dark and photoexcitation conditions at different electric field. The light intensity is 70 mW cm⁻². b) The orientations related to ferromeagnetism and paramagnetism by projecting the structure of C₆₀ molecules along 010 direction created by PyMOL.^[48] The 6–6 double bonds in upper layer are colored in red. c) The configurations of charge and spin in TCCTs. The interactions between molecules are presented as 1D chain. Purple circles, shaded yellow ovals, gray arrows, and purple arrows represent molecules, electron distributions, electric dipoles, and spins, respectively, d) The loading of electric field along the chain direction with the enhancement of both polarization and magnetization.