Tunable Mechanical and Optoelectronic Properties of Organic Cocrystals by Unexpected Stacking Transformation from H- to J- and X-Aggregation

Molecular stacking modes, generally classified as H-, J-, and X-aggregation, play a key role in determining the optoelectronic properties of organic crystals. However, the control of stacking transformation of a specific molecule is an unmet challenge, and a priori prediction of the performance in different stacking modes is extraordinarily difficult to achieve. In particular, the existence of hybrid stacking modes and their combined effect on physicochemical properties of molecular crystals are not fully understood. Herein, unexpected stacking transformation from H- to J- and X-aggregation is observed in the crystal structure of a small heterocyclic molecule, 4,4′-bipyridine (4,4′-Bpy), upon coassembly with N-acetyl-l-alanine (AcA), a nonaromatic amino acid derivative. This structural transformation into hybrid stacking mode improves physicochemical properties of the cocrystals, including a large red-shifted emission, enhanced supramolecular chirality, improved thermal stability, and higher mechanical properties. While a single crystal of 4,4′-Bpy shows good optical waveguiding and piezoelectric properties due to the uniform elongated needles and low symmetry of crystal packing, the significantly lower band gap and resistance of the cocrystal indicate improved conductivity. This study not only demonstrates cocrystallization-induced packing transformation between H-, J-, and X-aggregations in the solid state, leading to tunable mechanical and optoelectronic properties, but also will inspire future molecular design of organic functional materials by the coassembly strategy.

4,4'-Bpy and 450-650 nm for 4,4'-Bpy/AcA with a slit of 5 nm. The air was used as background and subtracted. DSC and TGA. Thermal stability of the crystals (4,4'-Bpy, AcA, or 4,4'-Bpy/AcA) was examined by melting and decomposition temperatures using a Differential Scanning Calorimetry (NETZSCH STA 449F5, Germany) at a heating rate of 5 K min -1 under nitrogen atmosphere with a flow rate of 50 mL min -1 and at a temperature range between 50 °C and 500 °C. Powder X-ray diffraction (PXRD). PXRD spectra of all the crystals (4,4'-Bpy, AcA, or 4,4'-Bpy/AcA) were recorded using a BRUKER d8 ADVANCE DIFFRACTOMETER equipped with Goebels mirrors to parallelize the beam and LYNXEYE-XE linear detector.
Raman spectroscopy. Raman spectra of the crystals (4,4'-Bpy, AcA, or 4,4'-Bpy/AcA) were collected using a spectrophotometer from Horiba Jobin Yvon LabRAM HR. The maximal power of the laser on the spot was 10 mW. The laser was a frequency doubled Nd:Yag with 532nm wavelength. Edge filter was used to cut the laser line. The detector was a Synapse CCD with thermoelectric cooling operating at -70 °C. The grating was 600 holes/mm (denoting the number of holes in the grating per millimeter). FTIR spectroscopy. All the crystals (4,4'-Bpy, AcA, or 4,4'-Bpy/AcA) were deposited onto a real crystal KBr IR card (International Crystal Labs, Garfield, New Jersey, USA). The FTIR spectra of all samples were recorded on a Nicolet 6700 FTIR spectrometer (Thermo Scientific, Waltham, Massachusetts, USA), from 4000 to 400 cm -1 at room temperature. The background signal was recorded using blank and subtracted to obtain each FTIR spectrum.
Nuclear magnetic resonance (NMR). The powder of single crystals and co-crystal (4,4'-Bpy, AcA, or 4,4'-Bpy/AcA) was dissolved in D 2 O at the concentration of 5 mg/mL. 1   Optical waveguiding. To evaluate the potential of the 4,4'-Bpy crystals as active waveguide materials, rhodamine 6G (Rh6G) was used as a probe. Rh6G (1 mg/mL) in double distilled water was incubated with 4,4'-Bpy needles for overnight, leading to the spontaneous accommodation of the dye within the needles. The crystals were then cleaned with double distilled water three times to remove residual Rh6G. The optical waveguide image was obtained using an Olympus FV1000 microscope with a CCD camera or a Leica DM IRBE microscope equipped with a digital camera. A laser with an excitation source of 488 nm was employed for the measurement.
A schematic diagram of the experimental setup for the 4,4'-Bpy crystal waveguide characterization is shown in Figure S19.
Piezoelectricity prediction. Calculations are performed using periodic DFT 8 with the VASP 9 code. Electronic structures were calculated using the PBE functional 10 , projector augmented S5 wave (PAW) pseudopotentials 11 with a plane wave cut-off of 600 eV, and k-point sampling of 4x4x4. A finite differences method was used to calculate the stiffness tensor, with each atom being displaced in each direction by ± 0.01 Å (plane wave cut-off of 600 eV, and k-point sampling of 2x2x2). Piezoelectric strain constants and dielectric tensors were calculated using Density Functional Perturbation Theory 12 (DFPT) (plane wave cut-off of 600 eV, and k-point sampling of 2x2x2). It is important to note that negative piezoelectric constants can appear counterintuitive but are becoming more accepted [13][14][15][16] . They can be seen in classical piezoelectric materials like AlN 17 and quartz 18 . Negative e ij values have been investigated in detail by Liu and Cohen 19 using DFT methods similar to those presented here. Assuming positive elastic stiffness constants, these materials will also demonstrate negative d ij (and g ij ) constants, as can be     Figure S6. Raman spectroscopy measurements using a laser to detect the signal of crystal surface.                   Table S9. Calculated charge tensor components e ij (C/m 2 ), strain tensor components d ik (pm/V), and voltage tensor components g ij (mV m/N), of the 4,4'-Bpy/AcA co-crystal.