Crystal Growth, Structure And Phase Transition Of Cyano-substitutent Distyrylbenzene Derivatives

Crystalline polymorphism of organic compounds is the phenomenon of an organicsolid material to exist in more than one form or crystal structures. This phenomenon iswell prevalent for which there are two reasons. On the one hand, the driven forces fororganic crystal growth are non–covalent interactions which are very weak and susceptibleto growth conditions, leading to the alteration in molecular packing; on the other hand,the organic molecular conformations are variable giving rise to the conformationalpolymorphs. Single crystal diffraction analysis can be viewed as one of efficient way toacquire the information of structure, but it suffers from the often insurmountablechallenges relating to the availability of suitable crystals. So it is essential to have anin-depth research on crystal preparation, crystal growth mechanism, the relationshipbetween structure and characters and the mechanism of phase transition, then obtaininghigh quality crystal for X-ray diffraction analysis. Also, it is a subtle way for modulationthe characters of materials through studying the principle of molecular arrangement incrystal in detail and realizing the control over the production of different polymorphicforms.Two kind of different emitting color α-CNDSB crystals have been prepared throughsolvent diffusion and physical vapor transport (PVT) method. Their crystal structures andmolecular conformations have been studied by single crystal diffraction and XRD, andfund there are two distinctions in the conformations of these two kinds of crystal: themolecule is very torsional in crystal Ⅰ, whereas it is planar in crystal Ⅱ. It is just theflexibility of the molecule endows it’s with the possibility of formation two kind of polymorphs. The condition of the crystal growth has been analyzed, and confirmed thatthe growth speed was determined by viscidity of the solvent. The theoretical studies of theconformational changes help to understand the mechanism under which the relative yieldof each polymorphic form is controlled by the external surrounding conditions. In gasphase, the energy barrier due to the rotation of the side phenyl rings is higher than that ofthe middle phenyl ring. The lower energy barrier of rotating the angle indicates that smallangle rotation of the middle phenyl ring along the cyano substituted ethylene plane ismuch easier to that of the two side rings. Therefore, in the absence of solvent, the Form IIcrystal that requires less energy cost from the optimized state in gas phase is favorable inthe PVT method. However, in solution, due to the effect of solvents, the energy barrier forformation blue is lower than that for green crystal. Therefore, the Form Ⅰ crystal isfavorable in the solution. Based on the growth speed, diffusion energy barrier and thesolution ability for α-CNDSB, it is reasonable to presume that the decreasing in activeenergy is lower in chlorobenzene then in other three kinds of solution. It is at anappropriate value, which was exhibited by bringing two kinds of crystal in it.Comparing the three pair crystals in this thesis, there are two common grounds:1. themolecular conformation in blue crystal is twisted while in green crystal is planar.2. Thetwisted molecules in blue phase crystal adopt edge–to–face stacking. The planarmolecules in the green phase crystal are stacked in a face–to–face arrangement. Thisconcept is very reasonable for the blue and green emissions in the blue and green phasecrystals, respectively, because of the relative extension of the π-conjugation in theG-phase crystal.Then, the tubular β-CNDSB crystal is successfully prepared through PVT method.High crystalline quality, highly ordered molecular uniaxial orientation, and hollow-liketopological structure make the crystal exhibiting optical wave guided emission behaviorswith low optical loss and highly polarized emission. Amplified spontaneous emission(ASE) characteristics of the tubular crystals were also studied, the net gain coefficient atthe peak wavelength and the threshold are91.7cm-1and36kW/cm2, respectively. Theunique morphology and molecular arrangement make tubular β-CNDSB crystal possibleapplications as optical waveguides organic lasers. Lastly, the heating-and pressure-induced phase transitions have been studied. Take thephase transition of α-CNDSB crystal as example, the structural evolution in the molecularand supramolecular levels as the crystalline phase transforms from the blue phase togreen phase under varied temperature or pressure was investigated in situ by X–raydiffraction, as well as with Raman, Fourier–transform infrared, and fluorescencespectroscopies. Spectral monitoring for the crystalline phase transition was performed,and direct evidence to elucidate the structural evolution was obtained. Pressure-inducephase transition is the dynamic balance between external pressure—flexible moleculeconformation—intermolecular interaction, and the phase transition is reversible. As theα-CNDSB crystal is heated, the molecules have minor adjustment before the temperaturerising to181oC. While above the transition point, there is an abrupt change in the lattice.Heating-induced phase transition is un-reversible, after the temperature decreasing to theroom temperature, the heated crystal could not return to the starting state. Then, thecourse of the phase transitions have been speculated, as follow: the molecule must getover the active energy barrier like the molecular conformation transformation in crystalgrowth when the phase transition induced by heating. But in pressure-induced phasetransition, it does not need get over this active energy barrier. Then, the course of thephase transitions have been speculated, as follow: the molecule must get over the activeenergy barrier like the molecular conformation transformation in crystal growth when thephase transition induced by heating. But in pressure-induced phase transition, it does notneed get over this active energy barrier. The intermolecular interaction has been brokendown partly in this phase transition, so the conformation has gradual changed. Thechanged conformation would establish new interaction, and the molecular packing hastaken the same changing. Due to pressure-induced phase transition not need getting overactive energy, the pressure of transition point is related to the molecule’s flexibility.