The Preparation And Optoelectronic Properties Of Organic Crystals And Nanocrystals Of The Oligo(Phenylenevinylene)s Derivatives

The high stability, high-quality and high carrier mobility of conjugated organic crystals attract great attention in the field of optoelectronic materials. In the mean while, due to their unique optoelectronic properties and diversity structure, nanocrystal materials based on small organic molecules have been developed as potential optoelectronic materials. The research on organic nanocrystal materials is still in the beginning and more exploring is necessary. Oligo(phenylenevinylene)s derivatives with excellent optoelectronic performance and crystallizable properties are the potential materials for the optoelectronic devices application. In this thesis, the preparation method and the optical properties of the organic crystals and nanocrystals of these materials have been studied:The first investigation of two-photon-pumped (TPP) amplified spontaneous emission of organic crystals has been reported. E, E-1,4-bis[4'-(N, N-dibutylamino)styryl]-2,5-dimethoxy-benzene (DBASDMB) organic crystals with high crystalline quality, large size and excellent optical properties are prepared. The unit cell of DBASDMB is triclinic, space group P1.The intermolecular interaction could cause a "brick wall" motif J-aggregate molecular stacking mode. The linear and nonlinear properties in the crystal are comparatively studied. The relaxation dynamics pumped by two-photon are very similar with that pumped by one-photon. In the solution or crystal, there are negligible differences in the shape and peak positions of the PL spectra excited by one-and two-photons, reveals that the emission stems from the same excited state. Further time resolved investigations (femtosecond fluorescence upconversion experiments) are employed to study the dynamic properties of one-and two-photon excited molecular transitions and relaxation in solution and crystals. The crystal exhibit very strong two-photon excited fluorescence and amplified spontaneous emission. The two-photon absorption in the slab crystal was investigated by nonlinear transmission (NLT) method. The measured two-photon absorption is about 588.7 GM. TPP amplified spontaneous emission of organic crystals has been observed. The spectral narrowing is accompanied with a simultaneous output jump. The threshold of TPP lasing is measured to 1.68 mJ/(cm2·pulse).Further more, size-dependent amplified spontaneous emission (ASE) from organic crystals has been observed. Specifically, N-(4-(4-(4-(diphenylamino) styryl)styryl)phenyl)-N-phenylbenzene amine (Ph-TPA2) organic crystals were used in the experiment. The ASE threshold was decreased with the decrease of width of the crystal at the same gain length, which reflects that total internal reflection plays an important role on the ASE properties in these slab organic crystal. The ASE properties pumped by one- and two-photon are also comparative studied. For the crystal with w=48μm, the threshold is 0.03 mJ-pulse-1·cm-2. With the increase of the width, the threshold increases, and reach 0.057 mJ-pulse-1·cm-2 for the crystal with 222μm in width. The threshold induced by two-photon absorption is about 0.12 mJ-pulse-1·cm-2 for varied size crystals with slightly different. We found that the thresholds of ASE in two-photon pumping condition are less size-dependent than those in single-photon condition because of the the nonlinear light generation processes in two-photon absorption processes.Meanwhile, we improved the organic crystal growth method. Large size single-crystalline materials with high quality were successfully prepared from a small organic functional molecule,1,4-Bis(4-methylstyryl)benzene (BSB-Me), by an improved physical vapor method. It has been demonstrated that the introduction of adsorbents can decrease the sublimation temperature of BSB-Me, and slow the weight-loss process markedly, which is proven to be indispensable in improving the quality of the as-prepared BSB-Me crystalline materials. The surface morphologies and structural information were characterized by optical microscope, atomic force microscopy (AFM) and X-ray diffraction (XRD) analysis. The prepared crystals are high transparency, low light scattering and exhibit self-waveguided amplified spontaneous emission with a relatively low threshold resulting from the excellent optical properties. The ASE threshold could be easily concluded to be 25μJcm-2. The results indicate that the growth mechanism of single crystals is a two-dimensional nucleation, elementary steps, large straight steps, layer-by-layer growth. Besides the BSB-Me,2,5-bis(4-biphenylyl)thiophene (BP1T). and 5,5'-bis(4-biphenylyl)-2,2'-bithiophene (BP2T) were also grown by this improved method.Besides, we also developed the preparation method and the optical properties of organic nanocrystals. Suspension of micrometer-sized 1,4-bis(4-methylstyryl) benzene (BSB-Me) was converted into colloidal nanocrystal solution by irradiation with an femtosecond laser (800 nm,1 KHz,29 mJ/cm2·pulse). The prepared nanocrystals were rectangular with～100 nm size. The same crystal structure with bulk crystals was confirmed by X-ray diffraction measurement. UV-vis spectra and emission spectroscopy of the nanoparticle dispersions in dichloromethane (poor solvent) were examined. The nanocrystals exhibite large quantum yield (89%). The nonlinear optical properties of nanocrystal were further studied by Z-scan technique with femtosecond laser duration of 120fs at wavelength of 800 nm. The results show that the nanocrystals exhibite strong nonlinear absorption.The nanoparticles of BSB-Me, DBASDMB and O-MSB have been produced by femtosecond laser-induced forward transfer (fs-LIFT) method. The prepared nanocrystals were rectangular with uniform size distributing. The influence of the pulse fluence on the size and morphology of particles had been researched. Fourier-transform infrared spectroscopy has been used to analyze the nanoparticles. The results show that the material has no change before and after laser operation. X ray diffraction patterns, transmission electron microscope and high resolution transmission electron microscopy have been used to characterize the morphology and structure of the nanocrystals, revealing the well defined crystal structure in the nanoparticles.