Preparation Of Organic Nanoparticles And Its Size Effects

Preparation and the size-dependence of organic nanoparticles are not investigated as well as those of inorganic quantum dots by far. In this thesis, using the pyrazoline derivatives as the model compound, a series of highly monodisperse nanoparticles with controlled size and shape had been prepared. Further, their optical size-dependent properties were studied by means of steady state and time-resolved spectroscopy, and were explained by the aggregate theory using molecular modeling calculations. The optical size effects exhibited by pyrazolines nanoparticles may prove useful in the future luminescence applications, and in the study of the fundamental process connecting both of these more conventional classes of materials, i.e., organic molecular crystals (OMCs).1. Global nanoparticles of 1-phenyl-3-((dimethylamino)styryl)-5-((dimethylamino)phenyl)-2-pyrazoline (PDDP) and cubic nanoparticles of 1,3,5-triphenyl-2-pyrazoline (TPP)ranging from tens to hundreds nanometers were prepared using the reprecipitation method. Their excitonic transitions responsible for absorption and emission as compared with that of dilute solution have been investigated as a function of nanoparticle size. We found that PDDP and TPP nanoparticles possess a special size-dependence in optical properties. We identified an extended charge-transfer state stemming from the model-molecules closely stacking in nanoparticles and observed it shift to high-energy side with decreasing nanoparticle size due to exciton confinement. At the same time, the molecular n-n* absorption of nanoparticles were also blue-shifted, accompanied by a hardly changing n-π^* absorption as a result of the reduced overlap of the pyrazoline ringπorbital and a decrease of intermolecular interactions. Moreover S1 and CT states were in equilibrium in the nanoparticles, and the probability of fluorescence from S1 increased with decreasing nanoparticle size.2. 1,3-Diphenyl-5-pyrenyl-2-pyrazoline (DPP) shows the emission characteristics of pyrene in solution or the emission characteristics of pyrazoline in films and bulk crystals. However, DPP nanoparticles were found to present multiple emission from pyrene at 385 nm, pyrazoline at 465 nm and an intermolecular pyrazoline-pyrene charge-transfer (CT) state at 570 nm, and each of them possesses individual optical channel. Moreover, the emission at 570 nm gradually vanished with increasing nanoparticle size due to the destroying of intermolecular pyrazoline-pyrene pair as a result of the formation of intermolecualr pyrazoline- pyrazoline pair.3. Nanosized fiber of 1,3-diphenyl-2-pyrazoline (DP) was prepared using the templates of cetyltrimethylammonium (CTAB) micelles. Upon controlling the specific value between DP and CTAB molecules, N=n_CTAB/N_DP, the diameter of DP nano-fiber can be controlled. For example, when N=18 and 9, the diameters of DP nano-fiber are 140 and 225 nm, respectively. XRD measurements indicated that DP molecules are highly ordered in nano-fiber, and arrange as J-aggregate of which the absorption gradually appeared with increasing the N value and was red-shifted from that of monomer. Moreover, the decay of DP nano-fiber fluorescence became faster and the emission width became narrower than those of the dilute DP/ethanol solution due to the formation of J-aggregate in nano-fibers.