Size Effects on the Optical Properties of Organic Nanoparticles

Nanoparticles of 1-phenyl-3-((dimethylamino)styryl)-5-((dimethylamino)phenyl)-2-pyrazoline (PDDP) ranging from tens to hundreds of nanometers were prepared by using the reprecipitation method. Their excitonic transitions responsible for absorption and emission, as compared with those of dilute solution, have been investigated as a function of nanoparticle size. We found that PDDP nanoparticles possess a special size dependence in their optical properties. We identified an extended charge-transfer (CT) state stemming from PDDP molecules closely stacking in nanoparticles and observed its shift to the high-energy side with decreasing nanoparticle size due to exciton confinement. At the same time, the molecular π−π* absorption of the nanoparticles was also blue-shifted, accompanied by an almost unchanging n−π* absorption as a result of the reduced overlap of the pyrazoline ring π orbital and decreased 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.