Size effects on the optical properties of organic nanoparticles

Nanoparticles of 1-phenyl-3-((dimethylamino)styryl)-5-((dimethylamino)pheny l)-2-pyrazoline (PDDP) ranging from tens to hundreds of nanometers were pre pared by using the reprecipitation method. Their excitonic transitions resp onsible for absorption and emission, as compared with those of dilute solut ion, have been investigated as a function of nanoparticle size. We found th at PDDP nanoparticles possess a special size dependence in their optical pr operties. We identified an extended charge-transfer (CT) state stemming fro m PDDP molecules closely stacking in nanoparticles and observed its shift t o the high-energy side with decreasing nanoparticle size due to exciton con finement. At the same time, the molecular pi-pi* absorption of the nanopart icles was also blue-shifted, accompanied by an almost unchanging n-pi* abso rption asa result of the reduced overlap of the pyrazoline ring pi orbital and decreased intermolecular interactions. Moreover, S1 and CT states were in equilibrium in the nanoparticles, and the probability of fluorescence fr om SI increased with decreasing nanoparticle size.