Tunable Emission from Doped 1,3,5‐Triphenyl‐2‐pyrazoline Organic Nanoparticles

Abstract

Recently, organic nanoparticles (NPs) have inspired growing research efforts due to the great diversity of organic molecules available, the flexibility in materials synthesis and preparation of NPs, in addition to their size-dependent optical properties.[1] Our group has prepared organic NPs from a series of pyrazoline compounds, which have been widely used as optical brightening agents for textiles and paper and as the hole-conducting medium in photoconductive materials and electroluminescent (EL) devices. It has been shown that the emissions of these pyrazoline derivatives can be tuned by changing the size of the NPs.[2] Such tunable emission is attributed to increased intermolecular interactions with increasing particle size. Here, we show that the emission of organic NPs can also be tuned by employing a doping technique. Doping techniques based on energy transfer have been proven to be an effective way to improve the luminescence efficiency and tune the emission color of EL materials [3] and are widely used in the fabrication of thin-film EL devices.[4]However, little attention has been paid to doped organic NPs, which may potentially serve as promising candidates for advanced optoelectronic materials.[5] Herein, we extend our research interests to doped organic NPs: 1,3,5-triphenyl-2-pyrazoline (TPP) NPs doped with 4-(dicyanomethylene)-2-methyl-6-(p-dimethyl-aminostyryl)-4H-pyran (DCM, a redlight-emission material widely used in EL devices) were prepared by the simple reprecipitation method. [6] The absorption spectrum of DCM shows good overlap with the fluorescence emission spectrum of TPP in the 400±550 nm wavelength range (Fig. 1), which is essential for the occurrence of energy transfer in the doping systems. The as-prepared doped NPs present tunable fluorescence emission, dependent on the DCM content, as well as efficient energy transfer. All the NPs, including pure and doped ones, were prepared through the reprecipitation method; mixtures of TPP and DCM in ethanol were used in the preparation of doped NPs. Sizes and shapes of the as-prepared NPs were observed by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Representative images of the TPP NPs with a DCM doping level of 10 % (molar) are presented in Figure 2. The NPs are approximately spherical with a mean size of about 30±40 nm. The electron diffraction patterns and X-ray diffraction (XRD) measure-COMMUNICATIONS

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“…From left to right, the doping concentrations of DCM are 0, 0.1%, 0.2%, 2%, 10%, and 100% (molar ratio). (Reprinted from [194].) REVIEW the preparation of composite nanoparticles, based on a derivative of photochromic spirooxazines (SO), a typical fluorescent dye DCM, and an emissive assistant molecule of 1,3-bis(pyrene) propane (BPP, see Fig.…”