Preparation of 1,4-bis(4-methylstyryl)benzene nanocrystals by a wet process and evaluation of their optical properties
© Baba and Nishida; licensee Springer. 2014
Received: 25 October 2013
Accepted: 1 January 2014
Published: 13 January 2014
Single-crystal 1,4-bis(4-methylstyryl)benzene is a promising material for optoelectronic device applications. We demonstrate the preparation of 1,4-bis(4-methylstyryl)benzene nanocrystals by a wet process using a bottom-up reprecipitation technique. Scanning electron microscopy revealed the morphology of the nanocrystals to be sphere-like with an average particle size of about 60 nm. An aqueous dispersion of the nanocrystals was monodisperse and stable with a ζ-potential of -41 mV. The peak wavelengths of the absorption and emission spectra of the nanocrystal dispersion were blue and red shifted, respectively, compared with those of tetrahydrofuran solution. Powder X-ray diffraction analysis confirmed the crystallinity of the nanocrystals. The presented 1,4-bis(4-methylstyryl)benzene nanocrystals are expected to be a candidate for a new class of optoelectronic material.
In contrast, we have investigated the preparation and evaluated the properties of nano-sized organic crystals, i.e., organic nanocrystals [8–11]. Organic nanocrystals show unique physicochemical properties different from those of the molecular and bulk crystal states [12–15]. Organic nanocrystals have been broadly used as optoelectronic materials as well as biomedical materials [16–22]. Recently, Fang et al. demonstrated the preparation of BSB-Me nanocrystals using a femtosecond laser-induced forward transfer method [23, 24]. The BSB-Me nanocrystals were directly deposited on a substrate to form a nanocrystal film, and their size and morphology were investigated as functions of applied laser fluence. The use of BSB-Me nanocrystals will be a promising approach for organic crystal device applications in the near future. However, according to Fang's report, the morphology of the prepared BSB-Me nanocrystals were multifarious, i.e., while most nanoparticles were cubic in geometry, others were tetrahedral shaped, truncated cubes, and truncated tetrahedra . To fabricate high-quality optical devices, such nanocrystals should ideally be homogenous in shape and in size because their optical properties are strongly affected by the crystal morphology. Additionally, there is a serious problem that the yields of nanoparticles prepared by laser ablation are smaller than those obtained by other nanoparticle synthesis methods because the nanocrystals are formed only in the small laser-irradiated spot . This is a weak point when considering mass production for device fabrication. Furthermore, the output power of laser ablation is not suitable for organic compounds because the high energy may degrade them [26, 27]. Wet processes using bottom-up techniques overcome these disadvantages. The solvent exchange method, known as the reprecipitation method, is especially suitable for preparing organic nanocrystals [18, 28]. Unlike laser ablation, no excess energy is necessary to form the organic nanocrystals, and bulk production is possible . Following a previously reported study, it is possible to prepare a nanocrystal-layered thin film for optical devices using the reprecipitation method . Instead of top-down laser ablation, the alternative approach of this bottom-up wet process is an attractive prospect for preparing BSB-Me nanocrystals.
The aim of this study is to demonstrate the preparation of BSB-Me nanocrystals having narrow size distribution with singular morphology by means of a bottom-up, wet process using the reprecipitation method. This method makes it possible to control the particle size and morphology of the nanocrystals. We prepared BSB-Me nanocrystal dispersions in water, and investigated the size, morphology, optical properties, and powder X-ray diffraction pattern of the nanocrystals.
BSB-Me (>98.0%) was purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan) and used without further purification. Tetrahydrofuran (THF) (>99.5%) was purchased from Wako Pure Chemical Industries, Ltd. (Tokyo, Japan). Purified water (18.2 MΩ) was obtained from a Milli-Q A-10 (Millipore, Tokyo, Japan).
BSB-Me was dissolved in THF (2 mM) at 50°C, and 100 μl of the solution was injected into vigorously stirred (1,500 rpm) poor solvent water (10 ml at 24°C) using a microsyringe. As a result, the BSB-Me suddenly precipitated to form dispersed nanocrystals. Syringe filter (pore size 1.2 μm; Minisart®, Sartorius Stedim Biotech, NY, USA) was used to remove small degree of aggregates from the nanocrystal dispersion.
The particle size and morphology of the BSB-Me nanocrystals were evaluated using scanning electron microscopy (SEM; JSM-6510LA, JEOL, Tokyo, Japan). To prepare specimens for imaging, the nanocrystals were collected from the water dispersion using suction filtration with a membrane filter (0.05-μm pore size), followed by platinum sputter coating (JFC-1600, JEOL). The average particle size, size distribution, and ζ-potential of the nanocrystal dispersion were evaluated using an ELSZ-1000 zeta-potential and particle size analyzer (Otsuka Electronics Co., Ltd., Osaka, Japan). Ultraviolet-visible (UV-vis) absorption spectra and fluorescence spectra were measured using a V-550 UV/vis spectrophotometer (JASCO, Tokyo, Japan) and F-2500 fluorescence spectrophotometer (Hitachi, Tokyo, Japan), respectively.
Results and discussion
Quantum yield, integrated intensity, optical density, and refractive index of the BSB-Me
Quantum yield (Q),%
Integrated intensity (I)b
Optical density (OD) at λ = 324 nmc
Refractive index (n) at 20°C
BSB-Me dissolved in dichloromethane (1 μM)
95 ± 1a
BSB-Me nanocrystal water dispersion (2 μM)
9.2 ± 0.1
We demonstrated the preparation of a BSB-Me nanocrystal dispersion in water by the reprecipitation method, which is a bottom-up, wet process for preparing organic nanocrystals. SEM observations revealed that the nanocrystals had a sphere-like morphology. The average particle size was 60.9 nm, measured using an ELSZ-1000 zeta-potential and particle size analyzer. The nanocrystal dispersion was stable with a measured ζ-potential of -41.62 mV using ELSZ-1000. The blue shift and red shift of maximum peak wavelength were observed in absorption and emission spectra, respectively. This optical feature may have arisen from supramolecular interactions like those caused by the herringbone structure, i.e., H-aggregation, in the nanocrystals. The photoluminescence quantum yield of the BSB-Me nanocrystal water dispersion was estimated to be 9.2 ± 0.1%. Powder X-ray diffraction analysis confirmed the crystallinity of the BSB-Me nanocrystals. In future work, these BSB-Me nanocrystals will be applied to crystalline-based optoelectronic devices. Measuring amplified spontaneous emission and nonlinear optical properties of single nanocrystals will be a particularly interesting topic for the near future. We will also investigate and discuss elsewhere the nanocrystal size distribution using Scherrer's equation based on the data of XRD measurements. Further detailed optical properties such as an absolute photoluminescence quantum yield, fluorescence lifetime, and radiative decay rates of BSB-Me nanocrystals will be discussed elsewhere. Furthermore, fluorescent BSB-Me nanocrystals could be used in biological applications such as fluorescent bioimaging of cells and tissue similar to that in our previous work.
KB is an Endowed Chair Associate Professor at the Department of Visual Regenerative Medicine, Osaka University Graduate School of Medicine, Japan, and KN is a Professor and a medical doctor at the Department of Ophthalmology, Osaka University Graduate School of Medicine, Japan.
This study was partially supported by a Challenging Exploratory Research (no. 25560223) and Grant-in-Aid for Young Scientists (A) (no. 24680054) from the Japan Society for the Promotion of Science. We thank Dr. Yasunobu Wada for his technical support to the experiments.
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