Polymer-controlled Fabrication Of Organic 1-(2-pyridylazo)-2-naphthol Structures

During the past few years, nano/microstructures represent attractive building blocks for the fabrication of functional nano/microscale devices. However, most of the reported nano/microstructures have been focused on inorganic materials or polymers. Recently, nano/microstructures based on small organic functional molecule have been attracting considerable and increasing attention due to their unique optoelectronic properties and applications compared to those of their inorganic counterparts. Among the few synthesis methods, the so-called"reprecipitation method"has been one of the most popular methods for organic nano/microstructures formation due to its easy and versatile operation. In this paper, 1-(2-pyridylazo)-2- naphthol (PAN) was employed as a model compound to fabricate its corresponding organic nano/microstructures by reprecipitation with the assistance of poly(N,N- dimethylacrylamide)-poly(N-isopropylacrylamide) copolymer (PDMA-b-PNIPAM). The effects of copolymer concentration and PAN concentration on the size and shape have been investigated. Furthermore, the growth process of the products was discussed.(1) The preparation of block copolymers. A series of thermoresponsive diblock copolymers, PDMA-b-PNIPAM were synthesized by RAFT polymerization using 2-propynyloxy-2-(ethylthiocarbonothioylthio)-2-methyl propanoate (PEMP) and 2,2'- azobisisobutyronitrile (AIBN) as the RAFT agent and initiator, respectively. The molecular weight and the polydispersity (PDI) were determined by gel permeation chromatography (GPC). The results indicated that all polymerizations proceeded with good control, as evidenced by low polydispersity. The thermally aggregation behavior of the copolymers were investigated by dynamic light scattering (DLS) and the results indicated that critical micelle temperature (CMT) decreased with the increasing of polymer aqueous concentration.(2) Fabrication of organic PAN nano/microtructures. Nano/microstructures of a small organic functional molecule, 1-(2-pyridylazo)-2-naphthol (PAN), were successfully prepared via the reprecipitation method with the assistance of thermoresponsive diblock copolymer PDMA-b-PNIPAM. The effects of concentrations of copolymer and PAN on the size and shape have been investigated. Scanning electron microscopy (SEM) indicated that the morphology of the nano/microstructures prepared as either nano/microfibres, nano/microrods, or bundle- like nano/microstructures can be readily controlled by varying the experimental conditions such as the concentration of the polymer or PAN. Fourier transform infrared spectra (FTIR) were used to demonstrate that the as-prepared nano/micro- structures were essentially pure PAN without a noticeable incorporation of PDMA-b-PNIPAM molecules inside the nano/micromaterials. The optical absorption and fluorescence emission properties of the PAN nano/microtructures were also investigated, and the time-dependent spectra of the precipitating solution for nano/microstructures formation were measured to monitor the self-assembly process of PAN molecules. The nano/microstructures present a red shifts absorption spectrum to that of the monomers, suggesting that these as-obtained nano/microstructures were J-aggregates. Meanwhile, these absorption bands of the molecules decreased gradually and the baseline of the absorption at longer wavelength gets higher with the aging time increasing. Furthermore, the fluorescence emission demonstrated aggregation-induced emission enhancement (AIEE) phenomenon, which were caused by the restriction of the intramolecular vibrational and rotational motions in an indirect manner. Besides, new emission bands were observed at longer wavelength. With increasing aging time, intensities of the emission increase gradually whereas the peak positions are consistent.