Figure1 shows the phthalocyanine dendritic system. The monomer, 1,2-bis(3,4-dicyanophenoxy)benzene, was firstly synthesized from the reaction of Catechol and 4-nitrophthalonitrile under the existence of K₂CO₃. In a flask, 1,2-bis(3,4-dicyanophenoxy)benzene(3 mmol)and zinc acetate(1 mmol)were dissolved in DMAc(40 mL), and then the mixture was stirred at 160 ℃ for 24 h. The reaction mixture was poured into 1 000 mL of DI water following by filtration, washed with water, and dried in vacuum. The bottled green hyperbranched ZnPc powders were finally collected after refluxing in methanol twice and dried in vacuum.

Fig.1 Synthetic route and structure of the dendritic Zinc phthalocyanine

ZnPc compressed pellets were made by cold isostatic pressing(Shenyang Kejing, China)with a pressure of 300 MPa. Au electrodes were sputtered on the top and bottom surfaces of the pellets for electrical measurements. The dielectric properties were measured by a frequency-response analyzer(Novocontrol, Germany). Poly(vinylidene fluoride)(PVDF)films were also fabricated and characterized for comparison.

The dielectric response for a compressed pellet of the hyperbranched ZnPc is shown in figure 2. As is shown here, a high dielectric constant ～15 can be obtained with a weak frequency dispersion. This value is larger than that observed in the existing high-k polymer, PVDF, which also shows very a large dispersion. In contrast, the dendritic ZnPc system has a very low dielectric loss over a wide frequency range. The loss is less than 0.002 at 10 MHz, which is much smaller than that observed in PVDF films and Pc oligomers(tan δ>0.5)^[5]. Our measurements of a different pellet's dielectric constant showed no apparent thickness dependence. A similar high-k response has also been reported in hyperbranched polyaniline materials by Goodson and Yan et al^[6]. Their investigation suggests that delocalized polarons may provide a way to enhance the dielectric response at a high frequency. The branched structure provides a way for long-range polaron delocalization, and, then gives rise to a small dispersion at high frequencies. However, further and detailed study should be performed to clarify the mechanism of the high-k response in the new dendritic Pc system.

Fig.2(Color online)Dielectric properties of the dendritic zinc phthalocyanine and PVDF at room temperature

Figure3 presents the differential scanning calorimetry(DSC)curve of the dendritic ZnPc. It can be observed that the glass transition(T_g)occurred at about 768.5 K, which is much higher than that observed in PVDF. The mass reduction(not shown here)of the ZnPc system starts at a temperature higher than 700 K, also indicating very excellent thermal stability.

Fig.3 (Color online)DSC curve of hyperbranched ZnPc