Elsevier

Polyhedron

Volume 30, Issue 6, 13 April 2011, Pages 1023-1026
Polyhedron

Temperature activated ionic conductivity in gallium and indium phthalocyanines

https://doi.org/10.1016/j.poly.2010.12.047Get rights and content

Abstract

The effects of introducing gallium and indium metals into phthalocyanine molecules were investigated via temperature and frequency dependent dielectric spectroscopy. The dielectric properties of Ga(III) and In(III) phthalocyanine pellets were measured at frequencies from 1 kHz to 1 MHz in the temperature range 300–530 K. The temperature dependence of the real part of the dielectric constant suggested that these compounds exhibit semiconductor behavior. The activation energy values were calculated from the Arrhenius plots at different frequencies. A distinct transition in these plots indicated the activation of ionic conductivity at higher temperatures.

Graphical abstract

The effects of introducing gallium and indium metals into phthalocyanine molecules were investigated via temperature and frequency dependent dielectric spectroscopy. The temperature dependence of the real part of the dielectric constant suggested that these compounds exhibit semiconductor behavior. The activation energy values were calculated from the Arrhenius plots at different frequencies. A distinct transition in these plots indicated the activation of ionic conductivity at higher temperatures.

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Research highlights

► Temperature dependent dielectric properties of ClGaPc and ClInPc compounds were analyzed. ► Ga and In embedded Pcs showed the standard semiconductor temperature dependency. ► The temperature dependence of the conductivities of both samples exhibited Arrhenius behavior. ► Our results show that the ionic conductivity is activated at elevated temperatures in both samples.

Introduction

Metallophthalocyanines (MPcs), a family of aromatic macrocycles based on an extensive delocalized 18-π electron system, are known not only as classical dyes in practical use but also as modern functional materials in scientific research. There is a growing interest in the use of phthalocyanines (Pcs) in a variety of applications, including non-linear optics [1], semiconductor devices [2], Langmuir–Blodgett films [3], electrochromic display devices [4], liquid crystals [5] and as photosensitizers in photodynamic therapy (PDT) [6]. For non-linear optical applications, MPcs have advantages over the currently used inorganic compounds due to their small dielectric constants [7], fast response times, ease of processability into optical components and their lower cost [1], [7]. The structures of MPcs can be modulated in many ways, by changing the peripheral and non-peripheral substituents on the ring in addition to changing the central metal and the axial ligands.

Heavy metals, especially diamagnetic metals, play a major role in photosensitizing and optical limiting mechanisms because they enhance intersystem crossing through spin orbit coupling. This enhancement is desirable as it improves the probability of forming a large population in the triplet state. Axial ligands in MPcs play a key role in preventing or minimizing intermolecular interactions, which causes aggregation in solution. Aggregation can result in the fast decay of excited states. Indium and gallium are useful central metals in MPcs complexes since they are diamagnetic and are able to host axial ligands. Gallium and indium phthalocyanines have been reported to have good photosensitizing and optical limiting properties [1], [7], [8], [9], [10], [11].

In the scope of this work, chlorogallium (ClGaPc) and chloroindium (ClInPc) phthalocyanine samples were examined with dielectric spectroscopy (DS). This method is shown to be a reliable tool for investigating molecular scale events and for the optimization of tailored materials [12], [13], [14]. The temperature dependence of the real part of their dielectric constants and dielectric loss were measured and analyzed. Activation energies of ClGaPc and ClInPc samples were also calculated at different frequencies. The conductivities and the activation energies of the samples increased at elevated temperatures, which were attributed to the activation of the ionic conductivity with increasing temperature.

Section snippets

ClGaPc

This compound was synthesized and characterized according to the method reported elsewhere [15]. Briefly, a mixture of phthalonitrile (5 g, 0.04 mol), anhydrous gallium trichloride (5.5 g, 0.03 mol), and 20 mL of quinoline (double distilled over CaH2, deoxygenated) was refluxed for 1 h (particular attention was paid to the exclusion of water during this step). After cooling the mixture to approximately 273 K, the reaction mixture was filtered. The mixture was washed with toluene and methanol and dried

Results and discussion

The complex dielectric expression, ε = ε  ′′, where ε′ is the real part, and ε′′ is the imaginary part of dielectric constant, was utilized in the analysis of the DS results. The real part of the dielectric constant was calculated from the equation ε = Cpd/(ε0A), where Cp is the parallel plate capacitance, d is the inter electrode distance, ε0 is the permittivity of free space and A is the area of the sample. The dielectric loss (tanδ), also a critical parameter in DS, was derived from the

Conclusions

Temperature dependent dielectric properties of ClGaPc and ClInPc compounds were analyzed to determine the dielectric properties of these Pc complexes after the insertion of gallium and indium metals. Ga and In embedded Pcs showed the standard temperature dependence characteristics of conventional semiconductors. The temperature dependence of the conductivities of both samples exhibited Arrhenius behavior. Our results suggest that the ionic conductivity is activated at elevated temperatures in

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