Amphitropic Mesomorphic Phthalocyanines—A New Approach to Highly Ordered Layers

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Phthalocyanines (Pcs), like many flat-shaped aromatic macrocycles, often arrange themselves into cofacial stacks (columnar arrangements) either in their crystalline and liquid-crystalline phases or in concentrated solutions. The main driving force for this particular supramolecular arrangement is s-overlap between the large p-systems of adjacent Pc macrocycles. Such a columnar architecture of pstacked Pc units is, among other potential applications, a promising structure for new materials with low-dimensional (photo)conducting properties. Therefore, they might present a new generation of organic photoconducting materials in xerographic processes or as components in organic photovoltaic cells of enhanced photovoltaic efficiency. The main advantage of Pcs, when compared with other aromatic macrocycles, is their strong absorption in the visible and near infrared (NIR) region. Pcs have long been known in industry as dyes, pigments, and catalysts, but more recently have found application as photoconducting materials in xerographic processes. However, the potential of Pcs for new functional materials with special optical, electrical, photoand semiconducting properties is currently being investigated. One of the main advantages of Pcs over many other discotic macrocycles is their strong absorption in the visible and NIR regions. For many of these new potential applications, thin layers of defined supramolecular order and reasonable stability have to be prepared. One of the most promising approaches for the preparation of ordered layers is to use the columnar liquid-crystalline phases (mesophases) of suitably substituted Pcs. In general, mesomorphic compounds have the advantage of good solubility and processability as well as a rather unique combination of supramolecular order and molecular mobility within their mesophase. However, the mesomorphism of Pcs needs to satisfy several requirements in order to obtain optimal materials properties. First, the columnar mesophase should display a hexagonal arrangement with a high degree of intracolumnar order; second, the mesophase should be easy to orient for the preparation of monodomains of macroscopic dimension and homeotropic orientation (the columns are oriented perpendicular to the surface of the substrate). This particular supramolecular architecture is probably the most favorable one for optical and electrical applications and is outlined in Figure 1. Finally, a afrozen ino mesophase (glass phase) rather than a return to the crystalline phase should be obtained upon cooling to ambient temperatures. This is important, because crystallization usually destroys the supramolecular architecture of the preceding mesophase and the molecular mobility within the mesophase has been found to disturb the conducting properties enormously. Although the thermotropic mesomorphic behavior of Pcs has been investigated extensively for 15 years, a satisfying synthetic approach covering all the aforementioned requirements has not yet been found. In fact, a molecular design that intrinsically leads to the ideal supramolecular behavior seems to be a contradiction. For example, most of the known mesomorphic Pcs display a very viscous mesophase with a clearing point above their decomposition temperature of about 300 C and are therefore not easy to orient. A fluid mesophase, or even an isotropic liquid phase, of monomeric Pcs could be introduced when the macrocycle was surrounded by bulky flexible side chains (e.g., very long or branched side chains) or was substituted at the non-peripheral positions. This kind of substitution pattern, on the other hand, led to less intracolumnarily ordered mesophases. In fact, it has been pointed out that alkane side chains are already responsible for what is in general, a low long-range order within the columns. This was explained by intracolumnar density fluctuations caused by the different optimal van der Waals distances between