Porphyrin loosely stacked

As discussed in section 2.2, a mixture of AMP and AA showed two solid condensed phases above and below about 30 mN m- [5,10]. A loosely stacked structure of two porphyrins was proposed for LB films prepared at higher surface pressures than 30 mNmr1, which was caused by squeezing-out of a monomolecular structure formed at lower surface pressure [5,10]. In this section, photoelectric characteristics of LB films containing AMP and AA deposited at two solid condensed phases will be discussed in relation to multilayer structure and the anisotropic intermolecular tunneling rates [87]. Seven monolayers of 1 5 or 1 10 mixture of AMP and AA were deposited at 20 and 50 mN m-1 on an ITO plate at 18 °C to form stable Y-type LB films. Aluminum was vacuum evaporated onto LB films as sandwich-type electrodes at 10-6 Torr. Steady photocurrents were measured in a similar manner as mentioned above. 

Figure 23 Schematic representation of the intermolecular tunneling interaction between a pair p to a pair qin LB films containing loosely stacked porphyrins (ellipses).

It was observed that the direct electrodeposition of H2(ETRPyP) from a DMF solution leads to poor quality films, due to the looseness of the presumably low molecular weight polymeric chains. Considering that the supramolecular porphyrin is able to form quite homogeneous films by dip coating (122, 167, 170, 184, 250, 286, 296) and that such films are poorly soluble in acetonitrile, an altenative procedure was devised. This procedure was carried out by an electropolymerization reaction on pre-formed films (53, 172), whose advantages are the high concentration of the monomer on the electrode surface and the presence of suitably preoriented molecules, which keep the n-stacking structure of the dip-coated films. Such characteristics increase the efficiency of the electropolymerization, since only one or two scans in the... 

Most of what has been aid so far applies to rod-like molecules, but similar phases exist for disk-shaped molecules, such as porphyrins and phthalo-cyanines. These tend to form phases in which the director is now orthogonal to the plane of the molecule. Thus, where the planes of the molecules are more or less parallel, but with no positional ordering into columns, the phase is a nematic discotic. If, however, the macrocycles form themselves into columns as well, we have a columnar smectic phase. A loose analogy of this phase is that it is like stacked coins, with the stacks arranged in a hexagonal lattice. 

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