Porphyrins lateral assemblies

Figure 6.3.1 Absorption spectra of the covalent porphyrin stacked and lateral pen-tamers shown in Scheme 6.3.3. Stacks show only short-wavelength shifts lateral assemblies often give, although not in the present case, both short- and long-wavelength Soret bands.

Only the lateral assembly produces an 850 nm signal upon flashing, indicating formation of a porphyrin anion radical presumably resulting from light-induced charge separation. 

The use of disulfide linked di-a-helical peptides for the self-assembly of a heme-peptide model compounds has also been explored by Benson et al. (109). Conceptually analogous to the larger heme-protein systems utilized by Dutton and co-workers, to be detailed later, the incorporation of C4 S5mimetric Co(III)-porphyrins, based on coproporphyrin and octaethylporphyrin, resulted in helical induction comparable to that observed in the covalent PSM systems. 

More work is required in order to clarify the molecular structure of these fascinating molecular assemblies which seem to be on the borderline between fluid and solid micellar rods. Their formation develops through a certain type of precipitation, also typical for solid micellar fibres. However, the binding forces between the head group molecules (tetraalkylammonium and phenol) are weak, meaning that the fibres are not as stiff and uniform as the crystalline fibres described later. Aqueous suspensions of the described fibres dissolve massive amounts of small hydrocarbon molecules, e.g. 20 mol % of hexane, but the dissolving of hydrophobic porphyrins in them has not yet been achieved. 

In the case of ort/io-disubstituted aldehydes, one obtains extremely dense n stacks with Soret band absorptions at 370 nm, whereas meta-disubstituted analogs give a lateral packing characterized by 450-nm Soret bands (Fig. 6.3.1). Such covalent porphyrin oligomers are useful references in structure elucidation of corresponding supramolecular assemblies, where one cannot hope for a... 

The exciton effects observed in absorption spectra of porphyrin monolayers are often different from those in stacked or lateral fibrous assemblies. One usually finds exclusively long-wavelength shifts of the Soret bands or no effect at all. Porphyrins either lie parallel to each other on the subphase (no shift) or are oriented in a slipped stack-of-cards configuration tilted by 10-20° angles against the subphase surfaces, which produce file observed 20-100 nm red shifts (Fig. 6.8.2). 

All of these examples show that the use of metal coordination at the periphery of the porphyrin chromophores can be extremely helpful in modular approaches targeting multiporphyrinic assemblies. As we will see later, the formation of coordination polymers has already found applications in the preparation of materials. 

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