Macrocycles 24-subunit porphyrins

Electrons are not the only charges that can be conducted. Protonic conductivity is also possible, and many proteins are known to be proton conductors. Such proton conductivity has been demonstrated with porphyrins specifically substituted with groups that allow the macrocyclic subunits to undergo intermolecular hydrogen bonding. The intriguing possibility exists that, by chemical manipulation of the macrocycle, it may soon be possible to achieve both electronic and protonic conduction in a porphyrin. 

The porphyrin ring system (the parent compound 1 is also known as porphin) consists of four pyrrole-type subunits joined by four methine ( = CH-) bridges to give a macrotetracycle. The macrocycle contains 227i-electrons from which 1871-electrons form a delocalized aromatic system according to Huckel s 4n + 2 rule for aromaticity. The aromaticity of the porphyrin determines the characteristic physical and chemical properties of this class of compounds. The aromatic character of porphyrins has been confirmed by determination of their heats of combustion.1"3 X-ray investigations4 of numerous porphyrins have shown the planarity of the nucleus which is a prerequisite for the aromatic character. 

In the previous sections, expanded porphyrins have been described in which the expansion is restricted to the pyrrole-linking bridges. The smallest expanded porphyrin in which the expansion is created by an additional pyrrole subunit in the macrocycle is the so-called orangarin, a [20]pentaphyrin(1.0.1.0.0).13 Orangarin 39 can be prepared via a hydrogen chloride induced... 

Templates possessing two hydrogen bonding subunits bind two substrates forming a ternary complex in which the substrates are positioned so as to facilitate bond formation between them [5.64a]. In a related way, the rate and stereoselectivity of a bimolecular Diels-Alder reaction are substantially increased by binding both the diene and the dienophile within the cavity of a tris-porphyrin macrocycle [5.64b]. 

According to a recently proposed definition, expanded porphyrins possess more than 16 atoms in the inner or, more precisely, smallest circuit of the macrocycle (porphyrin 1 contains exactly 16 atoms in the smallest macrocyclic circuit) [11], In addition to porphyrinoid macrocycles traditionally recognized as expanded, i.e., possessing more than four cyclic subunits or more than four meso bridges, this new definition encompasses some other systems, such as p-benziporphyrin 150. Regardless of the exact definition, expanded porphyrins constitute an unusually rich and diverse family of structures, and have been the subject of several excellent reviews [2, 5, 11, 146], The relationship between aromaticity and tautomerism in... 

In corroles and other systems described above, the macrocyclic ring is contracted by leaving out one of the four porphyrinic meso bridges. A more radical modification, namely an omission of one the cyclic subunits, results in even smaller macro-cyclic rings, collectively termed triphyrins. Boron subphthalocyanines (65, Fig. 25), known for 35 years [191], are an archetype of triphyrin structure. However, their... 

An interesting variation on this theme has been reported by Lehn and coworkers. In 1986, they reported the synthesis of macrocycle 33, which consists of a zinc porphyrin bearing two linked cyclic binding subunits [87]. It was later found that addition of silver triflate to a solution of 33 in methanol resulted in the incorporation of a silver ion in each of the binding subunits [88], Thus, the complex may be represented as Ag+-P-Ag+. The porphyrin fluorescence of the silver complex was quenched, and transient absorption studies demonstrated that the porphyrin singlet state was quenched with a rate constant of 5.0 x 109 s 1 to yield a charge separated state Ag°-P+-Ag+. Some quenching of the porphyrin triplet... 

The self-assembly of these oppositely charged macrocycles can be extended to form multilayer and Langmuir-Blodgett (LB) films [81-83], As shown by absorption spectroscopy, the chromophores are aggregated within the layer. Upon excitation of the porphyrin subunit, long-lived radical ions are formed in the layers as a result of the high polarity experienced by the complexes in this medium. 

The porphyrinic complexes are all essentially planar. However, the simple porphyrin and tbp macrocycles are somewhat flexible, and can adopt a ruffled, or saddle-shaped distortion through twisting at the methine carbon atoms. These deformations cause almost negligible changes in electronic structure, but the conformational mobility offers an added element of subunit variability. 

Macrotetracyclic cryptate 87 containing a Zn-porphyrin complex as a photoactive subunit and two lateral [18]-N204 macrocyclic receptors was developed by Lehn and co-workers.146 In a CHC13/ CH3OH (9 1) solution, the two receptors were able to complex Ag+ to form a polymetallic cryptate, 87-Ag+. The Ag+ ion in the polymetallic cryptate quenches the typical Zn- porphyrin fluorescence by a 10-fold factor via a PET process and by forming a long-lived charge-separated state. 

A very large class of macrocyclic thiophene derivatives is the thiophene-modified porphyrins. Several compounds (153-157) with ter- or quaterthiophene subunits <2006OL2325, 2006OL4847, 2007CC43> have been reported. All of them have been synthesized by the classical acid-catalyzed porphyrin synthesis. Solution and solid-state structures have been investigated using nuclear magnetic resonance (NMR) and ultraviolet (UV) spectroscopy, calculations, and X-ray structures. 

Throughout this book an effort has been made to use both the original authors trivial names as well as designations based on a Franck-type approach. Since the rationale for using this latter systematic nomenclature is to allow for easy inter-comparisons between classes of molecules, a formal definition is appropriate here. For a given macrocycle, the number of pyrrole or pyrrole-like substituents is first determined and the system designated as being a porphyrin (four subunits), pentaphyrin (five subunits), hexaphyrin (six subunits), etc., as warranted. This compound name is then preeeded by a number (in brackets) that indieates the... 

The review presented here has a more synthetic focus than its predecessors. Detailed information about the physical properties of the corrin-related macrocycles is, therefore, not included here. Instead, the reader is referred to the earlier reviews, as well as to a number of relevant papers, for detailed descriptions of the physical properties of corrin-type systems.Still, in this chapter, a complete, up-to-date discussion of corrole and heterocorrole synthesis and metalation properties will be presented. Also, two sections will be devoted to other synthetic contracted porphyrins, including isocorroles, and several systems that contain fewer than four pyrrole-like subunits in their macrocyclic framework. 

Vinylogous porphyrins are formally built up of odd-numbered carbon spacers separating four pyrrole rings. There are, however, several expanded porphyrins that contain four pyrrolic or pyrrole-like subunits that contain only even-numbered carbon spacers. These macrocycles will be grouped under the general heading stretched porphycenes , since they are typically prepared via the low-valent titanium mediated coupling of pyrrolic aldehydes just as are the porphycenes (cf. Chapter 3). 

Analysis of the products indicated that these new macrocycles consisted of six pyrrolic subunits linked in a (1.1.1.1.1.1) fashion by six methine subunits. As such, they may be referred to as being [26]hexaphyrins-(l.l.l.l.l.l) in the Franck-type nomenclature used in this book. Here, the key macrocycle name, hexaphyrin, came from Gossauer. Indeed, because this investigator viewed products 7.9-7.12 as being the next higher porphyrin homolog in the porphyrin-pentaphyrin series coined the term hexaphyrin for this class of macrocycles. 

In recent years, the desire to generate new expanded porphyrins has led to the preparation of ever larger macrocycles. Most of these have consisted of tetra-, penta-, and hexapyrrole-type macrocycles. However, a few higher order systems, expanded porphyrins containing more than six pyrrole-type subunits, are now known. It is a review of these systems, still limited to macrocycles containing eight and ten pyrrole-type subunits, that is the subject of the present chapter. 

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