Meso-substituted porphyrins

Meso substitution of porphyrines to give tetraazaporphyrines, so-called porphyrazines, modulates the electronic character of the macrocycle. While porphyrazines have received considerably less attention than porphyrines over many years, this has changed due to the development of efficient syntheses of soluble derivatives.1 6-1809 Also, various porphyrazines (and phtalocyanines) with peripheral groups for metal ion coordination have been prepared and used for the construction of multimetallic complexes.1806 Ni porphyrazines (695) typically show absorptions spectra with a strong Q band at around 615nm. 

Lambrechts SA, Aalders MC, Langeveld-Klerks DH, Khayali Y, Lagerberg JW (2004) Effect of monovalent and divalent cations on the photoinactivation of bacteria with meso-substituted cationic porphyrins. Photochem Photobiol 79 297-302. 

Merchat M, Bertolini G, Giacomini P, Villanueva A, Jori G (1996) Meso-substituted cationic porphyrins as efficient photosensitizers of gram-positive and gram-negative bacteria. J Photochem Photobiol B 32 153-157. 

If the a-meso regioselectivity of hydroxylation is not inherent to the heme group, as indicated by the fact that all four isomers are obtained by coupled heme oxidation (40), how is the regiochemistry of heme oxidation influenced by electronic factors One possibility is that the puckering or ruffling of the porphyrin caused by steric clashes between the meso and flanking substituents in the meso-substituted heme probes... 

In the case of porphyrins different substitution patterns induce relevant electronic variations the energy level of the two HOMOs of the porphyrin n system (a/u and a2u) are in fact reversed in going from a meso-substituted porphyrin, such as tetraphenylporphyrin, to a -substituted one, such as octaethylporphyrin [44]. Electronic variations may also derive from conformational changes Smith and coworkers have reported hybrid compounds substituted both at meso and P positions, the Zn2+ complexes of 2,3,7,8,12,13,17,18-octaethyl- and 2,3,7,8,12,13,17,18-octamethyl-5,10,15,20-tetraphenylporphyrin [46] (see Sect. 3.3). These complexes have a nonplanar conformation caused by the steric interactions of the substituents. Theoretical calculations indicate that such conformational changes cause a differential shift in the energies of the HOMO and LUMO and so modulate the electronic structure of the macrocycles. 

Most of the porphyrinoid molecules synthesized so far possess a number of peripheral substituents. In fact, many of the important macrocyclic systems were obtained with several distinct substitution patterns. Usually they correspond to one of the generic substitution types of regular porphyrins meso-aryl or p-alkyl, as exemplified by 5,10,15,20-tetraphenylporphyrin and 2,3,7,8,12,13,17,18-octaethylporphy-rin, respectively (Fig. 2). For the sake of brevity we will assume the following conventions in the entire manuscript ... 

Fig. 7 Synthesis of NH-linked Pc-porphyrins 9 substituted at the meso-phenyl group...

Kassab K, Ben Amor T, Jori G, Coppellotti O. Photosensitisation of Colpoda inflata by meso-substituted cationic porphyrins. Photochem Photobiol Sci 2002 1 560-4. 

Pasternack RF, Huber PR, Boyd P et al (1972) Aggregation of meso-substituted water-soluble porphyrins. J Am Chem Soc 94 4511 1517... 

Escudero C, El-Hachemi Z, Crusats J et al (2005) Zwitterionic vs porphyrin free-base structures in 4-phenylsulfonic acid meso-substituted porphyrins. J Porphyr Phthalocyanines 9 852-863... 

In addition to the above reactions, the preparation of metal complexes directly from 46 was also studied. Upon treatment of 46 with either Zn(II) or Cu(II) an immediate ring contraction occurred and the meso-substituted porphyrin derivatives 54 and 55 were again obtained. 

While these expanded porphyrins represent a unique and potentially interesting class of expanded porphyrins, their stability towards ring contraction severely limits their utility, except perhaps as intermediates in the production of meso-substituted metalloporphyrin systems. 

In terms of synthesis, the rosarins are most easily prepared via the condensation of three a-free bipyrroles with three equivalents of an aromatic aldehyde. For instance, rosarin 7.36 was prepared in > 70 /o yield via the condensation of bipyrrole 7.31 with benzaldehyde, followed by oxidation with DDQ (Scheme 7.3.1). In a similar way, the rosarins 7.37-7.39 could also be prepared in good yield. The preparation of rosarins thus bears formal analogy to the Rothemund-Adler-type methodology used to prepare meso-substituted porphyrins. " In marked contrast to what is observed under these latter porphyrin-forming conditions, the rosarins are obtained as 24 jt-electron, non-aromatic 4n 7r-electron species. According to the Franck notation, therefore, the rosarins are formally [24]hexaphyrins-(1.0.1.0.1.0). 

If Eqs. (5) and (6) describe the physics reasonably well, the pressure shift s should vary in a linear fashion with bum frequency Pb, and there should be a frequency, namely, p ac, where this shift vanishes. At p ag, the hole should broaden, only. We tested these predictions in a series of glasses with a variety of dye probes and found perfect agreement with the model.This encouraged us to apply the technique to proteins. We also stress that the conditions under which Eq. (5) holds can be derived from a microscopic model. Figure 4 shows how a hole burnt into the inhomogeneously broadened absorption spectrum of a protein (meso-porphyrin IX-substituted horseradish peroxidase) deforms under pressure variations up to 1.5 MPa. 

A similar incremental effect of porphyrin-quinone separation was observed with the systems shown in Scheme 36 which were prepared by Wittig condensation of the meso-substituted porphyrin 116 (as the nickel complex) with the phosphorus ylide 117 Demethylation, reduction of the double bonds and then oxidation furnished the free base porphyrins 118 and 119a, b. The rate of photoinduced electron transfer in such systems showed an inverse exponential dependence on the length of the chain In order to demonstrate a multistep electron transfer the bis-quinone porphyrin 120 was prepared in which the pair of quinone rings provide a redox potential gradient and may thus stabilize charge separation. Comparison with the mono-quinone etioporphyrin 119a... 

Fig. 1.7 Meso-substituted porphyrin dications distort in a different way to accept two more protons. [From D. Dolphin (ed.). The Porphyrins, Vol. 3, Academic Press, New York.]...

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