Photo-oxygenation of meso-Tetraphenylporphyrin Derivatives: the Influence of the Substitution Pattern and Characterization of the Reaction Products

Abstract

<jats:p> The photo-oxidation reactions of several meso-tetraphenylporphyrins have been studied in order to ascertain the chemical stability of this class of compounds towards singlet oxygen. The 2,6-disubstituted ones showed an excellent stability, whilst this is not the case for other porphyrins with different substitution pattern on the phenyl rings. This parallels what has been previously found using mono-oxygenated donors. The steric effects in protecting the macrocycle seems to be predominant over the electronic ones; even with electron-donating groups in the 2,6-positions, e.g. methoxy groups, the porphyrin macrocycle was not degraded. The reaction on the β-tetrabrominated meso-tetraphenylporphyrin also proceeded easily and yielded a bilinone derivative. The structures of the final products were all elucidated by mass, <jats:sup>1</jats:sup> H and <jats:sup>13</jats:sup> C NMR spectroscopy. </jats:p>

Metallophthalocyanines as Catalysts in Aerobic Oxidation

“…The system Fe(III)Pc-BuOOH could operate with a one-electron mechanism-starting from the homolytic scission of the peroxo bridge O-O-when oxidizing phenols to quinones, although it oxidizes to quinones some other substrates such as anthracene and xanthene with a two-electron mechanism (as a consequence of a heterolytic scission releasing OHand affording a very reactive hypervalent intermediate), as shown by the absence of oxidative coupling products, that are typical for one-electron (radical) mechanisms. On the other hand, the significant incorporation of 18 O from 18 O 2 along the oxidation of alkynes is a convincing proof of the radical mechanism. The reasons for such varying behavior of this and other MPc catalysts is still unknown.…”

“…In fact, MPcs are protected by their structure against the main threats jeopardizing redox-active metalloporphyrins, and that is the outstanding limitation towards an extensive use of the latter as industrial catalysts. First, the methinic bridges joining the pyrrole-type rings in porphyrins are the preferential targets of oxidative attack, leading initially to oxophlorins and later to macrocycle breaking and therefore to catalyst destruction [18]. Such methinic bridges are replaced in MPcs by aza bridges, which obviously are generally inert towards oxidative attacks.…”

Antimicrobial Photodynamic Polymeric Films Bearing Biscarbazol Triphenylamine End-Capped Dendrimeric Zn(II) Porphyrin

“…52,53 However, Silva et al demonstrated that the photooxidation reactions of several 2,6-disubstituted meso-tetraphenyl porphyrins have the chemical stability towards O 2 ( 1 Δ g ). 54 If positions 2 and 6 on the phenyl rings are blocked, the photooxidations do not take place. This excellent stability is consequence of steric effects, which protected the porphyrin ring.…”

meso-Aryl-substituted free-base porphyrins: formation, structure and photostability of diprotonated species

“…Photodegradation of the macrocycle in H 2 Por has usually been attributed to oxygen-mediated reactions that may be prompted due to absorption of photonic energy by both the porphyrin substrate and molecular oxygen. Since excited porphyrins may sensitize O 2 species to give rise to the formation of singlet molecular oxygen ( 1 D g ) along with some other radicals, it is frequently referred to as the principal reason for the photo-bleaching of porphyrins [39][40][41][42]. Hence, it is important to assess the stability range for the most promising porphyrin systems, including their protonated forms.…”