SUNLIGHT-PROMOTED PHOTOSENSITIZING AND PHOTOPHYSICAL PROPERTIES OF PORPHYRINS.
Corrispondence should be addressed to M. Tronchin
In order to start a "field" research line about potential porphyrin photopesticides six of them, namely Coproporphyrin (CP), Haematoporphyrin (HP), Protoporphyrin (PP), Zinc-Protoporphyrin (ZnPP), meso-Tetracarboxyphenylporphine (TCPP) and meso-Tetrasulphonatophenylporphine (TPPS) were chosen as representative of porphyrin main classes.
They were compared for theoretical sunlight-dependent singlet oxygen production effectiveness. For each molecule this parameter was calculated multiplying its singlet oxygen quantum yield by the overlap integral of the absorption spectrum with sunlight emission spectrum. With this parameter TCPP and PP seemed to be the more promising molecules. The overlap integral appeared to be the most important parameter to discriminate the theoretical sunlight-promoted efficiency. Only CP was limited by an other parameter: its singlet oxygen quantum yield.
In order to understand the influence of the more relevant photophysical properties,( i.e. triplet and fluorescence quantum yield, the energy of the first excited states, their lifetimes and quenching constants by molecular oxygen) of CP upon its singlet oxygen quantum yield we compared them with the ones of the other screened porphyrins.
Sun-activated pesticides, and particularly porphyrins, on the other hand, show interesting positive properties with respect to traditional pesticides. Light may not penetrate deeply in human body (1) and therefore only sun-exposed areas of accidentally-poisoned persons are in danger. Few millimeters of light penetration is a limited portion of vertebrate body, while can be half of the body, or more, in insects. Sun-activated pesticides may therefore be selectively poisoning toward invertebrates. Thinking about a new class of pesticides one should consider also pollution. Among all photosensitizing molecules porphyrins are mostly naturally-occurring molecules, some of them normally present in human (and not only human) blood (Protoporphyrin), excretions (Coproporphyrin, Uroporphyrin) and therefore are good candidates for environmental-friendly photopesticides.
In order to start a photopesticide research line with porphyrins we screened six of them (see fig. 1), taken as representatives of their porphyrin classes, to determine the best candidate to sunlight-produced photosensitization. In order to understand the "bulk" efficiency results and allow future developments with different molecules we also determined their most important photophysical parameters.