Abstract
The photoreactivity of zinc tetraphenylporphyrin (ZnTPP) in the presence of photoexcited CdS nanoparticles and molecular oxygen in 2-propanol has been revisited. The photodecomposition of ZnTPP in 2-propanol, in the presence of CdS(e-CB/h+VB)and molecular oxygen, is linked to isopropyl free radicals formed by both hole transfer from CdS(h+VB)to(CH3)2CHOH and the reaction of superoxide ion(O2−)with 2-propanol. Isopropyl free radicals react with ZnTPP, leading to the formation ofZnTPP•−. Intermediate paramagnetic species monitored by EPR spectroscopy(g=2.0029,ΔH=5.7G)were characterized as superoxides formed in the reaction ofZnTPP•−withO2. Further transformations of such EPR-active species lead to the formation of diamagnetic intermediate photoproducts of ZnTPP observed by absorption and emission spectroscopy.
Highlights
Photocatalytic reactions initiated by colloidal semiconductor nanoparticles have been studied extensively during the past two decades [1]
Ellis and co-workers have demonstrated that metalloporphyrin films on the surface of II–VI semiconductors can be used as transducer films for chemical sensing [4]
Among the metalloporphyrins used in the photochemical studies mentioned above [3], zinc tetraphenylporphyrin (ZnTPP) was studied in more detail in the presence of electronically excited CdS nanoparticles and molecular oxygen [3, (a)–(d)], leading to the formation of open-chain tetrapyrrole compounds and their metal complexes [3, (d)]
Summary
Photocatalytic reactions initiated by colloidal semiconductor nanoparticles have been studied extensively during the past two decades [1]. Among the metalloporphyrins used in the photochemical studies mentioned above [3], zinc tetraphenylporphyrin (ZnTPP) was studied in more detail in the presence of electronically excited CdS nanoparticles and molecular oxygen [3, (a)–(d)], leading to the formation of open-chain tetrapyrrole compounds and their metal complexes [3, (d)]. Furthemore, the recombination luminescence of CdS(e−tr./h+tr.) nanoparticles in solution was found to be quenched effectively by ZnTPP, both in the absence and presence of molecular oxygen [3, (a)–(c)]. Irradiation of CdS nanoparticles (hν > Eg) in the presence of ZnTPP may lead to CdS-induced photochemistry of ZnTPP via both electron and hole transfer. To distinguish between the contribution of photogenerated electrons and holes in the photochemistry of ZnTPP appropriate EPR studies were undertaken in the presence and absence of molecular oxygen and under conditions controlling the availability of photogenerated conduction band electrons (e−CB)
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