Porphyrins-cyclodextrin. 1. Photooxidation of zinc tetrakis(4-sulfonatophenyl)porphyrin (ZnTSPP) in cyclodextrin cavities: the characterization of ZnTSPP dication. Photolysis, radiolysis, and NMR studies

Abstract

NMR and spectrophotometric evidence for the formation of a 4:1 cyclodextrin-zinc tetrakis(4-sulfonatophenyl)porphyrin (ZnTSPP) (CD-ZnP) complex is reported. Photolysis of CD-ZnP at 422 nm at pH = 12 was found to lead to quantitative formation (99.9%) of 1,2-dihydroxyporphyrin, which exhibits intense absorption bands at 340 and 625 nm and an emission band at 642 nm. At much lower pH the photolysis was found to lead to the efficient formation of isoporphyrin or its hydroxylated form (pK = 6.0), which possesses characteristic absorption bands in the near infrared region. The nonfluorescing isoporphyrin is converted slowly via molecular rearrangement and further hydroxylation into the fluorescing 1,2-dihydroxyporphyrin (k = 1.1 M{sup {minus}1} s{sup {minus}1}). Emission and excitation spectra of the final stable products are reported. OH{sup {sm bullet}} radical also leads to the formation (although inefficient) of 1,2-dihydroxyporphyrin as shown by {gamma}-radiolysis and pulse radiolysis experiments. Pulse radiolysis experiments in the presence of Br{sup {minus}} were carried out as well. Inhibition of the electron transfer from ZnTSPP to the Br{sub 2}{sup {sm bullet}{minus}} radical in cyclodextrin solution was observed and investigated in detail. The photophysical properties of the inclusion complex and of ZnTSPP{sup 2+} were determined by using the time-correlated single-photon-counting technique and themore » nanosecond laser absorption spectroscopy method. The absorption spectra of ZnTSPP{sup {sm bullet}3+} and ZnTSPP{sup 4+} are also reported.« less