Comparison of the photochemical properties of phthalocyanine and hemiporphyrazine Zn(II) complexes

Abstract

In DMF (dimethylformamide) solutions containing concentrations of (NEt4)Br (Et = ethyl) between 0.001 and 0.05 M both zinc phthalocyanine (Pc) and hemiporphyrazine (Hp) complexes are dissolved and form species [Zn(Hp)(Br)]–, and [Zn(Pc)(Br)]–. The increase in formation of the hemiporphyrazine bromide adduct is revealed by a linear dependence of the UV–Vis absorption spectrum on the Br– concentration whereas, over the same concentration range, absorption measurements indicate the formation of the phthalocyanine bromide adduct in solution. The x-ray and molecular structure of [NEt4][Zn(Pc)(Br)](H2O)0.33 has been determined showing the coordination of the Br– to the metal center. Photo-induced processes of the Zn complexes were investigated with deaerated solutions in a time scale t ≥10 ns. The 355 nm flash irradiation of [Zn(Hp)(Br)]– produced transient spectra which showed a bathochromic shift of the absorption maximum from 460 nm to 440 nm and an increase of the spectrum rate of decay with increasing Br– concentrations. The initial spectrum is attributed to the (nπ*–LLCTBr→Hp)[Zn(Hp)(Br)]– excited state which reacts with H-atom donors forming a radical [Zn(Hp-H)(Br)]•–. On the other hand, a transient spectrum, λmax ∼600 nm, grows in a time t ≤0.6 µs when [Zn(Pc)(Br)]– is flash irradiated at 355 nm. The photoreactions of these complexes with SCN– and 2-propanol (IPA) were studied. Despite of the apparent similarity between both complexes, IPA was photo-oxidized by both complexes although by different mechanisms, an electron transfer with [Zn(Pc)(Br)]– and an H atom abstraction with [Zn(Hp)(Br)]–. The photo-oxidation of SCN– by [Zn(Pc)(Br)]– but not by [Zn(Hp)(Br)]– makes a remarkable difference between the photochemistries of these complexes.