Electrochemical and spectroelectrochemical behavior of methyl pheophorbide a-zinc phthalocyanine dyad

The electrochemical properties of a covalently linked conjugate (Dyad) composed of methyl pheophorbide a (Pheo a) and zinc(II) 2-(2-hydroxymethylbenzyloxy)-9(10),16(17),23(24)-tri-tert-butylphthalocyaninate (ZnPc) were studied. Cyclic voltammetry (CV) and square wave voltammetry (SWV) measurements revealed three reductions at -1.59, 1.78 and -2.03 V vs. Fc(+)/Fc and four oxidations at 0.06, 0.47, 0.57 and 0.87 V vs. Fc+/Fc in o-DCB containing 0.05 M TBAPF(6). The first reduction included two overlapped processes, the reduction of Pheo a followed by the reduction of ZnPc, separated by ca. 0.08 V. Compared to individual components, the ZnPc subunit in Dyad undergoes the first oxidation more easily with a cathodic shift of potential by 0.08 V, while Pheo a subunit, on the contrary, is more difficult to oxidize showing an anodic potential shift of 0.14 V. This indicates a certain interaction between the two subunits in Dyad, which is additionally confirmed by a slight bathochromic shift of the Q band positions in the UV-vis spectra of Dyad relative to its components. Moreover, the cathodic shift of the second oxidation potential of the ZnPc subunit concerning individual ZnPc reaches 0.35 V, indicating an intensification of the electronic interaction between [ZnPc](center dot+) and Pheo a in Dyad. The spectroelectrochemical experiment showed that the reversible first oxidation of the ZnPc subunit is followed by its irreversible second oxidation, which proceeds slightly ahead of the first oxidation of the Pheo a fragment. Thus, both electrochemical and spectroelectrochemical studies support the conclusion on the enhanced interaction between [ZnPc](center dot+) and Pheo a in Dyad, which even leads to a change in the order of their further oxidations.