Photophysical, electro- and spectroelectro-chemical properties of the nonplanar porphyrin [ZnOEP(Py)44+,4Cl−] in aqueous media

The photophysical and electrochemical properties of the tetracationic zinc 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetrakis(N-pyridiniumyl) porphyrin chloride (ZnOEP(Py)44+,4Cl−) were studied in aqueous solutions. The steady state and time-resolved absorption and emission measurements indicate that the porphyrin skeleton adopts a severely nonplanar conformation which minimizes steric crowding between the 12 peripheral substituents. The absorption spectrum of [ZnOEP(Py)44+,4Cl−] in water exhibits significant red shifts of the visible Q and Soret bands as well as considerable broadening and decrease in intensity of the latter compared to the spectrum recorded for the planar [ZnTMPyP4+,4Cl−] porphyrin. The S2 → S1 internal conversion is faster than the experimental resolution (<90 fs) while the S1 excited state has a lifetime of 170 ps. The electrochemical properties of [ZnOEP(Py)44+,4Cl−] were investigated in water at pH 6.5 and 3.0 by cyclic and differential pulse voltammetry as well as spectroelectrochemistry. Reductions take place initially at the pyridinium sites with four successive one-electron steps at pH 6.5 or a one-electron step followed by a three-electron process at pH 3.0. Both oxidation and reduction processes undergone by the porphyrin are irreversible.