FOURIER-TRANSFORM INFRARED-SPECTROSCOPY OF ELECTROGENERATED ANIONS AND CATIONS OF METAL-SUBSTITUTED BACTERIOCHLOROPHYLL-ALPHA

The monoanions (the terms monoanions (anions) and monocations (cations) used in this paper refer to pi-monoanion radical and pi-monocation radical, respectively) and monocations of transmetalated bacteriochlorophyll a [M]-BChla (M = Mn, Zn, Cd, Co, Ni, Cu, Pd) and the corresponding 13(2)-hydroxy derivatives [M]-OH-BChla were investigated by a combination of Fourier transform infrared (FTIR) spectroscopy and electrochemistry between 1800 and 1200 cm(-1). [M]-BChla undergoing only ring-centered redox processes (M = Zn, Cd, Ni, Cu, and Pd) exhibit FTIR difference spectra typical for the monoanions [M]-BChla(.-) and for the monocations [M]-BChla(.+). The exceptions are the Ni derivatives and the monocations of the Co derivatives. They are attributed to a deformation of the bacteriochlorin macrocycle from a planar toward a nonplanar conformer. [M]-BChla undergoing metal centered redox processes (M = Mn, Co) resemble the FTIR difference spectra of the [M]-BChla undergoing ring-centered redox processes in the carbonyl frequency region and below 1400 cm(-1), where the main contributions arise from C-N and C-C modes. The skeletal modes between 1400 and 1600 cm(-1) are strongly influenced by the metal-centered redox reactions. The 13(2)-ester absorption and difference band is proposed as a marker band for distortions of the planar macrocycle toward a nonplanar conformer.