The spectroscopy of the low-lying bands in the special-pair radical-cations of photosynthetic reaction centres

The spectra of the special-pair cation radicals P⁺ produced after photoexcitation of photosynthetic reaction centres and initial electron transfer can, in principle, provide important information concerning the function of the reaction centres. Extraction of this information requires detailed knowledge of the spectroscopy of the cations, however. We review our contributions to this field concerning the bands observed at near 2500 cm^–1 and 8000 cm^–1, and review results obtained from the study of porphyrin reaction-centre model complexes. We also consider the impact of recent experimental developments in these fields. However, our primary focus is to raise the possibility that the observed band at 2500 cm^–1 is either a composite of two independent electronic transitions or has both an allowed component and a forbidden component arising from vibronic coupling to intense high-energy transitions. The resolution of this dichotomy will have profound consequences for interpretation of the function of photosynthetic reaction centres.