Orientation Effects in Triplet−Singlet Difference Spectroscopy Measured with Absorbance-Detected Magnetic Resonance^†

Absorbance-detected magnetic resonance (ADMR) triplet-singlet difference (T-S) spectra are often measured with unpolarized light. We show that under nonsaturating conditions ADMR spectra are a function of the angles between the optical and microwave transition moments. T-S spectra measured in this manner are not in agreement with the widely accepted definition of T-S spectra as the angle-independent difference between triplet and singlet absorbance, In order to remove the dependence on the angle between the transition moments from the ADMR-detected T-S spectra, T-S spectra have been recorded as the sum of the change in absorbance with light polarized parallel plus twice the change in absorbance with light polarized perpendicular to the microwave field. Spectra are shown for the photosynthetic bacterium Rhodobacter sphaeroides R26 and the Fenna-Matthews-Olson (FMO) complex of the green sulfur bacterium Prosthecochloris (Pr.) aestuarii.