The orientation of the triplet axes with respect to the optical transition moments in (bacterio) chlorophylls

Chlorophyll a (Chl a), bacteriochlorophyll a (BChl a) and bacteriochlorophyll g (BChl g) were studied by absorbance-detected magnetic resonance (ADMR) and linear-dichroic (LD-) ADMR in zero magnetic field using two different glassy matrices. The microwave-induced triplet-minus-singlet absorbance-difference spectra were measured in the wavelength region 500–900 nm. With LD-ADMR the orientations of the near-infrared singlet-singlet and triplet-triplet transition moments relative to the in-plane triplet x and y axes were obtained. For all (B)Chls studied, the QY and near-infrared triplet-triplet transition moments are mutually parallel (within 5°), and oriented in the triplet xy plane. For BChl a and BChl g the QY-transition moment was found to be oriented within 20° parallel to the triplet y axis. For Chl a the orientation of the QY-transition moment with respect to the triplet y axis was found to be 35–45°, the precise value depending on the solvent used.