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Optically-detected magnetic resonance (ODMR) experiments in zero-field on the photosynthetic bacteria Rhodopseudomonas sphaeroides revealed triplet states belonging to molecules which have a prompt emission in the optical region from 590 to 700 nm and a delayed emission between 700 and 830 nm. The zero-field parameters of these triplet states are 29 less than [D] less than 34 . 10(-3) cm-1 and 4 less than [E] less than 8 . 10(-3) cm-1, the decay rates of the [D] + [E] transitions being in the order of 60-340 ms. The correlation between optical emission and radio-frequency was used to separate the total optical emission from 590-700 nm into individual emissions, belonging to molecules whose triplet states were studied by ODMR in this region. Comparing the fluorescence microwave double resonance (FMDR) spectra with the results of excitation spectroscopy, as well as comparing the zero-field parameters and the decay rates with that of Mg-porphyrins in matrices given in the literature, allowed the identification of the emitting molecules as Mg-porphyrins which are produced by the biosynthesis of bacteriochlorophyll in the cells.

Prompt and delayed emission of Rhodopseudomonas sphaeroides 'wild-type' strain ATCC 17023 was measured in the spectral range 550-950 nm at 1.7 K. The broad emission spectra could be resolved into separate bands by excitation spectroscopy. Through a comparison of the separated excitation and emission spectra of the present study with absorption and fluorescence spectra of photosynthetic pigments given in the literature, the emitting species could be identified. Beside the well-known pigments bacteriochlorophyll (BChl), bacteriopheophytin (BPh) and carotenoids, additional pigments could be detected. In the visible and near-infrared range, the fluorescence of pigments could be observed which were produced by the biosynthesis of BChl in the cells. Further, fluorescence bands at 753 and 813 nm are interpreted as originating from BPh (F-753) and BChl (F-813). Delayed emission signals between 700 and 800 nm were attributed to a metalloporphyrin phosphorescence. Surprisingly, the excitation spectra of the delayed BChl emissions measured at 872 and 912 nm showed the same excitation spectra as the short-wavelength emission, typical of metalloporphyrins. This is strong indication of an energy transfer between metalloporphyrins and BChl.