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From the isolated LH2 antenna of Rhodobacter sphaeroides , called B800-850, the bacteriochlorophyll a pigment complement BChl-B800, mainly responsible for the 800 nm absorption band, has been extracted. The remaining complex, with the main NIR absorption peaking at 850 nm (termed pure (p) B850), has a small but distinct sideband in the 800 nm region and contributes there about 20% to the total absorption of the integral B800-850 antenna. In the latter the energy transfer 800→850 nm is per se (at least) biphasic with the pB850 intra-aggregate relaxation as the fastest component known so far ( k ≈10 13 s −1 ).

The properties of one- and two-exciton states of the circular aggregate responsible for the B850 band in the peripheral light harvesting antenna (LH2), extracted from the purple bacterium Rhodopseudomonas acidophila/strain 10050, have been studied at room temperature by non-linear absorption, differential optical density spectroscopy as well as time-resolved fluorescence spectroscopy. As a main result, giant dipole moments for the transition between ground state and the one-exciton band as well as for the transition between the one-exciton band and the two-exciton band have been estimated, corresponding to a delocalization of the excitation energy over most if not all of the 18 BChl a molecules of the aggregate.

The excited-state and lasing properties of 20-chloro-chlorophyll a in ether solution were compared to those of chlorophyll a. Desactivation parameters and cross-sections were obtained from non-linear absorption spectroscopy in combination with a physico-mathematical methods package. The Cl substituent at C-20 (1) increases both intersystem crossing and internal conversion, (2) produces a blue-shift of the S1 absorption spectrum, and (3) leads to pronounced photochemistry.