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The covalently linked chlorophyll a dimer exhibits remarkably different properties in the folded and open configurations. In the folded configuration the absorption maximum is at 695 nm and the fluorescence maximum is at 730 nm. Laser output at 733 and 735 nm is obtained for solutions in wet benzene and 0.1 M ethanol/toluene, respectively. Measurements of fluorescence lineshapes, made with a transverse excited atmospheric (TEA) nitrogen laser for excitation, show the lifetime shortening associated with stimulated emission resulting from appreciable concentrations of molecules in S(1) excited states. In contrast, the open dimer has absorption and fluorescence spectra essentially the same as those of chlorophyll a monomer. Unlike either the folded dimer or chlorophyll a monomer, the open dimer shows no laser emission or fluorescene lifetime shortening. It does not appear that the behavior of the open dimer can be explained in terms of excimer or triplet formation or by nonradiative decay processes. It is suggested that absorption of the exciting radiation by S(1), leading to the formation of an exciplex or charge transfer state, may be involved. Significantly, no large changes in fluorescence quantum yield or fluorescence lifetime are observed for these dimers as compared to monomer chlorophyll. This suggests that concentration quenching and lifetime shortening in condensed chlorophyll systems involve more than the simple proximity of two chlorophyll molecules.

The zinc and magnesium tris(pyrochlorophyllide a) 1,1,1-tris(hydroxymethyl)ethane triesters have properties that are very similar to those exhibited by pairs of pyrochlorophyll a or chlorophyll a linked by ethylene glycol through their propionic acid side chains. In the open configuration, the fluorescence lifetimes and emission spectra of the triesters resemble those of chlorophyll a monomers. However, the triesters have a mechanism for energy dissipation not present in monomeric chlorophyll a, which prevents the build-up of significant concentrations of molecules in S(1) states. Optical pumping of these compounds in their folded configuration results in the formation of high concentrations of molecules in S(1) states and in laser light emission, which originates from the two folded macrocycles. The optical data indicate that efficient energy transfer occurs from the open to the folded configurations.

Stimulated emission and lasing have been demonstrated for chlorophyll a, chlorophyll b, and bacteriochlorophyll a as well as for the Mg-free chlorophyll derivatives pheophytin a and methyl pheophorbide a. Population inversion was obtained by optically pumping with an N(2) laser. For the three chlorophylls, lasing was observed for only the (0,0) vibronic region of the S(1) --> S(0) fluorescence band. In contrast, lasing was observed for both the (0,0) and (0,1) vibronic regions of the S(1) --> S(0) fluorescence band of pheophytin a and methyl pheophorbide a. Differences between the lasing behavior of the chlorophylls and the Mg-free derivatives probably reflect differences in aggregation behavior. Laser pulses of duration shorter than the fluorescence lifetimes were generated. This result suggests that stimulated emission must be considered in the interpretation of fluorescence lifetime data for in vitro and in vivo chlorophyll-containing systems.