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Pulsed laser-induced time-resolved fluorescence spectra for chlorophyll a (Chl a) in benzene are reported. These spectra show that, in addition to the monomeric fluorescence, emission bands are observed at 625 and 690 nm, with a broad spectral feature at -740 nm. Results from selective photochemical bleaching experiments with blue and with red pulsed laser light, together with consideration of theoretical calculations of the energy levels of Chl a dimers and oligomers, permit the assignment of the bands at 625 and 690 nm to the Chl a dimer in benzene. The emission at 740 nm is assigned to higher aggregates of Chl a. IntroductionThe aggregation of chlorophylls in solution is the subject of extensive literature. Much of the information concerning dimeric and aggregated species of chlorophyll a (Chl a) has been obtained by spectroscopic methods,'s2 including absorption and fluorescence measurements at

Pulsed laser induced fluorescence spectra of dimeric and aggregated bacteriochlorophyll a (BChl a) in solution are presented. Two fluorescence bands, at 783 and 583 nm for BChl a in hexane, are assigned to the BChl a dimer. These two bands correspond to the two fluorescence bands of the chlorophyll a (Chi a) dimer in solution at 688 and 625 nm.1 Comparison of absorption and fluorescence data for the two chlorophyll dimers allows assignment of the shorter wavelength fluorescence transitions at 625 and 583 nm to the dimer Qx transitions of the Chi a and BChl a dimers, respectively. The stronger fluorescence bands at 688 and 783 nm are the corresponding dimer Qy transitions. Qx fluorescence transitions are not observed for the monomer of either Chi a or BChl a in solution. The activity of the Qx fluorescence transitions in the dimers and the observation that the fluorescence lifetime of the Qy transition is longer than that of the Qx transition for each of the chlorophyll dimers are discussed in relation to nonradiative channels for deexcitation of the dimers and monomers.

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