only behavior which one might associate with concentration quenching as previously defined.At the LC-SC transition point a small recovery in intensity was noted, indicating that at least portion of the nonfluorescent chlorophyll is reorganized to fluorescent form. As was seen in Figure 6, this was accompanied by the appearance of two-component fluorescent decay indicating two new environments for the pigment. The short component shows a very high pressure sensitivity over a very small range of area change. From our previous studies in DOL, it is unlikely that this may be assigned simply to changes in the Forster energy-transfer rate, facilitating quenching by a set number of traps in the medium; rather it would appear more likely that one is observing a sharp increase in the number of trapping sites present which become available to the fluorescent population of chlorophyll over a small area/molecule range. Because one could not expect the total number of traps in the system to increase significantly over such a short concentration region, the observed decreases in lifetime may well indicate diminishing barriers to energy transfer across phase boundaries in this region of pressure.Somewhat more surprising-and perhaps the most interesting finding of this study-is the appearance in this region of the long-lifetime component whose magnitude is essentially that of unquenched chlorophyll in a nonaqueous medium. At this point one may only speculate on the type of organization which insulates this population from either concentration quenching or interaction with traps. It may be essentially free, Le., ejected from the lipid layer either into the subsurface region or into the region above the lipid layer. Alternatively, although less likely, regions which lock monomeric chlorophyll into small concentrations or into orientations highly unfavorable to energy transfer may be formed. The interplay between alterations in local dye concentration and crystalline domain growth as observed by Miller and Mohwald may play a role in creating some microenvironments of isolated chlorophyll as the transition from LC to SC takes place.25 Any explanation of such behavior must also take into account the observed reversibility as pressure is lowered.It may be seen, from both these and previous studies, that Chl a emission parameters respond markedly to changes in the lipid monolayer environment. While earlier studies in DOL indicated that self-quenching via a Forster transfer mechanism governed fluorescence behavior and was essentially independent of lipid packing within the single DOL LE phase, the present results have focused on the discontinuous changes in both lifetime and intensity that can accompany phase changes within the lipid matrix. These data, with those already available, may provide a basis for building more complex monolayer models involving other components of the photosynthetic apparatus from which an expanded understanding of pigment luminescence and its dependence on environment may be obtained.A nonlinear least-squares...