The photophysical properties of porphycene incorporated in small unilamellar lipid vesicles

Porphycene (a porphyrin isomer) has been incorporated into small unilamellar vesicles (diameter about 52 nm), and its first excited singlet and triplet states have been investigated under these conditions. Below a local concentration of about 4 mM in the vesicle, the photophysical properties of porphycene are as in organic solvents. Above this critical local concentration a second shorter-lived component appears in the fluorescence and triplet decays, and the fluorescence yield and lifetime and population of the longer-lived triplet are lower. Also, the rate constant of triplet quenching by oxygen is somewhat higher, and the yields of porphycene- and 2,7,12,17-tetra-n-propylporphycene-sensitized singlet molecular oxygen formation in the vesicles drop as well. Furthermore, the absorbance and fluorescence yield change with temperature when the porphycene concentration is greater than the critical limit: at just below the phase transition temperature of the lipid the two values rise steeply from low plateaus to plateaus about twice as high upon warming. The inherently less sensitive Beer-Lambert absorption plot curves downwards starting at a somewhat higher value than the critical local concentration (10 mM). The findings are discussed on the basis of (i) aggregation of porphycene at high local concentrations and (ii) localization at two distinct sites within the vesicle.