Intermolecular interactions in monolayers of porphyrins

Abstract

Monolayers containing solely porphyrins with or without aliphatic chains or porphyrins mixed with phospholipids are studied on water on solid surfaces. The systems are manipulated on the water surface and characterized by film balance techniques, fluorescence microscopy, fluorescence and absorption spectroscopy and, after transfer onto solid supports, by optical and electron microscopy techniques. The data presented contain information on the interrelation between molecular and monolayer structure and on porphyrin-porphyrin and porphyrin-lipid interactions. Homogeneous monolayers can be formed only for porphyrins containing aliphatic chains or in dilution with lipids in the fluid phase. An ordered structure is obtained for zinc 3,8-bis(1'-heptadecenyl)deuteroporphyrin dimethylester (ZnHDPDME) that exhibits two aliphatic chains attached to one edge and two (hydrophilic) ester groups to the opposite edge of the porphyrin moiety. If these groups are bound to the same edge, a disordered structure with little tendency to aggregate formation is obtained. None of the porphyrins are soluble in the solid phase of phospholipids. As this is presumably due to the high packing density given by the area per hydrocarbon chain this is suggested to hold also for other aliphatic matrices including fatty acids. A strong decrease in fluorescence yield with increasing molecular density is observed for ZnHDPDME diluted in phospholipids. This suggests a change in the coordination of the central zinc atom of the porphyrin. As the environment can be changed in a defined and continuous way these systems offer the possibility of studying the relation between the microscopic surroundings and the optical properties.