Ethane-Bridged Zinc-Porphyrin Dimers in Langmuir-Shäfer Thin Films: Structural and Spectroscopic Properties

This work reports on the structural and spectroscopic properties of ethane-bridged Zn porphyrin dimers (1) in Langmuir- Schäfer (LS) thin films by combining scanning force microscopy (SFM) with film balance, UV-vis absorption, fluorescence, and nanosecond laser flash photolysis measurements. Results show that depending on the surface pressure the Langmuir films of pure 1 can be arranged in two different condensed phases, whereas SFM of the LS films shows characteristic fractal networks constituted by nanoscopic aggregates. The spectral findings agree with a picture in which 1 is apparently present in the anti conformation but aggregated in a sort of H-type structure whose optical features resemble those of the syn conformer. This type of structure is not responsive to light stimuli. By diluting 1 in arachidic acid the porphyrin aggregation is significantly minimized with 1 exhibiting almost exclusively the anti conformation. As a result the LS films become photoresponsive, showing fluorescence emission and triplet-triplet transient absorption