Macroscopic expansion of tetraphenylporphyrin Langmuir layers stimulated by protonation

We report the relationship between the degree of areal expansion of floating Langmuir layers of tetraphenylporphyrins and their molecular structure, and show that their realisable expansion depends on the ratio of their experimental area per molecule (footprint) in the unexpanded film to their modelled molecular area when in the flat, face-down configuration in which the plane of the porphyrin ring is parallel to the plane of the water surface. Protonation of porphyrins leads to dramatic areal expansion of a Langmuir film.