Tim H. Richardson

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.

Seven different porphyrin compounds have been investigated as colorimetric gas sensors for a wide range of volatile organic compounds. The porphyrins examined were the free base and Mg, Sn, Zn, Au, Co, and Mn derivatives of 5,10,15,20-tetrakis[3,4-bis(2-ethylhexyloxy)phenyl]-21H,23H-porphine. Chloroform solutions of these materials were prepared and changes in their absorption spectra induced by exposure to various organic compounds measured. The porphyrins that showed strong responses in solution were selected, and Langmuir−Blodgett films were prepared and exposed to the corresponding analytes. This was done to determine whether they are useful materials for solid state thin film colorimetric vapor sensors. Porphyrins that readily coordinate extra ligands are shown to be suitable materials for colorimetric volatile organic compound detectors. However, porphyrins that already have bound axial ligands when synthesized only show a sensor response to those analytes that can substitute these axial ligands. The Co porphyrin displays a considerably larger response than the other porphyrins investigated which is attributed to a switch between Co(II) and Co(III) resulting in a large spectral change.

We report a molecular switching event that leads to a reversible large-scale expansion of a floating porphyrin Langmuir layer residing at the air–water interface. This vapour-induced protonation–deprotonation mechanism, responsible for the expansion–contraction cycle, could potentially be exploited in applications in which a physical shape-change is triggered by exposure to an acidic or basic gas.

The optical and electrical properties of dodecanethiol-stabilized nanoparticles (6 nm diameter gold core) have been investigated over a range of film thicknesses and temperatures. The surface plasmon resonance absorbance is found to be dependent on temperature. Heating of the nanoparticle film causes desorption of the thiol from the surface of the gold nanoparticle, resulting in irreversible changes to the absorption spectra of the nanoparticle film. Atomic force microscopy images of the samples before and after heating for different film thicknesses reveal structural changes and increased domain connectivity for thicker films leading to sub-10 Omega resistances measured for the 15-layer film.