Two asymmetrically substituted porphyrins (5,15-bis(4-nitrophenyl)-10,20-bis(3,4-bis(2-ethylhexyloxy)phenyl)-21H,23H-porphine, CAH3; 5,15-bis(4-aminophenyl)-10,20-bis(3,4-bis(2-ethylhexyloxy)phenyl)-21H,23H-porphine, CAH4) have been used in this work. Good Langmuir monolayers of these compounds have been prepared on water and transferred successfully to glass slides at much faster than conventional deposition rates (500 mm min -1 ). The monolayer behavior has been investigated by measuring surface pressure-area isotherms as well as by Brewster angle microscopy (BAM) and reflection spectroscopy. For both porphyrins, the intermolecular π-π interaction is sufficiently strong that significant preaggregation occurs prior to compression of the film. However, as the monolayers are compressed, changes in the molecular association and orientation are observed for CAH4 while no molecular reorganization is appreciated for CAH3. The different molecular packing of the Langmuir monolayers is maintained during the transfer. The UV-vis spectra of CAH3 and CAH4 solutions, respectively, were found to be sensitive to NO2 gas. However, only the Langmuir-Blodgett (LB) assemblies of CAH4 showed a response in the presence of the toxic gas. This different behavior has been explained in terms of the different molecular organization of the two porphyrins.

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In this paper we have investigated the NO 2 gas-sensing properties of LB film assemblies of 5,15-bis(4aminophenyl)-10,20-bis [3,4-bis(2-ethylhexyloxy)phenyl]-21H,23H-porphine (CAH4). The optical absorbance spectrum of these films is dramatically affected when exposed to low concentrations of NO 2 gas. LB films of CAH4 were prepared by using ultra-fast deposition and characterized by imaging ellipsometry. The high deposition rates employed (500 mm min 21 ) led to an inhomogeneous structure with high porosity. The LB film exposed to 4.6 ppm NO 2 showed a sensitivity of 60% relative absorbance change at 439 nm. The response was found to be faster than that measured in similar systems. The fast response can be explained in terms of the molecular structure of the porphyrin as well as the enhanced surface area of the porous film. The optical response of the CAH4 film gradually decreases as its temperature is increased, a result of a shift in the adsorption-desorption equilibrium towards desorption. An activation energy of 0.48 eV is obtained. Full recovery of the original spectrum after exposure to NO 2 is obtained and can be dramatically accelerated with gentle heating (353 K). The concentration dependence of the optical response over the range 0.46-4.6 ppm NO 2 obeyed a Langmuir adsorption model. Ageing experiments have shown that the basic response of the CAH4 assemblies is not affected over a time period of at least 1 year.

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