The optical gas-sensing properties of an asymmetrically substituted porphyrin

Abstract

In this paper we have investigated the NO₂ gas-sensing properties of LB film assemblies of 5,15-bis(4-aminophenyl)-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 NO₂ 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⁻¹) led to an inhomogeneous structure with high porosity. The LB film exposed to 4.6 ppm NO₂ 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 NO₂ 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 NO₂ 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.