Characterization and fast optical response to NO2 of porphyrin LB films

In this paper, we show that Langmuir–Blodgett (LB) film assemblies of 5,15-bis(4-aminophenyl)-10,20-bis[3,4-bis(2-ethylhexyloxi)phenyl]-21H,23H-porphine (CAH4) exhibit a fast, reversible and reproducible response to NO2 in the concentration range of 0.46–4.6 ppm. The monolayer behaviour at the air–water interface has been investigated by measuring the surface pressure–area isotherms as well as reflection spectroscopy (ΔR) and Brewster angle microscopy (BAM). It was found that intermolecular interaction between porphyrin rings directs the molecular organization even at submonolayer coverage. LB films of CAH4 were prepared using ultra-fast deposition. The high deposition rate employed (500 mm min−1) led to an inhomogeneous structure with high porosity. The LB film exposed to 4.6 ppm of 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. Full recovery of the original spectrum after exposure to NO2 is obtained and can be dramatically accelerated with gentle heating (353 K). 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.