Ž . Thin Solid Films 393 2001 259266 Taking advantage of optical and electrical properties of organic molecules for gas sensing applications Tim H. Richardson a,  , Colin M. Dooling a , Oliver Worsfold a , Liza T. Jones a , Keizo Kato b , Kazunari Shinbo c , Futao Kaneko c , Rose Treggoning d , M.O. Vysotsky d , Chris A. Hunter d a Applied Molecular Engineering Group, Uni  ersity of Sheffield, Hounsfield Road, Sheffield S3 7RH, UK b Graduate School of Science and Technology, Niigata Uni  ersity, Niigata 950-2181, Japan c Department of Electrical and Electronic Engineering, Niigata Uni  ersity, Niigata 950-2181, Japan d Department of Chemistry, Uni  ersity of Sheffield, Brook Hill, Sheffield S3 7HF, UK Abstract The discipline of molecular electronics has grown rapidly over the last 10 years and is driven by the promise of the enhanced applied physical properties of functionalised organic materials compared to their inorganic partners. The subject can be divided Ž . generally into two broad themes, namely active molecular-scale electronics or photonics , in which the control or generation of Ž . Ž . charge or photons at the nanoscale is attempted, and passive supra-molecular electronics or photonics , in which the specific functionality of the molecules is modified by some interaction or process. In this paper, an example of the latter approach to molecular electronics will be given and this will describe the gas sensing properties of a tetra-substituted porphyrin molecule. The Ž   . Ž . optical absorbance spectrum of LB film assemblies of 5,10,15,20-tetrakis 3,4-bis 2-ethylhexyloxy phenyl -21H,23H-porphine EHO Ž is highly sensitive to low concentrations of NO . LB films prepared at much faster than conventional deposition rates 1000 2 1 . mm min yield t response and recovery times of 25 and 33 s, respectively, and show a sensitivity of 60% relative absorbance 50 Ž . change at 430 nm for 4.4 ppm NO . The morphology of these films is revealed using atomic force microscopy to contain isolated 2 micron-size domains which are composed of grains of several nm in diameter. This unconventional structure leads to a useful sensing material as a result of the molecular functionality of the porphyrin coupled to the enhanced surface area of the porous film assembly. The EHO film shows a gradually diminishing optical response as its temperature is increased, resulting from the shift in the adsorption desorption equilibrium towards desorption. The spectrum recovers fully after exposure to NO . The rate 2 Ž . of recovery is slow at room temperature but can be accelerated dramatically with gentle heating 350 K for a few seconds. The kinetics of the gas sensing process have been modelled and found to fit Elovichian surface adsorption for an initial fast surface adsorption process. This is followed by a much slower diffusive process in which the NO molecules diffuse through the bulk of 2 the assembly. The concentration dependence of the optical response over the range 0.8 4.4 ppm follows a Langmuir model.  2001 Elsevier Science B.V. All rights reserved. Keywords: Porphyrin; LB film; Langmuir Blodgett; Gas sensor  Corresponding author. Tel.: 44-114-2824280; fax: 44-114-2728079. Ž . E-mail address: t.richardson@sheffield.ac.uk T.H. Richardson . 0040-609001$ - see front matter  2001 Elsevier Science B.V. All rights reserved. Ž . PII: S 0 0 4 0 - 6 0 9 0 01 01080-X