Organic thin films for optical gas sensing of nitrogen dioxide: comparison of sol–gel and LB layers

4-Alkoxy-4′-(phenylazophenethyl) alcohol derivatives respond rapidly and reversibly at room temperature when exposed as thin films to nitrogen dioxide and interrogated by change of absorbance at 475 nm. The temperature dependence of sensitivity is described by an Arrhenius activation energy. Values of activation energy obtained from a polymeric derivative deposited as a Langmuir–Blodgett (LB) film and from a low molar-mass derivative trapped in a sol–gel matrix are all close to 0.6 eV, indicating that aggregation effects are negligible, and that this figure represents a sterically-unhindered interaction between nitrogen dioxide and the azo chromophore.