Nitrogen dioxide gas-sensing studies on dip-coated films of novel chromophoric siloxane copolymers employing azo derivatives as side-chains

Siloxane copolymers having as side-chains azobenzene derivatives bearing di fferent electron-withdrawing and donating substituents were deposited as th in films by dip coating, and their behaviour upon exposure to 100 pm NO2 wa s studied by UV/visible spectroscopy. Electron-donating substituents at the ortho positions on the aromatic rings in the azo unit have significant inf luence on the absorbance changes produced by exposure to NO2, and this is e xplained by modelling, which suggests that interaction between the electrop hilic nitrogen atom in NO2 and the electron clouds of the azobenzene skelet on is responsible for the sensing process, rather than the formation of a W heland intermediate or other product of chemical reaction. Especially effec tive substituents are methoxy groups in either aromatic ring ortho or the a zo linkage, which also produce a significant increase in intensity of the l ong-wavelength n-pi(*) transition. This moves the optical interrogation sig nal to a wavelength range of particular benefit for potential applications. (C) 1998 John Wiley & Sons, Ltd.