Evaluation of solvatochromic materials as sensor components.

Sensors play an important role in industry and in academic research for measurement of various chemical and physical parameters in a variety of environments. Optical materials, whose properties (e.g. absorption, colour) change on interaction with a target analyte, are very useful for sensor applications. Among organic optical materials, there is a class showing electrochromic or solvatochromic properties, which have potential to be used as sensor materials. Normally, organic compounds, with electrochromic or solvatochromic properties, have a conjugated system linking donor and acceptor groups in the molecule. During excitation by a field (electric or light), an intramolecular charge transfer takes place resulting in an excited state with different dipole moment to the ground state. A number of such dye compounds have been synthesised in this work, some of them show considerable electrochromism or solvatochromism, such as 3-ethyl-5-{[1-dodecyl-2(lH)-pyridylidene]-ethylidene}-rhodanine. Its absorption maxima are 524 nm in acetonitrile and 584 nm in ether, having a 60 nm shift. It was found that solvatochromism of such compounds was increased with the increase of the chain length of their conjugated system. The conjugated compounds synthesised were deposited onto a transparent substrate either by solution evaporation or by Langmuir-Blodgett film technology. The films formed did respond to some toxic gases such as NO[2], resulting in an absorption wavelength shift in their uv-vis spectra. Pyridinium N-phenoxide betaine dyes are known as good probes for solvent polarity. The best example is ET(30). ET(30) values have been widely used for polarity measurement of solvents. In order to covalently link these betaine dyes to a solid support for convenience of use, a number of dyes with a functional amino group have been synthesised. This amino group forms the basis for linkage. Additionally, modifications were made to the phenoxide end of the molecule to reduce the phenolate pKa. Dichloro and difluoro variations from the parent diphenyl structure were made. Several methods have been used to try to covalently immobilise the betaine dyes synthesised onto a solid support. Some of them have been successfully immobilised onto a silica sorbent by a thiourea linkage or an amide linkage. Immobilised dyes showed reduced but still good solvatochromism. Their ET values demonstrated linear correlation with standard ET(30) values.