New solvatochromic and halochromic phosphonium betaine dyes.

This thesis is concerned with the synthesis and characterisation of three new types of solvatochromic systems, the 4-N-(O-phosphoniobenzylidene)aminophenolate betaines, the 4-(O-triphenylphosphoniophenylazo) phenolate betaines, and a comparison with the related 4-methyl(or hexadecyl)pyridinium-4-methylidene iminophenolate betaines. The coordination template effect in the nickel-catalysed formation of arylphosphonium salts has been reviewed and applied in the synthesis of the phosphonium betaine dyes. The structural confirmation of these betaine dyes was carried out by NMR and mass spectral analyses, together with the results of microanalysis. The solvatochromic behaviour of these dyes was investigated in detail by dissolution in a range of organic solvents and measurement of their longest-wavelength absorption bands using UV/Visible spectroscopy, which revealed that the phosphoniophenolate betaines have the possibility of two structures, e.g., the dipolar betaine form and a less polar form. The effects of substituents on their solvatochromic properties have also been discussed. Most of the betaines studied display highly negative solvatochromism as the solvent polarity increases, and their absorption maxima are linearly related with the standard E[n][t] values for the Reichardt betaine, which suggests a possible application of these betaines prepared as indicators of solvent polarity. However a few of the N-alkyl pyridinium betaines exhibited an unusual reverse solvatochromism effect. The negative halochromic behaviour of selected betaines has also been studied, a hypsochromic shift of the long-wavelength CT absorption band being observed on addition of salts. The ability of representative members of a family of N-hexdecylpyridinium iminophenolate betaines prepared in this study to form Langmuir-Blodgett films has been explored by Prof.G.Ashwell (Cranfield Univ.). Non-linear optical properties of such films have been observed, together with other properties that may form the basis of sensor techniques.