Retinylidene Iminium Salts and Related Systems

Abstract

<p>This thesis encompasses some investigations into the structure and chemistry of iminium salts. The interest in this work stems from previous investigations of the visual pigment rhodopsin and a related protein, bacteriorhodopsin. Both of these proteins have been shown to consist of an iminium salt linkage between the chromophore and the protein. Also, these compounds are able to absorb light in the visible region of the spectrum and undergo efficient isomerization processes.</p> <p>A series of iminium salts related to these natural chromophores were prepared and characterized by a variety of spectroscopic methods in both solution and the solid states. In particular, the ground state structure of these positively charged molecules was examined in terms of charge delocalization, cation anion interactions and chromophore conformation. These properties have been previously suggested to be important in the natural chromophores. One conclusion of this study was that positive charge in these "model" systems is localized in the iminium portion of the molecule. Secondly, it was found that the conformations of these molecules may be different in the solid state from that in solution. In at least one case, the conformation of the chromophore was found to be identical to that found in the natural pigment, bacterior-hodopsin.</p> <p>The photochemical and thermal reactivity of several of the prepared iminium salts was also investigated using high field ¹H NMR spectroscopy as an analytical method to directly monitor these reactions. Using this technique, it was found that several of the iminium salts studied underwent efficient trans to cis photoisomerization reactions. In one case, the efficiency of this reaction was very close to that found for bacteriorhodopsin. Moreover, the primary photoproducts formed in these reactions were found to be stable under the experimental conditions used. This was in contrast to previously reported investigations of similar compounds, where cis/trans thermal isomerizations occurred in the reaction medium.</p>