Oxidation of photochromic spirooxazines by coinage metal cations. Part II. Oxidation by gold(III) compounds and synthesis of gold colloids

Abstract

In our previous paper, we reported that colloidal silver can be prepared by thermal or photo-oxidation processes of spirooxazines (SO). Here we show that SO easily reduce gold(III) ions (1 : 1 equiv.) to monovalent gold(I) species. A salt composed of oxidized dye/Au^I was isolated and fully characterised by X-ray diffraction, ¹H and ¹³C NMR, UV-vis, fluorescence and FTIR spectroscopies. This salt was further reduced by another SO (2 : 1 equiv.) leading to gold(0) atoms that coalesce into gold powder. A mechanism for these two reactions is proposed. In the presence of a stabilising agent, such as poly(N-vinylpyrrolidone), the coalescence of Au atoms is limited and nanoparticles with an average size of 8–14 nm and a broad size distribution are formed. The colloidal solution is strongly fluorescent due to the presence of the oxidized SO in the medium. Changes in the absorption spectra from that of the sol and the free oxidized SO are interpreted as a consequence of the adsorption of the cationic dye on the gold surface.