Oxidation of photochromic spirooxazines by coinage metal cations. Part I. Reaction with AgNO3 : formation and characterisation of silver particles

Abstract

Thermal oxidation of the zwitterionic form of two photochromic spirooxazines: 1,3-dihydro-1,3,3-trimethylspiro(2H-indole-2,3′-[3H]naphth[2,1-b]-[1,4]oxazine) (1) and 1,3-dihydro-1,3,3-trimethylspiro(2H-indole-2,3′-[3H]phenanthr[9,10-b]-[1,4]oxazine) (2) has been studied in the presence of silver ions in anaerobic conditions. The products of the reaction have been identified and a mechanism for their formation is proposed. In alcohols (MeOH, EtOH), the major degradation product is the indolinooxazole salt together with either methoxy- or ethoxy-substituted spirooxazines at the C2′ carbon atom of the C2′N1′ bond. The four new spirooxazine derivatives display no photochromic properties although X-ray diffraction studies show that the bond lengths around the spiro carbon atom are similar to those observed for compounds revealing colouration. Photophysical properties of the indolinooxazole and of its hydroxylated derivative are also presented. High quantum yield and large Stokes shifts are observed. As a consequence of the reduction of silver ions to neutral atoms, silver colloids have been prepared after either thermal- or photo-oxidation of spirooxazine. They are stabilised by poly(N-vinylpyrrolidone). The formation and growth of silver colloids have been studied by electron microscopy, photon correlation spectroscopy and UV-vis absorption spectroscopy. The growth of silver particles of different sizes and morphologies is presented and the role of a photochromic electron donor on the growth process is discussed. The average particle sizes ranged from a few to approximately one hundred nanometers. The classical plasmon absorption band at 420 nm and a broad red-shifted peak at 790 nm due to the large non-spherical particles are thus observed. Both particle size and plasmon absorption are dependent upon UV irradiation. Size enlargement upon irradiation of silver colloids brings about colour changes from yellow through green to light blue.