Decomposition of metal tetrasulphophthalocyanines incorporated in SiO2 gels

The aggregation of metal 2,9,16,23-tetrasulphophthalocyanines, MTSPc (M = Fe, Co, Ni, Cu and Al), embedded in transparent monolithic gels of silica, prepared by the sol–gel method has been successfully controlled by the addition of N,N-dimethylformamide (DMF) or pyridine (py). The utilization of DMF produces materials less prone to fracture and with better optical properties than those obtained by the use of pyridine. In the case of CuTSPc, CoTSPc and NiTSPc species with DMF (4% v/v) some colour fading in the gels was observed. The comparative study of gels prepared with M(II)TSPc (M = Cu, Ni and Co) and (OH)M(III)TSPc (M = Fe, Al) species has shown that loss of colour in solution and in the monoliths depends on the oxidation number of the cation. An interaction between the DMF and the metal macrocycle, favoring the disruption of the conjugated double bond system of the organic macrocycle has been originally proposed, but now evidence is given that the possible decomposition of the complex is caused by internal conditions prevailing in the gel. Experiments in alkaline solution confirm that, when trapped in gels, (OH)M(III)TSPc forms μ-oxo dimers, and M(II)TSPc decomposes.