Photo-oxidative Stability and its Correlation with Semi-empirical MO Calculations of Various Tetraazaporphyrin Derivatives in Solution

The photo-oxidative stability of various annelated and substituted tetraazaporphyrin derivatives were investigated in N,N-dimethylformamide by irradiation in the presence of air. First-order rate constants were calculated. In addition, the positions of the HOMO and LUMO energy levels of the compounds were calculated using a commercially available program. A linear correlation between the experimental values of the rate constants and the theoretical values of the HOMO position exists. The method described allows one to predict the photo-oxidative stability by calculating their HOMO levels, which is very important for the use of macrocyclic metal complexes in photo-oxidation reactions in solution. From the calculated triplet energies it is considered that the macrocyclic metal complexes can convert by photoinduced energy transfer triplet oxygen to singlet oxygen. Experimentally, tetraazaporphyrin derivatives show high quantum yields of singlet oxygen formation under irradiation. No correlations of the quantum yields with the position of the HOMOs or with the rate constants of decomposition was observed. (C) 1997 by John Wiley & Sons, Ltd.