Triplet energies and the singlet oxygen quenching mechanism for 7H-pyrazolo[5,1-c]-1,2,4-triazole azomethine dyes

Abstract

Analysis of triplet energy transfer rate constants gives the triplet energies of six 7H-pyrazolo[5,1-c]-1,2,4-triazole azomethine dyes, with λ_max values in the range 546–633 nm in ethanol, to lie in the range 115–88 kJ mol⁻¹. Energy transfer rates from porphyrin and phthalocyanine sensitisers can be well approximated using the Balzani equation with a zero or small reorganization energy, and a transmission coefficient ca. 1/1000 that of the fully adiabatic value. A comparison of data on triplet energies of azomethine dyes suggests a relationship between the dye absorption energies and triplet energies of the form: E_T = 0.69(±0.04)(E_λmax) −33(±9) kJ mol⁻¹. A detailed study of the quenching of ¹O₂* by one of the dyes shows that this reaction is accompanied by isomerisation of the dye. This is interpreted as strong indirect evidence for an energy transfer mechanism for the process, a conclusion which is supported in a general way by the value of the ¹O₂* quenching rate constant.