Quenching of naphthalocyanine triplets by oxygen: application of the Sandros equation

Abstract

Recently the authors showed that the triplet (T/sub 1/) energy of bis(tri-n-hexylsiloxy)silicon 2,3-naphthalocyanine (SiNc, see below) is 1.0 kcal/mol lower than that of singlet molecular oxygen (22.5 kcal/mol). Sandros showed that for endergonic triplet energy transfer reactions the bimolecular rate constant decreases nearly exponentially with increasing gap between the donor and the acceptor. Thus, small variations in the triplet energies of metal 2,3-naphthalocyanines caused by changes in the nature of the metal and solvent are expected to have a significant effect on the efficiency of triplet quenching by O/sub 2/. Further, if the S/sub 1/-T/sub 1/ energy gap (E/sub ST/) remains constant within the 2,3-naphthalocyanine family, then the quenching rate constant should be related by the Sandros equation to the energy of the S/sub 0/-S/sub 1/ transition. The results described below show that the predicted relationship is obeyed with E/sub ST/ = 15.6 /plus minus/ 0.4 kcal/mol, in agreement with the value determined spectroscopically for SiNc. This is the first example of the application of the Sandros equation to the triplet quenching by O/sub 2/ and the first to correlate triplet energy transfer kinetics with singlet energy levels.