Oxidative fluorescence quenching of zinc tetraphenylporphyrin (ZnTPP) by trivalent lanthanide ions in several solvents: Role of lanthanide-induced singlet–triplet crossing

Abstract

The fluorescence emission arising from the S 1 -state of zinc tetraphenylporphyrin (ZnTPP) is quenched by trivalent lanthanide ions in different solvents. The quenching process involves several types of interactions. The paramagnetic Ln 3+-ions enhance the spin-orbit coupling of ZnTPP facilitating singlet–triplet crossing or induced system crossing ( k ind SC). Values of k ind SC measured for different Ln 3+-ions exhibit a fair correlation with the number of unpaired f-electrons in the Ln 3+-ions used. Induced system crossing reaches a limiting value at a critical [Ln 3+], which is of the order of (2–5)×10 −3 M, and it does not increase upon further addition of Ln 3+-ions. Contribution of electronic excitation energy transfer from 1 ZnTPP* to Ln 3+-ions is limited to Pr 3+, Ho 3+, Nd 3+, Er 3+ and Tm 3+ ions only, due to very unfavorable spectral overlap with remaining Ln 3+-ions. On the other hand, electron transfer from 1 ZnTPP* (and possibly 3 ZnTPP* ) to Ln 3+-ions, which is thermodynamically allowed for several Ln 3+-ions depending upon the solvent used, plays a significant role in the quenching of 1 ZnTPP* (S 1) in CH 3CN, a limited role in DMF and a negligible role in 2-propanol, in spite of favorable Δ G 0 el.tr. values in the latter. Values of k ind SC and ( k ET+ k el.tr.) in CH 3CN range from 1.6×10 7 s −1 (Tm 3+) to 7.0×10 7 s −1 (Gd 3+) and 1.2×10 10 M −1 s −1 (Eu 3+) to nearly zero (Gd 3+), respectively. Corresponding values in DMF are lower.