QUENCHING OF THE EXCITED-STATE OF HYDRATED EUROPIUM (III) IONS BY ELECTRON-TRANSFER

The quenching of *Eu(aq)3+ by several metal complexes and organic compounds takes place by electron transfer. The reactions are rather slow and only the most powerful reductants quench *Eu(aq)3+ effectively. The most reactive quenchers were Ru(NH3)62+, Fe(C5Me5)2, and Fe(C5H5)2. It was observed that Eu2+ and Cr2+ have similar reactivity with *Eu3+. Quenching with N,N,N',N'-tetramethyl-1,4-phenylenediamine (TMPD) and N,N'-diphenyl-1,4-phenylenediamine (DPPD) yields the strongly absorbing radicals TMPD+. and DPPD+., thus providing direct evidence for electron transfer. The slow rates of the *Eu3+ reactions imply that the electron-exchange rate constant for *Eu(aq)3+/Eu(aq)2+ is small, like that for the ground-state couple. The energy transfer quenching by metal ion complexes is also inefficient, as demonstrated by the lack of reactivity of non-reducing Mn2+ and Co(NH3)63+ complexes. Eu3+ in the ground state reacts effectively with excited state biacetyl by energy transfer to form *Eu3+.