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 comp ounds takes place by electron transfer. The reactions are rather slow and only the most powerful reductants quench Eu(aq)3+ effectively. Th e 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 absorbin g radicals TMPD+. and DPPD+., thus providing direct evidence for elect ron transfer. The slow rates of the Eu3+ reactions imply that the ele ctron-exchange rate constant for Eu(aq)3+/Eu(aq)2+ is small, like tha t for the ground-state couple. The energy transfer quenching by metal ion complexes is also inefficient, as demonstrated by the lack of reac tivity of non-reducing Mn2+ and Co(NH3)63+ complexes. Eu3+ in the grou nd state reacts effectively with excited state biacetyl by energy tran sfer to form Eu3+.