QUENCHING OF TRIPLET-STATES OF ORGANIC-COMPOUNDS BY COPPER(II) AND NICKEL(II) 1,3-DIKETONATES IN ACETONITRILE SOLUTION - ENERGY AND OR ELECTRON-TRANSFER/

Rate constants for quenching of the triplet states of 15 organic compo unds by copper(II) bis(2,4-pentanedionate), Cu(acac)(2), copper(II) bi s( 1, 1, 1,5,5,5-hexafluora-2,4-pentanedionate), Cu(hfac)(2), Ni(II) b is(2,4-pentanedionate), Ni(acac)(2), and Ni(II) bis(1,1,1,5,5,5-hexafl uoro-2,4-pentanedionate), Ni(hfac)(2), in acetonitrile solution have b een measured using the nanosecond laser flash photolysis technique. Th e quenching data were interpreted in terms of energy or electron trans fer alone or in terms of both as competitive processes. With a classic al framework for energy and electron transfer, the correlations of the quenching rate constants with the standard free energy changes for en ergy and electron transfer were used to evaluate appropriate intrinsic barriers and transmission coefficients for both processes. In acetoni trile, quenching by Cu(acac)(2) was suggested to occur mainly by elect ron transfer, whereas quenching by Cu(hfac)(2) was shown to involve en ergy transfer to the ligand-localized triplet in combination with elec tron transfer. Applying both the ''quadratic'' Marcus and the ''asympt otic'' Agmon-Levine free-energy relationships led to similar values of intrinsic barriers and to low values of transmission coefficients for the electron-transfer processes studied. The nonadiabatic character o f those processes was also confirmed using the recently developed semi classical approach by Tachiya and Murata which allowed for variable el ectron-transfer distance. A quantum mechanical model was also applied to the data within the phenomenological kinetics scheme, and the fitti ng parameters were consistent with the other approaches. Quenching by the NI(II) 1,3-diketonates was adequately described by energy transfer to metal-centered excited states or ligand-localized triplet states. The influence of changing of solvents from benzene to acetonitrile on the quenching was also discussed.