The energy transfer from biacetyl to Cr(en)3s+ has been investigated in aqueous solutions at 15°. The complex quenches the phosphorescent emission of biacetyl but leaves unchanged its fluorescence. Absorption of light by biacetyl leads to 2Eg -* 4A2g phosphorescent emission by the complex (sensitized emission) and also causes the aquation reaction of the complex (sensitized reaction). The quenching of biacetyl phosphorescence by Cr-(en)3s+, the intensity of the sensitized emission, and the quantum yield of the sensitized reaction increase with increasing complex concentration and decrease with increasing oxygen concentration. The limit quantum yield of the sensitized reaction (~0.8) is noticeably higher than the quantum yield of the direct reaction (0.40). In contrast, the ratio between the limit quantum yield of the sensitized emission and the quantum yield of the direct emission is <1 (~0.6). These apparently contradictory results may be explained assuming that (i) there is a process which quenches the biacetyl triplets and causes the sensitized aquation of Cr(en)38+ without involving electronic energy transfer from biacetyl to Cr(en)3s+ or (ii) the excited states of Cr(en)38+ produced by energy transfer are in a more favorable energetic and/or geometrical situation to undergo ligand dissociation than are the excited states produced upon direct excitation.
The quenching of the Cr(en)33+ photolysis by some transition metal ions has been investigated in aqueous solution at 15°. It has been shown that, as happens for the phosphorescence quenching, MnCla does not exhibit any quenching effect, whereas C0CI2 and FeCla do quench the photoreaction. It is also shown that the photoreaction is less quenched than the phosphorescence under the same experimental conditions. Specifically, 40% of the photoreaction directly originates from the lowest quartet excited state (4T2g) prior to intersystem crossing to the lowest doublet (2Eg), whereas the remaining 60% is due to molecules which pass through 2Eg and thus can be quenched. A quantitative analysis based on all the data which are available concerning the photochemical and photophysical behavior of Cr(en)33™ shows that, most probably, the quenchable part of the photoreaction also comes from 4T2g, after back intersystem crossing from 2Eg. Values of about 0.6 and 1 are obtained for the forward and back intersystem crossing efficiencies. The actual values for all the rate constants of the steps that depopulate the 2Eg state are discussed.
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