Sensitized photolysis and luminescence of the tris(ethylenediamine)chromium(III) ion in aqueous solution

Abstract

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.

Mechanistic Studies of the Photoinduced Quinone Trimethyl Lock Decaging Process

“…The reciprocal of the y -intercept in eq is designated as Φ sen lim (eq ) and describes the sensitized quantum yield for the disappearance of 1 in the limit where deactivation of the triplet sensitizer occurs exclusively through collisions with the quinone. Similar analyses have been presented previously …”

“…Similar analyses have been presented previously. 47 ϕ ϕ Determination of Φ sen lim from the fit is accomplished by averaging three independent samples. Although the standard deviation in Φ sen lim is consistently less than 10% (Figure 7, error bars), we note that the slope in the double-reciprocal plots is unexpectedly sensitive to the concentration of thioxanthone.…”

“…The y-intercept, however, is clearly unaffected. This kind of behavior has been seen before, 47 and it was not probed further.…”

Role of attractive interactions in endo-exo stereoselectivities of Diels-Alder reactions

“…fluxing ether, giving a 90 % yield of 6. Prior to reductive cyclization, the ethylene ketal of 6 was prepared13 which was then reduced with hydrogen-platinum oxide at 50 psi to give 7 in 50% yield from 6 (crystals from EtOAc-Et20, mp 134-136°). Removal of the ketal with trityl fluoroborate14 in methylene chloride gave 8 as an oil in 90% yield.…”

Mechanistic inorganic photochemistry, II. Applications of intermolecular energy transfer and excited state quenching

“…Basically, the approach consists of comparing the quantum yields for some observable photoprocess in the presence and absence of either a sensitizer or quencher. The relatively straightforward example given below should suffice to demonstrate the principle involved; the interested reader is referred to articles which discuss more complicated systems (20)(21)(22).…”