Phosphorescence implications

P-type delayed fluorescence is so called because it was first observed in pyrene. The fluorescence emission from a number of aromatic hydrocarbons shows two components with identical emission spectra. One component decays at the rate of normal fluorescence and the other has a lifetime approximately half that of phosphorescence. The implication of triplet species in the mechanism is given by the fact that the delayed emission can be induced by triplet sensitisers. The accepted mechanism is ... 

Furan-2-carbaldehyde has been much studied. A thorough analysis of the first two major electronic transitions has been carried out. Practical work is hampered by the resinification of the compound and by the presence of a trace impurity which gives rise to a long-lived pressure-independent component in the phosphorescence spectrum.23 The absence of n ->n excited emission and other facts implicate a very efficient double intersystem crossing.14 24 Whether or not sensitized by mercury, photodecomposition of the aldehyde gives much carbon monoxide, propyne, and allene. Small amounts of furan, carbon dioxide, and acetylene are also formed. 

An important implication from Eq. 2 is that the observed phosphorescence spectra are (always) already relaxed within the time resolution of the measurement of phosphorescence spectra. Therefore, the true triplet energy (center of the triplet DOS) is always higher than the apparent phosphorescence spectra, which sets a lower limit only. For PF2/6 the true triplet level is calculated to be 2.26 eV instead of the observed 2.15 eV [24] from phosphorescence measurement. This higher value however agrees very well with that measured using the pulse radiolysis energy transfer technique which yields triplet energies in the initial unrelaxed state [71]. 

Luminescence is generally less intense than incandescence, but it often emanates from extremely small amounts of matter, which has beneficial implications for analytical science. Nevertheless, the utilization of luminescence for analysis is quite a recent innovation. The following commentary describes the fundamental spectroscopic and chemical principles underlying luminescence in relation to its application in analytical science. As other articles will deal with atomic spectroscopy, this discussion will be restricted to analytical molecular luminescence spectroscopy including fluorescence, phosphorescence, and chemiluminescence (bioluminescence being a special case of chemiluminescence). 

They observed an initial fast decrease in conductivity (r, =lxl0 s) followed by a slower change (r, lOxlO" s). The major process (>67%) is the slower one and it has the same rate as the phosphorescent decay, which is from a doublet state. They suggested that the fast conductivity change is due to photoaquation from the quartet state and the slower and dominant decay is from the doublet state. The implication is that both doublet and quartet states can be photoactive, with their relative amounts depending on the efficiency of the intersystem crossing between these states, which in turn will depend on the ligands on the Cr(IIl). 

Two mechanisms have been put forward to explain the photochemistry of chromium(in) complexes. Their photoreactivity has been ascribed to excitation either to the lowest spin-forbidden excited state, Eg, or to the lowest quartet excited states, and Mxg. Several years ago some quenching experiments on the photoaquation of the [Cr(NH3)2(NCS)4]- anion indicated that a quartet state was at least partially involved. Recently two papers concerning the photochemistry of the [Cr(CN)6] anion, chosen for the known large energy difference between the doublet and quartet states, have provided strong evidence for a quartet state as the photointermediale. Thus the observations that pyrazine and xanthone sensitize the photoaquation of [Cr(CN)e] , but that Michler s ketone and [Ru(bipy)3] + do not, rule out photoaquation via a doublet state. Likewise a comparison of phosphorescence and photolysis of solutions of [Cr(CN)6] in dimethylformamide showed that the photolysis, to [Cr(CN)5(DMF)] , could not proceed via the same excited state, Eg, as phosphorescence. Both of these investigations led to the implication of the... 

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