Phosphorescence, definition

The results of this study show a definite quenching of the 418 nm phosphorescence emission of DMT. One would expect that the quenching effect, in a rigid glass, would fit the Perrin model (73). A plot in In 4>0/4> versus concentration of 4,4 -BPDC yielded a straight line, the slope of which was identified with NV. The radius, R, of the active volume of quenching sphere was calculated by the following equation ... 

Triplet—triplet energy transfer from benzophenone to phenanthrene in polymethylmethacrylate at 77 and 298 K was studied by steady-state phosphorescence depolarisation techniques [182], They were unable to see any clear evidence for the orientational dependence of the transfer probability [eqn. (92)]. This may be due to the relative magnitude of the phosphorescence lifetime of benzophenone ( 5 ms) and the much shorter rotational relaxation time of benzophenone implied by the observation by Rice and Kenney-Wallace [250] that coumarin-2 and pyrene have rotational times of < 1 ns, and rhodamine 6G of 5.7 ns in polymethyl methacrylate at room temperature. Indeed, the latter system of rhodamine 6G in polymethyl methacrylate could provide an interesting donor (to rose bengal or some such acceptor) where the rotational time is comparable with the fluorescence time and hence to the dipole—dipole energy transfer time. In this case, the definition of R0 in eqn. (77) is incorrect, since k cannot now be averaged over all orientations. 

The lifetime of fluorescence (or phosphorescence), r0, is defined using the rate constant kf as r0 = 1 /k. By this definition, the residual intensity according to equation (12.1) is only 36.7% of the initial intensity at r0 63.3% of the initial species have relaxed to a non-emissive state. Because fluorescence lifetimes are only a few nanoseconds, fluorimeters require measurements to be made at the same time as... 

We hope to accelerate the process of amalgamation by a discussion of terms which, from our point of view, are useful. In particular we have selected those which may cause confusion and also those which are not selfexplanatory to the average chemist. Consideration has been given to the common usage factor in language, to our own prejudices, and to certain recognized conventions. These factors are not always compatible and some confusion or contradiction still cannot be avoided (e.g., in definitions of fluorescence and phosphorescence). 

Confusion reigns when one examines the definitions of fluorescence and phosphorescence in different areas of the literature of the natural sciences. Many physicists in particular prefer the operational definition in which fluorescence is described as short-lived emission and phosphorescence is long-lived emission (4,15). However, the question arises as to what constitutes short-lived. A possible transition point may be radiative lifetimes, r0 (vide infra), of the order of 10-s to 10 6 sec. 

On the other hand, most chemists and many physicists leading with polyatomic organic molecules currently employ the mechanistic definitions advanced by G. N. Lewis and shown in Figure 1. Thus, fluorescence is defined as a radiative transition between states of like multiplicity, e.g., 5 x - So + hv. Phosphorescence is a radiative transition between states of different multiplicity. In organic molecules the process is usually associated with spin-forbidden transitions such as Ti - S0 + hv". 

Time-resolved CIDEP and optical emission studies provide further definitive characterization of the triplet and excited singlet states followed by their primary photochemical reactions producing transient radicals in individual mechanistic steps in the photolysis of a-guaiacoxylacetoveratrone. Both fluorescence and phosphorescence are observed and CIDEP measurements confirm the mainly n,n character of the lowest triplet state. The results indicate a photo triplet mechanism involving the formation of the ketyl radical prior to the P-ether cleavage to form phenacyl radicals and phenols. Indirect evidence of excited singlet photo decomposition mechanism is observed in the photolysis at 77 K. 

Another incompletely elucidated problem is that of the configuration of the free radicals S and S". Malrieu and Pullman stated that S " is tetragonally folded and adopts the configuration H-intra (see Section 111, A, 2) the nonplanarity of the radical has been claimed as a possible cause of the disagreement between calculated and experimental ESR spectra. However, Lhoste and Tonnard, though considering that no definitive conclusion may be drawn in this question, appear to favor the hypothesis that the radical is nearly planar. Preliminary data about the phosphorescent triplet state of the phenothiazine molecule support this view. 

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