Simple carbonyls radiative lifetimes

For most carbonyl compounds, kp is approximately independent of vibrational excitation energy (Eypp,), whereas kfjR usually increases with Evpp,. Therefore, p becomes smaller and ip becomes shorter as Evpb increases. Typically, highly sensitive technique of fluorescence excitation spectroscopy permits measurement of Op over an extensive range of Evib- Hence the rates of radiative transitions as well as the rates of radiationless transitions of SVLs and SRVLs can be readily determined (135). For simple carbonyl compounds with small amounts of vibrational energy in the Sp state, collision-induced processes can become important at pressures above a few torr, since the lifetimes (tp) are comparable to the mean collision times (tu) at these pressures. Rate data reported for a number of aliphatic carbonyls are summarized in Table 2. 

The formation of the triplet state can be directly followed in time through the observation of the transient triplet-triplet (T-T) absorption in flash or modulated photolysis or by the observation of the phosphorescence emission. A typical radiative lifetime of phosphorescence for simple carbonyls is 10 3 s. Therefore, it is extremely difficult to observe the "unrelaxed" phosphorescence emission without collisional relaxation, unless the triplet lifetime is significantly shortened by a competing radiationless process. Under these conditions, the correspondingly low quantum yield of phosphorescence makes such measurement rather difficult. Usually, "relaxed" phosphorescence from a molecule such as biacetyl is observed. Therefore, only the transient T-T absorption can provide useful data in the gas phase, although a determination of the absolute yield is rather involved and difficult. 

TABLE 11. Radiative Carbonyls and Nonradiative Lifetimes of Simple... 

When more conjugated diimine or pyridine ligands are used, the excited states of rhenium(I) carbonyl complexes can have substantial IL character. While the MLCT emission is often broad, with a lifetime in the submicrosecond to microsecond timescale, the IL emission usually has noticeable structural features, even in fluid solutions at ambient temperature. The emission lifetime is usually very long. A simple and widely applicable approach is to evaluate the ratio of the emission quantum yield and the emission lifetime (the product of the intersystem crossing efficiency and radiative decay-rate constant). Experimental values of... 

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