Triplet state population

Since the photon emitted by D is absorbed by A, the same rules will apply to radiative energy transfer as to the intensity of absorption. Because singlet-triplet transitions are spin-forbidden and singlet-triplet absorption coefficients are usually extremely small, it is not possible to build up a triplet state population by radiative energy transfer. For this... 

The rather long lifetimes of many triplet states often allow an appreciable buildup of the triplet-state population under constant illumination. Already in 1954 Craig and Ross 33) measured reliable T—T spectra on a single-beam recording spectrophotometer. The observation of new absorption bands in the visible part of the spectrum is not difficult and there is satisfactory agreement between the peak positions reported from different laboratories. The determination of triplet extinction coefficients requires additionally a knowledge of the stationary triplet concentration, which is much harder to obtain. The corresponding literature values are widely scattered. 

As already mentioned above, spectral sensitization may also become indispensable when the light absorption properties of a potentially photoreactive compound do not permit direct excitation in the desired wavelength region. By application of sensitizers with adjusted excited state properties, it is, for example, possible to induce photochemical reactions of otherwise colorless compounds with visible light. Another important application in photochemistry is the sensitized population of excited state levels, which are not easily reached by direct absorption of light due to the limitations of quantum chemical selection rules. This phenomenon has been extensively exploited in mechanistic and synthetic organic photochemistry, where enhanced yields of triplet state population could be achieved in various dye-photosensitized processes (98). 

As an alternative mechanism there remains the assumption of a very rapid Internal conversion either In the phenol or, more probably, in the quinone structure, preventing triplet state population (94). It should be noted that for no substance showing... 

On the much longer timescale over which phosphorescence is emitted, nanosecond pulses that populate Si look like 8-fimction pulses. In that case, the triplet state population is still firrmed virtually instantaneously and the phosphorescence decay remains undistoited and decays exponentially. Usualty the phosphorescence dec profile does not have to be described by a convolution integral unless one is using a xenon flashlamp fi r excitation, for which the pulses are a few tens of microseconds wide, with a weak 100 is tail. 

Regarding the photochemistiy of benzoin derivatives, starting from excited triplet states populated after intersystem crossing, Norrish Typ I bond scission is the main chemical reaction occurring under various experimental conditions [21-25]. As a consequence of this bond cleavage, benzoyl and ether radicals are formed. In the absence of the monomer, hydrogen abstraction takes place, leading to benzaldehyde, benzil, and pinacol... 

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