Collisional triplet quenching

The diffusion-controlled mechanism of triplet-triplet quenching (whether it be collisional or interaction at a distance) could still be applied to interpret these results, qualitatively at least, if one additional factor is taken into account. The additional factor is the extremely slow rate of diffusion in rigid medium which makes necessary the Consideration of nonsteady-state effects. Immediately after illumination is shut off, those triplet molecules situated close together will diffuse and interact more rapidly than others situated at greater distances. The more favorably situated pairs of triplets will thus be depleted more rapidly and the overall rate of interaction will be greater at shorter times than later when steady-state conditions will ultimately be approached. In fluid solvents at room temperature the steady state is reached after about 10-7 sec. In very highly viscous media, however, much longer times are required and this could explain the non-exponential decay observed with phenanthrene in EPA at 77°K. 

The collisional fluorescence quenching and phosphorescence induction processes have been later observed for a large number of small and medium-size molecules. The interpretation of these results was however rather confusing collision-induced intersystem crossing considered as a transition from the pure singlet to the pure triplet state is in apparent contradiction with the Wigner rule of spin conservation, at least in the case of light collision partners that cannot affect the intramolecular spin-orbit interaction. 

Next, we consider the case of collisional quenching. For triplet-triplet quenching (Lin and Eyring, 1975)... 

The acetone-sensitized photodehydrochlorination of 1,4-dichlorobutane is not suppressed by triplet quenchers (20), but the fluorescence of the sensitizer is quenched by the alkyl chloride (13). These observations imply the operation of a mechanism involving collisional deactivation, by the substrate, of the acetone excited singlet state (13,21). This type of mechanism has received strong support from another study in which the fluorescence of acetone and 2-butanone was found to be quenched by several alkyl and benzyl chlorides (24). The detailed mechanism for alkanone sensitization proposed on the basis of the latter work invokes a charge-transfer (singlet ketone)-substrate exciplex (24) and is similar to one of the mechanisms that has been suggested (15) for sensitization by ketone triplets (cf. Equations 4 and 5). 

Normally the quenching of the triplet states by the collisional process... 

Nonradiative Processes. The nonradiative pathways which depopulate are still a matter of some discussion. It has been pointed out that the short lifetime of precludes collisional quenching of this electronic state, and the failure to observe sensitized triplet emission from biacetyl (92) indicates that the... 

Bernstein et al. [82] have recently investigated the dynamics of collisional quenching of triplet Ceo by silyl derivatives of guanine and 8-oxoguanine in benzo-nitrile solution. The observed rates are 3.3 x 10 M s and 1.1 x 10 M s , respectively, in accord with an electron transfer mechanism in which the silylated nucleobase of lower oxidation potential (Table 3) is the better electron donor. 

Because of the long radiative lifetime of the lowest triplet state, most phosphorescence in fluid solutions is obviated by collisional quenching, especially by dissolved molecular oxygen. Phosphorescence, when it occurs, is usually observed at low temperatures (e.g., that of liquid nitrogen) in rigid matrices where it may demonstrate high quantum yields. In the past three decades, much interest has been focused on phosphorescence at room temperature (RTP), which sometimes can be observed in samples adsorbed on solid substrates such as filter paper. Unfortunately, the quantum yields observed in room temperature phosphorescence are low, leading to poor analytical sensitivity, and the method has not enjoyed wide popularity. Phosphorescent measurements at low temperatures... 

Stern-Volmer Model. When the chromophores are sufficiently mobile as in fluid solution, the bimolecular quenching process of A by a quenching molecule, B, including the triplet energy transfer and some collisional quenching, will result in the single exponential... 

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