Triplet quenching rate constants

As with arene-amine radical ion pairs, the ion pairs formed between ketones and amines can also suffer a-deprotona-tion. When triplet benzophenone is intercepted by amino acids, the aminium cation radical can be detected at acidic pH, but only the radical formed by aminium deprotonation is detectable in base (178). In the interaction of thioxanthone with trialky lamines, the triplet quenching rate constant correlates with amine oxidation potential, implicating rate determining radical ion pair formation which can also be observed spectroscopically. That the efficiency of electron exchange controls the overall reaction efficiency is consistent with the absence of an appreciable isotope effect when t-butylamine is used as an electron donor (179). 

Measurements of Relative Triplet Quenching Rate Constants. 

The rate constants of various reagents quenching a photoexcited triplet are important parameters in both CIDEP and CIDNP observations. Knowledge of these absolute and relative rate constants is obviously important in photochemistry. We shall illustrate in this section and with some details how CIDEP experiments using a slow-response conventional spectrometer and a rotating sector can lead to measurements of relative triplet quenching rate constants. 

Table 3. Free energy change, AG°ct, and rate constants, of photoinduced electron transfer from group 14 organometallic electron donors to C, observed rate constants, obs, triplet quenching rate constants, kq, and limiting quantum yields, Oco, in the photoaddition of the donors to in benzonitrile at 298 K [212].

The photoreduction of triplet duroquinone by the antioxidant vitamin C has been studied by time-resolved EPR in homogeneous ethylene glycol-water solutions and in AOT and SDS micelles. The triplet quenching rate constant is lower in the latter medium, due to slower diffusion. ... 

Porter and Wilkinson(56) measured the rates of quenching for a variety of triplet donors with triplet acceptors at room temperature in fluid solution by flash photolysis. The appearance of the triplet-triplet absorption spectrum of the acceptor and the simultaneous disappearance of the donor triplet-triplet absorption spectrum provided unequivocal evidence for the triplet-triplet energy transfer process. Table 6.5 provides some of the quenching rate constants reported in this classic paper. 

Triplet decay in the [Mg, Fe " (H20)] and [Zn, Fe (H20)] hybrids monitored at 415 nm, the Fe " / P isosbestic point, or at 475 nm, where contributions from the charge-separated intermediate are minimal, remains exponential, but the decay rate is increased to kp = 55(5) s for M = Mg and kp = 138(7) s for M = Zn. Two quenching processes in addition to the intrinsic decay process (k ) can contribute to deactivation of MP when the iron containing-chain of the hybrid is oxidized to the Fe P state electron transfer quenching as in Eq. (1) (rate constant kj, and Forster energy transfer (rate constant kj. The triplet decay in oxidized hybrids thus is characterized by kp, the net rate of triplet disappearance (kp = k -I- ki -I- kj. The difference in triplet decay rate constants for the oxidized and reduced hybrids gives the quenching rate constant, k = kp — kj, = k, -I- k , which is thus an upper bound to k(. 

Fig. 7. Temperature dependence of the triplet-state quenching rate constant (k,) for the [a(Zn), PiFe +HjO)] hybrid. Adapted from Ret [7d]...

One possible explanation for the lack of correspondence between emitting and reacting states is reaction of the singlet. In the case of benzophenone, there is little question that the reaction involves only the triplet state, since triplet quenchers can effectively inhibit the reaction. This need not be the case with all carbonyl compounds. For example, it has been shown that both the n,ir singlet and triplet states can be involved in the Norrish type II cleavage of alkyl ketones (25-27). At high concentrations, piperylene quenches only that part of the 2-hexanone cleavage which arises from the triplet. The rate constants for... 

The LFP of diphenyldiazomethane ( DDM ) in a variety of solvents produces triplet diphenylcarbene ( DPC, 14a), whose transient absorption is readily monitored. The optical absorption spectrum of DPC is quenched by methanol and yields the product of O—H insertion, suggesting that DPC is quenched by the O—H bond of methanol. The quenching rate constant (fex) is determined to be 6.8 X 10 M s in benzene. ... 

A representative plot is shown in Figure 1.15 this is known as a Stern-Volmer plot, and 0.16) as a Stern-Volmer equation. This method for obtaining reaction rate constants is again a comparative one, since there is competition between the primary reaction step and the quenching process. A value for the quenching rate constant needs to be known, but in many cases this is independent of the substrate and quencher because triplet quenching is controlled by diffusional collision of the two species. So for a particular solvent at a given temperature K, values are available in the literature as an... 

It has been suggested in the literature that the a-amino radical is the species that initiates polymerization [210], This view is supported by our observation that, in spite of the relatively high quenching rate constant of Eosin triplet by triphenylamine (Table 5), the system Eosin-triphenylamine does not sensitize the photopolymerization of multifunctional acrylates. Thus, it is necessary that the amine contains a hydrogen at the a-carbon to be released as a proton after oxidation of the amine by the dye triplet. This deprotonation prevents the back electron transfer and forms a carbon radical that is sufficiently long-lived to be captured by the monomer. 

A characteristic feature of triplet exciplexes 3(A — Q) is that they are quenched via energy transfer by molecules Q with a low-lying triplet level (e.g. dioxygen, azulene, tetracene) and the corresponding quenching rate constants kq reach the diffusion limited value kdif. 

Fast generation of the radical ions can be attributed to electron-transfer reaction from the singlet excited state and slow radical generation to that from triplet excited state. Fluorescence of both 326 and 327 was quenched in the presence of CCI4 according to the Stern-Volmer equation. The Stern-Volmer constants were estimated to be 1.55 and 17.7 M 1 for 326 and 327, respectively, and quenching rate constants were estimated to be 1010 and 2.7 x 1010 M 1 s 1. 

An exception are naphtholes as donors (e.g., 2-naphthol [2-NOH], 2,3- and 2,7-dihydroxynaphthalene, l,l -bi-2-naphthol [BN(OH)2]), which quenches the fullerene triplet with rate constants of only 106 M 1 s-1. The addition of pyridine increases the quenching rate to about 2-3 orders of magnitude [162], This observation can be explained by a reduced oxidation potential of the naphthols by 20 kcal/mol on addition of pyridine. This indicates that pyridine attracts proton from the naphtholes resulting in the increase of quenching rates [Eq. (3) [162],... 

The excited triplet state of 3,4-dimethoxyacetophenone is quenched by both phenolic hydroxyl and methoxyl functionalities. The rate constant for decay of the 3,4-dimethoxyacetophenone triplet state k, in the presence of a quencher Q is related to the rate of triplet state decay in the absence of quencher k, the quenching rate constant kq and the quencher concentration according to Equation 2. 

Table 7.17. Quenching rate constants of singlet ( (>81 ) ) and triplet (3(> Si ) ) luminescence...

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