Monomer quenching rate constants

Other groups may cause shortening of the lifetime. The phosphorescence of parvalbumin is quenched by free tryptophan with a quenching rate constant of about 10s M i s l (D. Calhoun, unpublished results). A more extensive survey of proteins or model compounds with known distances between tryptophans is needed to study how adjacent tryptophans affect the lifetime. It should be noted that at low temperature the phosphorescence lifetime of poly-L-tryptophan is about the same as that of die monomer.(12) This does not necessarily mean that in a fluid solution tryptophan-tryptophan interaction could not take place. Thermal fluctuations in the polypeptide chain may transiently produce overlap in the n orbitals between neighboring tryptophans, thus resulting in quenching. 

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. 

The quantum yield of fluorescence of many fluorescent substances in solution decreases with increasing concentration. In some cases, e.g., with aqueous solutions of thionine and Methylene Blue, this selfquenching is due to formation of stable dimers in equilibrium with the monomer.78 In other cases, e.g., with solutions of anthracene, perylene, and coronene in solvents such as benzene, chloroform, and kerosene, Bowen and co-workers have shown that quenching takes place by collision17 the self-quenching rate constants obtained are very close to those given in Table I (see also ref. 19). 

The overall quenching rate constant for monomer fluorescence, kg is given by koM (kiD +kFo)... 

The addition of -CD to an aqueous naphthalene solution caused the growth of its molecular fluorescence and the appearance of excimer emission [130]. By lowering the temperature of the solution, the excimer intensity grew at the expense of that of the monomer. The excimer emission was attributed to the association of 1 1 complexes to give 2 2 / -CD-naphthalene inclusion compounds. In air-saturated solutions, the three species have the following lifetimes 40 ns (free naphthalene), 48 ns (1 1 complex), and 68 ns (2 2 complex). The quenching rate constants derived from these lifetimes by the addition of I" were 6x 10 dm mol s , 3.9x 10 dm mol s , and 1.8 x 10 dm mol s , respectively, which confirmed the protection furnished by the cavity to the included molecules. 

The effect of excimer kinetics on fluorescence decays of monomers and excimers upon excitation with a short pulse was studied first by Birks et al. [119]. They took into account all the relevant processes that proceed after the excitation of a low fraction of monomers by an ultrashort pulse and derived the rate equations describing the monomer and excimer decays. Most processes involved in the Birks scheme are monomolecular and depend only on the concentration of the excited species and on the first-order rate constant one of them is a bimolecular process and depends on the concentrations of both the excited and ground-state molecules. They include (1) monomer fluorescence, (rate constant fM), (2) internal monomer quenching, M —>M, ( iM). (3) excimer formation, M - -M D (bimolecular reaction, i.e., the rate depends on the product of the rate constant and concentration of the ground-state... 

The quenching of pyrene monomer emission by 2-bromonaphtha-lene on dry silica gel has also been studied as a function of temperature. Linear Stern-Volmer plots are obtained either with t /t or with tJ/T], and t%/t2 vs tQ] where [Q] Is a surface concentration. Fig. 12 illustrates the f/f0 plot. The rate constants derived from... 

The high rate constants for chemical quenching of triplet ketones by amines provide two sidelights of considerable importance. Photoinitiation of polymerization has received widespread and varied industrial applications. One problem is that many vinyl monomers quench triplet ketones very rapidly either by charge transfer or energy transfer mechanisms, without forming any radicals. Most solvents cannot compete with the olefins for the triplet ketone. However, tri-ethylamine quenches at rates close to diffusion-controlled, so that radical formation and polymerization initiation are quite efficient 158>. 

Such polycyclic aromatic hydrocarbons as anthracene or heteroaromatics as acridine, phenazine and 2,4,5-triphenyl oxazole act as Jt-donors for the Jt-acceptors AN and alkyl methacrylates [50-53]. Again, the interaction of the donor excited states with vinyl monomers leads to exciplex formation. But, the rate constants (k ) of these quenching processess are low compared to other quenching reactions (see Table 1). The assumed electron transfer character is supported by the influence of the donor reduction potential on the k value (see Table 1), and the detection of the monomer cation radicals with the anthracene-MMA system. Then, the ion radicals initiate the polymerization, the detailed mechanism of which is unsolved,... 

Table 1. Rate constants (k ) for quenching of donor excited states by vinyl monomers and half wave oxidation potentials of donor (E° /2)...

Table 3. Life times of triplet states (3x), rate constants of self quenching (3k, ), of quenching with monomers (3k ) and of quenching with diphenyliodonium hydrogensulfate (3kJ,)...

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