Nonradiative quenching effects

A similar but smaller intramolecular quenching effect was seen by Phillips and co-workers 44,4S) for 1-vinylnaphthalene copolymers incapable of excimer fluorescence. The monomer fluorescence lifetime of the 1-naphthyl group in the methyl methacrylate copolymer 44) was 20% less than the lifetime of 1-methylnaphthalene in the same solvent, tetrahydrofuran. However, no difference in lifetimes was observed between the 1-vinylnaphthalene/methyl acrylate copolymer 45) and 1-methylnaphthalene. To summarize, the nonradiative decay rate of excited singlet monomer in polymers, koM + k1M, may not be identical to that of a monochromophoric model compound, especially when the polymer contains quenching moieties and the solvent is fluid enough to allow rapid intramolecular quenching to occur. 

The PL quantum yield r)pl. While r]pl of many dyes is close to 100% in solution, in almost all cases that yields drops precipitously as the concentration of the dye increases. This well-known concentration quenching effect is due to the creation of nonradiative decay paths in concentrated solutions and in solid-state. These include nonradiative torsional quenching of the SE,148 fission of SEs to TEs in the case of rubrene (see Sec. 1.2 above), or dissociation of SEs to charge transfer excitons (CTEs), i.e., intermolecular polaron pairs, in most of the luminescent polymers and many small molecular films,20 24 29 32 or other nonradiative quenching of SEs by polarons or trapped charges.25,29 31 32 In view of these numerous nonradiative decay paths, the synthesis of films in which r]PL exceeds 20%, such as in some PPVs,149 exceeds 30%, as in some films of m-LPPP,85 and may be as high as 60%, as in diphenyl substituted polyacetylenes,95 96 is impressive. 

Now we consider the kinetic information about photochemical processes that can be obtained by quenching studies. Fluorescence quenching is the nonradiative removal of the excitation energy from a fluorescent molecule and the elimination of its fluorescence. Quenching maybe either a desired process, such as in energy or electron transfer, or an undesired side reaction that can decrease the quantum yield of a desired photochemical process. Quenching effects may be studied by monitoring the fluorescence of a species involved in the photochemical reaction. 

Collisional quenching is particularly efficient when the quencher is a heavy species, such as iodide ion, that receives energy from the fluorescing species and then decays nonradiatively to the ground state. This fact may be used to determine the accessibility of amino acid residues of a folded protein to solvent. For example, fluorescence from a tryptophan residue is quenched by iodide ion when the residue is on the surface of the protein and hence accessible to the solvent. Conversely, residues in the hydrophobic interior of the protein are not quenched effectively by I". 

The rate of relaxation by nonradiative pathways can be increased by addition of quenchers. Quenching of fluorescence occurs by several mechanisms, many of which involve collision of the excited chro-mophore with the quenching molecule. Some substances such as iodide ion are especially effective quenchers. The fluorescence efficiency of a substance in the absence of a quencher can be expressed (Eq. 23-lb) in terms of the rate constants for fluorescence (fcf), for nonradiative decay (km), and for phosphorescence ( r )=... 

One of the outstanding properties of these substances is the extreme tunability of the electronic states under high pressure. In many cases a red shift on the order of 2000 cm-1/GPa has been observed (Yersin and Riedl, 1995). In addition, an effective nonradiative energy transfer from the cyano donor complexes to the f elements has been observed. In the case of Eu[Au(CN)2]3-3H20 this process even totally quenches the otherwise very intense and broad emission from the [Au(CN)2] layers. However, because of the very strong red shift of the donor electronic states, it is possible to shift the donor states over different levels of the f element. Especially, resonant and nonresonant energy transfer conditions can be achieved to study the transfer mechanism. 

It has been also reported that the emission intensity is enhanced as the emitting chromophore is diluted by other inert polymers in solid state [79]. The emission intensity of the blend polymer is increased as MEH-PPV is diluted by DSiPV because the nonradiative decays, especially intermolecular quenching, are reduced by the dilution effect. 

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