Quenching excimer

On close inspection the data of tables I and II show a number of inconsistencies with a three component model previously used to describe the excimer, quenched monomer and isolated monomer sites in other polymer systems (3 - 5.). For instance, if we consider the 25"C data for Poly(VBuPBD),Tj might be considered to represent the excimer, T2 the quenched monomer and T3 isolated monomer. However, B2 and B3 do not decrease proportionately as measurements are made at wavelengths further displaced from that dominated by monomer emission, though the high degree of monomer and excimer spectral overlap (Figure 1) means that B2 and B3 would not be expected to approach zero. In addition, the decay parameters clearly... 

Excimer quenching accompanies the formation of Product I in irradiated polystyrene films, but the rates of change of excimer fluorescence intensity in vacuum and in air are quite different. They are contrasted in Figure 3. Photoconversion of excimer sites, which act as... 

Non-linear Stern-Volmer plots in the quenching of 7,7/-polymethylenecoumarins (11) may also be the result of excimer quenching. If a non-linear plot is taken as indicative of the quenching of two different excited states, one could be the... 

This is a different view proposed for excimer formation by ethylenic polymers in which energy migration to excimer forming site is considered(9). Although the small Fg/FiD value for the anthracene polymers is in part due to the short lifetime, photocyclodimerization of anthryl groups(10, 11) has to be taken into account as an excimer quenching process. 

This premise is incorrect because kqj simply describes the quenching of an isolated excited phenyl ring while kqjj always describes the direct quenching of the isolated excited complex. The possible contribution to excimer quenching caused by monomer quenching before the excitation can reach an EFS is already included in kqx. Thus, kqj for the copolymer monomer should be approximately equal to kqj) for the PS excimer. MacCallum and Rudkin s conclusion shows that it is often easy to misinterpret quenching experiments. 

For anthracene the concentration quenching process could be the decomposition of the excimer to two ground state anthracenes, as shown in... 

The quenching of the trans dimer with oxygen and ferrocene indicates that this product is formed almost entirely from the triplet state. It is possible to calculate the amount of triplet-derived product in benzene by subtracting the amount of product obtained in the presence of oxygen from the amount of product obtained in the absence of oxygen. Such a calculation indicates that acenaphthylene triplets in benzene give both trans and cis dimers in the ratio of 74 26. The triplet state accounts for almost all of the trans product and about 10% of the cis product. The break in the slope of the Stem-Volmer plot for the trans dimer (Figure 10.3) may be attributed to the presence of two excited species which are quenched at different rates. These two species could be (a) two different monomeric acenaphthylene triplet states 7 and T2 or (b) a monomeric acenaphthylene triplet state 7 and a triplet excimer. This second triplet species is of relatively minor importance in the overall reaction since less than 5% of the total product in an unquenched reaction is due to this species. 

Meantime, we have to keep in mind that monomer and excimer are independent emitters possessing different lifetimes and that nonspecific influence of quenchers may be different for these two forms. For instance, dissolved oxygen may quench the long-lifetime emission of monomer but not of the excimer. 

Bimolecular reactions with paramagnetic species, heavy atoms, some molecules, compounds, or quantum dots refer to the first group (1). The second group (2) includes electron transfer reactions, exciplex and excimer formations, and proton transfer. To the last group (3), we ascribe the reactions, in which quenching of fluorescence occurs due to radiative and nonradiative transfer of excitation energy from the fluorescent donor to another particle - energy acceptor. 

The second group of intermolecular reactions (2) includes [1, 2, 9, 10, 13, 14] electron transfer, exciplex and excimer formations, and proton transfer processes (Table 1). Photoinduced electron transfer (PET) is often responsible for fluorescence quenching. PET is involved in many photochemical reactions and plays... 

Exciplexes are complexes of the excited fluorophore molecule (which can be electron donor or acceptor) with the solvent molecule. Like many bimolecular processes, the formation of excimers and exciplexes are diffusion controlled processes. The fluorescence of these complexes is detected at relatively high concentrations of excited species, so a sufficient number of contacts should occur during the excited state lifetime and, hence, the characteristics of the dual emission depend strongly on the temperature and viscosity of solvents. A well-known example of exciplex is an excited state complex of anthracene and /V,/V-diethylaniline resulting from the transfer of an electron from an amine molecule to an excited anthracene. Molecules of anthracene in toluene fluoresce at 400 nm with contour having vibronic structure. An addition to the same solution of diethylaniline reveals quenching of anthracene accompanied by appearance of a broad, structureless fluorescence band of the exciplex near 500 nm (Fig. 2 )... 

A. Weller and K. Zachariasse 157-160) thoroughly investigated this radical-ion reaction, starting from the observation that the fluorescence of aromatic hydrocarbons is quenched very efficiently by electron donors such as N,N diethylaniline which results in a new, red-shifted emission in nonpolar solvents This emission was ascribed to an excited charge-transfer complex 1(ArDD(H )), designated heteroexcimer, with a dipole moment of 10D. In polar solvents, however, quenching of aromatic hydrocarbon fluorescence by diethylaniline is not accompanied by hetero-excimer emission in this case the free radical anions Ar<7> and cations D were formed. 

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