Excimer species

Lochmuller and coworkers used the formation of excimer species to answer a distance between site question related to the organization and distribution of molecules bound to the surface of silica xerogels such as those used for chromatography bound phases. Pyrene is a flat, poly aromatic molecule whose excited state is more pi-acidic than the ground state. An excited state of pyrene that can approach a ground state pyrene within 7A will form an excimer Pyr +Pyr (Pyr)2. Monomer pyrene emits at a wavelength shorter than the excimer and so isolated versus near-neighbor estimates can be made. In order to do this quantitatively, these researchers turned to measure lifetime because the monomer and excimer are known to have different lifetimes in solution. This is also a way to introduce the concept of excited state lifetime. 

Only one excimer fluorescence peak and lifetime were observed l2) for R—CH2— CH2-CHX-R, where R = 2-naphthyl and X = H 121>, methyl, dodecyl, or p-phenylethyl. The same was also true for meso- and d/-2,4-bis(2-naphthyl)pentane12). A recent report128) attributing the fluorescence peaks at 345, 363, and 381 nm in the spectrum of poly(2- er/butyl-6-vinylnaphthalene) to a second excimer species must therefore be considered invalid. The polymer undoubtedly contains an in-chain impurity with vinylnaphthalene conjugation, as discussed in Section 2.1.9.2 for a similar claim involving P2VN 33). 

Figure 1. Energy-level diagram for excimer formation. Symbols represent hv, absorbed photon k emissive rate from the monomer species k, bimolecular rate coefficient for formation of the pyrene excimer k, unimolecular rate coefficient for dissociation of the excimer and k, emissive rate from the excimer species. Note no ground-state association is indicated.

In supercritical krypton the formation of excimers has also been time resolved but the results contrast with those for xenon.As in xenon, electron-ion recombination should occur rapidly. Again, electrons remain hot for many nanoseconds in krypton and the mobility of hot electrons is in the range of 150 to 400 cm /Vs. This leads to a theoretical range for of 1.4 to 3.6 x 10 m" s" at a density of 0.48 g/cm. In pulse radiolysis studies using optical detection, the concentration of intermediates is around 0.5 to 1 pM, thus the first half-life for recombination of electrons with ions is less than 10 ps in krypton. What has been observed is that an excimer species (A), the... 

The rare gas excimers readily transfer energy to various additives. Rates for transfer to nitrogen and hydrogen in krypton are known at 1 atm. Because excimer species have strong absorptions in the visible region, it is necessary to quench them when studying reactions of other intermediates by absorption spectroscopy. Ethane has been shown to be convenient for this purpose. The rate constant for excitation transfer from excimers to ethane in xenon was measured by the pulse-probe technique to be 3.4 x 10 ° molal s at pressures near 50 bar. Thus, addition of a small concentration of ethane can be used to reduce the absorption due to excimers to a small level at nanosecond times. 

It is clear that the time-resolved studies of those polymers are in an interesting and productive phase. With further work it should be possible to rationalize the kimt-ically distii diable monomer and excimer species in terms of the different conformation sites within polymers. 

Once again, time-resolved studies on copolymers have shown the existence of two kinetically distinct monomer species, an isolated chromophore capable only of excimer formation throu long-range interactions and a monomer capable of excimer formation which is also populated by reverse disrociation . An alternative explanation of observed behaviour is that two excimer species may exist which... 

More interestingly, triplet state studies have produced date which expand our experience of intramolecular triplet state interactions and are relevant to the issue regarding the existence of triplet excimer species in naphthalene compounds(25-30). The data may be summarized as follows ... 

Such behaviour has been reported in a wide range of polymers including both homo and copolymers labelled with napthalene (3 - 5) and styrene-methyl methacrylate copolymers ( 6). However, in all these cases no clear evidence for other than a single excimer species has emerged. In addition, a rise time in the fluorescence decay, which can be associated with excimer formation, has also been observed (7). 

The results presented above may be compared with previous studies on these polymers. It has been suggested that more than one spectrally distinct excimer species may exist in some naphthalene-containing polymers (6,22). Analyses of the spectral surfaces for the polymers studied in this work clearly indicate that only one spectrally distinct monomer and one excimer species contribute to emission. Phillips and co-workers (3,23) also found that the monomer fluorescence decay from PACE requires a minimum of three exponential terms to provide adequate fitting. They explained their results on the basis of a kinetic model involving two temporally distinct excited monomers (M3, M2 ) which can form the excimer (D ) as outlined in Scheme 1. Reverse dissociation of the excimer was also considered important. 

An alternative mechanism for sensing nucleotides relies on the formation or dissociation of highly emitting excimer species. This is the case of the pinzerlike benzene-bridged imidazolium receptor 111 depicted in Figure 25. ... 

Itagaki et al [76] have proposed that the decay kinetics observed for fluid solutions of poly(l-methoxy-4-vinylnaphthalene) (PMVN) may be explained in terms of two excimer species in addition to excited monomer. The proposal is based on variations in steady-state l,3-di(4-methoxy-1-naphthyl)propane (BMP) [77,78], presumes that the lower energy excimer comprises the normal structure with fully overlapped aromatic rings, whereas the second excimer is thought to comprise a partially overlapped structure. Steady state spectroscopy indicated a reasonable degree of evidence that three excited state species do indeed exist in the sterically hindered naphthalene systems. However the spectral and numerical deconvolution procedures adopted make the separation of intensity components used in the kinetic analysis, necessary for excited state assignment, rather optimistic. 

The rotations to TG/GT (also full overlap of the phenyl chromophores) are less favourable leading to less excimer species within the radiative lifetime of the excited phenyl groups. At 77 K, however, the time required for bond rotation to the appropriate conformation exceeds the radiative lifetime and therefore no excimer emission is observed. 

In this paper will be presented the results obtained in three families of elastomers, using probes of various sizes which exhibit different photophysical behaviour and give gise to different excimer species. The correlation times,ranging from 10 - 10 s,have been measured as... 

Excitation spectra of HPC-Py/26 in water revealed a red-shift in the excimer species relative to monomer. This, along with the lack of a detectable rise time in the fluorescence decay profile of the excimer species, indicated that the excimers exist predominantly as ground-state aggregates. This result is a very significant indication of the nature of the aggregates as well as the association mechanism. 

The degree of spectral overlap between the absorption spectrum of BDHM and the P2NMA monomer and excimer fluorescence emission, indicates that the BDHM chromophore is capable of acting as an acceptor in a Forster-type energy transfer process from both the monomer and excimer species. The values of the Forster critical transfer distance, Rq, for energy transfer... 

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