Transient effects on diffusion-controlled reactions. 11. Diffusion effects on pyrene excimer kinetics: determination of the excimer formation rate coefficient time dependence

Abstract

A numerical method is presented to recover the excimer formation rate coefficient from the experimental monomer and excimer decay curves. This method was applied to study pyrene monomer-excimer kinetics in cyclohexanol from 25 'C up to 85 OC. We observed that the time evolution of the rate coefficient deviates from the Smoluchowski equation, showing a minimum at high temperatures and concentrations, when the reversibility effects are more pronounced.K, using discharge-flow/molecular beam mass spectrometry techniques (D-F/MBMS). The ratios were derived from the observed decreases of MBMS CH2 signals upon substituting 25% of the helium bath gas by 0.5 Torr of methane, which selectively scavenges CH2('Al); in the absence of CH, the singlet CH2 is mainly collisionally converted to CH2(!Bl). In this way, the ratios of the total formation rates of singlet and triplet CH2 are obtained. As the rate of reaction r3 is linked to the rate of reaction r2b by the known fraction of HCCO reacting with H, in competition with 0, both the ratios kt/kzb and k3s/k3b can be. extracted from the data. Thus, including also probably systematic errors, the HCCO yield of C2H2 + 0 is found to be k2,/kz = 85?% and the CH2('AI) yield of HCCO + H, k 4 k 3 = 92 * 15% (95% confidence intervals).In addition, the rate constant of CH#B1) + H -CH(211) + H2 (rl) was denvcd relative to the known k(3CH2+O) from CH2 signals at different [H]/[O] ratios:kl = (1.6 f 0.6) X lo1' cm3 mol-' s-l.

Potential analytical applications of differential fluorescence quenching: pyrene monomer and excimer emissions as sensors for electron deficient molecules

“…In alcohols at room temperature, the rate constant for excimer dissociation, k rev , has values around 10 6 s −1 . 8, 12 Since the monomer and excimer singlet excited state lifetimes are ∼100 ns and ∼50 ns, respectively, under our conditions (vide infra), we expect excimer dissociation (k rev ) to be unimportant. A concentration of pyrene of 3 mM was used throughout, since comparable emission intensities for monomer and excimer (that this concentration achieves) ensure the best sensitivity in dual quenching experiments; further, under these conditions the criterion of negligible excimer dissociation is readily met.…”

“…The excimer 1 E* is a relatively stable complex between one excited molecule of pyrene and another in its ground state, 9 with a binding energy of 38-42 kJ mol −1 in non-viscous solvents. 10- 12 A variety of molecules are able to quench fluorescence emission from pyrene via charge-transfer or electron-transfer mechanisms. 13 In the case of electron-deficient nitromethane, the mechanism of fluorescence quenching was rigorously explored via computational studies by McGuffin and coworkers.…”

Membrane Binding and Oligomerization of the Lipopeptide A54145 Studied by Pyrene Fluorescence

“…Pyrene forms excimers when two molecules come within 10 Å of each other (17). Moreover, the fluorescence intensity and lifetime of the pyrene monomer undergo a large increase upon translocation from water to an apolar and highly viscous environment, such as the interior of lipid bilayers (18)(19)(20)(21)(22)(23)(24).…”

Long Range Polymer Chain Dynamics Studied by Fluorescence Quenching

“…27 In turn, RF−Q could then be related to the conformation of the peptide, a larger RF−Q value indicating a more extended conformation. 28 Similarly, excimer formation between pyrene molecules indicates that the pyrene labels have come within less than 1.0 nm from each other, 29 which for pyrene-labeled macromolecules implies that a macromolecule that forms an excimer adopts a close conformation.…”