Pyrene concentration quenching

The normal violet fluorescence band of pyrene solutions shows concentration-quenching which is accompanied by the appearance of a blue structureless emission band. Forster and Kasper40 showed that the blue band is due to emission from an excited dimer formed by the combination of an excited singlet molecule with a molecule in the ground state. Most of the light in both spectral bands has a relatively short lifetime but Stevens and Hutton87 observed a long-lived component of the dimer... 

For a given value of Iai, the lifetimes show an appreciable decrease with increasing pyrene concentration. This may be due to quenching of the triplet by the pyrene itself, or by traces of impurity present in the pyrene. The lifetimes are, of course, also critically dependent on the efficiency of deoxygenation of the solutions and slight variations from one cell filling to the next may thus account for part of the variation shown in Table XI. Such variations will not, however, affect the validity of the calculations. 

For years it has been known that the quantum yield of fluorescence for a number of aromatic hydrocarbons decreases with increasing concentration, but the cause of this concentration quenching was not well understood. In 1955 it was first noted by Forster that increasing concentration not only quenches the normal fluorescence of pyrene (5), but also introduces a new fluorescent component. 

Excimers are complexes/dimers of electronically excited molecules with molecules of the same type in their ground state. They only exist in the excited state and they dissociate into monomers upon radiative or non radiative deactivation in agreement with scheme shown in Figure 4.2. This phenomenon of association of chromophores is called concentration quenching. Since the discovery of the pyrene excimer by Forster and Kasper in 1954 [12],... 

Fig. 1.26 The effect of concentration on the emission spectrum of pyrene in air-equilibrated ethanol for 2exc = 330 nm. The spectra are normalised to the 372 nm peak and the concentrations are (1) lx 10 mol dm (2) 1 x 10 " mol dm (3) 1 x 10 mol dm . In dilute solution, a structured emission band between 350 and 450 nm is observed, which is assigned to the pyrene monomer. As the pyrene concentration is increased, a broad emission band between 425 and 550 nm emerges, which is attributed to emission from the pyrene excimer. The monomer to excimer ratio is dependent on both pyrene concentration and the excited state lifetime, which is reduced here due to oxygen quenching. Degassing of the solution to remove oxygen would result in an increase in the relative intensity of the excimer emission band...

Such a fluorescence dependence upon the concentration of quencher was observed for the pyrene fluorescence quenching by dimethylaniline in SDS micelles [25,26]. Rodgers and Bexendale [27] studied the IRuibpy) ] luminescence quenching by 9-methylanthracene and found that Eq. (24) fitted well in the case of sufficiently fast... 

The influence of the surfactant concentration upon the quenching process may be caused by the change of the micelles shape, rather than intermicellar exchange of quencher molecules, as it was supposed for pyrene fluorescence quenching by copper ions in SDS micelles [49]. 

Organic exciplexes are best identified by studying the variation of their emission spectra with concentration. This is illustrated in Figure 19 for pyrene (py). At low pyrene concentrations (<10 M), the major luminescence band is due to the monomer. The characteristics of the monomer emission include (i) vibronic structure is present, and (ii) the emission profile is independent of concentration at concentrations <10 M. As the pyrene concentration is increased above 10 M, the monomer emission is quenched and a new lower-energy emission band appears due to the formation of a [py-py] exdmer. The intensity of the excimer band increases with a concentration increase. The characteristics of the... 

The fluorescence of pyrene is quenched by Tl owing to an external heavy atom effect that requires a short-range interaction [30]. In the reference copolymer poly(A/Py), which adopts an open-chain conformation in aqueous solution, TC ions are electrostatically concentrated in the vicinity of the anionic polymer chain and can come into contact with the Py labels. Thus, the Py fluorescence in the reference copolymer is efficiently quenched by Tr [22]. In contrast, the encapsulation of the Py labels in the hydrophobic microdomains in the terpolymers shown in Scheme 3 causes a sharp suppression of fluorescence quenching by TL [22]. The TL ions cannot reach the Py sites, which are buried inside the hydrophobic microdomains. 

Elegant evidence that free electrons can be transferred from an organic donor to a diazonium ion was found by Becker et al. (1975, 1977a see also Becker, 1978). These authors observed that diazonium salts quench the fluorescence of pyrene (and other arenes) at a rate k = 2.5 x 1010 m-1 s-1. The pyrene radical cation and the aryldiazenyl radical would appear to be the likely products of electron transfer. However, pyrene is a weak nucleophile the concentration of its covalent product with the diazonium ion is estimated to lie below 0.019o at equilibrium. If electron transfer were to proceed via this proposed intermediate present in such a low concentration, then the measured rate constant could not be so large. Nevertheless, dynamic fluorescence quenching in the excited state of the electron donor-acceptor complex preferred at equilibrium would fit the facts. Evidence supporting a diffusion-controlled electron transfer (k = 1.8 x 1010 to 2.5 X 1010 s-1) was provided by pulse radiolysis. 

The spectra observed with four concentrations of pyrene are shown in Figures 18 and 19. The spectra of normal fluorescence are similar to those previously reported by Forster and Kasper. The relative intensity of the dimer band increases as the concentration of pyrene increases and the simultaneous reduction in fluorescence efficiency, n, of the monomer follows the Stem-Volmer quenching law with a mean quenching constant of 2.0 X 10s liter mole-1 (Table VII). 

In some cases, simultaneously with the quenching of the normal fluorescence a new structureless emission band appeals at about 6000 cm-1 to the red side of the monomer fluorescence spectrum (Figure 6.4). This phenomenon was first observed in pyrene solution by Forster and was explained as due to transitory complex formation between the ground and the excited state molecules since the absorption spectrum was not modified by increase in concentration. Furthermore, cryoscopic experiments gave negative results for the presence of ground state dimers. These shortlived excited state dimers are called pxcimers to differentiate them from... 

Fluorescence quenching in micelles. Consider an aqueous solution with a high concentration of micelles (Box 26-1) and relatively low concentrations of the fluorescent molecule pyrene and a quencher (cetylpyridinium chloride, designated Q), both of which dissolve in the micelles. 

Quenching occurs if pyrene and Q are in the same micelle. Let the total concentration of quencher be [Q] and the concentration of micelles be [M]. The average number of quenchers per micelle is... 

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... 

Pyrene can sensitize the photooxidation of 1,3-diphenylisobenzofuran (72) in DTAC micellar solutions 61>. The reaction involves sensitization of singlet oxygen by pyrene which diffuses into another micelle and reacts with (72). Indole and tryptophan, which also react with singlet oxygen, quench the above reaction in ethanol solutions. However, in micellar systems they enhance the rate of reaction. Because of the high local concentrations of the quencher, the pyrene excited state is quenched by indole and tryptophan which leads to the photooxidation of (72) by a Type I process. 

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