Pyrene ground state association

As Fig. 15b illustrates, the graphical relation appears to be linear for an interaction number of 3 to 4, if A 1. Alternatively, for A = 1, linearity is evident (Fig. 15c) when the interaction number is 5 to 6. Thus a large value of A is compatible with the smallest interaction number. Excimer formation occurs within the fluorescence lifetime, about 8 nsec. Within that time the pyrene-labeled amine side chains must approach within about 4 A of each other. For the 5.3% pyrenylpolyethylenimine derivative in ethanol, where no ground-state association occurs, the effective local concentration of pyrene on the polymer matrix is about 10-2 M, as calculated from excimer fluorescence. In aqueous solution, where clusters form within the polymer matrix, the effective local concentration of pyrene adduct must be even greater. The quantitative assessment of fluorescence intensities (Fig. 15) points to a minimum interaction number of 3 to 4 pyrenyl-labeled amine side chains, within the 8 nsec lifetime. Since A 1, it appears from (12) that kDM(A) kMD + kD. Thus excimer formation must be very rapid in the polymer environment. We can conclude, therefore, that the primary-amine side chains of poly-ethylenimine are very flexible and mobile. 

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.

As has been previously reported, when pyrene is adsorbed on silica gel there is evidence for ground state association which is not present In solution or the vapor phase.9-13 but which has been described as being present when pyrene is dissolved in a plastic medium. This is also a manifestation of surface inhomogeneity -some sites enhance the tendency to form a ground state bimolecular complex, whereas other sites contain isolated pyrene molecules. The interaction differences are sufficient to yield significant spectral shifts in absorption and the ground state complex emits with the characteristic pyrene excimer fluorescence. Fig. 5 shows a typical set of spectra Illustrating this association and Fig. 6 presents evidence that this observation Is not due to microcrystal formation. 

Figure 5. Ground state association of pyrene and pyrene "excimer" emission. Excitation and emission opectra of pyrene on silica gel ( excitation (observed at 390 nml...

Surface heterogeneity may be inferred from emission studies such as those studies by de Schrijver and co-workers on P and on R adsorbed on clay minerals [197,198]. In the case of adsorbed pyrene and its derivatives, there is considerable evidence for surface mobility (on clays, metal oxides, sulfides), as from the work of Thomas [199], de Mayo and co-workers [200], Singer [201] and Stahlberg et al. [202]. There has also been evidence for ground-state bimolecular association of adsorbed pyrene [66,203]. The sensitivity of pyrene to the polarity of its environment allows its use as a probe of surface polarity [204,205]. Pyrene or ofter emitters may be used as probes to study the structure of an adsorbate film, as in the case of Triton X-100 on silica [206], sodium dodecyl sulfate at the alumina surface [207] and hexadecyltrimethylammonium chloride adsorbed onto silver electrodes from water and dimethylformamide [208]. In all cases progressive structural changes were concluded to occur with increasing surfactant adsorption. 

Pyrene fluorophores are also used as probes. Derivatives of pyrene show /.max/ Xem 340/376 nm, e 4.3 x 104 M 1 cm-1, and environmental sensitivity, this fluorophore can be used to report on RNA folding [102]. Pyrene also displays a long-lived excited state (x > 100 ns), which allows for an excited pyrene molecule to associate with a pyrene in the ground state. The resulting eximer exhibits a red-shift in fluorescence intensity (A,em 490 nm). This characteristic can be used to study important biomolecular processes, such as protein conformation [103]. 

Eximer Fluorescence. Since Forster and Kasper discovered concentration-dependent long-wavelength emission resulting from association of an electronically excited pyrene molecule with another ground state pyrene molecule,39 the phenomenon of excimer fluorescence has been studied extensively.40 The mechanism for excimer formation and emission can be represented by... 

Figure 4.22 The potential energy of excimer formation and emission of pyrene. The excimer ground state So is dissociative and association takes place only from the La excited state when the transition to Lb is forbidden. hvm is the molecular emission, and hve the excimer emission...

In this paper, we present a preliminary analysis of the steady-state and time-resolved fluorescence of pyrene in supercritical C02. In addition, we employ steady-state absorbance spectroscopy to determine pyrene solubility and determine the ground-state interactions. Similarly, the steady-state excitation and emission spectra gives us qualitative insights into the excimer formation process. Finally, time-resolved fluorescence experiments yield the entire ensemble of rate coefficients associated with the observed pyrene emission (Figure 1). From these rates we can then determine if the excimer formation process is diffusion controlled in supercritical C02. 

In some cases excited state chromophores form supramolecular complexes either with ground state chromophores on the same molecule or one nearby resulting in the formation of an excimer (excited state dimer) or, if the two chromophores are different to one another an exciplex. Formally an excimer as defined as a dimer which is associated in an electronic excited state and which is dissociative in its ground state.1 Formation of the pyrene excimer is illustrated in Figure 11.3. 

This phenomenon was first identified and explained by Forster.120 The structureless emission is attributed to an excited pyrene dimer (1P - P) that is formed by collisional association of singlet excited pyrene P with a pyrene molecule P in the ground state. It was subsequently found that many aromatic molecules exhibit similar behaviour. The expression excimer (excited dimer) was proposed by Stevens to distinguish such species from the excited state of a ground-state complex. Excimer formation is prominent at relatively low concentrations of pyrene (Figure 2.22, left), because of its unusually long fluorescence lifetime, 1t = 650 ns, which allows for diffusional encounters of P with P even at low concentration. 

Time-resolved measurements of pyrene fluorescence show that only the structured monomer emission is observed immediately after excitation, because the pyrene molecules are not associated in the ground state. The broad emission then grows in as the excimers are formed by diffusional encounter and the equilibrium between monomers and excimers is reached. 

In addition to parinaroyl phospholipids, pyrene fatty acid derivatives may be used. Such phospholipids have a concentration-dependent emission spectrum (Roseman and Thompson, 1980). At low concentrations of the derivative within the bilayer, the fluorescence is maximal at a wavelength below 400 nm. At higher concentrations of the derivative, the excited state monomers can associate with a ground state monomer to form a dimer complex, or eximer, in a diffusion-controlled process. The maximum emission wavelength of the eximer shifts to approximately 470 nm. The ratio of the eximer to monomer fluorescent intensity is proportional to the concentration of the probe molecules within the bilayer. 

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],... 

Evidence for some ground state pyrene association was provided by Yamanaka et al. (6971) in doped Si02 thin films, based on time-resolved fluorescence spectroscopy. 

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