Pyrene excimer formation, effect

Figure 11. Effect of lowering the temperature on the pyrene excimer formation. The silica gel is covered with decanol (1 x 10"3 mol/g SI02). Pyrene iSo 2. 5 mg/g Si02. Emission spectrum of pyrene at 200° K. The excitation wavelength 345 nm.

Pyrene excimer formation still continues to be of interest and importance as a model compound for various types of study. Recent re-examinations of the kinetics have been referred to in the previous section. A non a priori analysis of experimentally determined fluorescence decay surfaces has been applied to the examination of intermolecular pyrene excimer formation O. The Kramers equation has been successfully applied to the formation of intermolecular excimer states of 1,3-di(l-pyrenyl) propane . Measured fluorescence lifetimes fit the predictions of the Kramer equation very well. The concentration dependence of transient effects in monomer-excimer kinetics of pyrene and methyl 4-(l-pyrenebutyrate) in toluene and cyclohexane have also been studied . Pyrene excimer formation in polypeptides carrying 2-pyrenyl groups in a-helices has been observed by means of circular polarized fluorescence" . Another probe study of pyrene excimer has been employed in the investigation of multicomponent recombination of germinate pairs and the effect on the form of Stern-Volmer plots ". 

P. Lianos and R. Zana, Use of pyrene excimer formation to study the effect of NaCl on the... 

Nakamura and Thomas have studied a stable aqueous suspension of the hexadecyltrimethyl-ammonium chloride (Ci63CiNCl)/laponite system using pyrene as a probe (126). Their suspension contained 2 mmol of Ci63CiNCl and 1 g of laponite, which was able to dissolve 0.1 mmol pyrene. The kinetics of pyrene quenching and pyrene excimer formation reactions suggests that the Ci63CiNCl forms a cluster-like double layer on the clay surface. Since the amount of CieSCiNCl used is twice the CEC of laponite, the effects of Cl on the photophysics of adsorbed pyrene cannot be excluded. 

Cheung S-T, Winnik MA, Redpath AEC (1982) The effects of solvent on end-to-end cycliza-tion of poly(ethylene oxide) probed by intramolecular pyrene excimer formation. Makromol Chem 183 1815-1824... 

Bright and coworkers investigated pyrene-excimer formation in supercritical fluids from a somewhat different angle using not only steady-state but also time-resolved fluorescence techniques (47,167). They measured fluorescence lifetimes of the pyrene monomer and excimer at a pyrene concentration of 100 p,M in supercritical ethane, CO2, and fluoroform at reduced densities higher than 0.8. Since the kinetics for pyrene-excimer formation was found to be diffusion controlled in ethane and CO2 and less than diffusion controlled in fluoroform, they concluded that there was no evidence for enhanced pyrene-pyrene interactions in supercritical fluids. The less efficient excimer formation in fluoroform was discussed in terms of the influence of solute-solvent clustering on excimer lifetime and stability. Experimentally, their fluorescence measurements were influenced by extreme inner-filter (self-absorption) effects due to the high pyrene concentration in the supercritical fluid solutions (35). 

To date, only three studies have appeared which examine the consequences of macromolecule concentration on the cyclization of labelled polymers. These have involved pyrene excimer formation end-to-end cyclization for polymers with Py groups on the chain ends, and internal cyclization for polymers containing Py- groups along the chain backbone. In none of these experiments are the chains long enough, or the solutions sufficiently concentrated, for entanglement effects to be important. 

Frank et al. [29] studied the effect of hydrophobic interaction by comparing the fluorescent properties of PMAA/PEO and with those of PAA/PEO . Here PEO denotes pyrene end-labeled PEO. Figure 3 shows the intensity ratio le/Im of inframolecular excimer pyrene for PMAA/PEO (9200) and PAA/PEO (9200). It is seen that when added, PMAA more markedly reduces intramolecular excimer formation in PEO than does PAA. This difference is thought to be due to a stronger abihty of PMAA to combine PEO and the consequent suppression of intramolecular cychzation of PEO. 

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. 

On the low density side of these traces (below pr = 1), the rise in WIm (as one approaches a reduced density of unity) is a result of pyrene continually being solubilized by the fluid (Figure 5). This observation is consistent with Johnston s report of pyrene solubility in CO2 (17). Again, recall that excimer formation is a generally believed (10,11) to be a bimolecular process, i.e., the rate is concentration dependent. Thus, because the actual analytical concentration of solubilized pyrene in the solution is lower due to solubility, the amount (fraction) of excimer is "artificially" lower. Thus, in the low density region (pr = 0.5 -0.8) the observed trends are simply a result of solubility effects. However, solubility does not help to explain the results seen over the remainder (pr = 0.8 - 1.8) of the density range investigated. 

Pyrene excimer fluorescence was used as a sensitive proximity probe in the intermolecular complex system. Upon addition of PAA solution, the intramolecular mobility of the PEG chain was suppressed resulting in decreased intramolecular excimer formation. Simultaneously, the local concentration of PEG is increased in the vicinity of PAA. The excimer formation seems to result from the arrangement of pyrene groups which is already pre-formed in the ground state. The effect of complexation is observed to be more pronounced in the PEG-PAA with a higher molecular weight of PAA. A more complete account of this work will appear elsewhere. (16,17)... 

Since increasing temperature leads to increased fluidity and thus to a faster probe diffusion, pyrene Hpids have been frequently used to study phase transition in membranes [161,162]. Phospholipid phase separation increases the local concentration of dye labeled Hpids and can, therefore, be investigated via the characterization of exdmer formation. The binding of proteins or ions, however, may induce phase separation as well as decreasing lateral lipid diffusion. Since these two effects are opposing in terms of excimer formation, the binding of such proteins or ions cannot be studied by the (Ex/Mo)-ratio. The time-resolved analysis of the monomer fluorescence of the labeled lipid, however, allows for the separation of... 

Pyrene Monomer and Excimer Emission. The emission of locally isolated excited pyrenes ( monomer emission, intensity Im) is characterized by a well-resolved spectrum with the [0, 0] band at 378 nm. The emission of pyrene excimers (intensity Ie), centered at 480 nm, is broad and featureless. Excimer formation requires that an excited pyrene (Py ) and a pyrene in its ground state come into close proximity within the Py lifetime. The process is predominant in concentrated pyrene solutions or under circumstances where microdomains of high local pyrene concentration form, even though the total pyrene concentration is very low. This effect is shown for example by... 

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