Pyrene excimer intensity, effect

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... 

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...

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. 

Incomplete coverage of the surface of such a fifth-generation POPAM dendrimer exposes hydrophobic areas of adamantyl units remaining uncomplexed by cyclodextrins on the dendritic outer shell [38]. Pyrenes were used as neutral fluorescence probes to examine whether this might lead to aggregation in water driven by the hydrophobic effect [39a]. Their inclusion in the dendrimer/cyclo-dextrin aggregate leads to changes in fluorescence intensity and in the vibrational fine structure. Formation of excimers was also observed. 

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