Pyrene fluorescence spectrum

The relative changes in intensity of the vibronic bands in the pyrene fluorescence spectrum has its origin in the extent of vibronic coupling between the weakly allowed first excited state and the strongly allowed second excited state. Dipole-induced dipole interactions between the solvent and pyrene play a major role. The polarity of the solvent determines the extent to which an induced dipole moment is formed by vibrational distortions of the nuclear coordinates of pyrene (Karpovich and Blanchard, 1995). 

The ratio Ij/ (intensities of the first and third bands in the pyrene fluorescence spectrum) changes from 1.8 in water to about 0.6 in aliphatic hydrocarbon solvents and decreases with increasing Cp. The high I /I3 (1.58-1.65) at low Cp (ca. 1.58-1.65) was related to the existence of premicelles. 

Figure 1. (a) Room-temperature fluorescence spectra of benzo(a)pyrene on 80% a-Room-temperature fluorescence spectrum of 500 ng of benzo(a)pyrene on 80% a-<7clodextrin—NaCl. = 300 nm. 

Tab. 7.4. Solvent dependence of the ratio / //m of the fluorescence intensities of the first and third vibronic bands in the fluorescence spectrum of pyrene.

The Na+ sensor M-9 has a structure analogous to that of compound E-4, but instead of two identical pyrene fluorophores, it contains two different fluorophores with a pyrene group and an anthroyloxy group. Resonance energy transfer (see Chapter 9) from the former to the latter is then possible because of the spectral overlap between the fluorescence spectrum of the pyrene moiety and the absorption spectrum of the anthroyloxy moiety. Upon addition of Na+ to a solution of M-9 in a mixture of MeOH and THF (15 1 v/v), the fluorescence of the anthroyloxy group increases significantly compared with that of the pyrene group, which permits a ratiometric measurement. 

Figure 8.2 presents the fluorescence of pyrene on silica gel. The loading is low so that pyrene is predominantly adsorbed as nonaggregated monomers (Mi). The backward fluorescence spectrum Fb of this sample is very comparable to the spectrum in polar solvents and not distorted by reabsorption. However, the forward spectrum Ft is almost completely suppressed in the region of overlap with the o -transition and hot sidebands of the weak first absorption band Si. The absorption coefficients of the sample vary widely from k" = 0.1 cm 1 (Si-band, Aa = 350-370 nm) to k = 25 cm-1 (S2-band, 1 290-340 nm), and in a first approximation the excitation spectrum of Fh reflects this variation correctly (Figure 8.2, left). The Ff-excitation spectrum, however, has only little in common with the real absorption spectrum of the sample. 

The normal (short-lived) fluorescence spectrum of 3 X 10 2M naphthalene at —105 °C. [Fig. 21, curve (a) ] shows not only the band due to the singlet excited monomer but also the broad dimer emission band, with maximum at 400 m which is similar to that observed by Doller and Forster46 in toluene solutions. The spectrum of the delayed emission at the same temperature [Fig. 21, curve (b)] also shows both bands, but the intensity of the dimer band is relatively much greater. When the concentration is reduced to 3 X 10 W, the intensity of the dimer band at —105 °C. is very small in normal fluorescence but is still quite large in delayed fluorescence.45 The behavior of naphthalene solutions at —105° C. is thus qualitatively similar to that of pyrene at room temperature. At temperatures greater than — 67 °C. (Table XII) the proportion of dimer observed in delayed fluorescence is almost the same as that observed in normal fluorescence, and presumably at these temperatures, establishment of equilibrium between the excited dimer and excited monomer is substantially complete before fluorescence occurs to an appreciable extent. The higher the temperature, the lower is the proportion of dimer observed in either normal or delayed fluorescence because the position of equilibrium shifts in favor of the excited monomer. 

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... 

Figure 8 shows a pair of typical time-resolved fluorescence decay traces for 100 / M pyrene in supercritical CO2 (Tr = 1.02 pr = 1.17). Note that the ordinate is logarithmic. The upper and lower panels show results for selective observation in the monomer (400 +. 10 nm) and excimer (460 + 10 nm) regions of the pyrene emission spectrum. Several interesting features are apparent from these traces. First, both decay processes are not single exponential. Second, the excimer emission has a significant contribution from a species that "grows in" between 30 - 75 ns this is a result of the excimer taking time to form (i.e., k in Figure 1). Third, the fits between the experimental data and the model shown in Figure 1 are good. Detailed analysis of these decay traces (10,11,21-26) yields the entire ensemble of photophysical kinetic parameters for the pyrene excimer in supercritical C02.

Figure 3. Fluorescence spectrum of benzo(a)pyrene in solution and in the living...

Figure 2. Fluorescence spectrum in a nitrogen matrix at 15 K (excited by a 2.5-kW mercury-xenon lamp) of an adsorption chromatography fraction from a coking plant water sample. Compounds BbF, benzo[b]fluorene C, chrysene BeP, ben-zo[e]pyrene P, pyrene BkF, benzo[k]fluoranthene BaP, benzo[a]pyrene U, unknown ( ).

Differential solvent interactions with ground- and excited-state molecules not only lead to shifts in the fluorescence maxima but also to perturbation of the relative intensities of the vibrational fine structure of emission bands. For instance, symmetry-forbidden vibronic bands in weak electronic transitions can exhibit marked intensity enhaneements with increasing solute/solvent interaction [320, 359]. A particularly well-studied ease is the solvent-influenced fluorescence spectrum of pyrene, first reported by Nakajima [356] and later used by Winnik et al. [357] for the introduction of an empirical solvent polarity parameter, the so-called Py scale cf. Section 7.4. 

Figure 4. MI fluorescence spectrum of LC Fraction IV from Synthoil at two excitation wavelengths. P = pyrene U = unknown.

Figure 6. MI fluorescence spectrum of LC Fraction VI from Synthoil at three excitation wavelengths. BaP = benzo[a]pxjrene U = unknown P = pyrene (artifact not present in fraction).

Figure 5.23. Absorption ( ) and fluorescence spectrum (—) of pyrene a) 10 mol/ L in ethanol, b) 10" mol/L in ethanol, and c) absorption and emission (—) of crystalline pyrene (adapted from FOrster and Kaspar, 1955).

Pyrene undergoes this type of dimerization in solution as the concentration increases the original fluorescence spectrum disappears and a new one appears at longer wavelengths. The latter has been shown to be associated with the excited pyrene dimer, which is formed by the diffusion-controlled reaction (5). 

Pyrene has been used widely as a photophysical probe because of its long fluorescence lifetime and great tendency for excimer formation. Emission characteristics of pyrene molecules depend on the nature of the solvent. The ratio of relative intensities of the 1st (373 nm) and lllrd (383 nm) peaks, Ijjj/Ij, in a pyrene emission spectrum decreases as the polarity of the solvent increases. This... 

The fluorescence spectrum of dilute pyrene (<10 4m) in degassed cyclohexane exhibits vibronic structure and has a maximum at 395 nm. As the concentration of pyrene is increased, the fluorescence quantum yield of pyrene decreases and a broad, structureless emission band with a maximum at about 480 nm gains in intensity (Figure 2.22, left). 

Another extremely useful method for cac determination, especially in the light of high sensitivity, is fluorescence emission spectroscopy [15]. Some aromatic water-insoluble dyes that are present in trace amounts in mixed polyelectrolyte-surfactant solutions have an ability to solubilize within the self-assembled surfactant aggregates and to change their photophysical properties because of the change of environmental polarity. Through this, they offer a very sensitive method for the determination of cac values. A typical and lately frequently used compound is pyrene, which is used as a fluorescence probe to assess various micellar properties. Pyrene exhibits a polarity dependent fluorescence spectrum with the ratio /,//3 (the ratio of the intensity... 

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