Pyrene emission spectrum

Fig. 17 Ratio of third to first vibrational peak of pyrene emission spectrum as a function of DTAB concentration in the presence of 1 g/L sodium polyacrylate...

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.

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... 

Another remarkable property of the pyrene fluorescence is that a change of polarity in the vicinity of molecular pyrene leads to a dramatic modification of the intensity of the 0-0 vibronic band (/i peak), without affecting the intensity of the /j peak. The /1//3 ratio of the pyrene emission spectrum is therefore an extensively used probe parameter to determine the micropolarity of the environment in which it resides. 

The fine and resolved vibronic structure of the pyrene emission spectrum coupled with the low water-solubility of this aromatic molecule, make pyrene a cheap, efficient, and polymer-cost effective fluorescent probe for characterizing micellar cores (eg, dielectric constants). 

Additional support for this disassembly mechanism was obtained by monitoring the release of the pyrene tail units by fluorescence spectroscopy. The confined proximity of the pyrene units in the dendritic molecule results in formation of excimers. The excimer fluorescence generates a broad band at a wavelength of 470 nm in the emission spectrum of dendron 31 (Fig. 5.25). Upon the release of the pyrene units from the dendritic platform, the 470 nm band disappeared from... 

Since the same dye molecules can serve as both donors and acceptors and the transfer efficiency depends on the spectral overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor, this efficiency also depends on the Stokes shift [53]. Involvement of these effects depends strongly on the properties of the dye. Fluoresceins and rhodamines exhibit high homo-FRET efficiency and self-quenching pyrene and perylene derivatives, high homo-FRET but little self-quenching and luminescent metal complexes may not exhibit homo-FRET at all because of their very strong Stokes shifts. 

A first generation poly(amido amine) dendrimer has been functionalized with three calyx[4]arenes, each carrying a pyrene fluorophore (4) [30]. In acetonitrile solution the emission spectrum shows both the monomer and the excimer emission band, typical of the pyrene chromophore. Upon addition of Al3+ as perchlorate salt, a decrease in the excimer emission and a consequent revival of the monomer emission is observed. This can be interpreted as a change in the dendrimer structure and flexibility upon metal ion complexation that inhibits close proximity of pyrenyl units, thus decreasing the excimer formation probability. 1H NMR studies of dendrimer 4 revealed marked differences upon Al3+ addition only in the chemical shifts of the CH2 protons linked to the central amine group, demonstrating that the metal ion is coordinated by the dendrimer core. MALDI-TOF experiments gave evidence of a 1 1 complex. Similar results have been obtained for In3+, while other cations such as Ag+, Cd2+, and Zn2+ do not affect the luminescence properties of... 

The width of a band in the absorption or emission spectrum of a fluorophore located in a particular microenvironment is a result of two effects homogeneous and inhomogeneous broadening. Homogeneous broadening is due to the existence of a continuous set of vibrational sublevels in each electronic state. Absorption and emission spectra of moderately large and rigid fluorophores in solution could therefore be almost structureless at room temperature. However, in some cases, many of the vibrational modes are not active, neither in absorption nor in emission, so that a dear vibrational structure is observed (e.g. naphthalene, pyrene). 

Fig. 13 Emission spectrum of pyrene in different solvents showing the change in peak intensity with the change in the solvent...

Weller24 has estimated enthalpies of exciplex formation from the energy separation vg, — i>5 ax of the molecular 0"-0 and exciplex fluorescence maximum using the appropriate form of Eq. (27) with ER assumed to have the value found for pyrene despite the doubtful validity of this approximation the values listed for AHa in Table VI are sufficiently low to permit exciplex dissociation during its radiative lifetime and the total emission spectrum of these systems may be expected to vary with temperature in the manner described above for one-component systems. This has recently been confirmed by Knibbe, Rehm, and Weller30 who obtain the enthalpies and entropies of photoassociation of the donor-acceptor pairs listed in Table XI. From a detailed analysis of the fluorescence decay curves for the perylene-diethyl-aniline system in benzene, Ware and Richter34 find that... 

The author has recently carried out some measurements in solution using naphthacene as acceptor.63 As donor, anthracene was chosen rather than pyrene, so as to minimize overlap of the delayed fluorescence of donor and acceptor. A typical delayed emission spectrum from a solution containing 5 X 10 W anthracene and 4 X lO M naphthacene... 

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.

Fig. 3. The 3 to 13 micron emission spectrum from the Orion Bar compared with the absorption spectra of the PAHs chrysene, pyrene and coronene suspended in KBr pellets. (Orion, Ref. [47] Chrysene, Ref. [38] Pyrene, Ref. [37] Coronene, Ref. [39].) A schematic representation for the absorption spectrum is used because the KBr pellet technique alters the spectrum compared to that of a free species...

Figure 4. Emission spectrum of a cyclohexane solution of pyrene (a) uncorrected (b) spectrally...

Figure 10.11 shows the fluorescence emission spectrum of pyrene in the absence and presence of two humic substance concentrations. The fluorescence intensity of pyrene decreases with increasing humic substance concentration. This decrease results from the binding of pyrene on the humic substance. 

Excited state complexes are relevant in connection with photo-induced electron transfer, since their formation frequently competes with or precedes electron transfer. The simplest examples, excited state dimers (excimers), were discussed by Kautsky as early as 1939 [77], The first organic excimer, the dimer of pyrene, was identified by its characteristic, red-shifted, structureless emission spectrum by... 

Solvatochromic fluorescent probe molecules have also been used to establish solvent polarity scales. The solvent-dependent fluorescence maximum of 4-amino-V-methylphthalimide was used by Zelinskii et al. to establish a universal scale for the effect of solvents on the electronic spectra of organic compounds [80, 213], More recently, a comprehensive Py scale of solvent polarity including 95 solvents has been proposed by Winnik et al. [222]. This is based on the relative band intensities of the vibronic bands I and III of the % - n emission spectrum of monomeric pyrene cf. Section 6.2.4. A significant enhancement is observed in the 0 0 vibronic band intensity h relative to the 0 2 vibronic band intensity /m with increasing solvent polarity. The ratio of emission intensities for bands I and III serves as an empirical measure of solvent polarity Py = /i/Zm [222]. However, there seems to be some difficulty in determining precise Py values, as shown by the varying Py values from different laboratories the reasons for these deviations have been investigated [223]. 

The solvent sensitivity of the emission spectrum and fluorescence quantum yield of pyrene and its derivatives have been used to sense the polarity of microphase interiors. By these methods, pyrene in SDS micelles is located within a microenvironment less polar than water, but more polar than typical hydrocarbons [53]. 

For some fluorophores (well-known examples are pyrene and its derivatives) the emission spectrum depends dramatically on their concentration. This is due to the formation of excited-state dimers, so-called excimers, consisting of a ground-state and an excited-state monomer. For instance, lipids that are substituted with a pyrene moiety in each of their acyl chains can be used to study intramolecular excimer formation. In this way information on lateral organization and intramolecular dynamics can be obtained. 

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