Pyrene normal fluorescence

P-type delayed fluorescence is so called because it was first observed in pyrene. The fluorescence emission from a number of aromatic hydrocarbons shows two components with identical emission spectra. One component decays at the rate of normal fluorescence and the other has a lifetime approximately half that of phosphorescence. The implication of triplet species in the mechanism is given by the fact that the delayed emission can be induced by triplet sensitisers. The accepted mechanism is ... 

Fig. 11. Luminescence of impurities in ethanol at 20°C. sensitized by 10 Mf phenanthrene. Rate of light absorption was 0.7 X 10- einstein liter-1 sec.-1 at 341 to 362 mji. (a) normal fluorescence (b) and (c) delayed fluorescence at 1000 times greater sensitivity (d) fluorescence of dilute pyrene solution. Photodecomposition of (i>) to give (c) was produced by irradiation for 30 min. with rate of light absorption equal to 10 - einstein liter-1 sec.-1.

The spectra observed with four concentrations of pyrene are shown in Figures 18 and 19. The spectra of normal fluorescence are similar to those previously reported by Forster and Kasper. The relative intensity of the dimer band increases as the concentration of pyrene increases and the simultaneous reduction in fluorescence efficiency, n, of the monomer follows the Stem-Volmer quenching law with a mean quenching constant of 2.0 X 10s liter mole-1 (Table VII). 

Fig. 18. Normal fluorescence of pyrene in ethanol.42 (1) 3 X 10 W, (2) 10-5M, (3) 3 X 10 4JI/, (4) 2 X 10 Af, The instrumental sensitivity settings for curveB 1 and 4 were approximately 0.6 and 3.7 times that for curves 2 and 3. The short wavelength ends of the spectra in the more concentrated solutions are distorted by self-absorption.

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... 

For years it has been known that the quantum yield of fluorescence for a number of aromatic hydrocarbons decreases with increasing concentration, but the cause of this concentration quenching was not well understood. In 1955 it was first noted by Forster that increasing concentration not only quenches the normal fluorescence of pyrene (5), but also introduces a new fluorescent component. 

Solute/solute Interactions are revealed by the forinatlon of exclmers. Exclmers are excited state dimers that result In a broad structureless band at significantly longer wavelengths than the normal fluorescence. While they are not dimers In the sense of a ground state conqplex, their existence does Indicate that there Is sufficient Interaction In the approximately 10 second lifetime of the excited state (33) to form the excited state complex. We have observed the formation of pyrene exclmers even at extremely low concentrations In supercritical fluids. Figure 7 shows the spectra of pyrene In SCF CO2 at two concentrations. 

Fig. 86 Fluorescence spectra of a pyrene-implanted PBMA surface as a function of laser pulse number. Pyrene was transferred using ablation of a triazene polymer. Laser flu-ence 100 mj creT2, (a) 5 pulses, (b) 10 pulses, (c) 15 pulses, (d) 20 pulses. The vibrational pyrene emission peaks are denoted (I-V). Inset Normalized fluorescence intensity of the V pyrene peak at 393 nm vs laser pulse number. Data are taken from the spectra in the main figure. REPRINTED WITH PERMISSION OF [Ref. 360], COPYRIGHT (1998) Elsevier Science...

I. Excimer Formation, t Dilute solutions of pyrene (1) show a normal fluorescence, corresponding (see Fig. 6.6) to the absorption spectrum of (1). In concentrated solutions, however, the normal fluorescence is quenched. 

Figure 9J2 Pyrene (1 pM) band 1-to-band 3 emission intensity ratio (Py ///j) as a function of [Cj( im][Cl] concentration in IM aqueous NaOH. Inset, normalized fluorescence emission spectra of pyrene =337 nm, slits=1/1 nm) in 1M aqueous NaOH in the absence of [Cj im]...

Fluorescence spectra and quantum yields of pyrene in supercritical CO2 have been determined systematically as functions of temperature, CO2 density, and pyrene concentration. Under near-critical conditions, contributions of the pyrene excimer emission in observed fluorescence spectra are abnormally large. The results cannot be explained in the context of the classical photophysical mechanism well established for pyrene in normal liquid solvents. The photophysical behavior of pyrene in a supercritical fluid is indeed unusual. The experimental results can be rationalized with a proposal that the local concentration of pyrene monomer in the vicinity of an excited pyrene molecule is higher than the bulk in a supercritical solvent environment. It is shown that the calculated ratios between the local and bulk concentrations deviate from unity more significantly under near-critical conditions (Sun and Bunker, 1995). 

The normal violet fluorescence band of pyrene solutions shows concentration-quenching which is accompanied by the appearance of a blue structureless emission band. Forster and Kasper40 showed that the blue band is due to emission from an excited dimer formed by the combination of an excited singlet molecule with a molecule in the ground state. Most of the light in both spectral bands has a relatively short lifetime but Stevens and Hutton87 observed a long-lived component of the dimer... 

We report on steady-state and time-resolved fluorescence of pyrene excimer emission in sub- and supercritical C02. Our experimental results show that, above a reduced density of 0.8, there is no evidence for ground-state (solute-solute) interactions. Below a reduced density of 0.8 there are pyrene solubility complications. The excimer formation process, analogous to normal liquids, only occurs for the excited-state pyrene. In addition, the excimer formation process is diffusion controlled. Thus, earlier reports on pyrene excimer emission at rather "dilute pyrene levels in supercritical fluids are simply a result of the increased diffusivity in the supercritical fluid media. There is not any anomalous solute-solute interaction beyond the diffusion-controlled limit in C02. 

Figure 2.22 Left fluorescence spectra of pyrene in cyclohexane. Intensities are normalized to a common value of < f. A, 10-2m B, 7.75 x 10-3m C, 5.5 x 10-3m D, 3.25 x 10-3m E, 10-3m G, 10-4m. Reproduced by permission from ref. 109. Copyright 1970, John Wiley Sons, Ltd. Right potential energy surfaces for excimer formation />... P represents the distance between two pyrene molecules. The vibrational levels shown for the monomers at large separation refer to degrees of freedom other than rP... P...

A similar experiment was performed for the PMMA film doped with 1-ethyIpyrene. As shown in Fig. 7, fluorescence spectra were composed of a structured monomer and red-shifted broad excimer bands. As molecular diffusion during fluorescence lifetime is negligible in film, the latter band is due to the ground state dimer of pyrene which is easily formed under its high concentration. It should be notified that the fluorescence intensity ratio of the monomer to excimer emissions under the TIR condition is larger than that under the normal one. This may indicate that the pyrenyl concentration in the interface layer is also lower than that in the bulk. 

Figure 5. The dependence of the rate of proton dissociation from excited 8-hydroxy-pyrene- 1,3,6-trisulfonate on the mole fraction of ethanol in water, and the respective proton conductivity of the mixtures. The rate of proton dissociation was measured by time resolved ( ) or steady-state ( ) fluorescence. The proton conductivity of the solutions (A) is normalized for pure water conductivity. Data taken from Erdey-Grutz and Lengyel (1977).

Excited singlet lifetimes, also called fluorescence lifetimes, of organic molecules are normally smaller than 10 ns. Notable exceptions are polycyclic aromatic hydrocarbons, such as pyrene and naphthalene. Due to their short lifetimes, fluorescent probes can only explore a small volume therefore, competition between the probe s decay to the ground state and dissociation from the supramolecular structure to the homogeneous phase occurs only infrequently. For example, in order to compete with the decay of the excited state, a probe with a 10 ns excited state lifetime must possess a dissociation rate constant from the supramolecular structure that is larger than 10 s. For this reason, fluorescent probes are normally assumed not to relocate during their lifetimes hence, explore a very limited volume. 

The term probe is normally used as a synonym of label. Thus, in contrast to labels, probes respond to their microenvironment or to a chemical species. Those probes responding to a chemical species such as oxygen, an ion, or to pH are also referred to as indicators. In this article, both terms will be used indistinctly. Different classes of fluorescent labels are available. Low molecular weight dyes include xan-thene (rhodamines, fluoresceins), cyanine, coumar-ins, sulfonated pyrenes, and metal phthalocyanine compounds, while high molecular weight labels include phycobiliproteins and other luminescent proteins. 

---