Exciplex peak

The exciplex intensity showed quite different behavior as the setting proceeded. A comparison of the (monomer peak/monomer peak) ratio to the (exciplex peak/monomer peak) ratio was quite illuminating. We considered the initial maximum wavelength of the exciplex emission at 540 nm, and compared its intensity to the monomer intensity at 405 nm as the dissolution/ polymerization proceeded. A substantial decrease in exciplex intensity, compared to monomer intensity, was observed over the first 40 min of the cure. The ratio then leveled off, indicating that the local viscosity had reached a maximum after 40 min and that the dissolution/polymerization was considered to have reached completion at the ambient temperature of the laboratory. Since the working time for the cement was considerably less than the 40-min time period over which the exciplex/monomer intensity ratio was steadily decreasing, the intensity ratios served as in situ monitors of the cure. 

Poly(ACN-co-vlnyl cinnamate) showed an increase in intensity of the exciplex peak (433 nm) on increasing the concentration of oxygen. This behavior is similar to the formation of a CT complex (16). When dlmethylaniline was used as the quencher, normal quenching curves were observed. 

The appearance of the new, red, long-lived exciplex emission becomes even more obvious when plotting three-dimensional, time-resolved emission spectra (TRES). Figure 2.12 shows that the PL emission of the pure polymers decays monoexponentially with no spectral changes throughout their lifetime, whereas the blend PL evolves into the red exciplex peak. 

Fig. 2.20 Exciplex photoemission energy Ex plotted versus the difference between the oxidation potential of the donor E x and the reduction potential of the acceptor E d (i.e. the difference between HOMO and LUMO). Shown are results for PFB F8BT TFB F8B"f F8 PFB, and PFB F8T2. The straight line is a fit to the data according to Eq. (2.14) and yields 6=0.067 eV To reflect the inhomogeneous broadening of Ex, error bars representing 1/4 of the estimated FWHM of the exciplex peak were introduced. The HOMO and LUMO levels were measured with an error of + 0.03 eV (see Section 2.1.4), which gives an error of 0.04 eV for Eqx - E d, and corresponding x-error bars are drawn.

Apart from the dominant exciplex PL, the bilayer EL spectrum in Fig. 2.29(b) still contains some F8BT exciton emission that is visible as a yellow-green shoulder of the red exciplex peak. We show that this is due to endothermic transfer from the exciplex to the exciton, by considering the dependence of the bilayer EL on device temperature at constant applied bias. From Fig. 2.31, one can see that the excitonic contribution to the emission spectrum is thermally activated and frozen out completely below 209 K. The activation energy of this process is found to be 200 50meV (see Arrhenius plot in Fig. 2.32). This is consistent with the values extracted with our time-resolved PL [13] and electric-field-depen-dent PL [14] measurements, and confirms the origin of the exciton EL to be endothermic transfer from the exciplex as depicted in Fig. 2.1(b). 

Fig. 2.48 shows time-dependent PL spectra for several blends as well as for the pure polymers. At short times (0-10 ns), the spectra are mostly dominated by the exciton emission but at longer times the PL from the blends evolves into the red exciplex peak. After —30 ns the spectra do not evolve any further and decay with the time constants raekyed given in Table 2.2. The delayed emission still contains an F8BT exciton contribution (visible as a green shoulder at —535 nm). Since pure F8BT shows no delayed emission at all this has to be due to the endothermic transfer from the exciplex. Hence, the delayed PL emission has the same origin as the EL emission (i.e. the exciplex) and the two phenomena are very comparable. 

Figure 3. PL of a bilayer of PVK and PPyVP(CO(X i2H25)2V as a function of both emission energy and excitation energy. The 3D plot shows three prominent features a peak due to the PVK (excitation energy from 3.6 to 4.2 eV, emission energy from 2.8 to 3.4 eV), a peak due to the copolymer (excitation energy from 2.4 to 3.0 eV, emission energy 1.8 to 2.2 eV), and the exciplex peak (excitation energy from 3.6 to 4.2 eV, emission energy 2.2 to 2.8 eV). Reproduced with permission from reference 23.

As expected [2-4], when the solid and liquid cement components were mixed, the anthracene-toluidine complex fluorescence increased in intensity over time as the cure proceeded and nonfluorescence pathways for energy disposal were blocked. Although the change in peak shape made it difficult to comment on the relative fluorescence intensity from the exciplex compared to that from independent molecules, it was clear that the exciplex... 

Figure 2 Anthracene monomer, 1(427)71(405) and anthraeene-toluidine exciplex to monomer, 1(540)71(405) peak height ratios from a single sample, scanned repeatedly over the first 90 min of cure.

The interaction of nondegenerate molecular or charge-transfer states is insufficient to describe the stability of photoassociation products of molecules with different electronic energy levels, ionization potentials, and electron affinities. On the other hand, treatments26-26 of the exciplex as a pure charge-transfer state afford a quantitative description of the shift in fluorescence peak with solvent polarity and with electron affinity of the (fluorescent) donor in the same quencher-solvent system (Eq. 13) moreover, estimated values for the dipole moment of the emitting species (Table VI) confirm its pronounced charge-transfer character. 

The exciplex is formed with a particle that enters the first coordination sphere of the excited partner. This sphere is distinctive from others even in pure liquids where it is marked by the strongest peak in the pair distribution function. In this case k are simply the rates of jumps in and out of the nearest shell. They are... 

The light generation in the bilayer device is attributed to the decay of exciplexes formed at the PVK/copolymer interface. Figure 9.6 compares the PL of a pure wrapped copolymer, pure PVK, and a bilayer of PVK/copolymer, as well as the EL spectra of the bilayer device. The PL of the PVK film excited at 3.6 eV has an emission peak at 3.06 eV. The PL of pure wrapped copolymer film excited at... 

In Fig. 2.7, the PL spectra of a PFB F8BT blend and its pure components are plotted. The blend exhibits a strong emission, peaked around —625 nm, that is not observed in the pure polymers. We show that this red emission is due to exciplex states at the PFB F8BT heterojunction. 

In Section 2.1.4.4 we have shown that exciplex formation also occurs in blends of PFB with F8. We further characterize this system by considering time-resolved emission spectra for different film temperatures, as we have done for the PFB F8BT and TFB F8BT systems in Fig. 2.23. An example, taken at 150 K, is shown in Fig. 2.27(a). At short times (0-15 ns after excitation) PFB exciton emission, peaked at 455 nm, is predominantly observed together with some weak F8 exciton emission at 425 nm. At longer times the spectrum evolves into a red-shifted, broad peak at 475 nm that does not show any vibronic structure. For times > 30 ns there is no further spectral evolution and the decay is found to be roughly monoexponential with 41 ns decay constant. As already shown in Fig. 2.17, this long-lived, red-shifted emission is not observed in the pure polymers and is due to exciplex states that form at the F8 PFB heterojunction. 

In order to quantify the retrapping phenomenon, we determine the ratio r of the exciton and the exciplex contributions to the delayed emission spectra in fig. 2.49. We assume the peak intensities of the spectra (at —630 nm) to be a mea-... 

The best characterized ion recombination systems are those involving irradiated rare gases. In the mid 1970 s rare gas-monohalide systems were extensively studied, due to their new-found application in u.v. exciplex lasers. Pulsed electron irradiation of these systems was the only major excitation method, as it allowed investigation under realistic laser pressure conditions of several atmospheres gas pressure. Typically these systems were investigated by monitoring of the time dependence of their characteristic peak fluorescence, as given in Table 12. 

Table 12. Summary of peak emission wavelengths of rare gas-monohalide exciplex species.

The simplest exciplex system for the study of gaseous ionic recombination is from the gas system Xe/SF. The XeF exciplex produced is formed solely from Xe2 /SFg ion-ion recombination there is no detectable emission from the reaction of xenon electronically excited states with SFg [68]. The emission from the coupled XeF (B,C) state was found to extend from 320 to 360 nm, with a peak at 351 nm. 

A typical emission trace for XeF is shown in Figure 5a, for 500 Torr of xenon and 0.50 Torr of SF. This curve has several components a X-ray signal, dimer rare gas fluorescence and ionic recombination formed exciplex fluorescence. The X-ray signal followed the time profile of the 3 ns. electron pulse, and was typically only a few percent of the total emission signal. The first emission peak was also observed in irradiated pure xenon, at all wavelengths across and outside the XeF emission spectrum, and was therefore assigned to the broad xenon dimer, Xe2 fluorescence. The decay of the dimer fluorescence was typically complete within several hundred nanoseconds, and its intensity varied greatly with the xenon pressure. The second peak in the emission curve was dose-dependent, and only observed across the known XeF ... 

In 10 P-CD, besides the naphthalene-like emission of 1-a-naph-thyl-3-dimethylaminopropane (34) in HjO, a broad band peaking at 500 nm was present. Only the naphthalene emission was present in a- or y-CD solutions. The long wavelength is attributed to an intramolecular exciplex. On the basis of the solvent polarity effect on the exciplex an alcoholic environment was inferred, which should indicate that 34 is located near the top of the CD cavity, in the proximity of the OH groups. The failure to observe 34 intramolecular exciplex emission with a- and y-CD is attributed... 

Figure 13.14 Molecular oxygen absorption spectrum over the spectrum of a free-running ArF exciplex laser. The absorption peaks in the molecular oxygen spectrum are Schumann-Runge absorption bands. (Courtesy of R. Kunz and R. Dammel...

Figure 27. Corrected fluorescence emission spectra of metal-free Li (a), of Li in the presence of Eu(III) (b), of metal-free Lj (c), and of L2 in the presence of Eu(lll) (d) in dry THF. Both (c) and (d) are showing exciplex emission. The extra peaks in (b) and (d) are due to Eu(lll) as shown.

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