Delayed-emission spectra

It is interesting to note that excimer bands of phenanthrene67 and of anthracene,124 which have defied detection in the prompt fluorescence spectra even at low temperatures, have been observed in the delayed emission spectra of these compounds at — 75°K. Presumably at the low temperatures necessary to observe these bands the high solvent viscosity completely suppresses photoassociation at the reduced concentration available, i.e., WM] 1 /r , whereas the reduced triplet-triplet annihilation rate constant mstationary concentration of the triplet state. 

Fig. 12. Eosin in glycerol (7 X 10 Af) and eosin in ethanol (1.5 X 10 W). (a) Fluorescence emission spectrum at +30°C. (6) delayed emission spectrum (DES) at +69°C. (c) DES at +48°C. (d) DES at + 18°C. (e) DES at -40°C. Delayed emission spectra at a sensitivity 600 times greater than that for the fluorescence emission spectrum. (/) Fluorescence emission spectrum at -J-22°C. (g) delayed emission spectrum (DES) at +71 °C. (h) DES at +43°C. (j) DES at +22°C. (Z) DES at — 7°C. (m) DES at —58°C. (s) Sensitivity of 1)558 photomultiplier with quartz monochromator (unite of quanta and frequency). Delayed emission spectra at a sensitivity 3000 times greater than that for the fluorescence emission spectrum.

Fig. 15. Delayed emission spectra of 10 W proflavine hydrochloride in glycerol (left) and ethanol (right).s [Excitation at 2.73 n l (366 m/ )]. (1) Fluorescence emission spectrum at 20°C. (2) delayed emission spectrum (DES) at 20°C. (3) DES at —18° (4) DES at —38° (5) DES at —76°. Delayed emission spectra at a sensitivity approximately 80 times greater than that for the fluorescence emission spectrum. (6) Fluorescence emission spectrum at 20° (7) DES at 20° (8) DES at 0° (9) DES at —21° (10) DES at —39° (11) DES at —74°. Delayed emission spectra at a sensitivity approximately 10,000 times greater than that for the fluorescence emission spectrum.

The first delayed emission spectra observed from a phenanthrene solution are shown in Figure 16.33 At —107 °C. a fairly intense broad emission band with peak at about 1.9 m-1 was present. As the tempera-... 

Fig. 16. Delayed emission spectra of 10 3M phenanthrene in ethanol.33 The left-hand sections of the spectra (delayed fluorescence) all were recorded at a sensitivity approximately 1000 times greater than that used for curve (1) (normal fluorescence). The right hand sections of the curves (phosphorescence) were recorded at the following sensitivities (2) —107° at 60X (3)...

Typical normal and delayed emission spectra from 5 X 10anthracene in ethanol are shown in Figure 17.38 Similar results were obtained in cyclohexane. The observed intensity of the delayed fluorescence band was about 0.28% of that of the normal fluorescence band and thus, applying the phosphorimeter factor of 3, the efficiency (6) of delayed fluorescence in this solution was approximately 0.8% of the efficiency (f) of normal fluorescence. In solutions with lower concentrations of anthracene the value of 6 was proportionately lower (see Table IV). 

Fig. 22. Sensitized delayed fluorescence spectra of anthracene in 10 SM phenan-threne solution.38 Intensity of exciting light was approximately 2.7 X 10-8 einstein cm. 2 sec. 1 at 3.19m-1 (313 dim)- Delayed emission spectra with anthracene concentrations of (1) 10 8Af, (2) 5 X 10 W, (3) 10-W, (4) 10- M, (5) 10 9M. Curve (6) Fluorescence emission spectrum of solution 1 at 260 times less sensitivity. (Owing to variation in the shapes of the cylindrical optical cells, the relative intensities of the delayed emission are only approximately proportional to 9A/fF.)...

Figure 8. Top, optical absorption and the corresponding prompt fluorescence spectra of diacetylene polymer molecules middle, excitation and the corresponding delayed emission spectra and bottom, transient absorption for a partially polymerized (broad line) and fully polymerized (narrow line) diacetylene crystal.

The results presented above show that the low-energy broad PL band at 480 nm in the OMBE-grown films is seen only in the time-delayed emission spectra and it stems from some defects in the material and its relative intensity depends on the sample morphology. This defect band can in principle be either due to structural defects in epitaxially-grown nanocrystallites or related to some sort of chemical defects, as fluorenone defects possibly created by oxidation of PSP molecules. 

We found that OMBE-grown PSP films on KC1 substrates at different substrate temperatures show a drastic difference in the delay emission spectra while their cw-PL spectra are rather similar. The broad structureless defect emission band dominates the delayed PL emission of PSP films consisting of lying molecules on the substrate and no such band has been observed in films composed of upright standing molecules. This clearly indicates a structure-related origin of the observed defects, implying that their concentration could be minimized in most perfect structures of PSP crystallites. 

Whereas under photoexcitation the exciplex is excited indirectly via energy transfer from the excitons, it is the primary neutral excitation in electroluminescence. This is shown in Fig. 2.24, parts (a) and (b), where the EL emission for both TFB and PFB blends is dominated by the exciplexes. This becomes particularly clear when comparing the EL spectra with the delayed emission spectra in Fig. 2.23, parts (c) and (d). In contrast, the time-integrated PL from similarly prepared blend films (also plotted in Fig. 2.24) is primarily due to bulk excitons. We note that exciplex EL emission has been observed previously, which suggests that these exciplexes may also be formed by the mechanism of direct electron-hole capture at the interface [37, 41, 42]. 

Fig. 2.49 Delayed emission spectra, integrated over 30-90 ns after excitation, for blends of different weight ratios. For low fractions of F8Bf the exciplex shifts to the blue, as is shown forthe case of a 97.5 2.5 blend. The spectra have been normalized.

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-... 

Fig. 3.26 Absorption and delayed emission spectra of PF2/6 in MTHF at 80 K (upper half). The emission spectrum was taken 5 ms after optical excitation at 3.05 eV DF corresponds to delayed fluorescence and Ph to phosphorescence. Absorption and delayed emission of a PF2/6 film at 80 K (lower half). The delay was 5 ms after excitation at 3.05 eV Reprinted from [142], copyright 2002, with permission from the American Institute of Physics.

Weak delayed-emission spectra of vacuum- or air-irradiated copolymer films were similar in intensity and showed a phosphorescence maximum at 432 nm with shoulders ca. 390 nm and 450 nm on excitation at 260 nm in addition to a very weak maximum at 505 nm excited at 380 nm. These spectra are close to those shown in Figure 4 for ketone phosphorescence in photooxidized polystyrene and agree reasonably well with phosphorescence spectra for model napthaleneones (15). Energetically, the quench-... 

FIGURE 20. Delayed-emission spectra at 77 K for (a) mixtures of poly(styrene) and poly(l-vinyl-naphthalene), (b) the corresponding co-polymer within each case 1 mo1e% vinyl naphthalene. (1) is poly(l-vinylnaphthalene) delayed fluorescence, (2) is poly(styrene) phosphorescence, and (3) is poly(l-vinylnaphthalene) phosphorescence [after figure in Macromolecules, 2, 187 (1969)]. 

The delayed emission spectrum reveals several interesting features (Figure 2) (note that these delayed emission spectra were taken with a S-20 response photomultiplier to enhance the red portion of the Py phosphorescence) ... 

Kim and Webber studied delayed emission spectra of poly(vinyl carbazole) that was doped with dimethylterephthalate and pyrene [252]. On the basis of their results, they concluded that at room temperature dimethylterephthalate does not completely quench the triplet excitation state of poly (vinyl carbazole). They also concluded that phosphorescent states of poly(vinyl carbazole)-dimethylterephthalate are similar, implying a significant charge-transfer character in the former. 

The fluorescence spectra of all films studied to date have the common feature that they are similar to the classical excimer band. In fact there may be several types of excimer bands, representing different chromophore orientations. Delayed emission spectra display similar excimer-like features for both delayed fluorescence and phosphorescence. Examples are reproduced in Fig. 8 for P2VN and PVCz. We also note that the delayed fluorescence is red shifted relative to the prompt fluorescence for both P2VN and PVCz. We (24) and others (25,26) have concluded that this is evi-... 

We start to present the delayed emission spectra to avoid, for the moment, the influence of the laser light on the emission behaviour and on the decomposition reaction. After a transient response as a result of the action of the laser light, the reaction will show features of the decomposing explosive. 

The broad continua and the line inversion shown in figures 12, 13 and 14 disappear when the light emission is recorded about 350 nsec after the start of the laser pulse. Then the laser pulse will have passed and the light emission is solely due to the decomposition process of the explosive. Particularly interesting are the initial and delayed emission spectra of mercury fulminate, shown in figure 15, which were recorded at z = 0 mm. 

The possible influence was also considered of the wavelength on the formation of species determined from emission spectra. The initial spectra, especially, may reveal some influence. Unfortunately, the bad timing characteristics of the Ruby-laser used did not allow for the recording of initial spectra. In figure 17 and 18 delayed emission spectra of picric acid and of RDX after irradiation with 248 nm and 694 nm laser light, recorded 270 nsec after the laser pulse, show identical emission spectra. If a different decomposition pathway was initiated by the Ruby laser pulse, then it apparently relaxes quickly to that... 

Delayed emission spectra in 1 1 diethylethertetra-hydrofuran glasses. Curve A Styrene-1-vinyl-naphthalene copolymer containing 1 mol percent 1-vinylnaphthalene derived groups. Curve B Mixture of polystyrene and poly(l-vinylnaphthalene) polymers 1... 

Fig. 5. Delayed emission spectra of three different poly(N-...

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