Anthracene delayed fluorescence from

There are two mechanisms by which the sensitized delayed fluorescence from anthracene can be produced ... 

Fig. 26. Sensitized anti-Stokes delayed fluorescence from anthracene.60 Normal fluorescence (curve 1) and delayed fluorescence (curve 2) from a solution containing 10- Af proflavine hydrochloride and 5 X 10-4M anthracene in ethanol excited by 436 m/ , 3 X 10-4 einstein liter-1 sec.-1 absorbed. Curve 1 at a sensitivity 3000 times less than curve 2. Temperature was — 66°C. 3°C.

Figure 3.95. The normalized initial time dependence of the delayed fluorescence from anthracene (3 x 1CT5M) in MCH/MCP at 115 K ( ). The theoretical curve (smooth line) was calculated with p = 2 x 1010,y = 0.0125, D/R T = 6.4 x 103 s-1. The weighted residuals are shown below. (From Ref. 262.)...

It has been known for some time that bimolecular collisions between triplet states in solution leads to quenching of the triplet state by a diffusion controlled process.71 Recently Parker and Hatchard have shown that delayed fluorescence from solutions of compounds such as pyrene, naphthalene, and anthracene, is due to triplet-triplet annihilation, i.e.,... 

Delayed fluorescence from a very-short-lived upper excited singlet state populated by hetero-TTA has been observed for the first time using the system A = anthracene and X = xanthone (Nickel and Roden, 1982). An energy-level diagram for this system is shown in Figure 5.32, and the corrected spectrum of the delayed fluorescence of anthracene and xanthone in trichlorotrifluoroe-thane is depicted in Figure 5.33. The band at 36,000-40,000 cm has been assigned to the delayed fluorescence of anthracene produced by Tf +... 

Parker CA, Hatchard C (1962) Delayed fluorescence from solutions of anthracene and phenanthrene. Proc Roy Soc Lond Ser A 269 574-584... 

The first observations of P-type delayed fluorescence arose from the photoluminescence of organic vapors.<15) It was reported that phenanthrene, anthracene, perylene, and pyrene vapors all exhibited two-component emission spectra. One of these was found to have a short lifetime characteristic of prompt fluorescence while the other was much longer lived. For phenanthrene it was observed that the ratio of the intensity of the longer lived emission to that of the total emission increased with increasing phenanthrene vapor... 

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). 

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... 

It has already been remarked that it is difficult to obtain reproducible values for the delayed fluorescence efficiency of phenanthrene from one solution to another. In ethanol, moderately high (though not reproducible) efficiencies have been observed but in highly purified n-hexane the efficiency was extremely low although anthracene gave similar results in both solvents. 

This phenomenon is possible in molecules like naphthalene, anthracene and Pyrene, each of which happens to have the lowest singlet energy level about twice hat of the respective triplet. For the generation of these triplets in high concentrations (T — T) type energy transfer from a suitable donor is necessary. Delayed fluorescence in naphthalene has been sensitized by phenanthrene. according to the following scheme ... 

No difference in the absorption spectrum is observed. The lifetimes of the two fluorescence emissions are both of the order of 10-8 sec. Another effect has been observed with hydrocarbons both as vapor and in solution, namely, a "delayed fluorescence emission with a life of a few milliseconds (56,63). This was at first interpreted as process 9, and the term excimer applied to the hypothetical excited dimer. However, C. A. Parker and C. G. Hatchard have shown that the intensity of the delayed fluorescence, which has the same spectrum as that from the excited singlet molecule, depends upon the square of the intensity of the exciting light (49). The mean life of the delayed fluorescence of anthracene solutions is (about) one-half that of the triplet state, and the effect is not observed in rigid media. These facts show that the delayed emission must be caused by an interaction between two triplet-state molecules ... 

This is due to the uniform initial condition k(t), which starts from its maximal value ki) Wrrd3r and drops with time, approaching the stationary limit from above. Contrary to this famous result, the experimental study of delayed fluorescence of anthracene in viscous solution [262] showed quite the opposite time behavior of k t). As it is initially much less than ko, the rate constant increases with time, approaching the long-time asymptote (3.56) from below. The authors of Ref. 262 called this anomaly the anti-Smoluchowski time behavior of the delayed fluorescence. They properly attributed it to a nonuniform distribution of triplets generated by the intersystem conversion from singlets that are preliminary quenched by the resonant energy transfer. 

In 1967, Johnson et al. found that the delayed fluorescence of anthracene crystal could be modulated by external magnetic fields below 2 T [11]. The magnitude of the MFEs was dependent on both the field strength and the orientation of its single crystal. These MFEs were successfully interpreted in terms of triplet exciton spin Hamiltonian [12]. The delayed fluorescence is induced from the lowest excited singlet state ( Si) by the annihilation of two lowest triplet excited states ( T ). 

Fig. 13-1. The influence of a magnetic field on the delayed fluorescence intensity from an anthracene crystal with 15 ms triplet lifetime at room temperature. The magnetic field was applied in the ac plane of the crystal in the direction at -17° with respect to the a crystal axis as shown in the insert. The dashed lines in the insert indicate the field direction which produce the greatest diminution in delayed fluorescence intensity. (Reproduced from Ref. [1] by permission from The American Physical Society)...

In fluid solutions, Faulkner and Bard first found the MFEs on the electrogenerated chemiluminescence of anthracence triplet-triplet annihilation [4], The general features of the MFEs were similar to those observed in crystals the intensity of delayed fluorescence was found to decrease with increasing B from 0 T to 0.8 T and to approach as3miptotically a value which was about 4 % below the zero-field intensity. They also found the MFEs on the anthracene fluorescence in the presence of doublet species such as Wurster s Blue perchlorate [5] the fluorescence intensity was found to be quenched by the doublet species, but the quenched intensity was found to be increased by magnetic fields. Their typical results are shown in Fig. 12-2. Although these MFEs observed in fluid solutions were smaller than those in crystals due to rotational motion of triplet molecules, the MFEs in solutions could also be explained by Eqs. (13-3) and (13-9). The magnetically induced decrease in the... 

Figure 5.32. Energy-level diagram of anthracene (A) and xanthone (X). Double lines denote the three different triplet pairs for which TTA processes are indicated asterisks mark the delayed fluorescence (DF) resulting from hetero-TTA (by permission from Nickel and Roden, 1982).

An observation of the triplet state of anthracene in benzene (27) is of special interest in that, in addition to the absorption spectrum of the triplet, a delayed fluorescence emission from the singlet at 4300 A. was recorded. The intensity of fluorescence, which was approximately proportional to the square of the triplet concentration at all times, was attributed to triplet-triplet quenching ... 

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