Delayed fluorescer addition

That an intermediate with a lifetime of some minutes (in the absence of a fluorescer) plays a key role in oxalate chemiluminescence was demonstrated by delayed fluorescer addition. Even when the fluorescer was added 70 minutes after the preparation of oxalate/hydroperoxide mixture, 53% of the quantum yield obtained by initial addition of the fluorescer was observed. On the other hand, the fluorescer evidently acts as catalyst in the chemiluminescent decomposition of that intermediate [2]. This intermediate could be transported by an inert gas stream from the oxalate/hydrogen peroxide mixture into a fluorescer solution, producing the fluorescence of the latter. In this case no consumption of the fluorescer took place. A charge-transfer complex between the fluorescer and the intermediate was proposed as early as 1967. The intermediate, as was mentioned above was assumed to be dioxetanedione (4) but it was not possible to identify it [2]. 

The delayed fluorescence efficiency of the donor will thus decrease rapidly on the addition of small concentrations of acceptor. 

It should not be supposed that crystal defects enter into the picture only as nuisances which the chemist seeks to avoid or eliminate. Actually, certain optical and electrical properties of oxides, sulfides, and halides have been found to depend strongly on the nature and extent of crystal defects. Indeed, semiconductivity, fluorescence (absorption of radiation and emission of less energetic radiation), and phosphorescence (delayed fluorescence) of some salts may be spectacularly increased, not only by a small stoichiometric excess of one of the constituents, but also by addition of very tiny quantities of a foreign ion. Perhaps the best known example is the case of zinc sulfide which, when precipitated from aqueous solution and dried at low temperatures, shows negligible fluorescence upon exposure to ultraviolet light. When the sulfide is heated to... 

Pyrylium and thiopyrylium salts show interesting emission properties when incorporated in a rigid polymeric matrix (85M12). In addition to a strong rapid fluorescence emission, a delayed fluorescence is observed that cannot be detected in solution, even at — 1%°C. 

This event occurs due to a radiational relaxation to the ground singlet (8i) state and in the 0.1 ms to 10 s time frame. Therefore, the emission is at even longer wavelengths than in fluorescence. Energy addition to the molecule in the form of heat or collisions of two triplet-state molecules can cause delayed fluorescence. 

In the PBA matrix, exponential decays were not observed at any temperature in excess of 140K. In addition, delayed fluorescence was observable from the naphthyl chromophores at higher photon fluxes. Considering the low concentrations of chromophore and the fact that for the labelled systems, diffusive motion of the naphthyl species is prohibited, these data are supportive of the MacCallum mechanism. 

Neat Polymer Films. It is interesting to compare the triplet photophysical properties of poly(N-vinylcarbazole) (PVCA)(16) on the one hand and poly(l-vinylnaphthalene) (PIVN)(17) on the other when each is examined as a pure polymer film. Both polymers exhibit a prominent excimer phosphorescence band as well as a distinct delayed fluorescence emission. In addition, the delayed fluorescence arises by a process of triplet-triplet annihilation for both polymers. Furthermore, the luminescence decay kinetics suggest that equilibria of the type... 

In order to demonstrate this effect to best advantage it was necessary to choose a PVCA sample having a relatively low molecular weight. In this way Interference of the phosphorescence emission by delayed fluorescence is minimized. These are provacative results because they indicate that there may be no well defined lowest triplet state in vinyl aromatic polymers unless special steric or electronic effects are present which nullify inter-chromophore interactions. On the other hand, they may provide an additional tool with which to investigate rates of energy migration in polymers and in some polymer/dopant systems as well. 

In the previous section, delayed fluorescence has been disregarded although it constitutes an additional deactivation channel for triplets in P1VN (1,56), P2VN (6,57,73), PVCA (4,5,7,13,59,65), P2NMA (74,75) but not in PACN ( ). 

Delayed fluorescence (DF) can be observed in several aromatic polymers in addition to phosphorescence after excitation via the singlet system or using triplet sensitizers. 

The fluorescence emission discussed so far is produced by direct excitation of a molecule M to one of its excited singlet states that, after IC to Sx when an upper singlet state was initially populated, emits prompt fluorescence from Si with a lifetime on the order of nanoseconds. In addition, several processes can be envisaged that permit repopulation of Sx following ISC to Tx, which can give rise to emission that has the spectral characteristics of fluorescence, but a lifetime much longer than that of prompt fluorescence, that is, to delayed fluorescence. These processes can be sub-classified as P-type and 4E-type delayed fluorescence,32 We do not consider artificial delayed fluorescence due to impurities, or due to ionization followed by recombination with the formation of 1M. ... 

With a glassy solution of poly-1-vinylnaphthalene, the delayed emission spectrum has been shown to consist of an emission having a mean lifetime of approximately 80 ms at the normal fluorescence wavelength, in addition to the phosphorescence having a mean lifetime of about 2 s [159]. The delayed fluorescence did not appear in the spectrum of 1-ethylnaphthalene. With the polymer it was found to be inhibited by piperylene, a well-known triplet quencher. These results have been explained by mutual annihilation of two excited triplet states produced by the absorption of two photons by the same polymer molecule. They are considered as strong evidence for migration of the excited triplet state in poly-1-vinylnaphthalene. In polyacenaphthalene, however, which is chemically very similar to poly-1-vinylnaphthalene (see p. 409), no delayed fluorescence could be detected in the same experimental conditions [155]. 

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 ... 

Triplet energy migration can be probed by addition of free quenchers to the polymer, and observing which kinetics, Stern-Volmer or Perrin (see Section II above), pertain. In the case of the quenching of naphthalene phosphorescence or delayed fluorescence in a variety of co-polymers by the dloleflns plperylene and cycloocta-1,3-diene at low temperature where material diffusion... 

One such method is sensitised fluorescence (see additional remarks on this subject in Chap. 6). Here, the detection limit for fluorescing compounds lies at less than 10" molecules/host molecule or ca. 10 impurity molecules per cm. Fig. 3.6 shows as an example of such a measurement the detection of -methyl-naphthalene in naphthalene. Still more sensitive is the method of sensitised delayed fluorescence (Sect. 6.9). Here, the detection limit is at ca. 10 molecules/host molecule or 10 impurity molecules per cm [5,6]. 

The generation of delayed fluorescence by triplet-triplet annihilation in conjugated polymers has been known for over 15 years,and can occur either by on-chain ° or inter-chain mechanisms. It has proved a valuable route to studying triplet state dynamics in conjugated polymers, and has recently been used to study the effect of these on the performance of fluorescent organic LEDs. In addition, the temperature dependence of phosphorescence and delayed fluorescence in a series of... 

In competition with radiationless deactivation, energy can also be lost in the form of radiation (F, P) [3], [28], Fluorescence occurs with molecules that either (1) have extended n-systems (such as polycondensed aromatics) (2) do not permit deactivation by torsional or rotational motion of parts of the molecule or (3) have no heavy atoms as substituents [32], 33]. In addition to these molecular properties, the environment also plays a part. Thus, the fluorescence intensity increases at low temperature and in solid matrices. This is even more important in phosphorescence, where the T] -> So transition is in fact spin-forbidden. If a higher vibrational level (v >0) is occupied in Ti, in accordance with the Boltzmann equation (Eq. 5),. so-called delayed fluorescence [28] can occur by backward intersystem crossing [T, S,(b = 0) S ]. 

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