The Energy-Transfer Mechanism

The direct photooxygenation of rubrene was also studied by two other groups of investigators.35-37 Schumacher36 confirmed the participation of two molecules of rubrene in this reaction. However, he postulated the formation of an unstable, electronically excited rubrene- 

Effective fluorescence quenching of polycyclic hydrocarbons by oxygen was observed by Terenin,45,46 who proposed a quenching mechanism according to which the overall result was the production of an electronically excited hydrocarbon-oxygen complex. 

A (singlet) + 02 (ground state triplet) - A (triplet) + 02 (singlet) 

The main feature of such a paramagnetic quenching is that not only an active 02 molecule is formed during this process, but that the second partner, i.e., the aromatic molecule, has also acquired chemical reactivity, having been transformed into the biradical A (triplet). We expect, therefore, that paramagnetic quenching will be accompanied by association or dimerization processes, induced 

Experimental results of an extensive study on the photoconductivity of solid dye films seemed to prove the existence of such an unstable addition product (A02) (triplet).45 

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For electric multipolar interactions, the energy transfer mechanism can be classified into several types, according to the character of the involved transitions of the donor (D) and acceptor (A) centers. Electric dipole-dipole (d-d) interactions occur when the transitions in D and A are both of electric dipole character. These processes correspond, in general, to the longest range order and the transfer probability varies with l/R, where R is the separation between D and A. Other electric multipolar interactions are only relevant at shorter distances dipole-quadrupole (d-q) interaction varies as l/R, while quadrupole-quadrupole interaction varies as l/R °. 

The concepts introduced in the previous section can be made more concrete by consideration of the termolecular formation of He according to the energy transfer mechanism,... 

Until recently, no direct proof for or against one of the two remaining mechanisms, the energy-transfer mechanism and the complex formation mechanism, was available (see Chart I, under II. Oxygen-Activation Mechanism). 

However, the ft values were probably incorrectly taken from Bowen s work.150 If the ft values obtained by this author are used, Pa = 0.42 and j3s = 0.057 (see Table VII), a better agreement between the calculated slope (=0.14) and the observed slope is obtained. The range of experimental error in determining the individual ft values as well as in obtaining a reliable slope from the competitive photooxygenation reaction appears to be fairly large. The difference between the calculated and the experimental slope, therefore, does not allow the abandonment of the energy-transfer mechanism. 

All the olefins involved in these studies are simple olefins to which energy transfer from ketones should be endothermic. Although the details of the mechanism of isomerization are still a matter of some debate,5,71 it is generally agreed that isomerization takes place by addition of the sensitizer to the olefin to form a new intermediate which may be of a biradical nature. Thus if energy transfer is not favored, there is another mechanism by which ketone sensitizers can induce olefin isomerization, and the observed quantum yields and photostationary states may differ sharply from those predicted by the energy transfer mechanism. 

Dye sensitization plays an important role in photography. The sensitization mechanism for ZnO-materials as used in electro-photography is obviously in complete correspondence with these electrochemical experiments as shown for single crystals under high vacuum conditions by Heiland 56> and for imbedded ZnO-particles by Hauffe 57). Even for silver halides where electron injection as sensitization mechanism has been questioned by the energy transfer mechanism 58> electrochemical experiments have shown that the electron injection mechanism is at least energetically possible in contact with electrolytes 59>. Silver halides behave as mixed conductors with predominance of ionic conductivity at room temperature. These results will therefore not be discussed here in any detail since such electrodes are quite inconvenient for the study of excited dye molecules. 

Two main models are usually discussed for the mechanism of the spectral sensitization. The excitation of the sensitizer by absorbed light and electron transfer from the excited sensitizer to the semiconductor is the first model. The alternative mechanism consists of the transfer of the excitation energy from the sensitizer to the semiconductor. This energy is used for photogeneration of the charge carriers in the sensitized photoconductor. In the first case the excited singlet level of the sensitizers has to be located above the conduction band of the semiconductor for realization of the electron transfer. For hole transfer the basic sensitizer level has to be located lower than the valence band of the sensitized photoconductor. The energy transfer mechanism does not need a special mutual location of the semiconductor and sensitizer levels. 

The analysis of the results obtained on sensitization of the organometallic compounds shows that their principal features coincide with the sensitization of the inorganic semiconductors. Apparently, the energy transfer mechanism from... 

The production of 02CA9) on photolysis at A = 2537 A of oxygen-ozone mixtures, observed by Izod and Wayne,70 is probably explained by the energy-transfer mechanism... 

Schlicke et al. [27] synthesized rod-like compounds, in which two metal-bipyridyl complexes were linked by >/igo(plienylene)s. The longest spacer (seven phenylene units) gave rise to a metal-to-metal distance of 4.2 nm. In this work, Dexter-type energy-transfer mechanism from [Ru(bpy)3]2+ to [Os(bpy)3]2+ was established. The energy transfer mechanism is essentially temperature-dependent and decreases exponentially with an attenuation coefficient of 0.32 A-1. 

Some of the initial work dealt with the formation of proton-bound dimers in simple amines. Those systems were chosen because the only reaction that occurs is clustering. A simple energy transfer mechanism was proposed by Moet-Ner and Field (1975), and RRKM calculations performed by Olmstead et al. (1977) and Jasinski et al. (1979) seemed to fit the data well. Later, phase space theory was applied (Bass et al. 1979). In applying phase space theory, it is usually assumed that the energy transfer mechanism of reaction (2 ) is valid and that the collisional rate coefficients kx and fc can be calculated from Langevin or ADO theory and equilibrium constants. 

Orange increases. This type of experiment is one of several ways to show the presence of the energy-transfer mechanism. 

Also, one can put into evidence the energy-transfer mechanism by recording the fluorescence excitation spectrum of the complex (donor-acceptor) (kem is set at a wavelength where only the acceptor emits) and comparing it to the absorption spectrum of the donor alone. In the presence of energy transfer between the two molecules, a peak characteristic of the donor absorption will be displayed in the fluorescence excitation spectrum. 

Membranes fusion can be studied using the energy-transfer mechanism. In fact, membrane vesicles labeled with both NBD and rhodamine probes are fused with unlabeled vesicles. In the labeled vesicles, upon excitation of NBD at 470 nm, emission from rhodamine is observed at 585 nm as a result of energy transfer from NBD to rhodamine. The average distance separating the donor from the acceptor molecules increases with fusion of the vesicules, thereby decreasing the energy-transfer efficiency (Struck et al. 1981). 

Let us consider kt as the rate constant of the excited state depopulation via the energy-transfer mechanism the measured fluorescence lifetime is thus equal to... 

The energy-transfer mechanism appears to affect the fluorescence lifetime, intensity, and quantum yield ... 

One of the concepts used to produce white light emission in OLEDs is based on the energy transfer mechanism from host molecule to guest molecule. The energy transfer in... 

Another interesting result obtained in experiments on these ratios of rate constants is the lack of the dependence of the ratio on the nature of the gas collisionally deactivating the vibrationally excited ozone isotopomer [46]. Different mechanisms have been postulated for the collisional deactivation, including the energy transfer mechanism used here and most commonly used elsewhere, and a chaperon mechanism in which the third-body collision partner forms collision complex with the Q or with the Q2 prior to the recombination step. Recently the chaperon mechanism was revisited for ozone formation, analyzing pressure and temperature dependent data on the recombination rate [47]. Since the ratios of rate... 

Fg/i FIi3/2 and " F9/2 FIi5/2 transitions of Dy +, respectively. The antenna effect was adduced to discuss the energy transfer mechanism between the ligand and the central Dy + ion. ... 

C2F4 plasmas are the well-investigated fluorinated carbon plasma systems by OES diagnostics according to literature with conventional plasma sources. However, without influence of ionization, these cases turned out to be the case 3 described earlier, i.e., the formation of chemically reactive species that do not emit photons by the energy transfer mechanism. 

Also in the case of Cu.lO", where a [2]catenand-type moiety is present, a two-step protonation reaction occurs, but in this case the luminescence of such moiety is quenched by the Cu-based one [66]. Unfortunately, it is not possible to determine the quenching rate and mechanism for a variety of reasons, including (i) the instability of the protonated forms under laser irradiation in CH2CI2 [56] (ii) the lack of electrochemical potentials of the protonated forms and (iii) the weakness of the MLCT emission band of the Cu-based moiety, which is partially masked by the tail of the emission of the protonated subunit, albeit quenched. However, the energy transfer mechanism is thermodynamically allowed and probably active. 

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