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Exciplexes deactivation

A or Q, acts as the electron donor. The radiative and non-radiative properties of exciplexes are very sensitive to solvent polarity. In sufficiently polar solvents, dissociation to free ions is the major pathway of exciplex deactivation. [Pg.302]

The corresponding reaction Scheme, Scheme (16) proposed for benzene solution is similar, the only additional reaction being the deactivation of the exciplex (CM ) by the banzene molecule (B). [Pg.248]

The acetone-sensitized photodehydrochlorination of 1,4-dichlorobutane is not suppressed by triplet quenchers (20), but the fluorescence of the sensitizer is quenched by the alkyl chloride (13). These observations imply the operation of a mechanism involving collisional deactivation, by the substrate, of the acetone excited singlet state (13,21). This type of mechanism has received strong support from another study in which the fluorescence of acetone and 2-butanone was found to be quenched by several alkyl and benzyl chlorides (24). The detailed mechanism for alkanone sensitization proposed on the basis of the latter work invokes a charge-transfer (singlet ketone)-substrate exciplex (24) and is similar to one of the mechanisms that has been suggested (15) for sensitization by ketone triplets (cf. Equations 4 and 5). [Pg.200]

Explain the deactivation of excited states by other molecules in terms of quenching processes, excimer/exciplex formation, energy transfer and electron transfer. [Pg.87]

In polar solvents, the quantum yields for the emission from the locally excited state of anthronyl-anthracenes 98 and 99 decrease drastically (see Tables 20 and 21), and a structureless, red-shifted exciplex emission is observed (see Figure 23). For the parent compound 98a in dichloromethane, for example, the quantum yield of emission from the exciplex state is 0.012, but that of emission from the locally excited state has decreased to 0.00058 (cf. Tables 20 and 22). Thus, intramolecular exciplex formation between the photoexcited anthracene moiety and the aromatic ketone in its electronic ground state represents the major mode of deactivation in polar solvents. [Pg.195]

This represents a second mode of deactivation of exciplexes, equivalent to an electron transfer reaction. Dipolar exciplexes are in fact often intermediates in such reactions. [Pg.109]

The present article reviews the photochemical deactivation modes and properties of electronically excited metallotetrapyrroles. Of the wide variety of complexes possessing a tetrapyrrole ligand and their highly structured systems, the subject of this survey is mainly synthetic complexes of porphyrins, chlorins, corrins, phthalocyanines, and naphthalocyanines. All known types of photochemical reactions of excited metallotetrapyrroles are classified. As criteria for the classification, both the nature of the primary photochemical step and the net overall chemical change, are taken. Each of the classes is exemplified by several recent results, and discussed. The data on exciplex and excimer formation processes involving excited metallotetrapyrroles are included. Various branches of practical utilization of the photochemical and photophysical properties of tetrapyrrole complexes are shown. Motives for further development and perspectives in photochemistry of metallotetrapyrroles are evaluated. [Pg.135]

Exciplexes and excimers have their own structure and properties (e.g. multiplicity, absorption and emission spectra, lifetime, stability constant, enthalpy and entropy content, pathways of deactivation) and can be regarded thus as new chemical species. [Pg.141]

The problem of the decay of exciplexes arises directly from their stability. There is no simple and unambigous relation between the electron donor ability of an excited complex A, electron withdrawing property of its reaction partner Q, and the decay rate constant kq. This experimental observation may be understood on the basis of two deactivation pathways of exciplexes (A — Q) a spin-forbidden conversion to the singlet ground state (GS) molecules A and Q, e.g. for a triplet exciplex 3(A — Q)... [Pg.146]

Leismann et al.[182] have recognized this problem in their publication of 1984, in which they describe a thorough and detailed investigation of the kinetics of formation and deactivation of exciplexes of. S) benzene or toluene and 1,3-diox-ole, 2,2-dimethyl-l,3-dioxole, and 2,2,4-trimethyl-l,3-dioxole. The evolution in time of monomer and exciplex fluorescence after excitation using a nanosecond flash lamp was analyzed, and again it was concluded that the formation of exciplexes is diffusion controlled their decay proceeds mainly (>90%) via radiationless routes. The polar solvent acetonitrile enhances radiationless deactivation, possibly by promoting radical ion formation. Because decay of benzene fluorescence is essentially monoexponential, dissociation of the exciplex into Si benzene and dioxole is negligible. [Pg.87]

Scheme 35 Formation of the arene-olefin (Ar-Ol) exciplex and its deactivation by charge-transfer effects induced by changes of olefin ionization potential (IP) and solvent polarity according to Leismann et al. (From Ref. 182.)... Scheme 35 Formation of the arene-olefin (Ar-Ol) exciplex and its deactivation by charge-transfer effects induced by changes of olefin ionization potential (IP) and solvent polarity according to Leismann et al. (From Ref. 182.)...
In recent years, these donor-acceptor correlations to the reaction mode [49], substituent effects to stabilize meta intermediates [50-52], exo/endo selectivity [53-55], kinetics by means of fluorescence study [56] and deuterium isotope effect [57,58], formation and deactivation studies of exciplex [59,60], influence of pressure [61], and theoretical calculations [62-64] have been extensively studied. [Pg.133]

In addition to unimolecular reactions, the excited state may participate in several bimolecular processes. At high concentrations, dimer formation, excimer formation, exciplex formation, solute-solvent complexation, energy transfer, and collosional deactivation may occur. The high-concentration conditions are often experienced when the guest molecules are loaded onto the layered materials with high coverages and specific examples will be provided shortly. [Pg.519]

Whether the role of the second olefinic partner is catalytic or a triplex intermediate is involved is not yet clear. The role of triplexes in photoreactions of aromatic compounds with olefins may be even more complex. For example, excited biphenyl forms both a fluorescent dual and triple exciplex with 2,3-dimethyl-2-butene [120]. The possibility of these two pathways of deactivation can explain the inefficient photocycloaddition in the particular case of 1biphenyl /cyclopentene [121]. [Pg.250]

Fig. 4. Electron transfer quenching via exciplex formation, a exciplex emission b exciplex radiationless deactivation c geminate ion recombination. For sake of simplicity, processes such as exciplex dissociation into A + B, exciplex reactions other than the electron transfer one and direct formation of a solvated ion pair from the encounter complex have been omitted... Fig. 4. Electron transfer quenching via exciplex formation, a exciplex emission b exciplex radiationless deactivation c geminate ion recombination. For sake of simplicity, processes such as exciplex dissociation into A + B, exciplex reactions other than the electron transfer one and direct formation of a solvated ion pair from the encounter complex have been omitted...
Scheme 2 is the simplest but not the only reaction scheme conceivable. The deactivation of, say, S may proceed via an excimer (SS) or a diastereomeric exciplex (SR). The same deactivation pattern holds, of course, for R, so that the deactivation in toto is nonasymmetric. If there were exciplex emission, circularly polarized fluorescence might be observed. Inoue discusses also a hot ground state path of deactivation [37]. [Pg.10]

Another example of intramolecular CT complex formation is provided by trans-4-dimethvlamino-4 -(1-oxobutvl)stilbene Solvent effects on the spectrum give a value of 22D for the excited state dipole moment. The effect of electric field on the fluorescence of 4-(9-anthry1)-N.N.-2.3,5,G-hexamethy1-aniline shows this compound forms an excited state whose dipole moment does not change with solvent . Chiral discrimination in exciplex formation between 1-dipyrenylamine and chiral amines is very weak . In the probe molecule PRODAN (6-propionyl)-2-(dimethylamino)—naphthalene the initially formed excited state converts to a lower CT state as directly evidenced by time-resolved spectra in n-butanol. Rate constants for intramolecular electron transfer have been measured in both singlet and triplet states of covalently porphyrin-amide-quinone molecules . Intramolecular excimer formation occurs during the lifetime of the excited state of bis-(naphthalene)hydrazides which are used as photochemical deactivators of metals in polyethylene . ... [Pg.17]

According to the following scheme, electron transfer, intersystem crossing and energy transfer can all compete with fluorescence in deactivating an exciplex (DA) formed from the molecules A and D. [Pg.283]

Heavy-atom quenching occurs if the presence of a heavy-atom-containing species enhances the intersystem crossing (MQ) HMQ) to such an extent that it becomes the most important deactivation process for the exciplex. Since the triplet exciplex is normally very weakly bound and dissociates into its components, what one actually observes in such systems is... [Pg.286]

If they are different. These complexes only exist in the excited state and they dissociate into monomers upon deactivation. While the existence of singlet excimers and exciplexes has been proved by many investigators, only a few reports deal with triplet complexes. [Pg.362]

The effect of solvent on the dipolar contribution to exciplex stability has been shown by measurements on a range of unsaturated compounds on the deactivation of the state of several naphthalene derivatives. Quenching of excimers of pyrene and... [Pg.11]


See other pages where Exciplexes deactivation is mentioned: [Pg.401]    [Pg.265]    [Pg.267]    [Pg.197]    [Pg.289]    [Pg.478]    [Pg.203]    [Pg.210]    [Pg.106]    [Pg.42]    [Pg.188]    [Pg.233]    [Pg.9]    [Pg.49]    [Pg.216]    [Pg.14]    [Pg.239]    [Pg.127]    [Pg.203]    [Pg.22]    [Pg.976]    [Pg.2267]    [Pg.2278]    [Pg.2280]    [Pg.16]    [Pg.278]    [Pg.188]    [Pg.16]   
See also in sourсe #XX -- [ Pg.171 ]




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