Anthracene exciplex formation

F. Pages, J.-P. Desvergne, and H. Bouas-Laurent, Nonlinear triple exciplexes Thermodynamic and kinetic aspects of the intramolecular exciplex formation between anthracene and the two anchored nitrogens of an anthraceno-cryptand, /. Am, Chem. Soc. Ill, 96-102(1989). 

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. 

On photoexcitation, the anthracene moiety is excited into a singlet state. Exciplex formation then proceeds probably via a folded conformation in competition with complete electron transfer in an extended conformation ... 

Such polycyclic aromatic hydrocarbons as anthracene or heteroaromatics as acridine, phenazine and 2,4,5-triphenyl oxazole act as Jt-donors for the Jt-acceptors AN and alkyl methacrylates [50-53]. Again, the interaction of the donor excited states with vinyl monomers leads to exciplex formation. But, the rate constants (k ) of these quenching processess are low compared to other quenching reactions (see Table 1). The assumed electron transfer character is supported by the influence of the donor reduction potential on the k value (see Table 1), and the detection of the monomer cation radicals with the anthracene-MMA system. Then, the ion radicals initiate the polymerization, the detailed mechanism of which is unsolved,... 

One of the problems facing spin chemists performing these measurements is that the observed field effects can be rather small. Thus the method of detection should be as sensitive as possible. In some systems, it is possible to use the inherent fluorescence of one of the species involved in the reaction as a probe of RP activity. The most common of these approaches is the situation with the formation of RlPs that can often lead to spin-selective exciplex formation via the singlet RIP. Systems involving conjugated aromatic molecules, for example, anthracene and pyrene as electron donors/acceptors, amines as electron donors, and substituted benzenes (e.g., dicyanobenzenes) as electron acceptors, have been commonly employed and are now extremely well... 

Weller and Zachariasse thoroughly investigated exciplex formation and luminescence for donor acceptor systems in THF [18]. A particularly interesting result from their work came from an examination of the temperature dependence of radiative charge recombination between 9,10-dimethylanthracene anion (DMA") and TPTA+ in THF [19]. They found that both exciplex emission and fluorescence from DMA were observed in solution at low temperature (ca. —50°C). As the solution temperature is raised, the excimer emission decreases in relative intensity, and at room temperature the emission is nearly completely DMA fluorescence. The monomer-to-exciplex emission intensity ratio as a function of temperature follows Arrhenius kinetic behavior and yields an activation barrier that is nearly the same as the energy gap between the exciplex and the DMA states. Thus, their model consisted of reaction of the solvent-separated ions to form an intimate emissive ion pair which could dissociate to yield the singlet anthracene derivative. 

A paper by Suppan draws attention to electrostatic interaction effects on condensed phase photoinduced electron transfer and the need to take account of the fact that solvent is not in reality a uniform dielectric material. Pressure effects on exciplex formation has been exemplified in the pyrene-p-cyanobenzene system. Ternary electron donor acceptor complexes are formed and in the case of anthracene-tetracyanoethylene gives rise to (DO ) dimer radical cations. Laser flash photolysis shows that perylene in acetonitrile undergoes three distinct electron transfer processes, (i) gives pt + MeCNT, (ii) gives... 

Electron-rich polyaromatic compounds such as anthracene, pyrene, and pery-lene [107] are suitable as photosensitizers as they give redox reactions with DPI salts through exciplex to finally yield the initiating species for photoinduced cationic polymerizations. Scheme 11.28 demonstrates the mechanism of a polymerization followed via exciplex formation through the excited sensitizer with the ground-state onium salt. 

Fig. 15 Examples of macrocyclic receptors showing exciplex formation, (a) Valine derived macrocyclic sensor (b) complex sensors bearing ferrocene and anthracene units (c) fluorescence enhancement at 414 nm of the sensin 51a upon addition of phenylalaninol, showing a high enantioselective response (reprinted from [66]. Copyright (2009) with permission from Elsevier)...

Three types of experiments have been applied to the study of diffusion controlled cyclization of polymers. These are excimer formation in polymers containing pyrene groups at both chain ends [Py-polymer-Py ], exciplex formation in polymers containing a pyrene at one end and a dimethylaminophenyl group at the other [DMA polymer-Py i ], and triplet-triplet [TT] annihilation between anthracene groups at both ends of a polymer [A -polymer-A 3]. A fourth obvious approach, as yet to be reported, is intramolecular phosphorescence quenching in a polymer... 

Exciplexes are complexes of the excited fluorophore molecule (which can be electron donor or acceptor) with the solvent molecule. Like many bimolecular processes, the formation of excimers and exciplexes are diffusion controlled processes. The fluorescence of these complexes is detected at relatively high concentrations of excited species, so a sufficient number of contacts should occur during the excited state lifetime and, hence, the characteristics of the dual emission depend strongly on the temperature and viscosity of solvents. A well-known example of exciplex is an excited state complex of anthracene and /V,/V-diethylaniline resulting from the transfer of an electron from an amine molecule to an excited anthracene. Molecules of anthracene in toluene fluoresce at 400 nm with contour having vibronic structure. An addition to the same solution of diethylaniline reveals quenching of anthracene accompanied by appearance of a broad, structureless fluorescence band of the exciplex near 500 nm (Fig. 2 )... 

A well-known example of an exciplex is the excited-state complex of anthracene and N,N-diethylaniline resulting from the transfer of an electron from an amine molecule to an excited anthracene molecule. In nonpolar solvents such as hexane, the quenching is accompanied by the appearance of a broad structureless emission band of the exciplex at higher wavelengths than anthracene (Figure 4.9). The kinetic scheme is somewhat similar to that of excimer formation. 

In anthracenocryptand 23, exciplexes are formed between the anthracene and nitrogen ion pairs.60,61,63,65,140 In MeOH, the quantum yield dramatically decreases due to the formation, via exciplex intermediates, of nonfluorescent radical ions. Upon addition of an excess of K+, Ag+, or Tl+ to methanolic solutions of 23, 1 1 cryptate-type complexes are formed.61,65 Complexation causes drastic changes in the spectroscopic properties. Cations such as Na+, for example, decrease the intensity of the exciplex emission and increase the intensity of the structured anthracene emission. Heavy-metal ions (Ag+, Tl+) interact strongly with the central ring of anthracene as shown by an exciplex-type emission observed for the Ag+ complex of 23. 

The spectra of anthracene-dimethylaniline shown in Figure 5.24 exemplify the new structureless emission at longer wavelengths due to the formation of an exciplex. This fluorescence is very similar to excimer fluorescence. 

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