Anthracene exciplex emission

In polar solvents such as chloroform, dichloromethane, acetone, and acetonitrile, the fluorescence quantum yields of 97a-d decrease by varying degrees (see Table 19). Moreover, in the case of the phenyl and acetyl derivatives 97c and 97d, the rather drastic decrease of the structured fluorescence from the locally excited anthracene is associated with the appearance of a structureless, red-shifted emission which is attributable to an intramolecular exciplex. For 97d, in which the electron acceptor properties of the aromatic carbonyl moiety are enhanced by p-acetyl substitution, exciplex emission is dominant even in toluene solution (see Figure 22). 

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. 

The fluorescence of anthracene in benzene is efficiently quenched by N,N-dimethylaniline and a strong exciplex emission appears in a longer wavelength than the emission of anthracene [382-384], However, the addition product was not obtained at all, except the (4 + 4) anthracene dimer (Scheme 114). In contrast, the addition product and reductive dimerization product of dimethylaniline to the anthracene ring are produced via photoinduced electron transfer, which was first reported by Pac and Davidson [385-387], In the case of V-mcthylaniline, some addition products are obtained both in nonpolar and polar solvents [386-389],... 

Some exciplexes, however, can be stabilized in polar solvents because of favorable orbital interactions, electrostatic effects, or a Kirkwood-Onsager solvation energy term [18]. For example, in polar solvents an exciplex structure with a large dipole moment can be stable with respect to solvent-separated and free ions (Fig. 8). The lifetimes of these exciplexes should then be sufficiently long to allow for observation. In fact, there is evidence to support exciplex emission even in polar solvents. Eisenthal and coworkers, for example, observed the exciplex emission of 9-anthracene-(CH2)3-iV,JV-dimethylaniline systems in acetonitrile [19] ... 

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. 

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. 

When the bridge itself contains a potential electron donor or acceptor, the situation can be more complex. Thus, Tsujiya et al. [15] studied the intramolecular electronic interaction between anthracene (An) and dimethylaniline (DMA) in bridged systems, when the bridge contained an ethereal oxygen. In these systems it was found that the appearance of exciplex emission depended strongly on the relative position of the ethereal oxygen with respect to the anthracene part of the molecule. For l-An-0-(CH2)2 DMA, one conformer was found to exhibit no exciplex emission at all in the jet, whereas another showed both locally excited and exciplex emission. For l-An-CH2-0 CH2-DMA (and also for 9-An-CH2-0-CH2-DMA), no exciplex emission was found in the jet, although the same molecules show exciplex emission in solution It appears that the interaction between... 

Several linked molecules in which the donor is an aniline derivative and the acceptor an anthracene moiety were investigated. In the case of anthracene linked to V,V-dimethylaniline by a trimethylene bridge, two molecules were investigated. In both the link was to the meta (3-) position of the aniline. In one anthracene was linked in the 9-position, (9-anthryl)-3-[w-(A, A -dimethylamino)phenyl] propane (9-An-m-DMA), and in the other the link was to the 1-position (1-An-w-DMA). In both cases hole-burning spectroscopy revealed two distinct species, that were assigned to two isomeric forms (possible structures are shown in Scheme 1). The emission spectra of both forms at the origin bands were essentially of the locally excited type. As the internal energy was increased, the exciplex emission spectrum... 

Jet cooling may reveal weakly fluorescing systems, since some non-radiative decay processes become less efficient in that environment, particularly bimolecular interactions. Thus, exciplexes formed between aromatic molecules acceptors and tertiary amine donors have long been known to fluoresce, whereas under ambient conditions exciplex emission from pairs in which the tertiary amine has been replaced by a secondary of primary amine is rare. Anner and Haas observed exciplex emission from an anthracene-ammonia adduct in a jet [40] and Drescher et al. [41] reported weak exciplex emission from a /i-methylstyrene-diethylamine adduct... 

Figure 5.24. Fluorescence and exciplex emission from anthracene in toluene (3.4-10 mol/L) for various concentrations Cq of dimethylaniline (by permission from Weller, 1968).

Interest continues in the intramolecular photodimerization reactions of polynuclear aromatic moieties joined by an alkane chain. The absorption and exciplex emission spectra of the naphthalene-anthracene sandwich pair have been previously studied this species was generated by the light-induced (254 nm) cleavage of the intramolecular photo dimer (139) of l-(9-anthryl)-3-(l-naphthyl)-propane at low temperatures in a rigid matrix.173 The generation and properties of the sandwich pair have been examined by other workers, using single crystals of the intramolecular cycloadduct (139).174 The exciplex fluorescence is broad and... 

The fluorescence of A33 in methanol is dual but the quantum yield is low ( p 0.04) compared with that of 9,10-dipropyl anthracene, the reference compound ( p = 0.76). By complexation with an excess of metal catirms such as K ( p = 0.30) or protonation with an excess of CF COOH ( = 0.76), a large fluorescence intensity increase is observed with the disappearance of the exciplex emission (Scheme 2) it is consistent with die fact that the nitrogen lone pairs interact no more with the aromatic ring but with the metal cations or the protons (8a). It is thus possible to use this fluorescence enhancement to evaluate cations as shown in Fig. 7 for K in CH3OH. Other systems closely related to these cryptands designed and prepared by Czamik (20a) and De Silva (2(m) are described in this book. 

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. 

Parker and Joyce125 have also observed a new band in the delayed emission spectrum of solutions of anthracene (A) and 9,10-diphenylanthracene (B) in ethanol at — 75°C, which they attribute to the exciplex of these species formed in the process of mixed triplet-triplet annihilation... 

Photoinduced electron transfer between amines and aromatic hydrocarbons occurs to generate radical cations of amines and radical anions of aromatic hydrocarbons. Pac and Sakurai reported the photoaddition of N,N-dimethylaniline to anthracene via photoinduced electron transfer [60]. In benzene, the 4n + 4n) photocyclodimer of anthracene is produced as a sole isolable product, although an emission due to the exciplex formed from anthracene and JV,N-dimethylaniline is observed. In acetonitrile, the addition of dimethylaniline to anthracene occurs via their radical ions to give 9,10- dihydro-9-(4 -dimethylaminophenyl)anthracene as the major product. However, the photoamination on anthracene takes place even in benzene when iV-methylani-line is used as an electron donor. Sugimoto and his coworkers reported the intramolecular photoaddition of anilines to aromatic hydrocarbons to give cyclic amino compounds (Scheme 16) [61-63]. 

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