Anthracene-dimethylaniline exciplex

The very first fluorescence spectra of jet-cooled exciplexes indicated the existence of two types of ground-state van der Waals adducts. For instance, the anthracene-dimethylaniline system displayed two types of cluster bands in the fluorescence excitation spectrum broad ( 150 cm ) and structureless, leading to typical ex-ciplex emission, and narrow (1 cm ), leading to resonance-type emission [10, 20]. It was assumed that they are due to different 1 1 adducts, distinguished by different geometries. Recently, laser-based techniques were developed that allow the discrimination of different species. One is hole-burning spectroscopy and another— mass-selected photoionization. 

Figure 2. Characterization of intermolecular exci-plexes by hole-burning spectroscopy. Adapted from Ref. [22a] (a) Fluorescence excitation ( emission set at 375 nm) and (b) hole-burning spectra of the R-isomer of the anthracene-dimethylaniline (An-DMA) adduct in a supersonic jet. The probe laser was tuned on the most intense line of the adduct. Lines marked with asterisks are due to bare anthracene, (c) Fluorescence excitation ( emission set at 450 nm) and (d) hole-burning spectra of the E-isomer of the An- DMA adduct in a supersonic jet. The probe laser was tuned to the maximum of the broad exciplex excitation band.

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. 

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] ... 

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]. 

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... 

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).

For example, naphthalene, anthracene, pyrene, perylene and other polynuclear aromatic hydrocarbons react in this way with donor molecules, such as dimethylaniline and other tertiary amines, methoxybenzenes, or tetracyanoethylene, in non-polar or low-polar solvents such as hexane or toluene. Such a complex is called an exciplex (from excited complex). The evidence is analogous to that summarised above for the formation of ex-... 

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