Anthracene excitation transfer

Hydrogen atom transfer from anthracene, excited into its lowest excited singlet state, to anthraquinone impurity molecules creates a radical pair that strongly quenches the fluorescence from anthracene crystals. The reverse transfer rate constant, found from measurements of fluorescence intensity and its characteristic lifetime at different moments after the creation of the radical pair, varies from 106 to 10s s 1 in the range 110-65 K, kc = 4 x 104 s 1, TC = 60K. The kc values drops to 102 s 1 in the deuteroanthracene crystal [Lavrushko and Benderskii, 1978]. 

The dimerization of anthracene can also be sensitized by biacetyl (Back-strdm and Sandros, 1958). Despite the fact that the rate of triplet excitation transfer from biacetyl, Et = 54.9 kcal per mole, to anthracene, Et = 42 kcal per mole, should be diffusion-controlled, the rate of sensitized dimerization is extremely slow. Since singlet excitation transfer can be ruled out rigorously on energetic grounds, the results indicate that either T anthracene adds to S anthracene much less readily than does S anthracene, or in the sensitized experiments the dimerization occurs upon triplet-triplet... 

The single-electron transfer from one excited component to the other component acceptor, as the critical step prior to cycloaddition of photo-induced Diels Alder reactions, has been demonstrated [43] for the reaction of anthracene with maleic anhydride and various maleimides carried out in chloroform under irradiation by a medium-pressure mercury lamp (500 W). The (singlet) excited anthracene ( AN ), generated by the actinic light, is quenched by dienophile... 

The low solubility of fullerene (Ceo) in common organic solvents such as THE, MeCN and DCM interferes with its functionalization, which is a key step for its synthetic applications. Solid state photochemistry is a powerful strategy for overcoming this difficulty. Thus a 1 1 mixture of Cgo and 9-methylanthra-cene (Equation 4.10, R = Me) exposed to a high-pressure mercury lamp gives the adduct 72 (R = Me) with 68% conversion [51]. No 9-methylanthracene dimers were detected. Anthracene does not react with Ceo under these conditions this has been correlated to its ionization potential which is lower than that of the 9-methyl derivative. This suggests that the Diels-Alder reaction proceeds via photo-induced electron transfer from 9-methylanthracene to the triplet excited state of Ceo-... 

Flegel, M. Lukeman, M. Huck, L. Wan, P. Photoaddition of water and alcohols to the anthracene moiety of 9-(2 -hydroxyphenyl)anthracene via formal excited state intramolecular proton transfer. J. Am. Chem. Soc. 2004, 126, 7890-7897. 

Basaric, N. Wan, P. Competing excited state intramolecular proton transfer pathways from phenol to anthracene moieties. J. Org. Chem. 2006, 71, 2677-2686. 

As discussed in Chapter 2, triplet excited anthracene transfers its energy to oxygen to produce singlet excited oxygen OA,). The singlet oxygen in turn attacks a ground state anthracene to form a 9,10-endoperoxide. 

Another explanation has been offered to explain the large proportion of cyclobutane derivatives produced by low-energy sensitizers, especially for the anthracene derivatives.<17) This is that energy transfer to diene occurs from the second excited triplet state of the sensitizer rather than the first. Experiments using a large number of anthracene derivatives as sensitizers... 

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 )... 

Time-resolved spectroscopy establishes that the fluorescence of the excited (singlet) anthracene ( ANT ) is readily quenched by maleic anhydride (MA), which leads to the formation of the ion pair ANT+, MA via diffusional electron transfer (see Fig. 12), i.e.,... 

Electron-transfer activation. Time-resolved spectroscopy establishes that irradiation of the charge-transfer band (hvCj) of various arene/0s04 complexes directly leads to the contact ion pair. For example, 25-ps laser excitation of the [anthracene, 0s04] charge-transfer complex results in the ion-radical pair instantaneously, as shown in Fig. 14218 (equation 76). 

Fig. 14 Transient absorption spectrum of anthracene cation radical (ANT+ ) obtained upon 30-ps laser excitation of the [ANT, OsOJ charge-transfer complex in dichloro-methane. The inset shows the authentic spectrum of ANT+ obtained by an independent (electrochemical) method. Reproduced with permission from Ref. 96b.

Ehrenfest dynamics with the MMVB method has also been applied to the study of intermolecular energy transfer in anthryl-naphthylalkanes [85]. These molecules have a naphthalene joined to a anthracene by a short alkyl —(CH)n— chain. After exciting the naphthalene moiety, if n = 1 emission is seen from both parts of the system, if n = 3 emission is exclusively from the anthracene. The mechanism of this energy exchange is still not clear. This system is at the limits of the MMVB method, and the number of configurations required means that only a small number of trajectories can be run. The method is also unable to model the zwitterionic states that may be involved. Even so, the calculations provide some mechanistic information, which supports a stepwise exchange of energy, rather than the conventional direct process. 

In complex organic molecules calculations of the geometry of excited states and hence predictions of chemiluminescent reactions are very difficult however, as is well known, in polycyclic aromatic hydrocarbons there are relatively small differences in the configurations of the ground state and the excited state. Moreover, the chemiluminescence produced by the reaction of aromatic hydrocarbon radical anions and radical cations is due to simple one-electron transfer reactions, especially in cases where both radical ions are derived from the same aromatic hydrocarbon, as in the reaction between 9.10-diphenyl anthracene radical cation and anion. More complex are radical ion chemiluminescence reactions involving radical ions of different parent compounds, such as the couple naphthalene radical anion/Wurster s blue (see Section VIII. B.). 

The first chemiluminescent paracyclophanes have been described recently 208> the compounds 138 and 139 both contain a phthalhydrazide group as that part of the molecule producing the excitation energy which is transferred to the substituted benzene resp. anthracene moiety. 139 chemiluminesces with about double the amount of 2.3-anthracene dicarboxylic hydrazide on oxidation by oxygen/potassium tert. butoxide... 

A general theory of the aromatic hydrocarbon radical cation and anion annihilation reactions has been forwarded by G. J. Hoytink 210> which in particular deals with a resonance or a non-resonance electron transfer mechanism leading to excited singlet or triplet states. The radical ion chemiluminescence reactions of naphthalene, anthracene, and tetracene are used as examples. 

Photosensitization of diaryliodonium salts by anthracene occurs by a photoredox reaction in which an electron is transferred from an excited singlet or triplet state of the anthracene to the diaryliodonium initiator.13"15,17 The lifetimes of the anthracene singlet and triplet states are on the order of nanoseconds and microseconds respectively, and the bimolecular electron transfer reactions between the anthracene and the initiator are limited by the rate of diffusion of reactants, which in turn depends upon the system viscosity. In this contribution, we have studied the effects of viscosity on the rate of the photosensitization reaction of diaryliodonium salts by anthracene. Using steady-state fluorescence spectroscopy, we have characterized the photosensitization rate in propanol/glycerol solutions of varying viscosities. The results were analyzed using numerical solutions of the photophysical kinetic equations in conjunction with the mathematical relationships provided by the Smoluchowski16 theory for the rate constants of the diffusion-controlled bimolecular reactions. 

However, there is an important subtlety in 4. While amine receptor-anthracene lumophore pairs produce excellent PET based quenching, a benzo-15-crown-5 ether receptor is demonstrably incapable of transferring an electron to an excited anthracene lumophore unless the latter carries an electron withdrawing substituent. Then, what is the secret of 4 s success First, only one rapid PET process is... 

Fig. 5 (A) Typical time-resolved picosecond absorption spectrum following the charge-transfer excitation of tropylium EDA complexes with arenes (anthracene-9-carbaldehyde) showing the bleaching (negative absorbance) of the charge-transfer absorption band and the growth of the aromatic cation radical. (B) Temporal evolution of ArH+- monitored at Amax. The inset shows the first-order plot of the ion...

Fig. 9 (A) Transient absorption spectrum of the cation radical from anthracene (AnH) in CH2C12 at about 35 ps following the 532 nm charge-transfer excitation of the 0s04 complex with 30-ps (FWHM) laser pulse. The inset shows the steady-state spectrum of AnH+- obtained by spectroelectrochemical generation. (B) The decay of the charge-transfer transient by following the absorbance at Amax = 742 nm. The inset shows the first-order plot of the absorbance decay subsequent to the maximum...

Picosecond time-resolved spectroscopy has defined the relevant photophysical and photochemical processes associated with the charge-transfer excitation of an arene complex such as anthracene with tetranitromethane... 

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. 

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