The Anthracene Triplet State

Up to this point we have concerned ourselves almost exclusively with the anthracene singlet state and its resulting photodimerization. However, we 

The amount of this endoperoxide produced depends upon the solvent and the anthracene concentration. 

Two mechanisms have been proposed to account for the formation of this product - - (for our purposes in this section we will neglect those steps leading to the dimer)  

The energy available from the anthracene triplet (42 kcal/mole) is sufficient to produce either of these states. The singlet excited molecule subsequently attacks a ground state anthracene to produce the observed endoperoxide. The state is believed to be responsible for the addition to anthracene to form the endoperoxide since it closely resembles a diradical species, while the state more closely resembles a dipolar ion. 

Proponents of each of these mechanisms attack the other with numerous criticisms. Those favoring the mole-oxide intermediate (mechanism A) claim that (1) the singlet oxygen mechanism does not adequately describe the observed kinetics ° and (2), it is eliminated since a chlorophyll derivative with a triplet energy too low to excite oxygen to its singlet level is still effective in producing product by the steps  

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An example of the equivalent (photoaddition) reaction following hetero-molecular photoassociation is provided by the photochemical addition of maleic anhydride to anthracene." Livingston and coworkers100 have shown that the anthracene triplet state is not involved in this reaction and that, in terms of Eq. (47) in the appropriate form, q%. = 0.03. However, if the excited complex XMQ formed directly by light absorption in the charge-transfer band is the reactive intermediate, this produces the adduct with a computed efficiency of 347 . 

The final state has a lifetime of about 2 ns, and decays to the anthracene triplet state. Similar behavior was noted for 23, where the lifetime of the final charge separated state in dichloromethane was about 47 ns. The intermediate exciplex state in these molecules is formally analogous to an Ar-D+-D species, which goes on by a second electron transfer to yield the final charge separated state. 

A rather important aspeet that should be eonsidered is that interfaeial quenching of dyes does not neeessarily imply an eleetron-transfer step. Indeed, many photoehemieal reactions involving anthracene oeeur via energy transfer rather than ET [128]. A way to discern between both kinds of meehanisms is via monitoring the accumulation of photoproducts at the interfaee. Eor instance, heterogeneous quenehing of water-soluble porphyrins by TCNQ at the water-toluene interfaee showed a elear accumulation of the radical TCNQ under illumination [129]. This system was also analyzed within the framework of the exeited-state diffusion model where time-resolved absorption of the porphyrin triplet state provided a quenehing rate eonstant of the order of 92M ems. ... 

Solution of this coupled set of differential equations allows the concentrations of each of the anthracene electronic states to be determined as a function of time. In a previous publication, Nelson et al 1 used this approach to investigate the relative importance of electron transfer from the singlet and triplet states of anthracene. In this contribution, we will use these simulations to predict profiles of the anthracene ground state as a function of time so that the simulation results may be compared with the steady-state fluorescence results presented above. 

Anthracene crystals are highly fluorescent (4>f = 1-0) but in dissolved state emission is much reduced ( / = 0.25). A recent explanation of this large difference is that the second triplet state T% of anthracene lies above the first singlet in anthracene crystal but below it in the dissolved state. thereby enhancing the nonradiative dissipative processes. Molecular adsorption on a substrate also enhances the fluorescence. Hydrogen... 

The reactive state of anthracene involved in the photoaddition reaction between anthracene and CC14 has been a point of controversy. An attempt has been made to establish the reactive state by populating anthracene triplet state by triplet-triplet energy transfer process ... 

As an example of the effect of level shifts in the crystalline state, as just described, consider the observed rates of radiationless transitions in anthracene.45 The first excited 1BSu of the isolated anthracene molecule is located about 600 cm-1 above the second triplet state. Hence, 8 < vv and the intersystem crossing process is quite rapid at room temperature. The fluorescence quantum yield is about 0.3 for this molecule in the gas phase and in solution. In the crystal the first excited singlet state is red shifted (from the gas level) by about 1880 cm- while the second triplet state is hardly affected, so that in this case the energy gap between those two states increases in the crystal. Then the coupling term, v, is smaller in the crystalline state than in solution, thereby leading to a decrease in the rate of the intersystem crossing. The result is that the fluorescence yield in the crystal is close to unity.40... 

Among the 1,3-linked bichromophoric anthracenes listed in Table 3, 1,3-di-9-anthryl-l-propanone 21a, l,3-di-9-anthryl-l-butanone 21b, and l,3-di-9-anthryl-2-methyl-l-propanone 21c are exceptional because their photochemical isomerization by intramolecular 4n+4n cycloaddition to give 22 is characterized by high quantum yields, viz. 0.65, 0.40, and 0.72, respectively. For photochemical cycloadditions of linked anthracenes, the quantum yield of 0.72 is the highest ever observed. Oxygen quenching and sensitization experiments indicate that 21a, 21b, and 21c undergo the 4n+4n cycloaddition in the excited triplet state (see Section II.C). 

Work conducted by Tiller and Jones (1997) demonstrated that the fluorescence of PAHs decayed over time under both under anoxic and oxic conditions. Typically, however, the presence of dissolved oxygen had a more pronounced influence on baseline fluorescence decay for all the PAHs studied. Moreover, certain PAHs (pyrene and anthracene) were more susceptible to this phenomenon than others. To date a mechanism to explain this phenomenon has not been identified, but it is probably a combination of complex pathways including the reaction of the analyte with reactive oxygen species formed from the excited triplet state DOM and the direct photolysis of the analyte by the excitation light source. Thus, the application of fluorescence quenching for measuring Kdom is probably limited to systems, which can be analyzed under anoxic conditions. 

The vibrationally excited singlet Si and triplet Tf, expected upon the annihilation of the lowest excited triplets Ti, have an energy below the autoionization threshold of typical aromatic crystals [26], thus, no intrinsic photoionization has been observed in these solids. However, the presence of intentional or non-intentional admixtures can allow the triplet-triplet interaction-induced photoionization forming a free carrier in the matrix and a trapped carrier on an admixture molecule. Also, such a process has been reported for the CT triplet states localized on the donor molecule, e.g. in polycrystalline samples of CT complex anthracene-tetracyanoben-zene, where the triplets are localized on the anthracene donor (3Di). Annihilation of 3Di results in the population of non-relaxed excited states of the complex 1(D+A ) and 3(D+A") , dissociation of which may lead to the formation of free charges D+ [214]. 

S can be one of the higher triplet states T , such as is the case for anthracene (cf. Example 5.3), and that in molecules with lone pairs of electrons either T, or T may be of the same type as S,. This is illustrated in Figure 5.13, where various possibilities for the eneiigy order of the and (n,jt )... 

Absorption spectra of electronically excited states may be observed in flash photolysis studies. Porter has established the existence of the triplet state in a wide range of organic compounds in the liquid and gaseous phases. For example, the first triplet state of anthracene is populated by radiationless conversion from a photochemically excited singlet molecule, and may be observed by the absorption to the second triplet level. Absolute measurements of the triplet concentration may be made by determinations, from the absorption spectra, of the depletion of the singlet state. Similar results have been obtained with a variety of hydrocarbons, ketones, quinones and dyestuffs. 

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