Anthracene triplet state energy

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 ... 

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. ... 

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... 

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 1Aff state is believed to be responsible for the addition to anthracene to form the endoperoxide since it closely resembles a diradical species, while the 1Ss+ state more closely resembles a dipolar ion. 

Chemically inert triplet quenchers e.g. trans-stilbene, anthracene, or pyrene, suppress the characteristic chemiluminescence of radical-ion recombination. When these quenchers are capable of fluorescence, as are anthracene and pyrene, the energy of the radical-ion recombination reaction is used for the excitation of the quencher fluorescence 15°). Trans-stilbene is a chemically inert 162> triplet quencher which is especially efficient where the energy of the first excited triplet state of a primary product is about 0.2 eV above that of trans-stilbene 163>. This condition is realized, for example, in the energy-deficient chemiluminescent system 10-methyl-phenothiazian radical cation and fluoranthene radical anion 164>. 

Figure 4. Electronic energy level diagram for anthracene illustrating the photophysical transitions (including reaction with the initiator from both the singlet and triplet states) and the associated kinetic constants.

In aromatic hydrocarbons, short-lived ](it, it ) is the lowest excited state and energy gap between (n, it ) and 8(it, it ) states is large. Both these factors are conducive to fluorescence emission and in general aromatic hydrocarbons are good fluorescer. Sometimes, the prediction may not come true if a higher triplet state T2 is available near the St state such as in anthracene. In such cases, fluorescence and phosphorescence both are observed at low temperatures in suitable solvent medium specially when S, and T, are states of different symmetry type. Some data correlating AEst and 4[Pg.148]

These preliminary experiments have established that anthracene triplets can be populated by energy transfer from suitable donor triplets. When the reaction is repeated in CC14 solution of anthracene, in absence of cut-off filter, the anthracene concentration decreases by 1.8% per flash but no reaction occurs in presence of filter. In presence of triplet energy donors and the cut-off filter, anthracene disappearance has been observed to the extent of 1.7% per flash. The results suggest that within experimental error, entire photoreaction with CC14 can take place via triplet state of anthracene, but they do not establish conclusively that the singlet state is not reactive. 

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... 

Several cases have been reported of sensitized chemical reaction from the triplet states of aromatic molecules. Anthracene triplets formed by triplet energy transfer from coronene react with carbon tetrachloride to yield, among other products, 9-chloroanthracene and hydrogen chloride,214 the same reaction which occurs upon direct excitation.215... 

Ferrocene forms ground-state complexes with dienes such as piperylene and isoprene. Irradiation of these yields a 1.25 trans/cis piperylene ratio in the first case and 92% cyclobutanes in the second,284 both indications of a high-energy sensitizer. However, ferrocene itself quenches anthracene triplets, so that it probably has a fairly low triplet energy. Therefore it has been postulated that an excited ferrocene-diene complex dissociates into an excited diene triplet which then proceeds to decay just as though it had been formed by normal bi-molecular sensitization. 

---