Anthracene triplet

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. 

With many aromatic hydrocarbons as solutes, excited state yields in alkane solutions are nearly equally divided between singlets and triplets, and these yields increase with solute concentration until -0.1 M (Salmon, 1976 Thomas et al., 1968). In these systems, both the solute anion and the solute excited state yields increase similarly with solute concentration. With anthracene as a solute, the rate of growth of anthracene triplet matches that of the decay of the anthracene anion. With aromatic solvents, on the other hand, solute ions play... 

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 . 

We know that the first mechanism (eqs. 81-83) must occur since the anthracene triplet lies below that of phenanthrene, and anthracene alone gives P-type delayed fluorescence. We also have evidence, to be discussed later, that the second mechanism (eqs. 81, 82, 84) is operative, at least with the system anthracene-naphthacene. We assume therefore that both systems are operative with the phenanthrene-anthracene... 

If the light is fully absorbed in 20 ml of solution, 2.5x 1019 molecules will photolyze to give as many intermediate particles and the concentration of intermediates will be 2.1 x 10-3 mol l-1. Therefore in order to be easily detectable the intermediates should have high extinction coefficient. For example, anthracene triplet has an extinction coefficient of 7 x 10 1 mol-1 cm-1 at 424 nm. For spectrophotometric detection, a 3 % change in transmitted intensity is required. If the cell length is 20 cm then from Beer s law we can calculate the limiting concentration that may be detected ... 

A flasn spectrum of anthracene triplets at various delay times are given in Figure 10.11. 

Figure 10.11 clash spectrum of anthracene triplet at different delaytimes. 

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 ... 

Fig. 11.7 Energy level scheme for triplet-triplet energy transfer from ar.thra-quinone and co.oncne triolets to anthracene triplet.

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. 

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. 

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. 

In a similar way, anthracene triplet (4>,gj3=0.71, z =6A,700Mr cmr ) and the naphthalene triplet (4>jg = 0.75, e j = 24,500 M" cm" ) in cyclohexane solution have been introduced as transient chemical actinometers for the third-harmonic (355 run) and fourth-harmonic (266 nm) output of Nd YAG lasers, respectively (44). In summary, transient chemical actinometers are ideal for accurately measuring the energy of single laser pulses, provided the quantum yields and extinction coefficients of the transients are well known (45 7). Thus, the well-established benzophenone actinometer (42-44) has been used as a reliable reference to calibrate the azobenzene actinometer (see section "Laser Intensity Measurements with the Azobenzene Actinometer" Doherty S, Hubig SM, unpublished results) and the Aberchrome 540 actinometer (48,49) for intensity measurements with pulsed Nd YAG and/or XeCl excimer lasers. However, such actinometer can only be used when a complete set of laser flash photolysis equipment including a kinetic spectrometer is available. 

The deviation from a single exponential curve for the phosphorescence decay of some chromophores dissolved in plastics was first noted by Oster et al.(19) Nonexponential decay curves were obtained for naphthalene and triphenylene phosphorescence in polyCmethyl methacrylate) (PMMA) at room temperature (20), while exponential decays at room temperature were observed in PMMA for anthracene triplet (21), pyrene (22), and coronene phosphorescence (20). Graves et al. (23) analyzed the temperature dependence of phosphorescence parameters for a number of aromatic hydrocarbons in PMMA from 77 to 400K and suggested the existence of intermolecular thermally assisted energy transfer from the chromophore to the host plastic in the higher temperature region. 

The overall decay of the triplet may include some quenching by free radicals. The yield of anthracene triplet was estimated to be 0.7 molecule/100 e.v. 

---