Anthracene triplet state

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 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 ... 

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. ... 

We have already discussed one of the earliest photoreactions to be studied, that is, the (4w + 4w) photodimerization of anthracene. That the singlet state was involved in this reaction was conclusively shown in the period 1955-1957. The first reaction in which the triplet state of the molecule was shown to be involved was the photoreduction of benzophenone by Hammond and co-workersa) and Backstrom and co-workers<2) 1959-1961. This was the first in a series of many papers from Hammond s laboratory... 

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. 

In Chapter 2 we discussed a number of techniques used to study the various photophysical and photochemical processes occurring in anthracene and similar molecules. In that discussion we were primarily interested in the singlet state. In this chapter we will discuss some of the techniques available for studying the photophysical and photochemical properties of the triplet state. Most of our discussion will be directed to the photochemistry of simple ketones. 

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... 

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>. 

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. 

While it is in the triplet state a molecule may undergo typical diradical reactions. This provides a plausible mechanism for radical-like reactions of substances that are largely diamagnetic. They are partly converted to the triplet state by light, or in the case of low lying triplet states, by heat. Probable examples of this mechanism are the photooxidation of rubrene and the photooxidation and dimerization of anthracene and higher members of the acene series.76... 

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. 

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.

Here A, lA, and3A represent anthracene in the ground state, the first excited singlet state and first excited triplet state, respectively. In addition, I represents the onium salt initiation, while Rs and Rt correspond to the reactive centers formed by reaction of the onium salt with the excited singlet and triplet state anthracene, respectively. 

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. 

The photochemical reaction can also proceed via the triplet state and in this case no cyclization is observed. Especially when acetophenone is added as a triplet sensitizer, 41 is not formed. Remarkable is the observation that in the presence of anthracene or pyrene as triplet quencher, the yield of the cyclization product 41 was not enhanced and that nitrene insertion into CH bonds of anthracene or pyrene was observed. When the photochemical cyclization reaction was performed with the tosyl azide derivative 42a or the azido nitrile derivative 42b (Scheme 6), only low yields of the tricyclic amide 41 (32% from 42a, 9% from 42b, respectively) were obtained <2001JCS(PI)2476>. 

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