The Triplet State Reactions

In parallel with respect to studies on the emission of dyes, the role of triplet was demonstrated also in chemical reactions. Protagonist here was the carbonyl chro-mophore, and the problem was again to demonstrate the participation of a non-spectroscopic state. 

A vivid recollection of that meeting and of other important moments has been published by Kasha [8, 27], 

As mentioned above, a few years later Lewis and Kasha reported on the phosphorescence in many photochemical systems, not only in rigid media, but also in some cases in fluid solution and in the gas phase and attributed such emission to the triplet state (a forbidden Ti So transition). This was the case, for example, of biacetyl, which phosphoresced, but was also known to abstract hydrogen from various substrates, e.g., from alcohols, and to give radical recombination products. Actually, in their key 1944 paper, Lewis and Kasha present their craiclu-sions by stating that they identified the phosphorescence emitting metastable state 

In fact, presenting photoreactions as involving biradical intermediates was natural. The energy of UV photons is of the same order as that of covalent bonds, and in photochemical reactions bonds were cleaved and formed. Thus, with aldehydes and ketones, the light-activated state was not uimaturally drawn as a biradical, with a large or small separation of the radical sites and the reaction pictured as a radical process, e.g., in hydrogen abstraction from alcohols to give pinacols [33, 34]. 

Backstrom measured the quenching of the emission (called for the moment long-lived fluorescence, although in later papers he used the term phosphorescence) [35] by alcohols, amines, and phenols and obtained quenching constants from 2.6 X 10 s (methanol) to 5.9 x 10 s (hydroquinone). Recalling 

---

The Norrish Type II reaction of aliphatic and aromatic ketones in isotropic solvents has been studied in considerable detail (26,43), and several aspects of the reaction depend on the conformational mobility of the excited ketone or the 1,4-biradical intermediates formed by y-hydrogen abstraction. In the case of aromatic ketones for example, the triplet lifetime can provide an indication of the facility with which the proper geometry for hydrogen abstraction can be obtained (29,43), the distribution of fragmentation O-cleavage) and cyclization products obtained depends on the conformations available to the triplet 1,4-biradical intermediate and their relative kinetic behavior prior to intersystem crossing (27-30,43-47), and the total quantum yield for the reaction is a function of both of the above factors. For practical reasons, product ratios are usually the easiest aspect of the reaction to monitor, and this is the approach that has been used most commonly in studies of Norrish II reactivity in ordered media (27-30,45). The pertinent features of the triplet state reaction arc illustrated in Scheme 1 (30). 

Perfluorobenzene 76 when irradiated in the gas phase (35 mm) at 25° produced the Dewar benzene 77 5 -58). The triplet state reaction of 76 gives only higher molecular weight materials. 

In two cases, where the singlet- and triplet-state reactions have been carefully looked at and separated, this proves to be true. Thus coumarin 8.59, in the excited singlet state, dimerises to give only the HH dimer syn-8.60, but in the triplet state it gives both HH isomers syn-8.60 and anti-8.60, with only a trace of head-to-tail (HT) products.1119 Acenaphthylene also gives the syn dimer from the singlet-state reaction and a mixture of the syn and anti dimers from the triplet-state reaction.1120... 

The intensity dependence of UV laser flash excitation of diphenylaraine in methanol shows the occurrence of two quantum photoionization involving the triplet state.Also the binding energies of the triplet excimers observed in poly(N-vinylcarbazole) films between 15 and 55 K have been estimated.Triplet states, biradicals, radical ions and heavy atom effects can all be involved in the photodimerization of aceanthrylene. It is established that the triplet state reaction mechanisms give rise to four stereoisomeric dimers. The basicity of the Ti state of phenazine is found to have a pKa of 1.9, which is appreciably different from an earlier value, by flash photolysis. The discordant results obtained in earlier work are satisfactorily explained. 

Related to the stilbene cyclization is the photoreaction of 2-vinylbiphenyls to give 9,10-dihydrophenanthrenes. The photocyclization of (141) and its Z-isomer is shown to be highly stereoselective on direct irradiation, but totally unselective when sensitized with benzophenone or xanthene. These results support the proposal that the singlet state undergoes conrotatory ring-closure followed by a suprafacial 1,5-shift of hydrogen, whereas the triplet-state reaction proceeds through a common intermediate, possibly a perpendicular triplet state. 

Triplet quenchers having triplet energies well below that of naphthyl halides and halogeno-biphenyls do not quench the triplet-state reactions of these molecules in the normal way, and it has been speculated that some reactionenhancing process counterbalances energy transfer.252 A study of triplet -> singlet energy transfer between benzophenone and perylene in vitreous solution... 

Equations 12.46 and 12.47 do not show the spin states of the unpaired electrons. In general, a cleavage can occur from both the singlet and triplet n,n states, but quantum yields are much greater for the triplet state reactions. Zewail and co-workers reported femtosecond studies and theoretical calculations of the Norrish type I reaction of acetone. They found a barrier of ca. 18 kcal/mol for a cleavage from the Si state of acetone but a barrier of only 5 kcal/mol for dissociation from the Ti state. Thus, they concluded that the photodissociation occurs from the triplet state and that intersystem crossing from Si to Ti is the rate-limiting step in the Norrish type I reaction of acetone. ... 

Of di-7c-methane and oxa-di-7t-methane rearrangements reported, that of greatest preparative value involves the photolysis of bicyclo[2,2,2]octadienones. The triplet-state reactions of these latter compounds have been examined for the first time in the absence of benzannelation. Bicyclo-octadienones (42) have available to them four di-7t-methane and two oxa-di-ii-methane modes of rearrangement, and, remarkably, only one di-7c-methane route is followed, to give (43) regiospecifically and in good (40—70 %) yields. 

Figure 104.37 shows the first and the second photodimerizations of the decyl ester (ESIO) in the thin soHd film (a, b, c) with the data in solution (e) and in polymer film (d). The rate of the photodimerization in chloroform solution was slow but proceeded completely (e). The photodimerization of thymine derivatives in solution is known to proceed by way of the triplet excited state. The short lifetime of the singlet state and the inefficient intersystem crossing are the reasons for the slow reaction rate for photodimerization of thymine in solution. The photodimerization was studied in the polymer film using poly(vinyl acetate) (PVAc) (d). The polymer film was obtained from a chloroform solution of the ester derivatives of thymine and poly(vinyl acetate) by spin coating on a quartz plate. The rate of photodimerization in poly(vinyl acetate) was faster than the rate in chloroform solution. In the polymer matrix, photodimerization occurs from associated thymine bases via the singlet state with a higher quantum yield than that from the triplet-state reaction. 

---