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.
Fig. 3.9 Schematic representation of the energy levels and transitions corresponding to a system where internal conversion leading to tautomer formation is possible. By permission from [26]
4A vivid recollection of that meeting and of other important moments has been published by Kasha [8,27].
52 3 The Framework of Photochemistry: State Diagram
In the period between the two world wars, the photochemical reactions of ketones had been extensively studied both in the gas phase and in solution and the role of radicals ascertained (removal of metal mirrors, trapping by iodine, product structure) [28], but the multiplicity of the reacting excited state had received less attention. However, it was becoming clear that such an excited state had a radical character. In particular, a study of the photoauto-oxidation of alde- hydes showed that this occurred with a quantum yield in the order of 104a clear indication for a chain process via radicals, although these did not seem to be formed in the primary act. In his studies on the thermal auto-oxidation of aldehydes, Bodenstein had suggested the intermediacy of an activated “upright” form of the aldehyde (Scheme3.1) [29,30]. When studying the ketone-photosensitized alde- hyde oxidation, Ba¨ckstr€om applied the same concept and suggested that the primary photochemical process “consisted in setting upright the carbonyl double bond” and the hole in the valence structure of the oxygen caused hydrogen abstraction from alcohols to form semipinacol radicals, “of which it had to be assumed that they were capable of existence as free species,” as well as from aldehydes (Scheme3.1). In the latter case, the acyl radicals formed added oxygen to give an acylperoxy radical that Ba¨ckstr€om identified as the “activated” intermediate of the thermal reaction. Thus, photochemical reactions could be understood as involving the formation of such valence-unsaturated species caused by absorption of a photon, while the slower thermal initiation probably involved the intervention of traces of impurities, in particular transition metals that gave access to the same species. Once arriving at the acyl and the acylperoxy radicals, a chain process followed in the same way, whether thermally or photochemically initiated. The same scheme could be adapted to the gas phase acetaldehyde oxidation [31]. The non-chain photoreaction of aliphatic aldehydes in solution likewise appeared to involve the formation of intermediates with “free valences” [30], and a similar path may be involved in the bimolecular reduction of ketones to pinacols via semipinacol radicals and their statistic recombination [32].
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 T1!S0transition). This was the case, for example, of biacetyl, which phosphoresced, but was also known to abstract hydro- gen from various substrates, e.g., from alcohols, and to give radical recombination products. Actually, in their key 1944 paper, Lewis and Kasha present their conclu- sions by stating that they identified the phosphorescence emitting metastable state Scheme 3.1 Formation of
radicals by hydrogen abstraction. Reprinted with permission from
[33]. Copyright 1938, American Chemical Society
3.3 The Triplet State: Reactions 53
as “the triplet or biradical state” and noted that “the physicist speaks of the triplet state, the organic chemist of the biradical state.” The triplet state implies a pair of electrons with spins parallel; a substance that has two “odd” electrons is called a biradical. The two names “are synonymous except that in the organic biradicals their two odd electrons may be sufficiently isolated from one another that their spin is independent of each other” [2]. Indeed, Lewis attempted to picture the chemical structure of excited states and identified a biradical nature as appropriate in several cases, such as alkenes and ketones.
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 unnaturally 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].
Ba¨ckstr€om 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 2.6102 M1 s1 (methanol) to 5.9109 M1 s1 (hydroquinone). Recalling his proposal that absorption of a photon led to a “biradical” (indeed the same designation used by Lewis and Kasha) or “bond upright” carbonyl, he suggested that the act of quenching actually consisted in “a chemical reaction between the biradical and the quencher involving abstraction of a hydrogen atom from the latter.
This accounted for the general parallelism between quenching power and inhibitory action in autoxidation reactions” [36,37]. Further support came from a study where triplet biacetyl was generated by energy transfer from benzophenone, as revealed by the appearance of both phosphorescence and photochemical reaction [35]. At the same time, Hammond studied in detail the photochemical reduction of benzophe- none (B) by benzhydrol (BH2), a convenient choice since hydrogen transfer gave a single radical (BH•) and coupling of this yielded a single product, the pinacol BHBH [38, 39]. The quantum yield of reduction (ΦB) was found to depend on the concentration of benzhydrol, and the simplest mechanism accounting for such result involved deactivation of the chemically active excited state by a first-order mechanism. Considering the excited state as an unstable intermediate gave the rate law:
1 ΦB
¼1
aþ kd
akr½BH2 ð3:1Þ
whereais the yield of the chemically reactive state,kdis the decay rate constant of such state, andkris the rate constant for hydrogen abstraction from BH2. A doubly reversed plot of ΦBvs [BH2] was actually linear, consistently with competition between thermal decay to the ground state and hydrogen transfer from the alcohol,
54 3 The Framework of Photochemistry: State Diagram
and the intercept at [BH2]¼ 1was 1, indicating that the yield of the chemically reactive state (a) was unity. The observed slope was too large for admitting that the reactive state was the singlet (estimated to have a lifetime0.2 ns), even if hydrogen transfer should occur at diffusion-controlled rate, which seemed unlikely.
Thus, it appeared that “even at room temperature, the excited state produced by the n!π* transition of benzophenone underwent quantitative intersystem crossing with the production of a long-lived triplet.” The identification of the triplet as the reacting excited state was consistent with the observed quenching by paramagnetic ferric dipivaloylmethide [39].
Starting from these studies, n!π* triplet carbonyls with their characteristic radical chemistry took the key role they have maintained in photochemistry. The
“chemical” aspect of the description of such excited states was strengthened by the analogy with alkoxy radicals. Thus, comparing the competitive consumption of different hydrocarbons in the photoreduction of benzophenone led to a reactivity series very similar to that observed witht-butoxy radicals. Triplet benzophenone was somewhat more selective and more sensitive to the electron-donating charac- teristics of the hydrocarbons, but the energetics was very close [40]. As for the
“physical” aspect, this received theoretical support by Oosterhoff, who calculated the lifetime of triplet–singlet emission for benzene and acetone and found that although the value obtained was too high in comparison with observed phospho- rescence, the calculated ratio for the two compounds agreed with the experiment.
Furthermore, the Hammond group could demonstrate that excited states of ketones were able to transfer energy to acceptors, such as dienes, and to cause sensitized cis–trans isomerization in such compounds. Energy transfer occurs with unitary probability when exothermic, at a lower rate when endothermic [41,42]. Further reactions were identified and inserted in the same paradigm, so that in a consistent picture of the chemistry of n!π* triplet carbonyls and their radical chemistry could be summarized. This included Norrish II fragmentation and hydroxycyclobutane formation, Norrish I cleavage followed either by CO loss and recombination of the fragments or by H transfer leading to a ketene and cycloaddition between α,- β-unsaturated ketones and olefins and between carbonyl and olefins. When the structure allowed it, these reactions were regiospecific but not stereospecific. This was in accord with a path via radical (or diradical when the reaction was intramo- lecular) intermediates, and the few instances of stereospecific course were proposed to involve the singlet [43].
Furthermore, studies by Kasha onN-heterocycles as well as carbonyl and nitro- aromatic derivatives had evidenced a transition corresponding to the excitation of a (nearly) nonbonding (heteroatom) electron to an antibondingπ* molecular orbital.
These had been designed as n!π* transitions and found to differ fromπ!π* under important respects. Although labeling the starting orbital as nonbonding is too stretched, the different chemistries exhibited by the two types of excited states offered an effective guideline in the exploration (and the prediction) of the photo- chemistry of these compounds (see Table3.1) [44].
3.3 The Triplet State: Reactions 55