4.3 Oxygenation Reactions and Singlet Oxygen
4.3.3 Mechanism
In the Dufraisse/Sch€onberg scheme (1933), a biradical adduct between oxygen and the rubrene molecule is indicated as the intermediate and results from oxygen trapping of the excited molecule, itself indicated as a biradical. This did not sound inappropriate considering the large amount of energy conferred by the absorption of a photon, well in the order of a chemical bond, that justified the broken bond representation of the excited state on one hand, and the well-known ability of trapping of organic radicals by oxygen in combustion processes. The diradical/triplet nature of molecular oxygen ground state explained the efficient quenching and thus a “chemical” activation of oxygen by forming a bond with the excited aromatic molecule. The same rationalization was then adopted by Schenck when he later discovered the photosensitized oxygenation of alkenes and dienes (see Scheme4.14) [102–105].
He invoked the intermediacy of a weakly bound peroxide biradical resulting from trapping of the biradical excited state of the sensitizer by oxygen. This was able to transfer oxygen to the substrate. In the same years, Lewis had identified the metastable state as a triplet, or, by adopting a term more often used by organic chemistry practitioners, a biradical (see Chap. 3). Thus, no wonder that such an intermediate may enter in radical reactions, and in fact a number of photochemical reactions of aldehydes and ketones had been explained exactly via such biradical Scheme 4.14 Photosensitized oxygenation of carvene
90 4 Some Paradigmatic Topics
properties of the excited (triplet) state (see Sect.3.2). In the extreme view, this corresponded to the representation that the two unpaired electrons of an excited state were independent one from another, in particular with regard to the spin, and dispensed with the notion of triplet, or, in a later formulation, this could be considered accessorial (what was formed was a biradical “from the chemical point of view” and formed an “adduct with biradical structure•SensSOO•”). Schenk considered that the sensitizer was “electronically photo-excited by unpairing two electrons, thus forming “photobiradicals •S• (1S, 3S)” [103, 105] (see Scheme4.15).I
However, another rationalization was possible. This involved the excited states of oxygen. The low-lying excited states of molecular oxygen were actually lower than the triplet state of most dyes, because they were singlet states of the same orbital occupancy as the ground triplet state. These states of oxygen that had been theoretically predicted were actually characterized in the atmosphere by spectros- copists in the 1930s (Scheme4.16) [106–108].
Thus, it may be thought that the triplet state of the sensitizer would transfer energy to gaseous oxygen and activate it in aphysicalway. In this mechanism, no bond formation was involved and the actual reagent were molecules of oxygen excited to the singlet state by energy transfer from the sensitizer. The reagent had thus to be a gas. As pointed out by Kautsky, experiments in solution were ill suited for investigating this point, because it would have been difficult to disentangle different effects [109,110]. Kautsky thus devised to use spatially confined sensi- tizers and prepared separately gel granules on which were absorbed either the sensitizer or the substrate to be oxidized so that the influence of molecular oxygen upon their fluorescence could be examined with no disturbance. He adsorbed dyes such as Trypaflavin (see formula24in Scheme4.17, actually used as disinfectant), chlorophyll, or porphyrins on solid silica gel or aluminum oxide gel, to which they remained firmly fixed. The materials formed were quite transparent and when carefully evacuated showed both normal and long-lived (in the order of seconds) fluorescence at room temperature and phosphorescence at low temperature (com- pare Chap. 3) (see below for the effect of oxygen).
Scheme 4.15 Biradical mechanism for the photosensitized oxygenation of organic molecules
Scheme 4.16 Sensitization of oxygen
4.3 Oxygenation Reactions and Singlet Oxygen 91
Colorless leucomalachite green (25) was likewise adsorbed and shown not to react when irradiated under these conditions, while when this adsorbate was mixed with that of the sensitizer in the presence of oxygen and irradiated the greenish-blue color of the oxidized dye appeared (the experiment was positive only for a small range of oxygen concentrations, however). This proved that an activated gas had been generated by the sensitizer and migrated from the place where it had been formed to where it had reacted. It could thus concluded that the sensitizer acted on oxygen, not on the substrate (Scheme4.18).
Candidates for this role were only the singlet oxygen states,1Σg(37.3 kcal mol, 766 nm) and1Δg(22.5 kcal mol, 1270 nm), that thus must be the actual reagents.
This conclusion was refuted by Gaffron, who studied the sensitized oxidation of different substrates, in particular allylthiourea and isoamylamine, and found that the rate was virtually independent of oxygen partial pressure. The reaction occurred with unitary quantum yield even under excitation at 760–800 nm and indeed even when bacterio-chlorophyll was used as a sensitizer, at 820 nm, where he thought energy was not sufficient for arriving at the excited state (incorrectly for the
1Δgstate) [111–114].
But in which way acted the sensitizer? As Kautsky observed, if energy transfer to oxygen was implied, then this occurred in competition with monomolecular decay from the excited state and one would expect that emission of the dye should be quenched by O2[109,110] Actually, for a unitary chemical quantum yield it had to be quenched by 100 % or thereabouts. This was clearly not the case, and the fluorescence of hematoporphyrin and chlorophyll was only marginally quenched.
The conclusion was valid for the case of dyes (such as uranin) for which fluores- cence was a main path, while the most effective oxygenation photosensitizers fluoresced weakly [115]. The fact that such a minor process was not quenched was thus not decisive for the mechanism. In the last group of sensitizers, the short- lived fluorescing state had little to do with oxygenation and the largest fraction rather converted to another, invisible, “quantum state.” It was this metastable state that lived long enough to be 100 % quenched by oxygen [116]. In fact, Kautsky found that several dyes showed a long-lived afterglow in very diluted solutions Scheme 4.18 Photosensitized
oxidation of Leucomalachite Green (25)
Scheme 4.17 Trypaflavin, a mixture of two dyes
92 4 Some Paradigmatic Topics
(up to 1/100 s) and this was quenched by oxygen, but not by the acceptors [117, 118]. In the adsorbed state, the afterglow lasted up to several seconds, and was quenched already by a 103 mm pressure oxygen, whereas with the short-lived emission (<108s) quenching was barely observed at 102mm oxygen pressure.
Thus, excitation mainly led to the metastable state that sensitized oxygen quite efficiently, and for this reason was detected only in carefully deoxygenated solutions.
Gaffron preferred the alternative rationalization that an excited dye molecule reacted with a ground state one and the excited dimer was the effective (long-lived) sensitizer. In studying the photoxidation of rubrene, Gaffron found that the oxy- genation rate was largely independent of oxygen concentration, but increased with a quadratic dependence on the hydrocarbon concentration, according to his postulate [106–108].
Both Gaffron and Kautsky concluded that some long-lived species was involved, but the former scientist attributed the quadratic dependence on the “increasing formation of double molecules” (an excited rubrene molecule + a ground state one) [106–108], while the latter one refused the dimer hypothesis and rather thought that in concentrated solution “a quantum oscillated between two molecules in close vicinity and these did not separate until they found the opportunity to interact with an oxygen molecule” [117,118]. This, thought Gaffron, was changing horse when a difficulty arose [96].
In retrospect, it is clear that photooxidation is a multifaceted phenomenon and different dyes under different conditions follow different mechanisms- and energy transfer to oxygen by the triplet sensitizer, excimers, the lengthening of excited state lifetime when adsorbed all may have a role, as well as hydrogen abstraction from the substrate by the sensitizer reactions followed by trapping of the radical by oxygen and the ensuing chemistry via peroxide radical and the hydroperoxy radical HOO•(initially proposed by Weiss [119]). (Chemical) activation of the substrate and (physical) activation of oxygen came to be known as the type I and type II photooxidation respectively (Scheme4.19).
Scheme 4.19 Different mechanism for photosensitized oxygenation reactions
4.3 Oxygenation Reactions and Singlet Oxygen 93
What Kautsky was proposing, with the physical activation of oxygen indeed applied to many dye-sensitized processes but was not accepted by the large majority of influential scientists of that period, mainly due to the still not recognized role of triplet sensitizers up to the 1930s. Furthermore, this mechanism perfectly suited the oxygen addition to alkenes and dienes developed from 1940 on by Schenk; indeed, these reactions are now felt as perhaps the most typical singlet oxygen reactions. Schenck, however, long resisted the introduction of the triplet energy transfer mechanism and insisted on the chemical activation [120].
Somewhat ironically, the Type II (physical) mechanism was strongly advocated by scientists such as Hammond and Turro, who recognized energy transfer sensi- tization as the most diagnostic test of the role of triplets (in the early 1960s) [121]. However, these scientists mainly studied the photochemistry of ketones, where in the presence of oxygen the short-lived triplets are in part physically quenched by oxygen, but some oxygenation may well occur via the Type I (radical) mechanism due to the efficient hydrogen abstraction from the substrate by the nπ*triplet (and in fact, the two unpaired electrons are orthogonal one to another, while this is not the case for dyes Schenk was using for the sensitized oxygenation of alkenes). Furthermore, the formally analogous addition of radicals to C¼C double bonds sensitized by ketones strengthened the proposal [122,123].
The Kautsky mechanism’s physical activation was a wonderful intuition, per- haps just too early for people to use, as Kasha later commented [124], but exper- imental support by the two-phase experiment and competition in solution was relatively weak. Further experiments based on the same idea, the physical separa- tion of sensitizer and acceptor, were later carried out by other authors, and supported the role of a gaseous intermediates, but the products were not unambig- uously identified, or were not diagnostic for singlet oxygen [125–127], or different rationalizations were put forward, e.g., invoking the role of vibrationally rather than electronically excited oxygen states [128].
However, a really solid demonstration of the singlet oxygen role was reached when Foote demonstrated on one hand that thermally produced singlet oxygen (from the reaction between sodium hypochlorite and hydrogen peroxide, ca. 10 % yield) gave the same product distribution as the photosensitized reaction [129], and on the other one performed a three-phase “Kautsky test” under better defined conditions that eliminated any doubt [130]. In this case, a well-investigated singlet oxygen reaction, such as was in the meantime become the ene reaction [131], was used, and products identification and separation of the reagents were ensured by having both the sensitizer and the alkene covalently bound onto polymers. Under these conditions, the hydroperoxide was formed both under 10 and 25 mmHg oxygen pressure (where the singlet oxygen lifetime was 0.56 and 1.4 s, respec- tively), but not in air (lifetime 0.088 s) or in carbon tetrachloride (lifetime 0.007 s) where1O2was too short-lived for migrating from the sensitizer site where it was formed to the alkene site. Thus, the various mechanisms can be distinguished, although in photobiology it may still be convenient to add up all of the effects as being due to “reactive oxygen species” (ROS).
94 4 Some Paradigmatic Topics