Carbonyl compounds intersystem crossing

Thus we see that in molecules possessing ->- 77 excited states inter-combinational transitions (intersystem crossing, phosphorescence, and non-radiative triplet decay) should be efficient compared to the same processes in aromatic hydrocarbons. This conclusion is consistent with the high phosphorescence efficiencies and low fluorescence efficiencies exhibited by most carbonyl and heterocyclic compounds. 

The first step of the reaction involves the (n, it ) excited state of the carbonyl compound reacting with the ground-state alkene. For aromatic ketones, rapid intersystem crossing from the excited singlet state to the excited triplet state occurs, forming initially a 1,4-biradical and then the oxetane ... 

In contrast to 2-alkylarylcarbenes, triplet carbonyl carbenes do not abstract H from 5- or e-CH bonds. Photolysis of diazo compounds (7) in methanol gave products due to Wolff rearrangement (8) and 0-H insertion (9). Sensitized photolysis led, in addition, to the H-abstraction product (10). Analysis of the results indicated that a large proportion of the insertion product (9) arises from the excited diazo compound and that spin inversion of the triplet carbene is faster than H-abstraction from the solvent. Intersystem crossing to the singlet state is a major reaction of all triplet carbonyl carbenes that are not rapidly scavenged intramolecularly. 

The rate of intersystem crossing is just as important as its efficiency. Obviously, if the rate of intersystem crossing is faster than that of diffusion in solution (usually on the order of 1010 sec"1), bimolecular reactions of the excited singlet are precluded. Unfortunately, the intersystem crossing rates are available for only a few carbonyl compounds.11,12 It is known that the rate of intersystem crossing for aliphatic carbonyl compounds (e.g., acetone) is slow (4-20 x 107 sec-1)30 in comparison to that for aromatic carbonyl compounds. Thus, aliphatic (and perhaps some aromatic) carbonyl compounds have an opportunity to react in the excited singlet state. 

The ,ir singlet may not be quenched to the same extent as the triplet. For some carbonyl compounds, as with acetone with maleic anhydride, where the rate of photocycloaddition is fast enough to compete with intersystem crossing, this state may play an important... 

Many photochemical reactions have been reported for unsaturated ketones294 but, despite the ease of intersystem crossing shown by most carbonyl compounds, it is not at all obvious that all the reactions occur from triplet states. We shall mention those that have been demonstrated to involve triplet excited states. 

As we shall see, n —> tt singlet and triplet states of carbonyl compounds play an important role in photochemistry. Aldehydes and ketones display all the characteristics of absorption, fluorescence, phosphorescence, and intersystem crossing (5, —> T,) illustrated in Figure 28-1. Generally, they are more efficient at intersystem crossing than are unsaturated hydrocarbons, perhaps because the energies of the S and T states involved are not widely different. 

Although carbonyl compounds are generally nonfluorescent because of fast intersystem crossing to generate the phosphorescent lowest n,n triplet excited state [187], irradiation of the absorption band due to Mg2+ complex of 1-naphthaldehyde (1-NA) or 2-naphthaldehyde (2-NA) formed in the presence of Mg(C104)2 causes strong fluorescence at 430-440 nm as shown in a general manner (Scheme 22) [188]. 

Conversion to the lower energy triplet (T,) by spin inversion (intersystem crossing) although formally forbidden this can occur with very high efficiency when the energy difference between the two states is small. It is most notable in carbonyl and aromatic compounds (e.g. intersystem crossing occurs with 100 per cent efficiency in the case of benzophenone). 

For most carbonyl compounds, we expect to have two near-lying excited states in the triplet manifold, which are either n-n or tc-tc in character. The n-7T states frequently show radical-like behavior. Benzophenone is an example of such a molecule which has an n-7t triplet state in which we see occurrences of hydrogen abstraction and very efficient intersystem crossing. When the lowest state is the 71-71 state, largely centered on the aromatic part of the molecule, as in the case of p-methoxyacetophenone, the reactivity decreases significantly (8,9). With the nature of lignin and the nature of the model we have chosen, we are mostly interested in molecules which have this type of behavior. 

Carbonyl compounds usually have intersystem crossing quantum yields, d 1Sc, of very close to unity. In the case of a-guaiacoxyacetoveratrcHie, we have shown that some of the chemistry is coming from the singlet state, which necessarily implies that die intersystem crossing quantum yield is no longer 1. 

In some cases, however, a high degree of stereoselectivity could be obtained even with pure triplet excited carbonyl compounds. In these cases, e.g., the photocycloaddition of benzophenone to several methyl vinyl sulphides 113, the intermediary triplet 1,4-biradical preferentially undergoes one of two possible cyclization modes after intersystem crossing (Sch. 34) [60]. 

Cycloaddition reactions of a,p-unsaturated carbonyl compounds to olefins have been studied in detail. Cyclic enones, undergo rapid and efficient intersystem crossing thereby providing easy access to the triplet state through direct excitation. The following cycloaddition reaction is proposed to involve a diradical intermediate formed directly or from an exciplex [102, 103] (see (29)). 

The introduction of the photochemically excited triplet mechanism leading to CIDEP of the resulting radicals has added a new dimension to the potentials of the CIDEP techniques in photochemistry. In liquid photochemical systems, very little is known experimentally about the exact nature of the intersystem crossing process, but the rate or efficiency of such ISC process can sometimes be estimated by chemical (86) and optical methods (51,105). The treatment of the phototriplet mechanism in CIDEP of radicals in liquid solution is consistent with the following conclusions (1) ISC occurs mainly by the spin-orbit coupling mechanism in carbonyl compounds, (2) spin polarization of the triplet sub-levels is obtained via the selective ISC processes, and (3) the chemical reaction rate of the triplet is at least comparable to its depolarization rate via spin-lattice relaxation. 

In many cases, the reactions of carbonyl compounds are interpreted in terms of the reactivity of the triplet carbonyl compound. However, the work on [123] in which a fluorescent excited charge-transfer complex was detected, and the finding that some amine radical cations react with the radical anions of carbonyl compounds to produce exciplex fluorescence (Zachariasse, 1974) shows that, although intersystem crossing in carbonyl compounds is usually highly efficient, they may participate in excited singlet-state reactions. 

Undoubtedly the most important distinction between (n, Jt ) and (n, n ) excited states of aromatic carbonyl compounds is the much greater efficiency of intersystem crossing (i.e. Si Ti conversion) in the former cases. This follows because, as already noted, triplet states are the longest lived and most useful intermediates in organic photochemistry. Two factors contribute to the high probability of intersystem crossing between (n, n ) states as compared to that between (jr, n ) states ... 

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