Photoreactivity of n,π* Excited States of Alkyl Ketones

ion of a hydrogen from the solvent are often observed (vide infra). Having discussed the energies and primary photophysical processes of the $1 and T1 states of alkyl ketones, we can now construct an energy diagram as shown for acetone in Figure 3. We shall assume that acetone is a good model for the photophysical behavior of other aliphatic ketones. The important features of this diagram are: (a) the $1 (n,~r*) state of acetone relatively low in energy compared to other simple chromophores; (b) the Tx (n,~r*) state of acetone is relatively high in energy compared to most chromophores (only simple benzene derivatives possess a higher triplet energy); (c) the lifetime of 31 is long enough to allow participation in bimolecular processes of low activation energy. Because of their high triplet energy, aliphatic ketones are commonly used as photosensitizers for unsaturated compounds possessing low absorption above 2800 QVANTUM YIELDS AND PtEAiCTIVITIE$ The quantum yield (~b) for formation of a photoproduct, a measure of the efficiency with which the photoreaction proceeds, is by definition equal to the number of molecules of photoproduct formed per photon of light absorbed. It can also be viewed as the probability that the initially generated electronically excited state will yield the given photoproduct. Take, for example, a unimolecular photochemical isomerization of A to B occurring www.annualreviews.org/aronline Annual Reviews A nn u. R ev . P hy s. C he m . 1 97 0. 21 :4 99 -5 60 . D ow nl oa de d fr om a rj ou rn al s. an nu al re vi ew s. or g by C ol um bi a U ni ve rs ity o n 03 /0 6/ 09 . F or p er so na l u se o nl y. PHOTOREACTIVITY OF n,,r* EXCITED STATES 505 exclusively from the Tt state of an alkyl ketone (sA). If every photon absorbed is assumed to result in the formation of the St state of A (1A), the quantum yield for formation of B ($B°) will be the product of the probability of 1A leading to 3A times the probability of 3A proceeding on to B. ~B° will then be given by Equation 5 where kr is the unimolecular rate constant for the photolsomerizatlon of 3A to B. ~B° = = dpsTkrrT 5. ~ + kS T + k~ kr + ka + k The reactivity of an electronically excited state toward a primary photochemical process is defined as the rate constant for that process. Thus for the photoisomerizafion of A to B, kr is a measure of the reactivity of aA toward the photoisomerization (or the primary photoprocess which proceeds the isomerization). The quantum yield for a photoreaction, which depends upon the relative rates of the various competing processes occurring from the electronically excited states (Equation 5), is not in general a good measure of either the absolute or the relative photoreactivity of excited states (27-29). The use of energy transfer quenching (30) has become an extremely popular technique for estimating the reactivity of excited states toward various primary photochemical processes. In this method the yield of product formation is decreased by the addition of a compound which is ~nown to quench a specific excited state by electronic energy transfer. For example, the quantum yield for the triplet state isomerization of A to B will be lowered by the introduction of the additional pathway for deactivation of *A shown in Equation 6, resulting in the expression for $~ given in Equation 7. Division of $~0 (Equation 5) by $~ (Equation 7) yields