Quantum yields for triplet formation

Capitano (18) has shown that the absorption band for TX around 380 nm corresponds to the lowest lying — x electronic transition. Herkstroeter, Lamola and Hammond (79) calculated the triplet energy for TX to be 65.5 kcal/mol. Amirzadeh and Schnabel (20) estimated the quantum yield for triplet formation of TX to be 0.85 and determined the rate constants for a number of the photochemical reactions that take place during the photocuring reaction. We have outlined a reaction scheme for TX based on our studies (Figure 2) which includes several pathways that occur during the curing reaction. 

At least four applications of this technique can be cited. Quantum yields for triplet formation in benzene108 and fluorobenzene109 have been estimated by comparing the phosphorescence yields of biacetyl produced by sensitization to that produced by direct irradiation. Intersystem crossing yields of a number of organic molecules in solution have been obtained by measuring the quantum yield with which they photosensitize the cis-trans isomerization of piperylene (1,3-pentadiene) and other olefins.110 As will be discussed later, the triplet states of... 

Riboflavin (2) and some related flavins are pigments of potential importance for aquatic photochemistry that exist at low concentrations in natural waters. These compounds have broad, moderately strong absorption bands in the visible region at about 440 nm and show high quantum yields for triplet formation (Heelis, 1982). 

The photoreduction of cyclobutanone, cyclopentanone, and cyclohexanone by tri-n-butyl tin hydride was reported by Turro and McDaniel.<83c> Quantum yields for the formation of the corresponding alcohols were 0.01, 0.31, and 0.82, respectively. Although the results for cyclopentanone and cyclohexanone quenching were not clear-cut (deviations from linearity of the Stem-Volmer plots were noted at quencher concentrations >0.6 M), all three ketone photoreductions were quenched by 1,3-pentadiene, again indicating that triplets are involved in the photoreduction. 

If et is not known, it is possible to obtain more accurate values of d>lsc than above by using a lower flash intensity such that all the molecules are not excited during the flash (70 20-100 J). For this method the intensity of the light absorbed Ia must be accurately determined from the absorption spectrum and the incident light intensity 70 determined by actinometry. The concentration of triplet molecules [A ] can be determined from A[A ] as above. Since Ia and [A ] are smaller than in the previous case, errors due to the underlying T0 -> Tx absorption are reduced. The quantum yield of triplet formation is now... 

The quantum yields for oxetane formation have not been determined in every case, and only a few relative rate constants are known. The reactivities of singlet and triplet states of alkyl ketones are very nearly equal in attack on electron rich olefins. 72> However, acetone singlets are about an order of magnitude more reactive in nucleophilic attack on electron-deficient olefins. 61 > Oxetane formation is competitive with a-cleavage, hydrogen abstraction and energy-transfer reactions 60 64> so the absolute rates must be reasonably high. Aryl aldehydes and ketones add to olefins with lower quantum yields, 66> and 3n-n states are particularly unreactive. 76>... 

Preparative photolysis of AETSAPPE (0.25 M aqueous solution) at 254 nm (Rayonet reactor) resulted in the formation of the disulfide product 2-amino(2-hydroxy-3-(phenyl ether) propyl) ether disulfide (AHPEPED) as the primary photoproduct Photolysis of AETSAPPE at 254 nm (isolated line of medium pressure mercury lamp) resulted in rapid initial loss of starting material accompanied by formation (analyzed by HPLC) of AHPEPED (Figure 12a and 12b) (Scheme IV). Similar results were obtained for photolysis- at 280 nm. Quantum yields for disappearance of AETSAPPE and formation of AHPEPED at 254 nm and 280 nm are given in Table I. The photolytic decomposition of AETSAPPE in water was also accomplished by sensitization ( x =366 nm) with (4-benzoylbenzyl) trimethylammonium chloride (BTC), a water soluble benzophenone type triplet sensitizer. The quantum yield for the sensitized disappearance (Table I) is comparable to the results for direct photolysis (unfortunately, due to experimental complications we did not measure the quantum yield for AHPEPED formation). These results indicate that direct photolysis of AETSAPPE probably proceeds from a triplet state. 

Data obtained from the photoreduction of benzophenone in isopropanol indicates that the quantum yield (for the formation of acetone) is nearly constant using irradiation at several different wavelengths between 366 and 254 mp..16 The quantum yield for phosphorescence emission from the carbonyl n,ir triplet state is independent of exciting wavelength.22 Furthermore, the singlet excitation spectrum for the phosphorescence emission has been found to be very similar (if not identical) to the absorption spectrum.23 There are other examples where irradiation in the region of a given transition has ultimately led to a triplet of a different type.24... 

The quantum yield for product formation from the sensitizer triplet, 0, is given by the expression ... 

The limiting < ao, for large [A] is obtained as the intercept in Fig. 11.4. The value 0.6 adds up neatly with R — 0.4 to give a total of unity suggesting no loss of A by internal conversion. Thus this limiting value may be identified with the quantum yield of triplet formation ... 

From the photolysis of cyclopentanone in the presence of O218 an upper limit of 0.03 has been estimated for the quantum yield for the formation of cyclopentanone-018 (33). This may be compared with the quantum yield of 0.45 for acetone-O18 formation under identical conditions (39). It has been suggested (but not established) that the reaction between acetone and 021S takes place with the triplet state of the molecule. On this basis, the low efficiency of the corresponding reaction in cyclopentanone has been interpreted to favor the idea that a triplet excited state is not important in the photochemistry of cyclopentanone. 

Cycloaddition is a singlet state reaction, triplet quenching yielding only stilbene isomerization. In the limit of high t-1 concentration, the quantum yield for the formation of 89 and 90 is 0.66 and no c-1 is formed. Nonradiative exciplex decay is proposed to occur by partitioning at the pericyclic minimum (Fig. 2) between products and reactants. In the limit of high c-1 concentration, 91 is formed with a quantum yield of 0.05 and the predominant exciplex decay pathway is dissociation to yield f-c, which decays to a mixture of t-1 and c-1. 

In the course of irradiation of acetophenone in the presence of 1-phenylethanol, the actual quantum yields for pinacol formation do not exceed 50%, but rise to 71% when PhCH(OD)Me is used for photoreduction of acetophenone in acetonitrile683,684. A conclusion has been reached from this inverse DIE that half the reaction of triplet acetophenone with 1-phenylethanol involves abstraction of an OH hydrogen followed by disproportionation of the initial radical pair back to reactants. A transfer of an O-bonded hydrogen to a triplet ketone is taking place (equation 318) besides the abstraction of hydrogen from... 

These molecules have a high quantum yield for the formation of their triplet state and usually rapidly dissociate into free radicals in the triplet state undergoing... 

The formation of triplets via excimers is still an area which is little explored. Medinger and Wilkinson (1966) have shown that pyrene excimer formation leads to a decrease in the quantum yield of triplet formation. A similar result has been obtained for benz(a)anthracene (Heinzelmann and Labhart, 1969). On the other hand, benzene excimer formation leads to an increased triplet yield (Cundall et al., 1972 Hentz and Thibault, 1973). There has been a recent theoretical study of intersystem crossing in molecular pairs (Bowman and Norris, 1978). 

Quantum yields for the formation of stable triplet metal-to-ligand charge transfer (MLCT) and/or ixu excited states are very high, usually almost unity. 

The formation of product C, on the other hand, proceeds in two steps with the efficiencies 1 r/isc. (Equation 3.32) and 3rjc 3/cr/(3/cr I 3/qsc I 3/cph + 3kqcq) where 3kr refers to the reaction from the lowest triplet state Ti leading to the photoproduct C, 3/cph is the rate constant of phosphorescence, 3/cisc that of intersystem crossing to the ground state and 3/cq is the second-order rate constant for quenching of I) by a quencher q. The overall quantum yield for the formation of product C is equal to the product of the efficiencies of the two reaction steps, (I visc Vc- Many authors do not distinguish between... 

Photoreduction of aliphatic ketones may involve both singlet and triplet excited states, but the quantum yield for product formation via singlet is usually low because other competing processes, such as radical pair recombination, are involved. The rapid intersystem crossing (ISC) in aryl ketones (Section 2.1.6) allows triplet reactivity. Ketones with n,7t lowest triplets, having an unpaired electron localized in an n-orbital on oxygen, are far more reactive than those with k.k lowest triplets.863... 

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