Isomerization quantum yield

Fig. 3. Photochemical and thermal isomerization products of vitamin D manufacture (49). The quantum yields of the reactions ate hsted beside the arrows...

The cyclization product is thermally unstable relative to Z-stilbene and reverts to starting material unless trapped by an oxidizing agent. The extent of eyclization is solvent-dependent, with nonpolar solvents favoring cyclization more than polar ones. ° Whereas the quantum yield for Z-E isomerization is nearly constant at about 35%, the cyclization... 

Since the quantum yield of the Norrish type I reaction is generally low, it has been assumed that the initial homolytic cleavage is a reversible process. Evidence came from an investigation by Barltrop et al. which has shown that erythro-2,3-dimethylcyclohexanone 12 isomerizes to t/zreo-2,3-dimethylcyclohexanone 13 upon irradiation ... 

The other commonly quoted industrial photochemical process is the production of vitamin D3 involving a photochemical electrocyclic ring opening followed by a thermal 1,7-hydride shift (Scheme 7.2). This is a further example of a successful low quantum yield process in this case there is no viable thermal alternative. Vitamin A acetate has also been produced commercially using a photochemical isomerization process to convert a mixed tetra-alkene precursor to the all-trans form. 

Recently, a photoisomerization reaction of azoferrocene was found to proceed in polar solvents such as benzonitrile and DMSO through both a 7t it transition of the azo-group with a UV light (365 nm) and the MLCT transition with a green light (546 nm) (Fig. 6) (Scheme 1) (153). The quantum yields of the photo-isomerization reaction at 365 nm and 546 nm were estimated to be 0.002 and 0.03, respectively. The transformation into the cis form causes the higher field shift of Cp protons in the 1H-NMR spectrum and an appearance of u(N = N) at 1552 cm-1. The cis form is greatly stabilized in polar media, and dilution of the polar solution of cis-25 with less polar solvents resulted in a prompt recovery of the trans form. 

However, bond b must cleave for both the cis and treats isomers since both produce the ring-opened product 3,4-diphenylcyclohexenone although the quantum yields for this process are considerably lower than those for isomerization by cleavages at bond a. Finally, the possibility of cleavage at bond c cannot be ruled out by these results. We will return to this possibility later in this section. 

A method for the determination of intersystem crossing quantum yields involving triplet-sensitized isomerization has been developed by Lamola and Hammond.<55) In this method the compound whose intersystem crossing... 

Tabu 6.6. Isomerization Quantum Yield Data for Compounds (7), n = 1-4... 

Table 9.1. Quantum Yields for Sensitized Stilbene Isomerization...

Table 9.3. Quantum Yields of Sensitized Isomerization of 2,4-Hexadienes

Irradiation of the cis isomer results in the isomerization with quantum yield, equal to 0.56. The fact that the quantum yields of cis and trans formation are similar and add to unity suggests the presence of a common intermediate, thought to be a twisted triplet state. Gas-phase photolysis of trans-... 

The quantum yield for isomerization in the direct photolysis was found to be d> = 0.94. The reaction could also be sensitized with acetophenone ( = 1.02) and quenched with piperylene, indicating a reactive triplet species with a rate constant kr of 3 x 10l°sec-1. With a 3-(p-methoxyphenyl) derivative two products were obtained<81) ... 

Quenching rate constants for dienes and quadricyclenes have similar sensitivities to the electronic and structural features of the excited aromatic hydrocarbon. However, during this process quadricyclene isomerizes to nor-boraadiene with a quantum yield of 0.52, whereas dienes usually remain unchanged/10 Hammond has suggested that vibrational energy which is partitioned to the acceptor upon internal conversion of the exciplex can lead to isomerization(10a,103) ... 

An interesting gas-phase study of the photolysis of ftmy-l-phenyl-2-butene has been published by Comtet.(39,40) He has found that (a) it was not possible to quench the formation of the cyclopropane product under conditions that reduced the fluorescence quantum yield, (b) sensitization by acetophenone only gave cis-trans isomerization, and (c) the quantum yield of cyclopropane formation in the direct photolysis decreases as n-butane is added to the reaction mixture. Comtet suggests that the data are consistent with a reaction from the second triplet state. 

The initial quantum yields for cis- to tam-stilbene isomerization (O0 T) and for trans to cis isomerization (4>T-.C) are consistent with Hammond s postulate that isomerization takes place from a common state, most likely the twisted or phantom triplet state ... 

If fluorescence and cis-trans isomerization (9.26)-(9.29) are the main competing reactions upon direct excitation, then inhibition of rotation about the central bond should produce an increase in the fluorescence quantum yield. The rigid systems (3) and (4) both have fluorescence quantum yields of 1.0 at room temperature.<44,52) While the fluorescence of /rmy-stilbene is a... 

The photostationary state composition for the benzophenone-sensitized isomerization of 2,4-hexadienes is given in Table 9.2. Table 9.3 gives the measured quantum yields for benzophenone-sensitized isomerization of 2,4-hexadienes along with the calculated quantum yields based on Eqs. (9.47)-(9.49) and the pss values given in Table 9.2. 

Photo-isomerism is also found in this group of compounds. Cis-Pt(NH3)2(H20)l+ photo-isomerizes to trans with a quantum yield of about 0.1 at 363 nm (40). Photoisomerism of Pt (glycine) 2 from cis to trans, but not its reverse is also reported. 

In addition, the results indicated that the efficiency of cis —> trans increased as the initial cis double bond configuration is shifted from the center of the polyenic chain, consistent with the 7j, triplet excited state potential curve that has a very shallow minimum at the 15-cis position compared to the deep minima at the all-trans position. The results strongly suggest that isomerization takes place via the 7j state of the carotenoid even in the case of direct photoexcitation, with their photosensitized process because of the very low intersystem crossing quantum yield, isc ([Pg.246]

Comparison of Photosensitized and Direct Photolysis Isomerization Quantum Yields (iso) of AH-trans and several c/s-lsomers of (3-Carotene in n-Hexane... 

Kuki, M., Y. Koyama, and H. Nagae. 1991. Triplet-sensitized and thermal isomerization of all-frans, 7-cis, 9-cis, 13-cis, and 15-cis isomers of (3-carotene Configurational dependence of the quantum yield of isomerization via the T1 state. J. Phys. Chem., 95 7171-7180. 

Alkenals also add to simple olefins to yield oxetanes, Eq. 43. Using acetaldehyde and the isomeric 2-butenes the quantum yield is low, and... 

A study of the photochemistry of 2-trimethylstanny 1-1,3-butadiene (10 in Scheme 1) shows that only isomerization is observed, with no Sn—C cleavage or polymerization26. Three different isomers are formed in approximately equal proportions, as shown in Scheme 1. In the case of the formation of 3-trimethylstanny 1-1,2-butadiene (11), an intermediate is observed, but not characterized. The intermediate subsequently either reforms 10, or forms 11. The quantum yield of the reaction shows a strong dependence on the concentration of 10. At [10] = 10 2 moll-1,... 

The effects of nitro substituents on the cis-trans isomerization of stilbenes has been reviewed70 (equation 63). The trans-to-cis isomerization occurs from a triplet excited state, whereas the reverse cis-to-trans isomerization occurs through a main route which bypasses the triplet state. A nitro substituent usually causes a significant enhancement of the quantum yield of the intersystem crossing. Nitro substituent effects on the photoisomerization of trans-styrylnaphthalene71 (equation 64), trans-azobenzenes72 and 4-nitrodiphenylazomethines73 (equation 65) have been studied for their mechanisms. 

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