Sensitized isomerization

Note 1. No cyanide was used to remove the copper salts, since the nitrile is probably very base-sensitive (isomerization to a conjugated diene). 

Deviation includes, in fact, the summation of steric and electronic effects, and basicity is somewhat a useful predictor for properties of complex dyes (solvent sensitivity, isomeric forms of trinuclear dyes) and gives also semiquantitative data for color structure relation (atomic)... 

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... 

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

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. 

Thus Saltiel has concluded that the small increase in [T]S/[C], in going from benzene to acetone indicates that a mixed mechanism is operative for acetone-sensitized isomerization, that is, both triplet energy transfer and, to a minor extent, Schenck intermediates are involved. When acetophenone or benzophenone is used as a sensitizer the pss is close to the thermodynamic... 

Fig. 8. Benzophenone-sensitized isomerization of E-pentadiene. UV.-absorption spectra and energy diagramm...

In the initial work135,136 on the sensitized isomerization of stilbene, it was found that quinone sensitizers gave photostationary states which were trans-rich relative to those predicted by energy transfer (Fig. 3). The original plot of sensitizer triplet energy versus isomer ratio of the photostationary state therefore showed several maxima and minima. Later work on the rates of energy transfer from various sensitizers to stilbene proved that the plot was in fact a smooth curve and that the photostationary states observed with quinones were not the true ones.137 Irradiation of benzene solutions of... 

Other systems for which the results cannot be explained by energy transfer include the ketone sensitized isomerization of / -methylstyrene, 1,2-dichloro-ethylene and 2-butene studied by Caldwell,147,148 deuterium exchange between acetone-d8 and tetramethylethylene reported by Japar, Pomerantz, and Abrahamson,149 and the acetone sensitized isomerization of 1-methoxy-l-butene examined by Turro and Wriede.71... 

A related phenomenon has been observed in the benzophenone sensitized isomerization of c/y-piperylene.150 The measured quantum yield of cis to trans isomerization increased from 0.55 to 0.90 as the concentration of piperylene increased from 0.08 to lOAf. This observation can be rationalized as arising from addition of the piperylene triplet to a ground state diene molecule to give a biradical intermediate which can either cyclize to the dimer151 or dissociate to give two molecules of the more thermodynamically stable trans-isomer. This mechanism predicts that the quantum yield for the isomerization of /runs-piperylene to cw-piperylene should decrease with increasing diene concentration, an experiment that has not yet been reported. 

A particularly interesting case of transfer from T2 has been noted238 in the 9,10-dichloroanthracene (DCA) sensitized isomerization of stilbene. As shown below, the DCA ground state which results from quenching of T2 by stilbene (St) can act as a quencher of stilbene triplets and thereby alter the stilbene cis. trans photostationary state. 

It has been assumed so far that the sensitizer acts by an energy-transfer mechanism, but in some cases other modes of interaction may occur. It is possible that electron transfer takes place to give the radical anion or the radical cation of the alkene, which is the species that subsequently isomerizes. This is likely to be the case in the chlorophyll-sensitized isomerization of vitamin A acetate, which is used commercially to obtain the required all-trans isomer 12.8) from the mixture of Isomers resulting from the synthesis. Unlike triplet-sensitized reactions, electron-transfer isomerizations frequently lead to a predominance of the most thermodynamically stable isomer. 

Triplet-sensitized isomerization can also be achieved, in which case the relative values of triplet energies are the most important factor. 

Many studies of benzene-sensitized isomerizations have been reported. One interesting example is provided by the behavior of cis,trans,trans-1,5,9-cyclododecatriene. With benzene photosensitization it is isomerized to the M-trans and the cis,cis,trans isomers, and a new, probably bicyclic, product is also formed in small yield.274 Another study employed a large number of sensitizers, many of which apparently reacted with the triene, and no particular trend was evident other than that the original isomer remained as the major geometric isomer in the photostationary state, with varying amounts of the all-trans and cis,cis,trans isomers being formed with different sensitizers.275... 

Cundall has done extensive work on benzene231,237 and acetone243 sensitized isomerizations of the 2-butenes, and in every case reported a photostationary or radiostationary trans/cis ratio of 1.27-1.37. Sato, however, has measured a value of unity for the benzene photosensitized isomerization.510 With higher homologs, from 2-pentene to 2-octene, benzene-sensitized isomerizations yield trans/cis ratios of 1.0,238 while acetone-sensitized isomerization of the 2-pentenes in solution yields a ratio of 1.65.244 At present no explanation is possible for the differences between 2-butene and 2-pentene. Until much more information is gathered relating to rates of triplet energy transfer as functions of olefin structure, sensitizer, and medium, the natural decay ratios of each olefin s common triplet cannot be deduced from photostationary trans/cis ratios. 

Yang proposed an alternative mechanism for sensitized isomerization of olefins by low-energy donors.25 He suggested that sensitizer (a ketone) and olefin form a a bond to give an intermediate biradical as in Equation 13.24. The intermediate can rotate about the original G—C double bond before decomposing... 

The actions of photoexcited semiconductor particles on organic compounds under oxygen is of significant importance from both practical and basic aspects. Semiconductors like titanium dioxide and cadmium sulfide were shown to induce oxidation of olefins and aromatic hydrocarbons under oxygen, and also to sensitize isomerization of unsaturated systems. The mechanisms of these reactions are discussed. 

In non-viscous organic solvents the fluorescence efficiency of most SNA derivatives is comparable to that of TS as are the yields of trans to ois photoisomerization (33,3A). The double chain derivatives, MSNA and MSN, show substantially higher fluorescence yields (0.2 0.02) in non-viscous solvents with a corresponding decrease in the trans to ois isomerization efficiency (3A). In the sensitized isomerization, the MSN derivatives investigated thus far show only photoisomerization but there is an increase in the triplet lifetime by a factor of 2 however there is no change in the decay ratio of the isomerization precursor and the sum of the benzophenone sensitized isomerization efficiencies of ASA is approximately unity. When water insoluble SNA or MSNA molecules are spread as a film, either pure or in mixtures with insoluble fatty acids, at the air-water interface, it is found that the films show comparable behavior on compression to those of pure fatty acid with an indicated area per molecule of the surfactant stilbenes very close to that of a linear fatty acid (oa. 202 /molecule) (35). This suggests strongly that the TS chromophore in these molecules offers suitable molecular dimensions to pack into a crystalline-like array of linear parafin chains. 

The fact that the intraligand transition is lowest in energy in ClRe(CO)3-(fnms-4-styrylpyridine)2 also allows a direct assessment of the effect of the coordination on the decay properties of the intraligand excited state. Data for both the photoisomerization by direct irradiation, Table 19, and the triplet sensitized isomerization, Table 20, of ClRe(CO)3(fra s-4-styrylpyridine)2 and frans-styrylpyridine... 

Table 20. Triplet sensitized isomerization of f/vwis-styrylpyridines and XRe(CO)3 (frans-sty rylpyridine)21)...

The study of the direct irradiation and triplet sensitized isomerization of styryl-ferrocene, reaction (46), is noteworthy.134) For either direct or benzophenone sensitized photoisomerization the photostationary state is exclusively the trans isomer. 

The 1,4-bifunctional intermediate is implicated further by the electron transfer sensitized isomerization of cis- to Irons-1,2-diphenoxylcyclobutane [115], and by the unique CIDNP effects observed during the electron transfer sensitized cleavage of the anti-head-to-head dimer of 3,3-dimethylindene (vide infra) [121]. 

The chloranil sensitized isomerization of geminal diarylspiropentane systems (55) gives rise to a pair of isomeric methylenecyclobutanes (56, 57) [240]. When carried out in the presence of molecular oxygen, this reaction gives rise to two oxygenation products (58, 59). All four products are compatible with a single bifunctional intermediate however, the available mechanistic data indicate a considerably more complex pathway. 

Re-examination of the benzene-sensitized isomerization of alkenes revealed that the photostationary EjZ ratio critically depends on the structure of alkenes and the triplet energy of sensitizers [69]. For instance, thermal E-Z equilibrations of 2-octene 19c and 3,4-dimethyl-2-pentene 20 give the thermodynamic EjZ ratios of 3.3 and 3.5, respectively, while the photostationary E Z ratios upon benzene sensitization are 1.1 and 1.6, respectively (Schs. 9 and 10). For practical runs, the use of /7-xylene or phenol, rather than benzene, in ether is recommended, as the yellowing of the irradiated solution, which retards the photoreaction, is substantially decreased. 

In the photoisomerization of l-(2-anthryl)-2-phenylethene [232-234] and l-(2-naphthyl)-2-phenylethene [235-237] 95, the s-cis/s-trans conformational isomerism plays an important role. (Z)-l-(2-Naphthyl)-2-phenyl-ethene is isolated in 70% yield in fluorenone-sensitized isomerization of the ( )-isomer (Sch. 39) [238,239]. The photochemical behavior of related (E)-stilbenoids (96-98) has been reported (Sch. 40) [240]. The direct and sensitized photoisomerizations of (Z)-styrylferrocene in benzene have also been reported [241]. 

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