Stilbenes, isomerization

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

Using the suggested mechanism for sensitized stilbene isomerization, show that... 

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 ... 

Fig. 9 Excess acidity plot against X for some A-SE2 reactions at 25°C a-methylstyrene hydration (open circles) 120,182 phenylacetylene hydration (open and closed squares) 118,160,183,184 cw-stilbene isomerization (closed circles) 120,185 and tritiated benzene T-exchange (open and closed triangles).117,147,186,187...

A Stern-Volmer plot obtained in the presence of donors for the stilbene isomerization has both curved and linear components. Two minimal mechanistic schemes were proposed to explain this unforeseen complexity they differ as to whether the adsorption of the quencher on the surface competes with that of the reactant or whether each species has a preferred site and is adsorbed independently. In either mechanism, quenching of a surface adsorbed radical cation by a quencher in solution is required In an analogous study on ZnS with simple alkenes, high turnover numbers were observed at active sites where trapped holes derived from surface states (sulfur radicals from zinc vacancies or interstitial sulfur) play a decisive role... 

Platinum porphyrin complexes can be prepared by reaction with PtCl2(PhCN)2. Purification of the final complex is by medium pressure liquid chromatography on alumina. The strongly phosphorescent platinum(II) porphyrin complexes are efficient sensitizers for stilbene isomerization. The quantum yields for the cis to trans process are greater than unity because of a quantum chain process in which the metalloporphyrin serves both as an energy donor and an acceptor.1110 Picosecond laser spectroscopy has been used to obtain time-resolved excited-state spectra of platinum octaethylporphyrin complexes, and to probe the excited-state energy levels.1111 Tetrabenzoporphyrin complexes have been prepared for platinum in both the divalent and tetravalent oxidation states. The divalent complex shows strong phosphorescence at 745 nm.1112... 

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. 

More recently, however, Saltiel has shown that direct irradiation of the stilbenes produces isomerizing excited states that are quenched by azulene via singlet-singlet dipole-dipole interactions (see p. 693). Thus it now seems more likely that isomerization of unsubstituted stilbene, following direct excitation, takes place from the Sx states.54 />-Bromostilbene, which undergoes ISC more readily than stilbene, isomerizes from both its S and its T1 states.55... 

Saltiel, J. and D Agostino, J. (1972) Separation of viscosity aand temperature effects on the singlet pathway to stilbene isomerization, J. Am. Chem. Soc. 94 6445-6456. 

A Stem-Volmer plot for photocatalytic stilbene isomerization also has both curved and linear components [183]. This requires at least two separate mechanisms... 

Fig. 14.8 The dependence of the activation energy of the frani-stilbene isomerization reaction on solvent (w-hexane) viscosity, t], and polarity expressed in terms of the refractive index n as shown. The input for this graph is obtained from rate, viscosity, and refraction index data over a wide range of temperature and pressure. (From J. Schroeder, J. Troe, and P. Vohringer, Chem. Phys. Lett. 203, 255 (1993)).

In applying ultrafast spectroscopy to kinetic measurements as done in the excited Zrany-stilbene isomerization, do we really look at thermal reaction rates The alternative is that the reaction proceeds before complete vibrational... 

Arylalkenes, such as stilbene derivatives, are important model compounds for the study of the E Z photoisomerization.529 559 560 The compounds absorb significantly over 250 nm, therefore direct irradiation is technically simple. Photolysis of unsubstituted stilbene in aliphatic hydrocarbons at 313 nm affords a photostationary state (PSS) consisting of 93% of (Z)-stilbene and 7% of ( )-stilbene (Scheme 6.4).112 In addition, a photoinduced 67t-electrocyclic (Section 6.1.2) formation of dihydrophenanthrene with a quantum yield of < > 0.10 competes with (Z)-stilbene isomerization ([Pg.232]

It has been deduced that the dimensionality of stilbene isomerization may not be one-dimensional, as commonly assumed, from an examination of the kinetics of isomerization of the meta-methyl substituted derivative of stilbene in decane as a function of... 

Whitten and co-workers [230] have amply investigated stilbene isomerization induced by excited porphyrins. Electron-deficient nitrostilbenes quench excited Zn or Mg etioporphyrins through exciplex formation. Light absorbed by the porphyrin affects isomerization of 4-nitro- or 4,4 -dinitro-stilbene with low quantum yields ((%trans)sens = 99.5), possibly via a stilbene exchange chain mechanism [230]. Azastilbenes coordinate to me-talloporphyrins in both the ground and the excited state (but do not quench it), and isomerize efficiently upon selective excitation of the porphyrin [231]. Complexed 1,4 -NPE gives d , = 6.6, = 0.2, and (%trans)sens = 96,... 

Stilbene isomerization. The (Z)- i (E)-Selective Wittig reactions. Tl Ph2S2 under visible light to maximize th Oxidative sulfenylation of fluorim the amines into sulfenylated imines, whi reagents and enolates. 

Park N S and Waldeck D H 1990 On the dimensionality of stilbene isomerization Chem. Phys. Lett. 168 379-84... 

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