Direct photoisomerization

Evidence that eliminates the triplet mechanism as the mode for the cis-trans isomerization of stilbene upon direct photolysis has been provided by azulene quenching studies.(48) Using the experimentally determined decay ratio a/(l — a) and the triplet mechanism, it is possible to calculate what the effect of azulene is upon the pss. The predicted and observed azulene effects on the direct photoisomerization are shown in Figure 9.6. The failure of the triplet mechanism in predicting the very small changes observed in the pss provides a crucial test that is the basis for rejecting the triplet mechanism. 

The only recent example of Forster transfer of photochemical importance is the demonstration by Saltiel163 that the ability of azulene to increase the photostationary transjcis ratio in direct photoisomerization of the stilbenes is due entirely to radiationless transfer of excitation from traw.y-stilbene singlets to azulene. As expected for Forster transfer, this azulene effect did not depend upon solvent viscosity. The experimental value of R0, the critical radius of transfer in Forster s formula,181 was 18 A, in good agreement with the value calculated from the overlap of stilbene emission and azulene absorption. 

The direct photoisomerization of substituted stilbenes has also received attention. Several 4,4 -disubstituted stilbenes in which one substituent is electron withdrawing and the other electron donating, such as 14, have quantum yields for cis -> trans isomerization similar to that of m-stilbene, but exhibit very low quantum yields for trans -> cis isomerization in hydrocarbon solvents and zero quantum yields in ethanol.250 Likewise, certain salts of 4 -amino-2-styrylpyridine, such as 15, do not undergo direct trans -> cis photoisomerization.251 The strong interactions between the ring systems in the ground states of 14 and 15 are probably increased in the excited states. Consequently the planar... 

Azobenzene readily undergoes both sensitized and direct cis-trans photoisomerization. However, the sensitized photostationary cis/trans ratios with high-energy sensitizers do not predict those obtained upon direct irradiation, indicating that much of the direct photoisomerization occurs from states other than the lowest triplet.455 As with stilbene, cyclization of the cis isomer apparently is a singlet-state reaction.456... 

The photoisomerization of stilbene is one of the most extensively studied photoreactions (25). Solvent effects have been thoroughly investigated for both the direct and photosensitized isomerizations, and a model has been developed which attributes these effects to solvent viscosity (26). Increased viscosity inhibits direct photoisomerization of the cis isomer, but facilitates that of trans-stilbene. As a result, the cis/trans ratio of the photostationary state increases with increasing solvent viscosity. The wide range of viscosities which are attainable by pressure manipulation of supercritical carbon dioxide provides an excellent opportunity to probe the effect of viscosity on stilbene photochemistry in the same solvent. 

The direct photoisomerizations are singlet-state reactions, as shown by their independence of oxygen and the addition of triplet quenchers. Ilge et a/.31 103 demonstrated that no potential barriers in the photoisomerizations, EE - EZ, and EZ — ZZ, were found. In contrast, there was a small barrier in the photoisomerization of ZZ - EZ or ZE a veiy weak fluorescence was detected at 77 K, and the fluorescence of the ZZ isomer disappeared at temperatures above 130 K. 

Prior to such a sophisticated attempt, a more straightforward strategy was examined in order to evaluate the chiral discrimination ability of the CDx cavity. Thus the direct photoisomerization at 185 nm of a 1 1 complex of (Z)-cyclooctene 30 with P-CDx was carried out in the solid state to give an E-Z mixture of E Z = 0.47 [122]. The ee of the obtained (E)-isomer 31 was low (0.24 %) [123], but this work paved the way for the supramolecular photosensitization of 30 with chromophore-modified 3-CDx derivatives 53-55 in solution [123,124]. 

Orlandi, G., Siebrand, W., Model for the Direct Photoisomerization of Stilbene, Chem. Phys. Lett. 1975, 30, 352 354. 

Three methods were applied to investigate the effect of these quenchers on direct photoisomerization ... 

Evidence has been obtained for the intermediacy of the triplet state in the direct photoisomerization of thioindigo dyes,228 a long-lived intermediate has been detected in the flash photolysis of 4-thiouridine in aqueous solution,224 and the p values of the lumiflavin triplet state have been determined by flash photolysis.225 ... 

Derivatives of indigo and thioindigo represent a related case. A transoid relaxed triplet geometry has been assigned to a number of them on the basis of triplet-triplet absorption behavior analogous to that of the 4-nitiostilbenes (65,77). In this case, however, O2 decreases for die direct photoisomerization... 

Direct photoisomerization of cyclo-octene gives a trans. cis ratio of 49 51, respectively, in the photostationary state, whereas photostationary trans cis ratios of 5 95 and 13 87, were observed on toluene photosensitization in the liquid and vapour phases, respectively. On irradiation, mixtures of isoprene and either cis-cyclohept- or cis-cyclo-oct-2-enone give the Diels-Alder adduct of the trans-cyclo-alkene similar Diels-Alder adducts were obtained using cyclopentadiene. ... 

Direct irradiation leads to isomerization via singlet state intermediates. " The isomerization presumably involves a twisted singlet state that can be achieved from either the cis or the trans isomer. The temperature dependence of the isomerization further reveals that the process of formation of the twisted state involves a small activation energy. This energy is required for conversion of the initial excited state to the perpendicular geometry associated with the state. Among the pieces of evidence indicating that a triplet intermediate is not involved in direct irradiation experiments is the fact that azulene, which is known to intercept stilbene triplets, has only a minor effect on the efficiency of the direct photoisomerization. 

Nobel Laureate Jean-Marie Lehn has conceptually reduced the light-powered motor to a very simple system the chiral imine (Figure 4) 25). As observed for chiral alkenes, absorption of a photon produces out-of-plane rotation of the tt bond in imines. Molecular asymmetry should cause the 90 rotation to occur with some preference for clockwise or coimterclockwise direction. Photoisomerization of the anti imine to the syn geometry constitutes directional, 180° rotation. At room temperature, the syn isomer would rapidly undergo in-plane inversion to the original anti form. This brilliant simplification of the molecular motor based on photochemical/thermal cycling has not yet been experimentally reduced to practice. 

Inoue, Y., Takamuku, S., and Sakurai, H., An anomalously high transxis photostationary ratio on the direct photoisomerization of cyclo-octene, /. Chem. Soc., Chem. Commun., 423—424, 1976. 

It was shown by sensitization experiments that —>Z and Z—> isomerization can proceed via the triplet state. As the quantum yield for sensitized and direct photoisomerization are different, it was concluded that direct photoisomerization proceeds mainly — if not completely — via singlet excited states. From the observation that < 1 nd from nonlinear Arrhenius plots, it was concluded that more than... 

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