Twisting photoisomerization

Reviews of the hula-twist photoisomerization of dienes have been published. New calculations dealing with the isomerism of -cw-butadiene have been reported.Molecular dynamic simulations of the photochemical behaviour of butadiene have shown that cis-trans isomerism occurs. The regioselectivity observed in the photochemical isomerism of trans,trans-l-fluorohexa-2,4-diene is proposed as due to electrostatic control. Direct irradiation of the fluorinated dienes (102) results in preferential conversion to the Z,3E isomers (103). DCA-sensitized irradiation of the butatrienes (104) in various alcohols afford the adducts (105). The simpler alcohols give moderate yields of product, while -butanol affords only a trace of the adduct. ... 

SCHEME 7 Proposed conformer specific Hula-Twist photoisomerization of Pre in a rigid medium. 

An X-ray crystal structure determination of calciferol (vitamin D-2,71) showed that steric crowding in the s-cis diene system resulted in a twisted conformation with a dihedral angle of 53° [59], On irradiation with a mercury lamp, it was partially converted into ergosterol (72) and tachysterol (73) [60, 61]. When a solution of calciferol in light petroleum containing a trace of iodine was exposed to diffuse daylight, the vitamin was photoisomerized to (74) [62],... 

Cholecalciferol (vitamin D-3) differs from calciferol only in the alkyl side-chain, so it was assumed to be in the twisted conformation (75a). In alcoholic solution, vitamin D-3 was irradiated with a mercury arc lamp through a cupric sulphate solution filter to give wavelengths above 250 nm. Six products were isolated. Conformation (75a) could reasonably give rise to the assigned structures (76a), (77a) and (78a) (Scheme 2.3). Photoisomerization could give conformation (75b), which would explain the isolation of (76b), (77b) and (78b). The report is confident on four of the new compounds, but notes that the cyclobutene structures (78a) and (78b) are tentatively assigned [63]. 

Intriguingly, the conical intersection model also suggests that E,Z-isomerization of acyclic dienes might be accompanied by conformational interconversion about the central bond, reminiscent of the so-called Hula-Twist mechanism for the efficient ,Z-photo-isomerization of the visual pigment rhodopsin in its rigid, natural protein environment101. A study of the photochemistry of deuterium-labelled 2,3-dimethyl-l,3-butadiene (23-d2) in low temperature matrices (vide infra) found no evidence for such a mechanism in aliphatic diene E,Z -photoisomerizations102. On the other hand, Fuss and coworkers have recently reported results consistent with the operation of this mechanism in the E,Z-photoisomerization of previtamin D3 (vide infra)103. 

The E,Z-photoisomerization of previtamin D to tachysterol has also received recent attention. Jacobs and coworkers examined the process in various solvents at 92 K and found evidence for the formation of a triene intermediate which converts thermally (Ea ca 6.5 kcal mol 1) to the more stable tEc rotamer of tachysterol (tEc-T equation 58)230. The rate of this conversion is viscosity dependent. They identified this intermediate as the cEc rotamer, produced by selective excitation of the cZc rotamer of previtamin D. In a re-examination of the low temperature ,Z-photoisomerization of previtamin D as a function of excitation wavelength, Fuss and coworkers have suggested an alternative mechanism, in which tEc-1 is produced directly from cZc-P and cEc-T directly from tZc-P (equation 59)103. This mechanism involves isomerization about both the central double bond and one of its associated single bonds—the hula-twist mechanism of Liu and Browne101 — and involves a smaller volume change than the conventional mechanism for ,Z-isomerization. The vitamin D system has also been the subject of recent theoretical study by Bemardi, Robb and Olivucci and their co workers232. 

The excited-state behavior of 1,1,2,2-tetraphenylethene (TPE) has been studied by means of picosecond fluorescence, absorption, and Raman spectroscopies and picosecond optical calorimetry. It has been shown that, like stilbene, TPE derivatives substituted with minimally perturbing stereochemical labels such as methyl groups undergo efficient photoisomerization. However, unlike stilbene, strong spectroscopic evidence exists for the direct detection of the twisted excited singlet state, 5ip herein but traditionally designated as of TPE. 

The stilbenes absorb light strongly, and their excited singlets cross over to triplets fairly efficiently, from which state photoisomerization occurs. The lowest energy T - S0 absorption bands measured for cis-and rnm-stilbene are at 57 and 48-50 kcal, respectively. However, it is quite likely again that both spectroscopically formed triplets relax very rapidly to a common twisted triplet which decays to both geometric isomers. 

When the twisting power of the closed-ring isomer is larger than that of the openring isomer, it is expected that UV irradiation should induce the phase change from the nematic to chiral nematic phases.1481 A diarylethene 31a, with two diarylethene units in a chiral cyclohexane, was incorporated into K15 and the phase change concomitant with photoisomerization was measured. 

The photoisomerization of several copolymers was studied, in order to determine the effects of the structure and switching of the chiral side chain on the helicity of the main chain. A delicate balance of parameters was found, including separation and nature of the stereocenters, solvent, and concentration of azobenzene moieties J77 Stereoselectivity was often greatly enhanced if the chiral moieties were closer to each other. Accordingly, it was found that the incorporation of the stereocenter into a short, two-carbon spacer resulted in much more pronounced helical preference, as well as CD effects at lower chiral chromophore concentrations. The greater helical twist and improved thermal stability of the cis form (half-life 40h at RT) are notable features. 771 It was also found that the relationship between the trans-cis... 

Despite great interest in azobenzene photophysics, the basic photoisomerization mechanism remains disputed [173] in contrast to the expectations of Kasha s rule, the isomerization quantum yield decreases rather than increases with increasing photon energy. In Fig. 22, the two possible isomerization channels, proceeding via either a planar pathway (inversion) or a nonplanar, twisted pathway (torsion) are shown. Previous studies determined that isomerization in the first excited state S1 state proceeds along the inversion coordinate [171]. The second excited state is generally thought to be... 

The photoisomerization of all-s-trans-all-trans 1,3,5,7-octatetraene at 4.3 K illustrates the need for a new mechanism to explain the observed behavior [150]. Upon irradiation, all-s-trans-all-trans 1,3,5,7-octatetraene at 4.3 K undergoes conformational change from all-s-trans to 2-s-cis. Based on NEER principle (NonEquilibrium of Excited state Rotamers), that holds good in solution, the above transformation is not expected. NEER postulate and one bond flip mechanism allow only trans to cis conversion rotations of single bonds are prevented as the bond order between the original C C bonds increases in the excited state. However, the above simple photochemical reaction is explainable based on a hula-twist process. The free volume available for the all-s-trans-all-trans 1,3,5,7-octatetraene in the //-octane matrix at 4.3 K is very small and under such conditions, the only volume conserving process that this molecule can undergo is hula-twist at carbon-2. 

The so-called torsion model by Kakitani and Kakitani [203] was developed to explain the high quantum yield of photoisomerization in rhodopsins as compared to isomerization yields observed in model protonated Schiff bases. The main feature of this model is the twist around double bonds in retinal when it is bound to opsin, which would make this substrate strained to be near the transition state for isomerization (Fig. 17). The most important twist as estimated by this model would be around the 11,12 double bond, although other double bonds would simultaneously be twisted to some extent ([204] and refs, therein). The protein was considered to be responsible for inducing the selective twist. This model predicted a... 

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