Photo-induced isomerisation

The photochromic behaviour of synthetic flavylium compounds, more properly the one of the trans-chalcone spiecies, only recently was perceived as a new photochromic system with many p>otentiahties.[7] In spite of its ap>p)arent complexity is a simple sequence of three well known elementary steps i) photo induced isomerisation ii) cyclization and iii) dehydration to give the coloured product, the flavylium cation, (or the very coloured quinoidal base, depending on pH), Scheme 2.7. 

In the absence of nucleophilic species, the radical cations generated in the photo-induced electron-transfer reactions may undergo other reactions. Thus, 1-phenoxypropene undergoes cis-trans isomerisation on irradiation in the presence of electron acceptors such as dicyanobenzene (Majima et al., 1979). Some alkenes undergo dimerisation giving cyclobutanes on irradiation in the... 

The photoisomerisation kinetics and other properties of the 1 1 inclusion complexes formed between aromatic derivatives of norbomadiene and P-cyclo-dextrin have been measured." (S)- or (R)-2-Chloropropiophenone affords partially racemised (S)- or (R)-2-phenylpropionic acid respectively by a photo-induced rearrangement via what is probably an ion or radieal intermediate," and (Z)-N-substituted benzoyl-a-dehydrophenylalanines such as (7) are photo-isomerised to 1-azetidine derivatives (8) by a 1,3-acyl migration. Irradiation of 9,9 -bifluorene-9,9 -diol (9) gives a mixture of fluoren-9-one and spiro[9H-fluorene-9,9 (10 -H)-phenanthren]-10 -one (10) whose composition is solvent dependent with the more polar solvents favouring (10). Laser flash photolysis shows the presence of two transients, one of which can be identified with the 9-fluorenyl cation (11), and which originates from photoheterolysis of the diol (9). There is also evidence to support the view that unimolecular rearrange-... 

In our work, we have used one of these photoisomers, [Ru(S02)(NH3)4(H20)j. [tosylate]2, as a test material in the development of photo-induced X-ray crystallographic techniques at the synchrotron radiation source (SRS) at Daresbury UK. Cop-pens and coworkers had shown previously that the tj -S02 group photo-isomerised to the metastable 17 -802 species upon application of light at 355 nm, with the crystal held at 100K [10, 11], During our tests, we first reproduced this result, and then varied several key experimental parameters to observe and to understand better their effect on the resultant refinement of the light-activated crystal stiucture. 

Although the success of the proposed model, there is no spectral evidence for the presence of cis double bonds in the yellow solution. Moreover, all attempts to produce the cis-structure by photo-induced or catalytic cis trans isomerisation led to a random chain scission rather than the intended structure. Probably increased amount of the ciis-double bonds decrease the stability of the polymer chain due to the repulsive interaction between the side groups and cause the observed chain scission. 

On the other hand, the optical anisotropic properties of all the samples prepared were also evaluated. Optical anisotropy is a photo-induced birefringence in the polymeric films resulting from a reorientation of the azobenzene moieties. Generally, linearly polarized light is used to provoke a trans-cis isomerisation followed by a molecular reorientation and a cis-trans isomerisation. The absorption and reorientation sequence will be repeated until the azobenzene molecules dipole moment lies in a direction which is perpendicular to the polarisation direction of the writing beam. In this way, optical anisotropy can be induced in the films [7]. 

The major cycloaddition reactions of a, P-unsaturated ketones is intermolecular of +71 ] type. These involve photo induced geometrical isomerisation followed by a ground state reaction of highly reactive frans-cyclic enone. 

Fleischmann EK, Zentel R (2013) Liquid-crystalline ordering as a concept in materials science from semiconductors to stimuli-responsive devices. Angew Chem Int Ed 52 8810-8827 Garcia-Amoros J, Velasco D (2014) Understanding the fast thermal isomerisation of azophenols in glassy and liquid-crystalline polymers. Phys Chem Chem Phys 16 3108-3114 Garcia-Amoros J, Szymezyk A, Velasco D (2009) Nematic-to-isotropic photo-induced phase transition in azobenzene-doped low-molar liquid crystals. Phys Chem Chem Phys 11 4244-4250... 

The other common photo-induced initial step is the cis-trans isomerisation as in the case of stilbenes and azobenzenes. Scheme 2.3. Other systems consist of two steps, a cis-trans isomerisation followed by cyclization, as in diarylethenes. Scheme 2.4, and fulgides. Scheme 2.5. In the case of diarylethenes the photoisomerisation step is avoided by "freezing" the compound in its cis configuration through an appropriate structural modification. This prevents the initial isomerisation process and the photochromism is only controlled by the cyclization reaction. Scheme 2.4, bottom. [4]... 

Irradiation of Ct (pH=4 for example) photo-induces the isomerisation as shown in Scheme 2.8. In sprite of the compretition between the direct recovery of Ct from Cc, part of Cc leads the photoproducts AH+ (A) which are thus accumulate under irradiation. The photostationary state is reached when the thermal rate of AH+ (A) disapprearance equals the rate of its photochemical production. 

Under free-radical conditions (often induced by photochemical means), thiol-ene reactions proceed via a typical chain process with initiation, propagation and termination steps (Scheme 6.3). The mechanism involves a thiyl radical cis-trans isomerisation, with concurrent formation of a thiol-ene product. Initiation requires treatment of a thiol with a photo-initiator under irradiation, and results in formation of a thiyl radical (R-S ). Propagation involves direct addition of the thiyl radical across... 

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