Photochemical isomerisation Mechanism

A DFT-based computational study has established the thermal and photochemical isomerisation mechanisms from the terminal P-bound phosphinidene oxide complex [Ru(tpy)(bpy)(POPh)] to the corresponding O-bound, [Ru(tpy)(bpy)(OPPh)] . Thermal isomerisation was found to be both kinetically and thermodynamically unfavourable, while photoisomerisation can readily take place by either adiabatic or non-adiabatic pathways. The different absorption spectra of the two isomers and the bi-stability of the system make this complex a good candidate for photochromism. ... 

The photochemical isomerisation of 5-membered aromatic heterocycles is associated with either of two mechanisms depending on the structure of the heterocycle. One of these (scheme 1) involves... 

As a team, consider the following historic preparation of a tris(l,l-dimethylethyl) derivative of Dewar benzene, B, by the photochemical isomerisation of l,2,4-tris(l,l-dimethylethyl) benzene by van Tamelen and Pappas (1962). B does not revert to A via either a thermal or a photochemical electrocyclic mechanism. Formulate a mechanism for the conversion of A to B and explain the kinetic robustness of B with respect to the regeneration of A. 

Mechanisms of the following photochemical isomerisations can be described as conversion of o-xylene into m-xylene and m-xylene into p-xylene, is accompained by 1, 2-alkyl shift. Similarly conversion of o-xylene into p-xylene and vice-versa is accompained by the 1, 3-alkyl group shift. 

Until very recently this type of conversion had not been observed for phosphine functionalised ferrocenes, although under certain conditions (solvent, acid, photochemical intiation, for example) similar conversions have been reported for related systems. [182] The bis-planar chiral ferrocenyldiphosphine bis(l-(diphenylphosphino)-T)-indenyl)iron(II) was found to undergo an isomerisation from the meso isomer to the rac isomer in tetrahydrofuran solvent at room temperature. [183] Among attempts to understand the mechanism of this... 

In contrast to thermal reactions, any photochemical reaction is accompanied by a number of photophysical processes which all have to be taken into account in the reaction scheme. Most of these processes are thermal reactions. The mechanism of the photoreaction and - as we will see in the examples - the photochemical quantum yield depends on these photophysical steps. One of the most simple photoreactions is a photo-isomerisation... 

The photoisomerisation of azobenzene in methanolic solution is a preferable photoreaction to be used in actinometry, since the mechanism has been well examined. The trans-cis photoreaction is photoreversible. The thermal reaction cis trans will not disturb photokinetics at normal conditions because of a half-life of approximately 1 week. Therefore a concentrated solution of trans-azobenzene in methanol at 6.4 x 1(H mol h, which totally absorbs radiation between 345 and 240 nm, can be used taking eq. (5.107) and the approximation given by eq. (5.109). In a first approximation the change in absorbance with time at a wavelength of observation is proportional to the intensity of radiation. This proportionality includes the photochemical quantum yields of the trans cis isomerisation step, the factor 1000 and the absorption coefficients at the observation wavelengths of the trans and cis isomers. 

In Section 5.6.1.1 quantitative results for the partial photochemical quantum yields of the photoreaction of stilbene-1 were given. A photoisomerisation was assumed to be the mechanism as a first step. This proposal is supported by a reaction chromatogram of the photodegradation reaction of this laser dye. The two parts of the diagram (see Fig. 5.58) prove a photo-reversible isomerisation as a first step [190]. This information was used for the evaluation mentioned. 

The classic photochemical reaction involving thiophenes is the isomerisation of 2-arylthiophenes to 3-arylthiophenes the aromatic substituent remains attached to the same carbon and the net effect has been shown to involve interchange of C-2 and C-3, with C-4 and C-5 remaining in the same relative positions scrambling of deuterium labelling is however observed and the detailed mechanism for the rearrangement is still a matter for discussion. 

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... 

Thermolysis of [(triphos)Ir(H)2Et] with styrene afforded [(triphos)IrH(ti2-CH2=CHI%)] which on photolysis underwent an insertion of iridium into a C-H bond to give a 1 1 mixture of the ( -) and (Z-) styryl complexes [(triphos)Ir(H)2(CH=CHPh)]. The ethene complexes lCpIr(L)(C2H4)] (L = CO, PPh3) were reported" to undergo two competing photochemical reactions in solution - isomerisation to a vinyl hydride and dissociation of ethene widi insertion of iridium into solvent C-H bonds. The mechanisms of these reactions were studied in detail. 

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