Excited states ethene

Fig. 13.3. Orbital correlation diagram for one ground-state ethene and one excited-state ethene. The symmetry designations apply, respectively, to the horizontal and vertical planes for two ethene molecules approaching one another in parallel planes.

Serrano-Andres, L., Merchan, M., Nebot-Gil, I., Lindh, R., Roos, B. O., 1993, Towards an Accurate Molecular Orbital Theory for Excited States Ethene, Butadiene, and Hexatriene , J. Chem. Phys., 98, 3151. 

The internal hydrogen bond (N—H—N) is responsible for suppression of cis-trans isomerization in the singlet excited state of cA-l-(2-indolyl)-2-(2-pyridyl)ethene (32) of reaction 13. Clearly, the reverse isomerization is possible125. On the contrary, irradiation of c -l-(2-pyrrolyl)-2-(2-quinolyl)ethene126 (35) induces the isomerization to 34. The isomerization (cis-trans) is possible because the excitation allows the tautomerization of 35 to 36 (equation 14). 

Electronic States of a Diatomic Molecule. In the picture of orbitals a one-electron excited state is described by the initial and final orbitals and by the multiplicity of the excited state. In an unsaturated molecule like ethylene (ethene) there will therefore be excited states labelled 3 (tt-tt ), 1 (xr—zr8 6), 3(7T-cr ), etc. in order of increasing energy. Two- or more electron excitations would require two or more such labels, but in practice the number of accessible excited states is quite small. For the purposes of photophysics and photochemistry it is almost always sufficient to consider one-electron excitations. 

When radiation of wavelengths on the order of 120 nm is absorbed by a molecule of ethene, the excited state has just sufficient energy (about 250 kcal... 

One may well ask why the isomerization of alkenes discussed in the preceding section requires a sensitizer. Why cannot the same result be achieved by direct irradiation One reason is that a tt — tt singlet excited state (5,) produced by direct irradiation of an alkene or arene crosses over to the triplet state (Ij) inefficiently (compared to n —> it excitation of ketones). Also, the Si state leads to other reactions beside isomerization which, in the case of 1,2-diphenyl-ethene and other conjugated hydrocarbons, produce cyclic products. For example, cw-l,2-diphenylethene irradiated in the presence of oxygen gives phenanthrene by the sequence of Equation 28-8. The primary photoreaction is cyclization to a dihydrophenanthrene intermediate, 6, which, in the presence of oxygen, is converted to phenanthrene ... 

Next, let s consider the photochemical version of this reaction. A photochemical reaction between two molecules involves the excited state of one component and the ground state of the other. (Because most excited states have extremely short lifetimes, the chances of two excited molecules colliding are exceedingly small, so reactions where both components are excited are highly improbable.) The HOMO of the excited ethene is n, and the LUMO of the ground-state ethene is also tt. The overlaps are as follows ... 

Similar to ketones, quinones have also been known for a long time to undergo PET processes with several donors such as aromatic hydrocarbons, olefins and amines etc. [10b, 11]. It is pertinent here to illustrate one of the synthetically important quinone-olefin [186] reactions which has been utilized for the preparation of Benz (a) anthracene 7,12-dione derivatives (237). In the present example, the excited state of naphthaquinone (234) reacts with ethene (235) to give 236, provided the electron transfer is thermodynamically allowed (ca. AGet < 0). In the follow up processes, (236) is transformed to (237) in 30-50% yield. Another report from the same group has described [187] a novel method of photoallylation of naphthaquinone by allyl stannane. 

Electron Transfer Processes - A study of the benzophenone/acetonitrile/t-butylamine system has shown that electron transfer occurs from the amine to the excited state benzophenone. Hydrogen abstraction processes within this system ultimately yield the CH2CN radical. When this species is generated in the presence of a diarylethene such as that shown in Scheme 1 then addition affords reasonable yields of the adducts (16). The mode of addition is dictated by the stability of the radical formed on addition to the ethene bond. Addition can also occur to alkenes such as (17) when the adduct (18) is obtained. SET processes also provide a novel synthetic path to 5,6-dihydro-4/f-l,2-oxazines. These reactions involve the DCA-sensitized transformations of the y,5-unsaturated oximes (19). The reactions are carried out in acetonitrile solution with irradiation through Pyrex for no longer than 30 min. This treatment yields the products (20) in reasonable yields. The likely mechanism for the process involves the formation of the intermediate (21) which cyclizes to yield the final products. The reaction is to some extent substituent dependent and when the oxime (19, R = H, R = R = R = Me) is irradiated under the same conditions for 3 h no reaction is... 

Benzo[l,2-b 4,3-b ]dithiophene is one of the products formed on irradiation of c -1,2-di(2-thienyl)ethene both in degassed and aerated solution. The analysis of the system suggests that a singlet excited state is involved. ... 

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