Four-state system photochemical reactions

Transform-limited (TL) visible pulses with as short as 3.9 and 4.7-fs duration have been generated from noncollinear optical parametric amplifiers (NOPA)[l-7], and they were applied to the study of various systems [8-14], We reported several new phenomena the direct observation of transition state during photochemical reaction by probing the change in the electronic transition probability induced by nuclear motion. In this paper four subjects are discussed. They are (1) NOPA, (2) polyacetylene, (3) polydiacetylene and (4) bacteriorhodopsin. 

Four-membered rings can be synthesised by [2 + 2] cycloadditions. However, thermal [2 + 2] cycloadditions occur only with difficulty they are not concerted but involve diradicals. Photochemical [2 + 2] reactions are common and although some of these may occur by a stepwise mechanism many are concerted. An example of a [2 + 2] reaction is the photodimerisation of cyclopent-2-enone. This compound, as the neat liquid, or in a variety of solvents, on irradiation with light of wavelength greater than 300 nm (the n - n excited state is involved) is converted to a mixture of the head-to-head (48) and head-to-tail (49) dimers, both having the cis,anti,cis stereochemistry as shown. It is believed that the reaction proceeds by attack of an n - n triplet excited species on a ground state molecule of the unsaturated ketone (Section 2.17.5, p. 106). In the reaction described (Expt 7.24) the cyclopent-2-enone is irradiated in methanol and the head-to-tail dimer further reacts with the solvent to form the di-acetal which conveniently crystallises from the reaction medium as the irradiation proceeds the other dimer (the minor product under these conditions) remains in solution. The di-acetal is converted to the diketone by treatment with the two-phase dilute hydrochloric acid-dichloromethane system. 

The lifetime of the state responsible for this emission could be determined by single photon counting and varied from 45 ns for n=2 to less than 10 ns for n=5. Kinetic analysis by quenching of the reaction and quenching of fluorescence indicated that the exclmer formation lies on the reaction coordinate leading to product. An analogous statement was made by Aladekomo in the dimerization of 9-methylanthracene (106). An interesting point is the observation of excimer emission in a photochemical reactive system in which the chain contains more than four... 

Solution (a) This is a photochemical electrocyclic reaction. Electrocyclic reactions are concerted, therefore they are completely stereospecific. The exact stereochemistry of the product depends upon the number of double bonds in the polyene molecular orbital theory allows us to predict this stereochemistry. Let us look at the electron configuration of butadiene, a four tt electron system, in the ground state and in the first excited state (achieved by the absorption of radiation) ... 

Applying this rule in the case of 1,3—butadiene, which has four electrons in the tt system, predicts that cyclization would occur as a result of con-rotation as was deduced earlier. The cyclization of 1,3,5—hexatriene (which has six 7T electrons) occurs by disrotation. It must be remembered that these predictions apply to thermally induced cycHzations and that photochemical excitation produces an excited state that causes the rules stated previously to be reversed for that type of reaction. 

I. Four-Electron Hiickel Systems. The literature abounds with examples of photochemical olefin addition reactions which proceed through transition states that are analogous to Hiickel cyclobutadiene e.g.. 

The reaction of two alkene molecules to give a cyclobutane ring has four ti electrons in the transition state. It is an example of a 4 7t system. Since each molecule contributes two ti electrons to the transition state, it is a [2 + 2] cycloaddition. A [2+2] cycloaddition occurs photochemically, not thermally. 

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