The organic photochemistry of benzene—I

Photodissociation of bromobenzene at 266 nm

“…27 However, in the case of aryl halides involving the electronic system of the aromatic ring, the early experimental results have shown that predissociation seems dominant over direct dissociation and the dissociation is suggested to complete on a picosecond time scale. [28][29][30] For example, Freedman et al 10 reported that the photodecomposition of C 6 H 5 Cl at 193 nm occurred from the first excited singlet state (S 1 ) which was internally converted from the third excited singlet state (S 3 ). Dzvonik et al 8 measured the angular distribution of iodine atom in iodobenzene photodissociation and pointed out that the dissociation was indirect and the dissociative lifetime was about 0.5 ps.…”

Photoaddition and Photocyclization Processes of Aromatic Compounds

“…For example the benzyl alcohol 254 gives the [3.1] meta cyclophane 255 in 54% yield by a process considered to involve electron transfer to the alcohol group followed by loss of hydroxide and radical cyclization. (200) As may be expected, the efficiency and direction of the process is very dependent on the chain length separating the arenes, and for 256 the [3,2] meta cyclophane product 257 is formed by attack at the dimethylamino substituent but the yield has dropped dramatically to 3%. …”

“…The only exceptions to this statement are found in the photoaddition of benzene to 1,2-dienes,(J1) the addition of SI naphthalene to dichlorovinylene carbonate, which yields the 1,2-(ortho) and 1,4-(para) cyc10adducts in parallel processes via a common intermediate, (2) and the additions of ethenes to the 9,10-positions of anthracenes. (3 ) In contrast, both ortho and meta photocyc1oadditions of ethenes to the SI state of benzene and its simple derivatives can, dependent on the particular system, exhibit both high chemical and quantum yields. Occasionally both 1: 1 adduct isomers are formed in approximately equal amounts, but in general the photoreaction shows selectivity and formation of one isomer is greatly favored.…”

The influence of “communicating states” on the dissociation yield in theS 1 of benzene

“…However, photoisomerization and photodissociation are also thrown into the debate to explain the mechanism of the spectral behaviour [2,[9][10][11][12][13]. Callomon et al [2] attempted to detect the formation of hydrogen by photolysis using a xenon arc as light source.…”