Effect of Heavy Atoms on Intercombinational Transitions in Aromatic Compounds

Effect of Heavy Atoms on Intercombinational Transitions in Aromatic Compounds 

The effect of the substitution of a heavy-atom directly onto the nucleus of aromatic compounds (internal heavy-atom effect) on intercombinational radiative and nonradiative processes can be seen by examination of experimental data obtained for naphthalene and its derivatives. The data obtained by Ermolaev and Svitashev ° and analyzed by Birks to obtain individual rate constants for the various processes are collected in Table 5.9. 

Cowan and Drisko have studied the photodimerization of acenaphthylene in detail and have concluded that the cis dimer is derived primarily from a singlet excimer state of acenaphthylene while the irons dimer is derived entirely from the acenaphthylene triplet state  

The amount of irons dimer formed upon the photolysis of acenaphthylene in heavy-atom solvents could be correlated with the square of the spin-orbit coupling constant for the heaviest atom in the solvent (see Table 5.11)  

Thus it can be seen that the heavy-atom effect, so useful in spectroscopy, can also be an important tool in photochemistry, not only to facilitate the study of a reaction mechanism, but also to control the major reaction product. This general system and other related 2n -i- 2w photoaddition reactions will be considered in more detail in Chapter 10. 

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