Transitions between pure spin states of different multiplicity are forbidden by the spin selection rule. There are, however, numerous examples in the literature of spin-forbidden or intercombinational transitions occurring in organic molecules. The occurrence of such transitions results from spin-orbit coupling, a relativistic phenomenon which induces a quantum mechanical mixing of states of different multiplicity. Thus, the spin states involved in an intercombinational transition are not pure states, but possess a small admixture of states of other multiplicities.2 Qualitatively, spin-orbit coupling arises from the interaction of the spin magnetic moment of an electron and the magnetic field resulting from the apparent motion of the n u~l e u s .~ Since the magnitude of the nuclear magnetic field is directly proportional to the nuclear charge and hence to the atomic number, spin-orbit coupling increases with increasing atomic number. Consequently, large increases in the rates of spin-forbidden radiative and radiationless transitions are often observed for molecules in the presence of atoms of high atomic n~m b e r .~ This phenomenon, which has been extensively studied in recent years, is known as the heavy-atom (H-A) effect.The presence of heavy atoms may influence both photophysical and photochemical processes in a molecule. For the purpose of discussion, a photophysical H-A effect is defined as one in which only photophysical processes are affected. A photochemical H-A effect is then defined as one in which changes in the photochemistry of the substrate are observed. In the latter case, changes in photophysical processes often occur as well.The discovery of photophysical H-A effects predated that of photochemical H-A effects by nearly 20 years. The first example of the former was reported by McClure in 1949. McClure's work involved an attempt to correlate the phosphorescence lifetime of various H-A substituted (internal H-A effect) aromatic compounds in rigid matrices (77 K) with the spin-orbit coupling parameter, C;, of the heavy atom. Although the quantitative results of this study were not good, McClure did observe a significant decrease in the phosphorescence lifetime, 7, of the compounds being investigated as the atomic number of the H-A substituent was in~reased.~ Further work by Ermolaev and Svitashev and later LaPaglia on the substrates originally investigated by McClure has revealed that in-Dwaine 0. Cowan, a native of Fresno, Calif., received the B.S. degree from Fresno State College in 1958 and the Ph.D. degree from Stanford University. After a postdoctoral year at the California Institute of Technology, he joined The Johns Hopkins University, where he is now Professor of Chemistry. His research interests include the organic solid state, organometallic chemistry, photochemistry, and electron transport in biological systems. Joseph C. Koziar received the E. A. degree and the R.D. in 1968 and 1975, respectively, from The Johns Hopkins University He is employed as a polymer chemist in the Plastics Inte...
