INTERSYSTEM CROSSING IN CHARGE-TRANSFER COMPLEXES

Abstract

“The quantum-yield ratio of phosphorescence to fluorescence, $\Phi_{p}/\Phi_{t}$ of the donor component has been investigated for charge-transfer complexes of naphthalene, phenanthrene and acenaphthene with the acceptors: \emph{sym}-trinitrobenzene, 2,4,7-trinitrnofluorenone, tetrachlorophthalic anhydride, tetrabromophthalic anhydride and tetraiodophthalic anhydride. It is shown that $\Phi_{p}/\Phi_{t}$ increases for fixed donor concentration as the concentration of the acceptor increases, and that the value of $\Phi_{p}/\Phi_{t}$ of the complexed emitter may be deduced from the observed rate of increase of $\Phi_{p}/\Phi_{t}$. By this means it is shown that complexation with \emph{sym}-trinitrobenzene increases the quantum yield ratios of naphthalene and acenaphthene by factors of approximately 30 and 75, respectively. An increase of approximately $10^{a}$ is observed upon complexation with the tetrahalophthalic anhydrides, and a heavy-atom effect is distinctly manifested within the anhydride series. It is possible to bracket values of the intersystem rate constant, $K_{18}$ whence it is shown that in charge-transfer complexes the increase in the intersystem-crossing probability makes it an important fluorescence quenching mode. It is also shown that the intersystem-crossing process is siginificantly more sensitive to complexation and heavy-atom effects than the phosphorescence emission or quenching processes. The observed effects are briefly discussed in terms of conventional theories. “