Energy Transfer in Charge-Transfer Complexes. III. Intersystem Crossing

Abstract

The quantum yield ratio of phosphorescence to fluorescence, φ_p/φ_f, of the donor component has been investigated for charge‐transfer complexes of naphthalene, phenanthrene, and acenaphthene with the acceptors sym‐trinitrobenzene, 2,4,7‐trinitrofluorenone, tetrachlorophthalic anhydride, tetrabromophthalic anhydride, and tetra‐iodophthalic anhydride. It is shown that φ_p/φ_f increases for fixed donor concentration as the concentration of the acceptor increases, and that the value of φ_p/φ_fΞy″ of the complexed emitter may be deduced from the observed rate of increase of φ_p/φ_f. By this means it is shown that complexing with sym‐trinitrobenzene increases the quantum yield ratios of naphthalene and acenaphthene by factors of approximately 30 and 75, respectively. An increase of approximately 10³ 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_i, 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 significantly 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. Charge‐transfer fluorescence of some crystalline complexes at room temperature is described, and the mirror‐image relationship to the charge‐transfer absorption shown by Czekalla et al. is verified. Finally, some thermodynamic data for the investigated complexes are presented, and from the behavior of extinction coefficient and stability of the complexes as a function of λ_max of the charge‐transfer bands it is inferred that the charge‐transfer band borrows much of its intensity from transitions proper to the donor.