Fluorescence decay of tetraphenylporphyrin (TPP) in toluene with added aromatic halides (RX)such as chloro-

bromo-

and iodo-benzene has been examined. It is found that the fluorescence of TPP is quenched by these additives and the quenching kinetical curve can be best fitted with a single exponential decay law with the decay time constant τ which is related to the nature of the halogen atom in the additives

but is independent of the viscosity of the medium. Furthermore

the relative intensity and the lifetime of the TPP fluorescence are decreased linearly with increasing the additive concentration in the low concentration range (Stern-Volmer relationship). But the concentration dependence deviates from this linearity with the further increase in concentration. All these observations appear to be understandable on the basis of a general analysis of bimolecular fluorescence quenching kinetics with an assumption of formation of a complex between the TPP chromophore and the additive molecule

provided that the rate constant of complex formation is much faster than those of processes leading to the fluorescence decay. Therefore

it is concluded that the observed fluorescence quenching of TPP in the presence of aromatic halides stems from the complexation between them rather than the enhancement of spin-orbital coupling behind the heavy atom effect as suggested in other cases. The linear dependence of kinetic parameters of fluorescence quenching by different halides on their electrochemical reduction potentials further implies that the complex formed is charge transfer in nature.