A phosphorimetric investigation of the external heavy-atom effect in aqueous solution and its correlation with phosphorescence intensity and quantum yield

Excitation and emission phosphorescence spectra, lifetimes, and relative quantum yields of benzene and ten monosubstituted benzenes were determined at 77 K in methanol/water 10/90 v/v and in sodium iodide/methanol/water solutions. Substituents CHO, COCH(3), COC(2)H(5), OH, OCH(3), H(3), C(2)H(5), CH(2)OH and N(CH(3))(2) were investigated. Aqueous sodium iodide solutions were found to be a suitable heavy-atom solvent for phosophorimetry. Heavy-atom enhancement factors ranged from 1.2 to 8.3 according to the molecular structure and a marked decrease of the lifetime was observed for nine compounds in aqueous sodium iodide solution. A linear log-log correlation was obtained between the relative corrected phosphorescence signal and the heavy-atom enhancement factor of the monosubstituted benzenes. A similar correlation was found between the quantum yields and the heavy-atom enhancement factors. The analytical usefulness of these correlations is evident for the prediction of the magnitude of the heavy-atom effect.