A Quenchofluorometric Study of Polycyclic Aromatic Hydrocarbons in Molecularly Organized Media Page: 2

                                
selected nonalternant fluoranthenoid and fluorenoid derivatives are quenched by
nitromethane significantly. Loss of selective quenching behavior of nitromethane is
attributed to the charge stabilization involving the negatively-charged headgroup of the
micellar aggregate and the developing positive charge on the PAH ring system during the
excited-state electron/charge transfer process. Effect of external additives is examined
through the addition of alcohol cosurfactants; "charge-screening" salts and "structure-
breaking" cosolvent. Loss of quenching selectivity in anionic micelles becomes less
severe as the "effective" surface charge density is decreased due to the presence of the
additives.
Alkylpyridinium cations, known as surfactant quenchers, are found to quench the
fluorescence emission of alternant PAHs only. Discovery of alkylpyridinium cations as
selective quenching agents is important from a chemical analysis point-of-view. Unlike
nitromethane, alkylpyridinium surfactants are optically transparent in the excitation
spectral region of many of the PAHs and therefore inner-filtering corrections are
minimized.
Fluorescence quenching is used to probe the structural features in mixed micelles
containing the various combinations of anionic, cationic, nonionic and zwitterionic
surfactants. Experimental results provide valuable information regarding molecular
interactions between the dissimilar surfactants.