Fluorescence quenching reaction of porphyrins in micelles: Ionic porphyrins quenched by nitrobenzene in ionic micelles

Fluorescence decays have been measured for 5,10,15,20-tetraphenyl-21H,23H-porphinetetrasulfonate (TPPS4−) adsorbed on the surface of the cationic micelles, decyl-, dodecyl-, tetradecyl- and hexadecyl-trimethylammonium bromide at 25 °C, and 5,10,15,20-tetrakis{4-[N-(trimethyl)ammmonio]phenyl}-21H,23H-porphine (TTMAPP4+) adsorbed on the surface of the anionic micelles, sodium decane-, dodecane-, and tetradecane-sulfonate at 55 °C, in the presence of nitrobenzene (NB). The fluorescence decay curves are well represented by a single exponential function. This is attributed to the shorter fluorescence lifetime of TPPS4− and TTMAPP4+ compared with those of Ru complexes and pyrene, where non-exponential decays have been reported. The quenching rate constants in the micelles, km, have been obtained and the dependence of km on the surfactant alkyl chain-length is calculated to be 0.21-0.23 per methylene unit. These values are much smaller than the values for the distance-dependence factor for electron-transfer reactions, i.e. 1-1.5 per methylene unit. From model calculations, it appears that NB molecules are localized primarily in the peripheral region of the micelles rather than being uniformly distributed within the micelles. This phenomenon, together with the diffusion of NB molecules within micelles, results in the quenching reactions being quasi two-dimensional and causes the small chain-length dependency observed in this work.