Effects of interaction with CTAB micelles on photophysical characteristics of meso-tetrakis(sulfonatophenyl) porphyrin

The whole set of photophysical characteristics of the metal-free water-soluble meso-tetrakis(p-sulfonatophenyl) porphyrin (TPPS4) in its protonated and nonprotonated states in homogeneous aqueous solutions and in the presence of cationic cetyltrimethylammonium bromide micelles (CTAB) was investigated with nonlinear optical Z-scan technique, UV–vis absorption and fluorescence spectroscopies, time-resolved fluorescence and flash-photolysis. The characteristics were: the cross-sections of the ground (σ0) and the excited singlet (σS) and triplet (σT) states; the rate constants of intersystem-crossing (kisc), internal conversion (kic) and radiative one (kr); the quantum yields (Φfl and ΦT) and lifetimes (τS and τT) of singlet and triplet states and quantum yield of internal conversion (Φic). The study was realized at pH 7.0 and 4.0, where TPPS4 in homogeneous solutions is nonprotonated and biprotonated, respectively. It was observed that, in spite of the fact that protonation changes all these characteristics, their values for TPPS4 bound with CTAB micelles were at both pH close to those of nonprotonated porphyrin in the solution. It is due to the fact that binding with CTAB shifts the pK of TPPS4 from 5.0 in homogeneous solution to 2.5 in the micellar one and the bound porphyrin appears in nonprotonated state at both pH used. We can conclude also that the change of the quality of the environment from homogeneous aqueous solution to the micellar one affects weakly the TPPS4 photophysical characteristics when it is not protonated.