Interactions between a tetraanionic porphyrin and the methylviologen dication in methanol studied by fluorescence quenching reaction

Abstract Fluorescence decay curves for 5,10,15,20-tetrakis(4-sulfonatophenyl)-21H,23H-porphine tetraanion (TPPS 4− ) have been measured in the absence and presence of the methylviologen dication (MV 2+ ) with various ionic strengths in methanol. In the presence of MV 2+ the fluorescence decays can be expressed by a double exponential function, I ( t = I 1 exp(− t /τ 1 ) + I 2 exp(− t /τ 2 ). The contribution by the faster decay component to the total fluorescence signal increases with increasing MV 2+ concentration. The faster decay process is attributed to fluorescence from the excited state of a solvent-separated ion pair (SSIP) formed between TPPS 4− and MV 2+ , and the slower process is attributed to fluorescence from free TPPS 4− ions in the solution. Rate constants for the quenching of fluorescence from free TPPS 4− by MV 2+ ( k q ) and formation constants for the SSIP ( K SSIP ) were calculated and both are found to decrease with increasing ionic strength. The decrease in k q and K SSIP values can be interpreted in terms of the shielding of electrostatic attraction between the ions.