Ground- and excited-state characterization of an electrostatic complex between tetrakis-(4-sulfonatophenyl)porphyrin and 16-pyrimidinium crown-4

We report the formation of an electrostatic complex between (16-pyrimidiniu m crown-4)tetranitrate (16PC4) and tetrakis-(4-sulfonatophenyl)porphyrin (4 SP) in aqueous solution. Ground-state complex formation results in a red sh ift of the 4SP visible absorption bands and a decrease in absorbance of the Soret band, The equilibrium constant for complex formation (determined fro m optical titrations) is found to be (2.0 +/- 0.2) x 10(5) M-1. In addition , the data fit to an expression describing a 1:1 stoichiometry. Excitation of the complex results in quenching of both the excited singlet and triplet states of the associated porphyrin, The singlet-state lifetime decreases f rom 10 ns for the free porphyrin to 1.5 ns in the presence of 16PC4 at low solution ionic strengths. In addition, evidence is presented for triplet-st ate quenching within the complex with k(q) = (1.1 +/- 0.1) x 10(4) s(-1). T he mechanism of quenching is tentatively assigned to electron transfer from either the excited singlet or excited triplet state of the porphyrin to th e ground state of the 16PC4.