Thermodynamic and Kinetic Study on the Protonation of Free Base Tetraphenylporpyrin Derivatives in Solution

摘要

The protonation of tetraphenylporphyrin (TPP) in acidic organic solutions was analyzed by acid titrimetric and temperature-dependent absorption measurements. Competition between the protonation of free base TPP (TPPH2) and the solvation of proton by near solvent molecules determines the equilibrium of the diprotonated TPP (TPPH42+) formation. The diprotonated TPP exists as an ion pair complex with the acid counterions, which are found to affect the degree of red shift of the Soret band. The rotation of the phenyl rings also plays an important role in the diprotonation, as suggested by the decrease in the degree of diprotonation for the fluorophenyl TPP derivatives whose phenyl ring rotation is significantly hindered relative to normal TPP. The difference of fluorescence lifetime between TPPH2 (ンFL=19.6 ns) and TPPH42+ (ンFL=2.1 ns) was used advantageously to measure the rate of protonation in the excited state. The protonation of TPPH2 are found to occur much slower than the diffusion of protons from bulk solution to the porphyrin ring. The monoprotonated TPP is suggested to be the transient species for the diprotonation process.