Mass spectrometric studies of competitive binding of C60 and C70 to mesosubstituted porphyrins

Competitive binding of C60 and C70 to meso-substituted porphyrins was studied by mass spectrometry (MS). Electrosprayionization MS was employed to acquire the mass spectra of 1 : 1 porphyrin–fullerene complexes formed in a mixture of mesosubstitutedporphyrin and fullerite to determine the ratio of complexes between C60 and C70. Matrix-free laser desorption ionizationMS was used to obtain the mass spectra of fullerite to measure the mole fraction of C60 and C70. The binding constant ratio (K70/K60)was determined from the mass spectral data. The difference in standard Gibbs free energy change, Δ(ΔGo)70–60, for the competitivebinding of C60 and C70 was calculated from K70/K60. Of the five porphyrins, tetraphenyl, tetra(4-pyridyl), tetra(4-carboxyphenyl),tetra(3,5-di-tert-butylphenyl), and tetra(pentafluorophenyl) porphyrins, the first three non-bulky porphyrins yield negative valuesof Δ(ΔGo)70−60, whereas the other two bulky porphyrins result in positive values of Δ(ΔGo)70−60. This result indicates that C70binding to porphyrin is thermodynamically favored over C60 binding in non-bulky porphyrins, but disfavored in bulky ones. Italso suggests that the binding mode of C70 is different between non-bulky and bulky porphyrins, which is in line with previousexperimental findings of the “side-on” binding to non-bulky porphyrins and the C60-like “end-on” binding to bulky porphyrins. KCI Citation Count: 0