Mass spectrometric studies of competitive binding of C 60 and C 70 to mesosubstituted porphyrins

Competitive binding of and to meso-substituted porphyrins was studied by mass spectrometry (MS). Electrospray ionization MS was employed to acquire the mass spectra of 1 : 1 porphyrin-fullerene complexes formed in a mixture of mesosubstituted porphyrin and fullerite to determine the ratio of complexes between and . Matrix-free laser desorption ionization MS was used to obtain the mass spectra of fullerite to measure the mole fraction of and . The binding constant ratio (/) was determined from the mass spectral data. The difference in standard Gibbs free energy change, , for the competitive binding of and was calculated from /. 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 values of , whereas the other two bulky porphyrins result in positive values of . This result indicates that binding to porphyrin is thermodynamically favored over binding in non-bulky porphyrins, but disfavored in bulky ones. It also suggests that the binding mode of is different between non-bulky and bulky porphyrins, which is in line with previous experimental findings of the "side-on" binding to non-bulky porphyrins and the -like "end-on" binding to bulky porphyrins.