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Noncovalent Binding between Fullerenes and Protonated Porphyrins in the Gas Phase

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journal contribution
posted on 04.11.2010, 00:00 by Sunghan Jung, Jongcheol Seo, Seung Koo Shin
Noncovalent interactions between protonated porphyrin and fullerenes (C60 and C70) were studied with five different meso-substituted porphyrins in the gas phase. The protonated porphyrin−fullerene complexes were generated by electrospray ionization of the porphyrin−fullerene mixture in 3:1 dichloromethane/methanol containing formic acid. All singly protonated porphyrins formed the 1:1 complexes, whereas porphyrins doubly protonated on the porphine center yielded no complexes. The complex ion was mass-selected and then characterized by collision-induced dissociation with Xe. Collisional activation exclusively led to a loss of neutral fullerene, indicating noncovalent binding of fullerene to protonated porphyrin. In addition, the dissociation yield was measured as a function of collision energy, and the energy inducing 50% dissociation was determined as a measure of binding energy. Experimental results show that C70 binds to the protonated porphyrins more strongly than C60, and electron-donating substituents at the meso positions increase the fullerene binding energy, whereas electron-withdrawing substituents decrease it. To gain insight into π−π interactions between protonated porphyrin and fullerene, we calculated the proton affinity and HOMO and LUMO energies of porphyrin using Hartree−Fock and configuration interaction singles theory and obtained the binding energy of the protonated porphyrin−fullerene complex using density functional theory. Theory suggests that the protonated porphyrin−fullerene complex is stabilized by π−π interactions where the protonated porphyrin accepts π-electrons from fullerene, and porphyrins carrying bulky substituents prefer the end-on binding of C70 due to the steric hindrance, whereas those carrying less-bulky substituents favor the side-on binding of C70.